Hi everyone,
I recently wound a OPT for 6336 SET amp with 2.5K:8ohm impedance ratio and 10W max output power, pretty similar to 300B. Yesterday I did some testing on this OPT and everything was exactly as the design, except one thing; Hi F resonance (I mean the series resonance of Leakage Inductance and Shunt Capacitance), was at around 34KHz which is lower than my previous builds.
So here is my question; what negative effects can this cause? (bearing in mind that there is no negative feedback in this amp). I know that it can cause Hi F roll off and you can see F response at 20Khz rolled off at about -0.3dB, but is it that important? What is the other side effects that I'm not aware of?
I attached some images from my measurements:
I recently wound a OPT for 6336 SET amp with 2.5K:8ohm impedance ratio and 10W max output power, pretty similar to 300B. Yesterday I did some testing on this OPT and everything was exactly as the design, except one thing; Hi F resonance (I mean the series resonance of Leakage Inductance and Shunt Capacitance), was at around 34KHz which is lower than my previous builds.
So here is my question; what negative effects can this cause? (bearing in mind that there is no negative feedback in this amp). I know that it can cause Hi F roll off and you can see F response at 20Khz rolled off at about -0.3dB, but is it that important? What is the other side effects that I'm not aware of?
I attached some images from my measurements:
Hi Walt, yes I use REW with a jig which I built to measure both impedance and F Response of OPTs. I use Focusrite 2i2's headphones output for measuring Primary impedance, and for measuring F Response and Distortion I use reverse method using a 300W class AB amp and a DC source.
Here is a picture of my setup:
Here is a picture of my setup:
Great!!!
I suggest this method that is simply
There are some thread about it
Walter
I tested a lot of methods for measuring OPTs and finally I settled on this arrangement which I found it gives the most accurate results:
1. Primary impedance (both with open secondary and shorted secondary): Forward method using soundcard's headphones output.
2. Frequency response: Reverse method using a low distortion class AB SS amp with 1W of power and also max power on secondary winding.(Secondary Voltage = √(P out * R seris))
Also a DC current source should feed secondary winding and it should be equal to: Rated primary DC current * Turn ratio.
3. Low Frequency distortion(THD): same as frequency response with full output power.
4. High frequency distortion: same as frequency response except that DC current should not be present.
1. Primary impedance (both with open secondary and shorted secondary): Forward method using soundcard's headphones output.
2. Frequency response: Reverse method using a low distortion class AB SS amp with 1W of power and also max power on secondary winding.(Secondary Voltage = √(P out * R seris))
Also a DC current source should feed secondary winding and it should be equal to: Rated primary DC current * Turn ratio.
3. Low Frequency distortion(THD): same as frequency response with full output power.
4. High frequency distortion: same as frequency response except that DC current should not be present.
No negative feedback? Good idea!
No instability to have to compensate for.
-0.3 dB @ 20kHz? Wonderful!
Most on diyAudio have lots more than 0.3 dB hearing loss at 20kHz.
Those who do not have that much hearing loss, need to have their hearing tested, and then they will become a Guinea Pig for the doctors to study and scratch their heads.
No instability to have to compensate for.
-0.3 dB @ 20kHz? Wonderful!
Most on diyAudio have lots more than 0.3 dB hearing loss at 20kHz.
Those who do not have that much hearing loss, need to have their hearing tested, and then they will become a Guinea Pig for the doctors to study and scratch their heads.
Can you clarify if the primary had rated idle DC current in any of your measurements? That would sort of seem to be a major comment to make about any testing, along with the signal voltage used (especially for impedance where low frequency inductance value is quite dependent on voltage as well as DC current).
You can enable component modelling for the impedance measurement, and that allows the inductance (or capacitance) to be calculated by REW at a suitable cursor location on the plot.
Perhaps better to show a distortion plot with just 2nd and 3rd harmonic levels, given you are wanting to show influence out towards the 34kHz resonance.
How does this OPT's primary shunt capacitance value (mid-band at circa 4-10kHz) compare to your previous builds, as well as the resonant frequency (and hence leakage inductance, given that REW may calculate a suspect value between 34kHz and 56kHz) ?
Are you planning to bandwidth limit the input signal to your amp, so that signal level above say 20kHz is being increasingly attenuated?
Also, the resonance centre frequency is usually defined at 0-deg impedance phase shift, rather than min impedance.
Ciao, Tim
You can enable component modelling for the impedance measurement, and that allows the inductance (or capacitance) to be calculated by REW at a suitable cursor location on the plot.
Perhaps better to show a distortion plot with just 2nd and 3rd harmonic levels, given you are wanting to show influence out towards the 34kHz resonance.
How does this OPT's primary shunt capacitance value (mid-band at circa 4-10kHz) compare to your previous builds, as well as the resonant frequency (and hence leakage inductance, given that REW may calculate a suspect value between 34kHz and 56kHz) ?
Are you planning to bandwidth limit the input signal to your amp, so that signal level above say 20kHz is being increasingly attenuated?
Also, the resonance centre frequency is usually defined at 0-deg impedance phase shift, rather than min impedance.
Ciao, Tim
This is the proto of test circuit with injection of dc to secondary acting primary
It is missing the external variable dc power
Walter
Attachments
These are diagram of one OT with different dc idle current and power
Also no dc of bias.
Reverse mode but dc on primary side
Idle current is 71 and 28 mA
Another aspect is the response with entire circuit; some changes on resonace will happen but this is a good start point
Also no dc of bias.
Reverse mode but dc on primary side
Idle current is 71 and 28 mA
Another aspect is the response with entire circuit; some changes on resonace will happen but this is a good start point
Hi, I used 1.67A on secondary which equals to 0.1A on primary (my rated value for design), for testing F response and also THD. Primary inductance has been measured using Focusrite Headphone out and 0.7 excitation voltage and also I measured it with 5 to 25V from the SS amp without much difference in any point except that Low F which impedance got a little bit higher with higher excitation voltage, The impedance measurements I posted is from the Headphone Out.
Yes I saw it in other article you wrote about Willamson transformers. With 0.7V excitation voltage, Primary inductance measured 25H at 20Hz, 24.2H at 30Hz, 22.8H at 50Hz, and 29.5H at 1KHz before the parallel resonance. It is interesting that other excitation voltages from 5 to 25V also gave me the same results.
As for the Capacitance, my measurements showed 1nF at 10kHz and 1.2nF at 20KHz.
Also L Leakage was measured around 10mH at around 1KHz which I guess is a little bit high I guess, I used 4P:3S arrangement for this design and I used perfectly layered wiring as always, maybe the reason is that all the secondary windings arranged in series instead of parallel, I'm not sure..
Both LL and Csh measured higher in this build compared to my previous builds. I normally get around nominal Z value in mH for LL which should be 2.5H in this case and I'm not sure why it turned out so high? As for capacitance I expected this number in my design because of the wide window of double C-core I used for this.
I'm planning to use this transformer in a design from this awesome website:Link
I will attach them in the bottom:
DC bias is a very important aspect in measuring F Response and Distortion of SE transformers as described by Patrick Turner in this article:Link
in figure 12 of this article he described the reverse method and DC bias.
The issue with adding DC bias to primary is that you need a very high impedance DC current source so it won't load down the primary, but if you use equivalent DC current on secondary you will need only around 48V DC voltage source and enough resistance to provide the current.
In the following I will attach my testing jig layout if you're interested:
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trobbins and everyone . . .
The circuit topology, circuit values including tube operating rp, power output, quiescent current, etc., should determine what factors are most important in selecting an appropriate output transformer.
How I use output transformers for my best amplifier designs:
Push Pull (Balanced amplifier), no negative feedback other than the common un-bypassed self bias resistor;
And . . . very importantly, I use a CD player with balanced XLR output, that CD player Drops Like a Rock for all frequencies greater than 22.05 kHz.
Although I use a fast rise square wave generator (read that as very high frequencies) to check the amplifier for high frequency "ringing", the result is not that important to me, because all frequencies above 22.05kHz are not present. Lucky Me.
As I said before, I would be willing to use any output transformers, SE or PP that are similar to waltube's designs.
My hearing has dropped like a rock above about 14kHz, a fractional dB at that frequency is not going to ruin my enjoyment of the music.
My life is getting to be much less complicated when it comes to my tube amplifier designs. They need to be functional, simple, reliable, and interesting and/or boring circuits.
The circuit topology, circuit values including tube operating rp, power output, quiescent current, etc., should determine what factors are most important in selecting an appropriate output transformer.
How I use output transformers for my best amplifier designs:
Push Pull (Balanced amplifier), no negative feedback other than the common un-bypassed self bias resistor;
And . . . very importantly, I use a CD player with balanced XLR output, that CD player Drops Like a Rock for all frequencies greater than 22.05 kHz.
Although I use a fast rise square wave generator (read that as very high frequencies) to check the amplifier for high frequency "ringing", the result is not that important to me, because all frequencies above 22.05kHz are not present. Lucky Me.
As I said before, I would be willing to use any output transformers, SE or PP that are similar to waltube's designs.
My hearing has dropped like a rock above about 14kHz, a fractional dB at that frequency is not going to ruin my enjoyment of the music.
My life is getting to be much less complicated when it comes to my tube amplifier designs. They need to be functional, simple, reliable, and interesting and/or boring circuits.
Here is something that you might be interested in ; I did some F response measurements using different output power levels (0.01W, 1W, 2W, 4W, 8W):
As the output power increases, Low F response gets better which I guess is due to higher core permeability resulted from higher excitation voltage. Another thing is that Hi F response gets worse with increasing the power, which I can't find any reason related to the transformer, what's your opinion on this one?
Also here is THD measurements on different Power levels:
As you can see Low F distortion gets lower with increasing the power which is odd to me! Also high F distortion gets bigger with increasing the power, and the reason to this one is also unknown to me...
Thanks
Sajad
As the output power increases, Low F response gets better which I guess is due to higher core permeability resulted from higher excitation voltage. Another thing is that Hi F response gets worse with increasing the power, which I can't find any reason related to the transformer, what's your opinion on this one?
Also here is THD measurements on different Power levels:
As you can see Low F distortion gets lower with increasing the power which is odd to me! Also high F distortion gets bigger with increasing the power, and the reason to this one is also unknown to me...
Thanks
Sajad
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Just a guess - the driving source unable to drive capacitive loads? How does it handle a high leakage inductance transformer?
The secondary is driven by ss trough a resistor that in my case is 6 ohms
Using some other value just to play
Of course checking with scope the signals
Using some other value just to play
Of course checking with scope the signals
In reverse method I normally use resistance equal to nominal secondary impedance in series with secondary winding and SS amp, and then load the primary with nominal primary impedance. Of course tube's plate resistance can be used on primary which will make the results a little better.
Ok
Same as me
But I test with nominal load and Rp tube
If you search an old thread by me on OT backward. there are other infos
And a lot of comments
Walter
Same as me
But I test with nominal load and Rp tube
If you search an old thread by me on OT backward. there are other infos
And a lot of comments
Walter
I love the reversible reactions of Chemistry, reversible reactions of Physics, and reversible calculations of Mathematics.
Some things are reversible, and some other things are not reversible.
One example of non-reversible electronic reactions:
If Smoke comes out, you can not stuff it back in to fix it.
Some things are reversible, and some other things are not reversible.
One example of non-reversible electronic reactions:
If Smoke comes out, you can not stuff it back in to fix it.
Hi again, I found something interesting; I wound another bobbin for this transformer with everything exactly the same, but another winding arrangement. This time I put all the secondary windings in parallel and the high frequency resonance changed dramatically from 37KHz to 72KHz!
The original arrangement was like this: 4P - S - 6P - S - 6P - S -4P (with secondary windings all in series).
The new one is with the same number of turns, but the arrangement is like this: S - 6P - S - 8P - S - 6P - S (with all of the secondary windings in parallel).
Here is the primary impedance comparison of the two transformers: (the purple graph is for the parallel arrangement)
And here is frequency response comparison at full output power with DC bias: (the purple graph is for the parallel arrangement)
The original arrangement was like this: 4P - S - 6P - S - 6P - S -4P (with secondary windings all in series).
The new one is with the same number of turns, but the arrangement is like this: S - 6P - S - 8P - S - 6P - S (with all of the secondary windings in parallel).
Here is the primary impedance comparison of the two transformers: (the purple graph is for the parallel arrangement)
And here is frequency response comparison at full output power with DC bias: (the purple graph is for the parallel arrangement)