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Old 23rd July 2003, 08:28 PM   #11
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what's the theoretical limit of a SE class A amp? without active loading?
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Old 23rd July 2003, 08:35 PM   #12
Fuling is offline Fuling  Sweden
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Limit of what?
Efficiency?
That would be 50%, using an inductive load.
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Old 23rd July 2003, 11:39 PM   #13
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Quote:
Originally posted by Fuling
I donīt think 1H is necessary. A while back I was messing around with a choke loaded source follower amp, those chokes where 100mH and they worked fine for an 8 ohm load.
How 'bout take your choke loaded source follower and hitch a step-up transformer to its input for voltage gain? A follower would be more linear and the only thing I can think of off the top of my head that could be a problem would be the gate capacitance of the MOSFET.

The Jensen JT-13K7-A would be a good choice.

se
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Old 24th July 2003, 06:53 AM   #14
aiwonsi is offline aiwonsi  Thailand
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what do you think about this schem

parameter

generator: 1K Ohm amperes: 2.618 Amp
device: IRF530S power@1%2/3rd: 7.558 Watt
z-Bias: 100K Ohm dissipation: 86.39 Watt
Bias Voltage: 25.2 Volt rendement: 8.749 %
z-source: 8 Ohm gain: 0.908 x
output capacitor: 4700uF Z-out: 0.2588 Ohm
supply: 33 Volt -dB @ 10 Hz: -0.6614 dB
z-speaker: 8 Ohm -dB @ 100 kHz: -0.0323 dB
rimple rejection: 0.648 %

THD@0.33Watt THD@1Watt THD@3Watt
0.125 % 2nd 0.223 % 2nd 0.432 % 2nd
0.006 % 3rd 0.022 % 3rd 0.079 % 3rd


http://home.zonnet.nl/tschrama/owndesign.html
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Old 24th July 2003, 07:15 AM   #15
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Quote:
Originally posted by aiwonsi
what do you think about this schem
That wouldn't meet Fuling's requirement that it be at least 35-40 percent efficient. You'd need to replace that load resistor with a choke.

se
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Old 24th July 2003, 09:00 AM   #16
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Quote:
Originally posted by Fuling
janneman, do you mean this one?

http://www.bonavolta.ch/hobby/en/audio/2sk135se.htm

Looks like something that I could rip...
Changing some voltages and currents and replacing the OPT with a choke and some output caps. Oh yeah, a coupling cap in the feedback loop, nothing fo the puritans
Yes, that's the one, Nemisis. Straight from L'Audiophile. It had a kind of cult following in the 80-s IIRW. The output transformer could probably be a mains toroid with the correct turns ration.

Jan Didden
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Old 24th July 2003, 09:25 AM   #17
UrSv is offline UrSv  Sweden
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Quote:
Originally posted by janneman


Yes, that's the one, Nemisis. Straight from L'Audiophile. It had a kind of cult following in the 80-s IIRW. The output transformer could probably be a mains toroid with the correct turns ration.

Jan Didden
I seem to recall that they used a Tango transformer.
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Old 24th July 2003, 12:18 PM   #18
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the transformer needs to cope with 800mA DC - no go with a mains trannie. unless you have another winding and put a compensating current through this. (but then of course you have more active devices), yes like in schematic no. 3 on bonavolta's page. i think that looks like a very interesting amp...

transformer coupled transistor amps are rare..
my new preamp has an output transformer..
so why not a nice efficient SE amp with tx coupling for my horns..

regards
keyne
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Old 24th July 2003, 04:48 PM   #19
Fuling is offline Fuling  Sweden
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Transformer, choke, whatīs the difference?
I donīt see the point of using a stepdown transformer in a solid state amp in times like this when we have mosfets that can handle enormous amounts of current.

Ok, if I had a pair of suitable OPTīs then Iīd use them, but I donīt.
Instead I have a pair of reasonably big chokes (nowhere near the chokes in Circlotrons "first ever class a amp", but probably big enough) that is sort of the fundaments of this project.

The big question is: Can this be made without using a stabilized PSU?
Letīs say I copy the topology of that Nemesis amp with itīs stabilized bias voltage and AC-coupled feedback, what would happen to the current through the mosfet if the Uds voltage goes up say 1V due to variantions in mains voltage?
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Old 24th July 2003, 06:11 PM   #20
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Hi All,

I would like to add my two cents to this thread.

For the past three years I have on and off been working on a inductor loaded single stage MOSFET amplifier. It still hasn't moved beyond the breadboard stage though. I first got the idea when I saw the schematic of a parafeed tube amp and thought "I can do this with a MOSFET and leave out the output transformer". Just as Fuling I wanted to build an amp using only one active device per channel. The first atempts were made using a aircore inductor from an old loudspeaker, which obviously lacked bass. Later I got some decent inductors and the amp now produces sufficient bass. I have included a schematic as the amp is now.

Any feedback on the design will be highly appreciated

The powersupply is unregulated and is consists of a few 16500 uF caps and a 2.2 mH aircore inductor.

Playing with the amp I have made several observations:

a) It needs some active circuitry added to it to keep the quiescence current stable. As the amp is now it is (too) sensitive to thermal drift and fluctuations in mains voltage. Unfortunately I havenīt come across an effictive and elegant solution yet.

b) The choice of MOSFET is more critical than in the Pass Zen amp (which I also build), but I donīt know exactly why. Through some experimentation I found that the input capacitance must be kept as low as possible otherwise you will get high frequency distortion. The IRF150 which worked well in my Pass Zen amp was a complete disaster in this amp.

c) The sound improves with higher quiescence current - I know this is nothing new :-) however it is my observation that any increase in current below 1A makes a nice difference, but the audible difference between 1A and 2A is very small. With 1A it just starts to distort a bit earlier, but a normal listening levels this is not noticeble.
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