My Gainclone PCB - Problems?

shyfx · 3rd July 2005, 08:03 AM

Hi guys, I just finished builing my gainclone on a PCB i designed in Protel.

I tested the amp before connecting it to a heatsink for DC offset and it was at ~20mV for both amps. Then I connected it to a speaker and this is when I noticed a hum sound which increased to a higher pitched sound over about 5 seconds. I figured I could test out the amp at low volumes before I attached it to a heatsink (which I am yet to purchase).

Can you check this amp at all without some kind of heatsinking or is it not going to work? Could this cause this weird sound I am hearing or is it my PCB design (which is attached)?

Any help would be great thanks

-Mikey

Netlist · 3rd July 2005, 08:27 AM

Mikey,
Which chip did you use? LM3886? Do you have a link to the schematic?

/Hugo

shyfx · 3rd July 2005, 09:48 AM

I used the LM3875

The schematic is attached.

-Mikey

shyfx · 3rd July 2005, 09:50 AM

A note, don't mind the three resistors and their values. I put it like that cause I figured I could match up to three resistors to get a niec precise value. I am using just one 220K 1% resistor in place of the three.

The signal and power grounds are connected

-mikey

Netlist · 3rd July 2005, 10:45 AM

I see the +in could use a resistor before hitting ground.
http://gainclone.com/inv_gainclone_1.gif

/Hugo

fedde · 3rd July 2005, 12:29 PM

I would advise to not use that chip without heatsink! It can easily damage that way, even considering the internal protection. Just use a metal bar or an old pentium cooler with a hole drilled in for testing.

Fedde

jackinnj · 3rd July 2005, 01:16 PM

Quote:

Originally posted by fedde
I would advise to not use that chip without heatsink! It can easily damage that way, even considering the internal protection. Just use a metal bar or an old pentium cooler with a hole drilled in for testing.

Fedde

Fedde is too modest -- look at his Chill Amp webpage to see a very nicely designed board -- http://jwg.student.utwente.nl/fedde/chill-amp.html

moamps · 3rd July 2005, 05:52 PM

Quote:

Originally posted by shyfx
... Then I connected it to a speaker and this is when I noticed a hum sound which increased to a higher pitched sound over about 5 seconds. ....

Hi,
Your amplifiers may oscillate. In an inverted setup, it is very important that the source (with a small output resistance) is connected to prevent oscillation.

Regards,
Milan

squadra · 3rd July 2005, 06:20 PM

Quote:

Originally posted by shyfx
A note, don't mind the three resistors and their values. I put it like that cause I figured I could match up to three resistors to get a niec precise value. I am using just one 220K 1% resistor in place of the three.

The signal and power grounds are connected

-mikey

The input impedance is only 56k // 10k == 8k5, isn't that a bit low?

shyfx · 4th July 2005, 04:21 AM

http://www.decdun.fsnet.co.uk/gainclonecircuits.html

That's the schematic my amp is based on. Maybe the minimised approach is suitable for point-to-point wiring only?

I will try tying the non-inverting input to gruond with a resistor and attach a source. I will go buy some heatsinks too, which I will need anyway and try this PCB and amp out again. Thanks for the help once again guys.

-Mikey

3rd July 2005, 12:29 PM	#6
fedde diyAudio Member Join Date: Jul 2002 Location: Eindhoven, the Netherlands	I would advise to not use that chip without heatsink! It can easily damage that way, even considering the internal protection. Just use a metal bar or an old pentium cooler with a hole drilled in for testing. Fedde __________________ http://www.fedde.nu, gainclone and non-oversampling DAC audio projects and kits...

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3rd July 2005, 08:27 AM	#2
Netlist diyAudio Moderator Emeritus Join Date: Jan 2003	Mikey, Which chip did you use? LM3886? Do you have a link to the schematic? /Hugo

3rd July 2005, 09:50 AM	#4
shyfx diyAudio Member Join Date: Apr 2003 Location: Melbourne, Australia	A note, don't mind the three resistors and their values. I put it like that cause I figured I could match up to three resistors to get a niec precise value. I am using just one 220K 1% resistor in place of the three. The signal and power grounds are connected -mikey

3rd July 2005, 10:45 AM	#5
Netlist diyAudio Moderator Emeritus Join Date: Jan 2003	I see the +in could use a resistor before hitting ground. http://gainclone.com/inv_gainclone_1.gif /Hugo

4th July 2005, 04:21 AM	#10
shyfx diyAudio Member Join Date: Apr 2003 Location: Melbourne, Australia	http://www.decdun.fsnet.co.uk/gainclonecircuits.html That's the schematic my amp is based on. Maybe the minimised approach is suitable for point-to-point wiring only? I will try tying the non-inverting input to gruond with a resistor and attach a source. I will go buy some heatsinks too, which I will need anyway and try this PCB and amp out again. Thanks for the help once again guys. -Mikey