My latest infatuation - transformers
Posted 25th October 2014 at 08:11 AM by abraxalito
Updated 6th November 2014 at 11:51 PM 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...
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...Total Comments 101
Comments
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I get the same subjective effect putting a trafo in line with an unbalanced interconnect - it cleans up the sound, giving more 'air'. I take it this is due to reduced noise currents flowing along the IC. So low capacitance is desirable in the trafo, or use a trafo in conjunction with CM choke.Posted 7th December 2016 at 02:06 AM by abraxalito
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output transformers arrived
2x30v pri 2x15v sec
and 2x30v pri 1x25v sec
its plainly obvious that step down is best
and the more the better
sounds like less noise, more dynamic, better clarity, less edge
more control
with the 25v secondary traffo is not a worthwhile improvement but noticeably better
i have a 2x30v pri 10x3v to test yetPosted 13th December 2016 at 09:58 PM by nige2000
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Didn't take too long then. Yep step down rocks, nice result.
You got 10 secondaries each of 3V?Posted 14th December 2016 at 05:23 AM by abraxalito
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yes 10 secondaries of 3 v
the primary is 2x30v
if i join the primaries like you would for 60v and put 30v in
will that make the secondaries 10 x 1.5v?
having unloaded secondaries have any unwanted effect?Posted 14th December 2016 at 10:07 PM by nige2000
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The voltage rating on the secondary windings doesn't change because you changed something on the primary. Yeah you get a higher step down if you put the two primaries in series but if you're then only going to put in 30V you're not fully utilising the trafo (meaning you've got too high resistance windings). Harmless to have secondary windings unconnected.Posted 16th December 2016 at 03:46 AM by abraxalito
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hmmn.....
ok with the trimpot out im getting 66v output from chipamp
which explains the clipped sine on single tone?
cos trafo is 60v?
trafo should be 66v pri?Posted 16th December 2016 at 03:08 PM by nige2000
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I"m confused here - the output of the chipamp depends on the power supplies you're using. No changes to them right, still the same array of batteries? In which case how can you suddenly get more output voltage?Posted 17th December 2016 at 12:23 AM by abraxalito
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sorry explaning it badly
the supply is 66v (+/- 33v)
i had a trim pot implemented like this in it
i guess what happened was the voltage divider was halving the signal say to 0.5v which caused max output of 30ish volts?
bypassing the trim pot raised the input voltage and therefore the output voltage to 66v?Posted 17th December 2016 at 09:43 AM by nige2000
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Can't see the image when I click on the link I get this message :
Block reason: Access from your Country was disabled by the administrator.
So are you saying you changed the gain somehow with the trimpot? In which case still the output maximum will be around 30V peak (60V peak to peak) due to the PSU.Posted 18th December 2016 at 01:23 AM by abraxalito
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not really important
if i halve the input signal it halfs the output?Posted 18th December 2016 at 08:54 AM by nige2000
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yep, for sure. You took out a pair of series Rs? That's bound to increase the gain, depends on the ratio of R to the pot value.Posted 18th December 2016 at 01:21 PM by abraxalito
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as well as higher volume its a livelier sound
lower signal input into higher voltage/gain chipamp better than visa versa?
guess thats difficult to quantifyPosted 18th December 2016 at 01:59 PM by nige2000
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im getting intermittent pops when using the two 30v primaries in parallel, its ok only using one primary
what could that be?
would there be any benefit slightly raising primary voltage say to 33v or is that over thinking it
and having 11 secondaries at 1.5vPosted 18th December 2016 at 03:13 PM by nige2000
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Maybe you're not AC coupling the trafo? If there's a DC offset the trafo won't much like that, putting two secondaries in parallel will pull twice the current (half the DCR).Posted 19th December 2016 at 04:34 AM by abraxalito
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Are you suggesting I try a cap in series on the primary?
Would a slight imbalance in the pos and beg supplies cause that much trouble ?
Funny when I use one primary there's no issue only when I use both in parallelPosted 19th December 2016 at 03:29 PM by nige2000
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I always use a series cap to ensure no DC on the transformer. Trafos hate DC. DC on your amp output isn't caused by an imbalance in the power supply, rather from offset voltage/current inside the LM3875.Posted 20th December 2016 at 03:33 AM by abraxalito
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What sort of cap do you recommend?Posted 20th December 2016 at 08:25 AM by nige2000
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Electrolytic, bipolar if you can get it if not then two back-to-back electrolytics with a high value resistor to the mid-point to bias them. The resistor to go to a -ve or +ve supply (whichever is more convenient.) I reckon you want 2200uF+2200uF (or 1000uF if you find a biploar cap). Working voltage 35VPosted 20th December 2016 at 02:42 PM by abraxalito
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Posted 20th December 2016 at 03:31 PM by nige2000
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Out of curiosity why is there no issue when only one primary is used?
is that not the same thing as if there was only one primary?
define high value resistor? 50k-1m?Posted 20th December 2016 at 06:30 PM by nige2000
