[sdiy] Re: Source of 2N5459 FETs

[Oren Leavitt]

Hi Fabio,

The way Z5 is  set up, as a differential amp with the 10.7K and 787K 
resistor dividers on both inputs, the input impedance will be 797.7K at 
either input regardless of the op amp used. Load on the filter ladder 
output needs to be kept minimal so that it will perform its best. That 
is what the JFETs or JFET input op amp buffers accomplish.
The capacitors, C6 and C7, block DC offsets from reaching the diff amp.

Oren

fmg wrote:

>Hi Oren and list,
>
>Why not just skip Q1, Q2, R20, R21, C6 and C7 and replace Z5 for a
>LF357 or 411, including the reprograming of R25/R26 and R24/R27 ?
>
>Does the FETs and LM301 participate substantially in the character
>of the sound? I think they influence a bit, but can someone say "this
>is an ARP filter" or "this is not an ARP filter" based upon this mod? [1]
>
>Just my thoughts. What do you (owners of ARP filters) think ?
>
>
>Fabio
>
>[1] (ok, I know in this list that's possible :)
>
>
>Oren Leavitt wrote:
>  
>
>>Hi Steven,
>>
>>Referring to:
>>
>>http://www.arptech.synth.net/pdf/4035.pdf
>>
>>Remove Q1, Q2, R20 and R21 from the circuit.
>>Using a dual JFET input op amp, such as the TL072 (8pin DIP),
>>connect pin 8(V+) to +15Vand pin 4(V-) to -15V
>>connect pins 1(-in1) & 2(out1) together and connect these to C7 +
>>connect pin 3(+in1) to where Q1's gate (G) went - pins 10 & 14 of Z4
>>connect pins 6(-in2) & 7(out2) together and connect these to C6 +
>>connect pin 5(+in2) to where Q2's gate (G) went - pins 7 & 5 of Z4
>>
>>That should do it!
>>
>>Oren
>>
>>Steven Downhill wrote:
>>
>>    
>>
>>>Hi Oren,
>>>
>>>
>>>
>>>      
>>>
>>>>Hi Steven and all,
>>>>
>>>>
>>>>        
>>>>
>>>
>>>      
>>>
>>>>You can probably ditch the JFETs all together and use JFET input op amps
>>>>as non-inverting buffers, too.
>>>>This would have been an expensive luxury back in 1974.
>>>>
>>>>
>>>>        
>>>>
>>>
>>>      
>>>
>>>>HTH,
>>>>Oren
>>>>
>>>>
>>>>        
>>>>
>>>You've lost me with this comment! (my electronics knowledge is limited at
>>>the moment).
>>>How would you go about doing this?.
>>>
>>>Regards
>>>Steven
>>>
>>>-----Original Message-----
>>>From: Oren Leavitt [mailto:oleavitt at ix.netcom.com]
>>>Sent: 19 March 2004 04:33
>>>To: Synth DIY (synth-diy at dropmix.xs4all.nl)
>>>Subject: Re: [sdiy] Re: Source of 2N5459 FETs
>>>
>>>
>>>Hi Steven and all,
>>>
>>>You can probably ditch the JFETs all together and use JFET input op amps
>>>as non-inverting buffers, too.
>>>This would have been an expensive luxury back in 1974.
>>>
>>>HTH,
>>>Oren
>>>
>>>Steven Downhill wrote:
>>>
>>>
>>>
>>>      
>>>
>>--
>>Oren Leavitt
>>oleavitt at ix.netcom.com
>>    
>>
>
>
>  
>

-- 
Oren Leavitt
oleavitt at ix.netcom.com