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

[Csaba Zvekan]

I have found some here: http://www.donberg.de/descript/2/2n_3958.htm


but 11.99EURO is alot of money....


Csaba

On May 28, 2008, at 8:05 PM, Csaba Zvekan wrote:

> 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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>
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music stuff
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