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How do you determine primary winding OHMS on an unknown Audio Transformer?

gary350

Well-Known Member
There are lots of audio transformers for sale on ebay but many of the sellers have no idea what they are selling. All they know is this transformer was removed from an amplifier and the secondary winding is 4, 8 and 16 ohms.

If I need a 5000 ohms primary winding with a center tap how do I determine if a transformer is 5000 ohms on the primary?
 

unclejed613

Well-Known Member
Most Helpful Member
most often, audio transformer impedances are measured at 1khz. the measurements are done with a bridge circuit. the unknown impedance is on one arm of the bridge, and a potentiometer is on the measurement arm. the potentiometer is adjusted for a null, and then disconnected and measured with an ohmmeter, (or on most commercially made bridges, the potentiometer resistance is read from a calibrated dial attached to the potentiometer shaft). or you can use an AC current source and measure the voltage across the inductor at 1khz with the current source set for 1ma. 5V across the inductor would be equivalent to 5k ohm impedance. of course there's a 90 degree lead in the voltage waveform, so the measurement is best done with an oscope in XY mode (current on x axis, voltage on y axis), with some math required for correct results.
 

MrAl

Well-Known Member
Most Helpful Member
Hi,

You could drive the primary with a small test voltage, like 1 volt, then measure the output voltage Vout. The primary impedance is then the output impedance times the square of the turns ratio.

TR=TurnsRatio=Vin/Vout
Rp=PrimaryImpedance=SecondaryImpedance*TR*TR

For example, say you drive the primary with 10 volts and the output is 1 volt, and you measure that 1 volt on the 8 ohm output of the transformer. This means the turns ratio is 10, and so the primary impedance is:
Rp=8*10*10=800 ohms.

This is an approximation and you may need to apply some resistive load on the secondary to get better results.
 
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Jon Wilder

Active Member
Audio transformers in and of themselves do not have "impedance" per se. The primary impedance with a given load is a REFLECTED impedance. That being said, we need to find the IMPEDANCE RATIO of the transformer, which is equal to the voltage ratio squared.

I drive my audio output transformers @ 120VAC across the primary, then measure the voltage across the secondary.

Find the voltage ratio by dividing the Vpri by Vsec, then square that to get the impedance ratio -

Impedance Ratio = (Vpri / Vsec) ^ 2

Once the impedance ratio has been determined, you can multiply the impedance ratio by the speaker load that you plan to drive with it in order to determine what the reflected primary impedance with that load will be.
 
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gary350

Well-Known Member
Hi,

You could drive the primary with a small test voltage, like 1 volt, then measure the output voltage Vout. The primary impedance is then the output impedance times the square of the turns ratio.

TR=TurnsRatio=Vin/Vout
Rp=PrimaryImpedance=SecondaryImpedance*TR*TR

For example, say you drive the primary with 10 volts and the output is 1 volt, and you measure that 1 volt on the 8 ohm output of the transformer. This means the turns ratio is 10, and so the primary impedance is:
Rp=8*10*10=800 ohms.

This is an approximation and you may need to apply some resistive load on the secondary to get better results.
I will write this down in my notes and make sure I don't loose it. This is great information. I learn something new every day.

40 years ago when I was working in electronics every day I had a good supply of about 200 used transformers on hand. I would put 6 volts across the 16 ohm secondary winding of a 5000 ohm transformer then read the voltage across the primary winding and if the voltage was 106 volts. Then I would get all the unknown used transformers of the same physical size and test them 1 by 1 doing the same thing if I got 106 volts on the primary then I assumed it was 5000 ohms too. This was an idea I figured out on my own it always seemed to be correct the used transformers worked just as well as the new transformer. I don't have 200 transformers on hand anymore I wish I did.
 
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MrAl

Well-Known Member
Most Helpful Member
Hi again,

Oh yes, good idea.

Also, what Jon mentioned about the impedance of a transformer is interesting, but with audio transformers they are often designed with a given load in mind. This means there is some optimization that goes into the design of many transformers regarding a given load impedance (and consequently a given source impedance). The difference between the theory and practice here is that the audio transformer does best when used as the manufacturer recommends, so if they say "16 ohms" then they mean 16 ohms and not 4 ohms. The difference comes in the form of added distortion and some loss of low frequency response. Whether or not this is going to matter for your exact application is a question only you can answer by test. For the lack of information though i would go with the recommended load impedance such as that stamped on the transformer body.
 

Jon Wilder

Active Member
Y'all are seriously making this out to be harder than it is.

Transformers just transform AC. They have absolutely no clue whether they're transforming audio signals, power fed from a wall socket, etc etc. They don't even know what impedance is. All they see is current and voltage, and impedance is something that is calculated from this.

If you have 40Vrms at the secondary and your load is drawing 2.5A from the secondary at this voltage (basically a 16 ohm load) with an impedance ratio of 106.25, the primary will see a plate-plate voltage of 412Vrms @ 121mA, which would be the equivalent of a 1.7K ohm plate-plate load, which would also yield a 425R load from each plate to the center tap.

They also have OT's with multi-tap secondaries that allow you to run 4, 8 or 16 ohm loads while yielding the same reflected primary impedance.

If you double the load impedance on the secondary, the reflected primary impedance ALSO doubles. If you halve the load impedance on the secondary, the reflected primary impedance also halves.

It really is that simple. Audio transformers are just power transformers that operate at audio frequencies. If you look at a Class AB push-pull valve amp design, you will see that it's nothing more than a full wave grounded center tap rectifier circuit flipped upside down and backwards.
 
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unclejed613

Well-Known Member
Most Helpful Member
the bridge method then would work if the secondary were terminated with the correct resistance, 8 ohms for instance the transformer the OP mentioned
 

colin55

Well-Known Member
I am just saying you need to know the wattage of a transformer to see if it is suitable for an application.
It looks like he wants an output transformer.
 

The Electrician

Active Member
If you double the load impedance on the secondary, the reflected primary impedance ALSO doubles. If you halve the load impedance on the secondary, the reflected primary impedance also halves.

It really is that simple.
Apparently it really isn't that simple.

I got an output transformer and measured a voltage transfer ratio of 24.35 @ 1kHz.

The square root of the primary to secondary impedance ratio should be the same as the voltage ratio

I then connected various load resistors to the secondary and measured the impedance at the primary side with an LCR meter, also @ 1kHz, obtaining the following results:

Code:
Load Resistance     Primary Impedance    Square Root of impedance ratio
                       (measured)
      1Ω                 1190Ω                       34.50
      2Ω                 1642Ω                       28.65
      4Ω                 2854Ω                       26.71
      8Ω                 4842Ω                       24.60
     16Ω                 8068Ω                       22.46
     32Ω                12215Ω                       19.54
     64Ω                15710Ω                       15.67
    128Ω                17896Ω                       11.82
The measured results aren't following the simple rule you gave, so apparently there's more to it than the simple rule.
 

unclejed613

Well-Known Member
Most Helpful Member
the DC resistance of the windings would have something to do with those results, as well as the coupling factor between the windings. these two factors (as well as others, such as parasitic capacitances) introduce losses which would show up as less than ideal results compared to the theory. did you notice that at 8 ohms, the results of the impedance ratio calculation closely matched the measured voltage ratio? the reflected primary impedance with a secondary load of 8 ohms was also very close to 5k.
 

gary350

Well-Known Member
Apparently it really isn't that simple.

I got an output transformer and measured a voltage transfer ratio of 24.35 @ 1kHz.

The square root of the primary to secondary impedance ratio should be the same as the voltage ratio

I then connected various load resistors to the secondary and measured the impedance at the primary side with an LCR meter, also @ 1kHz, obtaining the following results:

Code:
Load Resistance     Primary Impedance    Square Root of impedance ratio
                       (measured)
      1Ω                 1190Ω                       34.50
      2Ω                 1642Ω                       28.65
      4Ω                 2854Ω                       26.71
      8Ω                 4842Ω                       24.60
     16Ω                 8068Ω                       22.46
     32Ω                12215Ω                       19.54
     64Ω                15710Ω                       15.67
    128Ω                17896Ω                       11.82
The measured results aren't following the simple rule you gave, so apparently there's more to it than the simple rule.
The Transformer Design book I am reading says impedance changes by the cube of the core area of the transformer. I may need to go back and read that again to be sure that is correct. But anyway it seem the larger the transformer the higher the impedance. I think that means a larger transformer needs fewer turns on the primary to have the same impedance of a smaller transformer.
 
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Jon Wilder

Active Member
Apparently it really isn't that simple.

I got an output transformer and measured a voltage transfer ratio of 24.35 @ 1kHz.

The square root of the primary to secondary impedance ratio should be the same as the voltage ratio

I then connected various load resistors to the secondary and measured the impedance at the primary side with an LCR meter, also @ 1kHz, obtaining the following results:

Code:
Load Resistance     Primary Impedance    Square Root of impedance ratio
                       (measured)
      1Ω                 1190Ω                       34.50
      2Ω                 1642Ω                       28.65
      4Ω                 2854Ω                       26.71
      8Ω                 4842Ω                       24.60
     16Ω                 8068Ω                       22.46
     32Ω                12215Ω                       19.54
     64Ω                15710Ω                       15.67
    128Ω                17896Ω                       11.82
The measured results aren't following the simple rule you gave, so apparently there's more to it than the simple rule.
Remember...reflected impedance figures are CALCULATED...not "measured". Turns ratio never changes, therefore impedance ratio never changes either. Yet in your chart the turns ratio keeps changing. Why is this?

You have a voltage transfer ratio of 24.35, which constitutes a turns ratio of 24.35. Your impedance ratio would be -

24.35 ^ 2 = 593

Now...impedance ratio is the turns ratio squared.Therefore, your chart should have looked more like this -

Code:
Secondary Load		Reflected Primary Impedance

1R			593R
2R			1186R
4R			2372R
8R			4744R
16R			9488R
32R			18976R
64R			37952R
128R			75904R
colin55 said:
I am just saying you need to know the wattage of a transformer to see if it is suitable for an application.
It looks like he wants an output transformer.
Unless you know something about valve amplifiers, it would seriously be in your best interests to refrain from posting on this thread. Seriously.
 
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audioguru

Well-Known Member
Most Helpful Member
The output transformer of a vacuum tube amplifier causes a lot of problems because an 8 ohm speaker is 8 ohms only at a few frequencies.
The speaker and enclosure resonate at a low frequency with a high impedance that needs damping to avoid booming. The high output impedance of a vacuum tube amplifier provides poor damping but the extremely low output impedance of a solid-state amplifier provides excellent damping.
Each speaker has inductance then its impedance rises at high frequencies.
The crossover network also changes the impedance of a speaker system.

The output level from the output transformer of a vacuum tube amplifier changes when the speaker impedance changes. But the output level from a solid state amplifier does not.
 

Jon Wilder

Active Member
The output level from the output transformer of a vacuum tube amplifier changes when the speaker impedance changes. But the output level from a solid state amplifier does not.
Wow...if THIS doesn't defy every aspect of Ohm's Law.

Seriously, I think it would be in your best interests to retract this statement. I know you know better than this. ;)
 
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Nigel Goodwin

Super Moderator
Most Helpful Member
Wow...if THIS doesn't defy every aspect of Ohm's Law.
Ohms law doesn't apply to inductors, inductive reactance does.

Seriously, I think it would be in your best interests to retract this statement. I know you know better than this. ;)
He's got a serious hang up about valve amplifiers, but there's an iota of truth in what it says - just not anything like the degree he claims. Don't forget, as the impedance of the speaker changes with frequency, the impedance of the transformer changes as well.

The moral of this thread though is quite simple - don't buy a second hand valve output transformer off an Ebay seller who just rips it out of some gear, with no regard for what it is - at the very least he should specify exactly what it was removed from, and what the output valves were.
 

audioguru

Well-Known Member
Most Helpful Member
When an excellent output transformer with low phase shift is used in a vacuum tube amplifier then it can have some negative feedback to reduce its fairly high output impedance (and also reduce its fairly high distortion). Then changes in load impedance by changes in a speaker's impedance vs frequency will make less difference in output level.

A solid state amplifier has a lot of negative feedback that reduces its output impedance (and distortion) to almost zero so its output level does not change when a speaker's impedance changes.
 

Jon Wilder

Active Member
When an excellent output transformer with low phase shift is used in a vacuum tube amplifier then it can have some negative feedback to reduce its fairly high output impedance (and also reduce its fairly high distortion). Then changes in load impedance by changes in a speaker's impedance vs frequency will make less difference in output level.

A solid state amplifier has a lot of negative feedback that reduces its output impedance (and distortion) to almost zero so its output level does not change when a speaker's impedance changes.
Lots of valve amps also use negative feedback.

Point being...regardless of impedance ratio, power out = power in. With output transformers you do have to worry about the efficiency of the OT yes, so power out = efficiency % of power in. HOWEVER, speaker impedance changes on BOTH types of amplifiers yet output voltage holds constant. This means that output CURRENT will change with speaker impedance. Therefore output power will ALSO change when speaker impedance changes regardless of amplifier type.

I know you have a personal vendetta with valve amps so I don't know why you still feel compelled to come onto valve amp related threads and talk about all the supposed "problems" they have. But seriously, please leave your personal vendettas for valve amps at the door if you're going to continue to come onto valve amp related threads. We only want to hear valve amp FACTS here...not your personal hang ups with them

Nigel Goodwin said:
Don't forget, as the impedance of the speaker changes with frequency, the impedance of the transformer changes as well.
Yes this is correct but the RATIO at which it changes is the same as it is on the secondary side. If you have a 50% change in load impedance on the secondary side, there will be a corresponding 50% change in reflected primary impedance as well.
 
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