[sdiy] anti pop mosfets in minibrute

[brianw]

Hilarious. They're labeled "anti plop"

In my experience, this sort of anit-pop feature is designed with depletion mode FETs, which conduct to ground when there is no power. As soon as the device powers up, the Gate turns off the FET, and the signal reaches the output.

I learned about this feature the first time I repaired someone else's home stereo equipment. The cheap solution was to just remove the FET. It had failed such that it was conducting all the time, even when the Gate was positive. I suppose it must have been a p-channel, but I don't remember. I think that I later came back and installed the correct FET, just so the owner would not get a 'pop' on the channel with the failed (and removed) FET.

The Mini Brute schematic shows an enhancement mode FET, though, so the schematic is already "wrong" in terms of what I would expect. Is this designed to only mute the headphone output for a brief moment after the power is turned off, and then the mute disengages once the capacitor is drained?

Brian


On Apr 24, 2024, at 2:30 PM, Mike Bryant wrote:
> My guess would be those are the actually specialist MOSFETs without the body diode.  Rare but they do exist.
> 
> Or it's just a poor design 🙂
> From: Synth-diy <synth-diy-bounces at synth-diy.org> on behalf of Chris McDowell via Synth-diy <synth-diy at synth-diy.org>
> Sent: 24 April 2024 21:14
>  
> <preview.png>
> MiniBrute-10-OUTPUT
> PDF Document · 339 KB
> 
> 
> hello list, I've been looking at minibrute schematics today and I'm puzzled by the anti pop circuitry in the headphone driver. it -seems- like the body diode of the mosfets would conduct when the signal went below 0.6ish volts (something I've run into this using mosfets for dc signal muting).
> 
>  From the vca page, that signal can be up to 2Vpp.  https://hackabrute.yusynth.net/MINIBRUTE/analog-board/schematics/MiniBrute-08-VCA.pdf
> 
> What am I missing here? Is 2Vpp just fine in this case? Now reading the 2n7002 datasheet I see the forward voltage of the source-drain diode is typ 0.88V, 1.5V max. Did I just answer my own question? 😜
> 
> Cheers,
> Chris McDowell