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Old 20th March 2013, 04:56 PM   #31
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Quote:
Originally Posted by JMFahey View Post
Hi audiostrat.
Respect to the output cap, I also thought you would rely on the power amp input cap, and it works, but it's "good manners" putting the proper one at the preamp output (if you have DC there) so the preamp can be used with any other amp, or to go straight into a mixer, drive effects, whatever.
As of the hiss with the volume poy at "0" it indicates some grounding problem.
That explais hiss on 10 (normal, comes from the earlier stages), "0", because of grounding, and less at intermediate points because you are neither on 10 nor touching ground.
As an experiment, lift the Volume ground connection and with a piece of "flying" wire, grounding at the output jack ground pin.
*Should* work, although this kind of problems is never 100% certain.
Proper grounding is a sort of black art.
Good luck

By the way, you are the second Greek I answer to today, maybe that means I'll have to visit beautiful Greece again??

(Hope so )
Well, volume pot is not grounded, just goes to 15V. You think this could also be the cause? Improper 15V connection?

I hope you come visit our country again!
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Old 20th March 2013, 05:48 PM   #32
JMFahey is offline JMFahey  Argentina
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Ouch!! I forgot that.
Well, that's one of the problems coming from using a "1/2 supply virtual earth", you can't ground stuff where you want to.

As of the visit, it's in my plans, I'm visiting Europe next year with my kids, for them to catch first hand the origins of Argentine Culture, which are Spain, Italy and Greece.
Besides I have a standing invitation to visit Santorini island, so I'll already be quite close to mainland Greece.
Thanks.
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Old 15th June 2013, 02:56 PM   #33
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Quote:
Originally Posted by Ian Finch View Post
By "thermal noise", the OP is likely referring to hiss - also likely with his circuit arrangements.
What do you mean by circuit arrangements? High impedance input resistors that add a lot of noise?
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Old 16th June 2013, 12:47 AM   #34
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JMF has given you most of the detail you need to approach this problem. The arrangements I referrred to are simply how you designed the preamp, how you connected the varous sections of the preamplifier and particularly the mis-matched presence control. 'Just the basics but as JMF explained, there were some errors.

I usually had problems with the controls on modern guitar amplifiers with their mix of sharp and normal response curves and near instability. Perhaps simply duplicating the control circuit from a reputable brand amp. and getting experience with that first would be a better way to proceed and experiment with modifications.

Once you have a known working design operational, you then have a model to work on. Simulation might be cheap and easy but it won't tell you much about how this will work as a practical device for playing music - something you need a lot of experience to correlate usefully with graphical information. At least, that's how I find it.
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Old 16th June 2013, 12:44 PM   #35
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I am asking since I changed the topology a bit - I discarded the 1k resistor before the volume pot, and inserted in its place a 2u2 film cap. Then, grounded the volume pot instead of connecting it to 15V.

That modification worked: normal hiss from now on - zero with volume at 0, more as I rotate volume CW. So we can say everything is ok!

The thing is that overall hiss seems a bit to much to my ears - when guitar is at idle, I clearly hear it, even though it is well below my playing. I just don't remember whether old tube amps have so much or even more.
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Old 16th June 2013, 02:02 PM   #36
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Do you have access to another commercial amplifier with similar features to compare with? The more filter and buffer circuits the more noise, of course and that's why simple and crude circuits are often best, if they reduce the number of stages and stiil have good control over the effect, regardless of other imperfections.

Specific answers though, are probably best found in the "Instruments and Amps" forum where this topic really belongs.
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Old 16th June 2013, 02:26 PM   #37
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Quote:
Originally Posted by Ian Finch View Post
Do you have access to another commercial amplifier with similar features to compare with? The more filter and buffer circuits the more noise, of course and that's why simple and crude circuits are often best, if they reduce the number of stages and stiil have good control over the effect, regardless of other imperfections.

Specific answers though, are probably best found in the "Instruments and Amps" forum where this topic really belongs.
Sadly, no at the moment. I realise your point though.

Indeed it has become part of the forum you mention - still, when opened it had to do with - arguably - an oscillation.
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