Are there simple benchtop, out of circuit measurements that can be made in order to evaluate OPT quality?
If that question is too broad let me give background. I have Edcor XSE 25W 7.6K:8 transformers I purchased new. I also have EL84 SE transformers that came out of a vintage amp. They are roughly the size of the Edcors, maybe a little larger, and roughly the same ratio as the Edcors. Are there any simple measurements, say with a DMM or LCR meter and/or simple jig that will let me estimate performance or at least an "A is probably better than B" test?
If that question is too broad let me give background. I have Edcor XSE 25W 7.6K:8 transformers I purchased new. I also have EL84 SE transformers that came out of a vintage amp. They are roughly the size of the Edcors, maybe a little larger, and roughly the same ratio as the Edcors. Are there any simple measurements, say with a DMM or LCR meter and/or simple jig that will let me estimate performance or at least an "A is probably better than B" test?
Sorry, I didn't mean figuring out the impedance ratio. I meant something more like estimating FR, or if 1 likely has better FR than the other.
I am guessing that it could be simplified. Given the similarity, wouldn't the one with higher inductance at 1KHz as measured on the LCR meter have a lower F3, for example?
Inductance varies with frequency, so a measurement at 1 Khz does not tell you much.
Preferably measure at least not higher than 100 Hz.
Besides, primary DC current (it's about SE OPT's) also influences inductance.
Actually, the best way to compare is measuring the transformers in the amplifier circuit.
Preferably measure at least not higher than 100 Hz.
Besides, primary DC current (it's about SE OPT's) also influences inductance.
Actually, the best way to compare is measuring the transformers in the amplifier circuit.
Sorry, maybe this helps, I haven't read it but I suppose it should explain how to make the frequency response more extended. (see point 1.3.2 )
If it doesn't work, don't even answer me. I guess you've already searched the net enough. 🙂
https://www.jensen-transformers.com/wp-content/uploads/2014/08/Audio-Transformers-Chapter.pdf
Use a 50 Ohm signal generator, terminate it with 50 Ohms, that is 25 Ohms impedance. Connect a series resistor that is the same value as the plate resistance rp. That is within 25 Ohms of the nominal rp. Connect that to the primary.
Then load the 8 Ohm tap with an 8 Ohm non inductive resistor.
Check the frequency response with a scope (many DMM only work to about 1kHz).
With a scope probe on the primary, and a scope probe on the secondary, you can also check the phase versus frequency (if your [digital] scope has the phase measurement, you get an automatic measurement). Otherwise you have to eyeball it, or use the XY display.
Before that measurement, be sure to compensate both probes (with the probes on the exact channel they will be used on).
If you have a 600 Ohm generator, terminate with 600 Ohms. That is 300 Ohms impedance. and connect a series resistor, X Ohms, where rp - 300 Ohms = X Ohms. Terminate the secondary as in the first example, and use the scope to measure the frequency response.
Again, with 2 scope probes, you can check the phase versus frequency. Don't forget to compensate the probes first, as per the above example.
These above measurements will take into account the winding inductances, DCRs, distributed capacitances, and leakage inductance of the transformers.
What they do not do for a single ended transformer is account for the different primary inductance when there is DC current applied.
That is where a special setup is required, or where just as stated in one of the earlier posts in this thread says, drive the transformer in an actual tube amp.
A typical output transformer primary is self resonant at somewhere between 500Hz and 3 kHz. That is why you need to drive the primary with a resistance = rp of the driving tube you will use it with, and why you have to load the secondary tap with a resistance that is equal to the tap's rated impedance.
The driving impedance, and the load impedance will swamp out the self resonant frequency, so that you can measure the true frequency response of the transformer.
Estimating and Guesstimating are exactly what the words say.
Careful measurements, properly done, will give a more certain and sure answer.
Then load the 8 Ohm tap with an 8 Ohm non inductive resistor.
Check the frequency response with a scope (many DMM only work to about 1kHz).
With a scope probe on the primary, and a scope probe on the secondary, you can also check the phase versus frequency (if your [digital] scope has the phase measurement, you get an automatic measurement). Otherwise you have to eyeball it, or use the XY display.
Before that measurement, be sure to compensate both probes (with the probes on the exact channel they will be used on).
If you have a 600 Ohm generator, terminate with 600 Ohms. That is 300 Ohms impedance. and connect a series resistor, X Ohms, where rp - 300 Ohms = X Ohms. Terminate the secondary as in the first example, and use the scope to measure the frequency response.
Again, with 2 scope probes, you can check the phase versus frequency. Don't forget to compensate the probes first, as per the above example.
These above measurements will take into account the winding inductances, DCRs, distributed capacitances, and leakage inductance of the transformers.
What they do not do for a single ended transformer is account for the different primary inductance when there is DC current applied.
That is where a special setup is required, or where just as stated in one of the earlier posts in this thread says, drive the transformer in an actual tube amp.
A typical output transformer primary is self resonant at somewhere between 500Hz and 3 kHz. That is why you need to drive the primary with a resistance = rp of the driving tube you will use it with, and why you have to load the secondary tap with a resistance that is equal to the tap's rated impedance.
The driving impedance, and the load impedance will swamp out the self resonant frequency, so that you can measure the true frequency response of the transformer.
Estimating and Guesstimating are exactly what the words say.
Careful measurements, properly done, will give a more certain and sure answer.
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Hi 6A3
I have an amplifier with 4 and 8 Ohms output, but the impedance of the acoustic cabinets is 6 Ohms. I am intrigued to know why the manufacturer (Prima Luna) recommends "experimenting with different impedance loads, and letting the ear decide"
That does not go against a correct transfer of energy?
Sorry to the OP, for the OT
I have an amplifier with 4 and 8 Ohms output, but the impedance of the acoustic cabinets is 6 Ohms. I am intrigued to know why the manufacturer (Prima Luna) recommends "experimenting with different impedance loads, and letting the ear decide"
That does not go against a correct transfer of energy?
Sorry to the OP, for the OT
When in doubt, start by connecting the "6 Ohm" speakers to the 4 Ohm tap.
Many "6 Ohm" speakers have minimum impedances at several frequency areas of 4 Ohms.
And similarly, many "8 Ohm" speakers have minimum impedances at several frequency areas of 6 Ohms.
And many of the 6 and 8 Ohm speakers may have maximum impedances of 20, 30, 40 Ohms or more, at one or more frequency areas.
But many speakers can be more extreme, or less extreme than the above "typical" numbers.
Not all amplifiers are equal with respect to how they deal with varying loudspeaker impedance versus frequency.
And finally, in light of the above factors . . .
Just like the speaker company said, try the "6 Ohm" speakers on the 4 Ohm tap, and try them on the 8 Ohm tap.
"All system setups and connections are equally optimal, but some systems and connections are more equally optimal than others". Stolen from George Orwell's "Animal Farm", and modified by me.
Your mileage may vary.
Many "6 Ohm" speakers have minimum impedances at several frequency areas of 4 Ohms.
And similarly, many "8 Ohm" speakers have minimum impedances at several frequency areas of 6 Ohms.
And many of the 6 and 8 Ohm speakers may have maximum impedances of 20, 30, 40 Ohms or more, at one or more frequency areas.
But many speakers can be more extreme, or less extreme than the above "typical" numbers.
Not all amplifiers are equal with respect to how they deal with varying loudspeaker impedance versus frequency.
And finally, in light of the above factors . . .
Just like the speaker company said, try the "6 Ohm" speakers on the 4 Ohm tap, and try them on the 8 Ohm tap.
"All system setups and connections are equally optimal, but some systems and connections are more equally optimal than others". Stolen from George Orwell's "Animal Farm", and modified by me.
Your mileage may vary.
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academia50,
Attaching a 6 ohm load to the 4ohm or 8ohm secondary taps simply shifts the impedance of the OPT primary taps in the same ratio. 6ohm load on 4ohm tap will shift the OPT primary impedance by a factor of 1.5, and 6ohm load on the 8ohm tap will shift the primary impedance by a factor of 0.75.
The result of increasing the primary impedance by 1.5x is typically decreased power output and also decreased THD. The opposite is true when decreasing the primary impedance (0.75x). This all assumes that the increase or decrease of primary impedance remains within the operating range of the output tube topology and operating point.
Attaching a 6 ohm load to the 4ohm or 8ohm secondary taps simply shifts the impedance of the OPT primary taps in the same ratio. 6ohm load on 4ohm tap will shift the OPT primary impedance by a factor of 1.5, and 6ohm load on the 8ohm tap will shift the primary impedance by a factor of 0.75.
The result of increasing the primary impedance by 1.5x is typically decreased power output and also decreased THD. The opposite is true when decreasing the primary impedance (0.75x). This all assumes that the increase or decrease of primary impedance remains within the operating range of the output tube topology and operating point.
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The damping factor that is presented to a given speaker is higher when it is connected to the 4 Ohm tap.
The damping factor that is presented to the same given speaker is lower when it is connected to the 8 Ohm tap.
Speakers are not just resistive loads varying from say 6 Ohms to 40 Ohms.
They have capacitive, inductive, resonant, and resistive impedances, and combinations of that, RC, RL, RLC.
Negative feedback, especially Global negative feedback of a given amplifier will deal with that very well, or not very well.
A speaker that has a maximum impedance of 40 Ohms, may clip earlier on the 4 Ohm tap
(run out of voltage range) than the same speaker on the 8 Ohm tap.
A speaker that has a minimum impedance of 4 Ohms, may clip earlier on the 8 Ohm tap
(run out of current range), than the same speaker on the 4 Ohm tap.
Tradeoffs.
Your Mileage May Vary.
The damping factor that is presented to the same given speaker is lower when it is connected to the 8 Ohm tap.
Speakers are not just resistive loads varying from say 6 Ohms to 40 Ohms.
They have capacitive, inductive, resonant, and resistive impedances, and combinations of that, RC, RL, RLC.
Negative feedback, especially Global negative feedback of a given amplifier will deal with that very well, or not very well.
A speaker that has a maximum impedance of 40 Ohms, may clip earlier on the 4 Ohm tap
(run out of voltage range) than the same speaker on the 8 Ohm tap.
A speaker that has a minimum impedance of 4 Ohms, may clip earlier on the 8 Ohm tap
(run out of current range), than the same speaker on the 4 Ohm tap.
Tradeoffs.
Your Mileage May Vary.
I've had this configuration for a few years now and I have tested it on both terminals. I have not noticed any audible differences, not even when using the subwoofer amplifier taking the signal from the OPT. But I won't litter this thread with any more off-topic questions, thanks cogitech and 6A3 summer
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