I posted pictures and info of my newly built aleph mono amplifiers in another thread yesterday, here.
I have a slight speaker hum in one of the channels. The other channel is dead quiet. I mentioned this in my previous thread. The hum is only audible when I was within two feet of the woofer on that channel. This is not a major issue but I want it perfect.
I figured I would open a thread about this issue to see if anyone had any ideas on things to check. To recap my previous checks. The hum is present with an RCA shorting jumper installed, and it does not change audible intensity with the RCA shorting jumper pulled. I also tried pulling the XLR shorting jumper to see if the - 6 db of gain would be audible and it is not.
The only physical difference between the two amplifiers was that the mosfet output boards on the amplifier that has the hum had the wires for the Drain/Source twisted. The quiet amplifier had the same wires run straight without twisting. This morning I took the problem amplifier out and hooked up the scope to the output and it was showing about 10 mv of ripple on the speaker outs (into an 8 ohm resistor with the RCA jumper installed). I desoldered and untwisted the source/drain wires for each output board and the ripple upon retesting the ripples was approx. 25 mv, much worse. I took the amplifier inside and hooked it up to my Vandersteen's and the hum is quite audible with the wires run in the same manner as the quiet amplifier.
Aside from the fact I probably need to twist the wires back together in the amplifier that has the hum. Any ideas on isolating where this noise is coming from?
I have a slight speaker hum in one of the channels. The other channel is dead quiet. I mentioned this in my previous thread. The hum is only audible when I was within two feet of the woofer on that channel. This is not a major issue but I want it perfect.
I figured I would open a thread about this issue to see if anyone had any ideas on things to check. To recap my previous checks. The hum is present with an RCA shorting jumper installed, and it does not change audible intensity with the RCA shorting jumper pulled. I also tried pulling the XLR shorting jumper to see if the - 6 db of gain would be audible and it is not.
The only physical difference between the two amplifiers was that the mosfet output boards on the amplifier that has the hum had the wires for the Drain/Source twisted. The quiet amplifier had the same wires run straight without twisting. This morning I took the problem amplifier out and hooked up the scope to the output and it was showing about 10 mv of ripple on the speaker outs (into an 8 ohm resistor with the RCA jumper installed). I desoldered and untwisted the source/drain wires for each output board and the ripple upon retesting the ripples was approx. 25 mv, much worse. I took the amplifier inside and hooked it up to my Vandersteen's and the hum is quite audible with the wires run in the same manner as the quiet amplifier.
Aside from the fact I probably need to twist the wires back together in the amplifier that has the hum. Any ideas on isolating where this noise is coming from?
http://www.diyaudio.com/forums/pass-labs/256030-new-aleph-monos-completed.html#post3920660
Is it a pure hum or is there a buzz incorporated in the noise anomaly?
Try slight rotating the transformers and see if it gets worse or better.
Is it a pure hum or is there a buzz incorporated in the noise anomaly?
Try slight rotating the transformers and see if it gets worse or better.
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Seems to be a pure hum, but the wave form of the noise is not a sine wave. I'll try to get picture of the scope trace in a bit and post it. The way I installed the transformers and cut the wiring will make rotating them impossible, well maybe not impossible but surely painful. However the transformers in both amps are installed in an identical orientation and one of them is quiet.
In the amplifier that has the hum DC offset is 2 mv when hot, in the amplifier that is quiet 48 mv hot.
Here are a couple of pictures of the scope trace from the amp with the hum. 10 mv per division setting.
The noisier trace is with the RCA shorting plug installed, the quieter of the two traces is with the RCA plug removed. This seems backwards of what I would have expected.
Attachments
Are your input +- twisted? Doesn't looks like they are. Might be pixilation.
Twisting them could provide some shielding.
Could an output wire or power wire be too close to the input?
Does carefully wiggling wire connections in the "bad" amp change the intensity of the hum?
Let me re-phrase that. Does wire wiggling change hum intensity?
Twisting them could provide some shielding.
Could an output wire or power wire be too close to the input?
Does carefully wiggling wire connections in the "bad" amp change the intensity of the hum?
Let me re-phrase that. Does wire wiggling change hum intensity?
Well it looks like your suggestion on the input wiring is at least partially correct. I hooked up the scope this evening and watched the scope while I moved the unbalanced and balanced input wiring and moving it around dropped the noise to about 7 mv from the earlier > 20mv. I hooked it up to my speaker and the hum is still present but only when my ear is once again very near the woofer. I may try braiding the whole input wiring bundle to see how this changes the noise. I still think there is something other than that which is causing part of the low level hum.
As a test tomorrow I will check the quiet amp out by putting the scope on it and moving the input wiring. I am interested in whether it is as susceptible as the amp that is humming.
You are using an unshielded Antek, it's possible you are magnetically coupling noise into the wiring... Loose the nut and rotate the toroid around and see if that makes any difference. You probably don't have much wire to let it move very far, but it can help.
If you have the wire length, remove the toroid clamp and place the transformer on edge, if it's magnetic coupling a 90deg aspect change will make a huge difference.
If you have the wire length, remove the toroid clamp and place the transformer on edge, if it's magnetic coupling a 90deg aspect change will make a huge difference.
To see if the transformer might be causing the issues I was hearing and measuring I decided to try putting a shield in various locations to see how the noise responded. I have a number of brass shim stock pieces about 5" wide by 12" in length. I put a .020" thickness piece into a heavy ziplock bag to insulate it and started putting it into locations between the transformer and the circuit. I could in no way change the measured waveform on the scope by actively putting this makeshift electrostatic shield in various positions within the amp. In fact I was quite surprised that there was absolutely no variation in the noise good or bad seen. I can take my fingers and start moving the input wiring and I will see some small changes, but if I block the input wiring with the brass sheet it does nothing.
Based on the previous suggestions I shortened the unbalanced input wiring and twisted it with the ground. I also removed the balanced input wiring from the board for now as I'm not using it anyway. I just hard soldered a small jumper in across the minus balanced input to ground. After doing this the noise dropped from around 7mV on the output to about 2 - 3 mV. In fact its nearly identical to the quieter amplifier now on the scope which is showing about 2 mV of noise. After connecting the amps back into my system the ear test also confirms that the noisier amplifier is also now hum-free.
So I'm content for now....
Thank you for all of the suggestions!
Based on the previous suggestions I shortened the unbalanced input wiring and twisted it with the ground. I also removed the balanced input wiring from the board for now as I'm not using it anyway. I just hard soldered a small jumper in across the minus balanced input to ground. After doing this the noise dropped from around 7mV on the output to about 2 - 3 mV. In fact its nearly identical to the quieter amplifier now on the scope which is showing about 2 mV of noise. After connecting the amps back into my system the ear test also confirms that the noisier amplifier is also now hum-free.
So I'm content for now....
Thank you for all of the suggestions!
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