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Old 24th March 2007, 11:48 AM   #11
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Hi Patrick,

Iīll see if I can get some J174īs.
For now I will try without the cascode as the fets are seeing around 18V and input capacitance will at least be lower than with the 9610īs

I couldnīt get a 10R trimmer but Iīve just put two 10Rīs parallel to the 50R trimmer wich should work as well. I think I need it as I "only" bought 30 2sj74 and the matching isnīt that nice if you look at Idss and Vp.

Will inform if I have new results.

William
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Old 24th March 2007, 02:03 PM   #12
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I will take a closer look at what I have populated on the boards and post a schematic tonight.

I know I used some very high Idss jfets ( u1897 from mouser, and another) -- these I clearly remember worked with higher bias current. I need to doublecheck on the lower Idss j-fets used--I think they worked as well, operated in the pentode region, but I've got to check. I got rid of the flat spots on top of the sine waves by adjusting the gate voltage on the cascode mosfet. You can't do that with the self-biased jfet cascode.

For now, I've got to install some trim to keep the wife happy!
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Old 24th March 2007, 02:31 PM   #13
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Update,

so I removed the cascodes (2SJ103), lowered the source resistors to 8R and plugged the hole thing in.

Results are interesting and a lot better than before:

Bandwidth into 8 and 4 ohms:

2Volts 120kHz / 99kHz
10Volts 120kHz / 95kHz
20Volts 102kHz / --
15Volts -- / 74kHz

This is a lot better than yesterday. I also measured the voltage over the drain resistors wich was 4,7 volts and a bit higher than expected giving 6,9mA bias per fet.

So I changed these from 680 to 820 Ohms to lower the bias to a bit below 6mA.

This changed the bandwidth at 15V / 4 Ohms to 86kHz (from 76kHz)

So for the moment (until I get my J174 and maybe some V spec 2SJ74) I will leave it like this and do some listening.

Still not shure if I shouldnīt raise the McMillan resistors though. Absolute offset starts at around 2V and goes down quite fast.

William
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Old 24th March 2007, 02:53 PM   #14
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> Still not shure if I shouldnīt raise the McMillan resistors though. Absolute offset starts at around 2V and goes down quite fast.

I also use 10k as you. But by all means try 15k or 20k and see whether you hear anything different.

Although V spec 2SJ74s would allow you to lower the drain resistor further, it does not help to keep open loop gain. If you are really into more bandwidth, I would seriously consider putting 2 diff pairs in parallel.

But glad you are on the right track again.

Time for some listening first.


Patrick
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Old 25th March 2007, 02:39 AM   #15
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OK-

Here is a hopefully readable schematic of the cascoded BOSOZ. I made some changes to increase the gain as would be needed for an amp front end. This really increased the power dissipation across the jfets. These are vishay j105's -- that is all I had populated in the boards. ( I have some u1897's on hand, and also some 2n4416's....)

Ignore the scribbled out lines--the diagram was for testing a preamp circuit operating just above unity gain.

JJ
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Old 25th March 2007, 02:53 AM   #16
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Well I checked the schematic -- it is barely readable. I will try to fiqure out a way to post a more readable schematic with what I have on my computer.

Anyway, here is a pix of the input and output. The input is the smaller sinewave, and the major divisions are 1V. The larger sinewave is the output, and the major divisions are 5V. I backed off on the magnitude of the input sinewave until the flat spots showing up on the tops of the output sinewave were just about gone. I think you can see it a little bit if you look closely.

The jfets, vishay j105's are supposed to have an Idss min of 500mA.

Again, this circuit was tweaked for use a preamp circuit that was driven single ended -- I'm sure much better results would be had if it were optimized for an aleph x input. Cascode modulation seemed to work wonders, but it is a bit difficult to implement when using a single ended input.

Any comments would be appreciated.
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Old 25th March 2007, 12:23 PM   #17
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Quote:
Indeed...but aren't you driving the Gate negative?
Grey- Is there a particular problem with driving the gate negative -- actually, now that I think about it, I don't think I entirely understand the question -- the BOSOZ has n channel devices, and the J105's have Vgs off of app. -7 volts. So those gates are negative with respect to the jfet sources. But these are p-channel devices.......I am trying the understand the problem you were seeing.

Wuffwaff-
If I get a chance today, I'll try re-populating one of the boards with 2n4416's -- they are supposed to have an Idss of 10 to 15 ma -- I guess then I will see what the problem is.
Perhaps it just isn't possible to get the dc voltages needed to drive output stage??

JJ
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Old 25th March 2007, 12:35 PM   #18
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Quote:
Originally posted by jupiterjune


. . . , and the J105's have Vgs off of app. -7 volts.

So, your source output is allowed to see the +6.73V?


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Old 25th March 2007, 03:22 PM   #19
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Quote:
So, your source output is allowed to see the +6.73V?
Babowana, not sure how you got this number, I know the pix didn't come out too well so here are the voltages (DC), (a bit tough to read on the dwg):

gates of input jfets 0 (vishay siliconix j105)
source of input jfets 7.43 V
drain of input jfet 31 V, also source of cascode mosfet
gate of the cascode mosfet 35.6 volts (irf 610)
drain of the cascode mosfet 48.6 V

The voltage on gate of the cc mosfet had to be cranked up to get the roughly 12.5 volts output before clipping, but this dumped much of the load on the j-fet.

JJ

I'm going to mess around with this for a bit to see what I can figure out.

Wuffwaff--please let us know what you end up with and how it sounds compared to the stock aleph-X
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Old 25th March 2007, 06:39 PM   #20
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Hi,

changed the other mono this morning and had a bit of a listen this afternoon and tonight.

Iīm afraid this isnīt really a succes in this combination. The mids and highs are good to very good but it is a bit lacking in the low bass (< 50-60Hz).
The punch is gone and sustained bass notes are there but not as loud as before.
This wasnīt quite what I expected.

Iīll leave it like this for a few more days, then go for a 10 day cycling vacation and when I return Iīll listen a bit more and return to the former (standard) setup.

Maybe Iīll try a different bias for the input pair (drain resistors from 390R to 330R) and see what happens..........

I noticed another thing wich must have something to do with the active current sources:

if you suddenly raise the output voltage from 10V to 18V (10kHz, 4 Ohm load) the amp will only keep this output voltage for a short while (maybe one second). After that the level goes down and distortion sets in. I think that this has something to do with the bootstrap capacitor and the resistors that feed it and the active current source.
This only happens at higher frequencies when the amp is operated near itīs current limit.
Lowering R14/31 (hifizen) from 1k2 to 820R makes it worse, raising it to 1k8 makes it better.
All earlier Alephs (2,4,5) had a 4k75 resistor for R14 and a 1k5 for R15. A3, 30 and 60 used a 1k5 for both.
Could it be that at high frequencies the bootstrap capacitor is emptied by the higher current (because of the C.S.Fet capacities)?

more in a few weeks.....

William
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