I picked up a Pass Labs BOSOZ preamp for a good price.
He included the schematics and wiring diagrams, so I decided to review them.
I noticed that he has an unusual way of switching the inputs ( balanced & SE).
He has designed it that when the inputs are NOT being used he has the + ( Pin #2 of XLR) & - ( Pin #3 of XLR) shorted to ground via a Form 2C relay.
The NC contact is shorted to ground when that specific input is not being used. This seems dangerous for the CD Player/DAC's Analog stage is feeding into a dead short.
Please take a look at a typical input attached (it has 3 Balanced/SE inputs) and make any comments. There is an unnecessary switch to change between SE & Balanced inputs.... it seems to send the - Input to ground as well.
Any input is appreciated.
Thanks,
He included the schematics and wiring diagrams, so I decided to review them.
I noticed that he has an unusual way of switching the inputs ( balanced & SE).
He has designed it that when the inputs are NOT being used he has the + ( Pin #2 of XLR) & - ( Pin #3 of XLR) shorted to ground via a Form 2C relay.
The NC contact is shorted to ground when that specific input is not being used. This seems dangerous for the CD Player/DAC's Analog stage is feeding into a dead short.
Please take a look at a typical input attached (it has 3 Balanced/SE inputs) and make any comments. There is an unnecessary switch to change between SE & Balanced inputs.... it seems to send the - Input to ground as well.
Any input is appreciated.
Thanks,
Attachments
shorting live to GND - nothing wrong with that
grounding negative input is necessary when feeding with SE signal, for proper SE to BAL conversion
grounding negative input is necessary when feeding with SE signal, for proper SE to BAL conversion
Unless a source, playing into the unused input is not happy with its output shorted. There would indeed be very few such sources, but it is very reasonable to remove the ground connection on the input relays.
Last edited:
The proper way to deal with inputs is to disconnect both ground and live connections of the unused inputs from the preamp. The circuit as shown does something different, unneeded and potentially dangerous.
The reason I brought this up is when I connected to a Theta DS Pro Gen Va DAC (with discrete outputs) one of the balanced outputs (Left Channel +) started smoking... the MPS8099/8599 & 10 ohm bias resistors were vaporized and the transistors opened up. I replaced them and it played fine on the Adcom preamp (SE outputs). So I hooked it up the BOSOZ and it did it again on the same channel but different transistors on Balanced Left Channel Output. But when I hooked it up to the Adcom preamp after the 2nd repair, then it plays for hours and hours without a problem (using SE outputs)
It didn't do it with some other sources previously (CAL CD Player/Audio GD DAC) with the preamp but with the Theta (which is biased heavily into Class A) is probably more vulnerable then some other DAC/CD Player Output stages.
I was going to remove the switches and open end the NC contacts to isolate the signals from ground when that input is not used. It looks like the ground will be OK grounded unless I get a 3PDT contacts and open up all 3 wires ( +/-/G) but it may be unnecessary. I see Nelson just isolates the +/- signal and leaves the ground connected in his Aleph 1.7 design.
Any comments would be appreciated.
It didn't do it with some other sources previously (CAL CD Player/Audio GD DAC) with the preamp but with the Theta (which is biased heavily into Class A) is probably more vulnerable then some other DAC/CD Player Output stages.
I was going to remove the switches and open end the NC contacts to isolate the signals from ground when that input is not used. It looks like the ground will be OK grounded unless I get a 3PDT contacts and open up all 3 wires ( +/-/G) but it may be unnecessary. I see Nelson just isolates the +/- signal and leaves the ground connected in his Aleph 1.7 design.
Any comments would be appreciated.
Last edited:
then no further comments needed - you already know outcome of grounding unused hots
however , it must be said - what's happening with Theta is completely result of their incompetence ; output stage must cope with currents involved driving dead short and that is possible either making it robust enough , or placing current limiting resistor in series with output of stage itself
I found numerous examples in professional equipment where unused inputs are grounded , to prevent even tiniest chance of crosstalk
but, hey , it is logical that real sources are going to be used , constructed by real professionals , not boutique market oriented ones
however , it must be said - what's happening with Theta is completely result of their incompetence ; output stage must cope with currents involved driving dead short and that is possible either making it robust enough , or placing current limiting resistor in series with output of stage itself
I found numerous examples in professional equipment where unused inputs are grounded , to prevent even tiniest chance of crosstalk
but, hey , it is logical that real sources are going to be used , constructed by real professionals , not boutique market oriented ones
- Status
- Not open for further replies.