# Need help in muting the signal

1. Aug 8, 2014

### yungman

Hi, I am designing a channel switching all tube guitar amp that I have two channels and be able to switch between the two channels. For now, I am using mechanical relays for switching and it gives me a popping sound when I switch channel. So I want to design a muting circuit to mute out the signal to the power amp for a few mS to remove the "pop".

The circuit that drive the power amp are high impedance, it can be just a switch to short the output to ground to mute the signal. The difficult part is the signal from vacuum tube is quite large. it is +/-40V peak to peak. So most of the analog switch cannot handle it.

1) above is what I want to do. I just need to find a device to serve as the switch. So far, I have been looking at solid state relays like this kind: http://www.digikey.com/product-detail/en/
TLP222A-2%28F%29/TLP222A-2F-ND/1823577
. Or http://www.mouser.com/catalog/specsheets/Toshiba-TLP172A.pdf.

Ideally, I would like a photoresistor type that it becomes over 5MΩ in dark and low impedance with light. But I have not seen any one with build in LED. So I have to mechanically couple a light source to the photoresistor. If you know of any device like this, please give me a link.

I have seen muting circuit like drawing shown in 2). I don't understand how this would work. As I said, the problem is the signal is large signal and swing both polarity. Am I missing something that the FET only work in positive voltage?

If you have better suggestions, please let me know.

Thanks

Alan

Last edited: Aug 8, 2014
2. Aug 8, 2014

### Staff: Mentor

Yeah, circuit #2 won't work for AC signals, at least not without several modifications.

Can you open the circuit instead of shorting it to ground? Do you still get a pop?

3. Aug 8, 2014

### Staff: Mentor

Actually, maybe a better idea -- stay with the shorting circuit and use a zero-crossing-synchronized optocoupler to do the shorting. Use a zero-crossing-detector circuit to ensure that when you short out the signal, it is near 0V so that there is no transient into your amp.

4. Aug 8, 2014

### yungman

Thanks, can you give me a link to a device so I can read?

Thanks

5. Aug 8, 2014

### Staff: Mentor

Which one? Optocoupler or Zero-Crossing-Detector circuit? I think both should be described fairly well at wikipedia. Maybe give that a try? I have to bail out for the day.

6. Aug 8, 2014

### yungman

I guess I don't understand what you mean by zero-crossing-synchronized optocoupler.

Thanks

7. Aug 8, 2014

cheers
Dave

8. Aug 8, 2014

### yungman

I have not manage to find on Yahoo, Google or Wikipedia on " zero-crossing-synchronized optocoupler". I have been looking for over an hour.

9. Aug 8, 2014

### davenn

10. Aug 8, 2014

### yungman

That's a opto coupled DIAC!!! I even download this data sheet. But if Y you type in zero-crossing-synchronized optocoupler on Yahoo and it did not even come up with anything!!!

Thanks, it's the name I don't get.

11. Aug 8, 2014

### nsaspook

Why are you getting a 'popping' sound? Are you using DC relays without a diode snubber on the coil?

12. Aug 8, 2014

### davenn

its a triac actually

that's why I never bother with yahoo its useless

D

13. Aug 8, 2014

### jim hardy

14. Aug 8, 2014

### yungman

Ha ha, never in my life I use or design with triac. It's foreign to me. I need to go hit the book tonight.

15. Aug 8, 2014

### yungman

Hi Jim, long time no talk!!!

I have been looking at analog switches, but I need +/-40V, and more important, it's a tube amp. I only have +6V 3A, -60V 5mA unregulated DC voltage available.

Also I have another question, if you look at page 1 of this schematic:http://schems.com/manu/soldano/soldano_50-100_hotrod.pdf
What is the one with a LED and resistor that is labeled S1 and S2 of the second gain stage. The driving signal of the LED is in page 3 of the schematic.

Thanks

16. Aug 8, 2014

### yungman

The reason is because there is a lot of gain after the relay. Any switching cause a tiny glitch and gets amplified. I have snubber diode, I tried bypassing the voltage and all. This is a common problem in this kind of guitar amp with an over drive channel that has a lot of gain.

17. Aug 8, 2014

### nsaspook

Interesting, most of the older gear used Mercury Wetted Relays when contact bounce noise was a problem but it might be a little hard finding low-cost stock today except on E-bay.

Last edited: Aug 8, 2014
18. Aug 9, 2014

### jim hardy

Likewise Yungman - i've missed ya .

Two interstage audio transformers back to back with the switching done between their low sides would alleviate your high voltage trouble.

I'm wondering about a 'softer' switch,
here's one for low level signals.. won't work for you though.
http://electronicdesign.com/analog/eliminate-clicks-when-muting-single-ended-audio
just mulling for now

page 3 tells us that S1 is off when S2 is on...they're complementary

and page 1 i'd guess ....
when foot switch jack over on page 3 conducts,
S1 clamps V2's right hand grid to ground
and S2 interrupts or attenuates signal going from V2's right hand plate to V3's left hand grid
effectively turning off "overdrive"..

Is that plausible ?

old jim

19. Aug 10, 2014

### yungman

I think adding two signal transformers is too complicated. I think I am going for the mute right before the power amp.

I found out the name of the S1 and S2. It' called Vactrol.http://www.doepfer.de/a100_man/Vactrol.htm. I think ultimately, this is what I am looking for. No trigger timing and all, just a resistor switch where when I drive a current in, the resistance goes low. Then when you cut off the current, the resistance becomes very high.

Thanks for the suggestion.

20. Aug 10, 2014

### yungman

Sorry I have not come back on this thread. I want to spend the time studying SCR, Triac and phototriac before I come back and talk. I am looking at this part:http://www.vishay.com/docs/83627/il410.pdf.

I want to confirm from reading the application notes and the data sheet.

1) Both the zero crossing and non zero crossing act like a short circuit ( almost) as long as I supply $I_F\;\geq \;I_{FT}$ where $I_F$ is the LED current, and $I_{FT}$ is the minimum trigger current. So if I apply the current to the LED, I short out the signal.

2) I should not use zero crossing phototriac because there is a delay after the start of the trigger signal until the zero crossing. I should use the non zero crossing as the device turns on as soon as you apply the current to the LED.

3) There are still some voltage develop across the triac, so there is a clipped signal going into the power amp even the triac is on.

I think all in all, the Vactrol should be a better choice as it is just a resistor that becomes low resistance when the LED is on, and high resistance when LED is off.