PL 700 II Clair Bros Rising from the Ashes

[Mohawk]

Hey Peter, Keeping the AC lines away from the OP amps makes sense to me , In this build are you going to fit your custom "faraday cage" around the control board also ? Has Joe left his opinion on running the AC across the middle of the board ?

M

 

[Peter S]

Hi M, I have just finished the input wiring and will test the noise level next. I forget who may have commented on my AC wire routing but have a faint memory of someone mentioning that this was the original method, (not sure). My thoughts are that since the input is at the bottom of the board and the Op amp is at the top, nowhere is best.
I have not checked but as this is a multi layer pcb, the input signal pcb trace may not necessarily be on the bottom face. Could you confirm this Joe?

By routing the wiring at mid level latitude, I could leave the top of the "faraday cage" open to alleviate cooling issues for the PL14_20. I will test this amp for noise first, to see if the steel shield is needed at all.

 

[Mohawk]

Looking forward to the results !

 

[Gepetto]

Hi M, I have just finished the input wiring and will test the noise level next. I forget who may have commented on my AC wire routing but have a faint memory of someone mentioning that this was the original method, (not sure). My thoughts are that since the input is at the bottom of the board and the Op amp is at the top, nowhere is best.
I have not checked but as this is a multi layer pcb, the input signal pcb trace may not necessarily be on the bottom face. Could you confirm this Joe?

By routing the wiring at mid level latitude, I could leave the top of the "faraday cage" open to alleviate cooling issues for the PL14_20. I will test this amp for noise first, to see if the steel shield is needed at all.

The control board is a double sided board (2 layers), the midplanes are multilayer boards (4 layers).

 

[Peter S]

Results ! :
The second Clair Bros 700 II is alive and kicking. 62 VAC into 8 ohm dummy load Left, 61.9 VAC Right, @ 1 kHz.

Noise measurements made with 8 ohm speakers connected to the outputs. Scope connected to output terminals. A 1 k ohm resistor only, connected from pin 1 to 2 on each side of the PL14_20. Scope set to 0.5 mV per division. I will construct a steel "Faraday cage" asap with open top and bottom (for cooling) then post pics, with and without.

With the board up, the hum content appears to be about 2 mV P to P. IIRC, this is about 0.7 mV RMS. Don't know how this compares to other amps but considering the amp is wide open...

 

[Peter S]

So here is the second amp with a "rev B" steel shield. The top and bottom are open for ventilation. The hum/noise is pretty much the same as having the board flipped up.
I feel that non-ferrous shielded AC wiring would probably accomplish the same results under no-signal conditions. This noise was caused by electrostatic-ally from the AC hot wire. The ferrous box may shield magnetic induction when the amp is under load. (Not verified, I am shopping for a distortion/signal to noise analyzer).
I just noticed that my input wiring was under the PL14_20. It was disconnected at both ends but maybe tying it to the #2 pins would have some effect? I will test this asap. Thanks to COVID-19, I will have a lot more time on my hands.

 

[WOPL Sniffer]

non-ferrous shielded AC wiring??????? HUH????????

 

[Peter S]

Hi Folks;
Having an issue with the second Clair Bros 700 II; a slight hum from both channels when both inputs are connected to my miniDSP signal source. (About 2 mVAC on the Fluke meter). With only one input (either), amp is dead quiet.
Input wiring and jacks have been changed, inputs reversed, connecting cables changed, alligator clipped the grounds together at the input jacks, still hums with both inputs connected and quiet with either left or right only.

Any ideas? I do not have this issue with the first amp and I have checked that both amps are wired identically.

 

[MarkWComer]

Hi Folks;
Having an issue with the second Clair Bros 700 II; a slight hum from both channels when both inputs are connected to my miniDSP signal source. (About 2 mVAC on the Fluke meter). With only one input (either), amp is dead quiet.
Input wiring and jacks have been changed, inputs reversed, connecting cables changed, alligator clipped the grounds together at the input jacks, still hums with both inputs connected and quiet with either left or right only.

Any ideas? I do not have this issue with the first amp and I have checked that both amps are wired identically.

Sounds like ground loops to me. When you did your rebuild, did you remove the little ground bus from your input jacks?

 

[Peter S]

Hi Mark, new 1/4" phone jacks were installed. The amps had 1/4" jacks before but they were not floating from the chassis. I kept L & R shields separate (no bus) at the input jacks. An alligator clip between the two shields at this point did not reduce the hum.

Just remembered; This amp has separated ground wires from the back planes to the centre-tap ground. But that is supposed to be better, right?

 

[MarkWComer]

Just remembered; This amp has separated ground wires from the back planes to the centre-tap ground. But that is supposed to be better, right?

Yes, that’s correct. Last ditch thought- AC hot and neutral may be reversed?
Otherwise, I don’t know... Maybe the DSP is at fault. Do you get hum from other sources? Try the DSP with other amps.

Nevermind- I see you checked with other amps...

 

[Peter S]

Hi Folks;
Been a while, hope everyone is doing well. Both amps behaving well, perfectly quiet, and sounding great. Again, the transformer leakage does not seem to be an issue with the second amp either. The issue was resolved with proper input cables.
Installed the amps in the rack with plastic shims and 1/4 pvc tubing pushed over 10-32 all-thread.

 

[Peter S]

There's always a fly in the ointment. The miniDSP, below the Yamaha amp, has a switching power supply. There is about 53 VAC measured from the 12 VDC output connector, to the AC line ground. This seems to be a potential ground loop issue.

Would love hear thoughts regarding replacing this power supply with a convention transformer, bridge, and a monster filter cap? I am not having hum issues at the moment but more than half the time, the miniDSP seems to 'lock up' or refuse to be recognized by the laptop.

 

[J!m]

Switching power supplies are bad news for audio.

 

[Gepetto]

Switching power supplies are bad news for audio.

True dat...

 

[FredR]

I use these power supplies for 12 volts in my ham shack.

https://www.astroncorp.com/linear-desktop-w-meters

Be sure to fuse each device being powered. I'm pretty sure that 12vdc at 70 amps will turn a DSP into blue smoke.

 

[Peter S]

Hi Fred;
They look perfect for the job but I was hoping to build something from my scrap heap. I have a one rack-space cabinet and was wondering if a simple bridge with lots of filter cap would provide enough smoothing, without having to use a regulator circuit?

 

[Gepetto]

There's always a fly in the ointment. The miniDSP, below the Yamaha amp, has a switching power supply. There is about 53 VAC measured from the 12 VDC output connector, to the AC line ground. This seems to be a potential ground loop issue.

Would love hear thoughts regarding replacing this power supply with a convention transformer, bridge, and a monster filter cap? I am not having hum issues at the moment but more than half the time, the miniDSP seems to 'lock up' or refuse to be recognized by the laptop.

Is it a 3 prong plug on the mini-DSP or 2?

 

[Peter S]

It is a 3 prong (IEC jack) on the external power supply. But there is 55 VAC measured from AC neutral to the 12 VDC (negative, outside shell of the plug) output.
I did not consider using an IEC cord with the ground pin removed. I will try it. Can switching power supplies isolate the output from the line?

 

[Gepetto]

It is a 3 prong (IEC jack) on the external power supply. But there is 55 VAC measured from AC neutral to the 12 VDC (negative, outside shell of the plug) output.
I did not consider using an IEC cord with the ground pin removed. I will try it. Can switching power supplies isolate the output from the line?

The output is isolated from the line, for sure. That is a mandatory safety requirement. So it is 3 prong and you are connecting all three.

Switching PSUs nearly always have Y caps from AC line to safety ground for filtering purposes in order to pass the conducted EMI portion of the FCC regulations. This creates purposeful leakage current in the safety ground lead. It has to be kept below a certain level, also by regulation.