70 Volt audio transformers for impedance matching

[WA4WAX]

Perhaps this will be of interest to some.

https://www.youtube.com/watch?v=QDrocGzih_s

[WBear2GCR]

Seems very confusered??

Afaik the 70v side is about equal to 600ohms... One can just take the known "8 ohm tap" and
put an AC voltage on it, then measure the secondary side voltage (aka the other side) pop that
into the formula that converts ratio to impedance. Done.

[KB2WIG]

I checked it out, and fell asleep during the ohm's LAW marathon.


klc

There was somthin in Electric Radio a few years back....

[Opcom]

Good video for first principles of utilizing the 'line to speaker coil' transformer.

A little more work and a comprehensive spreadsheet can be made to do all of that calculating and more in one step. A few years ago I created a spreadsheet for choosing a line-to-voice-coil transformer for driving modulator grids. I believe I uploaded it here but it was a long time ago.

The worksheet saves a great deal of time as you can explore the all major and some oddball possibilities of a transformer at once, regardless of the application.

Edcor makes line transformers with 25, 70, 100, and 140V center tapped windings, and power ranges from 3 up to 400 or 600 watts, and I've been real pleased with them.

There are also caveats not mentioned in the video, such as up-sizing the transformer.
For example, the Hallicrafters SX-28 is an 8 Watt receiver, but with the kind of (subjectively?) good sound quality it has, I would not use a economy 10W transformer because of poor bass handling. I would probably use a 30-40W one to transform the SX-28s 5000 or 500 ohm output to a speaker impedance.
Now it is tricky because one would use the whole primary 'volts' winding to keep the inductance up, but have to choose a different secondary tap (4 or 8 Ohms, etc) than the actual speaker load attached. So, there's much more to the topic of calculations.


some opinions about the impedance meter shown:
==
The Tenma in the video outputs a 1KHz square wave as shown on the scope. In my opinion, that instrument is inappropriate for these kinds of tests including field work for two reasons:

1. Unpredictable results caused by load circuit reactances to the broad spectrum of the square wave.
2. The lack of a test to cover the lower audio communication/PA frequencies. Low frequencies are also where most of the power goes. If the Tenma output signal shown in the video is correct, then the meter ignores all load impedance characteristics below 1KHz.
2-way and even 3-way speakers are common today in ham-radio receiver extension speaker, background music, and public address applications.

The Tenma meter cost >$100. It looks to me like an inexpensive DVM with a square wave instead of DC for the Ohms range and everything else removed. I believe it was said that the accuracy is 2%, but that's not important when the measurement is incomplete or compromised.

==

An excellent instrument is a University Sound or Raymer "LWT". It is a portable analog instrument with a voltage-regulated (leveled) sine wave output and a balanced bridge with a zero center meter. One turns the knob until the meter is zeroed, and then reads the impedance of (and the 0.5 to 200W power required by) a 70V or 25V line from the dial scale. It easily measures 1 to 10,000 Ohms, so no worries there. Test frequencies built-in are 100 and 400Hz and there's an external jack for a frequency of one's choice. This is an older analog instrument, but gives a set of more meaningful results. It's still sought after today.

==
"good" impedance meters cost more, but some high-priced ones even at $400 are still using only one frequency, etc.. One has to be careful what one is buying and don't be fooled by a recognized name -check the actual specifications.

happiness!

[KA3EKH]

Not buying it, first issue is that he implies that the high impedance side is the secondary of the transformer where the taps are. Thousand years ago, I worked doing PA systems for NASCAR race tracks. That was fun because no matter what you did people did not hear any way but at that time my limited understanding was that the 70 volt primary just has two wires and that had a impedance of around 600 Ohms and the secondary where the wattage taps are had low impedance anywhere between 4,8, and 16 Ohms and depending on how you strapped the secondary that set how loud the speaker was. I always thought of the secondary as an autotransformer with fixed taps.
Long story short is that the primary is high impedance allowing you to ship large signals over small wire to all the speakers where you had low impedance voice coils with higher current and low voltage.
Also, don’t buy his math in finding impedance although may not understand what he is saying because he is trying to relate to impedance as being the same thing as DC resistance where I always thought impedance was a value that was determined at a given frequency?  That’s where the test meters that use the 1 kHZ signal come in. we use to have a Toa test meter that was used just to determine impedance on speaker feeds that used a 1 kHZ test signal with the assumption that the Q of those circuits was such that if you know the impedance at 1 kHZ It will be about the same at 100 HZ or 10 KHZ.
These days I have taken a giant leap backwards around the shop and use a primitive General Radio bridge from the nineteen forties and an external detector for determining impedance, you just crank on the bridge until you null the signal and there you go, that’s the impedance. The bridge also has the internal galvanometer so you can do crazy detailed DC resistance measurements. 

[WBear2GCR]

Agree it's around 600 ohms nominal for the "70v" side.
Usually two leads for that.

Taps for the other winding, the low(er) Z one.

The video did a whole lot of stuff and seems completely wrong on the impedances.
As I said, any low AC voltage and a DVM + the formula for turns ratio:impedance ratio,
done.

                      _-_-

[KA3EKH]

I don't know, just don't get this guy. went back and watched a couple other of his videos and cant see what his game is. Don't get me wrong because anyone who knows me knows I piss away way too much time doing videos for YouTube but that's maybe because where I work we have resources and once upon a time I use to do video work so take some enjoyment out of the process. I went back and looked at a couple of his videos like his one on "What is a carrier Frequency" and there is just something about his presentation I cant define but makes me wonder how he can take a half hour to say something that can be said in five minutes?

Maybe I am just bitter because I have like three followers on YouTube and he has over ten thousand? but there appears to be at least a couple dozen potential Mr. Science type out there explaining radio so go figure?

His explanation of carrier frequency, wavelength and Amplitude Modulation:

https://www.youtube.com/watch?v=xrigkCPZ6ms

Once again not trying to bust the guys  balls but just can not put my finger on what about him i don't like.

[Opcom]

Don't worry about guys who clock up a bunch of views. If you want a real laugh watch some BBI videos. The explanations of how things work is priceless.

[KA2DZT]

The voltage on the primary is always a nominal 70 volts.  The impedance on the primary depends on the wattage tap on the primary.

ExE/W = Z  70.7x70.7 = 5000,   5000/10 = 500 

You would need a 70 volt xfmr with a 10 watt tap to get 500 ohms on the primary.

These xfmrs load the 70 volt line based on the wattage.  The same xfmr using a 0.5watt tap would load the line at 10K ohms   5000/0.5W = 10Kohms.

Constant voltage speaker systems were 25 volt, 70.7 volt and 140 volt systems.

Fred

[KA3EKH]

I don’t know, I was trained that the only thing that mattered in a 70 volt distribution system was total wattage that was on each amplifier. If you had five speakers that were on 20 watt taps that would be 5 X 20 or 100 watts required for that amplifier and if your total load exceeded the amplifiers output bad things happened. To the same extent you can have the same amplifier driving 20 speakers strapped to 5 watts each and although that’s twenty transformers strapped across the output total wattage is still 100 and everything is good. Been twenty years so forgot about the power taps on the primary side for power and the impedance taps on the secondary side.
Looks like your right, no disagreeing with that but long story short I still find the little 2.5 or 5 watt transformers useful for going between lo-Z to 600 Ohm Balanced. One thing that I was also turned on to back in the PA days was the Bogen WMT-1 that did low level transformation from Hi-Z to Low Z balanced, they were usually intended to feed phone lines to non-balanced inputs on mixers. But they were one of the most useful devices around being they allowed you to go from things like non balanced audio feeds to balanced inputs on amplifiers, take non balanced audio outputs from mixers and ship it as 600 Ohm across phone lines or just provide isolation between devices. Back in the race track days it was not uncommon to have to ship sound between buildings or for several hundred feet across properties and we used a ton of those little transformers to go between mixers and amplifiers with most of the distribution over dry pair telephone lines. With the transformers on each end of the non shielded phone lines we were able to ship audio all over the place with no hum, ground loops or any other issues. The balanced audio line is an amazing thing.
Think the Bogen WMT-1 may still be available new and considering it was something that was around thirty years ago and is still available that’s a neat trick in todays world, and as a last thought solid state transformers suck. No substitute for real iron.

[KA2DZT]

I use to install Bogen sound systems.  I still have the catalogs.  The amp needs to be about 150% of the total speaker load.   My post was just to clear up the 500 ohm thing with these constant voltage xfmrs.  500 ohm load only occurs with the 10 watt tap and the correct speaker load on the secondary.  You could also get 500 ohms with a 5 watt tap and loading the 8 ohm secondary with a 4 ohm speaker.

Fred

[KA3EKH]

When I first started doing the NASCAR races this was back in the early eighties, also did sound work with Horse Race tracks at that time. Was one of the first things that I started doing right out of high school. I cannot remember the model but at that time ran across lots of the Bogen tube amplifiers, huge amplifiers maybe 100 or 200 watts. Big rack mounted amplifiers with gray solid front panels and huge heavy chassis with tons of 6L6 tubes on them and spent lots of time removing them and installing new 100 watt solid state amplifiers to take there place, Think at that time we ended up just throwing them in a dumpster at the tracks.
Wonder what those things would be worth in today's tube crazy world? Of course, I only remember the bad things like having to change out tubes, blown output transformers and microphonics. And how when we installed new solid-state amplifiers they would just work.

[WBear2GCR]

I use to install Bogen sound systems.  I still have the catalogs.  The amp needs to be about 150% of the total speaker load.   My post was just to clear up the 500 ohm thing with these constant voltage xfmrs.  500 ohm load only occurs with the 10 watt tap and the correct speaker load on the secondary.  You could also get 500 ohms with a 5 watt tap and loading the 8 ohm secondary with a 4 ohm speaker.

Fred

This bit about the "X-watt" tap?
These are just transformers - they don't know anything about "watts".
They know ratio. Voltage ratio and Impedance ratio.

N1 ^2 / N2 ^2 = Z1/Z2

Define one impedance, like a speaker at 8 ohms and then the ratio available will
define the primary impedance.

For the purposes of matching a single speaker to a given receiver output nothing
much else needs to be thought about... and an imperfect match will still be "good
enough".

Ymmv.

                          _-_-bear

[KA3EKH]

Maybe its just me but my brain can not picture things in terms of impedance with these systems, I look at it more along the lines of using transformers to match loads and voltages and tend to look at the primaries and secondaries of the transformers more along the line of a multi tap or autotransformer allowing you to select the range of voltage available to each load. Maybe all the stenciling of wattage and impedance is a misnomer being the load impedance will have a effect on the impedance on the primary or something like that. Just makes my head hurt. These are just transformers, they are not that smart. I ran transformers with a primary voltage of 240 or 208 and they have a reduced output when fed with 120 but they don’t seem to care?

[WD5JKO]

Here is something I picked up many years ago from an Ebay seller that was selling 70v Line Transformers:

_________________

0 watt output transformers 8KCT to 4 or 8 ohms, great for tube amps etc.

PRIMARY:  (at 8K CT impedance; plate to plate DC resistance about 230 DC ohms center tapped.  The impedance from B+ to plate and B+ to plate is 1.5 henry, and 6 henry plate to plate).

Can use these for SE single ended amps also (i.e. 0K and 8K primary, and put 22-47 ohm in series with plate to give it an air gap type sound).  (an air gap is a cut in single ended transformers to give a softer sound).

Common = 0K

0.625W = 8K primary impedance

1.25W = 4K primary impedance

2.5W = 2K primary impedance

5W = 1K primary impedance

10W = 500 ohm primary impedance

Push pull 6V6GT or push pull 6BQ5/EL84s are made for 8KCT.  This also has outputs of common, 4 ohm, and 8 ohm.

(for 8KCT, use 2000 ohm to B+, Common to one plate, 8000 ohm to other plate)

For single ended, use common to B+, 4 or 8K to plate, etc.

SECONDARY: Common, 4 ohm, and 8 ohm speakers

Impedance=Volts^2/Watts example 70*70 / 0.625 = 7840. Tap marked 0.625W is about 8000 ohms or 8K impedance vs. common.

p=E^2/R example 70 volts on 500 ohm tap would be 70*70/500 =10 watts output

if there is 70 volts on the the 500 ohm tap, there would be 280 volts on 8K (0.625W)tap.

with 70V on 10W taps, the voltage on 8K tap is in the ball park of 250 volts.

If push pull amp produces 280 volts RMS from plate to plate (C and 0.625W taps),

output would be E^2/R = 280*280/7840 = 10 watts output


Note, if 70V was on common and 500 ohm, the 8K primary would be about 280V.

Due to their plastic insulation, we've sold hundreds and never heard of arcing

with them run to 450 volts.  If using for a headphone amp, always ground your output

to be safe.  Current? We've run them up to 40 ma each plate (80 ma total), or 50 ma single ended.

___

For 8K primary, use the black, brown (CT), and purple wires (these measure ~230 DC ohms CT, and impedance from the brown CT to black (plate) or purple (plate) is 1.5 henry, or plate to plate impedance 6 henry.

We dont know the voltage rating, but have used these for push pull 6V6, push pull 6BQ5 amps.

Price is $7 each.   The secondary is marked "SEC C  4ohm  and  8ohm).  Please, no questions AFTER you buy.  If you've never built an amp or never thought about impedance transformers before, don't buy and ask a lot of questions as you buy your first amp.  Ask any questions before you buy. These are new, being sold as-is.

________________

Jim
Wd5JKO

[Opcom]

The guy was interested un-unloading his transformers. I disagree with his statement:

"Can use these for SE single ended amps also (i.e. 0K and 8K primary, and put 22-47 ohm in series with plate to give it an air gap type sound).  (an air gap is a cut in single ended transformers to give a softer sound)."

The air gap is to prevent saturation of the core by DC current in a single ended circuit. It spoils the magnetic path in the core and reduces the inductance of the winding.

In push pull, the currents are in opposite directions and balance out so little or no gap is needed.

The line-to-speaker type of transformer is designed for zero DC current imbalance in any winding. Sometimes the core is, or can be, rearranged to a solid E-I form and a piece(s) of paper slipped in between the E and I sections to gap it.

A mere 22-47 Ohm resistor would have little effect on anode current through a transformer or 'softening' the sound (reducing saturation thereby reducing harsh distortion). With the high impedance of the primary circuit (5000 Ohms) how could 47 Ohms affect it?

[Chuck...K1KW]

These work great as driver transformers and I've used several.  This is the one I use to drive the pair of 572B's in my 813 rig with the amp board below driven with my dBx 286.  Flat response to 30 Hz.

My favorite is the Thordarson 24S166.  Last one I bought on Ebay was this:  https://www.ebay.com/itm/193959371934

Drive the grids with the 1000 ohm secondary, used the 250 ohm tap as the center tap.  I drive the 8 ohm primary with this amp.  https://www.ebay.com/itm/143464647535  At 30 W and negative feed back including the mod transformer, really clean.  Very simple, cheap and almost fool proof....

Chuck

[WU2D]

I have only played with the old ones. All I can contribute is that they do not work as well as a proper 50C5 or 6V6 audio output transformer designed to handle Plate Current and Audio Power. They appear to be good for Audio Power alone, but putting DC current through them is the issue. So as an interstage - it depends on the particular build. Might work fine.