[qwerty870]

The gilmore dynamic I built makes a loud humming noise just like an electric shaver in both channels at the same volume regardless of the volume level setting. Does anyone have any idea what i might have done wrong? Thanks in advance.

 

[JohnFerrier]

Quote:

Originally posted by qwerty870 The gilmore dynamic I built makes a loud humming noise just like an electric shaver in both channels at the same volume regardless of the volume level setting. Does anyone have any idea what i might have done wrong? Thanks in advance.

It sounds like there is a ground loop between your source and amp. Or the power supply has a lot of ripple.


JF

 

[qwerty870]

I am using Gilmore's power supply. How do I measure ripple and waht can I do to reduce it?

 

[JohnFerrier]

Quote:

Originally posted by qwerty870 I am using Gilmore's power supply. How do I measure ripple and waht can I do to reduce it?

If you have a multimeter, switch it to AC volts and connect it to the power supply. I would expect there to be much less than 50mV ripple.

To check for a ground loop, disconnect the cable from your amplifier to your source and see if the noise goes away.


JF

 

[qwerty870]

I measure 0.04V of ripple on V+ and 0.03V on V- is this acceptable? There is no ground loop problem. Thanks for your help.

 

[Possum]

antness says to try removing the capacitors at the output of your PSU, since he had the same problem with many types of capacitors he's tried.

 

[qwerty870]

Shall I remove the pair of micas and the pair of Wima films at the end of the power supply? Or the micas and all four wimas?

 

[JohnFerrier]

Quote:

Originally posted by qwerty870 I measure 0.04V of ripple on V+ and 0.03V on V- is this acceptable? There is no ground loop problem. Thanks for your help.

If you are measuring 40mV and 30mV AC on the +/-16V power supplies, I would say this is too high. It is supposed to be an "ultra regulated" circuit. I would hope that it is less than 1mV.

If you've ruled out a ground loop between source and amp, it could be a ground loop in your PS. The output of the REF02 should be a solid 5Vdc with well under 1mV ac. If not put both leads directly on the REF02.

Notice in Gilmore's descrition: "In theory, there is a ground loop between the RCA jacks on the back plate and the headphone jack on the front plate. Although the 60 Hz hum is about 110 db down..."

Maybe there is something more wrong with your implementation.

There is no reason that the circuit shouldn't work well. Don't give up. These problems can be tricky to find, but once you fix it, it will be gone for good. Look carefully over the grounds in the power supply. In particular, make sure that the REF02 and OPA548 (or whatever your using) has a low resistance path to "ground". In the best case, you would want to have a star pattern from say the bridge diodes out to all the other circuits. Also, make sure all the 4,700 caps are connected.


JF

 

[Possum]

Quote:

Originally posted by qwerty870 Shall I remove the pair of micas and the pair of Wima films at the end of the power supply? Or the micas and all four wimas?

Relaying the answer: Just remove the ones after the OPA548.

 

[JohnFerrier]

Quote:

Originally posted by Possum antness says to try removing the capacitors at the output of your PSU, since he had the same problem with many types of capacitors he's tried.

Not sure if KG would approve of this but if it is oscillating because of the "ultrafast poly caps". Then you might try leaving them in and put two 0.01uF caps (ceramic X5R, or try whatever is handy) across each of the two feedback resistors (22.8k and 10k).


JF

 

[qwerty870]

Done. I still get the same unusual hum, but at least now I hear music at the same time. The volume control still has no effect on voulme level. Thanks for all your help so far. I am considereing replacing all the transistors in the amp.

 

[qwerty870]

Could I buy a +/- 16V power supply and use the amp with that until I fix my power supply problems. Does anyone know of an inexpensive one?

 

[JohnFerrier]

Quote:

Originally posted by qwerty870 Could I buy a +/- 16V power supply and use the amp with that until I fix my power supply problems. Does anyone know of an inexpensive one?

Inexpensive ones are likely to have the 40mV AC ripple that trouble you now. You'll need to concentrate on fixing the Gilmore PS.


JF