[sdiy] Slightly OT: High quality VC attenuator

[Sowa Roman]

Don't they sell SSM2018T where "T" stands for trimless version?

How about making attenuator with muxes and resistors?
If mux (DG408 for example) can't satisfy your quality requirements,
you could switch the resistors with reed relays. They make them so
small now...

or better yet - use motorized potentiometer.

Roman

""-----Original Message-----
""From: Henri Kovalainen [mailto:hekovala at kovalainen.fi]
""Sent: Monday, April 08, 2002 3:06 AM
""To: harry
""Cc: synth-diy at dropmix.xs4all.nl
""Subject: Re: [sdiy] Slightly OT: High quality VC attenuator
""
""
""On Sun, 7 Apr 2002, harry wrote:
""> The question is "how much attenuation range do you need"...
""
""> I'm guessing a LOT and that FETs will not have enough range.  I'd
""> think of a VCA using the standard differential pair techniques...
""
""Hmm, as far as I've thought about it, some -70dB would be 
""good. I could
""probably live with -60 dB, too. I'm not planning on using the 
""three-way
""active speakers as headphones. =)
""
""The H11F3's specifications promise something like >300M to 
""<200 Ohm range.
""The other resistor in the divider could be as low as 500 Ohm 
""or so. The
""opamp should drive a 600 Ohm load quite nicely, so there 
""shouldn't be any
""problem there. Besides, the inputs boost the signal +6dB (due 
""to the fact
""that the balanced inputs do this by their very nature and 
""this would still
""leave healthy headroom in the internal signal level) so if I trim the
""coupler to have a minimum resistance of 300 Ohm for example 
""it's possible
""to have a nice little gain in the design. Anyway, if the specs are
""correct, the amount of attenuation shouldn't be a problem.
""
""I'm more worried about the noise level, the actual distortion 
""figures and
""so on. It's a pain to test, too, because to measure it you would
""practically have to build a prototype so good that it already 
""matches the
""final design. Besides I don't even have proper instruments to test it.
""
""If it comes down to standard VCAs or the likes, I'll probably 
""dash to get
""a couple of SSM2018s (140dB gain range, 0.006% typical THD at 1k 
""unity). With
""proper trimming they have impressing performance. How to actually trim
""them is another problem.
""
""| henri kovalainen   | peltokatu 19 b51   | hekovala at kovalainen.fi
""| phone: 0400-683750   90120 oulu finland   hekovala at paju.oulu.fi
""
""| Grow your own dope - plant an economist.
""
""
""
""
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