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Calvin Buffer for Paradise

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#381
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.
 
#385
kumori said:
Still I get a relatively large DC offset at the output of around 350 mV on one channel and 450 mV on the other. The 2k trimpot only can adjust it within about +-50mV, so I can´t set it to zero.
Click to expand...

This should not be the case if the transistors from the upper half match the transistors from the lower half - in that case both halves should be identical and thus the offset zero..... For a stereo setup you will need 4 matched K170's and 4 matched 4391's, consequently....
 
#389
Hi Guys,
have been on holidays for the last month. tried to catch up by reading all the posts yesterday.
so those are the questions left at my side related to use the CALVIN BUFFER with the PARADISE

1) CALVIN BUFFER:
The resistor R15 is optional, when used as a buffer for Paradise it should NOT
be inserted.
does this mean R15 should be connected by wire, or it should be left unexploited at all?

2) PARADISE BOARD:
One advice for new paradise builders: Don't solder the smd-caps in before you have tested the regulator and the riaa-amp.
WHY?

3) PARADISE BOARD:
PradiseBuilders #2969 About the Paradise PSU using Calvin buffers.
is this Post reffered to R104/R204 on the Paradise Board? or does it mean other 10Ohm Resistors anywhere else?
the BOM says 8R2 für those Resistors R104/204

thank you!
michael
 
#392
AndrewT said:
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.
Click to expand...

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
 
#396
atupi said:
Can anyone answer to my question: wat is the desired current flowing into Q3,Q6 ?

Thanks!
Click to expand...

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.....
 
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