BOSOZ I/O and Switching Recommendations

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Hey Guys,

I'm building the Balanced Zen preamp and need help or suggestions for switching between balanced input and unbalanced inputs; what switch to use.

I'd like to have 2 balanced and 2 unbalanced inputs. Also, I'd like to have both RCA and XLR outputs: one set of each.

Diagrams or example would be helpful. Switch recommendations would help too.


I used this input switching arrangement on my BZLS. I have had a very minor hum issue with it, but after much trouble shooting I now believe that it is due to my overheating the switch during soldering a couple of connections.

I used a Grayhill switch from Digikey for both input and ouput switching, part number GH7104-ND.

I'll put together a sketch of my output switching / volume control arrangement for you, which has been working perfectly.

My arrangement is a little convoluted, but it removed any need for using jumpers at the XLR jacks when switching between balanced and SE connections.

Cheers, Terry
Metal Man's Lay Out

Yo Folks,

Just my two cents about the lay out of Metal Man's grounding solution :angel:. The minor hum may be the result of the two channels being too far away from each other. That is, the two thermistors may cause a ground loop somewhere in the system. The signal grounds of both channels should connect before the thermistors. So, it should be best to use 1 thermistor between the signal starground and the chassis ground. I experienced a minor humm in my X-BLS also, which was gone when I reached the solution I just described :D.

Hope this helps,


Thanks for adding your thoughts. I had the same idea along the path of trouble shooting and combined my grounds into a single star ground, but in my case it didn't make any improvement. Could be something about my specific layout in the chassis, but right now my leading suspect is the switch and my heavy handed soldering. I'll be testing out the theory this weekend, so I'll let everyone know if I'm right or not.

And here is my output switching / volume control scheme.

Cheers, Terry


  • output-scheme.gif
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Metalman, or anybody else who has figured this out:

Is it necessary to have a switched output (XLR vs single ended) ?

If you are going to use only one type or another at a time (for example, a balanced connection to an Aleph amp vs a single ended connection to a ZenV4) do you need a switch? If you don't use this swiitching arrangement, do you need to ground the negative output of the XLR connector, kinda like on the input XLR's if you didn't use a switch there?

Sorry. I've been puzzling over how to have both XLR balanced and Phono unbalanced inputs and outputs on my BOSOZ for some time, and still apparently don't get it. I guess my thinking is just unbalanced.:xeye:
Balanced and Unbalanced Outputs


If you want both unbalanced and balanced outputs the best way is to follow Nelson's recommendations in the BoSoZ-article. If I remember correctly, Nelson designed the pre-amp to have both balanced (XLR) and unbalanced (RCA) outputs. He used RCAs for each XLR-polarity, resulting in 2 RCAs per channel: XLR pin 1 to RCA-ground of both the RCAs, XLR pin 2 to the positive polarity RCA signal-pin and XLR pin 3 to the negative polarity RCA-signal pin. For single-ended poweramps you would just need to use 1 RCA per channel and because you have both the polarities of the balanced signal available, for each channel you can choose to use the inverting or the non-inverting output.

Hope this makes some sense,

Thank you, Fox. Actuallly, I already have a chassis which has space for one set of XLR inputs, and 4 phono jacks on the input. It also has space for one set of XLR outputs and 2 phono jacks on the output. That's it in terms of space.

I realize this eliminates the possibility of picking inverting or noninverting output when using unbalanced output. but am not concerned with that too much. My real dilemma is how I can use this chassis and whether I have to use switches or shorting jumpers to do so. I have the advantage of a (up to) 6 position, 8 deck switch. A real beauty made by Shallcross in the late 1960's that I bought as new old stock surplus from Fair Radio for $6 a few years back. I am switch-rich here, you might say, but space poor on the chassis.

I appreciate your help. :)
In my case with an output attenuator (2 pole, one L, one R) between the +, and - outputs when I used the single ended outputs I had to ground the un-used output or I got a nice hum. Took me quite a while and many posts to the forum before I could figure out this problem.

I don't know if this will be the case with your output control but something to keep in mind, or atleast not repeat.

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