Weird gate

[Kipper4]

I say weird because I have my doubts as to whether it will work.
Once I've finished messing with my effed up computer I'm having trouble with. I'll Breadboard it.
Meantime pedals are a poor second to getting my IT back up and working.

Here's a schematic

http://www.zimagez.com/zimage/schema79.php

Edit
Does anyone see a good reason why I shouldn't replace the 2x 2.2M bias resistors with a noiseless bias scheme and a 1M from Vbias to IC1a +input pin?


Anyone built it?
See any reason it will work or not?

Cheers fellas

[samhay]

>Does anyone see a good reason why I shouldn't replace the 2x 2.2M bias resistors with a noiseless bias scheme and a 1M from Vbias to IC1a +input pin?

No - that should work fine.


>See any reason it will work or not?

Haven't built it, but it looks like it will work, although I would use a single-supply op-amp rather than a TL072. I would not expect stellar performance.

[Kipper4]

Cheers Sam.
Breadboard project it is then.
Any improvements that might make it Stella?

[deadastronaut]

subbed, out of curiosity...

[Kipper4]

subbed, out of curiosity...

What you saying Rob?

[deadastronaut]

just curious as to the results...whether it just cuts off instantly , or has fair adjustability..

my breadboards are all full at the mo, otherwise i'd try it...

[garcho]

there are some decent Q&D gate designs with OTAs à la R.A. Penfold, just sayin'

[Kipper4]

Thanks Gary that maplins noise gates on my list Too. I bought some chips before Xmas.
Just canvassing for opinions before I bread it. I'll compare to the DoD Fx30 too.

[R.G.]

There is a built-in issue with using MOSFETs for a variable resistor. They're VERY good at this, but the inherent body diode acts like a silicon diode from source to drain. So for any signal up around 0.4V or bigger across the MOSFET, you get the beginnings of conduction in the body diode.

Boss has used a similar circuit, but rearranged to use a JFET instead of a MOSFET for the shunting element and take care of the opposite DC offset of JFETs.

In the schemo, IC1B biased at ground will have the potential to cause ugliness with the TL072 in particular, as this is outside the input common mode range of the chip. It might work OK, might not, or some makers' TO072s may work, others not. If you have issues, put in a dual opamp that has an input range that includes V-.

I would be very tempted to take IC1B's signal input from the output of IC1A through a cap, as this would lessen any input loading.

[samhay]

>Any improvements that might make it Stella[r]?
RG largely beat me to it regarding tweaks/issues.

Another advantage of taking the comparator after the input buffer is that you could give the buffer some gain - you will have to see how well the triggering works, but it might help to have a hotter (or colder) signal.
If it takes too long to recover after triggering - which it might, it should be easy enough to make the recovery adjustable.
It is also trivial to add a 'triggered' LED if you want to see when the gate is opening.

[slacker]

There's something wrong around the output isn't there? Q1 will shunt the output to ground through R10 but it looks like R10 is 22meg so it's not going to do anything useful, or am I looking at it wrong. Also what are R12 and C9 supposed to be doing, I wonder if it's drawn wrong and the pot is supposed to be a volume control.

On the plus side the Russian girl who wants to chat to me looks nice

[samhay]

>There's something wrong around the output isn't there?

It's a little weird, but looks like it will work after a fashion.

I read R10 to be 2M2. In any case, when the gate is closed, the output is LP filtered with R12 pot (0-470k) vs C9 (100n). This will get weird with R12 nearly CCW (low resistance), but otherwise is shunting the audio band to ground.
When the gate opens and the MOSFET conducts, the FET drain || R12 resistance becomes negligible and the corner frequency shifts way above the audio band and you pass signal.

Edit - Rich, you should try making C9 somewhat bigger if you find that it is not gating bass very well - try 1-10u.

[slacker]

Cheers, yeah that makes sense now.

[Kipper4]

Thanks guys this is at the back of a long queue. I'll get around to it

[Kipper4]

Just bread boarded it as per schematic and i cant get it to work yet. Its late and time to hit the hay so i'll have a gander in the morning revisit it and try some of the suggestions youve all made
Night all

[Kipper4]

finally back at the BB
i have to take out c9 for ic1a to even make noise.
now for ic1b

[Kipper4]

its  gating
things that made it better
taking the comparators input from the output of Ic1a through a 100nf cap I may need to increase or decrease this
changing c9 for a 1uf
its gating but the cut off is too fast most of the time other times its on off on off off
getting there
thanks everyone

[Kipper4]

for C9 ive tried 10uf, 1uf, 470nf, 100nf
1uf seems to work the best

pot1 set to 16.1>16.3k

the mosfet shuts off quick depending on where the pot 2 is set Im calling this gate.
i guess pot 1 sets the dc voltage for the IC1b its a very narrow margin with setting this up and with the pot 1 set up at above K ohms setting its dc is 4.3>4.7volts

Is there a way to slow down the mosfet on off time?
maybe changing C7?
maybe a further series diode with D3?
Maybe a differnat D3? Germ, led, another?

At some settings the mosfet is not sure if its on or off. It makes a distorted kinda diode conducting noise.

All trials with no other effects just my MIM Strat.
Lord knows if it will behave with another pedal before it.

As it is I'm thinking last on the pedal board because of the mosfets quick shut off.

BTW input for IC1b is from the output of IC1a through a 100nf series cap.

IIRC RG mentioned something about Boss usng a Jfet instead. Do i have to add any further componants or just replace the mos with a J Fet?
Thanks Guys
If this works out it may be the cheapest low parts working guitar clamp I've ever seen and I'm using a TL072.
No Pinching my Clamp name either.

[Kipper4]

Boss has used a similar circuit, but rearranged to use a JFET instead of a MOSFET for the shunting element and take care of the opposite DC offset of JFETs.

So can I just use a jfet without any further parts.
Thanks

[samhay]

It might work without modification with a P-channel JFET.
Otherwise, you will have to invert the comparator's behaviour.