P-Channel JFETs in mute circuits

[GainFreak]

Hi,

Most of the amps these days have a mute circuit that doesn't allow relay pops and clicks to be heard. Most of the time P-Channel JFETs are used. According to the schematics for example Mesa /and not only/ is using J175. They are usually in parallel with the sound to ground.
I built a similar mute circuit but instead of J175 I used J174 because I had several of them by hand. I placed mine in parallel with the Master volume /PI In/. Unfortunately I noticed that at higher levels the FET starts to distort and makes the sound flat. At lower levels there's no such problem. I tried several of them with the same result.
So I'm wondering if J174 are the wrong type JFET for the purpose or I messed up something?
Any comments would be appreciated.

[Jazz P Bass]

JFET on resistance

The Off & On characteristics of the two are slightly different. Maybe whats getting you is the on resistance. J174 is 85 ohms & the J175 is 125 ohms.
Check out the data sheet @http://www.fairchildsemi.com/ds/MM/MMBFJ177.pdf
John

[GainFreak]

The FET distorts when open /OFF/. The resistance in this case is MOhms.

[Steve Conner]

There are two possible causes:

1) The positive peaks are greater than the Off bias voltage that you're applying. It needs to be larger than the largest expected peak value, plus whatever the pinch-off voltage is, or the JFET will turn on and clip the peaks.

A JFET's drain and source pins are interchangeable: in a P-channel device the "source" is simply whichever terminal is most positive at any given instant. So this clipping can be thought of as the device acting like a source follower, even if you thought you'd connected the source to ground and the drain to the signal.


2) The negative peaks are greater than the device's BVdss rating and it's avalanching: you need a higher rated JFET.

[GainFreak]

I'm sorry but I'm not very much into theory. What I did is take the mute circuit from Mesa and implement it in an amp. J174 source is connected to ground, drain to Master volume. Even without signal when connecting the J174 in parallel with the Master there's an audible level /only hiss, no signal/ loss. Gate voltage is 12Volts.
The only difference is Mesa's JFET is J175.

[Steve Conner]

Well, another possible difference is that your amp has a higher signal level at the master volume than the Mesa amp you copied the circuit from. With your 12V gate voltage, you're limited to about 12V peak signal, before the FET turns on by itself. That's not a particularly large signal by tube amp standards.

[GainFreak]

Well, another possible difference is that your amp has a higher signal level at the master volume than the Mesa amp you copied the circuit from. With your 12V gate voltage, you're limited to about 12V peak signal, before the FET turns on by itself. That's not a particularly large signal by tube amp standards.

In the 3 channel DR schematic the FET is right after the channel volume where the max signal is more than 20V. My experience is if you feed such a strong signal to the PI especially coming from a high gain preamp your amp will oscillate like crazy - 6-7Volts are enough to overdrive the amp but that's another story.
In other amp like Nomad and Lonestar the mute FET is also "fed" strong signals.