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Old 10th November 2005, 06:26 PM   #1
Fuling is offline Fuling  Sweden
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Default OK, itīs not a PASS project but...

...I guess SE mosfet amps belongs here anyway.

A while ago I bough some 112mH 2A chokes (with airgaps, before anyone comments that).
Seems like a perfect source/drain load for a smallish SE amp.

The easiest thing to build would be a source follower with a separate gain stage, but that feels a little...boring

What do you guys think about this one:
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Old 10th November 2005, 07:02 PM   #2
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Excellent. How does it sound?
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Old 10th November 2005, 07:14 PM   #3
Fuling is offline Fuling  Sweden
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Excellent. How does it sound?
Thanks!

Er, I donīt know yet.
I came up with this circuit just hours ago, inspired by an amp designed by my my fellow countryman Circlomanen.
I basically chopped his circlotron circuit in half and replaced the missing half with the choke.


Iīll start building a prototype as soon as I can, I expect my power fets to arrive withing a couple of days.
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Old 10th November 2005, 11:04 PM   #4
Zen Mod is offline Zen Mod  Serbia
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mebbe I'm just grumpie,but- only 4mA in first stage looks insufficient for proper driving of second stage......

but-when you hear it ,you will tell us.....in any case-there are more than few commerciall amps with wimpy driver stages ,and not all of them are cheap
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Old 10th November 2005, 11:13 PM   #5
Fuling is offline Fuling  Sweden
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Yes, the low driver current concerns me a bit but itīs a compromise.
Higher current would lead to very low resistor values in the feedback network (and thus a seriously big capacitor)

At least itīs only a single mosfet that is to be driven, not 2 or 4 or...
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Old 11th November 2005, 04:35 PM   #6
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You can replace the choke with an (Aleph) CCS or a powerresistor and ad a negative powersupply to build a CFB two stage DC coupled amp.

Cheers, Johannes.
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Old 11th November 2005, 04:50 PM   #7
Fuling is offline Fuling  Sweden
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Tjenare Circlo!

Iīve thought about the CCS (Aleph type or not) and a negative rail, but I have a strange fetish for chokes.
BTW: Didnīt you post a circuit similar to that at hififorum some time ago?
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Old 16th July 2006, 07:49 PM   #8
Fuling is offline Fuling  Sweden
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Today I finally decided to build a prototype of this circuit.
Took me about an hour.
Results? Probably a great midrange amp, cuzīit has neither bass or treble

EDIT: Forget what I wrote about the frequency respone, I had made a wiring mistake in the feedback loop which messed things up quite a bit.
Now it works muuuch better!!
Iīm listening to a mono channel right now, and itīs definitely not bad!
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Old 16th July 2006, 08:11 PM   #9
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Perhaps there is something I'm not noticing.

The way you have your feedback applied is actually giving you positive feedback. It seems that it would be negative feedback because the output signal is out of phase with the source of the first stage. But that will actually increase the gain of the amp in this circumstance, which is positive feedback, not negative. I only noticed because I designed a tube amp that had the feedback the same way, only to find out later what was really going on.
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Old 16th July 2006, 08:35 PM   #10
Fuling is offline Fuling  Sweden
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I must disagree on both points.
If the feedback was positive the amp would probably suffer from extremly high gain, very high output impedance and most likely severe oscillations.
This amp has a voltage gain of ~8, "tight" bass response and no oscillations that can be detected on my īscope.

Regarding the choke, yes it would be nice with 10x the inductance but it would be practically impossible.
Some guy here made a fullrange amp with a pair of 85mH chokes some years ago, so I canīt see why it wouldnīt work.
But as you say, higher inductance would of course be a good thing.
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