BeePre - Stereo to Mono

[Chris65]

Found an interesting suggestion to add a stereo to mono 'switching' using a line transformer on the input to blend the signals.
The transformers used were vintage 600Ω units 1+1:1+1 or two 1:1 with the secondaries wired in series.

As the BeePre already has mounting holes for transformers for the balanced input option, I was considering trying this using something from Cinemag. They have options for 600Ω transformers or higher impedance units such as the CM-13102.

So, the question is, what would be more suitable, a low or higher (10K) impedance unit?

 

[Paul Birkeland]

If you don't need the balanced input, there isn't a big reason to use a line input transformer to switch from stereo to mono; you can just switch the signal hot of each channel together and make the preamp mono for the cost of a SPST switch.

-PB

 

[Chris65]

Thanks PB. No, I don't need the balanced input, but it is just something I'd like to try.
Yes, I could just short the input signals together, or use the blending resistor method, but some opinions suggest this is the best method to blend the signals.

So, if I can find a suitable transformer at the right price, would a lower or higher impedance be preferable?

 

[Paul Joppa]

600 ohms is very low for an input impedance. Very few tube sources can drive that with success; even 10K is marginal on most cases. Solid-state sources are more likely to be able to handle it, but check the specs. (Reduction and Eros specify a 50K minimum load impedance, but they will work down to 15K with only very slight sonic compromise.)

 

[Chris65]

Thanks PJ. That is what I wondered about, there is no mention of any impedance issues in what I've read.
If the balanced input is fitted to the BeePre using the recommended Cinemags, & an unbalanced cable is used to connect an Eros/Reduction, would that mean a slight compromise due to the lower impedance?

 

[Paul Joppa]

You will lose a bit of gain, and you'd likely want to use a larger output cap with Reduction. Eros already has a larger cap.

In theory, distortion will rise also, but the headroom on both phono preamps is large so the distortion is so small to begin with that I doubt anyone can hear the difference.