ARC passive IV ciruit question

[johnmarkp]

I was looking at some high res pictures of the ARC DAC8. They use this passive circuit for IV then it just goes into the Ref3/5 preamp circuit. I have all the component values except R7 and R8. The color bands suggest that the value is 475 something. 47.5R or 4.75R etc. It looks like it comes from a -15v supply that is used for the CCS for the preamp. Any idea why it is there and what the proper value would be.

[Zero Cool]

I'm not sure what you are looking at but an ARC Dac8 does not have ANY tubes!

[Ken Newton]

A wild guess would be that they might be using it to grid-bias the tube. As in, directly grounding the tube cathode and using a negative bias supply to the grid. If you can determine that the 6H30 cathode is tied directly to ground, then that is probably the explanation. If so, then the correct value would be whichever resistance correctly biases the 6H30 via the -15V voltage-divider formed by that unknown resistor and the 10k resistor connected to the DAC output.

[johnmarkp]

Oops, I meant the CD8. The Cathode is not grounded. It is supplied by the CCS.

[SoNic_real_one]

Another example of fine DIY. Putting negative current from -15V supply in the DAC output... nice. Whatever DAC is that, I bet it's internal curent sink is not happy.
Fobia of capacitors makes "monsters".

[Ken Newton]

Originally Posted by SoNic_real_one

Another example of fine DIY. Putting negative current from -15V supply in the DAC output... nice. Whatever DAC is that, I bet it's internal curent sink is not happy.
Fobia of capacitors makes "monsters".

Is not a DIY circuit. He said it was from one of ARC's digital products. In addition, the top T.I. DACs (the PCM179X series) feature a quiescent D.C. output current of up to -6.2mA, which could be sunk into ground, or a virtual ground, or a negative supply through the right resistance. Perhaps, this ARC design is using one of these T.I. DACs.

You're hardly even thinking anymore, Sonic, you're becoming just plain reactionary.

[tvi]

Perhaps, this ARC design is using one of these T.I. DACs.

See pic

[qusp]

i hope there is decoupling under the board, if not thats got to be one of the crappest dac layouts i've ever seen! what good are those caps all the way over there?

[Ken Newton]

Originally Posted by johnmarkp

Oops, I meant the CD8. The Cathode is not grounded. It is supplied by the CCS.

Okay, that makes sense. This 6H30 is undoubtedly configured as a differential pair with a CCS tail. Which means we can't determine what the grid-cathod bias voltage is from the too little information we presently have. Which further means that I don't see a way to be sure what the multiplier value is for that unknown 475 resistor.

Since you are hoping to clone this ARC circuit from so little information you are free to provide some of your own design parameters. Such as, arbitrarily setting the 6H30 cathodes near ground level, which could mean a grid-cathode bias of about -10 volts, depending on the operating conditions you want for each triode section. That unknown 475 resistor then would become 4.75k.

[johnmarkp]

Is not a DIY circuit. He said it was from one of ARC's digital products. In addition, the top T.I. DACs (the PCM179X series) feature a quiescent D.C. output current of up to -6.2mA, which could be sunk into ground, or a virtual ground, or a negative supply through the right resistance. Perhaps, this ARC design is using one of these T.I. DACs.

Could I just replace the resistor with a pot and then adjust it until the DC sunk into the supply? At what points would this be measured, signal to ground or between the + and - legs of the balanced signal? Would there be any risk to the tube section or DAC during the adjustment?