Ring Stinger transformer substitution?

[aion]

I'm working on a Ring Stinger, which uses the LT44 transformer from Eagle in the UK. This is a 20k:2kCT transformer. However, they're getting a little harder to come by than they were a few years ago, and there's no cost-effective way to get them in the USA.

They're the exact same form factor as the Xicon 42TM series, but there's no direct Xicon equivalent. The closest are the 42TM002 (10k:2k), 42TM006 (20k:1k), and 42TM019 (10k:600).

Here's the schematic for reference:
https://aionfx.com/app/files/schematics/StringRingerv12r.pdf

(It's a little hard to follow due to the net labels, but both connections on the left-side transformer come from the DPDT switch immediately above it.)

Most other ring mod circuits do not seem to be picky about the transformer used. I've seen a lot of them that call for 10k:10k or 3k:3k. I suspect the Ring Stinger was designed with the LT44 just because it was what was available in the neighborhood electronics shop while the circuit was being designed - the LT44 being a retail hobbyist model like we may have found at Radio Shack in decades past.

So, my question: is the Ring Stinger circuit picky about the transformer specs, and will one of the Xicon alternatives work the same? Would any other circuit changes be necessary, like scaling the 100k trimmer or the 100k resistor (R60)?

[Govmnt_Lacky]

What do you consider "cost-effective way to get them in the USA?"

I can get a pair of LT44s with some diodes for about $20 shipped from the UK to US. Let me know if you are interested and I will pass along the place where I have gotten them several times in the past. 

[aion]

If it's the eBay listing, then yeah, that's where I got mine too. It's doable, but $10 each and 3-4 weeks untracked shipping, vs. $2 each and having them in 2 days... it's worth exploring!

[iainpunk]

i really don't think the transformers are too critical, i'd go with the 20k:2k one, seems closest-ish

just order some different ones and try them out.

cheers

[PRR]

It needs a CT.

Try the 42TM002.

[iainpunk]

It needs a CT.

Try the 42TM002.

couldn't the center tap be faked with two equal capacitors?

cheers

[aion]

Aren't all the 42TM series centertapped except the 113?

I figured the datasheet specs are given between outer taps, and the center tap is assumed to be halfway between, but maybe I'm not looking at it right.

[antonis]

couldn't the center tap be faked with two equal capacitors?

Shouldn't double the voltage needed..??

[Govmnt_Lacky]

Aren't all the 42TM series centertapped except the 113?

I figured the datasheet specs are given between outer taps, and the center tap is assumed to be halfway between, but maybe I'm not looking at it right.

The generic Xicon datasheet is vague. It shows 2 different schematics in which the only difference is that Schematic A shows a center tap on the secondary and Schematic B does not. Nowhere on the datasheet does it specify which schematic refers to which transformer.

[aion]

The generic Xicon datasheet is vague. It shows 2 different schematics in which the only difference is that Schematic A shows a center tap on the secondary and Schematic B does not. Nowhere on the datasheet does it specify which schematic refers to which transformer.

It's done poorly and easy to miss, but the table says "Schematic A" above all of the models except for the 42TM113 which says "Schematic B".

[Govmnt_Lacky]

It's done poorly and easy to miss, but the table says "Schematic A" above all of the models except for the 42TM113 which says "Schematic B".

Doh! Yep.. there it is. It appears that you are correct 

[anotherjim]

Not familiar with the pedal, but the LT44 is 10k:2KCT as is the alternative 42TM002 Paul mentioned but it has a CT on the 10k side too (10KCT/2KCT) while the LT44 does not.

[iainpunk]

couldn't the center tap be faked with two equal capacitors?

Shouldn't double the voltage needed..??

no, the voltage is the same because we use the same ratio transformer, only difference is that there's a capacitive center tap instead of an inductive one.

but reading the posts above, it doesnt really matter here

cheers

[aion]

Not familiar with the pedal, but the LT44 is 10k:2KCT as is the alternative 42TM002 Paul mentioned but it has a CT on the 10k side too (10KCT/2KCT) while the LT44 does not.

I've seen inconsistent specs on the LT44 (10k or 20k primary, 1k or 2k secondary), but the consensus seems to be that it's 20k:2k. No official datasheet, but here's the manufacturer page:

https://www.electrovision.co.uk/public/template-product-details.php?product_id=P631M
(P631M and LT44 are the same thing)

[Rob Strand]

I've seen inconsistent specs on the LT44 (10k or 20k primary, 1k or 2k secondary), but the consensus seems to be that it's 20k:2k. No official datasheet, but here's the manufacturer page:

https://www.electrovision.co.uk/public/template-product-details.php?product_id=P631M
(P631M and LT44 are the same thing)

At the end of the day it's all very confusing.   Someone needs to do some measurements.

From what I can see there's two versions of the LT44: some have a primary winding DC resistance around 1k to 1k2 and others have a primary winding DC resistance around 600 ohms.   For the secondary I've seen DC resistances between 60 ohms and 130 ohms, that's measured to the outside of the secondary.   The turns ratio of impedance ratio is the most unclear aspect.

I have seen an Eagle spec sheet which had one of the LT44's specified as 20k primary and 1k ct secondary.   But that might not cover the LT44 variants.  When I read 1k ct I read that as 1k ohm to the outside windings.  Not 1k + 1k which I think gets converted back to 2k ct on some sites   



The DC resistance is a good way to estimate the transformer impedance.  The impedance is typically in the order of 10 to 20 times the DC resistance.   You can check this by looking at the Xircon  42TM transformer data.   However be warned
- there's bugs in the datasheet
- you might see the primary closer to 20*Rprimary  and the secondary closer to 10*Rsecondary but the 20* and 10* might flip.
  The difference comes about because one winding wound closer to the core so the length of the turns are shorter.

Anyway, the problem here is we can only estimate to within a factor of 2.    While a 20k primary is definitely on the list,  I can't yet discard the possibility of a 10k primary.

I don't think the 10k vs 20k is a big issue.   The main issue is the turns ratio or impedance ratio.   If we have 20k to 1k then a 10k to 500 ohm will have a similar turns ratio.   What changes is the inductance.   And whether that has a big impact on the frequency response depends on the circuit.


Here's the 10k version:

https://www.velleman.eu/products/view/?country=nl&lang=en&id=11604

This one looks yellow so maybe the 20k one is the blue one.


Reply # 9 in this thread confirms blue and yellow differences,
https://www.eevblog.com/forum/projects/missing-info-on-schematic-could-you-wing-it/

"Their primary resistance is only about 1K for the 'blue' variety and 500R for the 'yellow' variety, so a pair of LT44 blue,"


If we believe the two sources of data there is an uncertainty of a factor of two in the impedance ratio.  In fact the output impedance to the outside is 1k in both cases so the uncertainty is the primary impedance.   (The different secondary DC resistances isn't quite explained though.)

[anotherjim]

I have an original package label for the Eagle LT44 with its diagram & data printed on and that didn't quite match the transformer IIRC. I'll have a look later.

[Rob Strand]

I have an original package label for the Eagle LT44 with its diagram & data printed on and that didn't quite match the transformer IIRC. I'll have a look later.

The tape color might be useful.

[Govmnt_Lacky]

I think I have a Blue LT44 kicking around the parts bin. Let me know if anyone wants some resistance measurements 

[digi2t]

Regarding the blue or yellow xfos, here is the info supplied by the UK seller;

LT44 - Yellow

The primary side has a nominal impedance of 20K Ohms. The secondary side is nominally 2k Ohms and has a centre tap. The D.C. resistance of the primary is approximately 480 Ohms, and that of the secondary is 107 Ohms. The theoretical voltage and turns ratio is 3.2 and something close to this is seen when testing at 1kHz with the appropriate input and output impedances connected.  These have input and output impedances that are more useful for making ring modulators than the LT700s.


LT44 / P631M Blue

This is the same as the one with the yellow tape but has the more usual blue winding tape and the untapped primary side has a higher d.c. resistance at about 1.08K.  If you're looking for low internal resistances to minimise losses, you would prefer the yellow type.

These were originally used to match germanium transistor stages in radios, a role that no longer really exists but these are still useful to match crystal radio detectors to higher impedance magnetic or crystal earpieces.  This allows the detector diode to work at a higher impedance section of the circuit where the higher voltages overcome the forward diode drop more easily leading to higher efficiency.  The transformer also provides a d.c. path to ground when using a crystal earpiece, doing away with the need for a lossy bleed resistor across the crystal element.

Maybe a 42TM019 might be closest resistance and turnswise?

[aion]

The LT44s came in the mail today. I got both blue and yellow so we can lay the spec discrepancies to rest (although the original Ring Stinger definitely used the blue type). But... my Atlas LCR40 isn't detecting any of the pin combinations as a resistor or inductor, instead detecting it as a capacitor with values 0.5pF - 1pF. The plain old multimeter ohm test isn't detecting anything either, and since I'm surprised by this, clearly I need to learn a thing or two about how transformers work.

Whatever the case, I've got an LCR45 on the way which should be able to handle it. Will post results early next week if nobody else beats me to it.