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My latest infatuation - transformers

Posted 25th October 2014 at 08:11 AM by abraxalito
Updated 6th November 2014 at 11:51 PM by abraxalito

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I've found that some of the el-cheapo trafos (18rmb each) at a shop at the local electronics market are of split bobbin construction. This makes bodging up an audio OPT from two mains trafos a fairly straightforward matter.

I bought some with 9-0-9V and others with 0-12V secondaries. Then I disassembled them (fortunately they're not varnish dipped) and swapped out the 220V primary bobbin for the secondary of the other one. This gives me a trafo with 18V on the primary and 12V on the secondary, a step down of 1.5:1, impedance ratio of 2.25:1. So it makes a 4R drive unit appear as 9ohms to the chipamp.

And when I applied this to the output of the bass/mid of my chipamp (residing in the Phenix active speakers, its a TDA7265), apart from it sounding quieter I suddenly realized how much power supply noise I was still listening to. Incredible

So if you want to know if your chipamp PSU decoupling is really up to snuff, see how much difference a 1.5:1 trafo makes. Subjectively it 'distanced' the sound, Frank's 'disappearing speakers' trick once again

Update - bought some more 15VA trafos, this time with 15V secondaries. After taking them apart, I tore out the primaries (the wire is thin (0.15mm) so unwinding isn't recommended unless you have an hour or more to while away, just cut through the whole caboodle with wire cutters and recycle the wire as copper scrap). In lieu of the 220V primary I wound a 7.5V secondary (0.8mm wire, around 0.3R) making these 2:1 step down. The application is my now long-in-the-tooth D1080 MkIIs where a pair of TDA8947s drive the units. I'd recently discovered there was a substantial mis-match in that the units are 4R and the 8947 is not at all suited to such a low impedance load, given it has a bridged output. A 2:1 ratio trafo transforms these units into 16R, a perfect match. So far I've only done the trafo mod to the bass/mids but the effect is quite repeatable, much improved dynamics and a shifting backwards of the music so that the speakers now seem rather incidental to the goings-on. Don't forget a 3,300 or 4,700uF 'lytic in series with the primary though to prevent trafo saturation with the DC offsets typical from chipamps. The D1080s have undergone such a remarkable transformation that I shall have to buy up a few more pairs now....

Update2 - now have wound and installed the tweeter trafos. These are much easier and quicker to wind as the design equations indicate much fewer turns are required for HF duty. I use ferrite cores, similar to Ferroxcube EQ25 and 40turns for the primary, 20 for secondary. I'll post up a pic of the end result in due course. I've only wound a couple of pairs so far, the wire might benefit from optimizing as I'm concerned about HF losses and with the wire thickness I use (0.6-0.8mm) there may well be considerable proximity effect losses. Listening now and Leonard Cohen's voice has not been more engaging in its emotional impact, addictive stuff The soundstage depth is the best I've ever had from a pair of integrated electronics actives - these speakers have the active XO mods I talked about in an earlier post but still only use TL084s.

Update3 - I've now uploaded a pic of the tweeter trafo. As you can see its dminutive, under 30mm across and about 20mm high.

Update4 - I'm experimenting now with a smaller size of transformer. I found another vendor where their 10VA trafos cost 9rmb - half the price of the 15VA ones. But on disassembling the first one I found that they're not making the best use of the winding window - more copper can be got in These also aren't split bobbin which is an advantage for doing custom windings (rather than re-using ready made ones) because there's no shroud holding the two bobbins together, meaning more space for windings. So it may well be that a 10VA with optimized windings can equal or exceed a 15VA. Stay tuned


Update5 - added a pic of the 'unwrapped' electronics from my 2nd pair of D1080s - I figured I needed the electronics out in the open to better optimize the SQ. Besides that the cabinet was severely constraining how many caps I could tack on to the supply The trafos for bass/mid now are 9rmb 10VA types, wound for 2.5:1 step down. Needless to say the sound is awesome, a major incentive for me to persue higher step down ratios...

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Posted in Active speaker modifications, Power chip amps

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1. Good work there, :) ... . One gets blasé at times, thinking one's got the SQ in pretty good shape - I'm as guilty as anyone of doing this - and then something is done that pops the level so much further forward, yet again! The key is knowing, [I]really [/I]knowing, that no matter what barrier seems to be in front of one, in terms of "fixing" the sound - that it can always be bypassed, in some fashion ...

   Posted 25th October 2014 at 09:31 AM by fas42

2. I am really curious now how far I can push that trafo ratio. Chipamps tend to max out below 100V - LM4766 (74V) is a bit below the LM3886 (94V) but its a dual so more convenient to use. I reckon I can run that in bridged with a 3:1 step down ratio. So then each chipamp side sees 18R (for a 4R drive unit). Beyond that caps go on getting better - in terms of energy stored per unit volume. I found photoflash capacitors have about the best Joules/cc ratio available. Not as good as EDLCs but those are too low voltage to be useful except in large paralleled arrays in high power amps. LME49810 goes up to 200V operation but beyond that it will have to be discrete....

   Posted 25th October 2014 at 12:20 PM by abraxalito

3. 	I would also be curious as to whether the transformer in some fashion is acting as a filter, which allows the circuit to function more optimally - as an off the top of the head example, whether RF muck on the speaker side of things is blocked more from feeding into the output stages, say?
   	Posted 25th October 2014 at 10:07 PM by fas42

4. Good point yes, I also wondered about this. Trafos have leakage inductance so perhaps its isolating problematic capacitance from the amp's output? Another aspect is that speaker cables act as an antenna hung on the chipamp output - I haven't seen any series inductor in the speaker itself. So there may well be marginal stability issues which I'm fixing up by running through the trafo. Incidentally are you following Tom's thread about his 'Modulus-86' chipamp design? If so have you noticed the tumbleweeds there in response to my probing about opamp OPS-induced power supply noise? Interesting how Tom's put up so many deflections - like citing cherry picked regulator PSRR numbers to 'defend' his design, plucking regulator output impedance seemingly out of a hat. This all goes to tell me power supplies are a major lacuna when designed by 'objectivists'

   Posted 25th October 2014 at 11:44 PM by abraxalito

5. Indeed, yes. In fact, if you check earlier posts I've been pushing him to do extra, but unfortunately still standard, tests in the hope that perhaps something of significance might appear. It [I]is [/I]remarkable that people can't seem connect that ultimately it is all one circuit - they always break it down into submodules of functionality and behaviour, and treat each as isolated from anything real ...

   Posted 26th October 2014 at 12:00 AM by fas42

6. If you look at his PCB (since there's no released schematic) there look to be TO-92 3-terminal regulators adjacent to where the power comes in to the board. The 'THAT' chip is at the far end, furthest from those regulators. I'd hazard that the total load on the regs is going to be under 20mA (there are only 3 chips being powered) - regulator output impedance gets pretty shoddy when they're lightly loaded. Since Tom misconstrued my opening gambit - which was that there was going to be load-induced noise - and proceeded to defend the line rejection, this tells me there's probably a line rejection issue too, at HF. If the regs turn out to be LM317L/337L the line rejection at HF is notoriously poor. I just downloaded the DS for the LM337L - a pathetic travesty of a DS with no graphs whatsoever. So it seems to get graphic data on line rejection I need to turn to the LM137 DS. Line rejection looks from that to be 40dB at 20kHz falling to 10dB at 1MHz (assuming use of the ADJ pin cap). LM337 OP impedance looks to be around 30mohm at 20kHz, but note the graph is plotted at 500mA output current. A first approximation would suggest that at 20mA the Zout rises by a factor of 500/20, so around 750mohm would be my guess. Subjectively this fits with experience as when I upgraded my integrated amp's input opamp with shunts it cleaned up the sound substantially, that was previously fed with the standard 317/337 pair. Seems there's been revision on TI's part of these reg DS - no output impedance graph any longer for LM317, but there is a new one - ripple rejection vs OP current and that shows a substantial fall off in ripple rejection at light loading for the LM317 - looks to be around -58dB at 20mA. A quick sanity check on my Zout estimate for the LM337L is made by comparing with the LM317L which does have the requisite graph, taken at 40mA and shows just under 1ohm for 20kHz. This is the point where the OP cap takes over the impedance curve.

   Posted 26th October 2014 at 12:16 AM by abraxalito Updated 26th October 2014 at 12:57 AM by abraxalito

7. 	Well, here you go then, [URL="https://www.alldatasheet.com/datasheet-pdf/pdf/257427/LINER/LM337.html"]LM337 datasheet pdf datenblatt - Linear Technology - Negative Adjustable Regulator ::: ALLDATASHEET :::[/URL]. I have an old National Linear Databook - this has been a treasure over the years - and the graphs for the Linear version have been photcopied from there ... :D
   	Posted 26th October 2014 at 02:23 AM by fas42

8. I see Tom's put up the IMD results for you. He says the AP is responsible for most of the IMD but looking at his plots I must be missing something. On the first one (loopback) the 1kHz looks to be the highest distortion component at -115dB, the 18/21k tones are 1 or 2dB below that. Coming out of his amp there must be some anti-phase 1kHz to cancel the analyser's 1kHz distortion but the 18k and 21k tones are -84 and -82dB respectively. That puts the amp as having increased those components by at least 10dB. So the AP's 'raw' IMD is going to be below -112dB compared to Tom's own figure for his amp of -105dB. In actual fact perhaps the AP's ADC is responsible for some of the harmonics, depending on the range setting going into the AP. Really the test tones should be notched out for the FFT to have much meaning. My other concern is where the noise 'floor' sits. Normally the AP is using an FFT with at least 64k 'bins' giving an FFT gain over 30dB, typically around 36dB. So the noise 'floor' on the loopback test is then going to be at best -104dBr in the full audio bandwidth (-140 + 36). Not too resolving for testing a poweramp methinks.

   Posted 27th October 2014 at 05:51 AM by abraxalito Updated 27th October 2014 at 06:02 AM by abraxalito

9. Yes, I had similar sorts of concerns, the AP intrinsic distortion seemed to be interfering with the results - but I didn't wish to appear entirely ungrateful for him going to the effort of getting the extra info, in the first round anyway! My feeling is that a variation of the testing technique could bring up something genuinely meaningful, but unless I had all the apparatus myself to play with directly, I would be firing off too many wild shots and getting nowhere, suggesting other things to try ...

   Posted 27th October 2014 at 09:26 AM by fas42

10. I was curious about Mooly's 'loaded opamp' test, tried to download the files but seems in China, dropbox is blocked so no cigar. Elvee has posted up an interesting way of doing a difference, one I'd not considered before - using LTSpice to subtract the .wav files. So far everyone is talking about the distortion but in my experience, loading the opamp sags the power supply dynamically and I wonder whether that's what Elvee's found with his cursor on the waveform? All very interesting stuff I see you're on the case with Audacity already - excellent progress! Later - looks like your analysis got the tumbleweed treatment too, in favour of Pano's

    Posted 27th October 2014 at 11:41 PM by abraxalito Updated 28th October 2014 at 11:07 PM by abraxalito

11. When I was littl'un, we lived way out, in the bush, as we call it here. And I used to visit big ants' nests, where they were scurrying around, calmly carrying out their everyday activities - and I'd pick up a big stick, and poke it down the big centre hole of the nest, to see what happened ... Hmmm, don't know why I just mentioned that ... Ummm, ahh ... yes, bit of a shame, but will see how things develop, ;).

    Posted 29th October 2014 at 01:53 AM by fas42 Updated 29th October 2014 at 01:56 AM by fas42

12. 	When I was married (going back now a decade and a half) my wife remarked I was one huge wooden spoon.... Seems we're cut from the same cloth. Yes let's wait and see
    	Posted 29th October 2014 at 02:08 AM by abraxalito

13. 	More good stuff ... in one way it's just downright more interesting, prodding and pushing supposedly unpromising gear to deliver competent sound - one has no hesitation in butchering things, just to "see what happens". And the satisfaction in getting a cheapie to deliver is ... ;)
    	Posted 2nd November 2014 at 12:03 AM by fas42

14. Yes - 'just to find out what happens' is exactly it. Curiosity I find is a totally relentless master No sooner have I gotten some cost out of the system than I'm again asking 'do I really need this part?' or 'if I substituted Y for X would it still sound this good?' Active speakers with electronics inside though I've just about had enough of. I'm planning to move the electronics out of the speaker cab so that tweaking can be done much more interactively. The more tweaks I can try the faster I'll learn what's important

    Posted 2nd November 2014 at 01:43 AM by abraxalito

15. 	I wouldn't tolerate anything less than "awesome" from you now, Richard ... :)
    	Posted 7th November 2014 at 10:35 PM by fas42

16. Hmmm, I am concerned about the devaluation of my adjectives over time. Rather like the Martin Colloms scale which had to be extended beyond 100.... I'm fairly convinced now from these latest trafo experiments that the amps I'm using aren't transparent so its onward and upward on the path of more supply voltage and bigger step-down ratios.... Reflecting on this lack of transparency, given that opamps running a mA or so of output current are very sensitive to supply impedance, how much more of a problem is a poweramp running 1A or so into a load? Since my opamps (which are biassed into classA) have something around 10,000uF on the rails this tends to indicate that 10F would be required in a poweramp even if it were classA.....

    Posted 7th November 2014 at 11:27 PM by abraxalito Updated 7th November 2014 at 11:32 PM by abraxalito

17. Ol' diminishing returns will kick in at some point - if it doesn't, then my take is that the real, underlying problem is not being addressed, and adding so much capacity is only indirectly improving things - much better, and cheaper, to go to the heart of things ... That Bryston amp I heard was the most impressive I've experienced, in being totally untroubled by being asked for ever increasing volume levels - its supply reserves would have been nothing special, yet it was able to deliver realistic grunt seemingly without end. So, there are definitely smart ways of getting there ...

    Posted 8th November 2014 at 12:15 AM by fas42

18. Seeems we may be in search of rather different things - realistic grunt so far has not been on my agenda, just getting a nearfield experience to be most satisfying has been my aim. Most certainly realistic grunt will be on the list of things to do in future, scaling the experience up to far-field listening with bigger boxes (and going from 2-way to 3-way). However given that I'm hearing more now than I've heard before on seemingly familiar recordings - this morning it was the tinkling of the piano on Saint-Seans Symphony No.3 which drew my attention - why hadn't that particular sound registered with me before? - I'm continuing on the path which brought about these most recent improvements. I'm not yet adding outrageous capacitance, I shall most likely add a 'hexacap' later on today as there's only around 20,000uF at present on the supply. Capacitance when I added it before (on earlier chipamp projects) mainly had an impact on LF transients, making bass sound 'tighter' or 'tauter' but I may have to give up such lily-gilding now that I have the OPTs which probably are softening the bass. But they've given me so much more transparency its easy to 'sacrifice' the bass slam for hearing deeper into the recording.

    Posted 8th November 2014 at 01:39 AM by abraxalito

19. No, it is the one and the same thing: a satisfying nearfield becomes realistic grunt when scaled up - automatically. This is because the ear adjusts to higher intensity sound in a certain way, as part of its mechanism for protecting its delicate inner apparatus - this is something one can explore with live instruments, by listening at some distance away - would be termed nearfield sound from a speaker - and then going right up to the player of that real instrument. The 'correct' sound is for the bass slam [I]and [/I]hearing deeper into the recording to co-exist, they do not exclude each other; highly manufactured pop recordings are excellent for picking up progress made - very, very meaty instrument sounds are punching hard, and at the same time the vocalist exists as a separate sound element, with a soft, floating voice, and deep, deep echo behind him. It sounds like you might be losing some of the treble - I would do a test with a frequency sweep going up to the highest frequency that still registers to my ears, to make sure that there wasn't a significant fall away.

    Posted 8th November 2014 at 02:40 AM by fas42 Updated 8th November 2014 at 02:43 AM by fas42

20. 	What in what I wrote leads you to suspect I'm losing some treble?
    	Posted 8th November 2014 at 02:51 AM by abraxalito

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