Hi all,

I'm looking to make a custom audio output transformer. The size I'm aiming for is on par with the typical (though nearly obscure) Radio Shack #273-1380.

I don't necessarily need automation, but something that is smooth and true would be great.

Any and all advice. Thanks in advance

 

That's a small interstage audio transformer.
Why do you want to make a custom version?

 

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That's a small interstage audio transformer.
Why do you want to make a custom version?

I'm looking to make a 600:16

My output source has 600ohms and I need to affix just standard stereo headphones (16).

The 1000 half works ok but the 8 ohm side has me have to raise the volume knob further than I'd like. Still works--just trying to refine it. And learn a skill.

 

I'm posting here rather than on your other thread because that thread has degenerated.

Here's the best solution to your problem. You have a single Radio Shack transformer which is 1000 ohms primary to 8 ohm secondary.

If you had two of those, you could connect the primaries in parallel and the secondaries in series, which would give an equivalent transformer with impedances of 500 ohms primary to 16 ohms secondary. For this to work, the phasing of the connection of the two transformers must be correct.

I know you said you had obtained the last transformer in town, but I see on eBay: http://www.ebay.com/itm/2x-EI-14-Au...239402&hash=item25dab2a566:g:F-cAAOSwRMtZYFT5

This is for two transformers essentially the same as the one you have now--two 1000:8 transformers--just what you need for the parallel primary, series secondary trick.

The subject line for the auction says something about 32 ohms, but the specs further down don't mention that; it must be a spurious bit.

 

That's very clever. And yes, the other thread has de-evolved.

The 500 primary though--my output coming into this thing is 600. Will that cause a problem?

 

Unfortunately the reality is that by switching up from a 8 ohm secondary to a 16 ohm one is that you will likely see a audibly near imperceptible gain in volume.

I have a few old tube amps I play around with from time to time and I can say for sure that switching between a 4 ohm and a 8 ohm output tap with any speaker regardless of whether it's a 4 or 8 ohm impedance rated unit does nothing discernible for volume improvements at any normal volume.

Theoretically you have 4x the power going to the speaker but realistically not quite plus our hearing is logarithmic so a 4X power gain is only a few dB or so volume increase in the end.

If you want more volume you need to change the gain of the amplifier or just get used to having the volume knob turned up further than where you think needs to be. Afterall, it's just a variable resistance voltage divider so where it's set is purely arbitrary anyway.

 

That's very clever. And yes, the other thread has de-evolved.

The 500 primary though--my output coming into this thing is 600. Will that cause a problem?

No, having a primary of 500 ohms rather than 600 ohms won't cause a problem. You'd be surprised at how far the actual measured impedance of transformers like this is from the nominal.

Edit: Just to show what I mean about transformer impedances not being very tight.

Here is an example of an audio transformer I have: http://www.edcorusa.com/wsm_series

I have one of the 10k:10k transformers. Measuring the actual winding impedances with an impedance analyzer, they are 7260 ohms rather than 10k ohms. But that's ok, because the actual source impedance is going to be transformed by the turns ratio squared and if that comes out reasonably close to the load, then all is well.

 

Unfortunately the reality is that by switching up from a 8 ohm secondary to a 16 ohm one is that you will likely see a audibly near imperceptible gain in volume.

He's also decreasing the primary from 1000 ohms to 500 ohms so he'll get twice imperceptible gain.

 

For testing purposes, what kind of equipment would be in order? Never seen an impedance analyzer.

 

Also, the normal speaker on this intercom is 150ohms.

With my 8 ohm transformer I have to raise the volume quite a bit. Good quality, just a little weak compared.

 

A meter might look like one of these: https://www.bing.com/images/search?...ce+meter&qpvt=audio+impedance+meter&FORM=IGRE

Too much money to spend for a one time use. But you don't really need precision in transformer impedances. What really counts is the turns ratio. If you had an audio transformer that was specified by the manufacturer to match 500:8 ohms, it would work just fine for your purpose matching 1000:16 or 250:4. A 2 to 1 variation in the impedances used with the transformer would be ok. So don't bother trying to measure the working impedances of a transformer. It's not necessary to have a transformer that is actually specified by a manufacturer to be 600:16; 500:16 will be plenty close enough.

I know nothing about modern aircraft intercoms, except that I'm sure they are solid state devices; no vacuum tubes. Whereas the old tube amplifiers needed an output transformer to get an output impedance as low as 4, 8 or 16 ohms, a low output impedance just happens naturally with a solid state output stage and regular solid state audio amplifiers like you would have at home generally have very low output impedance, less than one ohm.

While it's certainly possible to design a solid state amplifier to have a 600 ohm output impedance, I would think that the intercom might be a normal solid state amplifier with very low (less than 1 ohm) output impedance, and when they say 600 ohms, they are just saying that traditional 600 ohm headphones (or 300 ohm) will work just fine even though the intercom output impedance might be less than 1 ohm.

When I have used headphones with a home solid state amplifier, there was simply a resistor in series with the headphones because it would be much too loud if directly connected. It's not necessary to match impedances with a solid state amp of the type with very low output impedance; just don't deliver enough power to headphones to damage your hearing. You could try just hooking up your desired phones without a transformer and see how loud it is. Start with the volume control set to zero and turn it up slowly; if you do that it shouldn't overload and damage the intercom output stage.

Take a selection of resistors, say 10, 33, 100, 200 ohms (or perhaps a 500 ohm variable resistor, a potentiometer) with you to the aircraft and be prepared to temporarily connect them (or it) in series with the phones. Assuming the output impedance is in fact very low, you would just test it with various resistances in series and see where the volume knob is set for normal volume. If the actual output impedance of the intercom is very low as is the usual case with solid state amplifiers, it might be that all you need to do is connect a suitable resistance in series with the phones to give you a comfortable volume.

Or, of course, you could order those two audio transformers from eBay, do the parallel primaries, series secondaries trick, and try that out.

It's possible that the your preferred headphones are less sensitive that standard aircraft phones, and you might just have to live with the volume control turned up more than for the standard phones.

If it were me, I would try the no transformer and resistor in series option first; it would be quick and might work just fine.

 

Well the normal headsets like the green ones everyone sees in movies are 300ohms wired in parallel for 150.

The output was confirmed at 600 by an aircraft radio shop.

Resistors could work?
A potentiometer too?

 

It would be so easy to try the series resistor technique, why not give it a try? It might just work for you.

A potentiometer is just a variable resistor. You would wire the potentiometer in series and adjust it to a comfortable volume. Then without changing the adjustment (don't bump it!), take it back home (or to the aircraft radio shop) and measure its resistance. Then get a fixed resistor or 1 or 2 watt size and wire that in series for the permanent fix. This is what would be a good solution for you if the output impedance of the intercom is actually low.

And, of course, as I said earlier, your preferred headphones might be less sensitive than standard aircraft units so that you might just have to live with the volume control turned up.

How did the radio shop confirm the output impedance? Did they actually measure it, or did they just look at the specifications for the intercom? As I explained, the specification of 600 ohms might just mean that using 600 ohm headphones gives suitable performance. If it is really important to know (that is, if the series resistor technique doesn't work for you, and you have to use a transformer), I would measure it myself.

There is a way to measure output impedance without a meter. See: https://forum.allaboutcircuits.com/threads/how-to-measure-transformer-impedance.37161/

Here's how you would do it. Get yourself a 1000 ohm potentiometer (or other suitable 1000 ohm variable resistor). Go to the aircraft with the potentiometer wired in series with your phones. Leave the connections exposed because you might have to measure the signal at the headphone leads with a voltmeter. Set things up so you are getting a reasonably steady signal into the phones. Maybe somebody can sing a single tone into the mike at the other end of the intercom. Set your potentiometer to zero ohms, adjust the volume control on the intercom to a comfortable level, and don't adjust it again while performing the rest of the procedure.

Here's the not-so-precise part of the procedure. Listen to the signal, voice or preferably a tone, for a while, and fix in your mind the level. Now adjust the potentiometer until the loudness in the phones is reduced to half. This is difficult because hearing is logarithmic and a 2 to 1 reduction in apparent loudness only sounds like a small reduction to us humans. The really correct thing to do would be to connect an AC voltmeter across the headphone leads--that's AFTER the potentiometer. Measure the AC voltage across the phone leads with the potentiometer set to zero ohms. Then adjust the potentiometer to cause the measured AC voltage to drop to half its previous value. If the signal you're using is speech instead of a steady tone, making the judgement as to when the AC voltage is half will be a little flaky. After adjusting the potentiometer for half signal , measure its resistance with an ohmmeter; that resistance is the output impedance of the intercom.

 

Man, that's a lot of data. It's going to take a me a bit to process all that. I really do appreciate the help too.

I'm on the road right now, but when I get back tomorrow night I will start poking around.

 

Ok, I found a very straightforward example of construction;

I still need to find another 1k : 8 transformer to try the series trick.