Calvin Buffer for Paradise

[atupi]

In the notes of building manual it written: "adjust with R5,R7 bias. If current is too low reduce resistors value ".
What is the targeted current to Q3,Q6 ?

[kumori]

Originally Posted by AndrewT

Kumori,
your jFETs should work OK.
The highish Vp will result in a highish Id for the k170.
Expect Id to be 80% to 90% of Idss (about 7 to 8mA). This is very much higher than the 4mA given earlier in the Thread.
Using that predicted Id value, check your voltage drops across the various resistors.
It may be that you need to change some resistor values to get sensible output offset.

Thank you for your support Andrew.
I tried to get my head around the problem, but unfortunately it seems I lack the knowledge to do this on my own. Which resistors should i target and change to reduce the offset?

But, there is also good news, I made another buffer with the transistors from the group buy, they work right away with 8 mV offset and can be easyly trimmed to zero offset. Thank you for the matching work.

I wanted to ask how people power their buffers when it is not used with the paradise PSU. First I tried a TPA placid shunt regulator which worked, but with a loud hum that I could not get rid of. I tried different decoupling caps on the psu rails (220 uf elko, 10 uf elko, no cap) but the hum stayed. Then I used a LM317/37 regulator and it worked flawlessly with out hum.

Is there an obvious reason, why the Placid shunt results in a strong hum in this application?
I would probably try the buffer with a Salas regulator Did anyone try this already and can comment on how to decouple this properly? I thought about maybe a small elko or a small film cap with serial R? Any help appreciated.

By the way, the sound is excellent

Regards
Florian

[CFT]

Anyone got a spare stereo set (or two) of pcb for sale?

[atupi]

Originally Posted by atupi

In the notes of building manual it written: "adjust with R5,R7 bias. If current is too low reduce resistors value ".
What is the targeted current to Q3,Q6 ?

Can anyone answer to my question: wat is the desired current flowing into Q3,Q6 ?

Thanks!

[Joschl]

Hi atupi,
the current through the complete circuit is approximately 30 mA. Let's say 5 mA for the JFETs. So you need about 25 mA for the BJTs.

[hesener]

Originally Posted by atupi

Can anyone answer to my question: wat is the desired current flowing into Q3,Q6 ?

Thanks!

Hi, the comments from Joschl are correct. R5 and R7 should be the values from the schematic. A few posts back Calvin explained the circuit in great detail, and there is a fine balance between R5, R6, and R1+R2 (and the same for the corresponding resistors in the other half). These resistors set the bias current, but also the amount of local feedback and thus the distortion behaviour, in conjunction with the characteristics of the selected transistors.

In short, don't change too much - if you want changes, you may want to simulate the circuit first, then try it out and measure the distortion, and see if you can get to the same magnitude Calvin did. Hope that helps.....

[sq225917]

Are there any boards left for these?

[Jim50hertz]

You can have mine, unopened, unused pm me

[flashgo]

I'm looking for a pair of boards as well. Thanks!

[buzzforb]

I have 4 pairs of what was the first batch