[2] PCB for MAT-04 VCA

[MAILER-DAEMON]

On Mon, 24 Feb 97 15:56:58 P gstopp at fibermux.com       wrote: 
>
>
>     Good job Chris.
>     
>     Well in the time between my last response and now I've tried a few 
>     more things. First of all it looks like it's best to keep the input 
>     signal at 4V p-p or lower, because of what appears to be a long-term 
>     minor DC drift in the input electrolytic cap. Second, on a whim I 
>     swapped out the MAT-01 matched transistor pairs and put in four 
>     randomly-chosen 2N2222A transistors instead. No change in behaviour! 
>     Absolutely identical performance. So - no expensive parts at all, just 
>     six cheapo transistors and two TL082's. You can keep the layout the 
>     same, Chris, as far as I can tell you can still stick four NPN's in a 
>     14-pin DIP footprint if you don't want to buy MAT-04's.
>     
>     Maybe I should point a heat gun at it next, and see if anything drifts 
>     over temperature?
>     

Sounds good, but there are plenty cheaper trannies than the 2N2222, 
plus the fourteen pin-dil layout is sensible, I get the feeling the 
circuit will be OK with a lot of things, but they reckon distortion 
goes up without tranny matching, so for audiophiles the MAT-04 will be 
good. If I can use standard TL074's then I can get a pair of these 
onto a Eurocard at a spacing that is easy for the home-etcher too. 
I'll switch my plans of doing combined VCF/VCA modules to doing a quad 
audio VCA and bolt something else onto the VCF spare space. One thing 
I want to do is to put something approximating to inserts in the 
resonance loops of the filters, I reckon that could be a good idea, 
but I guess it means checking the levels in the feedback loop, lifting 
them to +/-10V and then dropping them back again after the insert. I 
reckon it's actually easier to do that than put in any of the VC 
resonance circuits I've seen, then you could always just use a VCA for 
VC Res, but what would it sound like if the resonance loop was 
chorused, or delayed, or flanged or ring-modulated.......
Just a thought,
chrisc