[sdiy] LM13700/LM13600-based filters

[cheater cheater]

On Mon, Apr 26, 2010 at 17:10, Harry Bissell <harrybissell at wowway.com> wrote:
> 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...

Is that what you call a low-blow? :-P

D.

> 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
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