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Old 25th February 2004, 08:44 PM   #1
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Default new hybrid amp turn on circuit

Hi -

I worked out a conceptual turn on circuit that looks like it might be promising for my upcoming 6 channel dc coupled hybrid amp circuit, using a newer type Form B (normally closed) optocoupled relay made by NEC.

For each channel, this amp will use complementary Toshiba power mosfets as followers and my plan right now is to use a 6k11 compactron tube with the two high mu sections operating as a long tailed differential amplifier driving the medium mu section as a common cathode amp biased at around 5 ma which will drive the mosfets outputs with bootstrapping. I plan to sample the voltage at a resistor at the input or driver stage across which a voltage will develop only when the tube warms up that will be further delayed through a lp rc time constant then to an inverting small signal transistor that drives the dual optocoupled relay (one each for the p channel follower and the n channel follower).

So, what should happen is that the optocoupled relay will hold the mosfets cut off until the compactron warms up and biases itself. No current should flow through the current sense resistor until this point in time. Then at some point several seconds later, depending on the rc delay chosen the output stage will turn on with the tube input and driver already fully warmed up.

I plan to use a dedicated choke input 5v supply for the transistor/opto drive circuit that is arranged to turn off very fast when ac power is removed with its output collapsing to essentially zero before the supply rails for the tube sections drop more than 10%. So, on turnoff, the drive is immediately removed from the optocoupled relay LEDs & the power mosfets should be cut off before supply decay causes the dc feedback loop to fail.

Any comments or suggestions?
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Old 25th February 2004, 09:13 PM   #2
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Yes! This a question that cries out for a diagram. Can you sketch a diagram and scan it?
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Old 26th February 2004, 02:23 AM   #3
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Hi, ec8010 -

I sketched the following up. Sorry about no values because I haven't finalized them yet. I may also use a zener string in the level shifter to get a little more open loop dc gain instead of a resistor divider if dc offset wanders too much, but since I plan to use this for HT rather than critical listening, it's not a primary issue. But I think I'll have easily accessible adjustment pots from the top of the chassis for offset and bias adjust.

A couple of other features I plan to add are a phantom center channel input and bridging mode for four of the six channels if I ever want to use it in biamp mode.
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Old 26th February 2004, 11:51 AM   #4
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I suspect that your proposed scheme will produce a switch-off thump. Another thing you might like to consider is that MOSFETs have a large (and changing) input capacitance. Still, if you've got lots of them, try it.
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Old 26th February 2004, 08:56 PM   #5
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That's true about the power mosfets' input capacitance changing (presumably you're referring to its increasing with decreasing Vds in this configuration). Because of this, it's serendipitous that the instantaneous slew rate needed to reproduce a sine wave decreases as you move away from its zero crossing. As a result, I expect to be able to get full voltage swing at the output past 20khz. I've had some experience with a somewhat similar circuit in the past which, if I recall, slewed at about 15V/us at the zero crossing. Stability didn't seem to be a problem then, but I'm going to try to get unconditional stability with this design.
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