Tube OTL and Zotl amps
- Thread starter Mr. Moscode
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I agree but also, If the music is cooking it will transcend the medium. Dean Benedetti's wire recordings of Charlie Parker's live solos for instance.
Strickland uses the mosfets as amplifiers. Little wiggle in = bigger wiggle out (+14db). Moscode and Hafler use mosfets as followers - big wiggle in = slightly smaller (-1db) wiggle out. Totally different. Transnova is like a vacuum tube PP output, w/out the xfmer.
Never heard of “runaway” tubes ?
Mosfets may not suffer from that but still present a capacitive load depending on usage as mentioned.
If you want a fast amp you make sure that everything in the amp is able to keep up or better : exceed !
Best regards,
Frank
This is from the Zotl patent. It functions as an active DC resistance transformer allowing a tube with limited current capability to control a much larger amount of current. It is not Class D topology. There is no comparator to turn the audio into pulse width modulation. The clock F is fixed as is the pulse width. When the tube conducts the whole block it's attached to conducts through its current amplification factor. The actual speaker current goes through converter which is controlled by the loading of the tube. If the tube shorts the converter carries its max current, if the tube is open or off then the converter allows no current to flow through it.
The point is to reflect the transfer function of the power tube thru the converter to the speaker and reflect the load of the speaker thru the converter to the tube. The same as an iron core transformer but without the iron core limitations of parasitic capacitance, phase shift, core saturation, inductance and all the other xfmer downsides.
According to the reviews it does so quite well. However it is not a pulse width modulator class D amp.
This second circuit is a class D amplifier with a comparator and does not use the tube in the circuit that controls the speaker. All it can do is amplify what ever is fed to it and output that into the + input of the comparator. So if you have a really linear (perfect) tube circuit the output will amplify that. If the tube is non linear and distorts on its own the amp will output that. The load does not affect the tube character like the Zotl
In the first circuit the tube sees the load through the Zotl output circuit. In the second the tube is as relevant as tone or volume control. The amp will work without the tube in front of it.
That's how I understand the circuit.
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RE thermal runaway - a mosfet will do that if it is not well heatsinked.
Mosfet capacitive load is partially mitigated by it's own source to gate capacitance since the source follows the gate, bootstrapping the gate to some degree.
I also have a bootstrap bias circuit that raises the input Z to the mosfets allowing great bass extension. The cath follower is also bootstrapped on the output so it rarely sees much loading.
Is 10hz at - 0db to 250khz at - 3 db fast enough? So far no complaints.
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I thank you kindly. I hope Thor gets it. Really a brilliant design by Dave amplifying in the ether (meaning RF). Thanks for bringing this to my attention.
I don't know what the current amplification factor is but I imagine you would need some larger cores and wire for a tube that can pass 1400 ma like the PL509 or 6lf6. Dave's are built around lower current tubes.
You've built these. Can you share your schematic/ design or are you keeping it secret? My electric bills are out of sight right now thanks to Putin.
Thank you. I can't really measure below 10 hz on the bench. I have to look at phase shift to see the effects.
I now use double die mosfets to cut the ouput impedance in 1/2 and double the damping factor, also raises the power into 4 ohms. The power xfmer and heatsinks are the short term and long term power bottlenecks.
You can share my designs as long as my name is attached to them or mentioned, Beware that i am a hobbyist and therefore have the freedom to mix my design ideas with that of others that may be protected by copyrights.
I have glued some cores together to prevent saturation in case a PL519 or similar tube will be used.
Also you will need to parallel multiple converters to match the lower internal resistance of these tubes.
Best regards,
Frank
I have glued some cores together to prevent saturation in case a PL519 or similar tube will be used.
Also you will need to parallel multiple converters to match the lower internal resistance of these tubes.
Best regards,
Frank
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Depending on your ego or experience you could start with a headphone amplifier as i did. This way you can get the “feeling” of how everything interacts without the smell of evapurated silicon.
It is easy to get saturation of the cores in the beginning especially when things get hot.
So here is my advice: start with low voltages and work your way up even as an experienced technician.
Here an image of a headphone amp (NanoZotl, that name has hopefully not yet been claimed) with XLR inputs as i didn’t want to bother with any preamp or and phase inverters. So you need a symetrical signal source to drive it properly or add an input transformer (i did as an addition) But at least you can concentrate on the Zotl part.
Best regards,
Frank
It is easy to get saturation of the cores in the beginning especially when things get hot.
So here is my advice: start with low voltages and work your way up even as an experienced technician.
Here an image of a headphone amp (NanoZotl, that name has hopefully not yet been claimed) with XLR inputs as i didn’t want to bother with any preamp or and phase inverters. So you need a symetrical signal source to drive it properly or add an input transformer (i did as an addition) But at least you can concentrate on the Zotl part.
Best regards,
Frank
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I still have a professional interest in my circuits but I would share them with an agreement not to publish on the interwebs through a PM.
I've made plenty of silicon to charcoal converters but not so much any more. I test each sub circuit and triple check my connections. Biggest challenge is dislexia.
My Moscode 402 with double die mosfets can source +/- 42 amps. Are we into that territory?
What diodes are in the board, are they bridges in series? Do you have a global schematic? You could PM me if you want it private.
Is the amp with the 509s done? Output/impedance?
Can these run above the AM band -1700khz?
Thanks for your knowledge and kind words.
I've made plenty of silicon to charcoal converters but not so much any more. I test each sub circuit and triple check my connections. Biggest challenge is dislexia.
My Moscode 402 with double die mosfets can source +/- 42 amps. Are we into that territory?
What diodes are in the board, are they bridges in series? Do you have a global schematic? You could PM me if you want it private.
Is the amp with the 509s done? Output/impedance?
Can these run above the AM band -1700khz?
Thanks for your knowledge and kind words.
Please see PM,
The amp with the 509's is still under construction (i got ill with covid and still deal with the aftermath).
The oscillator runs on ca. 250Khz but the tube doesn't mind at all. I guess higher is fine too as these are used in RF transmitters as well.
But beware of switching losses of the mosfets.
Please check the schematics on David's website as these are very detailed.
Actually the patent is all revealing if you can decipher it.... and you proved that you could !
Best regards,
Frank
The amp with the 509's is still under construction (i got ill with covid and still deal with the aftermath).
The oscillator runs on ca. 250Khz but the tube doesn't mind at all. I guess higher is fine too as these are used in RF transmitters as well.
But beware of switching losses of the mosfets.
Please check the schematics on David's website as these are very detailed.
Actually the patent is all revealing if you can decipher it.... and you proved that you could !
Best regards,
Frank
No. The output stage is cap coupled to the driver K follower. It has its own floating bias supply and servo all on the output board. It all runs off the hi power windings on the power xfmer. The output and input of the tube circuit is fully muted during warm up and the mute relay to the output stage speeds the servo up during warm up. All muting is a short to ground so no audio goes thru relay contacts on play.
The load resistor of the driver K follower sits on the output creating a bootstrap and keeps the follower as almost a constant current circuit. It is directly coupled to the triode preceding it. The nfb is around the 2nd stage and output and is summed at the grid of the 2nd stage. The 1st and 2nd stages are fully cathode bypassed class A triodes.
The input tube filaments have and automatic switch over between 12 volt CT tubes or regular 6 volt filaments. Both sides have to have the same type of tube for this to work. This allows use of a variety of low to medium mu tubes. A tube roller's dream.
7 sec turn on inrush current limiting delay, 45 second mute for tubes and output stage to stabilize before playing, fold down front panel with interlock.
I use some Mundorff caps and some Vishay military super low noise Rs in the feedback loop for the deluxe version and double die mosfets for the super deluxe version. Imaging is big as a house. 150 watts idle power.
That's the way we do it, we do it, we do it!!
I hope that explains it clearly.
thor.zmt
Well-known member
thor.zmt
Well-known member
Precisely.
But with RF switching added.
No PWM, agreed. But still switching.
Are you sure that the tube does not drive the load?
But of course it does.
The "Class D" Amplifier operates as impedance converter or curent multiplier.
So the Tube "sees" the Speaker Impedance through the impedance multiplier, in effect like it would through a transformer. It can also use Linear amplification, in fact the earliest versions I worked with used linear amplification.
Thor
Thor: No PWM, agreed. But still switching.
Me: It doesn't matter
This question referred to a tube stage of any kind connected to the input of a class D amp, not the Zotl.
Yes. It has nothing to do with the Class D amp which will work with or without it.
But of course it does.
The "Class D" Amplifier operates as impedance converter or curent multiplier.
All amplifiers do. The example has nothing to do with tube. The only thing the tube is for is to A. let the owner think he has tube hifi, B. Do whatever the tube circuit does to the audio, warm it up, distort it or do next to nothing. You can bypass the tube stage and drive the D amp directly.
That said I stand by my analysis. Zotl and class D are 2 totally different animals.
I'm out.
