The OPT measures at close to 5K primary impedance into a presumably 4 ohm load. I don't know the actual secondary impedance but it measures about .5 DCR. This seems like a 4 ohm tap. There are no other secondary taps except the ground.
Anyway, the question is what would the reflected impedance be into an 8 ohm load? Sorry, i can never keep this relationship straight.
Thank you for clarifying things, 808.
Anyway, the question is what would the reflected impedance be into an 8 ohm load? Sorry, i can never keep this relationship straight.
Thank you for clarifying things, 808.
The impedance ratio varies with the square of the turns ratio. That DCR is part of the inevitable real world losses that are always encountered, but it's the turns ratio that matters.
If it is 5K with a 4 ohms load, It will be 10K ohms if you connect a 8 ohms load onto that 4 ohms tap.
Well, my other similar sized OPT measures about twice the resistance. So, that is why i am guessing that the output on this tranny is for 4 ohm speakers. Is it wrong to make that determination for this particular OPT?
There are good transformers, not so good transformers, and bad transformers.
Transformer design takes into account many factors: cost, weight, max power, insertion loss, bandwidth, inductance, saturation, DCR of each winding, etc. . . . and we did not even talk about impedance yet.
So the question remains, what are the targeted primary and secondary impedances.
5K to 4 Ohm
6.67K to 6 Ohm
10K to 8 Ohm
What is the manufacturer?
Is there a stamped / engraved part number, letters/numbers?
What is the weight?
What are the dimensions?
Is it single ended, or push pull?
What did it come out of?
Do you have a picture of the transformer you can post?
Measure the inductance of the primary with the secondary unloaded (at a low frequency, i.e. 30Hz).
Transformer design takes into account many factors: cost, weight, max power, insertion loss, bandwidth, inductance, saturation, DCR of each winding, etc. . . . and we did not even talk about impedance yet.
So the question remains, what are the targeted primary and secondary impedances.
5K to 4 Ohm
6.67K to 6 Ohm
10K to 8 Ohm
What is the manufacturer?
Is there a stamped / engraved part number, letters/numbers?
What is the weight?
What are the dimensions?
Is it single ended, or push pull?
What did it come out of?
Do you have a picture of the transformer you can post?
Measure the inductance of the primary with the secondary unloaded (at a low frequency, i.e. 30Hz).
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I wrote a simplified piece about The subject a few years ago,You Will find it here: Audio output transformer
Give the primary 12VAC and measure the voltage on the secondary.
If the secondary voltage was 1V then the turns ratio is 12:1. The impedance ratio is the square of the turns ratio, in this example 144.
You then multiply that by how many ohms the speaker is and voila! In this case 1152R:8R.
(((12/1)^2)8)=1152
Cheers.
Koda
If the secondary voltage was 1V then the turns ratio is 12:1. The impedance ratio is the square of the turns ratio, in this example 144.
You then multiply that by how many ohms the speaker is and voila! In this case 1152R:8R.
(((12/1)^2)8)=1152
Cheers.
Koda
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Yes, that is what i did to get the number of 5800, but assuming that the speaker tap is a 4 ohm. This is what i am guessing from the DCR reading of less than 0.5 ohm.
This is a Norelco transformer, from a tubed tape deck. I forget the brand, but the one made in Switzerland. Supposedly it is designed for SE EL84. Why do you want to know the inductance of the primary? Just remembered, supposedly, the OPT is from a REVOX.
That answers your question, then... SE EL84 needs around 5k primary impedance, so it's got to be a 4 ohm secondary.
Transformers transfer the power between windings, losing a bit in process. Voltage transfer is directly proportional to number of turns, current is reverse proportional to number of turns. So, if you multiply V*I you get almost the same power. But resistance is V/I so resistance "reflection" as the result is proportional to square of the ratio of numbers of turns.
However, you can not "reflect" any impedances equally on all audio frequency band, since you have to add to your calculations inductance of the winding in parallel, and loss inductance in series. Also, stray capacitances have to be taken in mind on the upper frequencies. Also, on lower frequencies the magnetic material would saturate starting from the certain voltage limiting the power that can be transferred. Plus, you have to keep in mind that some too high for the transformer voltages would cause sparks between layers. That means, if you have for example 2K/4 Ohm transformer, you can use it for 4k/8 Ohm, but expect lower frequency twice higher on the same power, or lower power on the same frequency.
However, you can not "reflect" any impedances equally on all audio frequency band, since you have to add to your calculations inductance of the winding in parallel, and loss inductance in series. Also, stray capacitances have to be taken in mind on the upper frequencies. Also, on lower frequencies the magnetic material would saturate starting from the certain voltage limiting the power that can be transferred. Plus, you have to keep in mind that some too high for the transformer voltages would cause sparks between layers. That means, if you have for example 2K/4 Ohm transformer, you can use it for 4k/8 Ohm, but expect lower frequency twice higher on the same power, or lower power on the same frequency.
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Max, have you come across a SE el84 output trans which measures about 0.5 ohm on an 8 ohm tap?
The DC resistance will vary from tranny to tranny by the power rating, not the tap rating. The gauge of the wire sets the resistance. Bigger wire for more power handling. Turns ratio is only part of it for wire length, too.
Maybe, but what´s your point? We are analyzing your transformer, not others. You are way too focused on DCR alone, which as told many times, is NOT the main parameter.
The main one is turns ratio, and your measurements so far seem to match :
1) 5k promary impedance, simply because that´s a common useful EL84 typical load impedance
and
2) IF primary impedance is 5k, THEN turns ratio indicates a 4 ohm tap.
You *seem* to be expecting somebody tell you "that´s an 8 ohm tap" ... sadly that does not seem to be the case.
^^^^^ this. Plus, most small speakers in vintage radios, tv, tape recorders, etc have a 3.2 - 4 ohm voice coil, 8 ohms is more common in hifi speakers.
It would be wrong to draw that conclusion for any OPT. Different designers compromise on different parameters, so measuring the OPT secondary DC resistance tells you very little about the intended load impedance apart from a very rough ballpark. 0.5ohms resistance could mean a lowish quality 4 ohm secondary or a highish quality 15 ohm secondary.DAK808 said:
The ratio of primary to secondary DC resistances can give you a very rough guide, because if you assume that similar winding space was used for each (which might not be true) then the resistances scale like the square of the turns ratio. Much better to actually measure the turns ratio, although even this only gives you the impedance ratio not the actual impedance.
Thank you for replying. I am not familiar with speakers so i was seeking more info. And as you also stated most hifi speakers are multi tapped or 8 ohm. With no printed info on my outputs, i am just trying to get as much as possible.
As others have stated, the EL84 needs approx. 5k in primary impedance. I think it's a fair assumption that this is the primary impedance of the transformer, and the secondary impedance can be calculated after determining the turns ratio.
As an aside, if you're building a little DIY amp with this transformer, and you happen to only have 8 ohm speakers, don't fret too much about using this transformer. Maybe in theory it wont' match the load perfectly, but all you will lose is a little power, it will sound just fine and won't hurt anything. I routinely connect 8 ohm speakers to 4 ohm taps of tube amps, when the better damping factor offered by the lower tap is more useful to me than the full output capability of the amp.
Thanx Max, for sharing that thought.
Well, to throw another curve ball at you all, i was thinking of using this OPT for running 7591 tubes. The requirements for that tube is not that close to the 6bq5 and so i was going to shelve that idea. But, i don't mind trying some "work arounds" to get the project working if you guys and girls are willing to help. This project is meant to rescue an already made amp where the performance is lacking, due to a poor circuit. I just want to do something different than the SE EL84 amp which i have made too many and want to do something with a different power tube. I have lots of 7591 tubes but if it is a waste of time then i will try another tube.
cheers.
Well, to throw another curve ball at you all, i was thinking of using this OPT for running 7591 tubes. The requirements for that tube is not that close to the 6bq5 and so i was going to shelve that idea. But, i don't mind trying some "work arounds" to get the project working if you guys and girls are willing to help. This project is meant to rescue an already made amp where the performance is lacking, due to a poor circuit. I just want to do something different than the SE EL84 amp which i have made too many and want to do something with a different power tube. I have lots of 7591 tubes but if it is a waste of time then i will try another tube.
cheers.
It's possible, but the 7591 really needs a bit more bias current, which will saturate the core of that little transformer.
If you want to have some fun with it, why not try something like a 6V6GT, or a 6AQ5, 6005, 6CM6, 6K6, 6F6, single ended type 45 could be fun, albeit a bit costly. Or you could do an ECL86 single ended amp. Or maybe make a "spud" amp with a compactron. You really have almost unlimited choices given how many different tubes have been made over the years, just try and keep the DC current below 45-50mA, to avoid ruining your bass response, and keep the voltage same or lower than EL84 values ,to avoid stressing the little transformer.
If you want to have some fun with it, why not try something like a 6V6GT, or a 6AQ5, 6005, 6CM6, 6K6, 6F6, single ended type 45 could be fun, albeit a bit costly. Or you could do an ECL86 single ended amp. Or maybe make a "spud" amp with a compactron. You really have almost unlimited choices given how many different tubes have been made over the years, just try and keep the DC current below 45-50mA, to avoid ruining your bass response, and keep the voltage same or lower than EL84 values ,to avoid stressing the little transformer.
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