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OPT resonance Q factor in the HF region - your opinions?
OPT resonance Q factor in the HF region - your opinions?
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Old 16th July 2018, 08:37 AM   #1
50AE is offline 50AE  Bulgaria
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OPT resonance Q factor in the HF region - your opinions?
Default OPT resonance Q factor in the HF region - your opinions?

What would you choose between these two options and why? I'm curious to have your opinions.

1. Optimized Q factor, no apparent ringing, optimized transformer for the schematic and tube, but lower resonant frequency. Of course, roll-off due to Ls or Cp at much higher point (optimal tube and load)
Some numbers:
Ls - 0.69mH
Cp - 2nF
Fres - 135.5kHz
Q - 0,77


2. Slight ringing, higher Q, ugly square wave, but also higher resonant frequency. The parameters in this situation are:
Ls - 1.13mH
Cp - 667pF
Fres - 183.5kHz
Q - 1.7
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Old 16th July 2018, 11:26 AM   #2
6A3sUMMER is online now 6A3sUMMER  United States
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I would have to know the amplifier topology before making a choice.

1. If the amplifier Does have negative feedback, I would probably choose transformer #1. You have to worry about square waves into resistive loads during a Reviewers testing (or a audio club member's testing). You also have to worry about the real world loading from loudspeakers and loudspeaker cables, when using the amp for listening to music.

2. If the amplifier does Not have negative feedback, And is either a single ended triode amp or a push pull triode amp, I would use either transformer. That is because the music sources I use do Not have frequencies higher than 20kHz. There will be No ringing in that application. And sense there is no negative feedback, you do not have to worry about stability. So how about some other criteria for selection of the transformer:
Which transformer is less expensive?
Which transformer has more inductance?
Which transformer has better primary and secondary DCR?
Which transformer has more laminations?

Because I am not making a commercial amplifier, with some Reviewer or audio club member trying to criticize the amp's bench performance, I do not have to worry about some ringing at ultrasonic frequencies.

Some persons will say I need to worry about the phase performance of the amplifier. I have a story about phase: Some early CD players used only one 16 bit DAC, and a switch that alternated the DAC output between the left and right channels. The DAC outputted signals 88.2K times a second (44.1K times per channel). At 20 kHz, the left and right signals had about 90 Degrees phase error relative to each other.

Sound is 1080 feet per second, about 1 foot per millisecond, which is about 2 cycles of 20 kHz per inch. 90 Degrees is 1/4 cycle, or about 1/8 inch at 20 kHz.

A set of left and right loudspeakers are positioned exactly the same distance from the listener. The listener turns his head 1/8 inch total (90 Degrees at 20 kHz). Once the correct phase is established, the listener must not to turn the head again, not even 1/8 of an inch.

If you are worried about the relative phase versus frequency of just one channel's amp and loudspeaker drivers, be sure not to move your head up or down even a very small distance, you will disturb the relative phase of the woofer versus the tweeter.

How we seem to worry about the inner layers of the onion, when the outer layers have much larger problems.
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Old 16th July 2018, 11:35 AM   #3
45 is offline 45  Italy
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Slight ringing can't result in ugly square wave unless there is no damping and/or F3 is much lower than Fres which would mean the amp might be unstable and/or the transformer is not really suited for this job.

For me, optimal behaviour is such that cut-off is just below resonance. Extending the FR beyond the resonance is pointless because the phase will rotate anyway and the primary load will show a big dip in module. No serious power can be had there and not good if fbk has to be used.

What is the source impedance in case 2? what is the primary load? Fres at 135 KHz is a rather good transformer by any standard.
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Old 16th July 2018, 12:07 PM   #4
50AE is offline 50AE  Bulgaria
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OPT resonance Q factor in the HF region - your opinions?
Quote:
Originally Posted by 6A3sUMMER View Post

1. If the amplifier Does have negative feedback, I would probably choose transformer #1.
You have to worry about square waves into resistive loads during a Reviewers testing (or a audio club member's testing). You also have to worry about the real world loading from loudspeakers and loudspeaker cables, when using the amp for listening to music.
Opinions are welcome for both cases - NFB available or not. Speaker and cable loading have a significance as well. I would probably strive to the optimal solution for music listening, compared to resistive load measurements. Cable capacitance is one evil and it's in parallel with the secondary inductance.


Quote:
Originally Posted by 6A3sUMMER View Post
2. If the amplifier does Not have negative feedback, And is either a single ended triode amp or a push pull triode amp, I would use either transformer.
That is because the music sources I use do Not have frequencies higher than 20kHz. There will be No ringing in that application. And sense there is no negative feedback, you do not have to worry about stability.
So how about some other criteria for selection of the transformer:
Which transformer is less expensive?
Which transformer has more inductance?
Which transformer has better primary and secondary DCR?
Which transformer has more laminations?

Because I am not making a commercial amplifier, with some Reviewer or audio club member trying to criticize the amp's bench performance, I do not have to worry about some ringing at ultrasonic frequencies.
Both transformers will have the exact same core, number of winding, labour, hence price. There will be a slight alteration in insulation quantity and type, changing Cp vs Ls.

Quote:
Originally Posted by 45 View Post
Slight ringing can't result in ugly square wave unless there is no damping and/or F3 is much lower than Fres which would mean the amp might be unstable and/or the transformer is not really suited for this job.
Of course, if F3 is much lower than resonance, this will be an unacceptable situation.

Quote:
Originally Posted by 45 View Post
For me, optimal behaviour is such that cut-off is just below resonance. Extending the FR beyond the resonance is pointless because the phase will rotate anyway and the primary load will show a big dip in module. No serious power can be had there and not good if fbk has to be used.
In my case, FR beyond the resonance, either by Cp or Ls was just a bonus, not a search. I agree with you.

Quote:
Originally Posted by 45 View Post
What is the source impedance in case 2? what is the primary load? Fres at 135 KHz is a rather good transformer by any standard.
It's a 3k/4R transformer, dedicated for tubes with a Rp similar to the one of 300B (650-800R)
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Old 16th July 2018, 02:41 PM   #5
45 is offline 45  Italy
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Quote:
Originally Posted by 50AE View Post

Both transformers will have the exact same core, number of winding, labour, hence price. There will be a slight alteration in insulation quantity and type, changing Cp vs Ls.
In that case just go for the one that performs better overall.

Quote:
Originally Posted by 50AE View Post
It's a 3k/4R transformer, dedicated for tubes with a Rp similar to the one of 300B (650-800R)
In general you can keep the option 2 for another tube. For example if you run current production KT66 in pentode mode with 250V on G2 and 300-310V on the plate at 85 mA you will get at least 9W into 3K. So it's 300B territory. If you use 10% Schade fb then for gm around 5-6 mA/V the Zout will be 1.6-2 K....
In general, for well behaved FR you don't need exactly Q=0.7. Anything <1 should not show any ringing.
All this always assuming that SQRT(Ls/Cp) is less than Rl (3K in this case, so it's good).
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Old 16th July 2018, 04:45 PM   #6
trobbins is offline trobbins  Australia
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Just for background - are they measured values, and is it SE or PP primary, and did you do a response curve as well?
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Old 16th July 2018, 04:53 PM   #7
smoking-amp is offline smoking-amp  United States
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If one were designing the OT for CFB driven by a pentode, then the cathode and plate OT windings would want to be optimized differently. Cathode being low Z would like low leakage L to secondary, and plate being high Z would like low distributed capacitance. Now you can optimize the OT performance far better when only one parameter needs to be minimized for each winding. One could optimize the cathode and plate winding Q parameters separately.

Last edited by smoking-amp; 16th July 2018 at 05:00 PM.
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Old 16th July 2018, 06:22 PM   #8
45 is offline 45  Italy
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Hey Don, with CFb if one is not using large amounts the low Ls is almost trivial. For example, the last OT I am going to get soon will have 1/6 CFb ratio and its Ls will be over 200 times smaller than the full 5mH. So it should be around 20 uH.
I think that should low enough.
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Old 16th July 2018, 06:46 PM   #9
smoking-amp is offline smoking-amp  United States
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Should be good.

If the CFB is small enough, it could even be bifilar with the secondary, with the small %CFB also taken back to the driver for more loop gain. Providing it remains stable.

With the pentode plates acting as current sources, the plate winding portion may not even need interleaving. Just fully minimize the distributed C. Just a little extra B+ headroom for the plate "current sources" to overcome the leakage L then, which doesn't really require any B+ change. No one wants full power at the HF end anyway. This could make for a cheap to wind OT, with reasonably high performance.
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Old 16th July 2018, 07:16 PM   #10
45 is offline 45  Italy
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Quote:
Originally Posted by smoking-amp View Post
This could make for a cheap to wind OT, with reasonably high performance.
Indeed.
I have a design for a small OT just big enough to get 18W @20Hz without saturation which I will get done in August when I' ll go back to Italy for holidays. 1/6 CBf with the 10GK6 in PP. Max Pout optimised for 5R (where total primary impedance is the standard 8K) basically 40:1 step down, including CFb. Core size 2.5x4 cm. Total Cp for the plates should be no more than 0.3 nF according to my estimate.
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