Need help bugfixing DIY Amp

gruni · 21st March 2009, 01:55 PM

Hello everyone!

I just built an amp based on this circuit:http://sound.westhost.com/project3a.htm
But i used MPSA42 transistors instead of the BC546s and a CCS instead of the bootstrap circuit.

now i have a problem: when power is applied to the amp, only a 50 oder 100hz hum is audible, when power is removed, you can hear the signal as long as the supply filter caps discharge.

does anybody know what causes this problem and how i can fix it?

jaycee · 21st March 2009, 05:36 PM

You probably have a ground loop.

Make sure all grounds return to a central point. Don't "daisy chain" grounds. The best place to have your "star" point is at the centre of your main power supply capacitors.

Make sure any signal input sockets you have don't contact a metal chassis, if you have this in a box. The signal grounds should go back to the P3A pcb. You may want to put a small 10 ohm resistor between signal ground and power ground.

gruni · 22nd March 2009, 04:12 PM

thank you for your hints.

in my design i ommited R1 and R3. after installing them, the amp works more or less. there is still one problem:
the bias controll seems to be useless. at every setting the voltage across R13 and R14 starts at about 110mv and drops rapidly. whats this?

jaycee · 22nd March 2009, 04:36 PM

Does R15 get hot in this situation?

If so, it's oscillating. The P3A has a particular kind of output stage (CFP or Sziklai outputs) which are particularly sensitive to this issue. PCB layout is critical with this amp.

Another modification that I found helps this, is to put some 220-330uF capacitors on the supply rails, on the PCB itself, close to the output transistors. Similarly, the 100nF capacitors should be close by.

What transistors are you using?

You cannot omit R1. R3 possibly, but you would degrade the amp's RF blocking.

gruni · 22nd March 2009, 08:47 PM

no noticeable warming...

well, instead of Q5/Q7 and Q6/Q8 i used a pair of complementary darlington transistors i had laying around (MN2488/MP1620)... my amp in fact has not very much in common with the esp project

21st March 2009, 01:55 PM	#1
gruni diyAudio Member Join Date: Feb 2008	Need help bugfixing DIY Amp Hello everyone! I just built an amp based on this circuit:http://sound.westhost.com/project3a.htm But i used MPSA42 transistors instead of the BC546s and a CCS instead of the bootstrap circuit. now i have a problem: when power is applied to the amp, only a 50 oder 100hz hum is audible, when power is removed, you can hear the signal as long as the supply filter caps discharge. does anybody know what causes this problem and how i can fix it?

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21st March 2009, 05:36 PM	#2
jaycee diyAudio Member Join Date: Apr 2004 Location: Norwich, UK	You probably have a ground loop. Make sure all grounds return to a central point. Don't "daisy chain" grounds. The best place to have your "star" point is at the centre of your main power supply capacitors. Make sure any signal input sockets you have don't contact a metal chassis, if you have this in a box. The signal grounds should go back to the P3A pcb. You may want to put a small 10 ohm resistor between signal ground and power ground.

22nd March 2009, 04:12 PM	#3
gruni diyAudio Member Join Date: Feb 2008	thank you for your hints. in my design i ommited R1 and R3. after installing them, the amp works more or less. there is still one problem: the bias controll seems to be useless. at every setting the voltage across R13 and R14 starts at about 110mv and drops rapidly. whats this?

22nd March 2009, 04:36 PM	#4
jaycee diyAudio Member Join Date: Apr 2004 Location: Norwich, UK	Does R15 get hot in this situation? If so, it's oscillating. The P3A has a particular kind of output stage (CFP or Sziklai outputs) which are particularly sensitive to this issue. PCB layout is critical with this amp. Another modification that I found helps this, is to put some 220-330uF capacitors on the supply rails, on the PCB itself, close to the output transistors. Similarly, the 100nF capacitors should be close by. What transistors are you using? You cannot omit R1. R3 possibly, but you would degrade the amp's RF blocking.

22nd March 2009, 08:47 PM	#5
gruni diyAudio Member Join Date: Feb 2008	no noticeable warming... well, instead of Q5/Q7 and Q6/Q8 i used a pair of complementary darlington transistors i had laying around (MN2488/MP1620)... my amp in fact has not very much in common with the esp project

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