An externally hosted image should be here but it was not working when we last tested it.
The image above, is the schematic for of a simple fail-safe supply which may be used as a direct replacement for many of the Pass Zen/Aleph designs. DIY builders may find the over-current protection useful when setting-up. It can also limit the extent of any damage should a fault develop. A dual, balanced version has been successfully used with LTPZen and is used to eliminate the loudspeaker decoupling capacitors. See http://www.pmeweb.co.uk/audio/psu.html for details. Details also available in pdf format: http://www.pmeweb.co.uk/audio/psu.pdf
Interesting idea.
I designed and built a loudspeaker protection circuit/anti thump ciircuit which uses a relay to disconnect the speakers in the event of a fault. One of the fault conditions is excessive dc. For this I used a second order low pass filter driving an op-amp schmitt trigger. The circuit is relatively simple and it is easy to adjust the threshold values to suit the settling characteristic of the amplifier. You need to scope this first, but generally this will be a hump which rapidly settles to a near zero, followed by a much longer thermal drift to what should be dead-on zero if the amp is correctly trimmed. I hope to publish details of the circuit at a later date.
Instead of controlling an output relay, I don't see why you can't trigger the triac from such a circuit. This would also protect your speakers in the event of an excessive dc fault. The triac gate can be triggerd both internally or externally and operates as a NOR gate. That is the gate of the output mosfet and hence the PSU output goes low when either the internal or the extrnal trigger goes high. I know this will work because the triac gates on the dual fail-safe PSU used on LTPZen work this way; they can be triggerd by excessive currents in either rail.
I designed and built a loudspeaker protection circuit/anti thump ciircuit which uses a relay to disconnect the speakers in the event of a fault. One of the fault conditions is excessive dc. For this I used a second order low pass filter driving an op-amp schmitt trigger. The circuit is relatively simple and it is easy to adjust the threshold values to suit the settling characteristic of the amplifier. You need to scope this first, but generally this will be a hump which rapidly settles to a near zero, followed by a much longer thermal drift to what should be dead-on zero if the amp is correctly trimmed. I hope to publish details of the circuit at a later date.
Instead of controlling an output relay, I don't see why you can't trigger the triac from such a circuit. This would also protect your speakers in the event of an excessive dc fault. The triac gate can be triggerd both internally or externally and operates as a NOR gate. That is the gate of the output mosfet and hence the PSU output goes low when either the internal or the extrnal trigger goes high. I know this will work because the triac gates on the dual fail-safe PSU used on LTPZen work this way; they can be triggerd by excessive currents in either rail.
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
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
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.
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.
> 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
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
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
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
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.
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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