Audio Switch Interference

[redart]

Hi All,
I have a switching circuit that I'm using to compare two audio amplifiers. The circuit comes courtesy of Elliott Sound Products.
My problem is that when the relays (K1 & K2) energised / de-energise, there is an annoying click that comes thru the speakers. It's especially bad when passing a 100Hz test signal.
Being an electronics newbie I know very little about curing this type of (RF?) interference. I've tried adding bypass caps between power and ground (0.1uF to 47uF) with little or no effect. Do I need an RF choke?. If so can I make one?. Power is 12V DC.
Any suggestions on how to filter this click out would be much appreciated.

 

[audioguru]

You are switching the signal when its voltage is not at zero, hence the pop.
Quiet switching involves using a circuit that ramps the audio down before switching then back up after switching. Another quiet switch circuit waits for the audio signal to reach a "zero crossing" then switches extremely quickly while the signal voltage is still zero.

There isn't much you can do with slow relays.

 

[justDIY]

where is this switch located, on the inputs to the amps, or on the outputs to the speakers?

i built a simple switch that selects one of four line level outputs (4 sound cards) and passes it to a line level input on an amp, I'm able to switch between them with no pops, in fact I can switch them so fast, they sort of all run together in a weird way ... However, I'm using an electronic switch - an analog multiplexer, not a mechanical relay

if you're interested in building yourself a new switch, try searching for 74HC4052 (or 4051), there are lots of audio projects based around these parts.

 

[redart]

The raison d'etre of the switching circuit is that it allows you to switch between amplifiers without a break in the music, thereby giving you the best chance of detecting any differences between them. As the relays switch in about 5 milliseconds there is no audible break in the music. The only clue that the amplifiers have been switched is the slight click/pop thru the speakers, which I presume is some sort of RF interference being picked up by the speaker cables?. Would an RF choke be any use?

 

[audioguru]

As the relays switch in about 5 milliseconds there is no audible break in the music.

The speakers are connected to nothing for half the wavetime of a 100Hz AC tone. Connected to one amp they might have a positive-going signal but when switched to the other amp then the speakers are completely disconnected then must suddenly be negative. There are probably two clicks for each relay movement: disconnect from the 1st amp then re-connect to the 2nd amp.

As a test, switch the speakers with the relays without a signal. Is there a click?

 

[redart]

Good point. 100Hz = 10ms period. No click when no signal. I'd been wondering why it only 'clicked' when there was a signal passing thru!.
So are you saying there's no way around it using relays?

 

[audioguru]

there's no way around it using relays?

I don't think relays can switch quick enough.

 

[redart]

Guess I'll just have to live with it for the moment. I'll look into JustDIY's suggestion of using a 74HC4052 (or 4051) when I have more time.
Thanks both for the advice.

 

[audioguru]

The Cmos transmission gates switch extremely fast but create a little distortion. Maxim have some newer ones with much less distortion. They switch line-level signals at the inputs of the amps. You can't switch speakers with them.

 

[redart]

Thanks again for the replies. Just thought I'd let you know my switch box is working 'good enough' now, with only the faintest click at the speakers on changeover. Fortunately this cannot be heard during music playback (more noticeable during test signal). A redesign of the circuit layout seems to have helped, as the prototype had a lot of long wires criss-crossing. The 5 millisecond switch time is also inaudible during music program, so the changeover is pretty much seemless. And for the record....Pioneer A400 vs Cyrus Two ?. No difference!. Well at least not to my ears. Would be interested to hear from anyone else who has tried this sort of A-B comparison. Off to find another amp to compare.....