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?
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?
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.
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.
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.
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.
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...
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?
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?
It's hidden in the articles section:
Audio Component Grounding and Interconnection
David Davenport - 21st April 2010
http://www.diyaudio.com/forums/diya...udio-component-grounding-interconnection.html
Audio Component Grounding and Interconnection
David Davenport - 21st April 2010
http://www.diyaudio.com/forums/diya...udio-component-grounding-interconnection.html
Dumb question and I cannot see it in the Grounding Article:
I ground the xlr sockets pin 1 to chassis. I ground the power amp pcb to chassis star ground. I sort out the pre-amp grounds.
I have a short lengths of balanced screened cable from pcb to xlr chassis sockets. Do I ground these at pcb or at chassis socket?
I ground the xlr sockets pin 1 to chassis. I ground the power amp pcb to chassis star ground. I sort out the pre-amp grounds.
I have a short lengths of balanced screened cable from pcb to xlr chassis sockets. Do I ground these at pcb or at chassis socket?
The Abletec amp is complete with power supply. The pcb has 6 mounting holes. 2 must be insulated. 4 are ground plane and have bright solder around the screw holes. I thought I was being smart mounting on insulated stand-offs. Now I know from the grounding article it should be connected to chassis. The mains cable earth wire is screwed to chassis (a star of 1 wire).
I could - a) run a short length of wire from pcb ground plane to this screw, b) replace the nearest stand-off (about 20mm away from earth screw) with a spacer and long screw thus grounding the pcb at 1 point or c) replace all 4 stand-offs with spacers and long screws.
I could - a) run a short length of wire from pcb ground plane to this screw, b) replace the nearest stand-off (about 20mm away from earth screw) with a spacer and long screw thus grounding the pcb at 1 point or c) replace all 4 stand-offs with spacers and long screws.
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