I've been studying a lot of OTL headphone amp circuits but having a hard time accepting that big output capacitor. So I want to do something using an OPT. So I contacted Edcor about ordering OPTs from their XSE line but with a 300 ohm secondary to match my Sennheiser 650's.
EDCOR - XSE Series Output Transformers
Assuming you had a SE OPT transformer with a 300 ohm secondary to match your favorite headphones.... What amp would you build in front of it? I have some ideas but still deciding what primary impedance to get since Edcor asks that you buy a minimum of five units for a custom winding. Before I drop the dime I figured I'd find out what tubes and circuits people like for a transformer output headphone amp. Then decide which standard primary to get. Basically I'm letting the OPT and your kind suggestion dictate the path to take.
EDCOR - XSE Series Output Transformers
Assuming you had a SE OPT transformer with a 300 ohm secondary to match your favorite headphones.... What amp would you build in front of it? I have some ideas but still deciding what primary impedance to get since Edcor asks that you buy a minimum of five units for a custom winding. Before I drop the dime I figured I'd find out what tubes and circuits people like for a transformer output headphone amp. Then decide which standard primary to get. Basically I'm letting the OPT and your kind suggestion dictate the path to take.
Headphones aren't designed to be driven by a source impedance which matches their own impedance. Rather they expect a lower impedance - a rule of thumb is the source impedance should be no higher than 1/8th of the headphones' impedance. That translates in your case to a source impedance below 38ohm.
Thanks! I did not know that. Glad I came here, I won't need to order special. Edcor has a 50 ohm option for these transformers already that may be close enough.
I have another question if you don't mind. Edcor rates these transformers at 70 hz for the low end, but at what power level or DC they don't say. For headphones we are talking milliwatts and these are rated 10 watts. Do you think the 70 hz limit would still apply at this level or can I expect better extension given such low operating power and sensitive phones?
Source impedance is determined by the impedance of the output tube circuit. The transformer is only a circuit element that step up the loading of the headphone to allow the tube circuit output impedance to match the headphone's loading. A 300 ohms output impedance does not mean the amp's source impedance is 300 ohms.
The number you should pay attention to is the primary impedance. For example, for a 5K:300R OPT, it means when you put in a 300 ohms headphone load, the tube see 5K. ie. If you want a damping factor of 10 (Rout 10X less than Rload), you should use a amp design which yield 500R output impedance. I would stick with a 300R OPT if you plan to use 300R headphone.
The number you should pay attention to is the primary impedance. For example, for a 5K:300R OPT, it means when you put in a 300 ohms headphone load, the tube see 5K. ie. If you want a damping factor of 10 (Rout 10X less than Rload), you should use a amp design which yield 500R output impedance. I would stick with a 300R OPT if you plan to use 300R headphone.
The Sennheiser headphones don't seem to follow this rule. I have run my HD600s in some pretty stupid configurations in a pinch and they have never had any real issues.
There are some fairly high source impedance amplifiers on the market that the HD6XX headphones seem to do really well with.
What @Allensoncanon said about primary impedance is correct - if you select a 50ohm output impedance and load that with 300ohm then the primary impedance scales up by the same factor (X6). So for example the Edcor 50ohm output version only comes as one type - with an 8k primary. This becomes 48k when you load the output only with 300ohm.
It depends how the LF extension is figured out. Could be by primary inductance or it could be by core flux (I'm sure they're related via the core material characteristics but haven't figured out the math myself). The shunt inductance of the primary limits the LF extension irrespective of signal level. The LF -3dB point is determined by the amp output impedance in parallel with the reflected load impedance - lower impedance gives more extension. Hence with your 300ohm load you'll get a poorer -3dB point than with 50ohm. The flux limit though is only a large signal phenomenon - meaning at lower voltage swings (as for sure headphones need fewer volts than speakers) you'll get more LF. (Perhaps now you're even more confused than when you asked!)
I have DT880s with 600ohm and they're also fairly insensitive to source impedance - the load impedance is relatively independent of frequency.
Speaker amps with headphone output sockets often have just a series resistor of a few hundred ohms to feed them. This solution gives broad compatibility with a range of headphone impedances. Given the choice though I'd still recommend a lower impedance source. The 8X factor is just a heuristic, not a hard and fast rule.
You could either run a standard OT in a 10R resistor (or whatever value to obtain optimal primary impedance to match your final tube for 1 watt* at low distortion) and connect the cans in parallel to this resistor. The normal rules apply w.r.t. damping factor: either penthode with gobs of FB to force LF up to par. Or, more appropriate, a triode with inherent low rP. Primary self inductance is the key for undistorted LF reproduction if you want to go without FB.
Then there is the push pull option with low wattage tubes, either penthode or triode. How large do you allow this amplifier to be?
Another option would be a 300R custom job for the OP winder but what do need to ask for? First consider a final tube and part from there.
Finally there's the OT-less stage which can be very good and can have a small footprint.
* 1 watt in 10 ohm, parallel to a 300 ohm headphone leaves the latter with circa 33mW, what should be enough for headache
Then there is the push pull option with low wattage tubes, either penthode or triode. How large do you allow this amplifier to be?
Another option would be a 300R custom job for the OP winder but what do need to ask for? First consider a final tube and part from there.
Finally there's the OT-less stage which can be very good and can have a small footprint.
* 1 watt in 10 ohm, parallel to a 300 ohm headphone leaves the latter with circa 33mW, what should be enough for headache
Attachments
That would be #45, probably driven with D3A (perhaps finished with Rod's Shunt Cascode board). I'd go for good quality standard iron (5K se) and use a 10R resistor.
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Assuming you run some "real" tube; '50, 6V6(tri); not some preamp or tuner tube---
K.I.S.S. Get stock 5k:16. No min order, no setup. 17:1 ratio with 300V on the tube will put 8V to the phones which is LOUD.
This works out because we can use a 2W-5W tube to drive 1/10th Watt headphones, so we do not need "optimum power transfer". The small power bottles are popular so (most) do not cost more than sturdy small bottles; the 5k:speaker ratio is STOCK which avoids custom costs.
+1.
8V peak - according to Ken Rockwell's review - gets you to 118dB SPL. Definitely plenty loud.
Sennheiser HD 650 review
I have a custom 5k:32 ohm output transformer that sounds amazing with all my headphones. Remember that many headphones are only 32 ohms.
However this is not neccessary, 5k: 8 ohm works just fine with any headphone. It's essentially just a speaker amp. As long as the power supply is quiet enough, it's already optimal.
However this is not neccessary, 5k: 8 ohm works just fine with any headphone. It's essentially just a speaker amp. As long as the power supply is quiet enough, it's already optimal.
PRR helped me tremendously on this same topic a few months ago!
I built a Tubelab SE - II with 45 output tubes.
I used 5K:32 opt's that I bought from ElectraPrint.
Sounds amazing with HD650 and ZMF Aeolus. Need more time to decide how they match with HD800S.
Anyhow, I ran them without any load on the output, so the 300R headphone presented close to 40K to the tube and I still got over 250mW before clipping, which is insanely high as it seems like only about 1mW is needed for regular listening.
I also experimented with ~70R resistors on the output which brings the reflected load to about 10K (closer to "ideal"). In this case, I still get over 250mW max power, but with slightly higher THD.
I built a Tubelab SE - II with 45 output tubes.
I used 5K:32 opt's that I bought from ElectraPrint.
Sounds amazing with HD650 and ZMF Aeolus. Need more time to decide how they match with HD800S.
Anyhow, I ran them without any load on the output, so the 300R headphone presented close to 40K to the tube and I still got over 250mW before clipping, which is insanely high as it seems like only about 1mW is needed for regular listening.
I also experimented with ~70R resistors on the output which brings the reflected load to about 10K (closer to "ideal"). In this case, I still get over 250mW max power, but with slightly higher THD.
Brilliant review link! (I have a pair of HD650 and it jives with my personal experience of them)
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Thanks for suggestions so far I have more options than I thought. And learning that a custom winding really isn't needed, much appreciated. Same for the load resistor. Will try out some real power tubes with good iron and see if the dummy load resistor is even needed by listening.
I really like my Sennheiser 650's I could go on and on with audiophile-speak but I find most of that to just be "creative writing" and I'm usually exhausted and no better informed after reading magazine reviews.
My wife and I hear a lot of jazz groups (at least 2 per week) all with real acoustic instruments (trumpets, saxes, stand up bases, bones, etc). In ballrooms as we are avid dancers and travel far and wide just to dance. Typically bands with no PA with only the guitar maybe using a Fender Bassman or Twin clean. Big band, small swing combos, gypsy jazz, 20's, 30's, 40's... to present styles, no ear-splitting rock or blues with electronic distortion. Listening to an intentionally-distorted musical source like Jimi Hendrix has never helped me judge a speaker or phones, personally I need a real acoustical source, a reference I'm familiar with.
So my wife and I judge what we hear at home with what we hear for real. And I can say I'm happy with these phones, they don't add or subtract is all I'll say. I think the fact that they are 300 ohms gives me even more options transformer-wise, resistor-loading-wise, and room to experiment.
I dont have any 45's in my stash at this time but I think I'll have to try that too.
I really like my Sennheiser 650's I could go on and on with audiophile-speak but I find most of that to just be "creative writing" and I'm usually exhausted and no better informed after reading magazine reviews.
My wife and I hear a lot of jazz groups (at least 2 per week) all with real acoustic instruments (trumpets, saxes, stand up bases, bones, etc). In ballrooms as we are avid dancers and travel far and wide just to dance. Typically bands with no PA with only the guitar maybe using a Fender Bassman or Twin clean. Big band, small swing combos, gypsy jazz, 20's, 30's, 40's... to present styles, no ear-splitting rock or blues with electronic distortion. Listening to an intentionally-distorted musical source like Jimi Hendrix has never helped me judge a speaker or phones, personally I need a real acoustical source, a reference I'm familiar with.
So my wife and I judge what we hear at home with what we hear for real. And I can say I'm happy with these phones, they don't add or subtract is all I'll say. I think the fact that they are 300 ohms gives me even more options transformer-wise, resistor-loading-wise, and room to experiment.
I dont have any 45's in my stash at this time but I think I'll have to try that too.
Does this apply to parafeed as well? Running simulations a 25K load for a 40Ω headphone(5K:8) has a peak in the low frequency, 300Ω is +16dB at 10hz.
Now no headphone will be able to actually run 10hz and 30hz is already normal but wondering if there's adverse affects in this case where an airgap OPT will be fine and preferable for headphone use.
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