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Neurochrome HP-2 ultra-low distortion headphone amplifier

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#182
The buzz is not loud but definetly audible. It bothers me that the headphones are doing work all the time whether the amp is on or off. I'll check the ground. When I touch the ground the buzz is less but does not completly disappear.

I don't have any 2N7000 in my parts supply. Can I use a LND150N3-G instead?
 
#183
LND150N3-G is a depletion mode MOS. It won't work. Something like IRF530 would work too if you have any power devices handy.

I agree that having the headphones buzz constantly would be annoying. I would not have released an amp that did that and I didn't hear any buzz with the amps I have built.

Do the headphones buzz when they just rest on, say, a table? Or do they only buzz when you wear them? If @KSTR's theory is correct the buzzing should stop when the headphones are placed on a non-conductive surface.

Tom
 
#187
tomchr said:
If the issue is indeed leakage current from the mains ground to the negative supply terminal, a Class II supply like the XP Power I mentioned earlier (ACM24US24) should solve the issue.
Click to expand...
It's more the other way round. When system GND is at or near PE, leakage current through the body will be low whereas when system GND is basically at mains potential (half-wave rectivied, even) the leakage current will be much higher, many orders of magnitude.

Class-II SMPS typically connect output GND to the primary side with 1nF or so (to pass EMC) and that rather large coupling capacitance is the offender here.
 
#188
Tested the power supply that buzzes and it is open circuit between safety and output ground. The supply that does not buzz shows near zero resistance between safety and ouput ground.

Hard to hear if headphones still buzz when just sitting on the stand. I can't hear anything but it's hard for me to find a quiet enough place to really test.

So is the answer to find a 28v supply with safety ground connected?
 
#191
It sounds like an easy solution would be to run a ground wire from the RCA ground (or XLR pin 1) to the protective earth terminal at the mains outlet. Not the most elegant, but it should work and is easy to try.

If you have a few of the TN2106, I'd try connecting one as described in Post #179. If this works, the coupling is through the internal power supply.

This will work for sure: Connect two TN2106 as follows:
  • The source of both devices connects to the output connector pin 2 (ground).
  • The drain of one TN2106 connects to output connector pin 1 and the drain of the other TN2106 connects to pin 3 of the output connector.
  • The gate of both devices connect to the gate of Q5.
This can be done neatly and easily on the bottom of the board. Secure the parts with hot glue once operation has been confirmed.

The gate of Q5 is a 'discharge' signal. It'll go high when the amp is turned off. That will turn on the two TN2106 which will then short the output of the amp, which will result in 0 V across the headphones. If this doesn't eliminate the buzz you really need an exorcist rather than an electrical engineer. 🙂

Tom
 
#193
I tried the easy solution since well it was easy. Ground from rca to safety and the amp is quiet with the power off and the original 28v supply. I am currently waiting for caps that will fit under the lid and when those get here I will also try the fix using the TN2106s and see how that works.

Glad I didn't need to call in an exorcist for this one, need to save him for when the tube amp acts up!
 
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