Hi guys!
For almost a year, I was considering building a tube headphone amp. I still don't know, what design to go with. OTL or with OT. What are your thoughts on OTL? Are they really better? And do they also sound good with low impedance headphones? And if should I go with OT, what OT should I choose? I know that for headphones, I need amp that has at leats 8 times lower output impedance than headphones has. So should I use OT with 32 ohms secondary or 4 ohms secondary? Thanks!
For almost a year, I was considering building a tube headphone amp. I still don't know, what design to go with. OTL or with OT. What are your thoughts on OTL? Are they really better? And do they also sound good with low impedance headphones? And if should I go with OT, what OT should I choose? I know that for headphones, I need amp that has at leats 8 times lower output impedance than headphones has. So should I use OT with 32 ohms secondary or 4 ohms secondary? Thanks!
Actually you can use a small toroidal mains transformer. I used a multicomp 1.6VA 120-0-120V, 0-12, 0-12 with the secondary in parallel. There's loads on DIY audio. If you want to drive 32R OT is better, however there's others.
A lot to unpack here.
- OTL only makes sense if you're using headphones that are fairly easy to drive (your typical ones: sennheiser hd650, various grado and the like) and really want to avoid the cost of transformers. It's difficult to get a really low output impedance output from tubes so you have to either parallel a bunch and/or use quite a bit of feedback. The requirement for a low output impedance can be reduced by using headphones with a fairly flat impedance vs frequency curve. Have a look here to get an idea of a fairly simple design: https://headwizememorial.wordpress.com/2018/03/20/the-morgan-jones-mini-tube-headphone-amplifier/
- An output transformer makes a lot of sense and allows tubes to operate in more optimal conditions. Something to clear up first: the quoted impedance for a secondary is not the same as the output impedance. It is the impedance that corresponds to the rated impedance of the primary of the transformer. For example, if we take a 10K-8R transformer, a 8R load will be reflected to the tube(s) as 10K. So if the design calls for a 10K transformer with a 32R secondary, you have to use headphones around 32R. That's why headphones amplifiers usually have output transformers with multiple secondary taps. But don't worry, the output impedance will always be lower, see this link if you want a starter on why: https://www.audiopax.com/e/pdfs/artigos/SEAmplifierOutputImpedance.pdf
- The bad news: good headphones transformers made for typical tubes amplifiers topologies are rare and expensive. Small market, and so on. But you can find stuff from sowter, electra-print.
- The good news: the two factors that significantly impact the cost of headphones transformers can be played around. The first problem is that most tube based amplifiers require some DC inbalance through the transformer. If you're willing to go parafeed, the transformers can be made way cheaper. Edcor has a nice little transformer with a 7K or 10K primary and 32 and 300R secondary taps. Works a treat in a topology similar to the one I used here: https://www.diyaudio.com/community/...r-parafeed-headphones-amp.251505/#post4037465
- You can go one further. Transformers also become less critical if you drive them from a lower impedance. The mapletree ear+ uses cheap hammond transformers at the output of a cathode follower. Which is an excellent way to go, not often seen. They kindly provide all the info here: https://www.mapletreeaudio.com/manuals/HDSuper2usersmanual.pdf
- OTL only makes sense if you're using headphones that are fairly easy to drive (your typical ones: sennheiser hd650, various grado and the like) and really want to avoid the cost of transformers. It's difficult to get a really low output impedance output from tubes so you have to either parallel a bunch and/or use quite a bit of feedback. The requirement for a low output impedance can be reduced by using headphones with a fairly flat impedance vs frequency curve. Have a look here to get an idea of a fairly simple design: https://headwizememorial.wordpress.com/2018/03/20/the-morgan-jones-mini-tube-headphone-amplifier/
- An output transformer makes a lot of sense and allows tubes to operate in more optimal conditions. Something to clear up first: the quoted impedance for a secondary is not the same as the output impedance. It is the impedance that corresponds to the rated impedance of the primary of the transformer. For example, if we take a 10K-8R transformer, a 8R load will be reflected to the tube(s) as 10K. So if the design calls for a 10K transformer with a 32R secondary, you have to use headphones around 32R. That's why headphones amplifiers usually have output transformers with multiple secondary taps. But don't worry, the output impedance will always be lower, see this link if you want a starter on why: https://www.audiopax.com/e/pdfs/artigos/SEAmplifierOutputImpedance.pdf
- The bad news: good headphones transformers made for typical tubes amplifiers topologies are rare and expensive. Small market, and so on. But you can find stuff from sowter, electra-print.
- The good news: the two factors that significantly impact the cost of headphones transformers can be played around. The first problem is that most tube based amplifiers require some DC inbalance through the transformer. If you're willing to go parafeed, the transformers can be made way cheaper. Edcor has a nice little transformer with a 7K or 10K primary and 32 and 300R secondary taps. Works a treat in a topology similar to the one I used here: https://www.diyaudio.com/community/...r-parafeed-headphones-amp.251505/#post4037465
- You can go one further. Transformers also become less critical if you drive them from a lower impedance. The mapletree ear+ uses cheap hammond transformers at the output of a cathode follower. Which is an excellent way to go, not often seen. They kindly provide all the info here: https://www.mapletreeaudio.com/manuals/HDSuper2usersmanual.pdf
Have you seen Pete Millett's ECC99 based headphones amp?
http://www.pmillett.com/ecc99_srpp_headphone_amp.htm
Cheers
Ian
http://www.pmillett.com/ecc99_srpp_headphone_amp.htm
Cheers
Ian
Well thank you all for your response. I think that I will use parafeed design because I got some 6l6 laying on my bench so they will be finally usefull. I'm still not exactly sure with usingn some hammond OT with 8 ohm output impedance if it will works well with 32 ohm headphones?
I think the bigest factor favoring output transformers with tubes is safety.Even the recording industry at its top makes use of coupling transformers happily trading distortions that you can't hear for the ones you like along with safety.
Parafeed, while allowing for some of the inductive magic, doesn't offer safety.Capacitors in high voltage circuits can fail and so will the headphones pair along with them.
Parafeed, while allowing for some of the inductive magic, doesn't offer safety.Capacitors in high voltage circuits can fail and so will the headphones pair along with them.
Hello Tom,
regards your OT question.
I use my Decware SE84b to drive my dynamic cans every night.
Sennheiser HD650, AKG 701 and 702. Have also used Grados and Fostex TH909 with no issues. I built a little Rat Shack box with 16 ga milspec wire from the binding posts to the 1/4 stereo headphone jack. Works like a charm and the sound is absolutely sublime.
Used to put a 12 ohm resistor across the BP to present a lower impedance ( more like a speaker ) to the tube. Forgot to put it in one night ( damn that 2nd martini ! ). Nothing happened, so I haven't put it in since.
When I said it sounds absolutely sublime, I mean it. I have a Stax rig ( Woo Audio WES, Stax SR Omega and SR Omega II ). It sits there, looking pretty, while I always listen to that little 2 watt SE tube amp.
Go for it.
Cheers, Crazy Bill
regards your OT question.
I use my Decware SE84b to drive my dynamic cans every night.
Sennheiser HD650, AKG 701 and 702. Have also used Grados and Fostex TH909 with no issues. I built a little Rat Shack box with 16 ga milspec wire from the binding posts to the 1/4 stereo headphone jack. Works like a charm and the sound is absolutely sublime.
Used to put a 12 ohm resistor across the BP to present a lower impedance ( more like a speaker ) to the tube. Forgot to put it in one night ( damn that 2nd martini ! ). Nothing happened, so I haven't put it in since.
When I said it sounds absolutely sublime, I mean it. I have a Stax rig ( Woo Audio WES, Stax SR Omega and SR Omega II ). It sits there, looking pretty, while I always listen to that little 2 watt SE tube amp.
Go for it.
Cheers, Crazy Bill
I've built a few tube headphone amplifiers, because I mostly use headphones. As Crazy Bill said, if your headphones are low impedence (32 ohm) you can use a small low-power SE amplifier meant for speakers - no load resistor is needed, but I use a 100 ohm one just in case. I liked my Decware clone at first, but then I replaced it with a "spud" amplifier with triode-connected EF184 tubes: 250V anode supply, 660 ohm cathode resistor. 6L6 will work with the exact same values, but lower gain. The output transformers are small 5k/8 ohm 5W made in China, midrange quality, about the same size and type of the Decware ones. The current on the output transformer is less than 20% of the transformer design value, so it behaves a bit like a parafeed. Due to the combined gain/power stage ensured by the EF184 (very linear triode-wired pentode), the distortion is low and the result is better than the Decware to my ears. For my 600 ohms headphones, I use a OTL amplifier based on the Glassware CCDA schematic. Both systems sound good.
Hi and bump! I've been looking at this headphone amp: Nobsound NS-08E Vacuum Tube Headphone Amplifier Hi-Fi Valve Headphone Amp Stereo Audio Preamplifier https://www.amazon.com/dp/B014FASL1A/ref=cm_sw_r_apan_i_BFEZWFTQE2VZWAP1GX3H?_encoding=UTF8&psc=1
And was wondering how it would compare to this one on eBay?
https://www.ebay.com/itm/3139188368...spIgLxWR5a&var=&widget_ver=artemis&media=COPY
It will be used with my Hifiman 400i 2020 headphones and my Cayin N3 or Hiby player. Thanks in advance.
And was wondering how it would compare to this one on eBay?
https://www.ebay.com/itm/3139188368...spIgLxWR5a&var=&widget_ver=artemis&media=COPY
It will be used with my Hifiman 400i 2020 headphones and my Cayin N3 or Hiby player. Thanks in advance.
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