An open letter to transformer manufacturers…
Dear transformer manufacturers,
Here’s a quick list of existing products that I have found with comments on their suitability for the DIY market. This is followed by blue-sky ideal output transformer specs.
Edcor PCW10k-7k/300-32: this is a nice low cost but suffers the same issue as all of Edcor, namely lead time and lack of availability overseas. The turns ratios and secondary taps are just about ideal for headphone applications (which I believe it was originally intended for). The board mount style, on the other hand, is less than ideal for most hobbyists. With a rating of 0.5W, 15Vrms on the primary, and no published inductance figures, it also hasn’t really seen much interest from a lot of the community. That said, it’s the only non-custom North American option I know of. This is non-gapped single-ended so it’s limited to parafeed applications. Honorable mention to Edcor’s WSM and XSM series, though.
Sowter options: by all accounts these are very nice non-custom options, but they are difficult to get in North America and the price is very high. The way the secondaries are wired also makes switching or wiring multiple jacks difficult. Because most headphone enthusiasts have multiple pairs at different impedance, using an output transformer with an easily switchable secondary tap is preferred to ensure consistent load reflected to the output tube. The Sowters don’t make this easy, unfortunately. Most of the Sowters are gapped for normal series-feed single-ended power. End-bells are nice.
Lundahl LL2765: this is a fancy c-core option with a secondary arrangement similar to the Sowters. The same caveats about easily switching the impedance configuration apply. The turns ratios and impedance options are close to ideal for the headphone market. I haven’t been able to find a price (Lundahl is distributed by K&K Audio in the USA, though LL2765 isn’t on their price list). I expect it’s in the $200+ each range. No end bells, so no fun in showing off the iron. It’s available with a gap (series-feed) and without a gap (for parafeed or push pull operation).
As a DIY headphone amplifier designer and enthusiast, what I’d love to see is an output transformer that is readily available in North America and Europe at an affordable price.
Headphones broadly fall into impedance categories of 200-300 and 32-75 ohms (with most recent products trending towards the lower impedance). I think secondary taps of 200 and 50 ohms would make sense as these would be a ratio of 4:1 for full winding and center tap, easily selectable with a switch. The center tap secondary, rather than asymmetrical taps, might make this transformer useful for other balanced applications as well (and if so, might as well put a center tap on the primary, too).
A primary impedance of around 8k would work well for a good range of tubes. If rated for 8k:200, we’d have a turns ratio of about 6.3:1. With 300 ohm headphones it would provide a 12k impedance and with 150 ohm headphones it would provide a 6k impedance. If using the 8k:50 center tap, it would provide a turns ratio of 12.6:1. That’s about 5k with 32 ohm headphones or 12k with 75 ohm headphones. All these numbers would work fairly well with small popular single-ended tubes (EL84, 6V6, 6S45Pi, etc).
The transformer would ideally be rated for 2-3W. Headphone users are as silly about their power as speaker users are. Never mind that they only need a few mW to reach 100db. Frequency should be rated 20-20k +/- a db or two or the transformers would be ignored by spec snobs. More similarities to the rest of the high end audio market there.
Gapped and rated up to 50mA would be excellent, but not if it compromises the frequency bandwidth through low inductance or the cost by making it huge. A non-gapped transformer for single-ended headphone amps works a little better than for speaker amps because we can use it as parafeed, keep the B+ voltage reasonable, and still make plenty of power. Non-gapped and an exotic metal core would be even better, but I don’t see it much in most transformer catalogs.
So, TL;DR, what I think would be ideal is:
– 8k:200ct impedance
– 2W+ rated output
– 20-20k rated bandwidth
– $100 per pair (less is better)
– option for end-bells
– gapped (~50mA if possible) or un-gapped
Turning that into a small product line, I could see it working out as something like:
– 2W gapped, no-end-bell
– 2W gapped, end-bell
– 4W un-gapped, no end-bell
– 4W un-gapped, end-bell
I’d imagine the cores for the above would be around similar sizes, so materials and winding are more or less consistent, but the more in depth transformer permeability math isn’t something I regularly do. The only differences would then be in adding a gap and/or adding end-bells, making a pretty efficient line in terms of manufacturing and materials.
As you probably know, the headphone market is growing pretty quickly. Perhaps not growing as fast as the turntable market, but the overall size is pretty huge as of now and is still on the uptick. I think the high-end headphone products category will continue to grow. Most headphone companies are trending upwards in price points and releasing more designs that are meant for home-use (and maybe that’s partly driven by the growing turntable market, too). I don’t think I’m the only one waiting for a good practical headphone output transformer from an established and well-distributed manufacturer.
So who will step up to the plate?