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Old 15th February 2014, 04:35 PM   #2421
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Quote:
Originally Posted by JCN View Post
Hi,

I have read a lot the last days... whats fixed:
- 3 devices parallel
- 20V rails
- JFet input stage
- DC Servo for common mode DC

Open questions:
- IRFP044 or 240?
- How many parallel 2SJ74? Two? Three? Four?
- Is it worth to get rid of the "McMillan Resistors" at all (found a good solution with only a additional Fet).
- May it increase the bass precision with more open-loop-gain or better stay on the low figures?
- The circuit is critical regarding input impedance and DC output. But I like no DC coupling capacitors. Is a differential-DC-Servo recommended?

Thanks for your experiences

Regards,

J-C
Hi,

I´ve tried increasingly higher values for the McMillans up until 39k. The Gain was significant until about 27k (with one pair of 2SJ74) but not in the bass, more in the highs. Must be somewhere in this thread. I think it is important to match them very closely to keep the common mode input to the diff pair as low as possible.

With 3 output fets it is probably better to use two pairs of SJ74 and get a higher bandwidth. Since my speakers are 100dB/Watt I disconnected 4 output fets (8 left) and just used one pair.

I had a DC servo for a while but took it out again because the solution with a PTY was good enough for me (max 1V of absolut offset from startup, still dependent on the rail (mains) voltage. I did not hear any negative effects with the DC-Servo though.

Hope this helps a bit,

William
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Old 16th February 2014, 03:16 PM   #2422
JCN is offline JCN  Germany
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Hello,

thanks a lot!

Since I can get 20 pcs of 2SJ74, I decided to use 3 in parallel. And to use a DC servo which enables me to eliminate all resistors at the input-CCS and keep the CMRR high. 2 CCS on drain side of the JFets to rise the open-loop-gain (at least for low frequencies) will be tested when its running.

This will be the amp for a friend. For me, I have decided to switch to annother project: A balanced amp like the Aleph-X, but with a 240/9240 output stage as source follower and the complete gain in a complete-symmetric JFet (4 parallel, cascoded K170/J74) input stage, loaded with several CCS to keep enough open-loop-gain. Will try out if it can beat my Aleph-X and my collegues Aleph-JX...

Regards, J-C
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Old 24th February 2014, 05:07 PM   #2423
Eric is offline Eric  United States
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Quote:
Originally Posted by JCN View Post
Open questions:
- IRFP044 or 240?
Waaaaaay back in the beginning of the thread someone reported on the sound difference between the 044 and the 240. The impressions are that the 044 sounds a little "darker" - less treble - than the 240 that was mainly attributed to the higher gate capacitance of the 044 when compared to the 240. The 044's have a high current and temperature rating than the 240's do.

William: I never was able to reclaim my "lost" output power when I moved my front end from 9610's to SJ74s. I've kind of given up on that and I'm paying attention now to ambient temperature differences and how that affects my offset. Sometime over the summer, I'll put a PTY solution in place.

I think I last adjusted offset during the summer, but I want to wait and re-measure things later to be sure. Right now, I get the following:

Jan, Feb: Min outside temperature near -18c

Cold sink temp 12c - amps live in unfinished area of basement

After 2+ hours of operation:
Front sink temp 44-45c
Rear sink temp 51-53c
Offset voltage 1.9v to 2.2v

Eric
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Old 24th February 2014, 06:40 PM   #2424
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Hi Eric,

when measuring absolute offset it is always nice to measure the mains voltage too as this has quite a big influence.

William
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Old 24th February 2014, 06:54 PM   #2425
JCN is offline JCN  Germany
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Hi,

surprisingly the data sheet from 044 shows not much more capacitance than the 240-s. I will stay at the 044-s.

But why has there not been found a relieable solution for the absolute DC-offset during so many years? I think a solution w/o feedback resistors will improve sound quality. Its only needed to control the CCS depending on the DC. Isnt there a proven solution? If my idea will work, and there is an interest, will post it.

Regards, J-C
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Old 24th February 2014, 07:06 PM   #2426
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Hi,

a DC-servo will work fine (0,00V). A PTC will work fine too (<1V) with a lot less parts. If the mains voltage is more or less stable the second solution is OK. If not I would go for a dc-servo.

I did not hear any degradation of the sound quality with my dc-servo which injected the current upstream from the IRF9610 and was heavily filtered.

Sound quality gains by getting rid of the McMillan resistors will depend on the values you are using. If they are low (4k7) gains will be high, if not (33k) they will be very subtle.

I think there will be interest for your idea! Just post it when you´re ready.

William
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Old 10th March 2014, 12:53 PM   #2427
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Gentlemen,

Will Peter Daniel's AJ-X front end work with the Aleph J Rev 3 DIY Store boards (After some modifciations.)? I am still trying to get a more than the 25w/ channel.
Thanks,

Ron
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Old 15th March 2014, 11:25 PM   #2428
Eric is offline Eric  United States
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J-C,

I would also be Interested in your servo solution.

Eric
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Old 12th April 2014, 09:09 AM   #2429
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How about using not-so-low-noise-but-cheaper fets like 2sj103 in the input stage? Will it be an improvement over irf9610?
My Aleph-x has single outputs and ~15V, so a single fet in the input could be enough? Eric's page talks about just replacing the irfs with jfets, what about the ztx550s?
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Old 12th April 2014, 09:34 AM   #2430
Zen Mod is offline Zen Mod  Serbia
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2Sj103BL** is having much lower xcconductance then , say usual , 2SJ74BL

if you have proper triples of 2SJ103 , put them instead of one 2SJ74BL

in that case , just toss them in (two triples) , instead of IRF , leaving same current through LTP


** they must be BL current range

edit : what about ZTX ?
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Last edited by Zen Mod; 12th April 2014 at 09:38 AM.
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