[sdiy] LM13700/LM13600-based filters

[Harry Bissell]

There are some major issues. One is that if you use
the LM13700, the bias of the buffers is preset and
may limit performance. In the LM13600 the bias is variable with
the Gm input, and might feed through to the output.

In all cases, there will be an offset voltage because of the
voltage drop of the darlington pair. This may not be a problem
(especailly for YOU, JH... in your allpass stages :^)
but if you run a lowpass filter into total cutoff, the output
offset volatge will drift and it will thump when you come out
of cutoff. A fatal flaw if you are going that low (I do) imho...

There is a Philips part (out of production) which does the
buffers differently, but the problems remain.

You can try the buffers, in many apps they will work fine. You
just can't push the limits with them.

ymmv

H^) harry


----- Original Message -----
From: Antti Huovilainen <ajhuovil at cc.hut.fi>
To: Tom Wiltshire <tom at electricdruid.net>
Cc: synth-diy diy <synth-diy at dropmix.xs4all.nl>
Sent: Mon, 26 Apr 2010 09:20:12 -0400 (EDT)
Subject: Re: [sdiy] LM13700/LM13600-based filters

On Mon, 26 Apr 2010, Tom Wiltshire wrote:

> One thing that jumps out at me is that very few designs use the darlington 
> buffers, mostly preferring to use a standard op-amp buffer instead (some 
> examples below)
>
> Can anyone tell me what the problem is with the on-chip buffers? Why not 
> use them if they're there? Are they *that* awful?!

They weren't present in the CA3080 and people seem to blindly copy those 
old designs. For cases where you need very high impedance, the buffers 
aren't that great, but for normal audio filters they should work just 
fine.

LM13700 could be used to build a dual voltage controlled ADSR too. All you 
need is to think of the EG as a filter with switched input and control 
signal and the rest is dead simple.

Antti

"No boom today. Boom tomorrow. There's always a boom tomorrow"
   -- Lt. Cmdr. Ivanova
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-- 
Harry Bissell & Nora Abdullah 4eva