Without differential scope amplifier/probe, audio out is missing something
in the bass range about 70-80 Hz. Line-out signal faithfully follows the
bass guitar input signal . Shure microphone, capable of those frequencies,
monitoring the sound from the speaker, does not match the input at those
sort of frequencies. But is it a failing of suspension or something of the
speaker or some inductive/capacitive interaction with the amplifier when
loaded with the speaker?
Not yet monitored the main rails to see if dipping on this bass amp.
I will then be subbing the speaker, as a test, but it still would not
determine whether an amp or a speaker problem , speaker is original to the
combo. Any simple component to put in series or parallel with the speaker to
check if an output matching problem ?

"N_Cook" wrote in message
...
Without differential scope amplifier/probe, audio out is missing something
in the bass range about 70-80 Hz. Line-out signal faithfully follows the
bass guitar input signal . Shure microphone, capable of those frequencies,
monitoring the sound from the speaker, does not match the input at those
sort of frequencies. But is it a failing of suspension or something of the
speaker or some inductive/capacitive interaction with the amplifier when
loaded with the speaker?
Not yet monitored the main rails to see if dipping on this bass amp.
I will then be subbing the speaker, as a test, but it still would not
determine whether an amp or a speaker problem , speaker is original to the
combo. Any simple component to put in series or parallel with the speaker
to
check if an output matching problem ?




Does this amp have a graphic equaliser? The line out may be pre graphic,
and one graphic fader may be faulty.



Gareth.

Gareth Magennis wrote in message
...

"N_Cook" wrote in message
...
Without differential scope amplifier/probe, audio out is missing
something
in the bass range about 70-80 Hz. Line-out signal faithfully follows the
bass guitar input signal . Shure microphone, capable of those
frequencies,
monitoring the sound from the speaker, does not match the input at those
sort of frequencies. But is it a failing of suspension or something of
the
speaker or some inductive/capacitive interaction with the amplifier when
loaded with the speaker?
Not yet monitored the main rails to see if dipping on this bass amp.
I will then be subbing the speaker, as a test, but it still would not
determine whether an amp or a speaker problem , speaker is original to
the
combo. Any simple component to put in series or parallel with the
speaker
to
check if an output matching problem ?




Does this amp have a graphic equaliser? The line out may be pre graphic,
and one graphic fader may be faulty.



Gareth.




Volume , tone controls etc are in the DSP sextion and as line out is fine ,
I assume not a problem in that piece of magic.

"N_Cook" wrote in message
...
Gareth Magennis wrote in message
...

"N_Cook" wrote in message
...
Without differential scope amplifier/probe, audio out is missing
something
in the bass range about 70-80 Hz. Line-out signal faithfully follows
the
bass guitar input signal . Shure microphone, capable of those
frequencies,
monitoring the sound from the speaker, does not match the input at
those
sort of frequencies. But is it a failing of suspension or something of
the
speaker or some inductive/capacitive interaction with the amplifier
when
loaded with the speaker?
Not yet monitored the main rails to see if dipping on this bass amp.
I will then be subbing the speaker, as a test, but it still would not
determine whether an amp or a speaker problem , speaker is original to
the
combo. Any simple component to put in series or parallel with the
speaker
to
check if an output matching problem ?




Does this amp have a graphic equaliser? The line out may be pre graphic,
and one graphic fader may be faulty.



Gareth.




Volume , tone controls etc are in the DSP sextion and as line out is fine
,
I assume not a problem in that piece of magic.






Is this a Line 6?



Gareth.

On Mon, 11 Feb 2013 14:55:42 -0000, "N_Cook"
wrote:

Without differential scope amplifier/probe,

Then get a differential scope. You have one in your computah in the
form of the stereo input of your sound card. Just install an
oscilloscope emulator program. Put some large value resistors in
series with the inputs so you don't blow anything up. Favorite
softwa
http://www.sillanumsoft.org
You'll also find it handy when troubleshooting constant voltage
speaker system.

Incidentally, looking at my pile of about 15 assorted scopes (dead and
alive), the only one that doesn't have a differential input is an
ancient Heath Vectorscope. Finding two scope probes that are matched
and/or haven't been broken by my mishandling is a bit more difficult.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

On Mon, 11 Feb 2013 14:55:42 -0000, "N_Cook"
wrote:

Without differential scope amplifier/probe,

Battery operated (AC) DVM?
Or the green/yellow wire in the scope plug...

"Geo" wrote in message
...
On Mon, 11 Feb 2013 14:55:42 -0000, "N_Cook"
wrote:

Without differential scope amplifier/probe,

Battery operated (AC) DVM?
Or the green/yellow wire in the scope plug...

NEVER do that.

On Mon, 11 Feb 2013 08:34:04 -0800, Jeff Liebermann
wrote:

On Mon, 11 Feb 2013 14:55:42 -0000, "N_Cook"
wrote:

Without differential scope amplifier/probe,

Then get a differential scope. You have one in your computah in the
form of the stereo input of your sound card. Just install an
oscilloscope emulator program. Put some large value resistors in
series with the inputs so you don't blow anything up. Favorite
softwa
http://www.sillanumsoft.org
You'll also find it handy when troubleshooting constant voltage
speaker system.

I guess I should offer a few warnings to prevent the usual disasters.
I sometimes use a sound card scope when sniffing around switching
power supplies, where the input section is usually ungrounded. It's
kinda difficult to see the waveforms due to the limited bandwidth of
the sound card, but I have a good imagination. However, there are a
few precautions.

1. Use a USB sound card dongle and not the device on the computah
motherboard. If you do something wrong, and burn up the motherboard
sound chip, you will not be very happy. A $3 dongle is essentially a
throw away device. It's also nice to have a long extension on the USB
cable, instead of a long cable on the probe.

2. Use a protective resistor in series with the input cable. It
protects against overcurrent, but also protects against idiot errors,
such as unplugging the 3.5mm jack while the probes are connected to
the AC mains. The jack will be shorted on its way out, and will make
a nifty spark.

3. Build an attenuator if you're playing with high voltages. My
guess(tm) is that the sound card input can handle about 5V peak to
peak. 10:1 for audio is fine. 100:1 for high voltage. Also, don't
count on the input capacitor, which is probably only good for about
15VDC.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

On 2/11/2013 10:37 AM, Jeff Liebermann wrote:
On Mon, 11 Feb 2013 08:34:04 -0800, Jeff
wrote:

On Mon, 11 Feb 2013 14:55:42 -0000,
wrote:

Without differential scope amplifier/probe,

Then get a differential scope. You have one in your computah in the
form of the stereo input of your sound card. Just install an
oscilloscope emulator program. Put some large value resistors in
series with the inputs so you don't blow anything up. Favorite
softwa
http://www.sillanumsoft.org
You'll also find it handy when troubleshooting constant voltage
speaker system.

I guess I should offer a few warnings to prevent the usual disasters.
I sometimes use a sound card scope when sniffing around switching
power supplies, where the input section is usually ungrounded. It's
kinda difficult to see the waveforms due to the limited bandwidth of
the sound card, but I have a good imagination. However, there are a
few precautions.

1. Use a USB sound card dongle and not the device on the computah
motherboard. If you do something wrong, and burn up the motherboard
sound chip, you will not be very happy. A $3 dongle is essentially a
throw away device. It's also nice to have a long extension on the USB
cable, instead of a long cable on the probe.

2. Use a protective resistor in series with the input cable. It
protects against overcurrent, but also protects against idiot errors,
such as unplugging the 3.5mm jack while the probes are connected to
the AC mains. The jack will be shorted on its way out, and will make
a nifty spark.

3. Build an attenuator if you're playing with high voltages. My
guess(tm) is that the sound card input can handle about 5V peak to
peak. 10:1 for audio is fine. 100:1 for high voltage. Also, don't
count on the input capacitor, which is probably only good for about
15VDC.


Can you be more descriptive about what you're doing?
If you're connecting a sound card to the line side of a switcher,
I can't imagine anything more dangerous...as in smoke/fire/death.

"N_Cook" wrote in message
...
Without differential scope amplifier/probe, audio out is missing something
in the bass range about 70-80 Hz. Line-out signal faithfully follows the
bass guitar input signal . Shure microphone, capable of those frequencies,
monitoring the sound from the speaker, does not match the input at those
sort of frequencies. But is it a failing of suspension or something of the
speaker or some inductive/capacitive interaction with the amplifier when
loaded with the speaker?
Not yet monitored the main rails to see if dipping on this bass amp.
I will then be subbing the speaker, as a test, but it still would not
determine whether an amp or a speaker problem , speaker is original to the
combo. Any simple component to put in series or parallel with the speaker
to
check if an output matching problem ?




I have a current transformer on one leg of the input to my dummy load /
power meter. This allows me to monitor non grounded amp outputs, using a bog
standard grounded scope probe into a grounded scope ...

Arfa

On Mon, 11 Feb 2013 13:29:03 -0800, mike wrote:

Can you be more descriptive about what you're doing?
If you're connecting a sound card to the line side of a switcher,
I can't imagine anything more dangerous...as in smoke/fire/death.

Left -------------------------/\/\/\/------ hot

Right ------------------------/\/\/\/------ common

Shield ------------------------------------ ground

(For a switcher, pretend the resistor is a 100:1 attentuator network
as I'm too lazy to draw it). As long as the sound card can handle the
voltage difference between hot and ground, and common and ground,
nothing will blow up. With the 100:1 attenuator, it should be no more
than about 5V on the inputs. Incidentally, using a series capacitor
for isolation doesn't work as the initial charging current will blow
up the sound dongle.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

Arfa Daily wrote in message
...


"N_Cook" wrote in message
...
Without differential scope amplifier/probe, audio out is missing
something
in the bass range about 70-80 Hz. Line-out signal faithfully follows the
bass guitar input signal . Shure microphone, capable of those
frequencies,
monitoring the sound from the speaker, does not match the input at those
sort of frequencies. But is it a failing of suspension or something of
the
speaker or some inductive/capacitive interaction with the amplifier when
loaded with the speaker?
Not yet monitored the main rails to see if dipping on this bass amp.
I will then be subbing the speaker, as a test, but it still would not
determine whether an amp or a speaker problem , speaker is original to
the
combo. Any simple component to put in series or parallel with the
speaker
to
check if an output matching problem ?




I have a current transformer on one leg of the input to my dummy load /
power meter. This allows me to monitor non grounded amp outputs, using a
bog
standard grounded scope probe into a grounded scope ...

Arfa


I suspect a current transformer would affect any PA/speaker/Zobell
resonance/interaction effect. I do have a scope with differential inputs but
as its not been used for 15 years I would bet good money on it having at
least corroded switch contacts.
I may sort out replacement rechargeable batteries for an ancient Tektronix
212 hand held scope
This is a Roland Cube Bass 30. There may be a faulty , going open, "shape"
switch and may sometime settle into the default at the bottom of the
resistor chain which is the Octave Bass setting which , may explain the odd
effect

On Tuesday, February 12, 2013 12:31:27 AM UTC-8, N_Cook wrote:
Arfa Daily wrote in message

...

I have a current transformer on one leg of the input to my dummy load /
power meter. This allows me to monitor ...

I suspect a current transformer would affect any PA/speaker/Zobell
resonance/interaction effect.

A properly loaded currrent transformer can be ten millliohms, or a few
hundred microohms. There needn't be any significant effect on the system
being measured.

"whit3rd" wrote in message
...
On Tuesday, February 12, 2013 12:31:27 AM UTC-8, N_Cook wrote:
Arfa Daily wrote in message

...

I have a current transformer on one leg of the input to my dummy load /
power meter. This allows me to monitor ...

I suspect a current transformer would affect any PA/speaker/Zobell
resonance/interaction effect.

A properly loaded currrent transformer can be ten millliohms, or a few
hundred microohms. There needn't be any significant effect on the system
being measured.


Quite. The one that I use is completely non-invasive to the system it's
monitoring - in this case an output stage into a dummy load. It's just a
fully encapsulated winding with a hole through the middle. One leg of the
input to my tester passes through this hole. The winding is terminated with
a couple of k, and my scope hooks directly across it, fully isolated from
the unit under test. It has zero measurable effect on the amplifier or any
networks connected in the amp's output circuit. I would not have designed it
in, and been using it without issue for some years, if it did ...

This is the exact one

http://ww.sourcingmap.co.uk/020a-inp...-p-133281.html

Arfa

Arfa Daily wrote in message
...


"whit3rd" wrote in message
...
On Tuesday, February 12, 2013 12:31:27 AM UTC-8, N_Cook wrote:
Arfa Daily wrote in message

...

I have a current transformer on one leg of the input to my dummy load
/
power meter. This allows me to monitor ...

I suspect a current transformer would affect any PA/speaker/Zobell
resonance/interaction effect.

A properly loaded currrent transformer can be ten millliohms, or a few
hundred microohms. There needn't be any significant effect on the
system
being measured.


Quite. The one that I use is completely non-invasive to the system it's
monitoring - in this case an output stage into a dummy load. It's just a
fully encapsulated winding with a hole through the middle. One leg of the
input to my tester passes through this hole. The winding is terminated
with
a couple of k, and my scope hooks directly across it, fully isolated from
the unit under test. It has zero measurable effect on the amplifier or any
networks connected in the amp's output circuit. I would not have designed
it
in, and been using it without issue for some years, if it did ...

This is the exact one


http://ww.sourcingmap.co.uk/020a-inp...edding-precisi
on-current-transformer-ta17-p-133281.html

Arfa



So you would pass a speaker lead through the hole, ignoring the primary
sense turns.
In the meantime I may as well try a solenoid without pole piece

"N_Cook" wrote in message
...
Arfa Daily wrote in message
...


"whit3rd" wrote in message
...
On Tuesday, February 12, 2013 12:31:27 AM UTC-8, N_Cook wrote:
Arfa Daily wrote in message

...

I have a current transformer on one leg of the input to my dummy
load
/
power meter. This allows me to monitor ...

I suspect a current transformer would affect any PA/speaker/Zobell
resonance/interaction effect.

A properly loaded currrent transformer can be ten millliohms, or a few
hundred microohms. There needn't be any significant effect on the
system
being measured.


Quite. The one that I use is completely non-invasive to the system it's
monitoring - in this case an output stage into a dummy load. It's just a
fully encapsulated winding with a hole through the middle. One leg of the
input to my tester passes through this hole. The winding is terminated
with
a couple of k, and my scope hooks directly across it, fully isolated from
the unit under test. It has zero measurable effect on the amplifier or
any
networks connected in the amp's output circuit. I would not have designed
it
in, and been using it without issue for some years, if it did ...

This is the exact one


http://ww.sourcingmap.co.uk/020a-inp...edding-precisi
on-current-transformer-ta17-p-133281.html

Arfa



So you would pass a speaker lead through the hole, ignoring the primary
sense turns.
In the meantime I may as well try a solenoid without pole piece


Yes, a speaker lead passes straight through the middle. In this use of the
device, there is no 'primary sense turns' issue. The wire passing through
the middle represents a single turn primary. With the ratio being 2000:1,
that provides a more than adequate output level for really very small power
levels - just a few watts in fact. I dare say that if you were bothered, you
could rectify the voltage appearing across the secondary, and hang a
suitable meter and calibration pot across the output, and get a fair
indication of relative, if not absolute power levels. The transformer is
rated to 20A, which I believe is the point at which the core starts to
saturate, so the output becomes non-linear, rather than where anything nasty
starts to happen. 20A allows a pretty powerful amp to be run through it.

You can try a solenoid. I'm sure that you will get something out, but I'm
not sure how much. CTs are usually torroidal in construction - at least the
small ones that I've seen - and I guess there's a reason for that. I mean,
at the end of the day, you can drink coffee out of a shoe, but a cup works
better ... :-)

Arfa

Basically, a coil of wire will sense electrical activity on any conductor
that passes thru the coil.
A coil also senses fields near it without having to wrap around a conductor.

A coil on a ferrous core will generally be more sensitive, but as little as
a few turns of wire will also work.
Coils with ferrous cores are used in amp-clamp-type meters and inductive
pickups, and some current probes.

Sencore marketed a product they named the Snoop Loop, which was a length of
coaxial cable with a few turns of wire at the end.. which could be attached
to a scope or other test equipment to sample signals in circuit components
such as tiny board-mounted inductors.

A suitable DIY accessory can be a few turns of wire connected to a female
BNC connector (panel type), so the loop can be easily be attached to a
common scope probe by inserting the probe tip into the connector.
When placed around a small inter-stage transformer the signal becomes
loosely coupled to the loop.

Several turns of wire placed around RF transmission cables is often
sensitive enough to get frequency or modulation readings. I used this method
and capacitive coupling (T connector) when bench testing CB radios years
ago.

--
Cheers,
WB
..............


"Arfa Daily" wrote in message
...

Yes, a speaker lead passes straight through the middle. In this use of the
device, there is no 'primary sense turns' issue. The wire passing through
the middle represents a single turn primary. With the ratio being 2000:1,
that provides a more than adequate output level for really very small
power levels - just a few watts in fact. I dare say that if you were
bothered, you could rectify the voltage appearing across the secondary,
and hang a suitable meter and calibration pot across the output, and get a
fair indication of relative, if not absolute power levels. The transformer
is rated to 20A, which I believe is the point at which the core starts to
saturate, so the output becomes non-linear, rather than where anything
nasty starts to happen. 20A allows a pretty powerful amp to be run through
it.

You can try a solenoid. I'm sure that you will get something out, but I'm
not sure how much. CTs are usually torroidal in construction - at least
the small ones that I've seen - and I guess there's a reason for that. I
mean, at the end of the day, you can drink coffee out of a shoe, but a cup
works better ... :-)

Arfa

"Wild_Bill" wrote in message
...
Basically, a coil of wire will sense electrical activity on any conductor
that passes thru the coil.
A coil also senses fields near it without having to wrap around a
conductor.

A coil on a ferrous core will generally be more sensitive, but as little
as a few turns of wire will also work.
Coils with ferrous cores are used in amp-clamp-type meters and inductive
pickups, and some current probes.

Sencore marketed a product they named the Snoop Loop, which was a length
of coaxial cable with a few turns of wire at the end.. which could be
attached to a scope or other test equipment to sample signals in circuit
components such as tiny board-mounted inductors.

A suitable DIY accessory can be a few turns of wire connected to a female
BNC connector (panel type), so the loop can be easily be attached to a
common scope probe by inserting the probe tip into the connector.
When placed around a small inter-stage transformer the signal becomes
loosely coupled to the loop.

Several turns of wire placed around RF transmission cables is often
sensitive enough to get frequency or modulation readings. I used this
method and capacitive coupling (T connector) when bench testing CB radios
years ago.

--
Cheers,
WB
.............

All that you say is of course true, but the key to the whole thing is
sensitivity. For sure, you can sense RF fields with just a few turns of
wire, but sensing small currents at audio frequencies is a whole different
ball game. Before settling on the CT that I used in my dummy load / power
meter, I tried all sorts of DIY solutions such as messing about with ready
wound solenoids as Nigel is proposing, and wrapping turns around a piece of
wire and so on. None of those trials produced anything like a sufficiency of
output as to be useful for much. The CT that I used is small, neat, readily
available, cheap, and very sensitive with its 2000:1 ratio. Basically, it
does the job it's designed to do, much like that coffee cup that I
mentioned, and I don't really see the point in trying to bodge up some other
device to serve the same purpose ...

Arfa

Arfa Daily wrote in message
...


"N_Cook" wrote in message
...
Arfa Daily wrote in message
...


"whit3rd" wrote in message
...
On Tuesday, February 12, 2013 12:31:27 AM UTC-8, N_Cook wrote:
Arfa Daily wrote in message

...

I have a current transformer on one leg of the input to my dummy
load
/
power meter. This allows me to monitor ...

I suspect a current transformer would affect any PA/speaker/Zobell
resonance/interaction effect.

A properly loaded currrent transformer can be ten millliohms, or a
few
hundred microohms. There needn't be any significant effect on the
system
being measured.


Quite. The one that I use is completely non-invasive to the system it's
monitoring - in this case an output stage into a dummy load. It's just
a
fully encapsulated winding with a hole through the middle. One leg of
the
input to my tester passes through this hole. The winding is terminated
with
a couple of k, and my scope hooks directly across it, fully isolated
from
the unit under test. It has zero measurable effect on the amplifier or
any
networks connected in the amp's output circuit. I would not have
designed
it
in, and been using it without issue for some years, if it did ...

This is the exact one



http://ww.sourcingmap.co.uk/020a-inp...edding-precisi
on-current-transformer-ta17-p-133281.html

Arfa



So you would pass a speaker lead through the hole, ignoring the primary
sense turns.
In the meantime I may as well try a solenoid without pole piece


Yes, a speaker lead passes straight through the middle. In this use of the
device, there is no 'primary sense turns' issue. The wire passing through
the middle represents a single turn primary. With the ratio being 2000:1,
that provides a more than adequate output level for really very small
power
levels - just a few watts in fact. I dare say that if you were bothered,
you
could rectify the voltage appearing across the secondary, and hang a
suitable meter and calibration pot across the output, and get a fair
indication of relative, if not absolute power levels. The transformer is
rated to 20A, which I believe is the point at which the core starts to
saturate, so the output becomes non-linear, rather than where anything
nasty
starts to happen. 20A allows a pretty powerful amp to be run through it.

You can try a solenoid. I'm sure that you will get something out, but I'm
not sure how much. CTs are usually torroidal in construction - at least
the
small ones that I've seen - and I guess there's a reason for that. I mean,
at the end of the day, you can drink coffee out of a shoe, but a cup works
better ... :-)

Arfa


I tried a solenoid but not enough output and poor at sub 100Hz. Do you know
what the response is like below 100Hz on your pass-thru Tx ?
Dug out a couple of microphone Tx and checked they worked down to 40Hz ,
taking readings, less response but not too much. Then some more important
jobs came in.
One for the phones out line ( so same f response) and one with series 1K or
so over the speaker, should work fed to a normal dual channel scope

All this ado about not much......

First of all just float the scope. For the signal levels and impedances you are dealing with just don't worry about it. Now of course this requires diligence to keep Murphy's law from being enforced, but in my 30+ years on the bench I have determined that it is usually better not to have things grounded on the bench.

What's more you don't even have to do that. This is all fine for an academic discussion but in reality there are a few things to consider before getting one's bonnet in a beehive. At least 99.999999 % of the time the same thing is going to be on the + and - side of that speaker, just inverted. If not you likely have a problem in the output stage after the phase splitter. If it is OK from that poit you can measure either speaker wire and derive anything you want, just do a little mental math.

Now since we are such the academic group here, there are some things about measuring speaker current which makes this inductive pickup method not so desirable. Unless this is some wierd tube (valve) amp with no damp(er)ing factor at all, the output is a voltage source. That means the voltage at the output is supposed to follow the voltage at the input, relatively regardless of the specific current drain at any given frequency. On the other hand the speaker is a complex load, in a bass bin or subwoofer it will be inductive with some resistive component. This affects the current drain at varying frequencies and makes measurements of amplifier perfomance near useless.

In practice the inductive component of the woofer(s) partially copensates for the fact that the radiating surface is much smaller then the wave it radiates. The lower the frequency, the less efficient it is, however the lower the frequency the higher current the voice coil draws. It somewhat offsets..

Tell you what, test the +/- theory just by getting two resistors, say a thousand ohms or so and put them in series across the speaker wires. Stick a scope at their junction and crank that baby up. If you see relatively a flat line and hear sound that means the output stage is OK and you can proceed. Now just scope one side or the other, doesn't matter. If you need to calculate output power just double whatever voltage you read, or quadruple the calculated wattage.

When dealing with this type of audio, I recommend aa 1 Khz square wave. It can tell you alot about frequency response. Any decent amp should reproduce it quite faithfully, at least at a glance. To get an idea how to read it, put it through an audio equalizer and watch the effect. You will get the idea of what it looks like when low frequency response is lacking, or whatever. Looking at the flat parts of the output waveform is almost like looking at a frequency response curve in reverse. That is with the highest frequencies showing at the beginning and the lowest at the end. Looking for these characteristics with the scope should allow you to isolate which stage is causing the problem.

If this is like most of this modern junk it probably has eighty-one digital preset EQs that are modelled after some blues player's amps from 1956 or some ****, but if the response deficiency is in all of them, the problem is most not within that little universe/mess. It would also be nice to know if the problem came on all at once or gradually.

People who write regulations are not techs. I do not want anything earthed on my bench. Nothing. I have good reasons for that which are numerous. I can understand the concerns of people who believe that every square inch of this world should be safe enough for a four year old to play unsupervised but I do not espouse that bull****ing****. This is my bench and keep your hands off. If you happen to get shocked, see rule number one. In fact you guys are lucky with the single ended 240 volt mains, over here we have to deal with voltage doublers which make floating anything much more exciting. I try not to do it, but I will do it when needed. Suffice it to say that isolation transformers are needed more in the US for a couple of reasons.

I would like to float everything, even the antenna/cable lines in the shop. Put an isolator on each one. In my plug mold I not only didn't connect the grounds, I cut the green wires between each and every outlet. Of course this was at my shop, where I am now doesn't have that luxury and I am too lazy to do it.

Maybe I am crazy, but it hasn't killed me yet.

I tried a couple of physically larger microphone Tx and the response is
essentially flat from 2Hz (the bottom limit of my sine gen ) and about 20KHz
, rapid tail-off after 30KHz.
So will try one or both of them when I get back to this speaker/PA problem.
RTFM time : the "shape" switch on a Roland cube 30 bass does not switch in
and
out the COSM selection. I think there was a problem in this sw and when the
wiper disconnects then the function stays with whatever was selected before,
the control lines stay high.
Incidently if o/c at switch on then disables the amp totally. The speaker/PA
is perhaps fine after all , but will try comparing at some point , to at
least see if microphone Txs are useful for this

I tried a solenoid but not enough output and poor at sub 100Hz. Do you
know
what the response is like below 100Hz on your pass-thru Tx ?

Your findings are as I predicted, and no more or less than I would have
expected. I've never swept the CT that I'm using, but when I'm mending sub
amps, I normally test at 80 Hz and 40 Hz, and it produces plenty of output
at those frequencies. I would guess that it's probably specced for use at
mains frequencies anyway. I do actually use one for mains sensing on my
shower pump, to ensure that the shower room extractor fan comes on, and it
works just fine, producing plenty of output. When I designed that circuit, I
did a lot of trials with various loads, and it produced enough output to
switch a 4000 series CMOS gate, with less than 30 watts of load and one leg
of the cable passing straight through. If you take a single 'proper' turn
through the hole, then the sensitivity increases. The project is detailed on
Wiki somewhere. John Rumm on uk.d-i-y put it up there. I guess searching
'current operated switch' or some such would find it.

When I'm mending general amplifiers as opposed to subs, My 'standard' test
frequencies are 800 Hz and 8kHz, and again, the CT produces plenty of output
at those frequencies as well. If I get time over the weekend, I'll sweep it
for you, and see just how flat it is.


Dug out a couple of microphone Tx and checked they worked down to 40Hz ,
taking readings, less response but not too much. Then some more important
jobs came in.
One for the phones out line ( so same f response) and one with series 1K
or
so over the speaker, should work fed to a normal dual channel scope


Not sure how they are going to help you, though. Even the low Z side of
those is a proper winding, and will likely upset the output stage that you
hook it in series with. As I said, I tried all sorts of DIY 'solutions'
before settling on the current transformer. Some of these were attempts to
repurpose various transformers and coils, and as soon as you put a few
'primary' turns on, it interacts with the output stage, sometimes making it
oscillate, and sometimes giving you a misleadingly distorted waveform across
the secondary. Aside from this, I would have thought that the current
handling capability of a microphone transformer was very small, both from
burny - burny let go the magic smoke, and core saturation angles.

At the end of the day, I've told you what works well. I've been using it
daily for some years, and it just does the job, without issue. In the time
that's been spent discussing it and trying to come up with DIY alternatives,
you could have had one ordered, and ready to drop on your doormat early next
week ... :-)

Arfa

Mic Tx and dropper worked well, no difference other than amplitude between
line out and speaker input in normal low colouration amp types (perhaps
10Kdropper for a more powerful amp). I will not be subbing the speaker as
the owner went through the same thinking process as me, thinking the "shape"
switch disabled any type selection.
Not only that but he usually uses this amp flat with no reverb or effects,
with an acoustic bass. To switch out reverb and flanger etc is a matter of
turning the pots fully ACW in this DSP amp. He did the same with amp type
following the same reasoning (plus shape off) but ACW selects the octave
bass , an effect rather than colouration and the source of the non existent
problem.