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Old 21st May 2003, 02:47 PM   #1
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Default simple hybrid idea

I have an idea to use a differential pair of P-channel MOSFETs to bias and drive a pair of push-pull 6L6's running class A. I was wondering if anyone had seen this done anywhere. Think it will work? Think it would sound ok?
Steve
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Old 21st May 2003, 02:57 PM   #2
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Default The hibrid amp...

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Think it will work?
Yes...me too!!

but the capacitor C1 must be connected in parallel with zener and not as in your schematic...as if it is connected as in the schematic you will ended with hum at the output!
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Think it would sound ok?
I hope so!!

cheers
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Old 21st May 2003, 03:20 PM   #3
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Yes, you think it will work, or yes, you had the same idea?
I don't understand why the capacitor where its at would be a problem. It is bypassing the current source mosfet's gate to ground. Maybe I should add another cap across the zener?
Thanks for the reply... there dosen't seem to be a lot of interest.
Steve
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Old 21st May 2003, 03:35 PM   #4
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Default what's is bypassing what...

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I don't understand why the capacitor where its at would be a problem. It is bypassing the current source mosfet's gate to ground.
Yes but the source of the Q3 mosfet is referenced to the + 40volts rail...so the bypassing capacitor must be connected to the same rail for the mosfet doesn't "see" the hum voltage...and don't amplifie it

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Old 21st May 2003, 03:38 PM   #5
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Thanks... I see it now.
Steve
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Old 21st May 2003, 04:01 PM   #6
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As shown, you are DC coupled to the 6L6, which is nice. But you have no way of knowing whether your anode currents are out of balance and causing transformer core distortion, or of correcting imbalance. If you use separate 1R cathode resistors, you can detect imbalance, and you could add a servo feedback loop for correction, or add a manual tweak.

What advantage do you expect to gain from your proposed circuit over traditional glass driver circuitry?

Usually, hybrids go the other way, with glass to drive MOSFETs in a desperate effort to get rid of the output transformer.
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Old 21st May 2003, 04:14 PM   #7
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As shown, you are DC coupled to the 6L6, which is nice. But you have no way of knowing whether your anode currents are out of balance and causing transformer core distortion, or of correcting imbalance. If you use separate 1R cathode resistors, you can detect imbalance, and you could add a servo feedback loop for correction, or add a manual tweak.
good idea... I was planning on adding a balance pot on the differential current source to give a little bit of an adjustment for balance. I think when I simulated it before, I used a 100 ohm with the wiper attached to the current source and the legs attached to each mosfet. This seemed to correct for DC imbalances, but AC is another problem. Any ideas?
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What advantage do you expect to gain from your proposed circuit over traditional glass driver circuitry?
Well, I guess the appeal is 1 gain stage in the front end with a fairly low output impedance. If I could find a way to balance it, I could drive multiple pairs of tubes with a lower impedance output transformer for more power without an additional driver circuit. 2 gain stages total- short signal path. Also, no caps in the signal path (not that it really matters, but it's kind of cool
)
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Usually, hybrids go the other way, with glass to drive MOSFETs in a desperate effort to get rid of the output transformer.
Yeah, I've looked into doing that, but the circuit gets really complex when you have to drive the mosfet gate capacitance (white cathode follower or something similar)... I don't really see the point. If you have the right speakers, transformer coupling isn't necessarily that bad of a thing.
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Old 21st May 2003, 04:26 PM   #8
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ok... version 2 with the good suggestions above included.
-added individual anode resistors to check for balancing.
-added bias balance pot
-corrected misplaced zener filter cap
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Old 21st May 2003, 04:27 PM   #9
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Yes, the 100R source resistor will do the DC trick.

Do you know how much gm you are expecting, and thus how much gain? AC balance might not be a problem...

I wonder what the input capacitance of your gain stage looks like?

Most people object slightly to output transformer defects, and a lot to how much they have to pay for them. That seems to be the main incentive for MOSFET output stages. As you say, the large, and changing input capacitance is a pain.
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Old 21st May 2003, 04:28 PM   #10
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oops... here's the image
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