RS Audio Transformer

[Half a Mullard]

I ordered some items from RS a while ago including one of these transformers. It’s been stored away for a while but I dug it out again today. Although it was advertised as an audio transformer I have a feeling that I read somewhere since that these are not in fact suitable for audio purposes on account of the laminations? I’m not sure what I should be looking for and don’t want to use it if I am likely to cause any damage. Any advice would be gratefully received.

[AdrianH]

Interesting, there is a spec sheet on oep's web pages under output transformer, looks as though 3.5 Watt max output single ended, max 40mA. Looking at the graphs certainly looks as though it wants to be used at higher input impedances 10 K input to 4 Ohms seems to be the best audio response otherwise it seems to drop quite rapidly above 3KHz. Which I guess if replacing a valve output transformer on an old am radio will be OK?

Adrian

http://oep.co.uk/page-content/datash...0issue%207.pdf

[m0cemdave]

Originally Posted by AdrianH

Looking at the graphs certainly looks as though it wants to be used at higher input impedances 10 K input to 4 Ohms seems to be the best audio response otherwise it seems to drop quite rapidly above 3KHz.
http://oep.co.uk/page-content/datash...0issue%207.pdf

You need to look at the vertical scales on those graphs, confusingly they are all different.
The ones that look bad are actually a lot better than they appear at a casual glance! It's just that they have used expanded scales.

Look at the -3dB points on each graph for a proper comparison.

[AdrianH]

You are correct, why they do that!

Should be reasonable for the am radios then.

Adrian

[Sideband]

Nothing wrong with them as output transformers as long as you know their limitations. 40mA rather restricts their use except in small to average sized radios.

[McMurdo]

ISTR fitting one of those to a Bush DAC90A

[Half a Mullard]

Thanks folks, that's very useful.

I can't find the reference to it now but I seem to recall that the laminations are not separated?? and can cause overheating?

If they are optimised at higher input impedance I wonder if they would be suitable for battery sets?

[trobbins]

SE operation at 40mA idle through total primary will dissipate 1.5W - so it will certainly get warm, but I doubt it would overheat or cause overheating, but that does depend on how it is thermally located.

[Herald1360]

I just pulled open the clamp on one of these- it's true that the laminations are interleaved, not butted and gapped. Presumably at the low DC involved they get away with not saturating the core (too much?).

Certainly small mains transformers make usable cheap bodge replacements for dead output transformers in low power applications.

[Radio Wrangler]

The 40mA limitation will most likely be due to saturation.

Doing a quick check it looks like the low frequency cutoff is around the 100-150hz mark for the high anode load impedance settings in the data sheet, but the bottom one on the table (5k) looks more like 500Hz. Those graphs look like they were measured with a low-Z signal source into the anode windings chosen, which extends the low frequency cutoff a lot.

One of those transformers where triodes will give less gain, but also more bass. Ever wondered why some people like output triodes more than pentodes with no feedback? The reason isn't fairy dust.

David

[Half a Mullard]

Originally Posted by Herald1360

I just pulled open the clamp on one of these- it's true that the laminations are interleaved, not butted and gapped. Presumably at the low DC involved they get away with not saturating the core (too much?).

Certainly small mains transformers make usable cheap bodge replacements for dead output transformers in low power applications.

Thanks, yes, this is issue I was concerned about - core saturation. Does this cause any damage/strain or does it simply impair performance or sound quality?

I suppose in simple terms is it worth hanging on to the transformer and I guess a proper rewind would be the way to go?

Visually, when examining a transformer what should I be looking for? (idiots guide)

Thanks!

[Station X]

I must have used half a dozen of those transformers over the years to replace faulty transformers in valve radios. They work absolutely fine, but sometimes it's necessary to increase the value of the cathode resistor to get the anode current below 40mA.

I think it's exceedingly fortunate that these transformers are still available.

https://uk.rs-online.com/web/p/audio...rmers/2106475/

[Ed_Dinning]

Possibly no need to rewind, but removal of the lams and stack them with a 2 to 4 thou gap (1 layer of elect tape)

Ed

[Half a Mullard]

Originally Posted by Ed_Dinning

Possibly no need to rewind, but removal of the lams and stack them with a 2 to 4 thou gap (1 layer of elect tape)

Ed

Thanks Ed, I didn't want to ditch it if it was of any use. I'm a bit wary of taking it apart though, where would the tape need to go exactly? (excuse my ignorance)

From the comments received it sounds as if it could be used for a small radio where sound quality is not a major issue?

[Radio Wrangler]

You would have a stack of all E lams with the nobbin on them, and a separate stack of all I lams. The spacing tape would go between the Es and the Is.

It will drop the inductance fairly significantly but put up the DC current possible before saturation sets in.

David

[Half a Mullard]

Originally Posted by Radio Wrangler

You would have a stack of all E lams with the nobbin on them, and a separate stack of all I lams. The spacing tape would go between the Es and the Is.

It will drop the inductance fairly significantly but put up the DC current possible before saturation sets in.

David

Thanks David, I'll try taking it apart, I've nothing to lose!

So presumably it will work up to a point before saturation sets in?

And what effect (presumably negative) does saturation have?

Sorry about this, i really must do some bedtime reading on transformers

[m0cemdave]

I'd leave it as it is, assuming you aren't going to put more than 40mA through it. Reducing the inductance will lose some of the bass response.

[Half a Mullard]

Originally Posted by m0cemdave

I'd leave it as it is, assuming you aren't going to put more than 40mA through it. Reducing the inductance will lose some of the bass response.

Thanks, I think I will. According to the Trader sheet the Bush DAC90a draws 27.5ma anode current and I imagine most small radios will be around a similar level. So the consensus seems to be as long you don't exceed 40ma it should be OK.

I will do some research into transformer theory though!

[McMurdo]

Here's mine with the OEP transformer.
Difficult to tell from a vid clip but it goes very loud before distortion, and that's with a speaker I rebuilt rather dodgily using a scalpel, sandpaper and glue.

https://www.youtube.com/watch?v=8jvOxhHKYOQ

[Radio Wrangler]

Difficult to say with other variables kicking around like the speaker, microphone and recording, but it does sound a bit light at the bass end, which I'd expect from the winding inductance values. Makes it sound nice and clear on speech, though.

Running closer to saturation with a single-ended output stage will give higher levels of even-order harmonics. Push-pull stages tend to cancel even-order terms and leave mainly odd-order harmonics. This is the probable reason behind why some people prefer single-ended hifi amplifiers.

David