Simple Class A Hybrid MOSFET Source Follower

I have bread boarded this simple circuit and found it rather nice sounding.
I proceeded to do the second channel and hooked it up to my ZU's.
It must be my ears getting old, but I am so pleased with the sound that I will make a proper build of it.:p
 

Attachments

  • Hybrid-1.PNG
    Hybrid-1.PNG
    76.6 KB · Views: 2,615
I would suggest replacing the resistor source load with a choke and reduce the voltage of the power supply . For the driver stage , you need a much higher HT rail and a much higher value load resistor . 6922 probably has far too much gain , look at some lower mu valves such as 12B4A or 6AH4 . I currently run a similar hybrid but I use RD16HHF1 RF MOSFETS which have low capacitance . Current driver is a type 26 which has mu of 8 , if your speakers are efficient like mine , this is all you need up front for enough system gain

316A
 
I would suggest replacing the resistor source load with a choke and reduce the voltage of the power supply . For the driver stage , you need a much higher HT rail and a much higher value load resistor . 6922 probably has far too much gain , look at some lower mu valves such as 12B4A or 6AH4 . I currently run a similar hybrid but I use RD16HHF1 RF MOSFETS which have low capacitance . Current driver is a type 26 which has mu of 8 , if your speakers are efficient like mine , this is all you need up front for enough system gain

316A
 
An inductor will double the efficiency, but at much higher cost - in Australia, I had two built, each of 100mH 0.47 DCR with 0.5mm gapping and GOSS for $250 EACH. I'm trying for 45W into 8R from 4A quiescent and 36V rails, 144W dissipation from each channel.

A 6922 has a rp of 2750 and mu of 33. They run at around 10mA and 100V, and the plate load should be around 3.5 times the rp for best sound, approx. 10k and a total B+ of 200V and an Av of 26, or 28dB, about right. Clearly the mosfet should be run from a separate power supply...... but this is hybrid, so you would expect this.

We are using the mosfet as source follower, so it's high Ciss is not too relevant because it is effectively bootstrapped, particularly if an inductor is used. I would use a simple hexfet with rating to 280W and at least 100V. Should be high gm too, so should be just fine.

Stephan, you are doing well.........

Hugh
 

Ranchu32

Member
2013-04-29 11:48 am
Interesting little circuit, I would like to hear what it sounds like...
The supply needs to be well regulated otherwise it will hum like crazy.
Personally I would give it a fighting chance by replacing the source load resistor (a sizeable component) with an active load. Spend another MOSFET!
 
Interesting little circuit, I would like to hear what it sounds like...
The supply needs to be well regulated otherwise it will hum like crazy.

The sound is as good as my other class A efforts and beats my AB builds so far.
The power supply I am using for the bread board is a simple CRCRCRCRCRC.:confused:, Ripple is approximately 60mV. Sitting back in the listening position I can hear no noise through my ZU Soul (99db).

6922 probably has far too much gain

316A

I was looking for more voltage gain in the driver stage as I was planing to use the MOSFET to add current.
Might be a wrong though process but to me this is all a big learning curve.

I would like to add here, that I always marvel at the knowledge you guys display on this forum in general. I have been coming here for some time now and I do a lot of reading and trying to understand. Within the confine of my limited knowledge and being a hand on person I just like to put stuff on the breadboard and take it from there.
Thanks also to Hugh from AKSA for your input. I hope to be able to listen to one of your amplifiers one day soon.
 
Thanks for sharing photo - it gives me a great view of how complex/difficult something like this might be to undertake. I am totally ignorant when it comes to tubes/valves so can you give more details of ancilliary equipment/components needed to fire up that single tube? (small transformer and diode bridge/caps)

I like how simple it is - what kind of power can you expect to get out a SE class A like this with a single IRFN540 at 54v? I have a +/-53v PSU (SMPS Abletec) that might be a perfect candidate for powering this.

Besides lightbulb, a hotplate coffee cup warmer might be useful to keep your team warm. :)
 
Thanks for sharing photo - it gives me a great view of how complex/difficult something like this might be to undertake. I am totally ignorant when it comes to tubes/valves so can you give more details of ancilliary equipment/components needed to fire up that single tube? (small transformer and diode bridge/caps)

I like how simple it is - what kind of power can you expect to get out a SE class A like this with a single IRFN540 at 54v? I have a +/-53v PSU (SMPS Abletec) that might be a perfect candidate for powering this.

Besides lightbulb, a hotplate coffee cup warmer might be useful to keep your team warm. :)

I use a small multi tap transformer that has got a 6.3V tap for the tube heater. The 6922 does not require DC for heating. You might just have to watch the current.

I would like to think the power is somewhere around the 5 watt. This is plenty for all my speakers (93db, 94db and 99db). If one takes into consideration that the average listening level is in the 80db region, amp power is overrated imho.

I am not sure how quiet you can keep the power supply with an SMPS, but if that is all you have for now, might as well give it a crack!
 
This sounds promising and fun! I think I will give it a go. How much current is needed for the heater? Could 1 amp from a 7805 regulator suffice? Besides tube heater, are any HV's required for grid, etc? Also, is the output MOSFET part number critical (I havean IRFP240 on hand)? I have a new trafo that just arrived, rectified and filtered should give me about 49v - probably can work just fine. Just need to find myself some old style lightbulbs for the resistor load.

Just need to order the tube... btw, just did a search - those 6922's ain't cheap! :) Is there a cheaper alternative?
 
Last edited:
Could 1 amp from a 7805 regulator suffice? Besides tube heater, are any HV's required for grid, etc? Also, is the output MOSFET part number critical (I havean IRFP240 on hand)?

Just need to order the tube... btw, just did a search - those 6922's ain't cheap! :) Is there a cheaper alternative?

A 7806 regulator would do, current requirement is only about 300mA.
Just keep in mind your max (35V) and min (8.5V) voltage to supply the regulator.

No HV required. Bias is done via the 1meg resistor on the grid (grid leak bias).

I could not see a problem using the IRFP240.

Enjoy and happy playing around and experimenting!
:confused::wrench:
 
Given that the vacuum tube valve implementation costs a pretty penny - is there a solid state alternative or will all require multiple stages of transistors and we are back to something like 4 transistors to get enough current gain? It seems the mosfet, being a FET should require no current at all but only voltage - which could be provided for example with implementing a JFET on the input like 2SK170?

I still would like to pursue an all solid state version but just found the 6922 for $16/pair shipping included. (Chinese brand of course for this price).
 
Last edited:
No SS device will sound like a simple tube...... this is why tube are still used for very high end audio amps. Running up to 200V and a 6V filament is not too difficult if you scout around for a surplus amp, or mix 'n match..... eg two trafos mains to 8-24Vac secondaries and back to back will give you high voltage at low current; a couple of 12VAs will do it. 120Vac then rectified and smoothed gives you 170Vdc.
 
No SS device will sound like a simple tube...... this is why tube are still used for very high end audio amps. Running up to 200V and a 6V filament is not too difficult if you scout around for a surplus amp, or mix 'n match..... eg two trafos mains to 8-24Vac secondaries and back to back will give you high voltage at low current; a couple of 12VAs will do it. 120Vac then rectified and smoothed gives you 170Vdc.

Thanks for tip on HV generation with the back to back small VA trafos. So for this particular tube, I need the 6v filament heater and the V1 voltage shown in the diagram in Post 1 requires either the two back to back trafos to get 200v or alternatively rectifying 120vac to get 170v DC is also required to get the tube to operate properly?
 
Thanks for tip on HV generation with the back to back small VA trafos. So for this particular tube, I need the 6v filament heater and the V1 voltage shown in the diagram in Post 1 requires either the two back to back trafos to get 200v or alternatively rectifying 120vac to get 170v DC is also required to get the tube to operate properly?

I did add a 68R cathode resistor to the 6922 and changed the plate resistor to 33K to tidy up the sine wave a bit and at the same time increased the gain.
However, I am tempted to increase the plate voltage to where it should be as I find that I can not get a nice output on the CRO.
But like always, the ear has got the last say. And it was not disappointed with the low voltage set up. :)

Thanks Hugh for reminding me about the back to back trany option, I was looking in to voltage doubling.