Roll your own Output Transformers

[All Ears]

Hi to the resident gurus of iron (you know who you are),

I am preparing to do a little bit of hand winding of output transformers. I plan to take standard E + I cores and wind a few bifiliar wound gapped output jobs much in the style of the LO1166...haven't done it before but I figure OT's will not be "too" hard (famous last words).

A few questions:

1) Lamination Insulation: I plan to just use lamination air dry varnish. I guess I paint both sides of each lam and let them sit. But I am not sure how to get a consistent thickness on the lams - I figure drips and bulges could really screw it up and may even make it too much of a squeeze to get the lams in the bobbin core. Any advice from old TX winders ?  I once read somewhere that you could even put kraft paper between each lam as insulation instead of varnish...I personally have never seen this. Anyone heard of this ? Is varnish superior to paper ?

2) Winding insulation: I thought that while I wound I would brush on some more of the lam varnish on the turns both as insulation (not that is is really needed in terms of voltage) and as a method of stopping any microphonics. Will this varnish work in that context or might it attack the enamel on the wire ? Is this kind of varnish even needed here or can you just wind the wire on bare ?

3) Gapping: The standard procedure I have heard is to put a shim of paper between the E and the I lams. Do I need to get into any complex maths to determine the gap width or will the old kraft paper gap be about the right size for an OT like this ?

4) DC capabilities: Anyone know what the DC capabilities of the LO1166 are ? In fact, anyone know what kind of DC current levels might be seen at a BA283 output stage ? (at idle or maxed, or anything).

5) What other well-respected solid state gear/output stages (apart from Neve stuff) needs gapped output transformers ? 

6) Anyone know of a project that would really make UTC A-22 OT's shine ? I have a pair of them and have been wondering where to use them. I would say they are gapped (they say 20mA DC capability) and it says +30dB level on the can...so...that definitely sounds like an OT to me...not sure if the current of 20mA is enough to handle single ended outs or various solid state stages that need DC gapping...

I guess that was too many questions...but I know there are certain mag heads here who relish questions - and thanks to those people these forums are a goldmine of iron info (mixing my metal metaphors there)

Cheers,

I'm all ears

 

[abbey road d enfer]

1) Lamination Insulation: I plan to just use lamination air dry varnish. I guess I paint both sides of each lam and let them sit. But I am not sure how to get a consistent thickness on the lams - I figure drips and bulges could really screw it up and may even make it too much of a squeeze to get the lams in the bobbin core. Any advice from old TX winders ?  I once read somewhere that you could even put kraft paper between each lam as insulation instead of varnish...I personally have never seen this. Anyone heard of this ? Is varnish superior to paper ?

To my knowledge, and according to my winder, lams are readily insulated by surface treatment. You don't have to do anything. Anyway varnish would either be too thick or too fragile.

2) Winding insulation: I thought that while I wound I would brush on some more of the lam varnish on the turns both as insulation (not that is is really needed in terms of voltage) and as a method of stopping any microphonics. Will this varnish work in that context or might it attack the enamel on the wire ? Is this kind of varnish even needed here or can you just wind the wire on bare ?

I don't think varnish is a good idea; in addition to the risk of damaging the insulation, it would take some space and impair the coupling factor. This is traditionally done when the transformer is finished, by impregnation. Professional winders use hot varnish but you could use wax.

3) Gapping: The standard procedure I have heard is to put a shim of paper between the E and the I lams. Do I need to get into any complex maths to determine the gap width or will the old kraft paper gap be about the right size for an OT like this ?

gapping is not something you use as pepper and salt in cooking. Proper gap should be determined by calculation, starting with the DC current, which then defines the magnetomotive force (Amp-turns). Knowing the magnetic circuit reluctance determines the static induction B, which must be in the linear part of the H/B curve. The magnetic circuit reluctance is in series with the gap's reluctance, so it's easy to calculate the needed reluctance of the gap, hence the length of the gap and the thickness of paper.
Now you could experiment with different thicknesses until you get the desired result, by measuring LF THD at elevated level.

4) DC capabilities: Anyone know what the DC capabilities of the LO1166 are ? In fact, anyone know what kind of DC current levels might be seen at a BA283 output stage ? (at idle or maxed, or anything).

It depends very much of the setting of the trimmer RV1, but in order to produce the specified output (+24dBu into 600 ohms) it should be close to 50mA idle.

5) What other well-respected solid state gear/output stages (apart from Neve stuff) needs gapped output transformers ?

Out of my head, none...I guess most of the other designers took advantage of the relative ease of solid-state push-pull (Langevin, CBS, ...), with the notable exception of UREI (1108, 1176 1st version). The cost of a complementary transistor is easily compensated by the reduction in cost of the xfmr and the increase of performance. ther are more cost-effective ways of generating second harmonics 

 

[CJ]

don't coat the lams, you can use electrical tape in between layers, gap for Neve Line OPT is 0.003 KP so no math needed,

there is an un-gapped Neve OPT, the LL-2567 for use off pin F,

gapped Nevc OPT is LL-1166, wound on a sq stack of 75 EI,

wound 1/2 Sec B -Pri B - Sec A - Pri A -1/2 Sec B 

where A is top winding and B is bottom winding as drawn on coil schematic,

turns are 262-311-523-311-262,  311 is reversed wound,

you can either use #30 - #28 - #28 - #28 - #30 wire if you want to play games with balanced DCR, or use #28 for the whole enchilada,

30/28 is Marinair, #28 only is used in the Carnhill version.

ungapped LL-2567 has same coil structure but use #27 for the whole enchilada,

turns go 240 - 290 - 469 - 287 - 240  with 290 rev wound.

you want a bit more bass add 1/8 inch more lams like the Sowter copy.

Neve OPTs's are wound with wire tensioned almost to breaking point for low leakage, this is why failure mode for this xfmr is always a broken wire, usually somewhere in the first layer,

H/B s/b B/H from above post,

 

[12afael]

what is the effect of the reverse wound? reduce de flux?

 

[abbey road d enfer]

what is the effect of the reverse wound? reduce de flux?

RW helps reducing the effects of inter-winding parasitic capacitance, by cancelling electrostatic field. It is somewhat similar in principle to the humbucking technique.

 

[12afael]

thanks Abbey.

 

[CJ]

you do not really need to reverse wind this Neve OPT, it is very low Z, so humbucking is not really an issue, and it is not a bridging transformer so symmetry is not the deal,
plus there are no screens which you usually see in a reverse wound xfmr,

i bet Rupert designed this thing, not Partridge,

 

[All Ears]

Thank you to Abbey Rd and CJ ,

According to both your advice, I will skip the varnish.  The less chemicals I have to handle, the better (not counting certain botanical chemicals).

Abbey Rd - Would this Langevin pre/output stage be a reasonably feasible DIy project in terms of the silicon needed ?  What would be another outstanding classic "solid state + output tranny" output stage IYO ?

CJ -  Quick thinking. You pre-empted my questions on turns and gauges. Ha  - years of practice I guess.
Does that "0.003" for the kraft paper represent a measurement in inches ? I have seen 0.05 and 0.08 grades so I guess those might be in mm.

That's a really interesting thought about the tensioning...I had not even really thought about the effect of that - but of course it must...coupling especially.  Does tighter tension = more bandwidth or what ?
Which leads me to think of another the question of comparing say:
a)"bifilar wound" 
b) "one over the other" 
c) "top bobbin/bottom bobbin".
Is it easy to see a characteristic sonic change between those choices or is it not so obvious ?
I had thought that the best performance might be had in a DIY situation by going bifilar and having two spools on the go at once. But it might be pretty tricky to get that high tensioning with two wires on the go I guess.

And on the matter of having separate bobbins on output transformers -  I have just found these cheap 120v/120v power iso transformers (in different VA ranges) with a top and bottom bobbin separated quite a lot for the safety aspect (over an EI core). I am thinking these might make interesting 1:1 output transformer 'test beds' (don't know inductance and all those other figures at the moment). The efficiency and the coupling for audio might be a bit crap due to big separation of the coils (?). WIll put some subjective audio opinions about these mains iso on this thread when i get/test them.

Anybody heard of Z11 steel laminations being used (successfully) for audio purposes ?  I believe Z11 is normally a power transformer specification (sort of like M6 is spoken of for audio). Would this kind of lamination be a bit too "steely" ?

Interesting also CJ your comment about Sir Rupe actually building those parts himself. Is that just a fact or is it based on some kind of design features you detect ?

Thanks for the informative posts  - good stuff

 

[CJ]

i think Ruperts first job was in a transformer factory,

kraft paper was in inches,

build an API before the Langevin, it is a lot easier, boards and parts available online, OPT can be wound DIY style, Langevin uses C cores for their transformers and the coils have to be perfectly balanced,

power transformers can be used for outputs, it has been done many times, 1:1 would work for API,

never heard of Z11,

tight wire results in better linkage = less leakage = more inductance = better bass, plus if i remember right, Maraniar is scatter wound for less C, when you scatter wind, you do not get as many turns per square inch or cm, so pulling the wire tight helps get back some lost turns,

bi fi is alright for low turns pri but excess C is a problem after 300 turns, good example would be K-241-D by Peerless which is a very nice coil,1:12 ratio, has a mid-range boost,  i use it on for the V72 input and it kicks Batman's azz, 

 

[All Ears]

Cj - you sound like The Joker

Here is a couple of references to Z11 - I think it is originally a Japanese silicon steel grading name

Here:
http://www.diyaudio.com/forums/tubes-valves/22877-transformer-steel-material.html

And here - Z11 being used (less typically?) for tube output transformers:
http://www.audiodesignguide.com/New2A3/new2A3.html

 

[All Ears]

I just had a thought...perhaps someone has already done it

If you can find a ferrite type PI bobbin (used in power supplies nowadays) that is big enough...it might be possible  to make a PI wound output transformer

If there is no such moulding available then it is not so fun...

I recall from CJ's dissection that the Telefunken V72 input had a PI type wind (also phase or polarity differences)...but I believe PI winding on outputs may also be beneficial to performance. Got the idea from here:
http://lenardaudio.com/education/14_valve_amps_5.html

 

[CJ]

you can make your own bobbins from a regular bobbin and then adding in dividers,

simply tape the base of the dividers down, then use wood spacers to keep the outside flanges from warping outward away from the windings, then remove the wood spacers as you fill up the chambers, you can add varnish to each pie to hold the wires, or tape the spacers up after the wind to keep the outer flanges in place after you remove the spacers,

if you have some old LP's (records) laying around, you can use that for the plastic.

just heat it up and carve it down to size, maybe the Monkee's Greatest Hits, no wait, the Monkees are cool, maybe a Barbara Striesand album, I'm a Woman In Love would be a painless sacrifice,  ;D

 

[abbey road d enfer]

you can add varnish to each pie to hold the wires,

CJ, thank you for using the right spelling.
PI is so confusing, suggests something entirely different (think EI lams).
I think the correct word is "honeycomb winding" or "basket-weave winding".
It offers reduced stray capacitance at the expense of copper density. For a given core, it results in reduced LF.
The reduced capacitance is an important factor in old radios.
In audio transformers, the benefit of reduced capacitance is mitigated by the increase of leakage inductance.

 

[CJ]

took me a while to decipher PI also,

ok here is how you work the vinyl separator thang,

sorry to all the Aldo Nova fans out there> 

 

[All Ears]

ha ha - that is bizarre

i guess you are adding some spirit of whatever the album was to your o/p transformer

I fear if you used Streisand that any signal passed through those coils would end up sounding like ****

 

[gyraf]

When winding transformers, how important is it to make use of the whole winding area? I see the importance for optimizing DC resistance by using as heavy a wire as possible - but other than that?

In applications where wire resistance plays a minor role, but you want lower inter-winding capacitance, can you use thinner wire and live with not filling the window without other unwanted effects?

Jakob E.

 

[abbey road d enfer]

In applications where wire resistance plays a minor role, but you want lower inter-winding capacitance, can you use thinner wire and live with not filling the window without other unwanted effects?

You may want to use the extra space for increasing the wind pitch, which would reduce stray capacitance, like honeycomb windind does.

 

[CJ]

it depends on the application, any daylight you can see in the window is leakage inductance, so a mic input transformer might want the window filled up to make it more efficient,

if it is an inductor for an EQ, you might want to fill the window to improve the Q,

if it is a mega henry plate choke, you might be better off trading some window area for smaller wire to keep C down,

 

[gemini86]

Just thought I would share since we're on the topic of making bobbins. Just had this 3d printed for me, waiting on wire and a winding machine to be built.

 

[abbey road d enfer]

Whatcha gonna do with it?