Author Topic: Plate follower buffer/preamp  (Read 4498 times)

mgalusha

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Plate follower buffer/preamp
« on: February 24, 2007, 07:21:03 PM »
A couple of years ago I built a version of the Decware ZSLA preamp. This is basically a cathode follower buffer and I was quite happy with the results except that I needed some gain and with that circuit the gain was only .9, so with a low output source I couldn't even approach full power with my amps.

I've recently been researching an active buffer with a little gain to use with a modified Behringer DCX2496. The modified DCX only has an output of about .7V RMS and this isn't close to enough to drive my amps, so a little gain was in order.

As part of this I started playing with TubeCAD. I bought this some time back but have never really used it much. Well after spending several days with it I ended up with a circuit using a Russian 6N1P dual triode. I chose this tube because that was the tube I used in the Decware clone and I have about a dozen of them. This is a very nice little triode, similar to a 6922 and 6DJ8 but with a higher plate voltage and greater dissipation. I was able to use the existing power supply with only minor modification.

The results are quite good for such a simple circuit, a single triode for each channel. Measured distortion is .02% at 4V RMS and only .05% at 30V RMS output. Even at 30V it's not clipping, so there is no way this will ever be reached in the real world and it has loads of headroom. It provides about 8dB of gain, which is perfect for my intended use. Frequency response is dead flat from 10Hz to about 50KHz and it's -3dB at about 160KHz.  The distortion is pure 2nd harmonic, so what is present is not offensive. Noise is about 1mV with the input shorted. Output impedance is 980 ohms, so this needs to be used with an amp having an input impedance of 10K ohm or greater, which is most amplifiers.

If anyone is interested I'll post a schematic. I've chosen a lower value for the cathode resistor than is suggested, so the tube isn't biased very hard but I liked the sound better with a lower value resistor vs the textbook resistor with a bypass cap.

Mike

Gordy

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Plate follower buffer/preamp
« Reply #1 on: March 20, 2007, 01:54:44 PM »
I really need to get over here more often!  Mike, I'd love to see your schematic, thanks!  I've been thinking of trying a tube buffer after my S&B tranny pre, do you think this would be appropriate for that? I really wouldn't more than about 3dB of gain if possible...

Gordy

Offline richidoo

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Plate follower buffer/preamp
« Reply #2 on: March 20, 2007, 02:02:19 PM »
Me too, thanks. I will need a preamp soon when my integrated is upgraded to separate amps. Your design sounds simple enough for a first tube project. Thanks!
Rich

mgalusha

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Plate follower buffer/preamp
« Reply #3 on: March 20, 2007, 02:36:50 PM »
I'll dig up my schematic. It's pretty simple circuit and sounds surprisingly good for what it is. I did a little more experimenting after I posted this and found I liked the sound even better with a different cathode resistor.

I think it's on my laptop, so I'll see if I can hunt it up this evening and post it.

Mike

Offline Mike B.

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Plate follower buffer/preamp
« Reply #4 on: March 27, 2007, 05:37:20 PM »
Yes, please post it. :lol:
Acme Audio Labs

mgalusha

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Plate follower buffer/preamp
« Reply #5 on: March 28, 2007, 07:53:43 PM »
I found the schematic and have prettied it up a little for public consumption and also added John Broskie's name since it primarily came out of Tube Cad though the power supply didn't.

I need to take my unit apart and verify the values of a few resistors before I post it. I kept experimenting and I think I put the right ones in the schematic but I want to be sure. :)

I'm hoping I can get some time Friday evening or Saturday morning. Sorry for the delay.

Mike

Offline richidoo

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Plate follower buffer/preamp
« Reply #6 on: March 29, 2007, 09:33:21 AM »
No problem Mike, thanks for taking all the trouble. No rush at all.... Might take me a year to build it anyway.
Rich

Gordy

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Plate follower buffer/preamp
« Reply #7 on: March 29, 2007, 12:37:36 PM »
Thank you Mike, your time and consideration is greatly appreciated!  Speaking of Tube Cad, do you have any experience with the Aikida pre?  It seems to be a very interesting project as well.

Gordy