Help Needed - Push Pull Transformer replacement

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I am a newbie here and is seeking help on the headphone amp section of my hifi preamp.

Basically, I got hold of a Cary SLP-05 preamp recently which states it has a dedicated tubes driven (6sn7) headphone amp section. However, the sound from this headphone section has severe bass rolloff (no low freq bass at all) and when I opened it up, I see the headphones output is driven by 2 Hammond 125C push-pull OT. Searching Hammond's site said it has a frequency response from 150Hz to 15kHz, and I assume the bass rolloff is due to this.

Question is could this to corrected / replaced by something or a better OT from another brand with a similar size? Any experience anyone? :confused:
Hammond Output Transformer 125CSE

Which taps are used? There are 10K/5K/2.5K primaries and 4/8/16/32 secondaries.

I'll bet you could find a nice upgrade here EDCOR - Single Ended Tube Output Transformers but you have to know what you're looking for. Even the budget series would be a big upgrade and probably perform better than spec into headphones. XSE series are $17 a piece, cheaper than the Hammonds.
I have no personal experience with the Cary, but that won't stop take this with a grain of salt.

The amount of iron matters for bass capability at a reasonable output level, but they probably get away with a small transformer because headpones use so little power that at those level even a small transformer can make some bass, and a lot of headphones can't even handle a lot of bass. But if you're pushing it hard and adding also low bass EQ (which can require hundreds or even thousands of times more power) it's not going to do it. Even at low levels the fletcher/munson "loudness" curves may require more bass boost than it can handle. So the first relevant question is the impedance curve and efficiency of the headphones. Next is the level you are driving them at. Next after that is the EQ, both loudness correction for your ears at low levels and boost for your personal tastes. Do you expect to turn on some "loudness" curve correction? Have you tried optimizing the output taps to match the headphones? Headphone impedances are all over the place. I assume you're not using self-energized electrostatics, which require real speaker-level drive.

Then the bottom line probably ends up what you asked in the first place LOL. Certainly upgrading the output transformer is likely to help bass a lot, and certainly can't hurt. You're sure it's push/pul and not single-ended?
Angela Instruments Online Catalog - Transformers - Hammond Universal SE and PP OT
The 125C is a puny 8 watt transformer. Hammond Output Transformer 125C
It may have decent bass response at .5 watt, as long as you don't turn on the "loudness" or "bass" knobs. The frequency specs on those pages are all wrong. If there is sufficient space you could certainly drop in the 15-watt 125E and just follow the color-coded wires the same. It weighs 3 times as much, which should really improve the bass. Hammond Output Transformer 125E
That's a good place to start anyway. Then if you're still not happy you can check coupling caps etc. and make sure they didn't limit the bass to accomodate the little transformer. That's about $80 for a pair, cheaper than looking for more efficient headphones.

Of course you could upgrade even more to a real hi-fi output transformer.

I like the Edcor, nice littel company, they make each one for you, good prices. Takes a few weeks. Hammond takes them off the shelf and the 125E wires up the same as the 125C, it's just 3 times as big in weight or volume (not 3 times as big in each dimension, which would be 27 times as large and heavy).
The EDCOR prices are better. Their equivalent XPP open-core goes up to a 25-watt. And yes, it would be better to go to a better series like the GXPP or CXPP. mY my look at the nice little EDCOR CXPP 10-watt transformers.

Find out what input impedance your 125C is wired for.
Indeed, it looks like the 125CSE is more for radios and single-ended guitar amps. Instead of just going bigger from the "C" to "E" etc. a real hi-fi single-ended transformer is in a whole different category; I'd expect a real hi-fi transformer to work a lot better, but it will be a LOT bigger. And have a LOT better bass. It will probably be much better in many details of how it's wound, core material, etc. in additon to the bigger core size. Sorry I'm not more help, I thought others would respond too; hope I didn't divert anyone from posting up a reply.
Thanks for the wealth of information. It gave me much needed confidence to take action :)
I opened up up the Cary again, and on the Hammond 125C, the legs 1&6 are connected, however, when referring to Hammond Mfg. - 125 Hook-Up Data I see a table listing a range of ohms. So question is if I want to go for the Edcor XPP transformers, which one should I get? Appreciate any suggestions/ or let me know what I would need to find out as I really want to get this right. Thanks guys


So how did you get on with changing the transformers.
I'm thinking of doing the same with mine to drive my high impedance Beyer DT990's which are either 250 or 600ohms.


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