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Old 28th August 2010, 09:21 PM   #1
icko is offline icko  Sweden
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Default Alexander amp

Anyone simulated the current feedback amplifier by Mark Alexander?

I tried just for the heck of it but it seems to oscillate with an frequency of 50-60MHz, with a Vpp with about 500mV. Anyone else tried to simulate this amp? Can't really figure out where the problem lies.

Don't really know how to add pictures.

AN-211:
http://www.google.se/url?sa=t&source...9s8ex8BlsQE2Lg

Last edited by icko; 28th August 2010 at 09:25 PM.
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Old 28th August 2010, 09:34 PM   #2
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Don't feel bad. I actually built one and it did that.
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Old 28th August 2010, 09:46 PM   #3
icko is offline icko  Sweden
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Interesting. Did you figure out why?


EDIT:
I mean, looking at the loopgain it seems that is far from unstable.

Also I tried to implement a CMCL but it did not seem to do any good really, maybe I did it wrong, gave it one hour or so...

Last edited by icko; 28th August 2010 at 09:49 PM.
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Old 28th August 2010, 10:24 PM   #4
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I never did figure out why. I built it on a breadboard and chalked it up to that, figuring to get it running I'd have to do a board. A decade passed and then some. Here we are. Let me know if you want a couple old Toshiba IGBTs.
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Old 30th August 2010, 10:54 AM   #5
icko is offline icko  Sweden
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Thanks, but I think I'll let it go, atleast for now.

I think the simplicity of the amp is very interesting, but i'll think I'll dig into descrete input stages. It seems there is plenty of them around to learn a couple of things from.

My opinion is that even though Mr. Alexanders AES article is fairly old, there are'nt many of this amps with OPAMPS as input buffers (atleast I have not seen many), so maybe this is a fairly common flaw with this type of amp. Would still be nice to figure out why it behaves this way.

Thanks for your fast reply, your answer made do something else with my sunday, instead of thinking of what I did wrong in my testbench.

/Icko
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Old 30th August 2010, 01:18 PM   #6
lineup is offline lineup  Sweden
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It is not unusual CFA oscillate.
Especially if you try to run the whole amp from same Voltage supply.

A good way to prevent oscillation
is to let the Output stage have a separate supply.
And the input+vas stages should have a regulated voltage supply.
This will make the amp have a good chance to be stable.
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Old 31st August 2010, 06:47 PM   #7
icko is offline icko  Sweden
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Tackar Lineup.

I shall have that in mind when stuff starts to get practical. Right now it seems that I am going to build a cfp amp. Atleast doing simulations of them is very enjoyable.

But, it was a pretty simple testbench I hade were the supply rails were ideal so I don't think that was the case this time.

/Icko
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Old 31st August 2010, 08:18 PM   #8
lineup is offline lineup  Sweden
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Okay.
Then the compensation is different for CF Amps.
The usual compensation small caps should be placed elsewhere.
But I never have learned and I am not sure how to put capacitors
to get best effect and keep 100% stable.

Best way is to look at some current feedback amps
and see where those small caps are.
And try to change value, if it does not work.

Here is another CFA from Norway.
( Från vårt broderland Norge ... han har en hel del bra läsning )
But it works in CLASS A and has got JFET, 2SK170 + 2SJ74 input.
LINK: Power Amplifier
The schematic:
Click the image to open in full size.
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Old 1st September 2010, 09:49 AM   #9
Jay is offline Jay  Indonesia
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Quote:
Originally Posted by icko View Post
I think the simplicity of the amp is very interesting, but i'll think I'll dig into descrete input stages. It seems there is plenty of them around to learn a couple of things from.

My opinion is that even though Mr. Alexanders AES article is fairly old, there are'nt many of this amps with OPAMPS as input buffers (atleast I have not seen many), so maybe this is a fairly common flaw with this type of amp.
If I can recall correctly, the Alexander was issued after the Quad 405, which is also a current dumping amplifier, and the Quad works almost perfectly stable. Later amp is the super high slew rate Stochino amplifier which is a discrete symmetrical LTP input. Different animal but I guess it could be inspired by the current dumping also.

Quasar v13 is relatively a modern implementation of the Quad ideas. The problem is that the quality of such amps really depend on the opamp. The old LM301 as I thought used in the Quad 405 is very musical but is very noisy.

For me, it is hard to decide whether this kind of amp worth to be kept or not. The opamp is in class-A and with low gain, but I think it is really a test for everyone who can hear the terrible sound of an opamp.
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Old 1st September 2010, 12:06 PM   #10
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Danish hifi magazine "high fidelity" did an DIY article on this amp. The magazine doesn't exist anymore. But the guys behind it, have an online mag now.

NOMONO | webmagasin om lyd, billede og musik

Perhaps they can help.
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