Hi all. I run a professional amplifier service shop. I've seen most
different topologies used for high power audio amplification. But, this
Peavey amp really confuses me. It's a Peavey PV-1.3k massive 4U high
stereo power amp. It came into my shop with one channel badly blown,
looked like a disaster. 10pcs of blown output-devices, generally all
emitter-resistors were open, pcb traces exploded, driver-board severly
damaged. Customer refused repair and let me keep the unit for scrapping
parts. As I really enjoy audio power amps I decided to repair it just
for fun, in case I some day need 1000W per channel into 2 ohms. ;-)
Said and done, all defective parts replaced, but I can't figure out how
the stereo output-stage is configured. I'll try to explain it.
The power-supply has dual centertapped secondaries, each channel uses
its own supply of +/- 90V rails. The rails are applied to the output
stage collectors like a standard emitter-follower output stage. Now the
hocus pocus begins. Each output-device has its own pair of
emitter-resistors connected to a COMMON rail on the output-board. This
rail is actually used by both channels, so appearently both channels
are in parallel. And this rail is connected to chassis ground. This
rail is also connected to one side of the speaker output jacks.
I haven't seen anything like this before. The other side of speaker
output jacks goes straight to the centertap of the transformer.
(channel power ground) These centertaps are not connected together.
Most preferably, I would like to see the schematics for this amp,
secondly, if anyone could tell me what's going on in this hocus pocus
output stage.

Best regards
Stefan

The Peavey company is really easy to deal with.... special parts,
manuals and schematics easily available from them directly.
electricitym
..
..

I forgot to tell that the amp is actually operating.

wrote in message
.com...
The rails are applied to the output
stage collectors like a standard emitter-follower output stage. Now the
hocus pocus begins. Each output-device has its own pair of
emitter-resistors connected to a COMMON rail on the output-board. This
rail is actually used by both channels, so appearently both channels
are in parallel. And this rail is connected to chassis ground. This
rail is also connected to one side of the speaker output jacks.
Best regards
Stefan

Obviously this would dump the rails directly to ground thru the output
devices and the emitter resistors right? You must have made a mistake
tracing this out, somebody rewired this incorrectly, or you are still
measuring a short across the outputs. Are you familiar with a bridged
output? That might be a bit confusing if you've never seen one before, but
there still should be no path to ground from an output.
bg

I think you are misreading it, not uncommon. They don't build things to
be self apparent in design. What you are looking at is a high current
version of the regular audio output stage.

Now the tricky part, beware of the thing blowing up again. You said
like all (or many at least) of the emitter resistors were blown (open I
assume).

Having experience with power, when you have say, five pairs of output
devices they are never matched perfectly. Almost all the time one of
the devices fails and of course you change the whole bank.

There are only certain ways all of them could be bad:

1. The amp was running at very high power and something shorted the
output, this is not that likely unless there is no current limiting at
all.

2. The driver stage, the one common to all output devices failed, I
guess intermittently, and dumped a whole ****load of bias into the
output stage.

3. Over time, the devices shorted one at a time and opened up their
respective emitter resistors but continued to operate on the remaining
devices. (yes this can happen)

I mention this because I have worked on a few really high power amps. I
don't use ECGs or anything. One was a Luxman. When I reported the
condition of the amp to the office I recommended a price of $350 to fix
it. There was literally over $100 in parts, but we used good OEM subs,
that is from B & D. When you talk 2SC945s you don't need to be picky,
but ten outputs times about $9 for the NPN and $12 for the PNP you need
good parts. Add $4 for carpet for the bench, the dummy load melted it's
shape into it during the full power test :-)

A Phase Linear 700b came into the shop the other day, I said "Don't
look at me", those things are an incredible PITA to work on. Then I
said "You know what you use for a dummy load ?, Get an electric dryer
heating element".

Anyway, if I had that scenario, with all the outputs blown I would
short the bias out to the outputs and run the unit literally for a week
continuous. Remember to also short the bias to the drivers and
predrivers or you might cause problems. I would suspect that eventually
you will see a wiff of smoke from one of the drivers or predrivers.
Don't worry about the voltage amps unless they can increase bias. In
some amps they can.

The reason to short the bias out is so that when the $1 part blows it
will not take all your expensive silicon at the end out with it.

Of course you may be lucky that some idiot hooked it up wrong.

JURB

Well, I don't know where to start, but I can tell you this: I've
worked with high power audio amplification for 15 years on a
professional level. I've been involved with warranty/factory service,
R&D department of a very big amp manufacturer, I design and test audio
amps for living, I use Audio Precision system one and two for design
verifications. So, it's NOT so that your information in any way is new
to me. As you now may understand, I do have a very broad and deep
competence regarding audio power amps. These days I work with designs
generating over 10kW! Using exclusively switch mode power supplies,
which often is far more complicated than the amp part.
The thing is that this Peavey amp, which I decided to repair just for
fun, very hooked up quite abnormal, and I thought it interesting to
have additional information about it.
Regards, Stefan

On 8 Apr 2006 13:26:00 -0700, wrote:

Well, I don't know where to start, but I can tell you this: I've
worked with high power audio amplification for 15 years on a
professional level. I've been involved with warranty/factory service,
R&D department of a very big amp manufacturer, I design and test audio
amps for living, I use Audio Precision system one and two for design
verifications. So, it's NOT so that your information in any way is new
to me. As you now may understand, I do have a very broad and deep
competence regarding audio power amps. These days I work with designs
generating over 10kW! Using exclusively switch mode power supplies,
which often is far more complicated than the amp part.
The thing is that this Peavey amp, which I decided to repair just for
fun, very hooked up quite abnormal, and I thought it interesting to
have additional information about it.
Regards, Stefan

Is this one of those high-efficiency designs that they started making
a number of years ago?

Tom

No, it's pure class B, no class H, full rail voltage applied to output
devices. The strange thing about this amp, is that both channels have
their emitter-resistors connected to a common rail. Though,
powersupplies are separated having their own rect and caps for each
channel, sharing the same power xformer.
Electrically, it looks like both channels' outputs are connected in
parallel.
You don't have access to the schematics for this unit? (Peavey PV-1.3K)

Regards, Stefan

Sounds almost like a transconductance output stage (voltage to
current using current feedback, thus the resistors tied to the rail.)
to drive a transformer output stage. I would need the schematic.
Describe it better?

I can assume the outputs that run to the transformer(s) are tapped at
the resistor/emitters joint? if so the amplifier is still stereo and is
a transconductance design where they are most likely using current
feedback. This topology will convert the input voltage to an equal
current output. Then I can only assume that current is intended for the
final transformer outputs.

FYI. Transconductance amplifier topology is not to be confused as a
"class"

- Matt

Matty-t skrev:

Sounds almost like a transconductance output stage (voltage to
current using current feedback, thus the resistors tied to the rail.)
to drive a transformer output stage. I would need the schematic.
Describe it better?

I can assume the outputs that run to the transformer(s) are tapped at
the resistor/emitters joint? if so the amplifier is still stereo and is
a transconductance design where they are most likely using current
feedback. This topology will convert the input voltage to an equal
current output. Then I can only assume that current is intended for the
final transformer outputs.

FYI. Transconductance amplifier topology is not to be confused as a
"class"

- Matt






Hi.. yes, it could actually be some kind of current-feedback design,
here's a little more info:
DC-rails are applied to collectors like an ordinary emitter-follower.
Then each output device has its own pair of emitter-resistors (2pcs of
1ohm in parallell) connected to a common ground-rail on the power
board. There are no output-transformers. This unit can drive 2 ohm
loads directly. The transformer I was refering to was the mains
transformer.

So, the total voltage gain in the amp will be load dependent by the
current feedback?

--- Stefan

wrote:
Matty-t skrev:

Sounds almost like a transconductance output stage (voltage to
current using current feedback, thus the resistors tied to the rail.)
to drive a transformer output stage. I would need the schematic.
Describe it better?

I can assume the outputs that run to the transformer(s) are tapped at
the resistor/emitters joint? if so the amplifier is still stereo and is
a transconductance design where they are most likely using current
feedback. This topology will convert the input voltage to an equal
current output. Then I can only assume that current is intended for the
final transformer outputs.

FYI. Transconductance amplifier topology is not to be confused as a
"class"

- Matt






Hi.. yes, it could actually be some kind of current-feedback design,
here's a little more info:
DC-rails are applied to collectors like an ordinary emitter-follower.
Then each output device has its own pair of emitter-resistors (2pcs of
1ohm in parallell) connected to a common ground-rail on the power
board. There are no output-transformers. This unit can drive 2 ohm
loads directly. The transformer I was refering to was the mains
transformer.

So, the total voltage gain in the amp will be load dependent by the
current feedback?

--- Stefan


Someone--early on--suggested contacting Peavy about this amp. I
strongly second that. Everyone whom I know who has had occasion to
contact Peavy's tech support has nothing but good things to say about
them. I expect a schematic would be easily obtained.

Back in the 70's these were very common; although not very well thought
of. They were advertised as being available for a dollar a watt...very
cheap back then.

I've used a few of them back then, they sounded okay, but not
exceptional. The fact that there are not many still available might be
a clue as to how well they were designed.

jak

In article . com, wrote:
Well, I don't know where to start, but I can tell you this: I've
worked with high power audio amplification for 15 years on a
professional level. I've been involved with warranty/factory service,
R&D department of a very big amp manufacturer, I design and test audio
amps for living, I use Audio Precision system one and two for design
verifications. So, it's NOT so that your information in any way is new
to me. As you now may understand, I do have a very broad and deep
competence regarding audio power amps. These days I work with designs
generating over 10kW! Using exclusively switch mode power supplies,
which often is far more complicated than the amp part.
The thing is that this Peavey amp, which I decided to repair just for
fun, very hooked up quite abnormal, and I thought it interesting to
have additional information about it.
Regards, Stefan

I have one of these in my basement. I forgot where I got schematics
or did I. Seems like there was a bit of novelty involved as you say.
It may be due to a bridge configuration, but its been a couple years
since I worked on it. The worst part is picking it up, 65 lbs

greg

In article .com, wrote:
Hi all. I run a professional amplifier service shop. I've seen most
different topologies used for high power audio amplification. But, this
Peavey amp really confuses me. It's a Peavey PV-1.3k massive 4U high
stereo power amp. It came into my shop with one channel badly blown,
looked like a disaster. 10pcs of blown output-devices, generally all
emitter-resistors were open, pcb traces exploded, driver-board severly
damaged. Customer refused repair and let me keep the unit for scrapping
parts. As I really enjoy audio power amps I decided to repair it just
for fun, in case I some day need 1000W per channel into 2 ohms. ;-)
Said and done, all defective parts replaced, but I can't figure out how
the stereo output-stage is configured. I'll try to explain it.
The power-supply has dual centertapped secondaries, each channel uses
its own supply of +/- 90V rails. The rails are applied to the output
stage collectors like a standard emitter-follower output stage. Now the
hocus pocus begins. Each output-device has its own pair of
emitter-resistors connected to a COMMON rail on the output-board. This
rail is actually used by both channels, so appearently both channels
are in parallel. And this rail is connected to chassis ground. This
rail is also connected to one side of the speaker output jacks.
I haven't seen anything like this before. The other side of speaker
output jacks goes straight to the centertap of the transformer.
(channel power ground) These centertaps are not connected together.
Most preferably, I would like to see the schematics for this amp,
secondly, if anyone could tell me what's going on in this hocus pocus
output stage.

So when the outputs are summed and grounded, the whole amplifier
stage goes up and down in average voltage causing the common of the transformer
to go up and down also. Since the amps are isolated, bridging can
easily be accomplished. I just now recall seeing that for the first time, a bit
different. Let me know if you want the schematic.

greg

Hi greg,
Yes I would really like to have the schematics, since I find the
topology quite interesting.
If you could mail me the schematics I would be most greatful.

--- Stefan

In article .com, wrote:
Hi greg,
Yes I would really like to have the schematics, since I find the
topology quite interesting.
If you could mail me the schematics I would be most greatful.

You should get email with directions.

greg

jakdedert wrote:
wrote:
Matty-t skrev:

Sounds almost like a transconductance output stage (voltage to
current using current feedback, thus the resistors tied to the rail.)
to drive a transformer output stage. I would need the schematic.
Describe it better?

I can assume the outputs that run to the transformer(s) are tapped at
the resistor/emitters joint? if so the amplifier is still stereo and is
a transconductance design where they are most likely using current
feedback. This topology will convert the input voltage to an equal
current output. Then I can only assume that current is intended for the
final transformer outputs.

FYI. Transconductance amplifier topology is not to be confused as a
"class"

- Matt






Hi.. yes, it could actually be some kind of current-feedback design,
here's a little more info:
DC-rails are applied to collectors like an ordinary emitter-follower.
Then each output device has its own pair of emitter-resistors (2pcs of
1ohm in parallell) connected to a common ground-rail on the power
board. There are no output-transformers. This unit can drive 2 ohm
loads directly. The transformer I was refering to was the mains
transformer.

So, the total voltage gain in the amp will be load dependent by the
current feedback?

--- Stefan


Someone--early on--suggested contacting Peavy about this amp. I
strongly second that. Everyone whom I know who has had occasion to
contact Peavy's tech support has nothing but good things to say about
them. I expect a schematic would be easily obtained.

Back in the 70's these were very common; although not very well thought
of. They were advertised as being available for a dollar a watt...very
cheap back then.

I've used a few of them back then, they sounded okay, but not
exceptional. The fact that there are not many still available might be
a clue as to how well they were designed.

jak


Sheez! I shoulda' read the OP. I thought for some reason he was
talking about the venerable CS 800 amps.

Still, the advice to contact Peavey is still--IMO--most sound. No use
reverse-engineering something for which info is readily available.

jak