Line Transformers for Digital Audio

My latest blog upload chronicles my use of line transformers between a CD player or DAC. I was inspired to do this in the 90s after seeing an ad in the pages of MJ Audio Technology magazine.


I started out with this pair of surplus Tamura transformers wired 300:600.


Even if my DIY DAC has an all-tube analog output section, the use of these line transformers still make digital sources more enjoyable!

Here's a video as proof of concept.

 

I'm lazy, and I'm not going to click through to your blog or watch a YouTube video just to get the gist.

Can you give us the CliffsNotes on why this is desirable?

 

Since the 90s, Asian audio hobbyists have been using line transformers to tame the analog output section of DACs, which is usually populated by op-amps.

The line transformer can be used as a 600:600 shunt or wired with a mild step up in the primary. It's an easy DIY project. All you need is a pair of line transformers that has decent audio bandwidth.

 

You won't get the gist from his blog or the video anyway. The highlight of the blog is "I found my CD's a lot more enjoyable" and the video is one of those "record my stereo system playing music" type videos... so no real info there either.

 

Unless you need isolation or impedance matching (or some strange idea of gain regulation), I don't see any way they are beneficial in analog audio. Especially 1:1 transformers - only for isolation. Otherwise they just add noise, no matter how little if they are quality made.
Transformers in digital audio are pulse transformers, like in network switches. Those serve isolation purposes. They can't do reclocking or anything like that.

But hey if you enjoy it more, feel free to put whtever makes you happy in the chain, right?

I can see no reason why this would help in literally any way, unless you have to break a ground loop. Do you have any idea as to what the "taming of opamps" means? I'm not sure using transformers reactivity as lowpass filters works in this case (or at all). I'd say it's the case of "opamps are bad and digital, look I put this nice expensive quality analog component here to make it less digital and more analog". Maybe these folks had a bad case of ground loops.

 

The noise probably just adds color that sounds subjectively better to some people. It’s like the XLR->SE converters, but without the practical benefit of converting to SE and only the color. Seems unnecessary but I guess whatever floats your boat.

 

As someone who goes builds a lot of amps with line level transformers and has spent the better part of the last month measuring nanovolt levels of noise, I'll throw in my two cents.

The noise contribution of a transformer is basically that of the Johnson noise of the winding resistance. As a rule of thumb, at room temperature, that works out to 0.13*sqrt(R) nV/sqrt(Hz), where R is the winding resistance. The forum favorite CMLI-15/15B has a winding resistance of 1.75k, which works out to 5.4 nV/sqrt(Hz). That's about 770nV over a 20kHz bandwidth. Let's say your signal level is 2V: the SNR of this system would be 128dB. This is even better SINAD than a Topping amp!

The CMLI-15/15B has a fairly high DCR. Doing the math with the Lundahl LL1684 gives an SNR of 142dB. This is basically as close to noiselessness as it gets in audio.

I ran a quick measurement of the 15/15B to confirm this, using an APX515 and balanced inputs. Take note of the Y axis scale. The 60Hz noise spike is slightly lower with the transformer than with no transformer, owing to the part's very high CMRR rejecting the small amount of noise picked up by the balanced cable.

I can't speak to the OP's iron sounding good or bad, but even in fully balanced instrumentation, transformers can yield a measurable reduction in noise. With single ended gear, the benefit can be dramatic.

 

It's funny, before I had experience with transformers, I was intrinsically biased against them. I ignorantly assumed they coloured the sound and added noise but over the years I have experimented with various transformer coupled designs and they sound anything but mediocre. The best dynamics and detail from solid state designs and ultimately led me to choosing a transformer based preamp. I have a few small signal transformers lying about, I may have to explore a little more and have some fun.

 

Alright, so the noise is basically not a problem at all. And if the bandwidth is fine too, that means they should be completely transparent (I suspect they are not a huge source of nonlinear distortions). Good info, thanks!

But even so, other than isolation, in best case scenario, it's difficult for me to see what these might realistically add.

 

A transformer changes everything. Literally and figuratively.

Think about it electrically. You went from RC coupling to LRC coupling. It will change the total load on the output stage of your dac. In certain situations, this could make your dac run more linearly.

A transformer will have insertion loss, second order distortion, high frequency filtering, the list goes on an on.

 

If I'm not mistaken, due to transformer action, loads (and other values like inductances) are reflected on the sides of the transformer, and for a 1:1(done right), should remain the same. Almost as if it wasn't there. Insertion loss will just drop the voltage by a few db or so(at the worst).
Distortion can be generated in other ways, and less is almost always good while trying to reproduce, unless it pleases you. But I'm not sure a proper isolation transformer will add audible distortion, there are other ways of doing that that make more sense. High frequency filtering will probably do nothing, if it even occurs. Most if not all from that big list should do nothing much in terms of effect to audio. It'd be good to see a proper comprehensive measurement of one good (or several, why not) isolation transformers, to actually see if and what's going on. It does some things for sure, just not sure what a difference it actually makes in the end, probably nothing.
Now it's not the output stage of the opamp that's driving the whatever is next in chain, it's the collapse of the magnetic field in the coil, so that definitively has a more "audiophile" ring to it. I have a couple of transformers but no gear good enough to test them.
If someone wants, I can send you one to to test, but they are really not that expensive to get.
And, it's still the output of an dac's opamp that's the main driver, it's not like you are keeping everything analog like with vinyl. Da conversion probably matters more (to a degree to which even that can matter). As usual I might be wrong, I'm just a cat owner.

 

God I love this forum more and more every day. You have probably one of the nicest most respectful replies I have seen out of someone that isn't on a dedicated technical forum in years.

You are 100% correct from a purely theoretical point of view. And yes, it can be challenging to test a small signal transformer and see a result without dedicated equipment. However, you can very easily see a difference with a larger signal transformer with a scope that has an FFT function. An interstage transformer can have a similar effect to what is being described in the OP.

Insertion loss can be a huge deal depending on the circuit.

Distortion is indeed almost always a bad thing. But it's usually not the worst thing and it can be used to cover up other defects.

High frequency noise is a problem. It is almost impossible to build an amplifier that doesn't have some sort of circuit that is prone to oscillation of some kind or another. You can use all the snubbers you want, but sometimes a little extra filtering between stages really helps.

As far as your comments about the DA being more important, yeah, I would agree. The core circuitry will always matter more than some add on. I don't think anyone here would disagree with that. I don't think anyone is saying that a transformer ALWAYS sounds better. I think the OP is saying a transformer can make an improvement and people should be willing to give it a shot.

 

Fair enough, why not. It's a useful tool to have somewhere, if nothing else.
From personal experience, more affordable scopes with fft are good for about just the most basic of stuff. A proper hq spectrum or audio analyzer would probably be required for any serious work I'm afraid (would like to be wrong here).

Also, thanks for your observations!

This forum is cool, but it can bite you in the ass sometimes. Usually for good reason, sometimes due to misunderstandings. I wouldn't worry too much. It's important to stop the back and forth on time if you see it's going nowhere.