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Old 3rd July 2005, 08:03 AM   #1
shyfx is offline shyfx  Australia
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Default My Gainclone PCB - Problems?

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
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Old 3rd July 2005, 08:27 AM   #2
Netlist is offline Netlist  Belgium
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Mikey,
Which chip did you use? LM3886? Do you have a link to the schematic?

/Hugo
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Old 3rd July 2005, 09:48 AM   #3
shyfx is offline shyfx  Australia
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I used the LM3875

The schematic is attached.

-Mikey
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Old 3rd July 2005, 09:50 AM   #4
shyfx is offline shyfx  Australia
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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
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Old 3rd July 2005, 10:45 AM   #5
Netlist is offline Netlist  Belgium
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I see the +in could use a resistor before hitting ground.
http://gainclone.com/inv_gainclone_1.gif

/Hugo
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Old 3rd July 2005, 12:29 PM   #6
fedde is offline fedde  Netherlands
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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
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Old 3rd July 2005, 01:16 PM   #7
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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
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Old 3rd July 2005, 05:52 PM   #8
moamps is offline moamps  Croatia
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Default Re: My Gainclone PCB - Problems?

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
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Old 3rd July 2005, 06:20 PM   #9
squadra is offline squadra  Netherlands
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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?
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Old 4th July 2005, 04:21 AM   #10
shyfx is offline shyfx  Australia
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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
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