Building a symmetrical PSU B1 buffer

[chatziva]

I think I misunderstood you then, so do you mean +/-1mV dc offset on the DCB1 output can be achieved without the need to trim supplies, even when DCB1 is fed by a source with high dc offset?

 

[Salas]

You don't feed DC offset to a DC coupled preamp without expecting it replicated on the output. Just its own DC offset is very low without trimming if built with well matched signal JFETs.

 

[chatziva]

Thanks for the clarification Salas.

 

[mrWagner]

Can anyone measure with ohm meter the V + and - side io the B1 ?
I'm about to power it up and just noticed that I measure 38ohm there. I guess it is not a problem because if I flip the ohm meter sides then I measure 36 ohm. So not the same, meaning the 2 2SK170 is doing this. Measuring 2 2sk170 same way, - and + side without B1, then I get 90 ohm, flipping the multimeter gives 70 ohm. I'm just a bit afraid I messed up something.

 

[stajo]

40,0

 

[mrWagner]

40,0

Then it is normal. Thanks.
Hope it works

 

[mrWagner]

Didn't blow up
DC offset drifts slowly to -1,5 and -0,7mV then settles.
Funny that the warmest are the resistors, about 40 deg, the IRFs are around 34.

 

[mrWagner]

Well it seems to be working
Just one little problem.
The switching.
When I switch the input, there is a rather loud crack first time. After repeating the switching then there is no crack. Should I connect all the earthes of the inputs and the outputs?

 

[Salas]

There should be no crack. Does the relay delay a bit to engage? Do you hear a click swiftly but distinctly a bit after power on?

 

[Salas]

For both relays in a Mez that is.

 

[mrWagner]

This is the setup
There is no relay, used actual switches. In a setup with relays, the earth is also switched, or it is common all the time between outputs?

Ahh ok. That can be the problem then. Thanks.

 

[Salas]

The output is grounded to mute for a sec during power on in a pcb Mez or Hypno. The hot signal side is switched in a Mez additionally and then routed through an onboard vol pot. OK yours is bare bones self arranged build so some cracks could be expected. Seems normal. Never switch it off if there is a dc coupled power amp in the chain that hasn't been switched off first. Remember that, the most and longest DC offset from no relay dcb1 will be at power down.

 

[mrWagner]

Yep, I measured that, about 0.4V. My problem was with everything turned on, then switch output to the amp. That didn't sound well. But I'll conenct earth together and see if I still get sound at switching.

 

[Salas]

There is nothing like that in the Mez so I could help out of experience, maybe your switches or the grounding, but you will be able to pinpoint it I hope. Have you used the one meg resistors across the outputs that you can see on the original PCB builds by the way?

 

[stajo]

Are the switches make b4 break or break b4 make?

 

[mrWagner]

Yep, exactly this schematic.
They are just on the other side of the little pcb.

Stajo, I think break b4 make.

 

[stajo]

Are inputs and outputs referenced to ground in one way or the other?

 

[mrWagner]

As soon as they are connected together yes, but if the switch is in the other way, then signal and ground is also separated. Or by ground you mean earth? Nothing is connected to earth.

 

[Salas]

Have a try switching only the hot side as you said.

 

[mrWagner]

Tried to connect the earthes, but no luck.
Thought it helped, because after turning everything on, then actually connecting the lines with the 2 switch, I heard nothing. Listened for a while. Sounds great.
Then before tunrning things off, wanted to disconnect things, so switched the amp side and BAMM. Freakin loud crack. I'll have to power off the amp before switching anything.