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Old 11th March 2013, 07:58 PM   #21
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Thanks... I'm on it. Was this a large contributor to the improvement in the sound quality from your memory or was there something else I need to tackle first ?
The older I get, the fitter I was !
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Old 11th March 2013, 08:29 PM   #22
rmgvs is offline rmgvs  Netherlands
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It's only now that I look at the circuit diagram of ColinF (linestage only, post #15). As far as I can see it's what I had but with 330k en 1M reversed. Simulation of the circuit with the values of Colin results in a voltage at the output (before cap) of around 5 (where it should be around 15V with a power voltage B+ of 30V).

Not your standard circuit, this one. Took me quite a time puzzling. It's a 3 stage amplifier: normal amplifying stage 1 (1 in circuit), another normal one stage 2 (4 in circuit), ending in a follower stage without amplification (2) with current source (3). A lot of voltage dividing networks to get the mosfets going and and a rather unusual feedback loop from the output back to stage 1 where another voltage divider seems to feedback the signal too (the upper part of this voltage divider could have been going to the B+ like all others, but instead is hung to the midpoint/output.

Anyhow: I'm not an expert in circuits so any explanation is welcomed. And the circuit works both in simulation and in practice. Lastly: I don't think anybody in the world has built this one from scratch on Veroboard (both using bs170 and the very expensive vq1000) except for this nut? Why should you?
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Old 11th March 2013, 08:42 PM   #23
rmgvs is offline rmgvs  Netherlands
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I can't remember what I did with it and how it worked out. For me the clue is not in fiddling around with the caps, they ar okay by themselves. The value and type of the 100p can make a difference, I had good result with silver mica caps. Of course you can change the value of this cap to tailor the sound a bit (higher value means slightly rolling off highs at the 20-50 kHz range).

I had the biggest improvements in changing the power supply. Not by just putting another bigger transformer at it, but by trying completely different topologies. Of course you can start with LM317 (just for quick testing the board, not bad at all). In the end I used a Kubota type power supply with mosfet as regulating element. Just google around, you will find a number of beautiful circuits that can be used.

It takes a lot of time and trouble to discover this preamp is very well from itself and not so easy to improve upon. Well, that's no surprise as it is coming from guru Tim de Paracivini himself.
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Old 11th March 2013, 09:00 PM   #24
rmgvs is offline rmgvs  Netherlands
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Default picture of working prototype

This was my first prototype on veroboard using bs170. Still working today.
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File Type: jpg preamp3-diy-prototype.jpg (525.1 KB, 116 views)
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Old 30th May 2014, 05:32 AM   #25
ColinF is offline ColinF  New Zealand
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Hi rmgvs,
It's been a while since I last was on diy audio. So you got the line stage going on a board, well done! It's quite possible on the schematic I drew that the 1M and 330k resistors are the other way around. That would make better sense to me to set the output voltage at around 15v. Is your circuit noisy?
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Old 16th September 2014, 06:26 PM   #26
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Default B&K CS115 Pramp

Dear friends, I looking for a B&K CS 115 Service Manual

Many Thanks Joaquín Obiol
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