JFETs as switch in 2-channel tube amp - Page 4 - diyAudio
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Old 25th September 2012, 02:33 PM   #31
TheGimp is offline TheGimp  United States
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Not trying to hijack the thread, but wouldn't a simple diode switch work as well?

The standard switch would introduce transients on turn on and turn off, so use RC delay to slow the switches down.
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Old 25th September 2012, 06:22 PM   #32
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Quote:
Originally Posted by Mooly View Post
The gate channel begins to conduct for real and to pass current. That gives a DC shift on the output which you don't want. Try it
True, but I think that's where the reverse-biased diode at the gate comes into play.

Anyway, I'll ask him for more details and get back to you, this is very interesting!
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Old 25th September 2012, 06:35 PM   #33
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Quote:
Originally Posted by muadib25 View Post
True, but I think that's where the reverse-biased diode at the gate comes into play.

Anyway, I'll ask him for more details and get back to you, this is very interesting!
I would have to see it drawn out fully
Your diodes labeled to Vc1 etc (I have assumed) go to the control voltage for the FET. They don't clamp the voltage at all.
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Old 25th September 2012, 06:37 PM   #34
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Quote:
Originally Posted by TheGimp View Post
Not trying to hijack the thread, but wouldn't a simple diode switch work as well?

The standard switch would introduce transients on turn on and turn off, so use RC delay to slow the switches down.
That's scary although diode switching works well for RF.
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Old 25th September 2012, 06:59 PM   #35
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I guess a better description would be abrupt turn on which will translate to a step function of the output to the level of the input signal at turn on. I expect this to behave like a transient to the system.

For instance if a clarinet note was at a peak value of 1V at turn on, the drive to the amp would appear as a 1V step function followed by the ensuing sin wave from that value on.

I don't think it would be very pleasant to hear.
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Old 25th September 2012, 07:28 PM   #36
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Quote:
Originally Posted by Mooly View Post
I would have to see it drawn out fully
Yessir! Here it is. The second one is what you guys propose.
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Old 25th September 2012, 07:41 PM   #37
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This is how this looks to me at first glance.

In the first one I would say D17 leaves the gate "floating". Imagine +15 volts on the cathode of the diode. The FET is ground referenced at both D and S (as in your circuit). So the diode is non conducting and the gate floats and is liable (guaranteed) to just drift to some voltage determined by leakage paths in the components.

If the diode were substituted by a resistor (say 1Meg) and the diode was now fitted from gate to ground then the diode would clamp the gate to 0.6ish volts. A bit to high. Use a germanium diode and the voltage clamps at nearer 150 to 200 mv. Perfect

Thats how I see the first diagram.

As far as I can see your diagram needs to do the same. The FET's are ground referenced and we need to drive the gate to around 0.2 volts for on and around - 8 volts or more for off.

Build one switch and do some measurements and see
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Old 25th September 2012, 07:43 PM   #38
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Quote:
Originally Posted by TheGimp View Post
I guess a better description would be abrupt turn on which will translate to a step function of the output to the level of the input signal at turn on. I expect this to behave like a transient to the system.

For instance if a clarinet note was at a peak value of 1V at turn on, the drive to the amp would appear as a 1V step function followed by the ensuing sin wave from that value on.

I don't think it would be very pleasant to hear.
The on waveform looks surprisingly clean I must say. Much better than I would have thought tbh.

Transients and switching a signal at anything other than zero volts causes abrupt changes in levels heard as clicks and thumps. Thats a problem facing any switch, electronic or mechanical.
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Old 25th September 2012, 08:17 PM   #39
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Zero cross detect would certainly be cleaner.

Here is slow turn on using an RC delay in the switch drive circuit.
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Old 25th September 2012, 08:28 PM   #40
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Quote:
Originally Posted by Mooly View Post
Build one switch and do some measurements and see
I will try and build the second one first, since no drilling or much messing with the board is required.
There are 4 more FETs located at the EQ section, that come from the cathode follower after the preamp which I neglected to show... sorries.

Anyway, I'll try this second topology and let you know.

Thank you again!
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