[45] Leaky AC128 Buffer Redux

Date: September 18th, 2007 | Comments : [9] | Categories: DIY.

AC128 transistor buffer

Another look at the leaky transistor buffer!

I was looking back at some of the recent articles and it suddenly occurred to me that the AC128 buffer using the leaky transistors that I posted a week ago could easily be adapted to work on a negative ground system.

All we have to do is flip the transistor over and it can be powered from the same supply as the rest of your pedals. Note that there are some slight changes to the ground reference of the pulldown resistors and also the orientation of the output capacitor C2.

The low gain (Av=0.9) will prevent this circuit from causing problems when used in this configuration, unlike what can sometimes be encountered with the high gain fuzz circuits used like this.

Try this simple buffer with your leaky germaniums!


9 Responses to “Leaky AC128 Buffer Redux”

[656] Ian C. Says: 11:24 am, April 11th, 2009

Jack, can the gain on this buffer be pushed up to unity? Does it start to clip? I’m liking the way it preserves all the content in my guitar signal, but would prefer it without the attenuation.

I should have built it with a variable in R4 to play with the gain…

If you’re interested here is the buffer off/buffer on comparison: http://dl.getdropbox.com/u/870088/sounds/name-that-effect.mp3

That’s over a long cable run (~45′) with the buffer sitting mid-way.

[668] admin Says: 5:35 am, April 16th, 2009

You can make the circuit go closer to unity by selecting a high HFE transistor, but it will never get to gain = 1, nor will any simple transistor buffer whether silicon or germanium.

[742] AndrewCE Says: 1:46 pm, July 9th, 2009

Will this only work with leaky germaniums? cause I prototyped this with 2N3906’s, and i get no sound. Interestingly, though, the amp does respond to me touching the strings and bridge in the usual way. (humming when i’m not touching them, stopping when i do).

Also, i used 10uF polarized caps.
Are “leaky germaniums” necessary or what?

[1990] yot Says: 7:46 am, December 29th, 2010

I’m curious about R5, why it’s 1M, What is the output impedance of this buffer?

[1999] admin Says: 2:13 am, January 2nd, 2011

R5 is the output pulldown resistor and is only really needed if you use a footswitch with the circuit to toggle it on/off, and is included to prevent popping when the switch is stomped.

[2506] vass Says: 1:38 pm, December 15th, 2011

Tnx for this post and work.

I built this bufer. Why this bufer turn sound down 15% then pickup sound in original.

[2609] Anton Says: 11:37 pm, January 22nd, 2012

You must have something wired wrong ‘vass’. I’ve built this and is wired into my Crybaby and it works fine. I should actually say that the wah sounds better with it. I love it. Thanks again, Jack!

[2674] vass Says: 4:23 am, March 6th, 2012

Tnx Anton,

I try to find a problem.

[3659] Sega_SVN Says: 3:54 am, December 9th, 2012

That looks interesting. I’m planing to build a Ge boost because I have some AC128 with Hfe 400 but I would like to keep negative ground. How could I change your plan into boost? Changing R4 with Potentiometer and Resistor would work?


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