Switching Two Transformers in one pre....

[glazer56]

Hi all,
So, here's the deal. I'm currenly designing and building a rack of 8 or 16 312 style preamps. I bought two different input transformers from Cinemag to see which ones we liked better, and the problem is, we like them both equally as well... While I could build half with one type and half with the other, I'm thinking about having them both in each pre, and being able to switch between the two... Sort of a modern sound/vintage sound sort of thing. I'm also thinking about adding a line in option and adding a summing feature so that we can mix through it, and would want all the same sound available on each channel.

So... I'm trying to decide which is the best way of switching input transformers, manually with a 4 pole switch, or via relay? Also, i would assume it's necessary to switch all inputs and outputs at once, since you wouldn't want any unnecessary loading on either the mic or the opamp. Or am I completely wrong? I did an extensive search and couldn't find and discussions about this, any insights would be greatly appreciated! Many thanks!

 

[Sleeper]

Personally I think relays would be a better solution... moving signal up to front panel mounted switches and back to 2 transformers is a lot of wiring and a lot of wire running around around and you would want high quality grayhill switches at least.
save yourself alot of money and time and maybe a bit of noise and use relays.
MBB (make before break) relays and you won't have to worry about a circuit going open and causing a POP when you switch between transformers. 
Even If you momentarily have both transformers hooked up in the front end and at the output of the circuit I seriously doubt you could damage an API style opamp.  Think about it for a second.  Is there any way to raise the load using an additional transformer.  It seems that no matter what you would be getting the same or a higher impedence ( the second transformer in series or in parallel). Good luck

 

[Benny]

I'm using a combination of sowter and triad input xformer in my 312. Switching is done manually with a lorlin switch and I switch both, primary and secondary windings.
Works fine for more than two years now. But relay switching would be a nice option, too.

 

[glazer56]

Many thanks! Good to hear some opinions.
SG ;D

 

[mad.ax]

Sleeper wrote: "MBB (make before break) relays and you won't have to worry about a circuit going open and causing a POP when you switch between transformers."

I understand how a MBB switch physically works (usually in a slider or rotary configuration), but I am not aware of how a simple MBB relay would work...
Care to share some more info?

All I can imagine for now would be expensive and/or complicated like slow action wet mercury contacts, double spring contacts, servo driven rotary switch, or using two relays (one wich close a contact, then one which open one) and some delay logic circuitery....

Axel

 

[Sleeper]

Hi Axel It's much simpler than that.
You buy MBB relays ;D
I think the contact slides back and forth and TYCO calls them continuous contact relays....
I googled some datasheets.
BUT now that I think about it  this is unnecessary in this application too.
Does it really matter if your input goes open circuit for the microsecond that it takes to switch.
Unless your phantom power is on AND not properly implemented all I can imagine that would happen is a slight burst of open circuit ground noise. 
Are you going to be switching transformers mid-performance?
I didn't think so.

 

[audiox]

If the switching noise is a problem, place 1M to 10M resistors across the switch terminals. That doesn't affect the signal performance but keeps them in the same DC potential.

 

[mad.ax]

Well, curiously, the Tyco website didn't return anything while searching for "continuous contact" and a search for MBB lead me to a page about ALCO switches!

Bit I'm glad I've learned something! MBB relays are made by OMRON or PANASONIC for instance, and sold for cheap...

The principle of operation is quite simple actually, silly me ;D!!
According to a Matsushita patent from 1985:
"In an M.B.B. type relay, a contact spring comprises a pair of flat spring parts integral at the base of the spring and separate at their free ends, each of which cooperates with a fixed contact. An actuating card drives both spring parts in the same direction and has a stepped portion so dimensioned that a first contact formed by one of the spring parts with one of the fixed contacts is closed before the second contact formed by the other spring part with the other fixed contact is opened."

Axel

 

[Sleeper]

et Voila!
I use the Omron's in all the gear I've built and I've never had one fail yet.
Bon Chance