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Old 27th November 2008, 09:24 PM   #1
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Default Methods of Input Selection?

Hi,

First year electronics engineering student here.

First post on this forum, but I've spent a lot of time the past two weeks reading tons of information from the forums while trying to construct ideas for a tube amplifier project.

I'm trying to figure out the best sounding, simplest way to switch between my 4 audio inputs that I want to have. I know about rotary switches, but would I need a 3 pole 4 throw switch or a 4P4T? I've never actually used a rotary switch, do they noticeably stop when you make connection or is it like a potentiometer (ie no way to tell exactly where it is.) Are there other methods of input selection like relays? I'd like to devise a method that only needs one knob to control.

Some links to projects that outline their input selection and other resources would be appreciated.

I hope to learn a lot from this forum, thank you for any help provided.

-Matt
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Old 2nd December 2008, 11:51 AM   #2
Mooly is offline Mooly  United Kingdom
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Normal rotary switches have a definite click action. You can get "break before make" and "make before break" types. You are probably looking at a 4 way 3 pole switch for what you want. Use the spare contact to drive a LED for indicating the selected input.
Over the years I have gone over to solid state switching, IMO it's capable of better results, you can put the switch on the PCB -- no wires -- and it's possible to add remote control.
My favorite is JFET's in a series/shunt arrangement.
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Old 2nd December 2008, 02:18 PM   #3
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Thanks for the reply.

But wouldn't a 3-pole switch be just enough to switch Right, Left and Ground? How would I attach an LED indicator to it if I need them all for audio signals?
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Old 2nd December 2008, 02:29 PM   #4
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Quote:
Originally posted by Mooly

Over the years I have gone over to solid state switching, IMO it's capable of better results,

Better results in every way but soundwise


The majority of midfi manufacturers use cheap relays. The better ones use expensive relays and the best - mechanical switches with solid siver contacts. Some pretty expensive preamps use solid state switches but this is only to maximise profit and to make remote control easy.

Why would anyone need an LED if the selection of inputs is mechanical? Just to add some extra wide band noise?
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Old 2nd December 2008, 02:48 PM   #5
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Quote:
Originally posted by mattthegamer463
But wouldn't a 3-pole switch be just enough to switch Right, Left and Ground? How would I attach an LED indicator to it if I need them all for audio signals?
If you want to switch grounds as well, you'll need 2 deck, 2 pole, 4 position rotary switch. Unless you are using a knob that indicates position, forget about the LED.
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Old 2nd December 2008, 04:40 PM   #6
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Without wanting to get involved in yet another debate on the whys and wherefores I will just say this
Over the years I have come to realise that what makes an amp - and other components "special" is the whole design and implementation.
If the J-Fet solution sounded inferior I wouldn't have gone down that route. I have used relays in the past. They were OK but the signal isolation left ( in absolute terms ) a little to be desired. The same goes for mechanical switching which also has problems with contact resistance over the years. The J-Fet can be placed where ever it is needed on the PCB. Correct implementation is vital however, which really means arranging the circuit so that no voltage appears across the device when it is on.
LED's ? If you can really say that an LED causes un-toward effects then there is something pretty seriously wrong somewhere.

But none of this is helping Matt.
The simplest way is the switch. Signal path only. All grounds common.

This is the Jfet switching I used
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Old 2nd December 2008, 06:07 PM   #7
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There are a zillion ways of switching things, the majority of which are audibly indistinguishable given reasonable care with regard to crosstalk.

More important are probably things like price, reliability, form factor, convenience, user interface and style.

There's a tendency amongst hi-fi enthusists to want to employ massive, primitive devices preferably made from precious metals on roughly the same basis that 'I know it's not unlucky to walk under a ladder, but why take the risk?'

Anybody who's ever twisted 2 wires together knows there's not a lot of magic in a switch.

w
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Old 2nd December 2008, 07:09 PM   #8
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Quote:
Originally posted by wakibaki
Anybody who's ever twisted 2 wires together knows there's not a lot of magic in a switch.
I have my monocrystalline, oxygen free, 110% pure copper wires silver-gold-platinum plated before I twist them together using a proprietary Fibonacci twist ratio. I cryo treat them after twisting as to not upset the balance of the universe.
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