I drawn this schematic with my imagination yesterday. Seems rather promising for testing in real. It can provide 4 watts rms output with THD >5%,
kmtang,
That looks real interesting.
It is a Cascode amplifier that does not have any negative feedback, so the plate impedance of the 2A3 will be very large.
The secondary output impedance will be large, so the damping factor will be low.
Just run your simulation 2 more times . . . Once with 4 Ohms on the 8 Ohm tap; and once more with 16 Ohms on the 8 Ohm tap.
You will easily see the effect of high output impedance.
The system performance of the amplifier/loudspeaker combination . . . will largely be dependent on the loudspeaker impedance variations from 20Hz to 20kHz.
That looks real interesting.
It is a Cascode amplifier that does not have any negative feedback, so the plate impedance of the 2A3 will be very large.
The secondary output impedance will be large, so the damping factor will be low.
Just run your simulation 2 more times . . . Once with 4 Ohms on the 8 Ohm tap; and once more with 16 Ohms on the 8 Ohm tap.
You will easily see the effect of high output impedance.
The system performance of the amplifier/loudspeaker combination . . . will largely be dependent on the loudspeaker impedance variations from 20Hz to 20kHz.
After further analysis of the circuit, it has very strange behavior. The output AC current seems constant independent of the load. That means, the output power is proportional to the loading impedance.
Also tried to simulate the DC analysis of the circuit. The plate V/I transfer curve is just like a Tetrode.
Please don't ask me what is the reason. The hybrid cascode feature works the magic. I believe it is better to stay with the classic/conventional design which is the sure Win.
Also tried to simulate the DC analysis of the circuit. The plate V/I transfer curve is just like a Tetrode.
Please don't ask me what is the reason. The hybrid cascode feature works the magic. I believe it is better to stay with the classic/conventional design which is the sure Win.
The behavior isn't exactly "strange" once you realize that this is a transconductance amplifier rather than a traditional voltage amplifier.
Ignore the tube and the output transformer for a moment, what's left is a constant current source that is modulated by an input signal.
Nelson Pass uses similar circuits in some of his First Watt amplifiers where a high output impedance is considered a better match for fullrange drivers.
Ham radio guys would recognize your circuit as a grounded grid amplifier, quite common in HF contexts where the Miller capacitance can become very problematic.
Nothing wrong with this topology, it's just not very well suited for driving speakers that require any electrical damping unless negative feedback is added.
Comparing it to a tetrode is perfectly correct, a cascoded triode pretty much becomes a pentode.
Ignore the tube and the output transformer for a moment, what's left is a constant current source that is modulated by an input signal.
Nelson Pass uses similar circuits in some of his First Watt amplifiers where a high output impedance is considered a better match for fullrange drivers.
Ham radio guys would recognize your circuit as a grounded grid amplifier, quite common in HF contexts where the Miller capacitance can become very problematic.
Nothing wrong with this topology, it's just not very well suited for driving speakers that require any electrical damping unless negative feedback is added.
Comparing it to a tetrode is perfectly correct, a cascoded triode pretty much becomes a pentode.
The IRFP240 looks a bit heavy for 50mA. I did a quick search on Mouser, filtering for TO-220 / 247 packages, and low gate charge <20nC, and the IRL510 came up, which looks interesting. Gate capacitance is about 5 times smaller. You could buy a small collection of different MOSFETs and do listening tests.
The gain of such a setup is approximately gm(mosfet) * Z(load).
The high gm of the mosfet makes single stage possible.
But you essentially current drive the speaker.
Which reveals all its electrical and mechanical resonances.
So the sound can be expected to be dominated by the mosfet and the free-wheeling speaker, less that of the tube.
I tried a similar thing, but the driven element being a high gm tube with a transistor as the pass device.
The purpose was primarily to find out whether one of my spakers would like to be current driven.
Well, not really ...
The high gm of the mosfet makes single stage possible.
But you essentially current drive the speaker.
Which reveals all its electrical and mechanical resonances.
So the sound can be expected to be dominated by the mosfet and the free-wheeling speaker, less that of the tube.
I tried a similar thing, but the driven element being a high gm tube with a transistor as the pass device.
The purpose was primarily to find out whether one of my spakers would like to be current driven.
Well, not really ...
Thank you all for your excellent answers which help me understand the circuit. Surely, this circuit isn't good for audio amp at all.
It's an amplifier, but you'd mostly be listening to the sound of a MOSFET, which seems like a waste of a 2A3!
Use a P-channel MOSFET and you may have something more interesting...
Use a P-channel MOSFET and you may have something more interesting...
Yes, the loudspeaker impedance variations affects the cascode amplifier's effective frequency response . . .
But frequency response is also affected by the output transformer's primary inductance, and primary distributed capacitance.
The normal 800 Ohm plate resistance of a 2A3 is not available in a Cascode circuit, so it can not swamp out the impedance variations of the loudspeaker, nor the impedance variations of the output transformer.
No, I did not forget the frequency response effect of primary to secondary leakage inductance, but low plate resistance can not swamp out leakage inductance.
For me, the two major advantages of a 2A3 are:
The low plate impedance, rp, 800 Ohms
The lower order exponent, 3/2, of Childs law for Triodes (versus the higher order exponents of multi grid tubes).
Those two advantages are what makes a practical SE amplifier that does not need negative feedback in order to get good performance.
Two stage or One stage?
The Cascode circuit has often been called a 1 stage amplifier.
An amplifier with 2 active devices has often been called a 2 stage amplifier.
In any case, this amplifier uses 2 active amplifying devices.
Just my $0.03
Have Fun building a different circuit topology 2A3 amplifier that Does use the major advantages of the 2A3 for a great "sounding" amplifier !
Plus a 2A3 looks pretty!
But frequency response is also affected by the output transformer's primary inductance, and primary distributed capacitance.
The normal 800 Ohm plate resistance of a 2A3 is not available in a Cascode circuit, so it can not swamp out the impedance variations of the loudspeaker, nor the impedance variations of the output transformer.
No, I did not forget the frequency response effect of primary to secondary leakage inductance, but low plate resistance can not swamp out leakage inductance.
For me, the two major advantages of a 2A3 are:
The low plate impedance, rp, 800 Ohms
The lower order exponent, 3/2, of Childs law for Triodes (versus the higher order exponents of multi grid tubes).
Those two advantages are what makes a practical SE amplifier that does not need negative feedback in order to get good performance.
Two stage or One stage?
The Cascode circuit has often been called a 1 stage amplifier.
An amplifier with 2 active devices has often been called a 2 stage amplifier.
In any case, this amplifier uses 2 active amplifying devices.
Just my $0.03
Have Fun building a different circuit topology 2A3 amplifier that Does use the major advantages of the 2A3 for a great "sounding" amplifier !
Plus a 2A3 looks pretty!