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Old 13th February 2011, 03:30 PM   #1
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Default Preamp with buffer design

Ok, after a week of searching and reading, following the suggestions made by forum members on various posts, I decided to make a passive preamp for my quad 405 (linestage only). Having said that, since I will build a tube preamp for my phono stage in any case, I'm thinking, let's add the possibility to switch from passive to tube buffer in the preamp for the linestage.

I have found a few possibly suitable tubes in my stuff (salvaged mostly): 2 x 6922(unused!), 2 x 6cg7 and 2 x 12ax7. I also have old preamp boards, unpopulated, that I would like to use, even if I leave a few areas unpopulated on them. The schematic is as follows, and I was thinking of having the first stage with V1 left blank and using only the second stage (cathode follower) for buffer purposes. Since both channels are using the same tube, where do I connect the B+ on that schematic? Also, what do I do with r6 r16 r18 (what is their use)?

I am tube newb, so a few things are still unclear to me... please assume I am 5 years old when you explain something to me (I'm afraid I'm good with numbers, but not all the technical terms as of yet...)

Thanks
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Old 13th February 2011, 05:30 PM   #2
DF96 is offline DF96  England
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Where did you get that circuit? It has two identical amps, one per channel, but with lopsided B+ arrangements. It is not surprising that you are confused. The B+ supply should probably be attached to the middle of R16, so that each channel sees the same voltage.
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Old 13th February 2011, 05:37 PM   #3
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As a first step I would be tempted to split R16 in two as you suggest and decouple the left and right ends of it and also repeat the left channel first stage PSU decoupling on the right channel first stage.

Cheers

Ian
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Old 13th February 2011, 07:09 PM   #4
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It is a preamp board I had in my random stuff pile, it looks like the picture attached. Sorry for the crappy quality, I was in a hurry, I'll take a better one if needed. I have two btw, so I can experiment on one. The circuit is quite visible, I'd like to remove the part under the toroid hole, cause I have two transfos, one for hv, one for lv.

Splitting r16 is feasible, what about c6 and r4?

Thanks.
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Last edited by Antimoine; 13th February 2011 at 07:11 PM. Reason: pic missing....
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Old 13th February 2011, 07:33 PM   #5
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Difficult to comment on mods to the PCB without a better picture. I can't read the the component legend.

Cheers

Ian
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Old 13th February 2011, 08:19 PM   #6
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A clearer pic has been taken! I'm also wondering about the power transistor Q2....
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Old 13th February 2011, 09:36 PM   #7
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That's a much better picture. Looking at the (single sided) layout I would guess that R6, R16 and R18 are meant to be zero ohm links.

Q2 is part of a simple regulator. D9 thru D12 are what I would guess are four zener diodes in series and Q2 is the series pass transistor.

Cheers

Ian
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Old 14th February 2011, 01:05 AM   #8
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You, sir, are making a lot of sense... the schematic looks way nicer that way. I thought I was missing something, and there it is, it's single sided!

Is the idea of bypassing the gain stage a good one? or is there another way of achieving unity gain apart from using only the cathode follower?

What are c6 and r4? Is there a name for such an arrangement?

Thanks

John
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Old 14th February 2011, 10:31 AM   #9
DF96 is offline DF96  England
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C6 is a decoupler. It reduces noise and hum on the HT (B+) supply, and also reduces unwanted coupling between different stages.

R4 will simply use up current. It may be that it was originally part of an HT dropper arrangement.

The only other way to get unity gain is to use an anode/plate follower. This will have a lower input impedance than a cathode follower, and possibly more noise. It wil invert the signal.
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Old 14th February 2011, 12:00 PM   #10
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Quote:
Originally Posted by DF96 View Post
R4 will simply use up current. It may be that it was originally part of an HT dropper arrangement..
It is probably there to ensure the HT capacitors get discharged when the power is turned off.

Cheers

Ian
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