Phono Stage

[Jamesuk1]

Hi Guys,
I have a valve phono stage and wish to use some Cinemag Input Transformers
My Phono stage is way too Noisey in Active mode so i need to use the MM input with the Cinemags.I still wish to use a MM every now and then too so i need to Switch between the two ,mm,mc .What is the best way to switch these unputs for the least signal loss.Whats the best Switch to buy please.
Regards James

[kevinkr]

Probably not what you want to hear, but at this signal levels there are very few switches that would be reliable for any length of time due to the extremely low signal levels.

I recommend you instead manually swap the connections as required unless you switch from one to the other type of cartridge on a regular basis in which case you need to look for a switch that is specifically designed to switch small signals and these can generally be recognized by the fact that their switched current rating is 10mA or less and the specification should state that.

Toggle switches generally do not have sufficient wiping force to keep the contacts in good conduction at these signal levels, but some rotary types from ITT Shallcross or Grayhill will do the job, but are expensive.

[Geek]

I've used CK silver plated rotaries (like Digikey CKC7004-ND ) successfully for line selectors for a while now with great success. Weather they'll live long enough under those low signal levels is anyone's guess.

Cheers!

[DigitalJunkie]

What about mercury wetted relays? You'd want to supply the coils on them with a nice clean DC voltage though,to reduce possible hum pickup.

[Jamesuk1]

Ok So a switch isn't a very good idea.Trouble is there is only one input on the board for mm .Could i conect both the transformers and a separate mm Input to this conection at the same time Via separate Phono inputs.Would this cause a problem if say im using mm would the Transformers be seen in the circuit path.....
...

[kevinkr]

Originally Posted by Jamesuk1

Ok So a switch isn't a very good idea.Trouble is there is only one input on the board for mm .Could i conect both the transformers and a separate mm Input to this conection at the same time Via separate Phono inputs.Would this cause a problem if say im using mm would the Transformers be seen in the circuit path.....
...

No that won't work, the signal generated by one cartridge would attempt to drive the other and vice-versa, levels would be substantially reduced by the loading, and this would probably result in particularly poor performance with the MM cartridge, but probably both.

I would use a reed relay specifically designed for switching extremely low amplitude and current levels whether or not mercury wetted. Note that mercury wetted relays generally have specific required mounting orientations as well. Coil power must be very clean dc, any ripple present in the coil will be both electrostatically and magnetically coupled into the signal path, and at these signal levels it won't take many uV of induced noise to create a problem.

[Eli Duttman]

James,

The high quality reed relays needed to switch at these MINISCULE signal levels are costly and the CLEAN coil power supply needed will be expensive too.

It's quite likely you will do better in acquiring a 2nd phono preamp and connecting it to a set of line level I/Ps.

[Jamesuk1]

Ok thanks for your help guys.I,m not going to fit the Cine-mags inside my phono stage,seems to be too much trouble.I will carry on using it off board.Then it will be esier to swap back to MM.I just wanted to eliminate the unnessesary cables etc...

Thanks again James