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Old 18th December 2008, 09:11 PM   #561
gl is offline gl  United States
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Hi Dave,

The 15 ohm resistors were used to prevent current hogging. I also ran the amps without them and didn't notice any difference in the sound. I also didn't notice that one 109 pair was heating up more than the other.

Putting a resistors into the circuit at this location reduces the overall gain (not required for the AX) and can also linearize the stage and reduce distortion. Neither of these factors entered into my thinking.

As you can see I didn't get too rigorous about the details. The big deal is the use of JFETs over MOSFETs. That's the magic twanger.

You could use your extra JFETs as cascodes if you want to. If it was me I would save my JFETs for something more worthwhile. Good JFETs are getting scarcer. Good BJTs still seem to be cheap and abundant.

You're absolutely right about not messing with something that ain't broken and sounding great. That's why I haven't done any more messing with my amps.

Have a great holiday Dave.

Graeme
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Old 18th December 2008, 09:25 PM   #562
gl is offline gl  United States
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Hi Tad,

The only JFET I know of that you could directly substitute for the 2SJ109 is a matched pair of 2SJ74's. I used 2SJ109BL's so it would be the 2SJ74BL's that you would want for this circuit. I just bought 100 of them from bdent for my stash.

The 2SJ109 pair is already matched but the match isn't super close. You can get close enough by just and matching the 2SJ74's for Idss. This is covered in a lot of places. Macka built an AX100 amp by using matched 2SJ74BL's and things worked great. Passlabs does this now too I understand. There is a thread here somewhere covering macka's (Ian's) journey.

There are other P-channel JFETs around but nothing currently available comes to mind as a quick and easy sub here.

And a great holiday back at you!

Regards,
Graeme
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Old 19th December 2008, 12:49 AM   #563
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Actually we still have a 2SK109 inventory, but when we run out, we'll
be matching 2SJ74's.
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Old 19th December 2008, 10:45 AM   #564
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I had mine matched by a very nice man in Germany.
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Old 20th December 2008, 06:10 AM   #565
EUVL is offline EUVL  Europe
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My solution :

http://www.diyaudio.com/forums/showt...39#post1690439

Patrick
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Old 1st January 2009, 02:15 PM   #566
daly41k is offline daly41k  United States
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Default aleph1.2 to AX100 jfet mod and initial check out

GL, I had some time over the holidays to exierment with replacing my 9610 mosfet front end with the J74 fets that I got matched from Erno Borbely. I used your schematic 01 as a start point.

The differences in my cirucuit from yours are:

1. I used 10uf input caps vs 4.7uf as this is what I had on hand.
2. I did not use the jfet source resistors as I didn't have the
15 ohm values on hand but I do have some 27ohm that I can
stick in later.
3. I used all of your resistor value changes for the macmillian and
feedback resistors.
4. I repalced your 845ohm R12 with a high quality 2kohm pot so I
I could adjust the gain between the plus/minus amps. Using a
1k square wave I adjusted the resistor until I nulled the two
output signals (add function on the dual trace scope), I had
amp connected to a 5ohm 100 watt resistor.

My first check out revealed the following after about 4 hours of running on the test bench:
1. Relative offset adjusted to 70-80mv but I believe I can get it
lower but it was New Years eve and time to party.
2. The absolute offset increased to 1 volt after warm up which is
about 300-500mv more than my mosfet front end set up.
3. The relative DC at the balanced inputs after the cap
and into the board is the same 1 volt. This is not
surprising as the feedback network is connected to the
100 ohm resistors to ground at the outputs.

Everything else looks the same when I checked the dc voltages against the mosfet version. This amp runs hard all the time, I have to check the source resistors on the output fets yet but I am using the 100k for R14/R33 so my out put is still around 120 to 140 watts.

I am not sure how to get the absolute dc offset lower so as to eliminate the input caps. You are correct that this is a mystery as the mosfet version did not need input coupling caps but was connected in the same way. The only difference is that the feedback voltage divider on the input was changed (10k to 22k and the macmillian resistor was increased to 22k from 10k. Perhaps this is the reason for the higher absolute values.

I want to run it for another day and keep adjusting it to see what I can do. Perhaps lowering the 100ohm resistors may help.

What happens to a speaker if one leg of the amp gets disconnected? Does that put a volt across the drivers or just nothing since there is no return path?

happy new year to all and I am sure that Nelson is drinking some doobie brothers wine issue from BR Cohn. somewhere in the valley. dave
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Old 1st January 2009, 02:31 PM   #567
daly41k is offline daly41k  United States
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Default aleph1.2 to AX100 jfet mod and initial check out

GL, one more detail, I used all jfets and did not use transistors as you showed. I had the fets on hand and didn't want to expeirment with the transistors I had left over from other projects. dave
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Old 1st January 2009, 06:17 PM   #568
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Default Re: aleph1.2 to AX100 jfet mod and initial check out

Quote:
Originally posted by daly41k
What happens to a speaker if one leg of the amp gets disconnected? Does that put a volt across the drivers or just nothing since there is no return path?
No return path, no action.


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Old 2nd January 2009, 08:46 AM   #569
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Hi Nelson & Graeme,

Sorry I have been away from this great thread for so long.

I wanted to ask a few question about current share with Jfet front end of an X Aleph

Have you found that proportions other than 50/50 are preferable?

Its been a while since I read up on this but does 60/40 mean the current source is sharing 60% of the a/c current share?


Ian
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Old 2nd January 2009, 10:43 PM   #570
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Are you talking about the front end JFETs or the output stage?

For differential pair inputs, the convention is constant current
source with 50% of the current to each of the diff pair.

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