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Old 1st March 2014, 06:45 PM   #1
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Default Humming DRV134 balanced output.

I have my Arduino controlled PGA2320 preamp.
Output is through DRV134 balanced drivers to Abletec ALC0300-1300 amplifiers (balanced input).
Problem is horrendous mains hum - I mean really loud, not volume dependant.
I was getting music out so something works!
I can no longer test on the Abletec's as I don't want to abuse my speakers (or my ears).
I am now testing on active pc speakers (I can turn their volume down), I figure since they are not earthed at the mains they should work from either the balanced or unbalanced source.
I find it dead quiet taking (unbalanced) signal before DRV134 but hum on balanced output.
I have checked all XLR connections throughout and reds go to reds, blues to blues...
Any ideas?
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Old 1st March 2014, 08:00 PM   #2
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Are you using a split supply for the DRV?
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Old 1st March 2014, 08:31 PM   #3
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Quote:
Originally Posted by russc View Post
I have my Arduino controlled PGA2320 preamp.
Output is through DRV134 balanced drivers to Abletec ALC0300-1300 amplifiers (balanced input).
Problem is horrendous mains hum - I mean really loud, not volume dependant.
I was getting music out so something works!
I can no longer test on the Abletec's as I don't want to abuse my speakers (or my ears).
I am now testing on active pc speakers (I can turn their volume down), I figure since they are not earthed at the mains they should work from either the balanced or unbalanced source.
I find it dead quiet taking (unbalanced) signal before DRV134 but hum on balanced output.
I have checked all XLR connections throughout and reds go to reds, blues to blues...
Any ideas?
Where's pin 1 of the XLR connected to?

Jan
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Old 1st March 2014, 08:52 PM   #4
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Yes, same split +15/0/-15v supplies the PGA2320, both DRV134 & op-amp input buffer before the PGA2320. Nothing is grounded to chassis. Power supply 0v is also unbalanced signal ground and pin 1 ground of XLR. None of the pin 1's are connected to shell of connectors.
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Old 2nd March 2014, 12:18 AM   #5
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The XLR pin 1 should be connected to the chassis at the connector. It is not part of the audio circuit nor the +15/-15V supply.
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Old 2nd March 2014, 11:22 AM   #6
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Whether pin 1 is should be grounded to chassis is controversial to say the least.
I have done further testing. First I reversed the ac mains on the ALC amp (AC1/AC2) The instructions give no clue but I have now put the live brown to the fused line on the circuit board. No improvement there. Then using pc speakers I tested from DRV - clean sound with no hum from unbalanced (pin 4 & 3), +out (pin 1 & 3), -out (pin 8 & 3) and finally balanced (pin 1 & 8). So the problem is introduced when connecting the ALC amp. Next, I disconnected ground from pin 1 of preamp out (well, lead should only need to be connected at one end to screen and if it was an earth loop issue this should fix). Reconnected ALC amp. Slight hum. Turned preamp power on and loud hum again. Preamp is in 1 box and ALC amp in a seperate box. Preamp is breadboarded so insulated from chassis. Power amp is insulated from its chassis and I have checked with meter for inadvertant shorts to chassis. So far as I understand it, there should be no need to connect either the preamp or the power amp to chassis (mains earth) and in fact believe not connecting to mains earth is supposed to help avoid earth loops. The 6mtr XLR lead joining the 2 units is a ready made cable and the shells are not connected.
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Old 2nd March 2014, 11:28 AM   #7
AndrewT is offline AndrewT  Scotland
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Pin 1 is no longer controversial. That was sorted out decades ago.
If some manufacturers continue to ignore the correct PIN 1 termination in their product, we do not copy their error.

Pin1 goes direct to Chassis at the socket entry/exit point.
This extends the screening effect of the chassis to include the screen of the cable.
When done at both ends, the two chassis and the "tube" between them become the enclosing chassis. All the Audio is INSIDE that screening Chassis.
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Old 2nd March 2014, 11:42 AM   #8
peufeu is offline peufeu  France
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Quote:
Originally Posted by russc View Post
Preamp is breadboarded so insulated from chassis. Power amp is insulated from its chassis and I have checked with meter for inadvertant shorts to chassis.
This is wrong.

Current will flow through the amp's power transformer inter winding capacitance. This current will return mains earth.

You choose the return path. If amplifier GND is connected to chassis and earth, then this will be the lowest impedance path.

If it is not connected, then the return path will be through the signal cables, sensitive ground nodes, etc...
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Old 2nd March 2014, 12:22 PM   #9
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Thanks for all your help. Problem (half) solved.
I have been re-reading Sherlock Holmes and "when you have eliminated the impossible, whatever remains, however improbable, must be the truth?" I played around with ALC connections and couldn't solve, it made no sense. I was getting hum and strange 1Hz oscillations. However this was only one of a pair. I tried the other - brilliant! Duff amplifier all along. Next job, get amplifier replaced, house the preamp and I will look into grounding methods. Perhaps Andrew you would be kind enough to point me to some easy to follow guidance?
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Old 2nd March 2014, 12:49 PM   #10
AndrewT is offline AndrewT  Scotland
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the grounding article in the header page of the Forum.
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