Since early 2000 I've been bugging Mikey to make a headphone transformer.  Finally he said draw up what you want and I'll wind it. Mikey was probably betting I would never get a "round Tuit" and I'd be quiet about the headphone transformer because it was now my problem to draw one up. I drew one up, it works OK (it was a bit capacitive, and could use another spin to perfect it) and I gave Mike permission to sell this design to others as is or on the next pass.

Items that are on my wish list:

1. I wanted taps for most common headphones but I didn't want to have to add whole bunch of double pole double pole switches to change impedances like I have to with my Sowter transformers. I wanted to use a 2 pole 6 position switch to change output impedance. So I'll need to pick 6 useful taps.

2. I wanted the leakage inductance and insertion loss to be fairly constant as the secondary taps were changed. To do this requires a fairly complex interleave of the secondaries and would drive the cost up, but would be worth it.

3. I also wanted a 40K to 50K reflected primary impedance for the 600 ohm tap. This will allow a reasonable range of impedances for the taps that are slightly mismatched. If a tap isn't perfectly matched, it isn't the end of the world. It will just sound a little different. With 5 to 6 taps, there will be one that is right.

4. If possible, I wanted to be able to put the primaries in parallel or in series so I can play with different tubes, but I didn't want this to screw up the sound.

5. I wanted the primary input capacitance to be fairly low, say around 200 pF. This was to keep the high frequency load line fairly reasonable. 200pF is 40 kohm at 20 kHz. With music (the energy rolls off around 5 kHz), we can actually live with 4 times more capacitance before it is a serious issue.

6. At full output power I wanted a primary inductance around 300 Henry (about 40 kohm at 20 Hz). This will keep the load line nice at low frequencies.

7. At very low output power, I want the primary inductance to be greater than 160 H for a small signal -3dB response of 10 Hz.

8. I wanted it to cost the same or less than the SOWTERs if possible. (This is not under my control.)

9. I wanted at least low nickel laminations for a nice low frequency load line and for low core loss in the midrange. (This I can control.)

A. To save money, I was planning to put the transformer under the chassis so I could live with out the nice looking end bells and painting. A standard brass channel would be just fine.

B. I didn't want really fine wire used for the windings. This will drive the size up slightly, but make a part that isn't delicate.

C. I prefer terminals to leads. Terminals will survive more abuse than leads. Since the transformer is under the deck, terminals won't be a shock hazard.

When you look at a list of the headphone impedances out there, we see there really isn't a standard impedance out there. 600 ohms was the highest impedance I found and the impedances drop all the way down to 8 ohms. The low impedance headphones I found weren't high end headphones. I doubt someone is going to spend several hundred bucks to build a tube headphone amp and then use it to power cheap headphones.

2:1 and 1.5:1 ratios between taps make for good manufacturability. 2:1 taps have 4:1 (or 1:0.25) impedance ratios and 1.5:1 voltage taps have 2.25:1 (or 1:0.444) impedance ratios.

600 ohms

230, 250, 300 ohms

100, 120, 150 ohms

50, 55, 56, 60, 64, 65, 70, 75 ohms

24, 28, 30, 32, 38, 40, 44 ohms

12, 16, 22 ohms
 

One easy way to deal with the low efficiency headphones is to just let the 20 Hz bass distort. With a tube amp, this some times makes the bass sound louder for we'll hear the upper harmonics of the note being played, not the note itself.

Another way is to roll the low frequencies off. If you let the 20 Hz be 3 dB down, that is 3 dB more power you can put out. If you let the 20 Hz be 6 dB down (-3 dB at 40 Hz), that is 6 dB more power.

The AKG K1000 is a 120 ohm 74 dB/mW headphone that requires 398 mW (7V rms) to get 100 dB out.

The Sony MDR-F1 is a 12 ohm 100 dB/mW headphone. This headphone only requires 1 mW for 100 dB out, but doesn't have a perfect impedance match. At 100 dB/mW, an impedance mismatch won't be very noticeable for it only takes 0.11V or 9.1 mA into the headphone for full volume.

 
What was the actual winding method?

After VSAC-2003 I updated my tube impedance table to include a 16 ohm headphone run with a 36:1 transformer.  I assumed 32 Hz would be the lowest full power frequency normally used and the primary inductance of the transformer would be 200 H. I used this loading to generate a new table of optimum bias points. From the looks of this table, I should consider increasing the bias current in the Version 5 headphone amp from 2.4 mA to around 4.0 mA.

The hardest three headphones in each category are highlighted in bold. This table assumes a Sowter 8665X (36:1, 18:1, 12:1) output transformer is used. The tube gain calculations do not include a factor of about 1.5 lost in the crossfeed circuit. So if the needed tube gain is 20, it turns into a needed gain of 30 because of the crossfeed. The AKG K1000 was designed to be driven by a power amp, so we can throw it out of the requirements table. If left in, it will only make it to a loudness of about 91 dB with the Version 5 Headphone amp. Once the K1000 is eliminated, we see that only 16 mW is needed to generate 100 dB in the headphones.
 

HeadPhone data mostly from 1993 October Audio	Ohm	dB/ 1mW	mW 100 dB	mV for 100 dB	mA 100 dB	Est Pri ratio	Est Pri Z	Est Pri Volts	Est Pri mA	Tube Gain re 0.7V	dB at 2.4 mA / 30V rms Pri
Aiwa HP-J9	16	105	0.3	71	4.45	36	20736	2.56	0.12	3.7	121
Aiwa HP-X1000	40	104	0.4	126	3.15	36	51840	4.54	0.09	6.5	116
AKG K1000	120	74	398.1	6912	57.60	12	17280	82.94	4.80	118.5	91
AKG K240M	600	88	15.8	3084	5.14	12	86400	37.00	0.43	52.9	98
AKG K271 Studio/ K240S	55	91	7.9	661	12.02	36	71280	23.79	0.33	34.0	102
AKG K280	75	92	6.3	688	9.17	18	24300	12.38	0.51	17.7	108
Audio-Technica ATH-M40fs	60	100	1.0	245	4.08	36	77760	8.82	0.11	12.6	111
Beyerdynamic DT 770 Pro DT 990 Pro	250	96	2.5	792	3.17	18	81000	14.26	0.18	20.4	106
Beyerdynamic DT211	40	118	0.0	25	0.63	36	51840	0.91	0.02	1.3	130
Beyerdynamic DT911	250	115	0.0	89	0.36	12	36000	1.07	0.03	1.5	129
Denon AH-D950	30	106	0.3	87	2.89	36	38880	3.13	0.08	4.5	120
Denon AH-D950	30	106	0.3	87	2.89	36	38880	3.13	0.08	4.5	120
Etymotic ER-4S	100	98	1.6	398	3.98	18	32400	7.17	0.22	10.2	112
Grado HP1	40	96	2.5	317	7.92	36	51840	11.41	0.22	16.3	108
Grado SR325	40	96	2.5	317	7.92	36	51840	11.41	0.22	16.3	108
Grado SR80	32	94	4.0	357	11.15	36	41472	12.85	0.31	18.4	107
Koss PRO/4AA	230	94	4.0	957	4.16	12	33120	11.48	0.35	16.4	108
Sennheiser HD-280 Pro	64	95	3.2	455	7.11	18	20736	8.19	0.40	11.7	111
Sennheiser HD490II	70	94	4.0	528	7.54	18	22680	9.50	0.42	13.6	110
Sennheiser HD540II	300	94	4.0	1093	3.64	12	43200	13.11	0.30	18.7	107
Sennheiser HD-600/ 580	300	97	2.0	774	2.58	12	43200	9.28	0.21	13.3	110
Sennheiser HD-650	300	98	1.7	708	2.36	12	43200	8.49	0.20	12.1	111
Sony MDR-CD1000	32	104	0.4	113	3.53	36	41472	4.06	0.10	5.8	117
Stanton SRS-275	100	101	0.8	282	2.82	18	32400	5.07	0.16	7.2	115

Last Updated on 10/11/03
By VoltSecond

A brief overview of the data:

Transformer impedances:
The 300 ohm Sennheiser headphones need about 3 times the voltage as the 40 ohm Grado headphones do to reach 100 dB. With three windings on a transformer secondary (Like the 8665X), all three could be in parallel for the Grados and all three in series for the Sennheiser. A 4 pole double throw switch could handle this or use two DPDT switches. With two DPDT switches we also have the option of two windings in parallel with one in series with the paralleled pair for 150 ohm headphones.

Another way to accomplish this impedance change is to wind the transformer with taps on the secondary. Properly wound, the unused taps don't hurt the sound. Poorly wound, the taps drastically increase the leakage inductance and kill the highs.

The AKG K240M is a potential problem child to drive with the Sowter and a 12AU7. A 10:1 transformer give us a 60K reflected primary impedance. A 5:1 transformer would give us a 15K primary. If you want to drive these real loud (>98 dB), look around for a 5:1 (15K : 600 ohm) nickel transformer. Pico or Magnequest may have something, or they may not. If 98 dB is loud enough, the Sowter will work.

I would like to see above a 16K primary impedance because I want to drive the headphone transformer with small signal tubes, not power tubes. Even with a 16K primary, we can still get reasonable performance with a 12AU7.

Again, remember, there are ways to wind transformers so that the taps don't add extra leakage inductance when they are not loaded. You just have to be smart enough to wind them that way. So if someone says taps make a transformer sound bad, it may be true for some transformers, but not for all transformers.

Interleaving the windings on a transformer to get lower leakage inductance can cause issues with voltage ripple and phase shift in the audio region (< 20kHz )   If I remember right, The Ian G. Masters article "The Audibility of Distortion" Stereo Review Jan 1989 showed moderate phase shifts just aren't that audible.