6SL7, 6J5, or 6SN7 unity gain buffer amp - diyAudio
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Old 16th July 2012, 02:10 PM   #1
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Default 6SL7, 6J5, or 6SN7 unity gain buffer amp

So I have a handful of the following tubes, 6SL7, 6J5, and 6SN7. For a fun small project I'm thinking of building a very basic unity gain line stage for my computer/desk. I have a small pro-sumer balanced audio interface and a pair of KRK powered studio monitors. The problem is I don't really want/need any gain between my DAC and powered speakers but I'd like to introduce a little tube sound to warm things up a bit. I've been playing around with load lines for the 6SN7 but even with the steepest line (low Va high Ia) I still end up with an 8-10 volt swing on the output (way too high for my monitors). So I'm wondering, is there an operating point or circuit using a single 6sn7, 6J5, or 6SL7 (one side for each channel) or other variant listed above? I'm trying to keep it as simple as possible and will use nothing more than a blocking cap on the O/P. Any thoughts?
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Old 16th July 2012, 03:43 PM   #2
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Perhaps a White Cathode Follower may be an option!!!
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Old 16th July 2012, 05:56 PM   #3
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I'm trying to post a schematic, see if it worked...
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Old 16th July 2012, 06:13 PM   #4
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Quote:
Originally Posted by soundguruman View Post
I'm trying to post a schematic, see if it worked...
How does the second triode get its anode power supply?
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Old 16th July 2012, 07:06 PM   #5
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Speaking of cathode follower, I found this:

Circuit of the Month: Zero Gain Tube Line Stage Amplifier

Not very simple though given all the solid state stuff on the tail.
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Old 16th July 2012, 10:28 PM   #6
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Quote:
Originally Posted by Osvaldo de Banfield View Post
How does the second triode get its anode power supply?
The first stage is a gain stage to saturate the grid of the second stage..that will give it some tube harmonic overtones.
The second stage is an attenuator, there is no anode voltage. It still must have a heater, though. That's why DC heaters...

Hahaha you are looking at that scratching your head. I recently started to design a similar circuit, and I had it running pretty good...
In fact it was plenty clean...even for hi fi.
It's probably ideal for a metal envelope tube, or use a shield around the second stage. As you can see the plate of the second stage might be sensitive to AC fields, but it's not a high gain amp, so as long as the power transformer is far enough away, it should work without introducing hum into the audio.

Last edited by soundguruman; 16th July 2012 at 10:39 PM.
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Old 16th July 2012, 11:45 PM   #7
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second way, more current?
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Old 17th July 2012, 03:01 AM   #8
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So I came up with this...

Click the image to open in full size.

Click the image to open in full size.

Feel free to tear it to pieces.
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Old 17th July 2012, 03:44 AM   #9
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by dannydanger View Post
So I have a handful of the following tubes, 6SL7, 6J5, and 6SN7. For a fun small project I'm thinking of building a very basic unity gain line stage for my computer/desk. I have a small pro-sumer balanced audio interface and a pair of KRK powered studio monitors. The problem is I don't really want/need any gain between my DAC and powered speakers but I'd like to introduce a little tube sound to warm things up a bit.
A unitiy gain buffer will NOT introduce much "tube sound". If you design it correctly it will have so little distortion as to be completely undetectable by ear. Every unity gain design (stating with the cathode follower) has "tons" of negative feedback so the thing will sound just like a length of wire.

If you want to introduce that "sound" you have to amplify the signal. Then if unity gain is designed attenuate the signal back to a net effect of "unity". The extreme case of this is the preamp section of a guitar amp. They create a voltage gain of say 60x and then send it to a pot to reduce it.

If I wanted a buffer to introduce a "tube sound" I'd use a 12AU7 common cathode gain stage and run the output to a voltage divider. The result will sound like a triode amp.
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Old 17th July 2012, 03:58 AM   #10
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by soundguruman View Post
second way, more current?
This circuit is EXACTLY what was asked for. The output pot is the voltage divider I was talking about. Two fixed resisters would work too as would a trim pot. (once you set this you will never need to change it so don't bother with a knob unless you are building a guitar amp.)


I built one of these. A cheap and easy why to power it is to salvage two 9 to 12 volt "power cubes" (aka wall worts) and remove the transformers. Wire the secondaries in series. Now to have a (in N. America) 120 volts AC for B+ and you can tap the 9V to 12V for the heater. Send it to a three terminal voltage regulator, you only need about 1/10th amp DC.

An even better design is to find AC (not DC) wall worts and put a coaxial power input jack in your box and the second wall wort inside the box to convert the 12VAC to 120VAC. Now you don't have any mains current inside the chassis.

I use mine for a guitar OD effect so I did place the output pot knob in a accessible place. Built the whole thing inside a case that used to hold and external CDROM drive. These preamp tube don't make much heat and you can enclose them inside a plastic box, if you make some vent holes
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