From: Tony Roe
Subject: Req: comments on using a MOSFET instead of JFET for audio compressor
Date: Sat, 18 Jan 2003 15:18:13 +1000
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I need to build many ultra-low cost audio compressor/limiters, so I looked at
various designs from the internet (some commercial, some diy) using a JFET as
the control element as Rds(on) changes with gate voltage.
But I needed better signal-to-noise than the classic designs (blame it on
digital audio), and I didn't want any setup or component selection: with their
huge spread in gate threshold voltage, JFETs were out. A MOSFET looked like it
might work, and in a hex inverter like the 74HC04 I would get 6 independent N
channel MOSFETs, which I could parallel, or even use balanced signals throughout
to reduce even harmonic distortion. I could also use one of the MOSFETs to
automatically bias the control voltage just below the gate threshold voltage,
for consistent performance. I know I need to keep the unused supply pin
somewhere near the average gate control voltage, partially modulate the gate
voltage with the (buffered) signal, and do all the clever side-chain stuff (no
But before I start, I would appreciate the benefits of the experience of others,
so save me wasting a lot of time. So, when using a MOSFET as a variable
Is a MOSFET less / same / more noisy than a JFET?
Can the MOSFET handle less / same / more voltage signal for a given distortion?
Do the same comments also apply to the MOSFETs in a hex inverter?
Do some hex inverters work better than others (4049, 74HC4049)?
Any other tips / traps come to mind?
Tony (remove "_" from email address to reply)