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-   -   ESP P3A amp, very noisy (http://www.diyaudio.com/forums/solid-state/7553-esp-p3a-amp-very-noisy.html)

DaViruz 12th November 2002 03:48 PM

ESP P3A amp, very noisy
 
http://sound.westhost.com/project3a.htm

I built this amp about a year ago, i am very satisfied with it, but under certain conditions it generates a very loud distorted 50Hz tone.
It must be about a year ago i built it now, never really used it, found this forum a couple of months ago, but not until now i have had the time and will to do a few tests.
It consists of two mono amps using the same PSU (24-0-24 trafo with two 4700F caps (one for + and one for -)).

When i ground both inputs (or have any resistance at all to ground) and short both input grounds (which is the case since i use a 2xCINCH to 3.5mm stereo plug cable) i get the following noise:
L: 50Hz. ~20mV positive spikes, about 1-2ms long.
R: 50Hz, ~20mV negative spikes, about 1-2ms long.
The noise is very audible, and is even higher when the bias current is correctly adjusted, the pots are at highest resistance now.

If i remove the short between the input grounds, and just connect the inputs to their respective ground, the following noise is present at the speaker terminals:
L: none at all
R: 50Hz, ~5mV positive spikes, not audible

Now, if i just short the input grounds and leave the inputs floating, both channels are clean.

This is done with a 8ohm speaker load.

I have measured all resistors, and they are OK.

Oh, i used the components in the schematics, except for the power transistors, which i replaced with motorola MJL3281 and MJL1302 instead, since the toshibas i purchased turned out to be counterfeits. (Don't buy 2SA1302/2SC3281 from www.jec.se)

paulb 12th November 2002 07:12 PM

It sounds like a ground loop may be responsible.
Have you asked Rod for help? If you bought his boards, he will probably respond with some suggestions.

DaViruz 12th November 2002 07:21 PM

Quote:

Originally posted by paulb
It sounds like a ground loop may be responsible.
Have you asked Rod for help? If you bought his boards, he will probably respond with some suggestions.

Yup, we exchanged a couple of mails, he didn't have a clue. I didn't buy his boards thogh, but i am sure my boards are correct. As long as i use the amps totally individually, it works great. :-)

NickC 13th November 2002 02:13 AM

50 hz could be not suffice caps on the ps
most probably grounding problem
what type of grounding u went with?
one star ground or multiply star ground and all route to the main star?
have u tried lifting the ground with 10 ohm resistor with a .1uf connected in parallel

swede 13th November 2002 06:56 AM

Quote:

As long as i use the amps totally individually, it works great. :-)
In that case, it IS a ground loop.

I had the same thing on my Leach Amp. I solved it by disconnecting the ground connection to the board from one of the input connectors and connected it to the other board's input ground, or something.

You have to isolate where the ground loop is originated and terminate it somehow.

Do you have pictures? It always helps to see what it looks like.

//magnus

hifiZen 13th November 2002 10:26 AM

Hmm... 4700uF is pretty slim too. I'd double this at least, even for just one channel. Personally, I'd have at least 15,000uF per rail, per channel.

swede 13th November 2002 10:28 AM

hifizen: Even though I don't argue with you about the amount of filtering caps, I don't think he'll ever sort out a ground loop by just adding more caps.

Best,
//magnus

DaViruz 13th November 2002 05:10 PM

Hm, weird thing is that the noise is still there even if i only connect one input ground thus breaking the ground loop (its just on one channel then though). I tried 10 ohm resistors in paralell, didn't help, havn't tried caps though.
Im fixing pictures as soon as i can figure out where the hell i put my digicam :-)
Oh, and the PS is designed from Elliots guidelines, 300VA toroid with 2x4700F caps for both amps.

Pics up: http://daviruz.homelinux.org/pics/amp/
This is just a test-setup chassis in case you wondered :-)

paulb 13th November 2002 05:28 PM

Quote:

Originally posted by swede
hifizen: Even though I don't argue with you about the amount of filtering caps, I don't think he'll ever sort out a ground loop by just adding more caps.

I agree with swede. This is a Class AB amp, it draws very little current with no signal, therefore small amount of ripple and the 4700s are fine. With Class A it's different, it draws current big-time even with no signal.

chris ma 13th November 2002 05:49 PM

From the picture if I guess correctly that the yellow wire from the 230AC plug is the Earth, correct? But I can not tell whether it is connected to the chassis or to the cap. If it is connected to the cap then I think that is the wrong place(-Ve).
Regards,
Chris


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