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Bensen 3rd October 2012 06:18 PM

Strange ripple on rails when amp is connected
2 Attachment(s)

My speakers are humming when connected to my last diyamp, the frequency of the hum is approx 160Hz, measured at the speaker output.

After a lot of testing and measuring, I can say the hum is comming from the power supply. When connecting my scope between ground and the + or - rail, I'm measuring a ripple with a frequency of +-320Hz and 60mVpp, see the picture.

When disconnecting the amp the ripple on the rails is gone. I tried an other transformer, rectifier bridge and caps, nothing changed this phenomenon.

The setup of the power supply is:
1. mains 240Vac/50Hz
2. EMI filter
3. soft start
4. Amplimo transformer 625VA/ 2x35V
5. rectifier bridge made with 4x HFA25PB60
6. 10000F+4700f +0.11Ohm/10W + 10000F
7. amplifier

Can anybody tell me what could causes this ripple and what I could do to remove it?


Mooly 3rd October 2012 06:35 PM

Have to ask :)

Is your scope calibrated correctly ? 60mv ripple on the rails sounds very very low. The frequencies don't make sense either.

tauro0221 3rd October 2012 06:36 PM

Short the input and see if the hums goes away. Also check the voltage at the speaker terminals.

tauro0221 3rd October 2012 06:38 PM

What kind of amplifier your are building?

DF96 3rd October 2012 06:58 PM

What is your mains frequency?

60mV seems quite low for PSU ripple. Does your amplifier have particularly poor PSRR?

Bensen 3rd October 2012 07:11 PM

1 Attachment(s)
Mooly and Tauro0221,
Funny, I din't think on that before. I connected the scope to my frequency generator, and indeed, the frequency reading of on the scope was totaly wrong. After cleaning the contact of the measuring cables/interfaces scope, all is fine.

Checked again the ripple ==> when input of amp is shorted

- ripple on rails = 60mVpp /100Hz
- speaker output = 10mVpp / 50Hz


Poor PSSR, I don't think that it will differ much with many other complementary symmetric amps. Or am I missing something?? Please have a look at the shematic below. I already changed the Fet drivers to BJT's, but his hasn't made a difference.

DF96 3rd October 2012 07:29 PM

The first stage CCS circuits could be a route for supply rail ripple to get into the circuit. More smoothing would help, either at the CCS or in the supply rail to the early stages. The other thing to check is exact details of grounding, especially the ground reference for the feedback.

Mooly 3rd October 2012 08:02 PM

I too think layout and grounding is probably the most likely cause and from what I can see from your picture it all looks really neatly built.

If the "hum" has a harsh sounding edge to it (harmonics present) then grounding is a prime suspect.

50 hz hum. If it were induced hum then I would think the output would be more sinusoidal and tbh at that level probably not audible. 50 hz as a sinewave is a very deep pure tone (if the level is high enough).

I really don't know what to suggest without first hand knowledge of all the wiring layouts. Every conductor has to be treated as a resistance and you have to ask that "if a current flows and volt drop develops", can that get into the signal path.

Probably not much help but this might give you ideas. You'll have to read it all as the thread wandered a bit.

tauro0221 3rd October 2012 08:50 PM

One more question. Do you have the AC earth ground wire connected to the chassis?

nigelwright7557 3rd October 2012 11:04 PM

I had trouble with a power amp humming and I found the input stage supply of the amp needed decoupling using 33r and 470uf.

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