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Old 15th December 2011, 05:55 PM   #1
franke is offline franke  United Kingdom
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Default Rotary source selector for audio

Hi,

I'm building a gainclone amp, and I need a rotary selector to switch between sources. I have three inputs to the amp, and I bought one of these:

Rotary Switches : Rotary Switches : Maplin Electronics

I got the 4-pole 3-way one and that's fine, but will this switch do for audio?

Can you recommend something else I can use for this purpose? This is my first attempt at something like this and I'm not sure if this switch will work or not, I'd like to be sure before I solder everything up.

Thanks,
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Old 15th December 2011, 06:06 PM   #2
Mooly is offline Mooly  United Kingdom
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It's fine to begin with. The downside is the risk of some "capacitive" breakthrough from the inputs you haven't selected when using simple switches. Relays have the same issue too.

That's being perfectionist though... in reality it will be just fine.

You can use the unused pole to add LED indication of selected source. 3 LED's and one resistor... easy
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Old 15th December 2011, 06:14 PM   #3
Pingrs is offline Pingrs  United Kingdom
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And keep the outputs diametrically opposite, to minimise inter-source crosstalk.

Brian.
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Old 15th December 2011, 07:47 PM   #4
franke is offline franke  United Kingdom
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Quote:
Originally Posted by Mooly View Post
It's fine to begin with. The downside is the risk of some "capacitive" breakthrough from the inputs you haven't selected when using simple switches. Relays have the same issue too.

That's being perfectionist though... in reality it will be just fine.

You can use the unused pole to add LED indication of selected source. 3 LED's and one resistor... easy
So, both left and right signal ground should both go into one pole?

Quote:
And keep the outputs diametrically opposite, to minimise inter-source crosstalk.

Brian.
Thanks for that piece of advice, that's the kind of thing I wouldn't know about.
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Old 16th December 2011, 07:24 AM   #5
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by franke View Post
So, both left and right signal ground should both go into one pole?



Thanks for that piece of advice, that's the kind of thing I wouldn't know about.
Not exactly

The signal grounds all have to be common. They don't connect to the switch at all.

With your switch you are switching the "live" signal feed by applying all the inputs of one channel to the outer pins of one pole of the switch. You will have one pin left in the group as its a four pole switch.

The signal feed to the amp is taken from the centre pin applicable to that group.

You then do the same for the other channel using either of the remaining poles of the switch.

This might help, from post #5

Also look at post #7 and imagine the LED's are your three signal sources (just one channel). The feed to the amp would be terminal A on the switch.


A couple quick questions as I begin my LM886
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Old 16th December 2011, 07:46 AM   #6
sandyK is offline sandyK  Australia
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"The signal grounds all have to be common. They don't connect to the switch at all. - Mooley"
Thats not always a good idea these days. If'your various sources use SMPS internally, there can be quite a lot of nasty high voltage AC at low current from the primary side of the SMPS to the secondary side, via the obligatory high voltage capacitor between them.Even plugging/unplugging one of those sources into a preamp and power amp[ where the P.A. is earthed, can give quite nasty little bites. The effect is additive unless you also switch the "earth" side of the inputs.A couple of days ago I measured 160V AC from the "earth" side of an Oppo 981HD's output RCA jacks to the earth side of the preamp's input socket.
This doesn't do wonders for S/N either.
SandyK
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Old 16th December 2011, 08:13 AM   #7
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by sandyK View Post
"The signal grounds all have to be common. They don't connect to the switch at all. - Mooley"
Thats not always a good idea these days. If'your various sources use SMPS internally, there can be quite a lot of nasty high voltage AC at low current from the primary side of the SMPS to the secondary side, via the obligatory high voltage capacitor between them.Even plugging/unplugging one of those sources into a preamp and power amp[ where the P.A. is earthed, can give quite nasty little bites. The effect is additive unless you also switch the "earth" side of the inputs.A couple of days ago I measured 160V AC from the "earth" side of an Oppo 981HD's output RCA jacks to the earth side of the preamp's input socket.
This doesn't do wonders for S/N either.
SandyK
Hi Sandy,

That's why I mentioned in post #2, "it's fine to begin with". Many commercial amps use nothing more than this too.

It's not always easy to gauge someones experience and ability.

Switching signal and ground is an ideal approach but needs care to be done correctly using suitable switches to ensure ground continuity is maintained at all times to avoid massive thumps/bangs/noise as the switch is operated.
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Old 17th December 2011, 01:27 AM   #8
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Quote:
Originally Posted by sandyK View Post
"The signal grounds all have to be common. They don't connect to the switch at all. - Mooley"
Thats not always a good idea these days. If'your various sources use SMPS internally, there can be quite a lot of nasty high voltage AC at low current from the primary side of the SMPS to the secondary side, via the obligatory high voltage capacitor between them.Even plugging/unplugging one of those sources into a preamp and power amp[ where the P.A. is earthed, can give quite nasty little bites. The effect is additive unless you also switch the "earth" side of the inputs.A couple of days ago I measured 160V AC from the "earth" side of an Oppo 981HD's output RCA jacks to the earth side of the preamp's input socket.
This doesn't do wonders for S/N either.
SandyK
That sounds a lot like the phantom voltage that often troubles inexperienced electricians when measuring unterminated power lines. Are you sure that those sockets had a path to earth or each other?
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Old 17th December 2011, 01:44 AM   #9
sandyK is offline sandyK  Australia
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Quote:
Originally Posted by Speedskater View Post
That sounds a lot like the phantom voltage that often troubles inexperienced electricians when measuring unterminated power lines. Are you sure that those sockets had a path to earth or each other?
Kevin
As stated, there is a low value, high voltage capacitor (sometimes 2) between the primary and secondary sides of an SMPS.It has to meet a maximum leakage criteria though.
If you want more information, email me and I will send you a copy of an article detailing the problems that SMPS can cause in DVD-A /CD players which use SMPS.
Alex
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Old 17th December 2011, 07:37 AM   #10
Mooly is offline Mooly  United Kingdom
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This leakage is a well known "problem" in the TV trade. Take a rooftop aerial and with the downlead feeder coax plug in one hand and touching the equipment in the other can result in quite a shock, particularly with wet hands. You can even draw an arc from the coax plug to the aerial socket outer even though the equipment is not earthed. There is probably inductive leakage via the SMPS transformer operating at hf as well as via the small caps Sandy mentions.

That said I still think a simple switching arrangement will do all that the op requires on a gainclone.
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