[sdiy] CEM3372 passband gain trick?

[Dave Manley]

If I keep asking innocent questions wiil you draw up the schematic, prototype and test it and send me a pcb???

:o)

-Dave

-------- Original Message --------
 From: Magnus Danielson <magnus at rubidium.dyndns.org>
 Sent: Mon, Aug 13, 2012 01:57 PM
 To: Dave Manley <dlmanley at sonic.net>
 CC: Tom Wiltshire <tom at electricdruid.net>; synth-diy at dropmix.xs4all.nl
 Subject: Re: [sdiy] CEM3372 passband gain trick?

>On 08/13/2012 10:13 PM, Dave Manley wrote:
>> The mini filter is discrete, so adding an opamp is easiest,  although it would interesting (perverse?) to keep it discrete.
>
>Keep your perversities to yourself and your MiniMoog. :)
>
>It's a two-tree transistors away, maybe four.
>
>A diff-pair (like the bottom of your Moog chain), with a resistor at the 
>top of the collectors on both sides, then a current mirror on the common 
>emitter side, and then a resistor from +15V to give the current for the 
>current mirror. The bases is the + and - inputs. There, done it. 
>Counting in 4 NPN transistors and three resistors. Keep the current to 
>around 1 mA to start with, so the current resistor should be 15 kOhm. 
>Start with 6k8 resistors on each arm and see how that goes. The current 
>mirror has the collector and base connected to the 15k resistor while 
>the emittor hits the ground rail, the other transistor has it's emitter 
>on the ground rail, the base to the first transistors base (and 
>collector) and then the collector hit's the transistor pairs common 
>emitters.
>
>This is a febrish head's back of the envelope sketch (without the 
>envelope!), so it hasn't been proven in battle. Using a current mirror 
>will improve performance to "just" using a resistor, for the cost of two 
>transistors. One of them may actually be replaced by a
>
>Cheers,
>Magnus
>