DIY Linestage

[xxJPMxx]

I know there has been some buzz around here about inexpensive DIY linestages. For those that can't wait here is a project using a cigar box as a chassis. I have seen some cigar talk around here so maybe you folks have some laying around ready to made into a top notch tube line preamp!   I have always enjoyed Pete's designs and can't say enough good things about what he does for the DIY community.

 

http://pmillett.com/el_escorpion.html

[xxJPMxx]

Hello William, I think part of the allure of this project is the fact it's powred by a 12v wall wart. But I do always use linear supplies myself, Pete did mention the added noise from harmonics produced by the 100kHz switching frequency, a simple low pass filter at 30kHz cuts the noise down 1/3. Getting rid of the switching supply for a more common linear supply wouldn't be hard at all.

 

I would try both outputs but personally I have found I almost always use the output at the upper triodes cathode. This is one of my favorite topoligies, as I have mentioned in the past the gain won't change with tube age, I use this type of circuit a lot and I believe I suggested it for the 12AU7 line stage you built   Some call it an SRPP which it is but only at a very specific load, any other time I believe it's called a half mu follower.

 

Here is the thing; When you take the output from the upper triode and when you optimise the stage for a specific load the distortion drops substantially due to the push pull nature of the stage. I am fairly certain the reason Pete is taking the output from the bottoms triode plate is to keep the stage single ended for that nice second harmonic distortion but still have an active load. If you were to take the output from the upper triode the way Pete has it setup up now the optimum load I believe is 1k9, which most of our amps don't have an input impedance that low. Since I use tube power amps I tend to try and keep my preamps as low in distortion as possible so I would just optimise the stage for a specific load and take the output from the top tube's cathode, ala a true SRPP with very low distortion and low output Z. This is the stage that is still very popular overseas.

[mike stehr]

I pinched a Japanese circuit offline for a 71A triode amplifier that used a 6SL7 in a SRPP arrangement, and did a breadboard with my James OPTs. I liked the sound...I used variable DC power supplies for the 71A filaments. It was quiet, and sounded rather nice on the LaScalas I had at the time.

 

Was is the term for SRPP? Series Reactance Push-Pull? Or Shunt Reactance Push-Pull? Nonetheless, I liked the 6SL7 in SRPP to drive a triode. It seemed to have enough grunt for 71A, or 45 I would think. Dunno about 2A3 or 300B...

 

I've been happy with my Goldpoint stuffed in a box.

 

Edited November 28, 2015 by mike stehr

[xxJPMxx]

I believe Mike is using the Goldpoint stepped attenuator instead of a stereo potentiometer, so no gain. I made some push pull monoblocks a few years ago I sold to a friend and he uses a Goldpoint stepped attenuator as a passive pre, he is very pleased with the results. I was thinking of going that route but the Goldpoint's are very pricey, I ended up buying an Alps Blue Velvet pot for $10 to use with my latest build. I have spoke with the gentlemen over at Goldpoint, I foget his name but he was a very nice guy and we ended up talking for quite a while. He offered to advertise my services on his website but I usually only do local stuff, I am a one man show over here and don't want to get too backed up with work because it's unfare to customers to have their stuff hanging around for 6 months.