[sdiy] Steiner filter mods

[Fredrik Carlqvist]

I don't understand how the Q4/Q5 connection works. Does it have a name?

Is Q4 used as a emitter-follower and Q5 as an inverting amplifier? Q4
should have "constant" voltage on the collector (15V - Vbe ~ 14.3V?). 

If so, what is Q5 amplifying? And why doesn't the Q5 emitter go directly
to the +15V supply?

Q5's collector has very low impedance to the emitter of Q4 or vice
versa, if the resonace pot is very low. I suppose this would be the
highest resonance setting?

Also, why are the diodes D1-D6 placed so assymetrically? Does it matter
at all if D1 and D6 are left out? Shouldn't the number of diodes be
equal between each access point, i. e. shouldn't D3 and D4 be duplicated
or D1 and D6 removed? (I remember seeing a version with eight diodes
inside the diffamp.)

Can someone help me out?

Thanks,
Fredrik C



-----Original Message-----
From: owner-synth-diy at dropmix.xs4all.nl
[mailto:owner-synth-diy at dropmix.xs4all.nl] On Behalf Of HOSHUYAMA
Sent: den 6 december 2005 16:52
To: fmg
Cc: synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] Steiner filter mods

Hi Fabio,

Important thing is just to keep the current of the differential pair.
Any type of current source should be OK.  Of course, you have to adjust
V/Oct scale.

Actually, R13 is not needed.  I used it just because the PCB from
Motohiko has the holes and I thought it will save the circuit in case of
accident.

If you used an JFET with large Idss for Q2, maybe you should connect the
gate of Q2 to -15v directly to get appropriate current.

Sam HOSHUYAMA

fmg wrote:
> HOSHUYAMA wrote:
> 
>>Hello List,
>>
>>I also built a Steiner-style filter with simple modifications.
>>
>>Schematic:
>>
>>  http://houshu.at.webry.info/200511/img_1.gif
>>
>>Constant current source Q2 reduces common-mode gain of the 
>>differential amplifier to almost zero, resulting in far less CV 
>>feedthrough.
>>
>>Another modification is the output point to the buffer U1.
>>Response of the resonance pot is more like Korg MS20.
>>
>>I also used a diode limiter as Yves Usson.
>>
>>Sam HOSHUYAMA
>>Saitama, JAPAN
> 
> 
> Sam,
> 
> Shouldn't gate of Q2 be directly to -15V?
> I don't understand how Q2 keeps current constant as is. Was a drawing 
> mistake? I'm confused.
> 
> Fabio
> {I want an anatomical zoom
> /Soda Stereo}
> .
> 
>