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Super simple single stage tube preamp
http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=3266
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Author:  mwhouston [ 14 Sep 2012, 03:09 ]
Post subject:  Re: Super simple single stage tube preamp

@Matt: Of course a log pot "should" be used but a linear works "electrically" just as well. In fact and it may just be my ears, I like the way a linear pot works for volume over a log pot. Strangely you 'tend to crank it more to get the same changes in volume level, but hey that's OK!


My new retro-34UL high gain tube amp would go well with a 4S preamp. I think "The Black" maybe the one to build. This where Matt got across the build and designed the preamp. It only uses the 12AU7 but I have grown to like the tube in a preamp. For me as a driver tube in a power amp give me a 12AX7!

Author:  etet [ 18 Sep 2012, 14:58 ]
Post subject:  Re: Super simple single stage tube preamp

mwhouston, Suncalc - OK thanks for the potentiometer info, the pictures helped. So I have put together a quick prototype using parts on hand (257VDC, 1K cathode resistor, 100K pot) until I get the proper values and some nicer caps. It sounds very good the way it is with a 12au7 running into my RH84.

Attachment:
P1010012.JPG

Author:  Soundbrigade [ 18 Sep 2012, 15:16 ]
Post subject:  Re: Super simple single stage tube preamp

If a log potentiometer is needed and you just have linears in your toubox, you can do this trick: http://sound.westhost.com/project01.htm

Author:  mwhouston [ 18 Sep 2012, 17:48 ]
Post subject:  Re: Super simple single stage tube preamp

The resistor across the linear pot works. I still don't mind a linear pot for volume attenuation.

etet: I would call the build a finished preamp amp. I have used the same caps on other builds including phono preamps with good results. Have you had a listen.

Author:  etet [ 18 Sep 2012, 18:22 ]
Post subject:  Re: Super simple single stage tube preamp

Quote:
...Have you had a listen?

Yes, very nice the way it is. Very transparent and not too much gain, it's just about right. You guy's have done an excellent job with this 4S pre..it is very simple and the results are very good.

Soundbrigade - Good link...ESP has some really good stuff...I find myself there often.

Author:  mwhouston [ 18 Sep 2012, 18:50 ]
Post subject:  Re: Super simple single stage tube preamp

etet wrote:
Quote:
...Have you had a listen?


Yes, very nice the way it is. Very transparent and not too much gain, it's just about right. You guy's have done an excellent job with this 4S pre..it is very simple and the results are very good.

Any Idea how much gain is there is?

You should get somewhere around 9db of gain. About 5db with Ck out of cct. I found these preamps sound better with the bypass cap out of cct. This provides local NFB.

Because my power amps only have about 3db of gain I can't use any of the 4S preamp but rely on my BoZSE (single jFet) preamps for greater gain.

Author:  nodz [ 19 Sep 2012, 06:52 ]
Post subject:  Re: Super simple single stage tube preamp

I was looking at the simplicity of the 4SPS and wondered how easily it would be to separate it from the 4S preamp and use it for other simple projects. I was looking at it to power a simple 12AX7/12AQ5 SE amp, as mentioned on other threads but the peak heater/cathode voltage of the 12AQ5 is 90V. Could the 4SPS be used but with an elevated heater/floating heater.
I believe that the following would be necessary (see diagram), making it essentially the same as the PS used in the SRPP preamp. The additional capacitor would be 10uF/400V and the resistors 100K and 220K .

Author:  mwhouston [ 19 Sep 2012, 17:12 ]
Post subject:  Re: Super simple single stage tube preamp

I'll leave this one to the experts. Hope it works for you.

Author:  nodz [ 19 Sep 2012, 17:18 ]
Post subject:  Re: Super simple single stage tube preamp

I was wondering also whether it was possible to put a DPDT switch in the circuit too. One side breaking the ground and the other the resistors to top of the 0.01uF cap in the heater circuits, so that if the power supply was to be used for multiple projects, then the heaters could either be used as in the original 4SPS of at the flick of a switch they could be elevated.

Author:  nodz [ 21 Sep 2012, 02:45 ]
Post subject:  Re: Super simple single stage tube preamp

Found this link, regarding the elevation of heaters. Looks like, I more of less got the circuit right. Except this link puts 100R resistors to each side of the heater winding
http://www.valvewizard.co.uk/heater.html

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