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Old 4th December 2009, 07:46 AM   #21
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by AKSA View Post
Very impressive work, Elvee, thank you! Can you do something with 50V rails?

Hugh
Hi AKSA,

I did all of the simulation at 20V, and most of the physical work at between 15 and 25V, approximately. Upgrading it to 50V shouldnt pose major difficulties, and I just made a quick test on the sim: I changed the 20V supplies to 50V ones, and nothing else (except the input level). As you can see, the result is rather encouraging: the linearity has degraded by just 2ppm, mainly second harmonic, probably caused by the onset of an asymetrical clipping.
The quiescent current has increased to ~300mA, which is too much obviously, but these are rather minor issues.
Of course, in the real world, the transistors would have to be upgraded to more powerful, 120V or 150V types, but this normal.
I will think about it, anyway.

The comment of Panomaniac also sparked me into thinking of a full-N, high voltage (+/-700V) version:

Quote:
Quote:
Originally Posted by panomaniac
As good as it is on difficult loads, wonder how it would do driving electrostatic panels? Should do well.
It should do well, but in this version, a transformer would still be necessary.
But it would also be possible to "unfold" the cascode, in order to use only high voltage/N devices in the driver and output stages. This should make a direct drive possible.
I think I'll be able to come up with something viable.
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Old 4th December 2009, 08:08 AM   #22
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by sakis View Post
is there anyone that can answer me a question regarding induction heating as seen in the example of the OP ????? please ????


the question is:
if a circuit like that consuming so litle power can make a screw actually glow why is there no one so far designed some device that has 4-5 similar coils and 4-5 similar screw or other metal parts, that your home water supply runs through this device cold at the input and boiling hot at the output ?????

i have seen various applications on the internet but nothing like that ...

is there any darwbacks ???? or something i ve missed ???
What makes you think the circuit was "consuming so little power" while doing that?
In fact, the amplifier was run at full power, and everything went hot like hell in a matter of seconds: the heatsink, the resonance capacitors, and the coil, whose enamel caught fire almost immediately.
Inductive heating is not new, but in general, a special power driver is required:
Google Image Result for http://www.inductionatmospheres.com/jpg/fusing_top.jpg
And the coil is normally water-cooled.
But achieving a similar result with a general-purpose, low distortion amplifier is a bit unusual.
Don't try this with your favorite amplifier....
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Old 4th December 2009, 08:43 AM   #23
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How sensitive is the amp to different parts?

I would like to build this amp, but i dont have the different transistors.
Can i build the same circuit but with BC556B/BC546B and IRFP250?? This is what i have at home at the moment.

This looks like it would make a very stable and powerful subwoofer-amp.
Something with 80 volt rails and 20 IRFP250 on the output?!
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Old 4th December 2009, 09:59 AM   #24
godfrey is offline godfrey  South Africa
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Hi Elvee

When you increase loop gain to reduce distortion, doesn't that hurt the stability margin? I remember thinking, when I saw your first circuit that it's stability into obscene loads was probably largely due to few gain stages and relatively low loop gain.

I would have thought that for heating screws, injecting noise into power lines etc, bullet-proof stability would be more important than ultra-low distortion? Or has this circuit gotten a wee bit side-tracked from strictly work-related uses?

On an unrelated note: what's with the 10k resistor connected to the emitter of Q9? Looks like it gives some semi-local positive feedback, but only when M2 is conducting.

Cheers - Godfrey
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Old 4th December 2009, 12:13 PM   #25
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by Circlomanen View Post
How sensitive is the amp to different parts?

I would like to build this amp, but i dont have the different transistors.
Can i build the same circuit but with BC556B/BC546B and IRFP250?? This is what i have at home at the moment.

This looks like it would make a very stable and powerful subwoofer-amp.
Something with 80 volt rails and 20 IRFP250 on the output?!
In general, it is pretty tolerant to most parts variations, but you have to be more careful with the components defining the quiescent current, especially for the simpler, non-servoed version. They include the threshold shifter reference D2, and the PNP VAS Q3: a larger area transistor will generate a higher common circlo bias current, hence a higher Iq.
In some sim versions Q3 is a 2N2905, and other use a "synthetic" PNP version of the 2N3019, and there are minor adaptations to accomodate differences. Physical versions use mostly the 2N2905.
The voltage of D2 and the common circlo bias current have to harmonize with the threshold voltage of your MOS transistors; some recalculations, simulations and tests will probably be necessary for optimum results with widely different devices.

BC546/556 are OK.

The 80V rails shouldn't be a problem, but if you build a "big" version of this amplifier, it is probably a good idea to have a less primitive reference for the current sources than D3/D4: either some conventional CCSs, or something like Q8 in the AB6 version will be more stable wrt supply variations.

Paralleling a large number of output devices would probably require a modification to the frequency compensation, due to the much larger input capacitance.
There too, I recommend you make some simulations and tests.

Also note that this amplifier has a wide bandwidth, and you cannot afford a sloppy PCB design. Another possibility is to "clip its wings", i.e. include generous gate stoppers, increase the frequency compensation, etc.
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Old 4th December 2009, 12:29 PM   #26
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Thanks for all the good info. I think this is a very clever way to use only N-fets in a pushpullamp. Very simple but clever.

I have been drawing variations of your amp for a couple of hours now, and i realy like it. (drawing variations is my why of learning about and working out what everything does in the circuit)

I only have BD139 and BD140 as possible replacements for Q1 and Q3.

I think i will have to try... Some magic smoke is a great way to learn.
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Old 4th December 2009, 12:51 PM   #27
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by godfrey View Post
Hi Elvee

When you increase loop gain to reduce distortion, doesn't that hurt the stability margin? I remember thinking, when I saw your first circuit that it's stability into obscene loads was probably largely due to few gain stages and relatively low loop gain.

I would have thought that for heating screws, injecting noise into power lines etc, bullet-proof stability would be more important than ultra-low distortion? Or has this circuit gotten a wee bit side-tracked from strictly work-related uses?

On an unrelated note: what's with the 10k resistor connected to the emitter of Q9? Looks like it gives some semi-local positive feedback, but only when M2 is conducting.

Cheers - Godfrey
Hi Godfrey,

The higher gain version may not be as bullet-proof as the simple one, but it still does a decent job.
Here is an example, only slightly different from the AB6: it uses a bjt as a driver, and is shown driving a 470nF capacitor directly.
The schematic is a bit fuzzy, because parasitics are included.
As you can see, it it is relatively well-behaved (OK, not perfect, but it is a 100KHz square wave).
This test has been made for real, and the result is quite similar, but the amplifier blows after seconds, due to excessive dissipation in the MOS, not instability.
This version uses a nested compensation which improves stability.

And BTW, heating screws was not included in the original requirements: the idea came along the way.

The 10K resistor compensates for the slight asymetry caused by the current-sensing resistor R4, and it lifts the gain when M2 is conducting, restoring the symetry.
It also compensates for the unbalanced zero-signal circlo driver current, made necessary by the threshold shifting circuitry.
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Old 4th December 2009, 03:58 PM   #28
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Quote:
Originally Posted by Elvee View Post
What makes you think the circuit was "consuming so little power" while doing that?
In fact, the amplifier was run at full power, and everything went hot like hell in a matter of seconds: the heatsink, the resonance capacitors, and the coil, whose enamel caught fire almost immediately.
Inductive heating is not new, but in general, a special power driver is required:
Google Image Result for http://www.inductionatmospheres.com/jpg/fusing_top.jpg
And the coil is normally water-cooled.
But achieving a similar result with a general-purpose, low distortion amplifier is a bit unusual.
Don't try this with your favorite amplifier....
WELL ....actually i did try that with a pro amplifier but due to the normal reasons i only used a 10 khz frequency on any available crossover coil and i did managed to make a screw warm .... so it is working ....

i ve seen on the internet huge bolts glowing powered from a 9 v battery ...efeciency is there with this topology ...still i dont know why not in a home appliannce
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Old 4th December 2009, 09:25 PM   #29
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"...still i dont know why not in a home appliannce"

Doesn't induction stove count?
Very efficient and fast. The drawback is that the pot needs to be ferrous.
Iron is nice here.

Back on topic.
Why did Circlomanen have to link this, now I'm interested too =)
Adapted to IRF240 (TO3) perhaps...
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Old 4th December 2009, 10:18 PM   #30
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Quote:
Back on topic.
Why did Circlomanen have to link this, now I'm interested too =)
Because im evil!!!!

No, this is a very intriging amp. I have dug out all the stuff from my attic.
I have a lots of IRFP260 and one IRFP250. I guess i have to try the IRFP260.
All i need to buy is some zeners and some 1 ohm power-resistors.

Is the amp sensitive to noise from the powersupply???

Im going to try some gatestoppers. 100 ohms or so. I hope that will be a little more forgiving to my layout and the stability. I dont need full power clean squarewave at 100 kHz. Maybe a single transistor input. I dont like LTPs very much. They are usualy not very stable.
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