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

[Csaba Zvekan]

Thanks Dave,Thanks Jay,

I have been at my local electronic store today . And the younger  
employes didn't really understood it either.So they got a hold of the  
older retired big boss of the store and he was explaining something  
like David said. That it was a shielded wire from the switch FET   
2N4392 going to the Dual J-Fet IMF3958(2N3958)A and the shield going  
to the other J-Fet IMF3958(2N3958)B and just hangs there loose.I  
immediately asked him why loose and no connection?. For a newbie like  
me that is very confusing. So he explained something about the  
capacitor C8 0.022uF Polycarbonate???(don't exist anymore) 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?

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 ?
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

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

Csaba



On May 28, 2008, at 6:01 PM, Dave Manley wrote:

> Ebay item: 160161196684
>
> The guard isn't so much a plane, as a copper trace that surrounds  
> the indicated pins.  If you look at page 3 of the Arp-Tech PDF you  
> can just barely see it (the quality of the image is poor).  In the  
> circuit, the guard ring surrounds the high-impedance node where the  
> charge is held. If you do any repairs in this area make sure to  
> clean the board of any residue which might form a leakage path and  
> cause the S/H output voltage to droop over time.
>
> -Dave
>
> Jay Schwichtenberg wrote:
>> Csaba,
>> It's a ground plane on the circuit board to isolate the sensitive  
>> sections
>> of the circuit from outside interference. High impedance on the op- 
>> amp input
>> can pick up noise or unwanted signals very easily. Also you want  
>> the signal
>> path between the switch (FET), cap and op-amp input to be as short as
>> possible.
>> Jay S.
>> -----Original Message-----
>> From: synth-diy-bounces at dropmix.xs4all.nl
>> [mailto:synth-diy-bounces at dropmix.xs4all.nl]On Behalf Of Csaba Zvekan
>> Sent: Wednesday, May 28, 2008 4:56 AM
>> To: Synth-diy at dropmix.xs4all.nl
>> Subject: [sdiy] Substitute for IMF-3958 ARP 4015 S/H SubModule
>> Hi everybody,
>> I am looking for the IMF-3958 or 2N3958 N-Channel J-FET for a ARP  
>> 4015
>> S/H Submodule (ARP2600). Is there a good substitute ?
>> Also looking at the schematics from ARP-TECH's site. I don't
>> understand what the "GUARD" means coming from the Source of the  
>> 2N4392
>> to the two Gates of the 2N3958.
>> Thanks
>> Csaba
>
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> Synth-diy at dropmix.xs4all.nl
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