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Old 14th February 2006, 06:13 AM   #11
andrei is offline andrei  Canada
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Sorry, when peak output current is twelve times the idle current, it is deep in class B. The usual drawbacks are high distortion, a change in output impedance as signal swings from Class A to Class B region (which leads to very unpleasant waveform distortion), and heavy requirements on power supply. Might as well switch to transistors in output stage. (BTW, if you are willing to put up with Class B, you still could get a lot more power with a tranformer if you just drop it in into your OTL design instead of direct speaker load...)
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Old 14th February 2006, 07:50 AM   #12
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Andrei,

What do you mean by "deep in class B"? an amplifier operates either in class A, AB or C these are clearly defined.

It seems that you have your own definition of class B, that is OK even though it must be confusing for others.

What is more important is of course the end result you can get, I don't know what experience you have from OTL amps but I recommend you to listen to a good one. When it comes to measurement results I think it is quite difficult to equal the performance in a good OTL using transformers, be it bandwidth or distortion, the remaining problem in OTL amps is low efficiency but that is something you need to live with.

Regards Hans
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Old 14th February 2006, 08:00 AM   #13
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Leadbelly,

The Philips amp can be built with other tubes, almost any output beam penthode will work, the higher Gm the better as it will affect the output impedance. The capacitor values are not that critical and can be increased if you want to, the output capacitor need to be a very big one as it should pass low frequencies.

If you have the components you should be able to put this together in short time, especially if you build it without a chassis just for test, that is how I did it, it will play very low with ordinary speakers but should work better with headphones.

Regards Hans
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Old 14th February 2006, 09:10 AM   #14
andrei is offline andrei  Canada
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I have enough experience with listening to OTL transistor amps not to be thrilled with the idea of emulating their design flaws using tubes instead of transistors.

As far as operating point classes go, drawing 2.5A current from stage idling at 200mA means that 95% of the time one of the tubes is in cut-off (assuming sine wave). So your nominal AB is made up of 5% A and 95% B at your rated power. That is no different from Class B for practical purposes. I'm willing to bet that there is a lot of high-order harmonics in the output, even without measuring or simulating the whole thing.

I don't see the point of chasing bandwidth in audio amplifiers. If anything, it should be limited. Do you really want to be amplifying RFI? (When I lived in Toronto, all my solid state audio gear would receive CBC broadcasts on its own and without benefit of having a tuner. True story.)
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Old 14th February 2006, 11:47 AM   #15
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Quote:
I have enough experience with listening to OTL transistor amps not to be thrilled with the idea of emulating their design flaws using tubes instead of transistors.
If you feel like that then I find it is strange that you take part in a discussion of how to build tube OTL amps, are you trying to discourage people to try?

You still don't seem to understand what is meant by different operating classes, class AB doesn't mean that it switch between A and B at some point, the definition is that current flows in each active element between 180 and 360 degrees of a period.

Quote:
I'm willing to bet that there is a lot of high-order harmonics in the output, even without measuring or simulating the whole thing.
I think you would find that high-order harmonics still can be made lower in an OTL amp than in a compareable transformer coupled amp.

Quote:
I don't see the point of chasing bandwidth in audio amplifiers. If anything, it should be limited.
Agreed that it should be limited but what is the required bandwidth? In order to get high IM caused by high frequencies e.g 19, 20 kHz I want the amplifier to have wide bandwidth before applying feedback and I also want to have minimimum phase shift inside the audio range.

A suitable open loop bandwidth then could be around 200kHz which is not that difficult to achieve in an OTL but very difficult in a transformer coupled amp.

Regards Hans
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Old 14th February 2006, 12:54 PM   #16
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Dear Hans

Let's back to the OTL.
Do you think four 6c19 work together as CF on an inductor
via four 5 ohms resistor as what the SE 1.5W OTL of
Transcendentsound can drive a 8 ohms speaker with good
damping?
How about replaceing the inductor with a CCS as I question
in the other thread.

Regards CHOW
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Old 14th February 2006, 05:13 PM   #17
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6C19 will work but it is not an ideal OTL tube, the transconductance is only 7.5mA/V and internal resistance is high at 400ohm. Each tube as CF will have an output impedance of about 100 ohm so 4 in parallell will have an output impedance of 25ohm. In order to get reasonably low output impedance you would need quite heavy feedback.

A single 6C33C have a Gm of 40mA/V and can handle the same current as 4 6C19, the output impedance is also about the same or about 22ohm, I think that is a much better choice as you don't get the problems of matching the tubes.

Max output power is determined by how much current you can run continously in the tubes, each 6C19 is specified to max 140mA so 4 of them can run 560mA. As we have a single ended class A stage the output current can never be higher than the idle current so the output power will be theoretically max (R*I^2)/2 or 1.25W, (in reality it will be lower as you can't use the whole available current swing). To get higher output power you either need to run the tubes above ratings or add more tubes.

Replacing the inductor with a CCS doesn't change the calculations, output power and output impedance will be the same but the frequency response and distortion can possibly be better, it depends on the quality of the CCS.

Regards Hans
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Old 14th February 2006, 07:49 PM   #18
andrei is offline andrei  Canada
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Quote:
If you feel like that then I find it is strange that you take part in a discussion of how to build tube OTL amps, are you trying to discourage people to try?
No, I'm merely replying to the original question - and that was why there are no single triode OTL power stages. The answer is you can do better.

If you really want OTL, why not a hybrid with MOSFET followers? Even a cheap IRF610 has transconductance 800mA/V, which is an order of magnitude better than 6C33C tube (not to mention size and heater power advantage)...

Quote:
In order to get high IM caused by high frequencies e.g 19, 20 kHz I want the amplifier to have wide bandwidth before applying feedback and I also want to have minimimum phase shift inside the audio range.
This is pretty much the standard solid state design philosophy. Global feedback strapped on top of a bunch of non-linear stages. Makes it all better, right?
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Old 14th February 2006, 07:52 PM   #19
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Quote:
Originally posted by andrei
This is pretty much the standard solid state design philosophy. Global feedback strapped on top of a bunch of non-linear stages. Makes it all better, right?
Please don't jack my thread with your preaching.
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Old 14th February 2006, 08:04 PM   #20
andrei is offline andrei  Canada
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I was not preaching, I was being sarcastic .

Anyway, my point is that I don't see a technical reason to be running a tube OTL stage. If you do, please explain it to me.

And BTW, I am serious about the MOSFET follower. Any comments on that?
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