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Old 29th June 2004, 04:19 PM   #1
Fuling is offline Fuling  Sweden
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Default Autoformer CF amp

My latest design idea (I guess itīs not "my" idea, but I havenīt 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 isnīt 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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Old 30th June 2004, 01:01 AM   #2
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This sounds really interesting !
Can you tell me more about the transformer - what about DC saturation and the like, does it need to be airgapped like a standard SE
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Old 30th June 2004, 06:44 AM   #3
Fuling is offline Fuling  Sweden
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I havenīt tried this one out yet but output tubes, sockets and power transformers are on their way. Iīm going to wind my own output transformers on EI84-cores. An airgap will be necessary, and adjusted by trial and error for best LF response.
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Old 30th June 2004, 07:26 AM   #4
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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!
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Old 30th June 2004, 07:59 AM   #5
Fuling is offline Fuling  Sweden
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When you mention it, I remember some design using a PP pair of EL34 cathode followers loaded by an autoformer. It was designed by some Finnish engineer I think.

Some DC yes, but I donīt think 200mA throgh maybe 100 turns of thick wire shouldnīt cause any sleep disorders
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Old 30th June 2004, 08:22 AM   #6
Colt45 is offline Colt45  Serbia
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100 turns wont be much inductance.. i imagine most of the signal will go to earth rather than to your speaker.

or am i missing something?
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Old 30th June 2004, 08:27 AM   #7
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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.........

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Old 30th June 2004, 09:19 AM   #8
Fuling is offline Fuling  Sweden
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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 wonīt 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, donīt you think?
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Old 30th June 2004, 12:33 PM   #9
Fuling is offline Fuling  Sweden
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Just another idea: How about a pentode CF?
A mosfet follower connected to the cathode could be used to drive the screen grids.
Comparing the pentode curves versus the triode curves I feel that the pentodes would be better suited to drive low impedance loads, am I right?
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Old 11th August 2004, 05:09 AM   #10
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Hi Fuling,

Have you completed this project yet ? What are the results ?
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