BOSOZ Hum Problem

[hifileslie]

When I finished my Bosoz and connected to a power amp there was identical hum on both channels which did not change when the volume was turned up. I could hear the music from a CD once it was louder than the hum!! I have been trying to find the source of the hum for some time and now need some advice please.

I now have only the amp board(both channels on same board) connected to the PSU ( 2 transformer design giving +57.3 and -56.8. The amp inputs are all grounded as are the -ve ouputs. 2 RCAs are connected to the +ve outputs with individual screened cable.

The hum is there on both channels still!!

The PSU and signal earth for both channels are common and are connected to chassis earth with a varistor.

Where should I start looking to sort this out please?

[Zen Mod]

nice picture of innards worth as many words

in moment of powering off poor creature , does hum gone or is still there for few moments , 'till caps are still charged?

that's plain old test is it hum induced from PS , both from ripple or from xformer field

[hifileslie]

Impossible to tell if the hum continues as switching off the BOSOZ causes a thump in the speakers and the power amp goes into protection mode.

[hifileslie]

I have checked all voltages as per the building instructions on the Pass site and all are correct. My circuit boards are identical to those on the Pass site. Any thoughts please?

[Zen Mod]

pictures please,or better xplanation how you made grounds

[Raoul]

It sounds like you are using this with unbalanced connections. Did you use a wire link to ground the - inputs and - outputs, or did you use a resistor? Also, did you remember to include R18 and R19 in your offboard connections?

[hifileslie]

1. The boards and earthing arrangements are identical to the Pass site. Both channels share earth on amp board-amp board earth linked to PSU board with single wire-both then linked to chassis earth via varistor.

2. I have grounded the inputs and -ve outputs with wire and not resistors.

3. I have zener protection on inputs but not R18 and 19!! My mistake -I wonder why they are not fitted on the main circuit board?

Could the missing R18/19 cause oscillation even with grounded inputs?

[Raoul]

If you examine the schematic included on page four of the construction manual .pdf found on the passdiy site, you will see that R18 should be soldered inline with the +IN signal, and R19 is intended to be used inline with the -IN signal. Since you are operating with an unbalanced signal, you have tied the -IN to GND. You still want the R18 resistors inline with the +IN signal. You will need one 221ohm resistor for each each +IN connection (two total). You actually would not need these resistors (R18) if you had not used the input zener protection. If you send a voltage directly to the zener, parasitic oscillations may occur (pg. 7 of 9, construction manual .pdf).

I am not sure why R18/19 are not included on the board, but they are easy to miss, I think.

The use of a resistor rather than wire when grounding the input (-IN) is to help prevent a ground loop. I forget what value I used offhand; I think it was something nominal like a 221ohm. Maybe someone else has a thought on that one.

[hifileslie]

Many thanks. At the moment there are no RCA or XLRs connected to the inputs and I have grounded the +ve and -ve inputs on both channels and the -ve outputs. RCAs are wired to the +ve outputs for testing purposes.

So to make a difference to the hum I am getting the 221 ohms would have to be connected from +ve and -ve inputs to earth?

[hifileslie]

Well I grounded all the inputs via 200 ohm resistors (nearest I had to 221) and no change to the hum, which is the same on both channels and does not change with increases in volume.

What next?