Why you’re not crazy to use repeating coils/bridging transformers between digital and analog audio.

I added an RC network to stop any resonances with high impedance loads you may plug in, I'm sure you can find somewhere on the board to tack in this network as it's well worth the trouble. With this setup things start to roll off around 22kHz and f-3 is 30Khz. If you want a more warm filter a 200mH inductor will bring f-3 down to 22kHz and it starts to roll-off at 16kHz.

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Is the 100k (R2) the load in this simulation?
 
Yes, R2 is the load.

Since you're using 1 meter cables the extra load capacitance isn't very significant unless they are some exotic type of cables with high capacitance/foot. The sim showed a minor shift lower
Nothing exotic. Bog-standard Sescom SC3RR Canare Star-Quad interconnects from Markertek. Specs are as follows:
  • Nom. Capacitance: 150 pF/m (between twin Blue & twin White)
  • Nom. Capacitance: 185 pF/m (between conductors to shield)
  • Nom. Impedance: 44 Ohm
 
Nothing exotic. Bog-standard Sescom SC3RR Canare Star-Quad interconnects from Markertek. Specs are as follows:
  • Nom. Capacitance: 150 pF/m (between twin Blue & twin White)
  • Nom. Capacitance: 185 pF/m (between conductors to shield)
  • Nom. Impedance: 44 Ohm

That's about what I used in the simulation so you're good there.

It's going to boil down to how much of the top end do you want to smooth out. If you are trying to go with a more band limited old school transformer sound then 200mH will work well. If you just want to reduce high frequency artifacts from the from the top edge of hearing and above then 150mH will be good.
 
The inductors shouldn't be expensive since this is a very low current audio application. What is the lead spacing and footprint size? Even a rough estimate is fine.
Lead spacing is 10mm. Footprint size (based off the other inductors) is about 5mm. That being said, there's a decent amount of real-estate around where the inductors were in the original circuit, so something a little chonkier could fit in that spot...

Would something like this 150mF Fastron 5% leaded radial choke work in this scenario? Lead spacing is small, but that's why I have needle-nose pliers.
 
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Would something like this 150mF Fastron 5% leaded radial choke work in this scenario? Lead spacing is small, but that's why I have needle-nose pliers.

That should work good. The datasheet specifies application for LPF signal filtering applications. I also went ahead and plugged the extra 205 ohms of DC resistance into the intrinsic properties of the inductor and increased the added capacitance in parallel with the load to 250pF, 185pF for the 3 meter cables and 65pF for strays and Miller. The -3db frequency is 29.8kHz and -1db is 22kHz.
 
That should work good. The datasheet specifies application for LPF signal filtering applications. I also went ahead and plugged the extra 205 ohms of DC resistance into the intrinsic properties of the inductor and increased the added capacitance in parallel with the load to 250pF, 185pF for the 3 meter cables and 65pF for strays and Miller. The -3db frequency is 29.8kHz and -1db is 22kHz.
OK. Parts ordered from Mouser. This should be a fun little distraction. :)
 
With larger transformers you'll have more copper wound around the core. The more copper the higher the interwinding shunt capacitance and leakage inductance. This is what kills your high frequencies response and it takes extreme care with special winding techniques like interleaving the windings to get even decent open loop high frequency extension. Most of the time if you are using transformers in the signal path like with tube amplifiers you are relegated to using a lot of negative feedback if you want extremely high frequency response and reduced phase shift. Are these EI core? C-core? Toroidal?

Larger transformers have higher primary/secondary inductance which won't roll off the low ends. I think this is what you are recalling about larger transformers, not better high frequency extension.
Agreed with these comments on transformer design. High pass is not anything I've found, (nor heard from builders in Asia)... to be worrisome, ever. Much more of a problem with the low frequency band issues – – so the larger transformers help with that. Even those transformers often do OK for those guys.

One of the famous ones – (I hate using them for an example, because besides being my least favorite super nice vintage brand) – – I (they had a very stoked marketing dept) That said... the UTC "Ouncer" series they actually do very well (real world) as I recall ... some were spec'd at close 20(!) to 20 K, but I don't know if they actually do it. UTC's LS other transformer line (again quoting off the top of my head – – some they claimed to 100 K! Again I'm not sure if they do it or not. But that is well outside the range of any band limitation I would think. I realize 100 K even is quite the bandwidth limitation from a scientists point of view. However again we're dealing with the lowest grade of "RF" science, audio..... (or should we call it pseudoscience ha ha.)
To check most of the stuff we need a little more than our ears. Breaking out even a 60 year old oscilloscope is often overkill.
Nonetheless, I'm eager to apply test equipment these days whenever there is an excuse. If any of this stuff can shed light on why some of these items are better than others – – I am eager to peel back the onion skin's worth of evidence.

Over the years...as a vintage audio re-seller ... the people who spend big bucks, for specific transformers have correlated transformer bandwidth (esp high frequency response) with exactly "zilch" in reference to their overall sound quality. It always surprises me – – people vote with their pocketbooks. And, it's not the floor of some Audio demo room or densely packed Audio show where a sales person... where dollars chase looks... (Western Electric 91 "E", anybody?)
We were talking utilitarian items here transformers – – conspicuous consumption influence is probably fairly low down on the price tag of these. People hear these transformers and then they want them. I know that's pretty strange.

I remember being with Walt buying a Western Electric 197A technician who swept– – on his bench – – hard roll off at 10 K. I never confirmed that., not did we care, (I always doubted it was that bad) – – but there's little doubt that this transformer is absolutely magical sounding in the Western Electric 106A pre (and other broadcast pre's).
 

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I forget to add this. If you have the ability to run a Fourier analysis and show us an FFT plot with and without the transformer that could be very useful and tell us probably all we need to know, which is how pure the sine wave is or if there is other wave forms mixed in with the test tone.

Thanks!
FFT analysis is another one of those things that I really do intend to get around to more often. You are 100% right – – this will shed a lot of light on many audio components – – not just transformers. Many years ago Walt wanted to integrate FFT analysis into tube testing rather than just checking tubes at a single (industry standard 1000hz sine wave)...but we did add the audio output on the Amplitex to do it... We just could never spring the big bucks for the color Tektronix scope n the mid 2000s – – and now that they are so cheap and cheerful, I can. I'll give some of the stuff a try with my awful Rigol clone. So you suggest running FFT on a fixed sine wave? Typically with pink noise – – which isn't a single sign wave... would mix to much speculation into the pot.
This might shed some light on colorations that an audio component might contribute or detract from – for sure – but, I don' think it's going to resolve the above discussion regarding Fourier math/Nyquist / anti-aliasing /stairstepping that was so apparent on the old analog scope.
If you're (very likely correct, hypothesis holds true – – and I'm able to magnify the same stair steps at finer/microvolts – with the 2225 (still not here) – then I'd consider the transformer coupling a big analog success.
And yes this practice (passive or active buffering/filtering) adds coloration.... we all know it and the above thread was a great discussion on it. We never did it here ... as we were always, always testing – on the bench.. and for demos..... Walt's favorite CD player was a 1987 Toshiba XR-P9 (slant face) set up that had no oversampling, no anti-skip... it was very crude one of the first cd players that made mass market as a component. Would be interesting to take a look at the schematic – – but I do know that if you touched it with a feather it would skip and take a while to start playing again.
We compared it to many different CD players that came through here over the years and always came back to it for testing and general listening as well.
 
So you suggest running FFT on a fixed sine wave?

Any signal shape (waveform) you can imagine, no matter how complex is nothing more than multiple sine waves of different frequencies summed together. With a fairly perfect low distortion sine wave we want to see if the transformer is filtering out some sort of high frequency artifacts the DAC is adding to any signal passing through it. An FFT plot just breaks down all the separate sine waves that the complete waveform consists of. Using a sine wave that we know is for the most part as close to a pure tone as we have, this allows us to know what should be in the FFT plot, that would be the fundamental frequency and nothing else. If the DAC is adding information to the signal it should show in the FFT plot, you'll see the fundamental frequency of course but then you'll see some other frequencies along with it at some -db below the fundamental. Then do the same test with the transformer and see how far down those the other frequencies are and we know that the transformer is helping to filter out those unwanted frequencies/artifacts.

So for your initial experiment, my guess was when you zoomed into the sine wave you were actually seeing a much higher frequency riding along top that 200Hz test tone. When you added the transformer you filtered that frequency down lower and so the scope isn't picking it up at that resolution. That doesn't mean that frequency isn't still there, it could be down say -20db and the resolution of the scope, or lack there of isn't allowing you to see it on top of the sine wave anymore but if you could zoom in more, it's possibly still there just a lot smaller. Viewing it from an FFT plot, you would see the 200Hz fundamental test tone then down say -60db would be for example some other 'grass' from high frequency artifacts and harmonics. Then when you do the same FFT plot with the transformer those other high frequencies would be reduced from -60db to say -100db. They are still present, just much smaller.

Really if you think about it that's all a transformer can really do. I know those dashes looks like the signal is stopping and starting but a transformer could never fill that in. My guess is those dashes is just how the scope trace looks when a higher frequency is summed together with the fundamental frequency and instead of seeing the detail of that higher frequency it just looks dashed. When you add the transformer, you're essentially adding a low pass filter and reducing that high frequency down so far you can't see it riding on the fundamental sine wave at that resolution anymore. Either way you are showing the transformer is doing something, but if you model a transformer there is really nothing it can do besides filter, it can't magically fill in information that was never there, it can only filter it out. Well actually, of course the B-H curve of transformers aren't linear so at low frequencies especially where saturation may set in you'll see waveform distortion so it will add harmonics in that regard, which also would show on an FFT plot. To me, and my experience with old scopes it looks like you are reducing some information by that dashed line anomaly disappearing through the transformer. I could be wrong but I don't see what else the transformer can really do.
 
Now that I mentioned it, harmonic distortion from core saturation at low frequencies could explain a sound difference between the DAC with and without the transformer. This would be something completely different than the dashed line anomaly we are trying to figure out. I'm just saying if the transformer is altering the sound quality it could very well be from added harmonic distortion at low frequencies. Run a 20Hz signal through the transformer and see how much distortion is there.
 
FWIW if you have a decent sound card in your computer you could always download the free version of ARTA which you could use to take distortion measurement and FFT plots. Even if you only had a .05% THD capability that's still -66db which if we are seeing that on a scope it should be enough to see it on an FFT plot. The only problem is you might be limited to 22kHz on the graph so we won't see anything above it. But it could still show us something and worth a try.
 
To me, and my experience with old scopes it looks like you are reducing some information by that dashed line anomaly disappearing through the transformer. I could be wrong but I don't see what else the transformer can really do.
Yes I agree – – I'm familiar with this effect and I am aware that most analog scopes don't show square waves all that well. They do show up often times as dashes. Usually if you play around with a frequency enough, you can get a decent enough view with "legs" however. I didn't even remotely bother to do this during my 10 minute "experiment".

On the other hand, I'm also not looking at or for Square waves. Rather a sine wave coming off of a non HiFi DAC (probably has none of the filtering or AA). When shown on a proper oscilloscope they can go down to the "uV" region, looks more like a stairstep or a set of stairs. Definitely not a square wave, but as you were speculating – – I guess yes it would be shown or manifested as such possibly on this old machine.. And there's no actual "missing" or gaps in information... it's just a nasty looking shape that you wouldn't want to listen to (under the magnifier. Attached is a screen grab of EEV blogs demonstration of this with the 10 X magnifier feature, looking at a DAC based signal generator... (to be fair, this gave me the idea, partially, to even give it all a look-see)
This is where the main argument comes up – – because like you said looking at the big picture it's a beautiful sign wave from either signal source and there's absolutely no way that there's any appreciable missing information. It's all a good point – – but ears hear sound via a different mechanism vs electronic devices and I believe they may be sensitive enough to detect the difference between two identical sine waves (aka the "big picture" ) one being composed of a course signal and one being composed of a fine (a.k.a. analog, never sampled)) signal chain.

Really if you think about it that's all a transformer can really do. I know those dashes looks like the signal is stopping and starting but a transformer could never fill that in.
Now that I mentioned it, harmonic distortion from core saturation at low frequencies could explain a sound difference between the DAC with and without the transformer.
Now you're getting at my overall hunch as to what the transformer is doing. Actually I believe, in one sense, a transformer can " fill in information that is not there" or "missing" ... We would do this is by mechanism of the inductor ( primary coil) ringing of the coil – – adding harmonics ... By this very mechanism would also tend to smooth out the stair steps.. Which of course would be considered "distortion" by most bench geeks (no offense intended here, as we should all strive to be bench "geeks" :) . One man's distortion is another man's reverb – – and well ... as we all know, Elvis wouldn't be Elvis without a bit of reverb ;) .
This would also explain as to why results are so all over the road... depending on the transformer chosen – – and why things can take on such coloration.
Western Electric 91 is a good example of this (and your aforementioned 13k roll-off SE amp) . Pump square waves into it and it looks absolutely atrocious. The ringing of the transformer in an amp with no feedback? Not pretty on the scope. The amp sounds more than fine however. As a friend into square wave testing has recently taught/reminded me – – a lot of amps even with feedback can get pretty nasty when you pump square waves in. But that's another topic.
If that's what it is, I am OK with that. Because I think analog is the way to go for anti-aliasing – – so to speak. If you can do it with inductors or transformers, it usually sounds better to most people. That method is not popular with manufacturers because it's very expensive. Generally, for most audio engineering / pro audio folks, and those who live in studios – – transformer/inductor based audio processing has been the favored method for eons. EQs, Compressors, Mic Pre's, and also line-level isolation... big arse transformers they love.



I forget what he said this was ... but I think he said 100khz, with 10x magnification. Zoomed out, it's a normal sine wave.

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Whether we describe what a transformers properties are via impulse response and time domain terms or in frequency domain terms it will tell us the same information.

So for example here we can see some high frequency noise modulating the sine wave output. I know you don't have access to an FFT which will clue us in to what frequencies are in there you may also get a half decent idea by counting the number of "steps" in one full cycle of the sine wave.

So okay we add a bandwidth limited transformer to the output and filter out most of that high frequency noise and it looks better on the scope but the million dollar question is can we hear that difference? Most of us can't hear above 15kHz so even with those steps modulating the sine wave I'm sure it still sounds just like a 200Hz sine wave and in a blind test couldn't be differentiated between the two.

Transformers by nature cannot have infinite response since they are bandwidth limited so we can see that in an impulse response or frequency response. We can also see nonlinear functions adding harmonics so of course there is added information, or distortion. So which properties will have the largest effect on sound is the million dollar question.

Of course I'm with you, I'd probably rather have the high frequency noise gone even if I can't hear it, but there are much less expensive ways to do that besides a transformer. But if the other properties of the transformer, say nonlinear low frequency response adding second harmonic distortion is what is really causing your ear to like it then go ahead with the transformer.

My big thing is I find it funny that for example in your last comment about one mans distortion is another persons reverb; many people want distortion and like it or they like a non-flat frequency response yet they focus on things that are not audible to stress over. So first step is to find out "is it audible?" and then "what else is audible that's causing the sonic differences?".

Talking about that Western Electric amp, so you are investigating it's impulse response via square wave testing and the results are awful. That aspect of the amp is only one part of it's characteristics, we can then view the nonlinear aspect of it and it's harmonic distortion profile. Say it's quite high with 2H at -26db below fundamental; that's going to also majorly contribute to the sound of the amplifier. With simple music it'll sound warmer but with complex multi instrument music IMD can reach a point where now music is blurred and incoherent.

All I'm saying is it's extremely difficult to say which properties of a device are the most important aspects of a subjective sound quality. I'm not saying objective because objectively we want the output the same exact as the input only larger but that's not the state of affairs in our modern audio enthusiast world. There are many flavors of amplifiers and equipment to suit everybody's needs. I try and view an amp or a DAC as a black box with input/output characteristics, you can have two completely different types of amplifier designs and as long as you know the I/O properties you can force one to sound exactly the same as the other. Bob Carver did this, he used his famous coffee can amplifier and only had I think a day to analyze a $10,000 tube amp, of course he didn't know it was a tube amp as he could only measure the in/out characteristics. Well wouldn't you know he got his little cheapo Silicon coffee can amp to sound exactly like the tube amp. An easy way to test the output to be certain they are exactly the same is by doing a null test. What's nice about the null test is you can even use real music so the naysayers that hate sine waves can't gripe about the test. All one needs to do is invert one of the outputs 180° and combine them and see what's left over. If you have a complete null down past -100db then you know the in/out characteristics are for the most part identical enough for our ears. Nobody could tell the difference between the two amps in a blind test. That is what put Bob Carver on the map.

The output transformer is probably the largest part of what makes a tube amp sound like a tube amp, so it stands reasonable enough that adding a transformer to any device, say a DAC will tend to give you the sound you like if you are into tube amplifiers. I'm being extremely general here as not all tubes amp are the same obviously, but the transformer is a large aspect of the sound. I'm not afraid of feedback since I took the time to learn proper feedback theory and how to make it stable. It's when feedback is inappropriately applied is when I find that it does more harm than good. I've been more fascinated lately with local feedback loops and have been working on a design to get the transformer out of the feedback loop to see if my ear can hear the difference. Another reason why is because I am finding people instantly lose interest in my amplifiers the moment they find out it has global feedback, they turn their nose up at it. So if they see there is no global feedback but I can still get close to the same performance I want it's a win win.
 
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Apologies for the blurry photos (hard getting good pictures with stuff that small with a phone camera), but mission accomplished.

New 150mH inductors (L500, L501) and 220pF coupling caps (C550, C551):

20230813_235239475_iOS.webp

27k resistor and 10nF capacitor in series between positive signal output and ground. I realized after the fact that I accidentally put the RC as a CR, but from what I remember from physics in college, order should not matter in a series RC circuit as long as there isn't a tap between the resistor and capacitor - the electrical relationship between the signal and ground should theoretically be identical regardless of order:

20230813_235125609_iOS.webp

New 0.5uH inductor (L200) on Coax 1 input:

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Apologies for the blurry photos (hard getting good pictures with stuff that small with a phone camera), but mission accomplished.

New 150mH inductors (L500, L501) and 220pF coupling caps (C550, C551):

Photos are fine, good job!

Sound any different? My guess would be no since it's rolling off pretty high up in human hearing range and most of us older gentleman don't get much above 15kHz. The main benefit of course is now the output is loaded properly which is important for stability and so now it can't send off any nasties up into the amp chain.
 
Got a pair of Tamura isolation transformers and wired 'em up. They're now between Schitt Modi Multibit and my integrated; source is a cheap little Audioengine streamer. I think it's better! Just sounds more "of a piece," though only listened for 30 minutes or so. Fun project for ~$50, at any rate.

mPrgNgv.jpg

81PsEvV.jpg
 
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Got a pair of Tamura isolation transformers and wired 'em up. They're now between a cheap little Audio Engine streamer and a Schitt Modi Multibit. I think it's better! Just sounds more "of a piece," though only listened for 30 minutes or so. Fun project for ~$50, at any rate.

mPrgNgv.jpg

81PsEvV.jpg
I might have to try something like this downstream from my Magnavox CDB650 in my main system.
 
Nice post @hifitown👍 It made me feel very sane.😉

Now I can share my collection of digital filters.
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IIRC, I got this pair of Tamura 600:600 from HifiTown eBay store when I registered in the late 90s.
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UTC A20s
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Western Electric 111Cs - I use these with my nosTDA1541tube DAC with plate choke loaded loctal output in my main hifi rig
@je2a3 I'm assuming that a UTC A-43 would also work well for this application (600 ohm: 600 ohm line matching with 20-30kHz F/R), yes?
 
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