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Old 22nd February 2007, 08:54 AM   #11
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This is an interesting circuit. However, I have a question and a couple of comments for you.

Why do you include a diode in parallel with the MOSFET body diode? The body diode is a bug or a feature depending on how the FET is used. In your circuit it appears to be a feature!

At say 2A, the FET will burn 10W and drop ~5V for a typical IR part, based on the datasheet curves. This seems to be a stiff penalty to pay for the joy of having a source follower Cap-Mult.

If the output voltage trapped on the output cap is greater than 20V; you may destroy the gate as the triac fires. This stems from the Vgs rating of +/- 20V for the IR version of this part. I see that a Fairchild version of this part is rated at +/-30V (but it may be obsolete).

Finally if you choose to address the power losses with a logic level gate drive part, then the Vgs specification becomes more of a problem as these parts are normally +/- 10V rated.

Maybe you can find some minor tweak for this circuit that minimizes these potential drawbacks.

Thanks for letting me see your circuit idea.
VSR
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Old 22nd February 2007, 09:40 AM   #12
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On remote triggering. You could use a pair of momenary acting switches, a push to make via a series resistance to the inut caps to trigger the gate into standby mode and a second, push to break to reset the triac and power up again. Else devise some logic to do the same. I think you may end with a rather harsh turn-on characteristic when you power back up. I prefer to power up and down via the mains switch.

Thanks to sherelec for pointing out a flaw. The diode across the output fet is to prevent it being reverse biased on tripping. It was left there from my first implementation which used darlingtons. There should be a diode connected from source to gate to protect the gate. I hadn't noticed a problem, because I use a dual supply with 20V rails. I will ammend the schematic accordingly.

You are correct abou the voltage drop across the mosfet and the power loss. However since we are talking about A class amplifiers, it is a drop in the ocean in comparison with the power lost in the amplifier output transistors. The power loss is still better than with a regulated version using zener diodes across the gate.

Lastly, I agree that Aleph amps should have have a preset quiescent current in the output transistors, of 2Amps or thereabouts, plus a Amp or so for the CSS signal and you so you should not have a problem. However the circuit is particularly useful for setting up and will limit the extent of any dmage should a fault occur.
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Old 22nd February 2007, 11:30 AM   #13
EUVL is offline EUVL  Europe
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> At say 2A, the FET will burn 10W and drop ~5V for a typical IR part

One could easily half that by using Japanese FETs, such as 2SK1530, 2SK2955, ..... especially the "O" type which has low Vgs (less than 2V at 2A).

I have yet to figure out why people are so fond of IR FETs other than that they are cheap. But then what % of the total cost of a Class A Amp is in the MOSFETS ?

My experience tells me that even if the FETs cost 5 times more, and you buy a hundred to match, it is still a small fraction compared to heatsinks and power supplies. And it is for me the best investment one can made to improve the sonic quality of the Amp.

And yes I use Japanese transistors these days almost exclusively.


Patrick
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Old 22nd February 2007, 01:53 PM   #14
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Quote:
Originally posted by EUVL


I have yet to figure out why people are so fond of IR FETs other than that they are cheap.

My best reason is that I need rather high Vgs.
When I bias the mosfet with resistor divider, I could better
secure the input impedance.


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Old 22nd February 2007, 02:09 PM   #15
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With the IRFparts shown, what is the maximum voltage and current that this circuit can supply? Any advantages in using it for class A/AB amps?

Thanks.
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Old 22nd February 2007, 02:23 PM   #16
EUVL is offline EUVL  Europe
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> When I bias the mosfet with resistor divider, I could better secure the input impedance.

Better than a current source and resistor or a low-noise Zener or voltage reference (the latter 2 for Cascode bias, e.g.) ?

Patrick
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Old 22nd February 2007, 02:49 PM   #17
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Quote:
Originally posted by EUVL


Cascode bias

Patrick

Cascode bias is of cource different.
The total potential diff between the upper and lower fets could be
high. Then, the R divider could have reasonable size wrt the
input impe. And, the low Vgs of the upper does not matter.

Just a small opinion!


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Old 22nd February 2007, 02:55 PM   #18
EUVL is offline EUVL  Europe
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I thought we were discussing the power supply circuit as posted.
In which case I do not understand your argument that a high Vgs is of advantage.

If you were discussing an amplifier circuit like Aleph-X, then the higher Vgs of the second stage allows you a slightly higher gain of the first stage (say 3dB), but that is easy enough to get around.


Patrick
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Old 22nd February 2007, 03:03 PM   #19
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Quote:
Originally posted by EUVL


I thought we were discussing the power supply circuit as posted.

Patrick

O, Sht! Sorry, Patric. I'm drunk.

Right! with zener reference, the Vgs is a no big deal.
Forgive me . . .


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Old 22nd February 2007, 04:32 PM   #20
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Hi Samuel etc,

The maximum current you can supply depends on the value you choose for the sense resistor ~ 0.6/Rs. I use a pair of 0.22, 3W wirewound resistors in parallel to trip a +/-20 V nominmal supply at about 5.5Amps. The output devices shown have a 200V Vds rating, so you could use the circuit to supply much higher voltages.

As sherelec pointed out, with higher voltages, the schematic needs ammending to include a Vgs clamp to reduce the risk of gate oxide rupture at the point of triggering.
I will try and do this soon.

The circuit has no -ve feedback and will have an output impedance equal to the inverse transconductance ~ 0.2 ohms. This isn't brilliant, but with A class amps, the PSU load currentvaries linearly with the audio signal and because of this resistance you will see a very small full wave audio signal imposed on the PSU output. Whether this matters or is audible depends upon the PSRR of the amp. I use the circuit with LTPZen which has a reasonable PSRR figure thanks to the differential input stage. However it should be fine with any of the Zen Aleph designs.
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