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-   -   JFETs as switch in 2-channel tube amp (http://www.diyaudio.com/forums/instruments-amps/220199-jfets-switch-2-channel-tube-amp.html)

muadib25 22nd September 2012 09:56 PM

JFETs as switch in 2-channel tube amp
 
Hello!

I built a 2-channel tube amp using 2N5638 JFETs as switches, first time using them. The switches turn on and off seamlessly but I have distortion and attenuation problems, I think because the signal swings ~1-2 Volts, plus or minus...

Here is the way I use them:
http://imageshack.us/a/img826/920/jfetswitch.png

How can I correct this?

Thanks! :)

SemperFi 22nd September 2012 10:05 PM

The 'correction' will be not to use SS devices as 'mute switches'. Their non-linear capacitances make for horrid sound. No BS, use a relay if you must mute the output.

(One of the best and most audible mods I've ever done to commercial gear was to remove the muting transistors on the output of a CDP. Better to have a little 'thump' than have these muting devices.)

muadib25 22nd September 2012 10:42 PM

Relays are my last option, if this doesn't work in the end... Just for the sake of it, do you think a small capacitor from Gate to Source would help couple the AC ripple and keep the Gate-Source voltage at 0V?

Wavebourn 22nd September 2012 11:42 PM

You forgot a couple of resistors. You need DC path through resistor from gate to source to discharge it when the switch is "on". Also, you need DC path to ground from it's source. But now you have a problem with some DC on output. To solve this problem you need more than one JFET for the switch, like "T" of 3 JFETS, couple of horizontal and one vertical controlled by inverted levels.

muadib25 23rd September 2012 12:07 AM

(sigh)... And to think that I have 8 FETs on the system...

By the way, I just came across something called 'J-Fet Zero Signal Switch', which is one series and one shunt FET at each node.

I guess I have opened Pandora's box here... or maybe I'll just use Relays.

DUG 23rd September 2012 12:20 AM

Yes,

47K from input to gnd (or some DC path)

47K from output to gnd (or some DC path)

1M from gate to control V

control swings -13V to +13V

But Wavebourn is correct, a "T" configuration would insure a greater "off" state

mlloyd1 23rd September 2012 12:23 AM

it's possible to successfully use JFETs in this application.
do a search for some mcintosh preamp schematics.

mlloyd1

muadib25 23rd September 2012 12:37 AM

Quote:

Originally Posted by DUG (Post 3175282)

control swings -13V to +13V

I guess I forgot to say that there are 2 groups of FETs. When Group1 has -13VDC, Group2 has 0VDC. As far as I can tell, the Control voltages are VDC, there is no swing although I don't have an oscillator to verify this.

I am probably wrong in this, but wouldn't a small capacitor couple the changes of the signal voltage, so that Vgs is always 0V when there is no negative voltage to turn it off?

Just a noob thought...

DUG 23rd September 2012 01:01 AM

Sorry I was unclear

-13V for off state and +13 for on state

Then a few volts of signal will not create a -Vgs condition.

muadib25 23rd September 2012 12:00 PM

Here is the circuit that provides DC to the gates.

http://imageshack.us/a/img600/9792/gatesupply.png

This switching works perfectly and there is no voltage swing. The wanted here is to offset the negative side of the signal to the gate voltage for every JFET.

A friend suggested the placement of another diode like this:

http://imageshack.us/a/img6/17/jfetoffset2.png

I will choose to try this first due to available room issues on the eyelet board. Otherwise I'll start experimenting with the suggestions you guys provided here.

I'll post feedback when I'm done! :D


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