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Old 8th January 2006, 01:53 PM   #1
Skorpio is offline Skorpio  Denmark
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Default bi-Aleph

Have seen the interest fro the Aleph J, with J-FET input stage instead of the normal MOS-FET. Giving 15dB less feedback with same objective performance.

How about a bipolar input stage with 3 pcs. BD140?

Any guess om objective perforamnce?
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Old 8th January 2006, 08:58 PM   #2
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That's funny, all my Alephs are heterosexual...
...ahem...
The idea of using bipolars in the front end of an Aleph comes up about every six months or so. You'll have a couple of things to take into account. One is that the average bipolar has a lot more gain than either MOSFETs or JFETs. You'll need to decide what to do with that gain. Emitter resistors to degenerate the devices are one option. Another is to burn it off with feedback. If you choose that route, the numbers will look better, but it's open to question as to whether it will sound better. If you're a meter-reader, you'll probably think it's an improvement. If you're a listener, you might not feel that it's all that good.
You'll need to balance things very carefully as far as the biasing of the output stage. Shoot for about 4V above the negative rail in order to get the output to crank up properly.
Good luck.

Grey
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Old 9th January 2006, 02:33 PM   #3
Skorpio is offline Skorpio  Denmark
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I was thinking the emitterdegeneration way and going for the 15dB lower feedback as on the J-type.

If this idea comes that often, funny noone built it yet....my guess is that it will beat the power MOS input stage serverly....
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Old 9th January 2006, 03:28 PM   #4
Skorpio is offline Skorpio  Denmark
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Quote:
Originally posted by GRollins

One is that the average bipolar has a lot more gain than either MOSFETs or JFETs. You'll need to decide what to do with that gain. Emitter resistors to degenerate the devices are one option.
Grey
Just made a quick simultation on the input circuit with the standard MOSFET and a version with BC556 as input pair:

-Standard has 40dB gain (105x)
-BC556 has 28dB gain (x28) without emitter resistors.

The output resistor (392Ohm) is the same in both and also the current source, therefore the DC-bias (4.5V) for the output MOSFET is also the same...

Benefits:
-No zener protection
-12dB lower feedback (close to J-version)
-Input capacitance no problem
-Better performance on total circuit (no frequency comp. cap needed, as for J-version)

Perhaps BC556 will better be replaced with a BD139 for better reliability....any experiences?

This could be a Aleph-J competitor!!
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Old 11th January 2006, 11:52 AM   #5
Skorpio is offline Skorpio  Denmark
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I have been searching for treads regarding bipolar input on the Aleph/Pass, but found none.....anyone with links?
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Old 11th January 2006, 12:45 PM   #6
Dr.Gone is offline Dr.Gone  Netherlands
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I haven't tried the diy search engine, but its results are usually not that usefull, maybe a search with google on this site will give you better results:

http://www.google.com/search?hl=nl&q...e+zoeken&meta=


regards,
Jacco
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Old 12th January 2006, 06:16 AM   #7
Skorpio is offline Skorpio  Denmark
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Thank you Dr....

I found something, but only wrong statements like "what willl you do with the extra gain"....there is no extra gain, it is the other way around...bipolar is a step towards JFET input...and they might be better?

You can drop 3 bipolar devices directly into the circuit without changing anything other than the feedback level and input capacitance -in the right direction...
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Old 18th January 2006, 07:55 PM   #8
Skorpio is offline Skorpio  Denmark
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Not much responce....has anybody tried this input stage modification?

If not, why? It seems to offer much of the advantages of the new Aleph-J circuit.....

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