Here’s a design inspired by Arthur R. Bailey’s 1968 Wireless World article “The Bailey 30-30”. The intro to the article had a little picture of a bipolar transistor power stage driven by an interstage transformer. I’ve also seen bits and pieces of this type of design on John Broskie’s excellent TubeCad website.
However, JRB isn’t a fan of interstage transformers, and I’m expecting a lot of people on this forum aren’t either. Hold back the knee-jerk reaction when you see this topology!
I ordered a bunch of iron for my new tube amp. My Lundahl LL1660S interstage transformers showed up early, and I still have five weeks to wait for my Plitron output transformers. What to do, what to do?
Use MOSFETs in the interim, of course! Using my 6C45pi tubes to drive my interstage transformer, I can prototype the design in the schematic below. Don’t concentrate on the tube part; look at the transistor output stage.
Why would I do this instead of heading over to the Pass forums? For a big reason: a push-pull topology can have much higher efficiencies than the Pass designs. This amp can work in class A or AB, and it doesn’t have to heat my whole house to do it.
Should I waste my time with an interstage transformer? Maybe, for a number of reasons:
(1) Use the tube to do what a tube function – voltage gain. Concentrate using the transistors for a transistor’s meaning of life - current gain.
(2) Excess gain in the tube stage can be reduced through the transformer’s winding ratio, therefore, reducing the effect of the transistor’s gate capacitances.
(3) Two N-channel FETs can be used, getting rid of P-channels.
This amp is not complete. The bias may be difficult to tune as the amp heats and cools. Any bias mismatch between the top and bottom transistor will cause a DC offset at the output. In the Zen version 5 (push-pull Zen), Nelson Pass uses a potentiometer to adjust bias current of the two devices. John Broskie has an example using an op-amp in a servo loop. Both of these examples are conventional push-pull with N-channel on top and P-channel on bottom.
This design may work with a potentiometer to fine-tune the bias, but an op-amp servo loop would probably be the best worry-free solution. Some safety features must be looked at, also. If one of the FETs fail, the output slams to a rail, potentially destroying the loudspeaker. This is where a good feature of the Zen amps comes into play. If the current source FET fails, the amp just doesn’t work. If the amplification FET fails, then the current source just shunts to ground. Either way is safe.
Also note there is no feedback in the output stage. Some local feedback may be useful, but I’d steer away from global feedback in this amp.
So, I’m asking you to resist the knee-jerk reactions, and objectively look at the topology. This is not a thread to debate the evils/advantages of interstage transformers and/or tube driver stages, there’s probably a better place for that debate…
However, JRB isn’t a fan of interstage transformers, and I’m expecting a lot of people on this forum aren’t either. Hold back the knee-jerk reaction when you see this topology!
I ordered a bunch of iron for my new tube amp. My Lundahl LL1660S interstage transformers showed up early, and I still have five weeks to wait for my Plitron output transformers. What to do, what to do?
Use MOSFETs in the interim, of course! Using my 6C45pi tubes to drive my interstage transformer, I can prototype the design in the schematic below. Don’t concentrate on the tube part; look at the transistor output stage.
Why would I do this instead of heading over to the Pass forums? For a big reason: a push-pull topology can have much higher efficiencies than the Pass designs. This amp can work in class A or AB, and it doesn’t have to heat my whole house to do it.
Should I waste my time with an interstage transformer? Maybe, for a number of reasons:
(1) Use the tube to do what a tube function – voltage gain. Concentrate using the transistors for a transistor’s meaning of life - current gain.
(2) Excess gain in the tube stage can be reduced through the transformer’s winding ratio, therefore, reducing the effect of the transistor’s gate capacitances.
(3) Two N-channel FETs can be used, getting rid of P-channels.
This amp is not complete. The bias may be difficult to tune as the amp heats and cools. Any bias mismatch between the top and bottom transistor will cause a DC offset at the output. In the Zen version 5 (push-pull Zen), Nelson Pass uses a potentiometer to adjust bias current of the two devices. John Broskie has an example using an op-amp in a servo loop. Both of these examples are conventional push-pull with N-channel on top and P-channel on bottom.
This design may work with a potentiometer to fine-tune the bias, but an op-amp servo loop would probably be the best worry-free solution. Some safety features must be looked at, also. If one of the FETs fail, the output slams to a rail, potentially destroying the loudspeaker. This is where a good feature of the Zen amps comes into play. If the current source FET fails, the amp just doesn’t work. If the amplification FET fails, then the current source just shunts to ground. Either way is safe.
Also note there is no feedback in the output stage. Some local feedback may be useful, but I’d steer away from global feedback in this amp.
So, I’m asking you to resist the knee-jerk reactions, and objectively look at the topology. This is not a thread to debate the evils/advantages of interstage transformers and/or tube driver stages, there’s probably a better place for that debate…
Attachments
Hi Kashmire,
The circuit should work, thought you need to be careful with the biasing divider design othewise the power supply and temperature variations will change the idle current dangerously
. I've seen this topology published before, about 15 years ago, with a transistor driver stage and MOSFET's on the output, with good results achieved.
I always was particularly interested in a PP design using same transistors on the output - my very first proper amplifier was a JLH class A circuit. Have a look here for my own design with N-channel MOSFETS push-pull class AB output stage:
http://www.diyaudio.com/forums/showthread.php?threadid=14320
(not using a transformer - probably not useful for you right now )
Cheers
x-pro
The circuit should work, thought you need to be careful with the biasing divider design othewise the power supply and temperature variations will change the idle current dangerously
. I've seen this topology published before, about 15 years ago, with a transistor driver stage and MOSFET's on the output, with good results achieved.I always was particularly interested in a PP design using same transistors on the output - my very first proper amplifier was a JLH class A circuit. Have a look here for my own design with N-channel MOSFETS push-pull class AB output stage:
http://www.diyaudio.com/forums/showthread.php?threadid=14320
(not using a transformer - probably not useful for you right now
)Cheers
x-pro
Run the bottoms of the driver transformer secondaries to the drains instead of to the sources. This will give the output stage a gain of ~1 as it will be a pair of source followers, with all the usual advantages.
Shift the lower end of the upper fet bias resistor from earth to the upper fet source. The supply to the top end of the top bias resistor should have it's negative end to the top fet source as well.
Shift the lower end of the upper fet bias resistor from earth to the upper fet source. The supply to the top end of the top bias resistor should have it's negative end to the top fet source as well.
Thanks for responding to the thread. I'm going to cook up a schematic with Circlotron's suggestions. Circlo's right: I made a mistake by not drawing the amp as source follower.
For now, I'll address the feedback. I've had success using interstage transformers in tube amps. Usually I'll include local feedback at the driver stage to incorporate the interstage into a feedback loop. There's some tricks with tubes that naturally take the transformer into account.
I perfer local feedback with my tube amps - the driver stage has its own feedback that takes the interstage into account, and the output stage has feedback that takes the output transformer into account. I do this because of the natural ability of tubes and transformers. Since this amp will have a driver stage with local feedback, the obvious direction (for me) was for the output to have it's own local feedback.
I suppose global feedback can be used in this case.
Regarding the bias: an op-amp DC servo loop may be the way to go. In my experience, an amp like this will drift like crazy....
For now, I'll address the feedback. I've had success using interstage transformers in tube amps. Usually I'll include local feedback at the driver stage to incorporate the interstage into a feedback loop. There's some tricks with tubes that naturally take the transformer into account.
I perfer local feedback with my tube amps - the driver stage has its own feedback that takes the interstage into account, and the output stage has feedback that takes the output transformer into account. I do this because of the natural ability of tubes and transformers. Since this amp will have a driver stage with local feedback, the obvious direction (for me) was for the output to have it's own local feedback.
I suppose global feedback can be used in this case.
Regarding the bias: an op-amp DC servo loop may be the way to go. In my experience, an amp like this will drift like crazy....
On this issue of feedback:
I am a firm believer in building the device to desired performance without feedback, and then later add some feedback to correct for small things.
Feedback should be for fine-tuning or fine-correction, not to "fix" problems that could have otherwise been addressed with good design. This is why the tube amps and solid-state of the 1970s sounded so bad. Amp designers were saving costs with cheap designs, and relying on gross feedback to correct for all the problems. Great amps sound good without feedback, because the designs generally don't have gross problems.
Summary: this amp should work without global feedback to desired (or close to) desired performance. Only after that can global feedback be justified.
I am a firm believer in building the device to desired performance without feedback, and then later add some feedback to correct for small things.
Feedback should be for fine-tuning or fine-correction, not to "fix" problems that could have otherwise been addressed with good design. This is why the tube amps and solid-state of the 1970s sounded so bad. Amp designers were saving costs with cheap designs, and relying on gross feedback to correct for all the problems. Great amps sound good without feedback, because the designs generally don't have gross problems.
Summary: this amp should work without global feedback to desired (or close to) desired performance. Only after that can global feedback be justified.
Think it would be better to do without the coupling cap when you use transformer coupling.
See e.g.
http://www.homestead.com/whaan/files/page3.html
See e.g.
http://www.homestead.com/whaan/files/page3.html
The amp at:
http://www.homestead.com/whaan/files/page3.html
does use coupling capacitors. The transformer coils are attached to the transistor gates on one end, and ground on the other ... connecting to ground through the 1000uF capacitors.
Redraw the schematic with the transformer "whole", and it becomes much more clear.
Side note: This design uses both P-channel and N-channel FETs anyway, which is not my goal. However, there are several things to observe.
(1) there is no feedback, not even local feedback.
(2) the MOSFETs operate with the signal applied between the gate and source.
(3) DC bias is set with the 2K variable resistor, which can result in bias drift, therefore, DC offset at the output.
This amp could use some improvements:
(1) include local feedback at the driver stage and power stage. Global feedback is an option, and I'd probably include it to experiment.
(2) make the output stage a source follower, letting the tube do all of the required voltage gain.
(3) Use an op-amp DC servo loop to control the bias current.
I'll post a modified schematic soon ... as soon as I hook my scanner back up ...
http://www.homestead.com/whaan/files/page3.html
does use coupling capacitors. The transformer coils are attached to the transistor gates on one end, and ground on the other ... connecting to ground through the 1000uF capacitors.
Redraw the schematic with the transformer "whole", and it becomes much more clear.
Side note: This design uses both P-channel and N-channel FETs anyway, which is not my goal. However, there are several things to observe.
(1) there is no feedback, not even local feedback.
(2) the MOSFETs operate with the signal applied between the gate and source.
(3) DC bias is set with the 2K variable resistor, which can result in bias drift, therefore, DC offset at the output.
This amp could use some improvements:
(1) include local feedback at the driver stage and power stage. Global feedback is an option, and I'd probably include it to experiment.
(2) make the output stage a source follower, letting the tube do all of the required voltage gain.
(3) Use an op-amp DC servo loop to control the bias current.
I'll post a modified schematic soon ... as soon as I hook my scanner back up ...
Was suggesting to get rid of the coupling caps when you already use a transformer for coupling. Depends on your design at the end you may also have some filter bypass caps for smoothing the bias voltage for the upper FET to set your output offset voltage and to smooth bias voltage for the lower FET to set bias current. May be able to do without these filter cap using DC servo. (But I suppose it is still better to use some filtering caps for the bias.)
By the way there was a design published in Wireless World using N-ch MOSFET push-pull (without a P-ch as pre-driver for the N-ch) . It was a bit like the JLH 10W class A but with N-ch MOSFET.
By the way there was a design published in Wireless World using N-ch MOSFET push-pull (without a P-ch as pre-driver for the N-ch) . It was a bit like the JLH 10W class A but with N-ch MOSFET.
Ok. The article was titled "Better Audio from non-complements?" by Bengt Olsson, Electronic World + Wireless World Dec. 1994, pp.988-991. Sorry don't have a scanner.
I think there is (was, not sure lost my bookmark) an Australian website published power amplifier design using the ideas from this article. It may be
http://www.aussieamplifiers.com/n-channe.htm
Not sure since their site says downloads temporary removed when I tried to see the schematic.
On the same topic, there were several designs also using N-ch FET push pull.
"A 40W All-MOSFET POWER AMP" by William Chater, The Audio Amateur, Part 1 in Feb/88 pp.7-18 Part 2 in March/88, pp.35-41. Again I don't have a scanner. Some guy (who knows tubes and sand state) built this one before and highly recommended it.
"A Simple Direct-Coupled Power MOSFET Audio Amplifier Topology Featuring Bias Stabilization" by Bill Roehr, IEEE Transaction on Consumer Electronics, Vol. CE-28, No. 4, Nov. 1982, pp.546-552.
Now come to think of it, N-ch MOSFET pushpull designs are not too scarce.
I think there is (was, not sure lost my bookmark) an Australian website published power amplifier design using the ideas from this article. It may be
http://www.aussieamplifiers.com/n-channe.htm
Not sure since their site says downloads temporary removed when I tried to see the schematic.
On the same topic, there were several designs also using N-ch FET push pull.
"A 40W All-MOSFET POWER AMP" by William Chater, The Audio Amateur, Part 1 in Feb/88 pp.7-18 Part 2 in March/88, pp.35-41. Again I don't have a scanner. Some guy (who knows tubes and sand state) built this one before and highly recommended it.
"A Simple Direct-Coupled Power MOSFET Audio Amplifier Topology Featuring Bias Stabilization" by Bill Roehr, IEEE Transaction on Consumer Electronics, Vol. CE-28, No. 4, Nov. 1982, pp.546-552.
Now come to think of it, N-ch MOSFET pushpull designs are not too scarce.
Here is the link to another thread - I did upload there Olsson's schematics idea as it was printed in Ben Duncan's book:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=3075&perpage=15&pagenumber=48
and my own schematics from 1992 (at the bottom of another page in the same thread) :
http://www.diyaudio.com/forums/showthread.php?s=&threadid=3075&perpage=15&pagenumber=36
Merry Christmas!
x-pro
http://www.diyaudio.com/forums/showthread.php?s=&threadid=3075&perpage=15&pagenumber=48
and my own schematics from 1992 (at the bottom of another page in the same thread) :
http://www.diyaudio.com/forums/showthread.php?s=&threadid=3075&perpage=15&pagenumber=36
Merry Christmas!
x-pro
Thanks Jacky and X-pro!
The MOSFET-JLH circuit, X-pro's circuits, the Creek website and now this thread have made me very interested in a N-channel buffer..
X-pro's circuit simulates as a complete stable but fast and highly lineair buffer, indeed with high inout impednace and low output impedance..
I have saved Olsson's circuit to disk,thanks. Must be a interesting article.
Merry Christmas
,
Thijs
The MOSFET-JLH circuit, X-pro's circuits, the Creek website and now this thread have made me very interested in a N-channel buffer..
X-pro's circuit simulates as a complete stable but fast and highly lineair buffer, indeed with high inout impednace and low output impedance..
I have saved Olsson's circuit to disk,thanks. Must be a interesting article.
Merry Christmas
,Thijs
Please note: This post refers to the complimentary N- and P-channel amp at:
http://www.homestead.com/whaan/files/page3.html
Some previous posts pointed out some things in this design, such as potentially unstable bias. Attached is a modified schematic of the output stage using complimentary pair (N-channel and P-channel) with a DC servo for bias control.
The time constant of the bias control is set by the capacitor in the op-amp’s feedback loop.
Note that the interstage transformer is still coupled through capacitors as the original design! The two capacitors on the left side of the schematic AC couple the transformer secondaries to ground. There are two possible connection schemes: one connection scheme AC couples the secondaries to the DC bias point. The second scheme AC couples the secondaries to ground. This connection is drawn with a dotted line.
I didn’t show grid stopper resistors or secondary winding load resistors in this schematic.
A little more work, and this output stage could be used as source-follower.
However, the title of this thread is using only N-channel, so my next post will deal with getting rid of the complimentary pair.
http://www.homestead.com/whaan/files/page3.html
Some previous posts pointed out some things in this design, such as potentially unstable bias. Attached is a modified schematic of the output stage using complimentary pair (N-channel and P-channel) with a DC servo for bias control.
The time constant of the bias control is set by the capacitor in the op-amp’s feedback loop.
Note that the interstage transformer is still coupled through capacitors as the original design! The two capacitors on the left side of the schematic AC couple the transformer secondaries to ground. There are two possible connection schemes: one connection scheme AC couples the secondaries to the DC bias point. The second scheme AC couples the secondaries to ground. This connection is drawn with a dotted line.
I didn’t show grid stopper resistors or secondary winding load resistors in this schematic.
A little more work, and this output stage could be used as source-follower.
However, the title of this thread is using only N-channel, so my next post will deal with getting rid of the complimentary pair.
Attachments
Please note: This post refers to the complimentary N- and P-channel amp at:
http://www.homestead.com/whaan/files/page3.html
Now, another adjustment has been made. The AC coupling capacitor have moved, so the interstage transformer secondaries are coupled to the MOSFET drains, therefore, source-follower configuration.
With the DC servo loop keeping the bias in check, and the MOSFETs operating in source-follower mode, this topology is getting a little more appealing.
However, it is still using a P-channel FET, in which we are interested in replacing with another N-channel. In my next installment, I’ll get back to topic, and get rid of the P-channel.
http://www.homestead.com/whaan/files/page3.html
Now, another adjustment has been made. The AC coupling capacitor have moved, so the interstage transformer secondaries are coupled to the MOSFET drains, therefore, source-follower configuration.
With the DC servo loop keeping the bias in check, and the MOSFETs operating in source-follower mode, this topology is getting a little more appealing.
However, it is still using a P-channel FET, in which we are interested in replacing with another N-channel. In my next installment, I’ll get back to topic, and get rid of the P-channel.
Attachments
Now let’s get rid of the P-channel and replace it with an N-channel.
John Broskie of Tube CAD Journal has an article on some ideas:
http://www.tubecad.com/index_files/page0021.htm
This is (Circ’s gonna love me) a Circlotron amp. A generalized schematic is shown below. The power supplies induce a circular current through the MOSFETs. If the bias currents of the FETs are exactly equal, the output is zero. Any unbalance between the MOSFETs appear at the output as signal.
This amp (as drawn by Broskie) suffers from the same problem as other amps: potentially unstable/mismatched bias and no local feedback. In addition, we’d like the amp to work in source-follower mode.
It’s easy enough to convert this amp to source-follower mode, but it’s a bit more of a trick to control the bias (therefore, control DC offset at the output).
My next installment will be this schematic as source-follower.
John Broskie of Tube CAD Journal has an article on some ideas:
http://www.tubecad.com/index_files/page0021.htm
This is (Circ’s gonna love me) a Circlotron amp. A generalized schematic is shown below. The power supplies induce a circular current through the MOSFETs. If the bias currents of the FETs are exactly equal, the output is zero. Any unbalance between the MOSFETs appear at the output as signal.
This amp (as drawn by Broskie) suffers from the same problem as other amps: potentially unstable/mismatched bias and no local feedback. In addition, we’d like the amp to work in source-follower mode.
It’s easy enough to convert this amp to source-follower mode, but it’s a bit more of a trick to control the bias (therefore, control DC offset at the output).
My next installment will be this schematic as source-follower.
Attachments
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