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Old 11th June 2006, 08:05 PM   #1
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Default Preamp finished but it wont play..

Hello.
I just finished this preamp.
http://www.albertkreuzer.com/preamp.htm

And now for the big question.. It wont play.
The problem is not in the psu. That delivers +/- 15V as expected.
Should I take off all potmeters and switches and put them on one by one or how would you errortest it?
When I've had power on it and I turn it off, the threshold LED turns on for ˝ sec as the caps are discharging I suppose.
When I put in a pair of headphones and turn up the gain I hear a click-like sound when the meter is about 2/3 turned up.. P1 and P7 are both in middle position. As well are all meters.
Any ideas or could the problem be anywhere?
Thanks
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Old 11th June 2006, 08:30 PM   #2
kubeek is offline kubeek  Czech Republic
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First check the voltage on all + parts of the transistors, and the both supplies of the op-amp.
If it doesn´t help, then I would suggest making a signal-probe. As it is a bass preamp, you could take a tape player and connect it to the input. Then connect a jack-jack wire to some combo, make the shield grounded to your circuit, and start touching the signal part of the ciruit with the tip of the jack, and listen where the signal ends.
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Old 11th June 2006, 08:38 PM   #3
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Quote:
Originally posted by kubeek
First check the voltage on all + parts of the transistors, and the both supplies of the op-amp.
If it doesn´t help, then I would suggest making a signal-probe. As it is a bass preamp, you could take a tape player and connect it to the input. Then connect a jack-jack wire to some combo, make the shield grounded to your circuit, and start touching the signal part of the ciruit with the tip of the jack, and listen where the signal ends.
What do u mean by +parts?
Do you mean measureing the gate on the fets? And in that case how should i measure? What should i look for.

I just tested the opamps.. they get +/- 15V on the correct legs.
Im a beginner so bare with me
Thank you
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Old 11th June 2006, 08:42 PM   #4
kubeek is offline kubeek  Czech Republic
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by the +parts I meanrt the legs of the transistors which are connected to the +voltage, check the voltage by multimeter.
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Old 11th June 2006, 08:55 PM   #5
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Quote:
Originally posted by kubeek
by the +parts I meanrt the legs of the transistors which are connected to the +voltage, check the voltage by multimeter.
ok the drainvoltages are

8,4V on T1
5,89v on T2
15,25V on T3
but i dont know if this is what its suppose to be..
Any ideas
Thanks
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Old 11th June 2006, 08:58 PM   #6
kubeek is offline kubeek  Czech Republic
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seems OK. Now you should do the signal check, and you can also check the G-S voltages.
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Old 11th June 2006, 09:11 PM   #7
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Ok first the G and S voltages..

T1:
8.4 D
1.21 S
0.02 G

T2:
5.89 D
5.10 S
5.10 G

T3:
15.25 D
4.52 S
1.99 G

Ill see if I can figure out how to do that signal check now.
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Old 11th June 2006, 09:24 PM   #8
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about this signalcheck.. Could I plug in a mp3 player, and then use a pair of headphones, grounding the common and one of the speakers and using the tip on the headphonejack to probe on my pcb? That way just listen how "far" the signal gets in the path.
Or how would you do it?
Thanks
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Old 12th June 2006, 12:15 AM   #9
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Quote:
Originally posted by XXLPeanut
about this signalcheck.. Could I plug in a mp3 player, and then use a pair of headphones, grounding the common and one of the speakers and using the tip on the headphonejack to probe on my pcb? That way just listen how "far" the signal gets in the path.
Or how would you do it?
Thanks
I did this and I could only get a signal until i hit the first resistor... after this I coudnt find any signal anywhere.
Any ideas?
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Old 12th June 2006, 12:21 AM   #10
anatech is offline anatech  Canada
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Hi XXLPeanut,
Are you sure your jacks are wired with the resistor between them and the summing point? Are the resistors the correct value?

-Chris
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