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Old 25th March 2007, 06:51 PM   #21
EUVL is offline EUVL  Europe
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William,

The improvements in mid and high are what you should expect.
The loss in bass is due to your loss in open loop gain. To get that back you need to get rid of the source resistors of the diff pair altogether, and increase the drain resistors to 900 ohm to 1k. Your open loop gain would then be comparable to before. Only now you have a lower open loop bandwidth for the second stage. But that may not be a problem once the loop is closed.

As to the bootstrap cap, put a 2200uF cap in and try to see whether it makes any difference.

Don't give up as yet.


Patrick
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Old 25th March 2007, 08:19 PM   #22
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Hi Patrick,

I said the mids and highs were good to very good. Before the change the mids and highs were good to very good too.......

It is always difficult to tell the difference if some part (like the bass) is lacking.

I tried without the source resistors and got 132mV of offset wich is a bit too much for my taste. I will have to search a bit further for some better matched pair with high enough Idss. The 30 pieces Ive got are a lot further apart as Im used to with IRF parts

At the moment the drain resistors are 820R and bias is 4,7V/820 = 5,7mA. If I use a 900-1000R drain resistor bias will be 4,7-5,2mA so I could use a diff pair wit an Idss between 9 and 10mA. Ive got 7 of those so maybe Ill find some that work without source resistors. (2SJ109 would be welcome.......)

Could you explain why the open loop gain of the first stage has an influence on the bass performance? I must admit that I only looked at 1kHz signals and above cause I didnt expect any difficulties here.

As for the cap in the active current source. Going to 2200uF would probably change the time from 1 to 10 seconds before distortion will be setting in. I dont think this behaviour is important for normal listening and I will have a look if my Aleph 5 has the same "problem". It would just be nice to understand.

William
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Old 25th March 2007, 09:55 PM   #23
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Quote:
Originally posted by wuffwaff
...........

check AC gain of output CCS
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Old 25th March 2007, 11:43 PM   #24
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Quote:
Originally posted by Babowana


Babowana, not sure how you got this number

Sorry, I have read "app.-7V" as "app -.7V"


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Old 26th March 2007, 04:21 AM   #25
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> Could you explain why the open loop gain of the first stage has an influence on the bass performance?

It has been a long time since I did this, so I am only recalling from memory.

If I remember correctly, the standard AlephX-100 has about 30-40dB NFB on 8 ohms. This drops proportionally with load impedance. If your TOTAL loop gain is further reduced (since you have changed nothing for the second stage, this can only be a result of the first stage changes), AND you have a very reactive load which drops to say 1 ohm during large amplitude transients, your amplifier will run out of gain very soon.

Anything below 20dB NFB is asking a lot from the JFETs, as you would be putting something like +/- 0.2V at their gates. I think you would be driving one of them over Idss, and the other completely off (and hence deeply into its non-linear region.

Also, your damping factor suffers as the amount of NFB drops, if I am not wrong.

I have relatively efficient speakers (B&W 804s), and I do not need a lot of bass for my taste. So for me it was never an issue. I can recall (this has been years) comparing the Rollins Aleph-X with 4x 044s and 2x 9610s with the 100W AJX and for me it was definitely an improvement in terms of bass.

Compared with the AlephX100 with 9610s, I found highs much cleaner and sweeter. But that is only my observation with my own setup. AND I was (am) using 2SK1529s and NOT IRFP240s.
JFET was 2SJ109, no source degeneration, drain 910R.


Patrick
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Old 26th March 2007, 08:14 AM   #26
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Zen Mod,

ac-current-gain: checked and i.O. (51%)
bias: checked and i.O (6,9A)

Nothing was changed at the active current sources so this is as expected.

Patrick,

dont think my speakers (Thiel CS6) are so difficult to drive at lower frequencies. The lowest impedance (3 Ohms) occurs at higher frequencies.

I will try the 910R / no Rs version when Im back from holiday.
Do you know how much the gain will change from 820R / 8R?
Do the 2sk1529 have a lower input capacitance? Are they still available?

William
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Old 26th March 2007, 02:55 PM   #27
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The gain for the first stage with Toshiba JFETs (J74s or J109s) is roughly :

R drain / (30ohm + R source).

You can then work out the difference between different versions easily. The 30 ohm comes from 1/Yfs at 5mA bias, which is about 33mS.

Yes 2SK1529 is still widely available. And they have much lower capacitances. Datasheet is available on the internet (too large to post).

If you get 100 pieces, they are only slightly more expensive than IRFs (in Germany). But this was 3 years ago, I have to say.


Patrick
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Old 26th March 2007, 05:29 PM   #28
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Hi,

only found some 1529s at 4,57 a piece at Kessler.......

So gain now is 820/40 = 20,5 (26dB)
Gain 910/40 = 22,75 (27dB)
Gain 910/30 = 30,33 (29,6dB)

so I could raise the open loop gain by 4dB, and loose some in the second stage.

Ill just try. BTW the bass loss is not only appearant on high level listening but also at lower levels.

William
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Old 26th March 2007, 06:43 PM   #29
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> only found some 1529s at 4,57 a piece at Kessler.......

http://www.fibra-brandt.com/
List price 3 Euros + VAT. But ask him how much for 100.

Anything less than 100 you will not get good match (from experience); so I suggest you try to find out the cause to the bass loss first, which I admit is a bit of a puzzle to me as well.

Or maybe you should wait until Nelson publishes the Aleph J circuit for some hints.


Patrick
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Old 26th March 2007, 07:06 PM   #30
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Hi Patrick,

did you compare the 1529 to the IRFP240s (Ive got plenty of the latter.....)

One hint concerning the Aleph J could be the use of 2 output fets instead of 3 (6?)

Lets wait until Ive tried the 910R plus no source resistors to see if this helps. If not I can return to the 9610s and see if the dissappeared bass returns.

I will build a pair of Aleph 30s for a friend in the near future. Will do further experiments with the JFets in these.......

William
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