Mini Aleph J build using J174's - how to tweak the VAS - diyAudio
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Old 10th September 2013, 05:02 AM   #1
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Join Date: Sep 2010
Location: Perth, Western Australia
Default Mini Aleph J build using J174's - how to tweak the VAS?

Hi there all,
I've been doing some playing around with Jfets to substitute in for the sj74's in an Aleph-J and after doing some investigating Mystery 'sj74'? jfets I stumbled across a local supplier with a few J174's for sale, which I bought.

I'd like to build me a Mini-Aleph-J using these, but having done a fair bit of reading and simming I'd like to clarify my understanding of what's needing to be changed to get this functioning. I'll reference the attached pic. Rail voltage is expected to be approx 15V.

1) Adjustment of R8 can be used to alter the DC offset, and does so by altering the current through the LTP. My question here is 'whats the best way to set this given the J174 has a much higher Idss than the SJ74'? Do I just set this to a value to zero the offset or do I mess around with R7 (3) as well. I assume this is moving the operating point along the Jfet curve, but I dont actually understand what I'm doing here.

2) J174's - quick test on two suggests an Idss of 30mA-40mA and a Vp around 6V. As i understand it these are high emough Gm parts, but obviously the curve is very different to the original sj74's

3) This value also alters current through the LTP and offset, and I assume the voltage swing to the mosfet gate to some degree, as well as the bias of the mosfet q8. Whats the best setting here? what are we trying to achieve by altering it? Do I just set it to a get a particular bais current for Q8 and then work the R8 value to zero offset?

4) I've removed the second pair of output devices as per other Mini-A's, but would there be a reason to keep them? I'll be using doing a dual mono build with 24000uf per rail and a 200VA transformer per channel, so I assume I'd be able to supply enough current to bias up a pair, and I'm thinking of using a fan forced tunnel heatsink, so heat should be manageable. I guess the questions here are can I use a pair of output devices (ie is there a point/will it help for a low impedance loudspeaker?) or do I stick to one output and spare myself parts and heat?

5)I seem to see that mini-a's only use two resistors here, whereas four are used in the original schematic. I haven't as yet found an explanation as to what these do in the circuit, nor do I understand.

6) Just making it clear that I'll have 15V rails not 24.

Phew - if anyone held on to the end here and can explain it to me I'd appreciate it!
Attached Images
File Type: png Mini AlephJ J174.png (83.1 KB, 167 views)

Last edited by aspringv; 11th September 2013 at 08:14 AM.
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