Don's source follower

[Michael Lloyd]

Hi Don:

Nifty circuit! You asked if any has seen it before. I've seen it in my old
(from about 1982) harmon kardon CD301 cassette deck. The audio circuits
(except dolby, of course) used discrete devices instead of the monolithic op
amps.

Actually, it was a really good machine, performance wise. 3 heads, dolby C,
dolby calibration tones as well as bias level adjustment settings tones. I
played around with a number of cassette machines over the years. This was
the only deck I got anything that looked like a square wave when playing
back a recorded square wave. Tapes made with this machine sounded good! I
think they used 2SK170 (N)  and 2SJ74 (P) JFETs.

I ended up tossing the thing some years ago when it developed mechanical
problems that I
1. couldn't find parts for it anymore
2. didn't have time to work on it anymore
3. couldn't find a shop that I felt would fix it properly

But I did salvage the semiconductors! This is a DIY list, after all :-)

Michael
mlloyd1 at enteract.com

I'm curious; what are the ten features. I don't think I could come up with
ten :-)


----- Original Message -----
From: "Don Tillman" <don at till.com>
To: <synth-diy at node12b53.a2000.nl>
Sent: Monday, June 19, 2000 12:37 AM
Subject: Don's source follower


> Hey, electronics folks...
>
> This is a rough sketch of a building block I'm starting to use a lot
> lately; it's a complementary source follower:
>
> http://www.till.com/random/follower.gif
>
> The FETs are hand-matched.  One circuit I have here matches them to
> Id = 1.8 mA at Vgs = -0.4 V.  For more drive you can match to a higher
> current and reduce the values of the source resistors, even to zero.
>
> While it doesn't come close to the performance of an opamp voltage
> follower for control voltage applications, for a high fidelity audio
> path this follower has about ten really interesting features.  (Name
> them!)  Typical applications would be a buffer for guitar pickups, an
> input buffer for audio processors, an OTA output buffer, a follower
> stage for a Sallen-Key active filter, that sort of thing.
>
> Question: Has anybody seen this circuit used before?  I've never seen
> anything like it, but it's so simple I can't believe I invented it.
>
>   -- Don
>
> --
> Don Tillman
> Palo Alto, California, USA
> don at till.com
> http://www.till.com
>