Looking for HV fets for direct drive - Page 2

bentoronto · 03:44 PM

I built a Sanders-like 8023 (??) tube amp to drive my Dayton-Wright 6-panel DIY speakers and later, to drive my off-the-shelf D-W XG8 speakers. B+ was 2400 and being direct connected, that reduced the negative bias I needed. I think the clean swing voltage was like 600vrms. BTW, DC-coupled all the way, if I recall. I drove the speakers roughly 140 Hz to 3500 Hz, with sharp crossover curves. It played plenty loud in a medium sized live room but not as loud as I wanted, now and then.

DANGEROUS.........

Best sound I ever made and after no small amount of fiddling with feedback loops, etc. the amp ran with no troubles for almost 20 years. I am convinced direct drive is audibly superior to anything using a step-up transformer. There are compelling reasons to manufacture ESLs with transformers but those reasons don't include striving for best quality sound.

It is conceivable that a half-way step-up would be a good compromise if you can't achieve the full swing you need to drive ESLs. More than 40 years ago, Mike Wright made such a tube amp coupled to his first speakers.

Remember, an amp with distortion anybody would consider weakish by today's standards is a perfect match for even great ESL speakers.

maudio · 07:12 PM

been there, done that... All the way up to that 16 mosfet cascaded output stage running from a 4000V power supply, putting out more that 7.5 kV.

The irfbg20 and BFC40 were the best options available at the time I worked on this (couple of years ago). Could be there are better alternatives available now. You need high voltage and low gate capacity, approx 2A is ideal.

Take into account that for fullrange you need approx. 25 mA per kV, for a decent output you need at least 8kV voltage swing (4 kV power supply and bridging). That requires a current capacity of 200mA ruling out class A (2*.2*4000 = 1600W dissipation at least, per channel!). You have only N-channel fets available so class B is almost impossible to construct in such a way that it is reliable and low-distortion.

In short: Don't do it. IMHO best option is an impedance compensated transformer in a feedback loop.

I summed it all up in this topic:
Another direct drive thread

hella356 · 7th October 2012, 07:28 PM

Have you considered an Acoustat Servo clone?

maudio · 7th October 2012, 07:51 PM

The Acoustat is a very compromised design. Nowhere near enough output current. Not very good sounding either.

Quote:

Originally Posted by madtom1999

When I find some more I'll just run a lot of spice sims and see what I can get to work best

Forget spice for high voltage mosfet applications. Reliability is zero. The internal capacities of MOSFETs are voltage (and even current) dependent. At these high voltages this effect is very pronounced. The models used for MOSFETS in spice do not account for that. My experience is that the real world and spice are truly two very different things

madtom1999 · 9th October 2012, 06:08 PM

Thanks for your input so far folks - getting more confused by the day!
maudio - there are a few new models around that may (or may not) account for these effects. I've found most manufacturers make an effort to get things right these days.

maudio · 9th October 2012, 07:25 PM

The problem is that you have to drive a capacitor. At low frequencies you need many kV's swing to get decent output. At high and midrange frequencies you need hundreds of mA current. Combining the whole spectrum in one amplifier means it has to provide both, making it impossible to use any class A (or single ended) design.

So the challenge of building a good direct drive amplifier really comes down to how to build a push pull output stage using only one polarity of devices that see the full voltage. So far I have not found a satisfactory solution for this without using a transformer. Very interested in how you get along... So good luck and keep us posted!

bentoronto · 9th October 2012, 07:33 PM

Quote:

Originally Posted by maudio

The problem is that you have to drive a capacitor. At low frequencies you need many kV's swing to get decent output. At high and midrange frequencies you need hundreds of mA current. Combining the whole spectrum in one amplifier means it has to provide both, making it impossible to use any class A (or single ended) design.

So the challenge of building a good direct drive amplifier really comes down to how to build a push pull output stage using only one polarity of devices that see the full voltage. So far I have not found a satisfactory solution for this without using a transformer. Very interested in how you get along... So good luck and keep us posted!

Ummm, drive a capacitor? All you need is to run a big clean voltage. I drove a large bunch of power resistors push-pull which had an ESL speaker (not too big as capacitors go) in parallel.

Ben

maudio · 9th October 2012, 07:45 PM

why would you place resistors in parallel? To make the load more resistive?

maudio · 9th October 2012, 07:49 PM

just noticed that the tread I linked above is not complete any more, it is truncated after page 2. No idea what happened to the rest. Maybe old topics are truncated to save server space?

bentoronto · 9th October 2012, 07:58 PM

Quote:

Originally Posted by maudio

why would you place resistors in parallel? To make the load more resistive?

I'm not sophisticated in these things, but I thought that was what you always wanted to do - drive a well-behaved resistor.

Ben

7th October 2012, 03:25 PM	#11
bentoronto diyAudio Member Join Date: Jan 2008 Location: Toronto and Delray Beach, FL	I built a Sanders-like 8023 (??) tube amp to drive my Dayton-Wright 6-panel DIY speakers and later, to drive my off-the-shelf D-W XG8 speakers. B+ was 2400 and being direct connected, that reduced the negative bias I needed. I think the clean swing voltage was like 600vrms. BTW, DC-coupled all the way, if I recall. I drove the speakers roughly 140 Hz to 3500 Hz, with sharp crossover curves. It played plenty loud in a medium sized live room but not as loud as I wanted, now and then. DANGEROUS......... Best sound I ever made and after no small amount of fiddling with feedback loops, etc. the amp ran with no troubles for almost 20 years. I am convinced direct drive is audibly superior to anything using a step-up transformer. There are compelling reasons to manufacture ESLs with transformers but those reasons don't include striving for best quality sound. It is conceivable that a half-way step-up would be a good compromise if you can't achieve the full swing you need to drive ESLs. More than 40 years ago, Mike Wright made such a tube amp coupled to his first speakers. Remember, an amp with distortion anybody would consider weakish by today's standards is a perfect match for even great ESL speakers. __________________ Dennesen ESL tweets, Dayton-Wright ESL (110-3200Hz), mixed-bass Klipschorn w/param EQ plus giant OB using 1960's Stephens woofer HiFi aspirations since 1956 Last edited by bentoronto; 7th October 2012 at 03:44 PM.

7th October 2012, 06:59 PM	#12
maudio diyAudio Member Join Date: Apr 2006 Location: Netherlands	been there, done that... All the way up to that 16 mosfet cascaded output stage running from a 4000V power supply, putting out more that 7.5 kV. The irfbg20 and BFC40 were the best options available at the time I worked on this (couple of years ago). Could be there are better alternatives available now. You need high voltage and low gate capacity, approx 2A is ideal. Take into account that for fullrange you need approx. 25 mA per kV, for a decent output you need at least 8kV voltage swing (4 kV power supply and bridging). That requires a current capacity of 200mA ruling out class A (2.24000 = 1600W dissipation at least, per channel!). You have only N-channel fets available so class B is almost impossible to construct in such a way that it is reliable and low-distortion. In short: Don't do it. IMHO best option is an impedance compensated transformer in a feedback loop. I summed it all up in this topic: Another direct drive thread Last edited by maudio; 7th October 2012 at 07:12 PM.

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7th October 2012, 07:28 PM	#13
hella356 diyAudio Member Join Date: Feb 2009	Have you considered an Acoustat Servo clone?

9th October 2012, 06:08 PM	#15
madtom1999 diyAudio Member Join Date: Oct 2012	Thanks for your input so far folks - getting more confused by the day! maudio - there are a few new models around that may (or may not) account for these effects. I've found most manufacturers make an effort to get things right these days.

9th October 2012, 07:25 PM	#16
maudio diyAudio Member Join Date: Apr 2006 Location: Netherlands	The problem is that you have to drive a capacitor. At low frequencies you need many kV's swing to get decent output. At high and midrange frequencies you need hundreds of mA current. Combining the whole spectrum in one amplifier means it has to provide both, making it impossible to use any class A (or single ended) design. So the challenge of building a good direct drive amplifier really comes down to how to build a push pull output stage using only one polarity of devices that see the full voltage. So far I have not found a satisfactory solution for this without using a transformer. Very interested in how you get along... So good luck and keep us posted!

9th October 2012, 07:45 PM	#18
maudio diyAudio Member Join Date: Apr 2006 Location: Netherlands	why would you place resistors in parallel? To make the load more resistive?

9th October 2012, 07:49 PM	#19
maudio diyAudio Member Join Date: Apr 2006 Location: Netherlands	just noticed that the tread I linked above is not complete any more, it is truncated after page 2. No idea what happened to the rest. Maybe old topics are truncated to save server space?