[sdiy] Substitute for IMF-3958 ARP 4015 S/H SubModule

[Roy J. Tellason]

On Wednesday 28 May 2008 14:05, Csaba Zvekan wrote:
> So he explained something about the capacitor C8 0.022uF Polycarbonate???
> (don't exist anymore)  

Yeah they do...

I even have some,  though I'm not sure if they're that value or not.

> that holds the charge while that loose wire is holding a steady state of
> blablabla impedance and ratio??? I honestly did not know what he was 
> talking about. But then he analyzed the circuit further more and was
> suggesting to use a regular J-FET OP-AMP like the TL071 and hook up
> the Q5A line to the Non-Inverting Input and the Q5B to the Inverting
> Input. He also sold me some 2N5457 J-FETs to replicate the circuit and
> experiment with those and the LM301A. But the the problem there is
> matching the FET's .I know how to match NPN Transistors very good.
>
> http://www.csaba.ch/index.php?option=com_content&task=view&id=14&Itemid=30
>
> But N-Channel FET'S ? Hmmm...???.I have no idea?

I suspect that it's not so much matching as it is thermal tracking that has 
them using a dual FET in that circuit.  You could try it with two separate 
ones attached together (like they do with the expo pairs in the VCOs and 
such) and see if that works out okay.  The feedback from the op amp is going 
to pretty much self-correct any imbalance anyhow.

> So this is what I have so far for replicating/cloning the ARP 4015 S/H
> SubModule:
>
> LM301A 		LM301A or LM351 I have been told that the LM351 work just as
> good so I replaced all the OP-AMPs with the LM351I am not sure about
> the compensation caps though ? Do they need to be eliminated ?

Look at the datasheet for the part you're using and see what's connected to 
those pins.

> 2N5172               NPN  BC547B   or BC182   which ones are better or
> does it matter ?
> 2N6076               PNP  BC178B  is that OK ?
> 2N4392               J-FET Switch I have found the originals 2N4392

See my parts pages for datasheets on at least some of these,  probably many of 
them (I can't say offhand without looking).

> also I was gonna ask if the 20pF and 30pF is critical in value since I
> have either 18/22pF or 33/27pF?

That's not that critical either.

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