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Old 5th July 2011, 11:20 PM   #1
Minion is offline Minion  Canada
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Default Channel switching options ??

Hi , Im working on a 2 channel guitar amp build and am trying to figure out a good way to do the channel switching .....

I first considered using some sort of fet switching but Ive heard it can alter the sound so now I"m thinking relays ......

First question .....

Are relays suitable for audio channel switching ??

If so will it cause any poping noise when switching ??
If so is there a way to minimize it ??

I was going to use one of the Power amp power rails to power the relays and was wondering if it will cause noise though the power supply when engaging the relays ??

Thanx
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Old 6th July 2011, 11:19 PM   #2
Enzo is offline Enzo  United States
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RElays don't really add noise, but unterminated components that they switch will. Relays work just fine for this. Look through a few Peavey amp schematics for examples. Many companies use relays for channel switching.

There are plenty of amps using JFETs for switching as well. "Alter the sound"? Well if you mean that two identical amps, one switching with JFETs and the other with relays, might have some subtle sonic difference, well maybe, but any difference will be tiny. The on resistance of a JFET is maybe a couple hundred ohms. Put that in series with high impedance signal lines and not much happens. SLM/Crate used JFETs a lot in various switching applications, look at some examples there.

A relay is simply a switch that is activated by a magnet rather than your finger. The signal path you switch is dealt with the same either way. If a cap comes from somewhere carrying a signal, and you switch to or away from that cap, you need to include a high value resistor to ground to complete a charging circuit for that cap. Otherwise it will pop every time you switch it in.
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Old 7th July 2011, 02:07 AM   #3
Minion is offline Minion  Canada
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Hi , Thanx enzo ..... My (tube) preamps have a 1m to ground and no input cap so hopefully it will be ok .....

Cheers
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Old 7th July 2011, 11:27 PM   #4
Enzo is offline Enzo  United States
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I was not talking about the amp input, I refer to the point in the circuit switching takes place. If a coupling cap is wired to the switch/relay without a terminating resistor, every time the switch is thrown, the cap must recharge and a pop results. The amp input circuit will have no effect on a switching circuit between two stages in the middle of the amp.
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Old 7th July 2011, 11:36 PM   #5
Enzo is offline Enzo  United States
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Look for example at the schematic for the Peavey 5150 (6505) or the 5150-2 (6505+), at the effects loop switching relays. Both schematics have the same parts numbers there. Note the preamp signal feeds into the loop relay through C65, and exits the other half of the relay through C11. Without R33 and R85 to keep a charge on C65 and C11, the thing would pop every time the relay switched. The circuit would work without the resistors, but would pop.

A similar trick is done when switching cathode bypass caps in and out.
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Old 8th July 2011, 02:40 AM   #6
Minion is offline Minion  Canada
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I don"t think I"m smart enough to know what you are saying ...... Why would I connect coupling caps to the relay ??
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Old 8th July 2011, 08:52 AM   #7
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Quote:
Originally Posted by Minion View Post
I don"t think I"m smart enough to know what you are saying ...... Why would I connect coupling caps to the relay ??
So you're only switching AC, and not DC (which would cause a 'pop').

Relays are fine for doing it, as are CMOS switches - discrete jfets are a bit 'old hat' now.
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Old 8th July 2011, 10:27 PM   #8
Minion is offline Minion  Canada
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Quote:
Originally Posted by Nigel Goodwin View Post
So you're only switching AC, and not DC (which would cause a 'pop').

Relays are fine for doing it, as are CMOS switches - discrete jfets are a bit 'old hat' now.
Yes just AC , and DC for a couple LED"s to indicate what channel is selected and I"ll also ground the input of the unused channel using a relay .... I"m useing two 5v DTDP relays , I have a unused 6.3vAC tap on one of the transformers that I"ll use to power the relays .....

Thanx guys .....
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