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Old 26th September 2002, 02:48 AM   #1
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Smile My first ever Class A amp.

The time had finally come for me to see what all the fuss was about when it comes to Class A. For all their shortcomings they must have *something* going for them to have such a loyal following. So I dredged through the junk box and stuck a few bits and pieces together to see just what would happen. The results were so pleasing that I started putting it all together in a chassis as a proper project. This thing is nominally a 50 watt per channel stereo setup with a source follower output stage and an inductor in the source lead. It runs a 28 v rail with 3.5 amps current so that's about 100w dissipation. The advantage of using an inductor is that you only need half the supply voltage because the inductor will swing the missing half rail for you. Therefore the efficiency is at best a horrible 50% rather than a woeful 25%. The downside is that suitable inductors are heavy and expensive. But I already had two of them so that was lucky.

How does it sound? Very smooth and relaxed, crank up the volume and it doesn't get stressed at all. Easy to listen to. Most enjoyable. Not as nice as AKSA's amps that I had a listen to the other week (they are just soooooo good) but the development time on mine was about 2 days if you add it up. I sit there and look at the small handful of parts that is producing all that nice sound and think the results are way out of proportion to the simplicity of it. When I get it further assembled (it is only a rats nest at the moment) I will post a picture or two. In the meantime here is the schematic. Any comments, criticisms, suggestions etc are most welcome.

GP.

P.S. I edited this post and for some reason the schematic fell off. There are other ones later in the thread though.
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Last edited by Circlotron; 27th April 2012 at 01:58 PM.
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Old 26th September 2002, 08:47 AM   #2
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Interesting but have you drawn the schematic right? Now, it seem that the speaker does take DC(theorethically)?
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Old 26th September 2002, 11:50 AM   #3
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Default DC offset

Yes, the speaker does in fact have about 600mV across it. Doesn't worry me but it might make some people's hair stand on end. It was either that or use an electrolytic cap to couple to the speaker. I *think* I chose the lesser of the two evils. The resistance of the choke is a little under 200 milliohms. It only gets about 2 watts dissipated in it and seing there is about 4-5kg of copper (about ~100 metres of 1/8 inch thick wire) it doesn't warm up much. I am eventually going to low pass filter the dc voltage across it and use it basically as a sense resistor to control the source follower bias so it's current will remain stable regardless of temperature.

Since the last posting I put a 1000uF 16v cap across the 100R source resistor of the first fet. The ac open loop gain increases quite a bit loudness-wise but with the feedback loop closed there is practically no difference gain-wise but it does make the NFB loop work that much harder so it looks like a worthwhile mod.

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Old 26th September 2002, 01:04 PM   #4
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Default Re: DC offset

Quote:
Originally posted by Circlotron
Yes, the speaker does in fact have about 600mV across it.
I must have been dreaming. I measured it again and it's 300mV. So the choke has about 1 watt dissipation at 3.5 amps. A voice coil with 5 ohms dc resistance would dissipate 18 milliwatts With my 10 inch test loudspeaker the cone offsets about 1/2 mm. Not worth losing sleep over, hey?

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Old 26th September 2002, 01:53 PM   #5
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Default caution: backseat designer

A possible benefit to using an electrolytic coupling cap is that this would turn the output section into the type of circuit used by Pathos, in which the current through the output device is apparently held to a nearly constant value by the storage and release of energy from the resulting LC tank network. I've always been curious about this circuit, but never tried it myself.

http://www.pathosacoustics.com/inpoleng.htm
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Old 26th September 2002, 03:06 PM   #6
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Default problems, problems...

If there was a tank cct there, it would go to a very high impedance at only one frequency, not what you need for a flat frequency response! That company claim to have a worldwide patent but I couldn't find any reference to it in a quick look at the USPTO.

A constant current load for the source follower would indeed be a good thing. I was looking at some of Nelson Pass' amps with the current source at the top and I was thinking if it were arranged to be a negative resistance device that complemented the changing current drain of the load with changing voltage across it, then there is half a chance you could have a constant current load. That would be excellent. How it would go with a reactive loudspeaker load is another story though. Still, I think it's an interesting idea.

Cranked up the amp tonite with sinewaves and an 8 ohm dummy load and it seems it will swing down to -25 volts and then clip (that's fine), but it will only go to +15 volts before becoming decidedly nonlinear. Especially at 15 kHz upward it seems to lose a whole lot of slewing capacity above +15v (i.e.when Vds is < ~13v). But then I am using a *very* large die 500v Hexfet that has been optimised for *switching* not linear service. Once the drain-source voltage is less than about 13 volts in this case, and the drain current would be about 2 amps (perhaps not relevant) and the Vgs is about 6 volts and rising quickly because the negative feedback is starting to take action, the fet just seems to go into lazy mode. Admittedly, the gate is normally supposed to be rammed up to 15 volts or so in about 20 nS and down at the same rate as per it's intended usage. Even then this fet never was particularly fast which is why I ended up being given them. Turnoff time was in the order of 150 nS. The ones that were chosen in place of them turn off 15 amps 500v in 30 nS. So they're a bit of a slug all in all.

It's only in the last 10v of decreasing Vds that the wheels fall off with linear usage. At this voltage the nonlinear Drain-Source capacitance starts to go through the roof. Given the fact that the gate voltage is still in the linear region, perhaps this capacitance is showing up really badly. I might just try a low voltage hexfet tomorrow night and see what happens. Drat! Oh well, at least I'm learning things and nothing has blown up. Actually this amp seems very resistant to blowups and I have accidentally shorted things inside the cct several times. Not like the CDA amp; you only have too look at that and it pops the output fets. It's 1:25 AM. Yoicks! Way past bedtime!

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Old 26th September 2002, 03:10 PM   #7
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Talking I *think* I chose the lesser of the two evils.

Hey if it makes you happy thats what is important. Dare to think outside of the box. Good for you.

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Old 26th September 2002, 03:53 PM   #8
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Default Re: problems, problems...

Quote:
Originally posted by Circlotron
If there was a tank cct there, it would go to a very high impedance at only one frequency, not what you need for a flat frequency response!
Perhaps I used the term "tank" too loosely given that the load is in series with the C and would reduce the Q of the circuit. I presume that this, combined with the damping influence of the output follower MOSFET, is what makes it all hang together. But again I'm just speculating, having never built or tested one.
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Old 26th September 2002, 08:18 PM   #9
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Yeah, nice original thinking.
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Old 27th September 2002, 06:11 AM   #10
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I agree with Nelson, original but I wonder also how much you can "kräma ut" before your ears hurts by distortion (max useable output power)? Bandwith?

Do you have a picture of your "monster"?

Your choke, where did you get it?

300 mV DC, nothing to scared of and if you have to choose between the electrolythic cap I would have done the same thing...but I have had very big problems to sleep....0.0000001 mV is my kind of style.
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Old 27th September 2002, 06:39 AM   #11
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Question 0.0000001 mV

That's 0.1 nanovolts..... Anyone here who can tell me which Fluke meter to measure that with?

Anybody but me feel that 0.3 volts is kind of a lot of DC offset?

2 Amps though an (ferrous core?) inductor?

Just curious, not trying to be noty.

H.H.
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Old 27th September 2002, 06:48 AM   #12
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Hope you treble unit is not DC - coupled.

I would personaly accept a max of 50mV. But Congratulation with you home made amp..

Always nice when somebody design a amp from the scratch! This is very rare around here!

.1nV ... What is the noise level of a 50Ohm resistor!?!? ... Okay.. so you have no thermal drift or is it a snapshot of 1 ps?

Sonny
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Old 27th September 2002, 07:02 AM   #13
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Question Tall tales and low voltages

".1nV ... What is the noise level of a 50Ohm resistor!?!? ... Okay.. so you have no thermal drift or is it a snapshot of 1 ps?"

I guess that question would be for PerAnders? I am kind of curious though.....

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Old 27th September 2002, 07:08 AM   #14
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yes this one was for PerAnders, but i think maybe it was spelling error with mV.. maybe it was V?!?

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Old 27th September 2002, 08:32 AM   #15
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Default Re: 0.0000001 mV

Quote:
Originally posted by HarryHaller
That's 0.1 nanovolts..... Anyone here who can tell me which Fluke meter to measure that with?

Anybody but me feel that 0.3 volts is kind of a lot of DC offset?

2 Amps though an (ferrous core?) inductor?

Just curious, not trying to be noty.

H.H.
Yeah, I mean 100 pV, noooo.

Personally I feel better if the offeset is not audible (no pops or clicks at switch on), < 10 mV

It's cool when I read 0.0 mV on my Fluke 77 when I measure the output from my DAC with a OPA627! No trimming!
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Old 27th September 2002, 08:34 AM   #16
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Default Re: 0.0000001 mV

Quote:
Originally posted by HarryHaller
That's 0.1 nanovolts..... Anyone here who can tell me which Fluke meter to measure that with?

Anybody but me feel that 0.3 volts is kind of a lot of DC offset?

2 Amps though an (ferrous core?) inductor?

Just curious, not trying to be noty.

H.H.
Yeah, I mean 100 pV, noooo.

Personally I feel better if the offset is not audible (no pops or clicks at switch on), < 10 mV

It's cool when I read 0.0 mV on my Fluke 77 when I measure the output from my DAC with a OPA627! No trimming!
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Old 27th September 2002, 08:56 AM   #17
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agree! ;O)

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Old 27th September 2002, 12:00 PM   #18
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Quote:
Originally posted by peranders
Do you have a picture of your "monster"?

Your choke, where did you get it?
I'll try and post a picture on Saturday if I don't feel to lazy. the chokes were originally made as filter chokes for a 350vdc 15 amp power supply that was part of 2 UPS's. They ended up never getting used and so I hauled them home. Even took them with me when I shifted house 8 years ago. Originally they measured 22 mH each and had a 1/4 inch wide airgap between the facing EE cores. I pulled out the spacer and tapped the two halves of the stack together untill the centre legs of the E's met. This left about 1mm gap between the outer legs. Applied 40vac and measured the current to come up with the 85mH. Just lucky actually. They will actually take 240vac @ 50Hz without saturating although they do buzz a little bit. Overkill? You bet!

Now, as far as that distortion problem is concerned, talk about looking for a complex reason for the problem! I got to thinking about things and now I think it is because when the Hexfet starts to run out of puff on the positive half cycle - guess what? The output waveform starts to round off and so of course the bootstrapping starts to fall on it's face too! So now at the very moment the driver stage has sit up and work the bootstrap cct goes on a holiday. What a fair-weather friend! What I think I will do now is make some sort of current source load for the driver instead of the 1k resistor. Maybe a current mirror using a pair of pnp's. Bootstrapping would not be particularly necessary then perhaps. But of course I will still try it in conjunction with the current source to try and get the most linear cct possible. The gain of that stage should be quite high by then I think.

GP.
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Old 27th September 2002, 12:07 PM   #19
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Quote:
Originally posted by Circlotron
Overkill? You bet!
You too? Is it because of me? Is it a contagious? I asure you that I haven't tried to infect you by purpose.
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Old 27th September 2002, 01:29 PM   #20
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Default Just lucky actually.

"Originally they measured 22 mH each and had a 1/4 inch wide airgap between the facing EE cores. I pulled out the spacer and tapped the two halves of the stack together untill the centre legs of the E's met. This left about 1mm gap between the outer legs. Applied 40vac and measured the current to come up with the 85mH. Just lucky actually."

Hmmmm......I wonder what that 1/4 inch air gap was in there for?

Can you tell us Peranders?

H.H.
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Old 27th September 2002, 01:51 PM   #21
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Default Re: Just lucky actually.

Quote:
Originally posted by HarryHaller
Hmmmm......I wonder what that 1/4 inch air gap was in there for?
Can you tell us Peranders?
You askin me? You asking ME? Taxidriver? Robert de Niro

I don't know wether you are kidding or are really asking?

1/4" seems much but it's not forbidden to have VERY wide air gap. Normally we talk 1 mm (0.039") or so.
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Old 28th September 2002, 11:35 AM   #22
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Default dem's dat choke...

Note TO-3 device sitting on top for scale.

Buwahahahahaha!!!!!!...
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Old 28th September 2002, 11:40 AM   #23
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Default Chokes, caps, tranny and psu choke

.
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Old 28th September 2002, 11:50 AM   #24
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Default 2200uF 35v times fifty-two.

Copper shim joining caps is I think 23mm x 0.25mm. Approx same cross sectional area as 3mm diameter wire but extra good at high frequencies because of low skin effect. Total ESR of caps is way lower than one big cap. Besides, that's what the junk box had.

GP.
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Old 28th September 2002, 02:18 PM   #25
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I get hate email due to my overkill stuff. I hope you won't get any because this is overkill in a beautiful way. I like it!
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Old 28th September 2002, 03:25 PM   #26
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Default really asking........

http://www.coilws.com/magneticandhow.html

"So, it is very important in a choke or inductor design, not to drive the core into saturation by increasing the current (AC or DC). Usually it is the DC current that saturates the core since it is a constant current, and drives the core to a certain flux level."

Sorry I will ask someone else next time since you don't understand my concern with large DC currents in ferrous inductors.

http://www.maneng.com.au/dcchokes.htm

H.H.
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Old 28th September 2002, 08:11 PM   #27
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Default Re: really asking........

Quote:
Originally posted by HarryHaller

Sorry I will ask someone else next time since you don't understand my concern with large DC currents in ferrous inductors.
What were you asking about really? This big mamas was designed for much DC, or? I think they were designed for MUCH more than 3 A. Check the wire dimensions!

I wonder how the frequency characteristics are? The inductors are made of laminated iron.
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Old 28th September 2002, 08:19 PM   #28
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Default Compact capacitor bank

Circlotron, since you are overdoing it , haven't you though about of making a "multilayer" capacitor bank? You will then get the shortest possible connections. Just a thought, but it looks nice now. Why ruin it?
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Old 29th September 2002, 11:29 AM   #29
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Default Re: really asking........

Quote:
Originally posted by HarryHaller
http://www.coilws.com/magneticandhow.html

"So, it is very important in a choke or inductor design, not to drive the core into saturation by increasing the current (AC or DC). Usually it is the DC current that saturates the core since it is a constant current, and drives the core to a certain flux level."

Sorry I will ask someone else next time since you don't understand my concern with large DC currents in ferrous inductors.

http://www.maneng.com.au/dcchokes.htm

H.H.
If the core had no airgap then it would indeed saturate easily with dc and what's more it would store very little energy. We need to have stored energy to swing the mosfet source and consequently the load below the zero volt rail on the negative half cycle. With a gapped magnetic circuit the vast majority of the stored energy is contained in the airgap flux. A gapped inductor is much much more resistant to dc saturation than an ungapped one. However for a given amount of turns and iron the inductance is much lower. If I reduce the gap on my chokes from 1mm to zero the inductance would rise by a factor of perhaps 100. Great for AC, rotten for DC.

I seem to remember (from 1990) that those chokes with the original large gap were good for 25 amps dc. Actual working current was 15 amps dc. Not sure what current they would saturate at with the smaller gap but it seems to be a least 10 amps judging by the fact they will tolerate 240vac @ 50Hz.

GP.
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Old 29th September 2002, 11:42 AM   #30
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Default Re: really asking........

I say, splendid explanation. Spot on with your conclusions. 1 mm
is a relatively large air gap actually. You shold be working on a patent application and not giving you design away.

Guy
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Old 29th September 2002, 12:08 PM   #31
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Talking Nothing exceeds like excess...

Quote:
Originally posted by peranders
Circlotron, since you are overdoing it , haven't you though about of making a "multilayer" capacitor bank? You will then get the shortest possible connections. Just a thought, but it looks nice now. Why ruin it?
Not sure what kind of multilayer you mean, but I was thinking of putting a second layer of caps along with theses ones. Then the junk box is just about exhausted for that type of cap. Also there will not be enough space left for the tranny and other stuff. Each (100 watt) heatsink will have it's own fan so maybe I should put "Twin Turbo" on the front panel.

I finally got the driver stage going properly tonite (Sunday) took all weekend it did. Uses a N-channel fet current source load. Has an open loop gain of about 630. One version had a gain of over 2000! Not bad for a single stage :-P but the distortion was a bit ordinary and it had a bit of high frequency fuzz on the waveform. This one will swing 80v p/p with almost no visible distortion. It clips at 120v p/p using a 145v dc supply and a 50mA drain current. The closed loop gain of the whole amp is x10 and even though I have used no pole splitting caps or other compensation band-aids it seems to be as stable as a rock. The sound is really nice - to my biased ears anyway. Look out AKSA, when its done I will definitely lug it over to your place, that is if shifting it's weight 20 miles or so doesn't upset the balance of the whole city of Melbourne.

Also, in the spirit of overkill, and excessiveness generally, I am thinking of bootstrapping the top of the current source. That way the constant current load fet will have nearly a constant voltage across it. So the constant current will be *very* constant and it's slope resistance will be *very* high (currently about 22k or so) The stage gain will then be something the far side of ridiculous. We'll see.

On a more sensible note, I am thinking of sort of bootstrapping the output stage too. Put a second mosfet above the first with the top fet drain to dc supply and it's source to the drain of the existing (now bottom) mosfet. Bias the top fet about 10v above the bottom one and drive both with the same audio signal. What happens is the bottom mosfet now has a more or less constant voltage across it during the audio waveform! One less thing for it to worry about. Seeing the drain source voltage won't decrease to low values during high positive signal swings it's transconductance won't nose-dive and so there should be less distortion. The setup will need a higher dc supply voltage though so the efficiency will end up being about the same as a steam locomotive. heh heh egad, this is fun!!!

GP.
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Old 29th September 2002, 12:27 PM   #32
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Question Gate-source capacitance modulation?

With my driver cct I am using a 100k & 1M gain setting resistors and if I run the thing open loop the distortion is reasonable but if I short the 100k in series with the signal, the distortion improves quite a bit. What is happenening here? Is the gate-source capacitance changing with the instantaneous voltage on it, and a low impedance drive just bulldozes through, but with the 100k in series the capacitance variation distorts the signal? Sort of like capacitors causing distortion as per another thread here? I have never come across this before. The only solution I can see it to use a buffer upstream to drive lower value feedback resistors. I don't really want to do that, so what can I do?

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Old 30th September 2002, 02:13 AM   #33
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Default New output stage.

Going to try this output stage as per 2 posts ago. Also I found out my output stage distortion problems that I thought were caused by an unsuitable fet, or the bootstrapping going bad; well it turned out I had been ultra-conservative and left a 10k gate resistor on the fet. I mean duhh... So now it's 470R and works much better.

GP
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Old 30th September 2002, 12:41 PM   #34
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I tried bootstrapping again and the open-loop gain rose from 190 ( I think I got it wrong in an earlier post) to 260. I thought it would go sky high so that was a surprise. Maybe I should disconnect the output stage and see what it is then. Maybe it is loading the driver a little.

For my next trick, as a current source I am going to use a 3.3v low dropout 3 terminal regulator wired as a current source, i.e. put a 75R resistor from output to ground leg and the (floating) reg sinks constant current between input and ground pin. The one I tried on the bench had a slope resistance (*change* in current versus *change* in voltage) of about 4 megohms so that was good. The specs show 60 dB ripple rejection to about 10kHz then gradually falling above that. So I think it would work alright at audio frequencies.

The reg will only stand about 35 volts max input to output so what I will do is put it in the source leg of the existing current fet in place of the resistor. In theory this should multiply the 4 megohms of the reg by the voltage gain of the fet! On the bench though it actually showed about 300k slope resistance. Not sure what's going on there. Any ideas?

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Old 30th September 2002, 12:50 PM   #35
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Default Re: Nothing exceeds like excess...

Quote:
Originally posted by Circlotron


Not sure what kind of multilayer you mean, but I was thinking of putting a second layer of caps along with theses ones. Then the junk box is just about exhausted for that type of cap. Also there will not be enough space left for the tranny and other stuff. Each (100 watt) heatsink will have it's own fan so maybe I should put "Twin Turbo" on the front panel.
I ment cut the long copper band into smaller pieces. Make this "cap band" like a square (if you really want to go extreme). This is not important, only cool.
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Old 30th September 2002, 06:59 PM   #36
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Circlotron,
Intriguing Schwartzenegger-esque project! And progress is apace: seems to be developing by the hour - how about an updated schematic to show the latest state?
I would like to ask about the output bootstrap. What was the reason for this again? You see, at first glance it seems to me that since both devices are loading the driver stage that gains through shielding the lower FET might be lost on the upper FET. Perhaps if the upper FET was not loading the driver stage it would be a more effective circuit. For example, if the gate of the upper FET was tied 15V above the source of the lower FET. Might be a little more complicated to implement.
My worth.
BAM
(still trying to figure out what that photo is...woman in lilac dress in front of a control panel?)
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Old 1st October 2002, 12:06 AM   #37
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Graham,

Interesting circuit; constant voltage source follower, no less.

You go to a lot of trouble to avoid Vgs (bias) changes by placing a constant voltage across the lower FET, but in fact it will change considerably anyway because of the variation in current.

This will cause second harmonic distortion, which is no big deal, but rather than lose headroom and sacrifice efficiency with a second FET atop the source follower, why not run your source follower in CFP with a bipolar driver, thus reducing to very low levels the variation in Vgs, and scotching the constant voltage idea which in truth contributes far less distortion than the bias changes caused by current variations through the FET?
The circuit is below, and works well; the current source is replaced by your inductive load (with a blocking cap for the speaker, incidentally). I have tried it, and it forms the basis of my Glass Harmony SE amp using mosfets, which you must hear some time....

A PSpice analysis would reveal this, I feel, but I don't have such a beast on my PC to check it out.

Cheers,

Hugh

Aspen Amplifiers
aksaonline.com

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Old 1st October 2002, 02:32 AM   #38
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Quote:
Originally posted by traderbam
(still trying to figure out what that photo is...woman in lilac dress in front of a control panel?)
The woman is Brigitte Helm in Fritz Lang's 1926 film Metropolis. She is an evil robot, the "twin" of a woman of which butter wouldn't melt in her mouth. She is about to sabotage the underground city's pumping station control panel. The web has a jillion references to this nifty movie.

GP.
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Old 1st October 2002, 02:38 AM   #39
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Should have included this on the last one. Movie has much period style electrical stuff too.
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Old 1st October 2002, 02:43 AM   #40
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Default Current schematic

.
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Old 1st October 2002, 02:47 AM   #41
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This is way off topic, but where did you get the images from Metropolis? Is there a digitilized copy of the movie floating around somewhere?
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Old 1st October 2002, 05:26 AM   #42
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Ah,

Metropolis..... A wonderful movie! I loved Giorgio Moroder's soundtrack too.

Notice the knife switch in the background in the shape of a crucifix - now there's an image of modern life for you!

Graham, this is the beginning of a very fine SE amplifier, with low order distortion only, primarily H2. It will sound sensational, and if you visit me again soon I'll play one for you. You thought the AKSA was good; wait until you hear this one! Here's a few points you might like to ponder.

1. If you use the bipolar/mosfet hybrid in CFP your Zout will drop to below 50 milliohms. This means - you guessed it - gate drive problems are all but eliminated, and you need no global feedback! You can then even consider a mouse power tube to perform the voltage amplification - a lofty choice, I can assure you!

2. A bipolar transistor CCS in the +ve rail will give you superior performance, in my view, since the drain/collector impedance will not be affected by the protection zener you have from gate to drain. Further, the supply voltage can be lowered considerably, obviating two separate power supplies (although the voltage amp mosfet circuit will need to be thoroughly decoupled from the output device supply, of course.)

3. A CFP output stage requires no Zobel for stability, since there is no GFB and tolerance to reactive loads is nothing short of monumental. This amp will drive an ESL63 speaker very well.

4. Because the tempco of the mosfet is self-limiting AND its transconductance is limited, you can get away with very small DC resistance in the drain of a bipolar/mosfet CFP hybrid for current control. This means you can use the DCR of the choke to good advantage; as little as 300 milliohms is sufficient. In my experience an IC servo control here is quite unnecessary; I use a simple LED voltage reference. While this will give you about a volt of output offset for 3.3A of standing current, it will open up the possibility of a bipolar power supply, with the cold end of the choke feeding a negative 1V supply, thus eliminating the output cap. In practice, this means judiciously center-tapping the present single AC supply.

I think your design direction is very impressive!!

Cheers,

Hugh R. Dean

aksaonline.com
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Old 1st October 2002, 12:26 PM   #43
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Great photo - thanks for enlarging upon it (in both senses).
"...if I short the 100k in series with the signal, the distortion improves quite a bit. What is happenening here? "
The thing I noticed immediately about this circuit is that it looks like it derives from a valve circuit. The simple single-ended design, the big inductor, the use of 100k+ bias resistors and so on. And this is fine as long as compensation is made for the differences between FETs and valves. To my mind FETs are far more closely related to BJT than valves and come with a similar but different plethora of non-linearites.
All other things being equal (grounding, psu, etc in good shape) then I'd hazzard a guess that the distortion effect you observe is due to variation of *both* Cgs and Cgd, especially Cgd if you have very high voltage gain in the driver. Both capacitances vary quite a lot with voltage/charge. This is the ugly face of the FET. You may notice in BJT V gain stages that an auxilliary cap (say 20-50pF) is added from C to B and this is to swamp the BJTs variable capacitance. In general, BJTs have significantly lower parasitic capactiances than similarly rated FETs.
My advice would be to find out the nominal and likely variation of capacitance values for your FET and then check the circuit still works as you want it to.
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Old 1st October 2002, 12:33 PM   #44
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Hugh,
Would you explain how you calculated 50m-ohms output Z for the "current circuit" when using the CFP you suggested?
BAM
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Old 1st October 2002, 12:53 PM   #45
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Traderbam,

I built one with dual IRF mosfets top and bottom, using a current source as shown, and measured it into an 8R resistive load! Actually, it was 38 milliohms with a total stage current of 3A. I'd imagine with a single device it would be around 50 milliohms.

This is 1 impressive circuit......

Cheers,

Hugh

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Old 1st October 2002, 02:28 PM   #46
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Default resistance is futile

If you have made this circuit and measured it then that must be right. But my estimation for dc output resistance disagrees. It goes like this:

Let's say the CFP has a BJT with beta of 100, the GS resistor is 500-ohms and the Gm of the FET is 10. Educated guesses. Then this makes the beta of the CFB = 100*500*10 = 500k.
Now the driver stage has a collector loading of 100k. So the CFB is effectively fed from a 100k source. The output resistance is approximately the input resistance divided by the output stage beta = 100k/500k = 0.2 ohms. (This is slightly too low because the gm of the BJT hasn't been taken into account).

In the existing circuit (with no CFP) the output resistance is determined only by the Gm of the output FET, which, using my earlier estimate, would be 1/10 = 0.1 ohms.

So adding the CFP atleast doubles the dc output resistance.
Where have I gone wrong?
BAM
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Old 1st October 2002, 09:35 PM   #47
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Bam,

I'm referring to the diagram I posted on the board; not Graham's schematic.

And how come 50 milliohms is twice as much as 0.1 ohms??

Cheers,

Hugh
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Old 2nd October 2002, 02:30 AM   #48
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Default Off topic? Nonsense!

Quote:
Originally posted by moses
This is way off topic, but where did you get the images from Metropolis? Is there a digitilized copy of the movie floating around somewhere?
Metropolis is alway on-topic with me! I am the proud owner of a properly purchased Giorgio Moroder video, a CD soundtrack of same, also a video of a version with the crookest piano soundtrack you ever heard, a paperback screenplay, and text version of Frau von Harbou's original novel. I wish I could live in that movie sometimes...

GP.
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Old 2nd October 2002, 02:33 AM   #49
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Default Evra-buddy wants ta get inta de act...

Will you look at this.
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Old 2nd October 2002, 02:35 AM   #50
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Haha, I love the Betty picture. I'm afraid I have no idea how to get a copy of the movie on video, I've never seen it available. I would love to see the orginal, I've only seen the anime remake of it.
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