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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

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...

Posted in Active speaker modifications, Power chip amps

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	That Nichicon bipolar looks to be just the ticket. I only have a guess about why you only get the issue when two primaries are used, as I said earlier - putting two secondaries in parallel will pull twice the current (half the DCR). (Mistake here I should have said 'primaries' I think.) High value - in the range 100k to 1Mohm.
	Posted 21st December 2016 at 02:06 AM by abraxalito

	so you think the little bit of dc was getting through on single primary, but with two with half the dcr it causes a little pop ever 2-3 sec
	Posted 21st December 2016 at 09:04 AM by nige2000

	put the 1000uf nichicon bipolar in, seems to have positive effect actually seems louder bass quantities seems more but quality is still great is there a reason for capacitance value?
	Posted 23rd December 2016 at 09:40 PM by nige2000

	The 1000uF is calculated not to roll off the bass too much - the load 'seen' by the cap is 4X (coz your trafo is 2:1) the speaker impedance. I did a quick LTSpice sim and it shows you get about 0.3dB loss at 20Hz from this value, nothing really noticeable subjectively. Having no DC on the trafo is most likely the cause of the perceived bass quantity improvement.
	Posted 26th December 2016 at 07:12 AM by abraxalito Updated 26th December 2016 at 07:15 AM by abraxalito

well proper listening range starts at 9v so would be more like 3:1?
using two 30 v primaries in series
or 30 v into a 60v primary had loss of definition of the sound
using two primaries in parallel seemed to have no benefit/difference over using single primary

do you ever feel u lose some resolution or detail using a cap in series in the signal chain?
even though there is a resounding benefit to the ac coupling

Posted 2nd January 2017 at 09:28 PM by nige2000 nige2000 is offline

	So far I haven't noticed any downside effect of having the cap. But that could be because I don't know what to listen for. Nothing jumps out at me which takes away from the SQ by having the cap there.
	Posted 3rd January 2017 at 12:23 AM by abraxalito

	right i got two more transformers 30v pri and 11 x 1v sec more or less 3:1 what value cap do i need ? or is stacking them ok? sounds ok with 2 x 1000uf in the other has 11 x 1.4v sec havnt tested it yet
	Posted 12th January 2017 at 12:50 PM by nige2000

	Step down 3:1 is going to give a higher impedance than with 2:1 so just stick with one of those 1000uFs as you've already got them. You'll get a tad more bass than with 2:1 but probably not noticeable.
	Posted 12th January 2017 at 02:25 PM by abraxalito

	Weird thought I had less bass Clarity and Hf is better though
	Posted 12th January 2017 at 04:36 PM by nige2000

	Yes its the HF (due to poor PSRR) on chipamps which is their weakest point in my experience. The best HF I've had from a chipamp was a rather low powered one (TDA8566) running on 18V, bridged. I got the HF good because on this chip the supply pins are adjacent so I used dozens (literally) of 1206 ceramics across the rails.
	Posted 13th January 2017 at 01:10 AM by abraxalito

encountered an issue
when using my amp bypassing the input signal volume pot and using the output tvc
on input signals of more than 1v into the amp chip i get some type of distortion and approaching 2v is a real mess
im assuming the gain i had set on the lm3875 (22k and 680 r) x 33 is too much for the supply
just wondering now whats the lesser evil
reducing the gain (i read x20 is the min for stable lm3875) or a resistor voltage divider on input?
when reducing the gain also reduces potential step down on tvc

also was wondering had you tried the stepdown output as an autotransformer

an initial test showed some promise of better detail, clarity and deeper bass

the guys at the transformer shop suggest to go with a "bigger core to improve nominal induction"
think there's any merit in that?

Posted 29th January 2017 at 05:23 PM by nige2000 nige2000 is offline

You're right that having X33 gain is just too high, I suggest reducing it to X20. Increase the 680R to 1k2. I read the DS, you can go as low as X10 if you want (20dB). Putting a resistor divider on the input is a greater evil than gain reduction - with lower gain you get better PSRR.

I haven't tried auto trafo on the output so far no. Auto trafo has advantage when the step down ratio is low - saves copper. But for higher step downs, not much in it.
I don't know what the guys at the shop mean by going for a bigger core. It means you can use thicker wire, so get lower copper losses, but core losses will go up and they get quite high at the top of the audio band due to eddy current effects. So I prefer to go as small as possible.

Posted 30th January 2017 at 12:22 AM by abraxalito abraxalito is offline

	looks like i have to set the gain to suit the input voltage then pain in the **** for using the amp with different sources or testing on other guys systems
	Posted 30th January 2017 at 12:00 PM by nige2000

	ha... ! that wasnt worth censoring
	Posted 30th January 2017 at 12:01 PM by nige2000

	Good reason to get yourself an input transformer with taps for gain no? Keep the chipamp at 20dB and adjust accordingly.
	Posted 30th January 2017 at 02:19 PM by abraxalito

	reducing the gain seemed to do the trick tried the autoformer method again fairly sure i prefer it what sort of spec would i need for the input is 20va too big of a core?
	Posted 30th January 2017 at 11:01 PM by nige2000

	Input transformer you can make the size of the ones I brought to your place in September - around 25mm core size. PQ2020 may well work fine. If you tell me your input and output peak voltages and lowest full-power frequency I'll design it for you. Its a signal transformer so we don't care very much at all about the winding resistance, so we can make it small using very fine wire.
	Posted 30th January 2017 at 11:36 PM by abraxalito

	still have them edcore trafos 15k:15k with center taps is it worth trying? at 15k : 7.5k suppose id want inputs of 1 and 2v and output of 1v and 2v? sometimes i notice the output trafos unconnected to speakers make some playback noise is that normal?
	Posted 1st February 2017 at 12:28 AM by nige2000

	Yes, worth trying the edcores in step down. Not sure what you mean about playback noise?
	Posted 1st February 2017 at 12:40 AM by abraxalito

	I get sound From the transformers Like a 50hz trafo hum only it's whatever is playing through the amp Is YouTube blocked there too?
	Posted 1st February 2017 at 10:39 AM by nige2000

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