I am looking to build a tube amp that can be used to drive low impedance headphones. I currently have some that are 18 ohms. I have already built a hybrid amp and would like one that is strictly tube driven.
Thanks for the suggestions.
Thanks for the suggestions.
Do you have any other requirements? Is parafeed acceptable? Single-ended? OTL? OTL will probably not hack 18 ohms without heroic efforts, if you're fussy about damping factor.
Direct or indirect heated tubes?
Direct or indirect heated tubes?
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I just traveled this road, I wanted to try and just use parts from my junk box but I lacked the proper tube to get a low Z out without using output iron. So I am left with either going hybrid or wait for Edcor to make me some iron. I breadboarded an amp similar to Pete Millet's Starving Student amp and it sounded real good, I didn't do any measurements yet.
I too want all tube so I am thinking of pulling the trigger on some transformers🙂
I am interested in another Pete's designs, I recommend you take a look yourself: Jonokuchi desktop amp
I too want all tube so I am thinking of pulling the trigger on some transformers🙂
I am interested in another Pete's designs, I recommend you take a look yourself: Jonokuchi desktop amp
Simply make a SE amp with OPT. Most low Z headphone would have impedance of 32 ohms.
Hammond 125 OPT would have all different output tapping for 4, 8, 16, and 32 ohms that matches with speaker and headphone.
Hammond 125 OPT would have all different output tapping for 4, 8, 16, and 32 ohms that matches with speaker and headphone.
What headphones do you have?
I don't think that circuit you posted will be up to the task of driving the 18 ohm load
I don't think that circuit you posted will be up to the task of driving the 18 ohm load
I have Sennheiser momentums. As far as I can tell they sound great with the hybrid tube amp I built that is driven by a 12V battery but I want something more portable.
I see a lot of review state they are very bass heavy but mine have almost no bass. I don't know if that is because the source is an iphone and the amp is a tube hybrid.
Transformer output is your best bet. Impedance matching is only required for maximum power transfer - Since any SE power stage will output a watt or more, delivering all of that to the headphones will result in permanent ear (or headphone) damage. Just terminate the transformer with 4 Ohms and connect your headphones in parallel with that. May still deliver too much power (and S/N will suffer) so you may need to attenuate further.
A 6EM7 should work well for this, gain stage and power stage in one tube. If you'd rather use 9-pin, 6DE7 or 6FD7 is similar. Common, cheap TV tubes designed for linear amplification (vertical output) - distortion would turn the TV picture into a funhouse mirror.
A 6EM7 should work well for this, gain stage and power stage in one tube. If you'd rather use 9-pin, 6DE7 or 6FD7 is similar. Common, cheap TV tubes designed for linear amplification (vertical output) - distortion would turn the TV picture into a funhouse mirror.
What do you really want ? Something portable or something all tube driven ? The latter will not be portable at all...
Broskie's Aikido PP (not the plain variant) could do the trick. Use a high-mu tube at the input and a high current one at the output. Apply tons of NFB to lower Zout. Hum cancellation will give super quiet output, which matters for cans. Only problem is you need huge output caps to drive 18 ohms, probably 1000uF, so big ole lytics or some huge motor run MKPs in parallel..
My tube guru friend suggests using a EL84 with an output tranny to drive low Z phones. Personally I just grabbed a "headphone amp" IC from TI or NS and wired it up and it sounded great to me, but what a pain in the rump to work with SMD parts, which is the only way it's available. Tubes are arguably not portable, perhaps mostly because of how fast they will eat a battery.
I built the "optimized version with feedback" using Sovtek 6922 tubes and a fairly simple CRC power supply (Capacitor-Resistor-Capacitor) with 100uF of filtering total, and using DC on the filaments. Hum is virtually non-exisitant with my 106db/mw 63 ohm Sonys. It sounds wonderful when fed with a decent DAC and isn't hard or expensive to build. IIRC, the article says it can be used with headphones as low as 16 ohm... (The only change to the schematic I did was to add .47uF film caps across the output capacitors which resulted in clearer and extended highs. )
Might just be what you're looking for.
Might just be what you're looking for.
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This type of simple amp is very cheap, very low distortion with no gNFB, and will drive anything from speakers to any headphones.
Tubes can be 6SN7 or variants, 2C51, 6N3P, ECC88 or variants, or a number of other linear triodes. Whatever is cheap / at hand. 6N3P is supremely cheap any as linear as pretty much any indirectly heated double triode. Fits the bill perfectly in my opinion.
OT can be very cheap with good results. C2 is optional. First stage can be biased with a LED for cheapness and simplicity; maybe share one LED for both channels.
Tubes can be 6SN7 or variants, 2C51, 6N3P, ECC88 or variants, or a number of other linear triodes. Whatever is cheap / at hand. 6N3P is supremely cheap any as linear as pretty much any indirectly heated double triode. Fits the bill perfectly in my opinion.
OT can be very cheap with good results. C2 is optional. First stage can be biased with a LED for cheapness and simplicity; maybe share one LED for both channels.
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I'd avoid the OTL with 16ohms.
I'd make something similar to the Bottlehead crack (or buy the kit), which means a 12au7 gain stage, a 6as7 ouput cathode follower. Then a pair of hammond 119DA at the output.
The output transformer will ensure you don't have too much gain and kill noise nicely.
I'd make something similar to the Bottlehead crack (or buy the kit), which means a 12au7 gain stage, a 6as7 ouput cathode follower. Then a pair of hammond 119DA at the output.
The output transformer will ensure you don't have too much gain and kill noise nicely.
I also built this one many moons ago but sold it because I found better... the schematic has been optimized by Morgan Jones for use with headphones down to 32 Ohms, but not lower.
Not recommended as 00940 already said. My version had LED bias for the first stage which sounded a bit better (with high impedance 'phones).
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