Help with tube preamp input selector switch

Kennyg2209

Kennyg2209

Well-Known Member
I'm building my first preamp and am a little unsure of the best way to wire the selector switch. There will be a phono stage before the selector switch . I'm going to go with 4 inputs. Should I use a 4 pole switch and switch the signal paths and grounds all separate? Can I get away with a 2 pole and switch the grounds separate, or something in between? Thanks


Kennyg
 
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normally the grounds are not switched.

I've given consideration to doing it with a relay input panel at the back and having the rotary selector only operate the relays. Seems like it would make the internal wiring a whole lot less messy.
 
gadget73 said:
normally the grounds are not switched.

I've given consideration to doing it with a relay input panel at the back and having the rotary selector only operate the relays. Seems like it would make the internal wiring a whole lot less messy.
Click to expand...
You're always the one to help . I've thought and seen the signal paths themselves switched most of the time also. The only threads that I've read talked about switching the grounds. I think that makes it a lot more complicated. Your idea of using relays seems like a good one. You could probably do that with a single pole rotary switch. Use a 2 pole relay and have them right in the signal paths from the input jacks. Do you think that the coils in the relays would induce noise? Thank you



Kennyg
 
gadget73 said:
normally the grounds are not switched.

I've given consideration to doing it with a relay input panel at the back and having the rotary selector only operate the relays. Seems like it would make the internal wiring a whole lot less messy.
Click to expand...
I have a large inventory of NOS protection relays I might be able to make work. They mount on a circuit board, but I could make a small box with them in with the input jacks on. Except for the phono. Got me thinking now.


Kennyg
 
Usually if its got grounds going to the switch, its for a setup that shorts the un-used inputs. Basically instead of the moving finger, you get a contact that looks like a pie with a piece missing. The open goes to the active input, the rest get shorted to ground so there is no signal bleed.

I was thinking mini relays just for space reasons. You can actually buy those boards pre-made for small money on ebay. They show up if you search for input selector relay. Never tried one so no direct experience but I do have a pair of boards that are supposed to be copies of the Marantz 7c circuit, and one of these days I would like to put them together in a case with such a switch arrangement. Not sure if those are set up to ground the un-used inputs, but if the relays have NC and NO contacts, it would be trivial to make them do that.
 
Yes, I understand the shorting part. With NC and NO contacts it would be easy. I'll check that out on EBay.


Kennyg
 
gadget73 said:
normally the grounds are not switched.

I've given consideration to doing it with a relay input panel at the back and having the rotary selector only operate the relays. Seems like it would make the internal wiring a whole lot less messy.
Click to expand...

Relays complicate things somewhat (there's just more stuff to solder and mount), but they do offer other opportunities for signal switching, that might be difficult with just a switch. Things like muting inputs, routing signal (taking preamps into and out of circuits, for cinema bypass or anything like that), and such. With switches, diodes and relays, a lot of pretty intricate things are possible...

Regards,
Gordon.
 
Be aware that relays for use in low-level signal switching duty must have contacts specified for 'dry-circuit' applications, which usually means gold. Without that feature, they will start making poor/intermittent contact after a while.
 
Kennyg2209 said:
I'm building my first preamp and am a little unsure of the best way to wire the selector switch. There will be a phono stage before the selector switch . I'm going to go with 4 inputs. Should I use a 4 pole switch and switch the signal paths and grounds all separate? Can I get away with a 2 pole and switch the grounds separate, or something in between? Thanks


Kennyg
Click to expand...

Reed relays with Arduino control is the best option. Any mechanical switch in open air is invitation for problems in the future.
 
There are a lot of cheap kits available via eBay for switching inputs with relays. I've used them on preamps I've built with success.
 
BuzzK said:
There are a lot of cheap kits available via eBay for switching inputs with relays. I've used them on preamps I've built with success.
Click to expand...
Which did you go with? Balanced or unbalanced? If you go with balanced, You could switch the grounds. Unbalanced only switches the signal. Just wondering if it's worth switching the grounds also. I know the difference between them.
 
Not being a person that owns an unbalanced amp or any unbal sources, I wouldn't have any need for a balanced preamp. I've never had anything that switched grounds either.

For what its worth, I'm pretty sure balanced stuff doesn't switch the grounds either, it switches signal + and signal -, which isn't quite the same thing.
 
gadget73 said:
Not being a person that owns an unbalanced amp or any unbal sources, I wouldn't have any need for a balanced preamp. I've never had anything that switched grounds either.

For what its worth, I'm pretty sure balanced stuff doesn't switch the grounds either, it switches signal + and signal -, which isn't quite the same thing.
Click to expand...
I've just been trying to understand why a couple of the only threads I've found spoke about needing to switch the grounds . I never thought it would be necessary, but I'm always open minded. I ordered one on Ebay with 6 sources possible. Don't know if I need that many, but I may want to. I've seen some using one for a built in frequency generator. I didn't order one with its own power supply either. I don't think it would be good having it so close to the inputs. I'll put the psu with the main psu, shielding it from the preamp section. I like that it has the option to use LED indicators. I'll let you know how it goes. Thanks for your input.


Kennyg
 
Kennyg2209 said:
I've just been trying to understand why a couple of the only threads I've found spoke about needing to switch the grounds . I never thought it would be necessary, but I'm always open minded. I ordered one on Ebay with 6 sources possible. Don't know if I need that many, but I may want to. I've seen some using one for a built in frequency generator. I didn't order one with its own power supply either. I don't think it would be good having it so close to the inputs. I'll put the psu with the main psu, shielding it from the preamp section. I like that it has the option to use LED indicators. I'll let you know how it goes. Thanks for your input.


Kennyg
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I suggest that as research, download and study the schematics of lots of famous tube preamplifiers - Marantz, McIntosh, ARC, Luxman, Sansui, Audio Note, Sonic Frontiers, etc.

Consider using a 4-6 pole, double throw make before break rotary selector switch. Instead of relays, another option is to extend the shaft of the switch such that it goes all the way to the back of the preamp, and mount the switch itself there, adjacent to the input jacks.

I think relays and digital logic will only add to cost - and my experience with repairs of vintage gear says they're not more reliable long term than a simple selector.
 
maxhifi said:
I suggest that as research, download and study the schematics of lots of famous tube preamplifiers - Marantz, McIntosh, ARC, Luxman, Sansui, Audio Note, Sonic Frontiers, etc.

Consider using a 4-6 pole, double throw make before break rotary selector switch. Instead of relays, another option is to extend the shaft of the switch such that it goes all the way to the back of the preamp, and mount the switch itself there, adjacent to the input jacks.

I think relays and digital logic will only add to cost - and my experience with repairs of vintage gear says they're not more reliable long term than a simple selector.
Click to expand...
I understand where you are coming from. I actually did look at some schematics of some older top of the line preamps. I also doing repairs have never had to replace a faulty selector switch, but I'm sure relays will go bad. Just something to try. Using a rotary selector, wouldn't you want a break before make, so that you don't get 2 different signals combined? Thanks for your reply.


Kennyg
 
Kennyg2209 said:
I understand where you are coming from. I actually did look at some schematics of some older top of the line preamps. I also doing repairs have never had to replace a faulty selector switch, but I'm sure relays will go bad. Just something to try. Using a rotary selector, wouldn't you want a break before make, so that you don't get 2 different signals combined? Thanks for your reply.


Kennyg
Click to expand...

After looking at data for selector switches, I see that break before make, or non-shorting, seems to be what people go for. I suggested make before break, to equalize DC and get rid of any possible "popping" sound, but if the high end parts suppliers like DACT suggest to use the non-shorting option, I'd default to what's standard practice. In any case, it's not like you're going to have all the inputs live, and switch between them with the volume up... what you really need is just a way to change inputs.

Here's an overpriced audiophile version of what you should be looking for:
https://www.partsconnexion.com/DACT-76366.html
 
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