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'[EE]: Audio amplifier power supply problem'
2019\06\30@233050 by Bob Blick

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part 1 2180 bytes content-type:text/plain; charset="iso-8859-1" (decoded quoted-printable)

There's this 2-channel audio power amplifier about 100 watts per channel I built about 35 years ago for PA system use. It always had a little hum on the output but nothing you'd notice when used for live music, maybe 5 millivolts or so. I always chalked it up to the power transformers being general-purpose types rather than ones designed for low magnetic emission. It's been sitting in a closet for 20 years, but recently I needed to use it indoors in a studio environment and decided I'd find out once and for all what was going on with the hum. I connected speakers to it, turned it on, and it was totally silent. I plugged an input cable into it and put some signal in and it worked fine, still no hum. But when I plugged a second signal cable into the second channel, a slight hum came from both channels. Even without a signal source, just taking a patch cable from input 1 to input 2 would produce the hum. So there's a ground loop in the amp. This amplifier has separate power supplies for the two channels. The only connection between the two channels is a couple of resistors and capacitors from channel ground to chassis ground. I chopped them out temporarily and it didn't fix the hum. Running the amplifier on a Variac, I dropped the input from 120 volts down to 80 volts to change the magnetic emission from the transformers. No change in hum. At this point I decided the hum was coming from capacitive coupling either from primary to secondary, or from core to secondary, with a differences in transformer eddy current making a potential difference in the two channel grounds. I checked the transformer primary phasing to rule out the first one, but I didn't feel like unbolting the transformers from chassis to check the second. But here's where I'm stumped. I planned to switch the phase of the power transformers, and the terminals were shared with .01 uF caps, so I pulled those out first. Just for giggles, I powered up the amp and the hum was gone. These caps are marked C1 and C2 on the schematic I've attached. I am totally at a loss why this would fix the problem. All wild ideas welcome. Thanks! Bob

part 2 25434 bytes content-type:image/jpeg; name="AmplifierPowerSupply.jpg" (decode)


part 3 197 bytes content-type:text/plain; name="ATT00001.txt"
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'[EE]: Audio amplifier power supply problem'
2019\07\01@014046 by Bob Blick
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Hint: reversing the AC plug does produce a little hum in one direction.

Bob

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2019\07\01@034139 by Manu Abraham

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HI Bob,

I've had a similar experience (the 50Hz ? hum getting prominent when
the transformer primary wires to mains was swapped) about 2.5 decades
back, with audio power amplifiers. Those days I had concluded it
occurred due to ground loops. The issue appeared when the ground was
made common near the power supply, but the issue became less prominent
when all the grounds were terminated together near the signal input.
It appeared weird to me at that time. Didn't have much of an
explanation then. But that said, it was only a single transformer,
powering both channels.

Still, dont have any good explanation on the behaviour, except for the
faint acceptance that it could've been due to ground loops.

Cheers,
Manu

On Mon, Jul 1, 2019 at 11:14 AM Bob Blick <spam_OUTbobblickTakeThisOuTspamoutlook.com> wrote:
>
> Hint: reversing the AC plug does produce a little hum in one direction.
>
> Bob
>
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2019\07\01@034623 by Richard Prosser

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"Mr Carlson" (Youtube) is pretty keen on making sure that any caps are
connected with the most outer foil connected to the most grounded side -
even if they are not marked. He has a device for figuring it out.  I'm not
sure how effective it would be in your case with a 2 pin supply but you
could try swapping them end-end & see if it makes a difference.
I'm also having trouble with the separate "grounds" for the 2 channels but
can see it should be OK as long as there's no other connection to the
chassis. If connected externally there will be a loop formed that could
introduce noise.
RP

On Sun, 30 Jun 2019 at 17:44, Bob Blick <.....bobblickKILLspamspam@spam@outlook.com> wrote:

> Hint: reversing the AC plug does produce a little hum in one direction.
>
> Bob
>
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2019\07\01@114905 by Bob Blick

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The caps on either side of the transformer are fairly typical in this application, they are to reduce the pops and clicks that can happen when a motorized appliance such as a washing machine or refirgerator cycles. The idea being that these spikes make it through the transformer and exceed the main rectifier's breakdown voltage.

I don't think it's as big a problem as it was back in the 70's and 80's. I think the AC line in our homes is much more clamped, most of us have a dozen surge strips plugged in, all with MOV's, and lots of other electronic devices that tend to make the AC line more capacitive than it used to be. And rectifiers are better now. Usually.

As far as my hum problem goes, I'm tending to believe that these transformers have lots of feedthrough from primary to secondary, as evidenced by the reversing of the AC line making a difference. And of course being general purpose types they have no interwinding shield.
But why I would get a ground loop between the two channels leads me to believe that, although identical-looking, the transformers are far from identical. Perhaps when I get more time (haha funny!) I will try to investigate that aspect of it.

Thanks to all.
Bob

________________________________________
From: piclist-bouncesspamKILLspammit.edu <.....piclist-bouncesKILLspamspam.....mit.edu> on behalf of Bob Blick Sent: Sunday, June 30, 2019 10:41 PM
To: Microcontroller discussion list - Public.
Subject: Re: [EE]: Audio amplifier power supply problem

Hint: reversing the AC plug does produce a little hum in one direction.

Bob


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2019\07\01@132106 by Jim

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Bob,

I have a comment, but I don't want you to think i'm be a smart a**. I'm just stating basic fundamentals.
You mention "Feed through from primary to secondary".  Isn't that the
whole principle of a transformer?
A transformer is supposed to transfer, or actually, transform the
activity on the primary into a somehow  different form of itself onto the secondary.  So, it seems to me feed
through would be expected.

Regards,

Jim

> ---{Original Message removed}

2019\07\01@160726 by Bob Blick

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Hi Jim,
I was implying common-mode crosstalk, which these particular ones seem to have a lot of. The weird part is that there appears to be a large difference between these two, even though they are identical.
Best regards, Bob

________________________________________
From: EraseMEpiclist-bouncesspam_OUTspamTakeThisOuTmit.edu <piclist-bouncesspamspam_OUTmit.edu> on behalf of Jim
Sent: Monday, July 1, 2019 10:19 AM
To: Microcontroller discussion list - Public.
Subject: RE: [EE]: Audio amplifier power supply problem


Bob,

I have a comment, but I don't want you to think i'm be a smart a**.
I'm just stating basic fundamentals.
You mention "Feed through from primary to secondary".  Isn't that the
whole principle of a transformer?
A transformer is supposed to transfer, or actually, transform the
activity on the primary into a somehow
different form of itself onto the secondary.  So, it seems to me feed
through would be expected.


Regards,

Jim

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2019\07\02@082642 by Jim

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Oh, I see.  Just curious...Are these two transformers from the same lot
at mfg?
If not, that might explain the differences, or at least, some of the
differences.



Regards,

Jim

> ---{Original Message removed}

2019\07\03@091329 by David Reaves

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You may be over-analyzing the problem as to it being somewhere in the transformer/power supply. It may be an unseen ground-loop problem.

I have found, by strictly following high-quality analog audio wiring practices, that having a **single point ground** and *any and all* ground connections being single-wire “star” connections to that ground (usually located at the low end on the largest filter capacitor), that in almost all cases hum problems do not show up in the first place (Don’t use loops, either)! Every subassembly’s ground connection should have its own individual wire to that point, and if it's a relatively high-current-demand section of the circuit, the wire should be substantial and direct, to keep the impedance low. This philosophy is similar to the one-connection-only practice where the digital and analog grounds on a D/A or A/D converter have to be tied together on a circuit board.

Also, not using the outer shield to carry the low side of the input signal may be of help, with the shield being connected to “ground" only at the end with the highest noise susceptibility (receiving end, usually), and the actual circuit return being brought in on a separate, but closely adjacent, conductor. Two-conductor shielded cable used for balanced circuits works for this technique, too.

Good Luck
David Reaves
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2019\07\03@133452 by Bob Blick

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Hi David and all the other responders,
Thanks for your input. The thing that really made this weird is that it's just two single channel amps that only share a chassis and a power cord, with a couple of resistors and capacitors tying the two grounds(from the main filter caps) to chassis, which is a pretty typical thing in dual-monaural amps. The hum only happens when the two amps are connected together.

Last night I followed up on my thought that the two transformers were different from each other and generally pretty crappy with regards to common-mode isolation. Jim suggested I check the lot numbers, and indeed, they are from different production batches.  
Pulling the chassis connections, I tested the ground voltage of the two amps referenced to AC neutral, and one side was about .7V and the other about 2V. Reversing the AC line I got about 70 and 90 volts. It looks like the two transformers are definitely different, and there is enough common-mode current to make some hum, especially when the AC line is reversed.

Looks like there's not much to be done except to put a polarized line cord on the amp. Since I cut those two .01 caps off the input to the transformers the hum is pretty much gone. I really have no idea why that would make a difference, but I'm not going to drive myself crazy trying to figure that one out.

Thanks again!

Bob
________________________________________
From: @spam@piclist-bouncesKILLspamspammit.edu <KILLspampiclist-bouncesKILLspamspammit.edu> on behalf of David Reaves
Sent: Wednesday, July 3, 2019 6:14 AM
To: RemoveMEpiclistTakeThisOuTspammit.edu
Cc: Reaves David
Subject: RE: [EE]: Audio amplifier power supply problem

You may be over-analyzing the problem as to it being somewhere in the transformer/power supply. It may be an unseen ground-loop problem.

I have found, by strictly following high-quality analog audio wiring practices, that having a **single point ground** and *any and all* ground connections being single-wire “star” connections to that ground (usually located at the low end on the largest filter capacitor), that in almost all cases hum problems do not show up in the first place (Don’t use loops, either)! Every subassembly’s ground connection should have its own individual wire to that point, and if it's a relatively high-current-demand section of the circuit, the wire should be substantial and direct, to keep the impedance low. This philosophy is similar to the one-connection-only practice where the digital and analog grounds on a D/A or A/D converter have to be tied together on a circuit board.

Also, not using the outer shield to carry the low side of the input signal may be of help, with the shield being connected to “ground" only at the end with the highest noise susceptibility (receiving end, usually), and the actual circuit return being brought in on a separate, but closely adjacent, conductor. Two-conductor shielded cable used for balanced circuits works for this technique, too.

Good Luck
David Reaves


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2019\07\03@150652 by Jim

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Just a thought.  Could they somehow be connected out of pahse with each
other?


Regards,

Jim

> ---{Original Message removed}

2019\07\03@153914 by Bob Blick

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part 1 2714 bytes content-type:text/plain; charset="iso-8859-1" (decoded quoted-printable)

They both trended the same regarding ground voltage when I reversed the AC line, so they seem in phase. They have dual primaries in parallel and secondaries in series(see attached image), and I haven't tried swapping the arrangement of the secondaries on one transformer. It might be worth a try although it's working pretty well right now. It all goes to show how a perfect transformer is impossible to make, and you can't just grab one that has the right voltage and current specs and expect it to be the right one for your application. If these had an interwinding shield there wouldn't be this problem. Cheerful regards, Bob ________________________________________ From: spamBeGonepiclist-bouncesspamBeGonespammit.edu <TakeThisOuTpiclist-bouncesEraseMEspamspam_OUTmit.edu> on behalf of Jim Sent: Wednesday, July 3, 2019 12:04 PM To: Microcontroller discussion list - Public. Subject: RE: [EE]: Audio amplifier power supply problem Just a thought.  Could they somehow be connected out of pahse with each other? Regards, Jim > ---{Original Message removed}
part 2 4203 bytes content-type:image/gif; name="transformer.gif" (decode)


part 3 197 bytes content-type:text/plain; name="ATT00001.txt"
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2019\07\03@164623 by AB Pearce - UKRI STFC

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This suggests on primary has been wound the opposite way to the other, such that the line side of one transformer is against the secondary winding, while the other has the neutral side against the secondary.


{Original Message removed}

2019\07\03@165105 by enkitec

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    The hum appears when both inputs cables are connected?
    I had the very same problem with a single transformer stereo amplifier.
    Modifying the input ground connections eliminated the hum. There
were two wires connected to the central ground, one from each input. I
eliminated one of them and connected the two input grounds together.

    Mark Jordan


On 03-Jul-19 14:34, Bob Blick wrote:
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2019\07\03@173613 by Bryan Hood

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How closely spaced are your transformers? I wonder if one is magnetically
coupling to another. There is a bit of a tank circuit with the transformer
winding and parallel capacitor. Chances are that the inductors and caps are
not identical so they will have different resonant frequencies. They may be
creating a beat frequency by coupling together. Is the hum at exactly 60Hz,
or some other frequency? Does it change or go away if you remove only one
of the two caps?

Bryan

On Wed, Jul 3, 2019 at 1:53 PM <RemoveMEenkitecEraseMEspamEraseMEgmail.com> wrote:

{Quote hidden}

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2019\07\03@201250 by Bob Blick

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This gives me some interesting thoughts. There are two secondary windings, they could be layered or bifilar. If they are bifilar wound, it wouldn't matter how I connected the two in series. But if they were layered, I would want the start of the first secondary to be closest to the primary winding.
And I'd want to have the end of the primary to be connected to neutral.

Thanks for the ideas. But I think tonight I will clean the pots in my parametric equalizer instead of tinkering with the amplifier. All this stuff that's been in my closet for 20 years seems to want attention.

Best regards, Bob

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From: RemoveMEpiclist-bouncesKILLspamspammit.edu <piclist-bouncesSTOPspamspamspam_OUTmit.edu> on behalf of AB Pearce - UKRI STFC Sent: Wednesday, July 3, 2019 1:47 PM
To: Microcontroller discussion list - Public.
Subject: RE: [EE]: Audio amplifier power supply problem

This suggests on primary has been wound the opposite way to the other, such that the line side of one transformer is against the secondary winding, while the other has the neutral side against the secondary.


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2019\07\03@203335 by Bob Blick

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I should add that the start of the first secondary should be what I connect as the grounded center tap.

________________________________________
From: spamBeGonepiclist-bouncesSTOPspamspamEraseMEmit.edu <KILLspampiclist-bouncesspamBeGonespammit.edu> on behalf of Bob Blick
Sent: Wednesday, July 3, 2019 5:12 PM
To: Microcontroller discussion list - Public.
Subject: Re: [EE]: Audio amplifier power supply problem

This gives me some interesting thoughts. There are two secondary windings, they could be layered or bifilar. If they are bifilar wound, it wouldn't matter how I connected the two in series. But if they were layered, I would want the start of the first secondary to be closest to the primary winding.
And I'd want to have the end of the primary to be connected to neutral.

Thanks for the ideas. But I think tonight I will clean the pots in my parametric equalizer instead of tinkering with the amplifier. All this stuff that's been in my closet for 20 years seems to want attention.

Best regards, Bob

________________________________________
From: EraseMEpiclist-bouncesspamEraseMEmit.edu <@spam@piclist-bounces@spam@spamspam_OUTmit.edu> on behalf of AB Pearce - UKRI STFC
Sent: Wednesday, July 3, 2019 1:47 PM
To: Microcontroller discussion list - Public.
Subject: RE: [EE]: Audio amplifier power supply problem

This suggests on primary has been wound the opposite way to the other, such that the line side of one transformer is against the secondary winding, while the other has the neutral side against the secondary.


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