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Diabolical Artificer 15th Jul 2019 3:26 pm

"Matching" transformer.
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I've several transformers labelled "matching tfmr" and another labelled "speaker isolation tfmr"; your bog standard output tfmr is essentially an impedance matching tfmr, these are the same so why the designation and traditionally why were they used in the past but not now?

There is a little booklet in the box that says this - paraphrased - This auto tfmr is designed to match any combo of load and OP Z's in the range 2-16 ohms..." Most of the ones I have are auto tfmr's but I do have a "isolation matching tfmr", in the booklet it says a matching/iso tfmr can be used to connect an ext lodspeaker to a TV set.

I'm guessing in the 50's most speakers were 16 ohm and the "new" transistor amplifiers of the 60's early 70's were designed for a nominal 8r speaker Z so maybe these were sold then to save loss of power Op or similar.

One last thing, I had a trannie PA amp that has an OPT between OP stage and spkr terminals, one sec for 100v line but also for 4 and 8 ohm spkr's, it sounded superb.

Any thoughts welcome, Andy.

kalee20 15th Jul 2019 4:03 pm

Re: "Matching" transformer.
I think you have virtually answered your own questions!

Speaker isolation transformer - to provide isolation from a live chassis item to an external speaker. (Many of these won't meet current safety standards, but almost certainly nobody has died yet as a result of insulation failure).

Matching transformers would match, say, a 15Ω loudspeaker to an amplifier designed for 3Ω. Impedances in common use are 3Ω, 8Ω, 15Ω but there is also the 100V standard where a matching transformer might be used to match an 8Ω speaker to a 100V line at 10W.

The output transformer in your valve amplifier is of course a matching transformer, but to avoid confusion it would probably be referred to as an "output transformer" rather than a "matching transformer."

Synchrodyne 16th Jul 2019 1:47 am

Re: "Matching" transformer.
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Originally Posted by Diabolical Artificer (Post 1160643)
I'm guessing in the 50's most speakers were 16 ohm and the "new" transistor amplifiers of the 60's early 70's were designed for a nominal 8r speaker Z so maybe these were sold then to save loss of power Op or similar.

My recollection is that it was considered unwise to use these kinds of speaker impedance matching transformers with early solid-state amplifiers. I am not sure about this, but one potential problem may have been that at very low frequencies, they offered only a very low impedance to the amplifier, not a problem with most valved units but potentially destructive for some solid-state units.

I suspect that a major use for these transformers in the 1950s and 1960s was in connecting say a 15-16 ohm speaker of the hi-fi type to the external speaker outputs of typical radio receivers and radiograms, which were typically designed for speakers in the 2.5 to 3 ohm range. With limited AF power available, often around 3 watts, impedance matching was worthwhile.

Some of the Wharfedale cabinet speakers of the era were intended for such use as well as with hi-fi systems. This advertisement for the W3 is indicative:

Attachment 186763

The W3 was a 15 ohm speaker, so use of the WMT1 autotransformer would have been advisable when it was to be fed from a domestic radio receiver optimized for say 3 ohms.


Originally Posted by Diabolical Artificer (Post 1160643)
One last thing, I had a trannie PA amp that has an OPT between OP stage and spkr terminals, one sec for 100v line but also for 4 and 8 ohm spkr's, it sounded superb.

A classic example of a transistor power amplifier equipped with an output transformer intended to match a wide variety of loads was the Quad 50E.


Radio Wrangler 16th Jul 2019 3:21 am

Re: "Matching" transformer.
Just be careful with the word 'Matching'

The technically exact meaning is the connection together of two things where the output impedance of the source is equal to the input impedance of the load. Note that there is an implied halving of efficiency, because as much power must be being dissipated in the source impedance of the source as goes into the load. This is not an ideal situation when your only aim is to stuff power into the load. So, many sources, including radio transmitters, are designed to shove the required power into the specified load, but their output impedance is NOT matched to the load. The freedom is used to be more efficient, use smaller devices, have smaller electricity bills. This either means that the output impedance is appreciably lower than true match, or appreciably higher.

So why the fuss about matching?

The first reason is simple science. If I have a length of cable, it will have a characteristic impedance. If I send a wave down this cable, it temporarily looks to whatever is driving it as Z0. down the cabe go a pair of waves, one voltage, one current. at any point, at any time, the ratio between the voltage and the current are Z0. So that's what flies out the far end and hits the load. IF the load impedance is equal to Z0 (The cable and the load are matched to each other) The load sees the right ratio between the current and the voltage and absorbs the lot. Yummy! IF the load does not equal Z0, then there is either too much voltage for the current, or too much current for the voltage. The load cannot absorb all that hits it. Now the power sent down the line was put in an amount of time earlier and as we aren't about to discover the keys to a time machine, there isn't anything we can do about that. There is only one place for the power the load doesn't accept to go... back up the line. The line will only accept a reverse wave as a voltage/current pair in the magic Z0 ratio, so you have to solve a couple of equations to work out what fraction of the original power gets to the load, and how much turns back. Double entry bookkeeping for the voltages and currents at the point where the line connects to the load shows that the books balance, and from far on high, Mr Kirchoff is smiling. So now we have a smaller wave coming back and hitting the source. The wave sees the source as a new load to try to get off into. If the source is truly matched to the cable, it absorbs the reflected wave totally, and the tale ends here, provided the source can take the applied voltage and current vector-added to the voltage and currents of its intended output. But what if the source isn't matched to the cable? Its output impedance is wrong for the reflected wave hitting it and it's that partial reflection thing all over again. Our reflected energy bounces to-and-fro losing some at each bounce until the level is negligible.

Sounds complicated? It is, but there are tools for handling it. Also, it only needs bothering with if the cable length is a significant fraction of a wavelength at your highest frequency. At audio, this means you are going to need a very very large lounge to justify those immensely expensive speaker cables :-)

The second reason is that in some uses of cables you don't want the hall-of-mirrors thing going on. Good RF measuring instruments need to suppress it because it upsets their accuracy with ripply frequency responses. Sig gen designers do an output amp with as low a Zout as they can, then pad it up to 50 Ohms with a resistor. True matching makes delivery of calibrated power levels possible.

There is something called "The maximum power transfer theorem" It's dead simple. It says that if you have a power source, with an output impedance of Zout, driving a load of Zload, then adjusting Zload to equal Zout will extract the maximum power it is mathematically possible from that theoretical source.

Sounds good, dunnit?

It's what we came across earlier where our cable met its load, so we want to do it in some places.


The max power transfer theorem cares not at all about efficiency and the size of your power bills.

The max power transfer theorem cares not at all about whether your source can stand the voltages and currents it sets, or if the thing goes up in smoke. Your source is a real world one and not quite the same as an idealised mathematical model. Real world ones are much more expensive.

So in a hifi setup, the speaker cables are negligible, The speaker impedance should be within the amplifier's happy region. The amplifier is under no obligation to present that value as an output impedance. Damping factor is a measurement of how FAR it is away from a true match. If all is well, your amp is loaded within its capabilities and there is no matching going on. Matching is a two-way thing. It takes two to Tango...

In an amateur radio setup, you want to adjust your antenna so its feed impedance matches your feeder cable, so there is no reflection back to the transmitter. Where the transmitter output is marked "50 Ohms" read that as "Please connect me to 50 Ohms" but don't assume that the output looks like its from a 50 Ohm source, it could be a long way different.

So there you have it. "Matching" is a very special word with a precisely defined meaning in the electrical world. Unfortunately it gets used a lot in places where matching is not going on. This causes so much confusion, that whenever you spot it, you should question it.

Sometimes, what's going on is loading, not matching.


joebog1 16th Jul 2019 3:50 am

Re: "Matching" transformer.
As always, David explains things incredibly well!! 8-).
I wish some of my uni lecturers had had that skill, it is a wonderful gift that only a few possess, and many that do, dont use it.
Thanks again David

your humble student,


Diabolical Artificer 16th Jul 2019 5:29 am

Re: "Matching" transformer.
Thanks all and thanks for a good explanation David.


kalee20 16th Jul 2019 7:07 am

Re: "Matching" transformer.
David's explanation is why I was very careful in my choice of words, " an amplifier designed for 3Ω" rather than "an amplifier having 3Ω output impedance."

Basically, the matching transformer enables you to use an amplifier designed for one particular load, to a speaker you happen to have which is unfortunately of a different impedance.

Synchrodyne's comments about the transformer having a very low DC resistance are worth bearing in mind, but that should only be a problem if you push very low frequencies through the amplifier, or the amplifier puts a large surge current through the load such as, on switch-on. A real speaker will limit this, a transformer might briefly saturate and allow an unusually big current to flow through the output transistors. Of course, such an amplifier is not what you'd want to use with expensive speakers in the first place!

Radio Wrangler 16th Jul 2019 7:31 am

Re: "Matching" transformer.
To give the psychologists a titter: 'Sometimes a transformer is just a transformer'


If I'd thought of that one sooner, I might have had it as a .sig file.

Synchrodyne 18th Jul 2019 11:08 pm

Re: "Matching" transformer.
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Some background to the purpose of the Wharfedale "matching" transformers is provided in this excerpt from the Gilbert Briggs book "Loudspeakers":

Attachment 186906Attachment 186907


Radio Wrangler 18th Jul 2019 11:59 pm

Re: "Matching" transformer.
On top of all this there are transformers designed to pass chosen amounts of max power to speakers in 100v line systems. Systems can have many speakers and with different amounts of power assigned to different speakers. Just tot up the total power the speakers are set for and you need a 100v line amp that is comfortable at that power.

There is no real 'matching' in the max power transfer theorem, or in the zero reflection senses. The impedance of a given driver's voice coil is simply transformed into te right impedance needed to take the chosen (tap-selected) power from the defined voltage common line.

Seen as an RF assemblage it would be a right mess with reflections bouncing between all points. But at audio frequencies and the cable lengths involved, it works fine.

100v line is too low a Z for comfort with valves, so output transformers are used in most amps.

100v line is too high a Z for comfort with power transistor voltage ratings, so transistor amps had to be transformer-based which was common in the early days. Transformerless amps are doable, now, but are still less convenient.


Diabolical Artificer 19th Jul 2019 5:02 am

Re: "Matching" transformer.
Thanks Sync, that's answered the historical bit, one forgets sometimes that there was an age before the 70's stereo hifi with ubiquitous 8r speakers and trannie amps.


Synchrodyne 20th Jul 2019 1:16 am

Re: "Matching" transformer.
No problem!

I guess that history does have some relevance in a vintage forum...

Re "one forgets sometimes that there was an age before the 70's", this made me chuckle, as I am approaching the point when it will be true for me in a different context.


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