My latest design idea (I guess it´s not "my" idea, but I haven´t seen it around before): A meaty cathode follower loaded by an autoformer.
Not too different from a choke loaded SE OTL, but the point here is that we can make it without the output coupling cap.
How come?
The part of the autoformer that the speaker connects to shall be wound with very thick (>1mm) wire so that the cathode current that passes through it will only cause a small DC offset, small enough to be neglected. Below 100mV should be easy to achieve.
The rest of the winding can be made with a wire gauge more suitable for the current (0,3-0,4mm) since the drop across the whole winding isn´t important.
Another major reason for using an autoformer instead of a choke is that we get a very welcome impedance transformation.
Instead of loading the tubes with 8 ohms we load them with 200 ohms (if we use a 4:1 autoformer which has a "real" ratio of 5:1)
This of course calls for a driver stage that knows how to swing some voltage, and a choke loaded PC86 sure knows that.
Not too different from a choke loaded SE OTL, but the point here is that we can make it without the output coupling cap.
How come?
The part of the autoformer that the speaker connects to shall be wound with very thick (>1mm) wire so that the cathode current that passes through it will only cause a small DC offset, small enough to be neglected. Below 100mV should be easy to achieve.
The rest of the winding can be made with a wire gauge more suitable for the current (0,3-0,4mm) since the drop across the whole winding isn´t important.
Another major reason for using an autoformer instead of a choke is that we get a very welcome impedance transformation.
Instead of loading the tubes with 8 ohms we load them with 200 ohms (if we use a 4:1 autoformer which has a "real" ratio of 5:1)
This of course calls for a driver stage that knows how to swing some voltage, and a choke loaded PC86 sure knows that.
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Very interesting idea.
I've seen autoformers used for driving push-pull output stages from a single-ended source with very good results.
Without a question, the autoformer will need to be airgapped so as to avoid saturation.
There will be a little d.c. across the speaker but not much.
Good luck!
I've seen autoformers used for driving push-pull output stages from a single-ended source with very good results.
Without a question, the autoformer will need to be airgapped so as to avoid saturation.
There will be a little d.c. across the speaker but not much.
Good luck!
Yup. I'd use 100T for around 10 ohms on... about any core. Bigger core = more power and wider response. Of course, you can use more turns on a smaller transformer to get the same response, but there's just something, not worth it about bothering with that. Go figure.
But I digress. You'll need at least 1kT for that, and the "primary" has to be interleaved with the secondary just as with any other transformer. You gain very little in using an autoformer as opposed to a transformer...which has the added benefeits of isolation, no DC output, can be placed in the plate circuit, etc.........
Tim
But I digress. You'll need at least 1kT for that, and the "primary" has to be interleaved with the secondary just as with any other transformer. You gain very little in using an autoformer as opposed to a transformer...which has the added benefeits of isolation, no DC output, can be placed in the plate circuit, etc.........
Tim
The 100 turns would be the secondary winding, if such things exists in autoformers (the part that connects to the speaker). The total winding would be more like 600 turns or something. Also remember that these numbers are picked out of thin air.
I believe that I can get away with quite low inductance because of the following reasons:
1: Low Rp tubes in //
2: Cathode follower CCT, the 100% degenerative feedback should help I think.
3: My speakers are biamped so this amp will only work from about 100Hz and up. I will aim for fullrange use, but if the bass gets a bit thin it won´t be a disaster.
The low turns ratio should at least give good HF response
To give a hint of my point of view I might add that the idea for this project came when I was messing around with (imaginary) SE OTL amps. An autoformer should have several benefits compared to a choke / cap combo, don´t you think?
I believe that I can get away with quite low inductance because of the following reasons:
1: Low Rp tubes in //
2: Cathode follower CCT, the 100% degenerative feedback should help I think.
3: My speakers are biamped so this amp will only work from about 100Hz and up. I will aim for fullrange use, but if the bass gets a bit thin it won´t be a disaster.
The low turns ratio should at least give good HF response
To give a hint of my point of view I might add that the idea for this project came when I was messing around with (imaginary) SE OTL amps. An autoformer should have several benefits compared to a choke / cap combo, don´t you think?
Fuling
Do u happen to know the rp of the pl504 and pl500 and the pl509? Looks interesting to work with. Something along your schematic but with a transformer.
I think that your turn is to low too should be atleast more than 1K. E84 is a small core. With 500t i expect henry to be around 0.1-0.5 henry but should be fine for biamp systems. This is assumption is a general assumption, i don't know the rp of the tube. If it is around 20-30 that the value should be allright to have 500 turns then.
However i find that there will be offset voltage if u wind in this way. Thus you would require a cap to couple to the speaker.
I would follow Tim advise. Go for a transformer, safer and also offer isolation too.
Anyway for 200ma of current, start by gapping at 0.3-0.4mm
Anyway pentode should make better cathode follower as their gain should approach almost 1 too. But would the choke and pentode fight each other? Drawing a straight line across the curves of a pentode shows the point. However i seen this done in ss, some italian company makes their amps with a emitterfollower loaded by a choke.
Do u happen to know the rp of the pl504 and pl500 and the pl509? Looks interesting to work with. Something along your schematic but with a transformer.
I think that your turn is to low too should be atleast more than 1K. E84 is a small core. With 500t i expect henry to be around 0.1-0.5 henry but should be fine for biamp systems. This is assumption is a general assumption, i don't know the rp of the tube. If it is around 20-30 that the value should be allright to have 500 turns then.
However i find that there will be offset voltage if u wind in this way. Thus you would require a cap to couple to the speaker.
I would follow Tim advise. Go for a transformer, safer and also offer isolation too.
Anyway for 200ma of current, start by gapping at 0.3-0.4mm
Anyway pentode should make better cathode follower as their gain should approach almost 1 too. But would the choke and pentode fight each other? Drawing a straight line across the curves of a pentode shows the point. However i seen this done in ss, some italian company makes their amps with a emitterfollower loaded by a choke.
Macintosh used autoformers for years in their solid state amps (maybe they still do?). It would be interesting to dig around and see what tricks they came up with in the transformer design.
Tim, do you have any idea of why Mac didn't just go to a transformer and gain the advantages you cite?
Tim, do you have any idea of why Mac didn't just go to a transformer and gain the advantages you cite?
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