Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

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** Hi,

recently I wasted much time trying to get a small, off-line SMPS to work
properly. The supply was part of a 1000W class D audio amplifier and built
onto the same PCB smothered in mostly SMD parts.

The design employed an 8 pin DIL chip (Infineon ICE 2A165) that did almost
everything and it along with some SMD diodes and two 1W resistors had
failed.

After replacing all the damaged parts - it would operate briefly, then shut
down and seemed to be very fussy about the amount of load applied and DC
supply voltage too. All the waveforms look pretty good on my scope except
that across the 1 ohm current sense resistor - which showed a lot of
ringing at several MHz.

I even removed the ferrite transformer from the PCB to test it independently
and confirm it was OK.

Finally, I tried a 47nF cap directly across the 1 ohm resistor - which made
a big improvement to the behaviour. Smelling a fat rat, I removed the
resistor and broke it apart to find if it was really metal film - bingo !!

Although my resistor looked almost identical to the original, I had
accidentally swapped a film resistor for a wirewound type. It had about 12
turns of wire on the ceramic core instead of the 3 turn spiral a MF type
typically has so a tad more inductance.

Both resistors had near identical, smooth, light grey bodies with the same
coloured bands in exactly the same places - what a trap.

With a genuine MF type fitted, ringing was suppressed by about two thirds
and the SMPS finally worked as intended.



..... Phil








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On 07/26/2014 10:21 PM, Phil Allison wrote:
** Hi,

recently I wasted much time trying to get a small, off-line SMPS to work
properly. The supply was part of a 1000W class D audio amplifier and built
onto the same PCB smothered in mostly SMD parts.

The design employed an 8 pin DIL chip (Infineon ICE 2A165) that did almost
everything and it along with some SMD diodes and two 1W resistors had
failed.

After replacing all the damaged parts - it would operate briefly, then shut
down and seemed to be very fussy about the amount of load applied and DC
supply voltage too. All the waveforms look pretty good on my scope except
that across the 1 ohm current sense resistor - which showed a lot of
ringing at several MHz.

I even removed the ferrite transformer from the PCB to test it independently
and confirm it was OK.

Finally, I tried a 47nF cap directly across the 1 ohm resistor - which made
a big improvement to the behaviour. Smelling a fat rat, I removed the
resistor and broke it apart to find if it was really metal film - bingo !!

Although my resistor looked almost identical to the original, I had
accidentally swapped a film resistor for a wirewound type. It had about 12
turns of wire on the ceramic core instead of the 3 turn spiral a MF type
typically has so a tad more inductance.

Both resistors had near identical, smooth, light grey bodies with the same
coloured bands in exactly the same places - what a trap.

With a genuine MF type fitted, ringing was suppressed by about two thirds
and the SMPS finally worked as intended.



.... Phil

I worked at a pro audio company using wirewound power resistors to test
Class D power amps. I couldn't convince them to consider the reactance
effects at hypersonic frequencies, not only the switching artifacts but
the Power Factor Controllers too, create a rich VLF stew. It won't hurt
speakers but it may affect efficiency and heat load. Am I nuts?

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dave wrote:
On 07/26/2014 10:21 PM, Phil Allison wrote:
** Hi,

recently I wasted much time trying to get a small, off-line SMPS to work
properly. The supply was part of a 1000W class D audio amplifier and built
onto the same PCB smothered in mostly SMD parts.

The design employed an 8 pin DIL chip (Infineon ICE 2A165) that did almost
everything and it along with some SMD diodes and two 1W resistors had
failed.

After replacing all the damaged parts - it would operate briefly, then shut
down and seemed to be very fussy about the amount of load applied and DC
supply voltage too. All the waveforms look pretty good on my scope except
that across the 1 ohm current sense resistor - which showed a lot of
ringing at several MHz.

I even removed the ferrite transformer from the PCB to test it independently
and confirm it was OK.

Finally, I tried a 47nF cap directly across the 1 ohm resistor - which made
a big improvement to the behaviour. Smelling a fat rat, I removed the
resistor and broke it apart to find if it was really metal film - bingo !!

Although my resistor looked almost identical to the original, I had
accidentally swapped a film resistor for a wirewound type. It had about 12
turns of wire on the ceramic core instead of the 3 turn spiral a MF type
typically has so a tad more inductance.

Both resistors had near identical, smooth, light grey bodies with the same
coloured bands in exactly the same places - what a trap.

With a genuine MF type fitted, ringing was suppressed by about two thirds
and the SMPS finally worked as intended.



.... Phil

I worked at a pro audio company using wirewound power resistors to test
Class D power amps. I couldn't convince them to consider the reactance
effects at hypersonic frequencies, not only the switching artifacts but
the Power Factor Controllers too, create a rich VLF stew. It won't hurt
speakers but it may affect efficiency and heat load. Am I nuts?


unless these power resistors were very weird, as long tube shaped things
they'd still have less inductance than a speaker winding.


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"dave"

I worked at a pro audio company using wirewound power resistors to test
Class D power amps. I couldn't convince them to consider the reactance
effects at hypersonic frequencies, not only the switching artifacts but
the Power Factor Controllers too, create a rich VLF stew. It won't hurt
speakers but it may affect efficiency and heat load. Am I nuts?


** The lower the resistor's value, the more significant inductance becomes.

Egs:

I use a pair of 100W rated, 8 ohm, tubular, wire-wound resistors as a dummy
load - wired in series for 16 ohms and in parallel for 4 ohms. The ceramic
tube is about 1 inch dia and 4.5 inches long with 34 turns of flat strip
conductor would along it, so the inductance is about 6uH.

At 20kHz, the error caused by inductance is under 0.5% while at 100kHz, the
error is still only 10%.

For the 1 ohm wire-wound sense resistor, the inductance was about 200nH.
However the ringing frequency was around 3MHz making the inductive reactance
almost 4 ohms - four times more than the value of the resistor !!!


..... Phil








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"For the 1 ohm wire-wound sense resistor, the inductance was about 200nH.
However the ringing frequency was around 3MHz making the inductive reactance
almost 4 ohms - four times more than the value of the resistor !!! "

You twitwitted unsuccessful doppelganging lowlife untermention !

No idea of the parts you are putting in MY equipment ? No wonder they need consumer watchdogs organizations.

Actually I am just joking dude. If I had that problem, I would have found it btu likely it would take longer. they do not give you the specs on a print for the voltage across that usually. They CERTAINLY don't give you a spectrum ananlysis.



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On 07/26/2014 10:21 PM, Phil Allison wrote:
** Hi,

recently I wasted much time trying to get a small, off-line SMPS to work
properly. The supply was part of a 1000W class D audio amplifier and built
onto the same PCB smothered in mostly SMD parts.

The design employed an 8 pin DIL chip (Infineon ICE 2A165) that did almost
everything and it along with some SMD diodes and two 1W resistors had
failed.

After replacing all the damaged parts - it would operate briefly, then shut
down and seemed to be very fussy about the amount of load applied and DC
supply voltage too. All the waveforms look pretty good on my scope except
that across the 1 ohm current sense resistor - which showed a lot of
ringing at several MHz.

I even removed the ferrite transformer from the PCB to test it independently
and confirm it was OK.

Finally, I tried a 47nF cap directly across the 1 ohm resistor - which made
a big improvement to the behaviour. Smelling a fat rat, I removed the
resistor and broke it apart to find if it was really metal film - bingo !!

Although my resistor looked almost identical to the original, I had
accidentally swapped a film resistor for a wirewound type. It had about 12
turns of wire on the ceramic core instead of the 3 turn spiral a MF type
typically has so a tad more inductance.

Both resistors had near identical, smooth, light grey bodies with the same
coloured bands in exactly the same places - what a trap.

With a genuine MF type fitted, ringing was suppressed by about two thirds
and the SMPS finally worked as intended.



..... Phil


Thanks for the tip - I've learned something today! Mind you most of the
stuff I deal with doesn't work in those frequencies, but still, good to
have in the back of one's mind when troubleshooting or installing
replacement parts in items like switching power supplies or other medium
to high (and RF of course) frequency things...

John :-#)#
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John's Jukes Ltd. 2343 Main St., Vancouver, BC, Canada V5T 3C9
(604)872-5757 or Fax 872-2010 (Pinballs, Jukes, Video Games)
www.flippers.com
"Old pinballers never die, they just flip out."
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On 27/07/2014 3:21 PM, Phil Allison wrote:
** Hi,

recently I wasted much time trying to get a small, off-line SMPS to work
properly. The supply was part of a 1000W class D audio amplifier and built
onto the same PCB smothered in mostly SMD parts.

The design employed an 8 pin DIL chip (Infineon ICE 2A165) that did almost
everything and it along with some SMD diodes and two 1W resistors had
failed.

After replacing all the damaged parts - it would operate briefly, then shut
down and seemed to be very fussy about the amount of load applied and DC
supply voltage too. All the waveforms look pretty good on my scope except
that across the 1 ohm current sense resistor - which showed a lot of
ringing at several MHz.

I even removed the ferrite transformer from the PCB to test it independently
and confirm it was OK.

Finally, I tried a 47nF cap directly across the 1 ohm resistor - which made
a big improvement to the behaviour. Smelling a fat rat, I removed the
resistor and broke it apart to find if it was really metal film - bingo !!

Although my resistor looked almost identical to the original, I had
accidentally swapped a film resistor for a wirewound type. It had about 12
turns of wire on the ceramic core instead of the 3 turn spiral a MF type
typically has so a tad more inductance.

Both resistors had near identical, smooth, light grey bodies with the same
coloured bands in exactly the same places - what a trap.

With a genuine MF type fitted, ringing was suppressed by about two thirds
and the SMPS finally worked as intended.


**Yikes! Was it a WES resistor?


--
Trevor Wilson www.rageaudio.com.au
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"Trevor Wilson"
Phil Allison wrote:
** Hi,

recently I wasted much time trying to get a small, off-line SMPS to work
properly. The supply was part of a 1000W class D audio amplifier and
built
onto the same PCB smothered in mostly SMD parts.

The design employed an 8 pin DIL chip (Infineon ICE 2A165) that did
almost
everything and it along with some SMD diodes and two 1W resistors had
failed.

After replacing all the damaged parts - it would operate briefly, then
shut
down and seemed to be very fussy about the amount of load applied and DC
supply voltage too. All the waveforms look pretty good on my scope except
that across the 1 ohm current sense resistor - which showed a lot of
ringing at several MHz.

I even removed the ferrite transformer from the PCB to test it
independently
and confirm it was OK.

Finally, I tried a 47nF cap directly across the 1 ohm resistor - which
made
a big improvement to the behaviour. Smelling a fat rat, I removed the
resistor and broke it apart to find if it was really metal film - bingo
!!

Although my resistor looked almost identical to the original, I had
accidentally swapped a film resistor for a wirewound type. It had about
12
turns of wire on the ceramic core instead of the 3 turn spiral a MF type
typically has so a tad more inductance.

Both resistors had near identical, smooth, light grey bodies with the
same
coloured bands in exactly the same places - what a trap.

With a genuine MF type fitted, ringing was suppressed by about two thirds
and the SMPS finally worked as intended.


**Yikes! Was it a WES resistor?


*** Yep.

" 1W wirewound, low inductance type, KNP ....... "

Because it looked so similar, it was stored along with MF types. I actually
suspected the resistor pretty quickly, got another and tested it with my Bob
Parker ESR meter - it read spot on. I did a 200kHz square wave test too and
still missed it.

The particular SMPS operates at 100kHz while the ringing frequency is
3.5MHz - it has many outputs and runs ALL the low power electronics
including a valve !!

FYI:

it's internal to a Chinese " Ampeg SVT-7 pro" bass head, which has a
bridged pair of class D amps rated at 1000W into 4 ohms and a 1.2kW SMPS on
the same PCB as the small supply.

The original fault was a mid air arc-over between the leads of two 1W
resistors, spaced 8mm apart. So from +340VDC ( from the main SMPS's electro
bank ) to the IC end of the 1ohm sense resistor. Blew 200mm of 2mm wide
track right off the PCB, among all the other carnage.



..... Phil





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Default SMPS repair tipwirewound load

On 07/27/2014 11:57 AM, Cydrome Leader wrote:
dave wrote:

I worked at a pro audio company using wirewound power resistors to test
Class D power amps. I couldn't convince them to consider the reactance
effects at hypersonic frequencies, not only the switching artifacts but
the Power Factor Controllers too, create a rich VLF stew. It won't hurt
speakers but it may affect efficiency and heat load. Am I nuts?


unless these power resistors were very weird, as long tube shaped things
they'd still have less inductance than a speaker winding.

A very complex reactance anyway. The amps were ca 10KW 12KW bridged into
4 Ohm loads made of 8 Ohm cabs series parallel. There were a lot of
wirewound power resistors, in metal enclosures per OSHA, "switched" with
Banana plugs. I like the resistors you mount on heat sinks like power
transistors.


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On 07/28/2014 02:34 AM, Phil Allison wrote:
"Trevor Wilson"
Phil Allison wrote:
** Hi,

recently I wasted much time trying to get a small, off-line SMPS to work
properly. The supply was part of a 1000W class D audio amplifier and
built
onto the same PCB smothered in mostly SMD parts.

The design employed an 8 pin DIL chip (Infineon ICE 2A165) that did
almost
everything and it along with some SMD diodes and two 1W resistors had
failed.

After replacing all the damaged parts - it would operate briefly, then
shut
down and seemed to be very fussy about the amount of load applied and DC
supply voltage too. All the waveforms look pretty good on my scope except
that across the 1 ohm current sense resistor - which showed a lot of
ringing at several MHz.

I even removed the ferrite transformer from the PCB to test it
independently
and confirm it was OK.

Finally, I tried a 47nF cap directly across the 1 ohm resistor - which
made
a big improvement to the behaviour. Smelling a fat rat, I removed the
resistor and broke it apart to find if it was really metal film - bingo
!!

Although my resistor looked almost identical to the original, I had
accidentally swapped a film resistor for a wirewound type. It had about
12
turns of wire on the ceramic core instead of the 3 turn spiral a MF type
typically has so a tad more inductance.

Both resistors had near identical, smooth, light grey bodies with the
same
coloured bands in exactly the same places - what a trap.

With a genuine MF type fitted, ringing was suppressed by about two thirds
and the SMPS finally worked as intended.


**Yikes! Was it a WES resistor?


*** Yep.

" 1W wirewound, low inductance type, KNP ......."

Because it looked so similar, it was stored along with MF types. I actually
suspected the resistor pretty quickly, got another and tested it with my Bob
Parker ESR meter - it read spot on. I did a 200kHz square wave test too and
still missed it.

The particular SMPS operates at 100kHz while the ringing frequency is
3.5MHz - it has many outputs and runs ALL the low power electronics
including a valve !!

FYI:

it's internal to a Chinese " Ampeg SVT-7 pro" bass head, which has a
bridged pair of class D amps rated at 1000W into 4 ohms and a 1.2kW SMPS on
the same PCB as the small supply.

The original fault was a mid air arc-over between the leads of two 1W
resistors, spaced 8mm apart. So from +340VDC ( from the main SMPS's electro
bank ) to the IC end of the 1ohm sense resistor. Blew 200mm of 2mm wide
track right off the PCB, among all the other carnage.



.... Phil


Can't wait to see one of those. I fixed an SVT3Pro with fried
amplifier stage (everything north of the tube). Learned a few things.
That was over a year ago and it still works. Knock on MDF.



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I couldn't count all the times I ordered low value resistors and put on the page "NON INDUCTIVE". but rewally, just HOW non-inductive do I want it ?

Wire wound resistors already are wound with a bucking configuration. If you want a coil they make all the turns go the same way, if you want a resistor they do not.

Alot of times we were taliking equipment that was not really working at all that high a frquency. I have always been careful about components that go into decent audio equipment for example. I do not use ECGs or SKs or wahtever, which are fine for TV deflection circuits usuall, a few other things. But for the audio path I pay alot of attention to what I use.

But now this comes up. We knew it would happen. I mean we knew if you used a wirewound resistor in a certain application this might happen. What is jus a bit surprising is that the voltage eveloped across your resistor is not filtered. I see such designs and kinda wonder, do they really need this current response so fast that they can't use an integrator to smooth it out ? that method allows alot more configs for current limiting.

Of course that is not the trend anymore. there are SMPSes that damnear shut down in one cycle in case of a short. This of course allows them to avoid overbuilding them. Need 20 watts diddipation, get a 20.0001 watt device. that is engineering. You can't be a wasteful engineer - for long. None of my projects are ever likely to see a production line, but they will probably outlive the designer and builder.

But that's not what sells.
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diddipation ? lol

dissipation of course

It's getting bad folks
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I couldn't count all the times I ordered low value resistors and put on the
page "NON INDUCTIVE".

**Only way to get that is to order carbon composition types.


Wire wound resistors already are wound with a bucking configuration.

** That is so rare I have never come across one.

If you want a coil they make all the turns go the same way, if you want a
resistor they do not.

** Fraid they do, resistors are nearly all coils - but it hardly ever
matters.


..... Phil


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"Phil Allison"
"Trevor Wilson"

FYI:

it's internal to a Chinese " Ampeg SVT-7 pro" bass head, which has a
bridged pair of class D amps rated at 1000W into 4 ohms and a 1.2kW SMPS
on the same PCB as the small supply.

The original fault was a mid air arc-over between the leads of two 1W
resistors, spaced 8mm apart. So from +340VDC ( from the main SMPS's
electro bank ) to the IC end of the 1ohm sense resistor. Blew 200mm of 2mm
wide track right off the PCB, among all the other carnage.


** See link to good pic of the insides of one:

http://www.talkbass.com/threads/nad-....877128/page-3

Bottom of page, click on image to enlarge.

The high power SMPS uses just two TO220 pak IGBTs while the 1000W bridge amp
uses only four TO220 Mosfets.

There are no class D amp chips and the mode of operation of the output stage
is mysterious.



..... Phil




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Default SMPS repair tipwirewound load

dave wrote:
On 07/27/2014 11:57 AM, Cydrome Leader wrote:
dave wrote:

I worked at a pro audio company using wirewound power resistors to test
Class D power amps. I couldn't convince them to consider the reactance
effects at hypersonic frequencies, not only the switching artifacts but
the Power Factor Controllers too, create a rich VLF stew. It won't hurt
speakers but it may affect efficiency and heat load. Am I nuts?


unless these power resistors were very weird, as long tube shaped things
they'd still have less inductance than a speaker winding.

A very complex reactance anyway. The amps were ca 10KW 12KW bridged into
4 Ohm loads made of 8 Ohm cabs series parallel. There were a lot of
wirewound power resistors, in metal enclosures per OSHA, "switched" with
Banana plugs. I like the resistors you mount on heat sinks like power
transistors.


I hope you used special gold plated, cyogenic magical copper cables for
all that.

I just toured a plant that had gigantic resistor arrays mounted into what
appeared to be bread racks. All the resistors were sealed into oil filled
glass tubes, in house. Any like anything used on a shop floor, it had lots
of banana plugs, but with upto tens of thousands of volts across them when
energized. Each jack for each tap into the string was Dymo labelled. I
can't imagine how much it cost to make each one.

The really scary stuff was done in rooms with dented expaned metal cages.

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Phil Allison wrote:

"dave"

I worked at a pro audio company using wirewound power resistors to test
Class D power amps. I couldn't convince them to consider the reactance
effects at hypersonic frequencies, not only the switching artifacts but
the Power Factor Controllers too, create a rich VLF stew. It won't hurt
speakers but it may affect efficiency and heat load. Am I nuts?


** The lower the resistor's value, the more significant inductance becomes.

Egs:

I use a pair of 100W rated, 8 ohm, tubular, wire-wound resistors as a dummy
load - wired in series for 16 ohms and in parallel for 4 ohms. The ceramic
tube is about 1 inch dia and 4.5 inches long with 34 turns of flat strip
conductor would along it, so the inductance is about 6uH.

At 20kHz, the error caused by inductance is under 0.5% while at 100kHz, the
error is still only 10%.

For the 1 ohm wire-wound sense resistor, the inductance was about 200nH.
However the ringing frequency was around 3MHz making the inductive reactance
almost 4 ohms - four times more than the value of the resistor !!!


well, it's known that giant ceramic wire wound power resistors are not the
choice for RF use.


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Cydrome Leader wrote:

well, it's known that giant ceramic wire wound power resistors are not the
choice for RF use.



Carborundum resistors are used for RF.


--
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"Cydrome Leader"
Phil Allison

** The lower the resistor's value, the more significant inductance
becomes.

Egs:

I use a pair of 100W rated, 8 ohm, tubular, wire-wound resistors as a
dummy
load - wired in series for 16 ohms and in parallel for 4 ohms. The
ceramic
tube is about 1 inch dia and 4.5 inches long with 34 turns of flat strip
conductor would along it, so the inductance is about 6uH.

At 20kHz, the error caused by inductance is under 0.5% while at 100kHz,
the
error is still only 10%.

For the 1 ohm wire-wound sense resistor, the inductance was about 200nH.
However the ringing frequency was around 3MHz making the inductive
reactance
almost 4 ohms - four times more than the value of the resistor !!!


well, it's known that giant ceramic wire wound power resistors are not the
choice for RF use.


** Sure - but this is a current sense resistor in SMPS circuit where most
of the current is at 100kHz but with a strong harmonic at about 30 times
that frequency. Problem being, the over-current trip was being activated
prematurely because the peak value of that harmonic was being exaggerated by
a factor of 3.

While 1W metal film resistors have low enough inductance to work OK in the
circuit, typical wirewound types do not.

BTW:

I could have added a RC network ( ie a Zobel ) across the 1 ohm wirewound to
bring it line at 3MHz and well beyond. A 47nF cap and any 1 ohm, 0.5W film
type would do fine.

Same goes for the 100W 8 ohm power resistor, a Zobel using 68nF and 8.2 ohms
corrects it to beyond 5MHz if that was ever needed.


..... Phil




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On 07/28/2014 11:01 PM, Phil Allison wrote:
"Phil Allison"


uses only four TO220 Mosfets.

There are no class D amp chips and the mode of operation of the output stage
is mysterious.



.... Phil

What is a Tripath amp?

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Default SMPS repair tipwirewound load

On 07/30/2014 03:31 PM, Cydrome Leader wrote:
dave wrote:
On 07/27/2014 11:57 AM, Cydrome Leader wrote:
dave wrote:



I hope you used special gold plated, cyogenic magical copper cables for
all that.


No. 14g stranded usually. 12g for the woofs.

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On 07/30/2014 10:59 PM, Michael A. Terrell wrote:

Cydrome Leader wrote:

well, it's known that giant ceramic wire wound power resistors are not the
choice for RF use.



Carborundum resistors are used for RF.



My dummy load is full of tab resistors clamped to aluminum heat sinks.
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dave wrote:

On 07/30/2014 10:59 PM, Michael A. Terrell wrote:

Cydrome Leader wrote:

well, it's known that giant ceramic wire wound power resistors are not the
choice for RF use.



Carborundum resistors are used for RF.



My dummy load is full of tab resistors clamped to aluminum heat sinks.



That's OK at low frequencies, but can have a high VSWR t VHF and up.


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Phil Allison wrote:

"Cydrome Leader"
Phil Allison

** The lower the resistor's value, the more significant inductance
becomes.

Egs:

I use a pair of 100W rated, 8 ohm, tubular, wire-wound resistors as a
dummy
load - wired in series for 16 ohms and in parallel for 4 ohms. The
ceramic
tube is about 1 inch dia and 4.5 inches long with 34 turns of flat strip
conductor would along it, so the inductance is about 6uH.

At 20kHz, the error caused by inductance is under 0.5% while at 100kHz,
the
error is still only 10%.

For the 1 ohm wire-wound sense resistor, the inductance was about 200nH.
However the ringing frequency was around 3MHz making the inductive
reactance
almost 4 ohms - four times more than the value of the resistor !!!


well, it's known that giant ceramic wire wound power resistors are not the
choice for RF use.


** Sure - but this is a current sense resistor in SMPS circuit where most
of the current is at 100kHz but with a strong harmonic at about 30 times
that frequency. Problem being, the over-current trip was being activated
prematurely because the peak value of that harmonic was being exaggerated by
a factor of 3.


I agree with this- for the power supply problem- who'd have expected this
to happen?

While 1W metal film resistors have low enough inductance to work OK in the
circuit, typical wirewound types do not.

BTW:

I could have added a RC network ( ie a Zobel ) across the 1 ohm wirewound to
bring it line at 3MHz and well beyond. A 47nF cap and any 1 ohm, 0.5W film
type would do fine.

Same goes for the 100W 8 ohm power resistor, a Zobel using 68nF and 8.2 ohms
corrects it to beyond 5MHz if that was ever needed.


I wonder how much that would confuse the next repair tech?

Bizarro repairs are always fascinating to come across.


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"Cydrome Leader"

BTW:

I could have added a RC network ( ie a Zobel ) across the 1 ohm wirewound
to
bring it line at 3MHz and well beyond. A 47nF cap and any 1 ohm, 0.5W
film
type would do fine.

Same goes for the 100W 8 ohm power resistor, a Zobel using 68nF and 8.2
ohms
corrects it to beyond 5MHz if that was ever needed.


I wonder how much that would confuse the next repair tech?

Bizarro repairs are always fascinating to come across.


** I have sometimes added a Zobel ( ie 22nF and 56ohms, 0.5W) ) across the
output of an amplifier that did not need it.

Why ? As a tell tale in case of supersonic oscillation.

Incorrect installation and bad cabling practices can cause almost any power
amp to break into oscillation at an inaudible frequency resulting in rapid
overheating and failure of output transistors etc

It's nice for a repairer to have proof when this has happened.


..... Phil


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On 07/31/2014 05:52 PM, Phil Allison wrote:
"Cydrome Leader"

BTW:

I could have added a RC network ( ie a Zobel ) across the 1 ohm wirewound
to
bring it line at 3MHz and well beyond. A 47nF cap and any 1 ohm, 0.5W
film
type would do fine.

Same goes for the 100W 8 ohm power resistor, a Zobel using 68nF and 8.2
ohms
corrects it to beyond 5MHz if that was ever needed.


I wonder how much that would confuse the next repair tech?

Bizarro repairs are always fascinating to come across.


** I have sometimes added a Zobel ( ie 22nF and 56ohms, 0.5W) ) across the
output of an amplifier that did not need it.

Why ? As a tell tale in case of supersonic oscillation.

Incorrect installation and bad cabling practices can cause almost any power
amp to break into oscillation at an inaudible frequency resulting in rapid
overheating and failure of output transistors etc

It's nice for a repairer to have proof when this has happened.


.... Phil



I listen to distant AM radio in the shop. An amp that breaks into
oscillation (often caused by "replacement" actives with way too much
bandwidth, BTW) will cause interference on the AM (MW) radio. It is as
reliable an indicator of oscillation as fat blurry traces on the Oscope.
Also, the amp pulls more current than it should when it breaks into
oscillating. Now a SMPS without EMI suppression already makes the
interference so all bets are off.
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"dave"


I listen to distant AM radio in the shop. An amp that breaks into
oscillation (often caused by "replacement" actives with way too much
bandwidth, BTW) will cause interference on the AM (MW) radio. It is as
reliable an indicator of oscillation as fat blurry traces on the Oscope.



** Errrr - that is "parasitic" oscillation at radio frequency.

My post was about a quite different matter.

Maybe you have never seen it - it kills output stages in 30 seconds.




...... Phil






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"There are no class D amp chips and the mode of operation of the output stage
is mysterious. "

Not really. That is only a one channel amp. Even operating in BTL, if you sit down and figure out a class D drive circuit using discrete components, the count is not all that high. Class DB might take a little ore but since this is a bass amp the filters can be miles away from the operating frequency so DB is not really all that much advantage. Did you stick a scope on it and find out just what the amp itself chops at ?

"** I have sometimes added a Zobel ( ie 22nF and 56ohms, 0.5W) ) across the

output of an amplifier that did not need it.

Why ? As a tell tale in case of supersonic oscillation. "

Good thinking if you suspect that. I take it you mean mainly in analog amps..

I have done similar in amps with fuses internal. When they do not have current limiting amd people stick like a 2 ohm load on them and ****, it ain't my responsibility if the waste more silicon and they pay. I almost never do that, but have in cases of multiple recalls. One multiple recall was due to a leaky muting transistor at the input of the power amp. I ALMOST got to accusing the customer but it was always the same channel. The boss said they shouldn't be turning it all the way up, but I disagree. They are entitled to use every last watt that amp can put out. Since it was after the volume and tone controls the problem would only show up at very high levels. The ****er was rectifying the audio and imposing DC on the output. But that was where everyythignweas under warranty, a rental outfit. there is no simply siying screw it and give them a refund to divorce a piece of junk. We would have had to actually replace it. Another company found that out about selling service contracts. If you pay your housde insurance and your house burns down, just refunding your premiums does not cut it.

"Incorrect installation and bad cabling practices can cause almost any power

amp to break into oscillation at an inaudible frequency resulting in rapid
overheating and failure of output transistors etc "

This just happened to a buddy of mine recently. It was making a funky kind of distortion which cleared up at higher input levels. I said "Shut that off NOW !". Tunred out a speaker wire was running parallel to a line level input, but that was still downstrean of the gain control. It seems that luckily, the oscillation was high enough not to fry his tweeters. I cleaned that thing up and it is now quite happy, except for the one amp working into four ohms.

"Maybe you have never seen it - it kills output stages in 30 seconds"


Think about what would happen with piezo or electrostatic tweeters. Even ribbon tweeters. When it happened to my buddy, it must have been high because if it was only 50 kHz or so, with the power, those tweeters should heve been toast.
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"Phil Allison"

"There are no class D amp chips and the mode of operation of the output
stage
is mysterious. "


Not really.

** Then kindly explain how the PWM signal is being generated.


Did you stick a scope on it and find out just what the amp itself chops at
?


** There is very little HF at the output to go on, just some harmonics at
300 to 500 kHz.


"** I have sometimes added a Zobel ( ie 22nF and 56ohms, 0.5W) ) across the

output of an amplifier that did not need it.

Why ? As a tell tale in case of supersonic oscillation. "

Good thinking if you suspect that. I take it you mean mainly in analog amps.

** Is there another kind ???

( Apologies to Jack Nicholson ..... )


"Incorrect installation and bad cabling practices can cause almost any
power

amp to break into oscillation at an inaudible frequency resulting in rapid
overheating and failure of output transistors etc "

"Maybe you have never seen it - it kills output stages in 30 seconds"


Think about what would happen with piezo or electrostatic tweeters. Even
ribbon tweeters.

** Almost instant destruction.

When it happened to my buddy, it must have been high because if it was only
50 kHz or so, with the power, those tweeters should heve been toast.

** 50kHz or thereabouts oscillation does no harm to typical woofers - the
inductance of the VC protects them.




...... Phil







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On 08/01/2014 08:51 AM, Phil Allison wrote:
"dave"


I listen to distant AM radio in the shop. An amp that breaks into
oscillation (often caused by "replacement" actives with way too much
bandwidth, BTW) will cause interference on the AM (MW) radio. It is as
reliable an indicator of oscillation as fat blurry traces on the Oscope.



** Errrr - that is "parasitic" oscillation at radio frequency.

My post was about a quite different matter.

Maybe you have never seen it - it kills output stages in 30 seconds.




..... Phil


I know exactly what we are talking about. Radio Frequency starts at
10KHz. My traces fattens. The radio hisses. And the dim bulb glows
brighter. The dim bulb tells me to cut power way before any destruction
in 30 seconds happens. Modern IC opamps have too much bandwidth.

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"dave"
Phil Allison wrote:

I listen to distant AM radio in the shop. An amp that breaks into
oscillation (often caused by "replacement" actives with way too much
bandwidth, BTW) will cause interference on the AM (MW) radio. It is as
reliable an indicator of oscillation as fat blurry traces on the Oscope.



** Errrr - that is "parasitic" oscillation at radio frequency.

My post was about a quite different matter.

Maybe you have never seen it - it kills output stages in 30 seconds.



I know exactly what we are talking about.


** No you don't.

Radio Frequency starts at 10KHz.



** No it does not.

My traces fattens. The radio hisses.


** That is RF parasitics - not FULL POWER supersonic oscillation due to
positive input / output feedback.

And the dim bulb glows brighter.



** ROTFL.....


..... Phil



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"** Then kindly explain how the PWM signal is being generated. "

Relaxation oscillator to generate a sawtooth wave, comparator to chop it up into PWM. It's not that hard really. The chips that do it are just easier to use than to design it with discrete components. After tha you got your PWM square waves, then you design the drive circuit for the outputs. What's more, even though it is BTL, you only need one, it is simply inverted for the other phase.

Looking at the picture, there are plenty enough transistors and whatnot on the board to do it.

Of ocurse the question is why. Why didn't they just use some off the shelf chip for that ? To answer that we need to know things we are not going to find out. Amybe some parameter was too hard to get out of the chip, maybe their chopping frequency is lower than the chips like to operate at. Maybe certain effects were easier to implement, though I'm having a hard time fathoming any at the moment.

Or it may be attributable to audiophoolery. Some people like discrete components, myself included. I know damn well thaat what's in the chip is the same as what's in the transistor package, but...

Like if you bought a Marantz that uses an STKXXX instead of a Luxman that uses the same STKXXX, what did you get different ? Maybe us nuts just like to make the engineers work harder.

But really, among musicians you will find a healthy dose of audiophoolery, so it may be discrete for marketing purposes.

Of course in class D it doesn't mean a thing as far as I can tell. But then if ONEE golden ears can prove he hears the difference, the market will beat a path, to a certain door.
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dave wrote:

Modern IC opamps have too much bandwidth.



No, they don't. Some people just don't know how to use them
properly.


--
Anyone wanting to run for any political office in the US should have to
have a DD214, and a honorable discharge.
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"** Then kindly explain how the PWM signal is being generated. "


Relaxation oscillator to generate a sawtooth wave, comparator to chop it up
into PWM.

** Huh ??

You cannot ****ing make guesses about what the schematic is.

**** off fool.



...... Phil









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**** you.

You make an oscillator with perhaps two or three transistors. Then you get matchdf transistors and connect the emitters together. Whatwever their collectors pull current from becomes "on" time. IOr poisssiblky "off" time. I have coibsuidered boith actuyally becasue of trying to make my pet single ended push pull stage work, if you remember that. Diploma or not, I amn pretty sure it is beyond you.

How it works is a mystery my ass dude. What, you z tube jockey and fuse changer now ? I know better than that.

OK mother****er - the circuit :

An astable mul;tivibrator switching at about 50 kHz. Generate a triangle wave by simple integration. REGUALTE IT, it determines the open anded gain of a class D circuit.

Apply the input to another leg of the OP AMP you built, or just mix it with the damn triangle wave. It simply does not matter.

One of the most impportant things about engineering is to understand what matters, and what does NOT.

You know, I ight just up and get a paper and draw up something right now and upload it so you can see it. I just want to illustrate (hmm, that might do iut wh ? ) that is just ain't all that hard. ;I can concieve the entire schematicv of it in my head right now with not trouble. Putting it into Spice is a different story.

Maybre I should make it apoint to learn Spice better, that way because you cna run siulations and whit, you would better understand some of my concepts.

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Actually class D amps with less negartive feedback are excllent cnadidates for soft clipping. Just change the original sawtooth or triangle waveform. Flletten it at the peaks. What's more it is impedance independent. Anything that ****s with the gian of the output stae will change with load. When you do it this way, it is constant. ;

You can sett your watcxh to it.
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