Hello, so I'm thinking for my second project with tubes, I want to use the pair of EL84s I have to make a low powered single ended amp for my headphones. My headphones are 32 ohm Sennheiser eH 250s, and while they're really old, I really enjoy how they sound. The other reason I want to build a headphone amp is that the transformers aren't that expensive because the wattage is quite low.
For the input stage, I'm thinking a single 6SN7 would probably be fine, as it can provide the gain, and I've heard it's more linear than a 12AU7. I think I could use one tube, with one of the triodes for each channel. For the power stage, I plan to run the EL84s wired as triodes, because I like the sound of triode wiring better than pentode wiring based on some experiments I did, and I don't need much power. I will consider adding a Triode/UL switch though.
If I run at a plate voltage of 250 volts, it seems like a 5k load should be about right. That brings me to my question. What Edcor transformer should I get. With the power from a triode wired EL84, I should be able to easily drive headphones, and also very efficient speakers at reasonable volume. Should I get a GSXE10-5k with an 8 ohm or 16 ohm tap? Or should I get a GSXE10-2.5k or similar with a 16 ohm tap, which should provide a primary impedance of 5k with a 32 ohm load. I hope that wasn't too confusing!
I'll figure out a power transformer later when I get to that stage, as well as any chokes I need. For the rectifier, I'm thinking about using a 5R4, as I have a few that are NOS, but I could also use a 5U4 or 5Y3, but I don't think a 5Y3 can supply enough current for a stereo amplifier. A 5U4 might be overkill, but it would totally work, and I also have a couple of those.
Thanks, and please let me know what you think! I have no idea if this is really the design I should go with, and if it isn't, please let me know!
For the input stage, I'm thinking a single 6SN7 would probably be fine, as it can provide the gain, and I've heard it's more linear than a 12AU7. I think I could use one tube, with one of the triodes for each channel. For the power stage, I plan to run the EL84s wired as triodes, because I like the sound of triode wiring better than pentode wiring based on some experiments I did, and I don't need much power. I will consider adding a Triode/UL switch though.
If I run at a plate voltage of 250 volts, it seems like a 5k load should be about right. That brings me to my question. What Edcor transformer should I get. With the power from a triode wired EL84, I should be able to easily drive headphones, and also very efficient speakers at reasonable volume. Should I get a GSXE10-5k with an 8 ohm or 16 ohm tap? Or should I get a GSXE10-2.5k or similar with a 16 ohm tap, which should provide a primary impedance of 5k with a 32 ohm load. I hope that wasn't too confusing!
I'll figure out a power transformer later when I get to that stage, as well as any chokes I need. For the rectifier, I'm thinking about using a 5R4, as I have a few that are NOS, but I could also use a 5U4 or 5Y3, but I don't think a 5Y3 can supply enough current for a stereo amplifier. A 5U4 might be overkill, but it would totally work, and I also have a couple of those.
Thanks, and please let me know what you think! I have no idea if this is really the design I should go with, and if it isn't, please let me know!
How much power do you need to drive your 'phones to an earsplitting volume? I'm guessing its milliwatts.
If so, common cathode EL84's with an SN7 in front of them are overkill by a factor of a gazillion or so.
If so, common cathode EL84's with an SN7 in front of them are overkill by a factor of a gazillion or so.
The thing is you need just a couple of mW to drive those 112dB/mW headphones to the insane levels.
Triode-wired EL84 itself (without the driver stage) with 5k load will provide about 50mW at 1Vrms and 200mW at 2Vrms input. That's already overkill indeed.
So if you want just the headphone amp, I'd go for just the EL84 with 5k:8 or 5k:16 OPT - you'll get more than reasonable levels even with 10k or 20k load. You probably don't want to use 2.5k:something OPT as those are likely to have lower primary inductance.
And for the "universal" (speakers and headphones) solution I'd go for two-stage amp designed to drive the speakers in the first place with some kind of switchable attenuator at the output for the headphones - like 6.2R in series with the output and then 2.2R parallel to the headphones.
Triode-wired EL84 itself (without the driver stage) with 5k load will provide about 50mW at 1Vrms and 200mW at 2Vrms input. That's already overkill indeed.
So if you want just the headphone amp, I'd go for just the EL84 with 5k:8 or 5k:16 OPT - you'll get more than reasonable levels even with 10k or 20k load. You probably don't want to use 2.5k:something OPT as those are likely to have lower primary inductance.
And for the "universal" (speakers and headphones) solution I'd go for two-stage amp designed to drive the speakers in the first place with some kind of switchable attenuator at the output for the headphones - like 6.2R in series with the output and then 2.2R parallel to the headphones.
Hmm okay, I'll probably go ahead and design for speakers and add a Speaker/Headphone output switch. However, I'm not opposed to getting my hands on some other tubes if that might be better. I just figured that I would like to use them as I breadboarded out a circuit with them that I really liked the sound of. Any suggestions?
Thanks!
Thanks!
Also, TG or anyone who can confirm this, the resistors in the attenuator should be rated at the amplifiers output wattage plus some, correct? So it would be prudent to use 5+ watt resistors?
That is correct.
I personally would be more comfortable with the 10W resistors (especially for that 6.2R which would take most of the stress), but since you are unlikely to play full-power white noise through the amp all of the time, you could probably get away with 5W resistors.
I personally would be more comfortable with the 10W resistors (especially for that 6.2R which would take most of the stress), but since you are unlikely to play full-power white noise through the amp all of the time, you could probably get away with 5W resistors.
What TG said in post #3 can work very well. I was goofing around with a similar idea.
If you take an EL84 and triode-wire it, then put it into an OPT with an 8k:50 impedance ratio (or 5k:32, close enough), you'll lose enough gain from the stepdown that it will come out pretty close to unity gain. I did that with a triode-wired 12GN7A and it turned out really good, even with lowly $20 Edcor OPTs. I used a choke input, standard passive psu. Single supply for the stereo amp, but with separate decoupling for each channel. AC heaters too. Extremely simple and it came out with very low noise levels. I don't have great hearing any more, but I hear no hum at all even with the amp cranked all the way up.
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If you take an EL84 and triode-wire it, then put it into an OPT with an 8k:50 impedance ratio (or 5k:32, close enough), you'll lose enough gain from the stepdown that it will come out pretty close to unity gain. I did that with a triode-wired 12GN7A and it turned out really good, even with lowly $20 Edcor OPTs. I used a choke input, standard passive psu. Single supply for the stereo amp, but with separate decoupling for each channel. AC heaters too. Extremely simple and it came out with very low noise levels. I don't have great hearing any more, but I hear no hum at all even with the amp cranked all the way up.
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Attached is my schematic so far. Please let me know anything I should change! I'll add the power supply tomorrow.
Thanks!
View attachment EL84Headamp.pdf
Thanks!
View attachment EL84Headamp.pdf
1) I'm not sure you need C1 at all (though it might be useful if there's a chance of having DC voltage at the source output)
2) 12.5k plate load seems a bit low for 6SN7 (unless you're trying to lower the total THD by compensating for the output stage nonlinearity or something)
3) If you have 8R speakers, the OPT should be 5k:8. If you have 16R speakers, you should change the attenuator resistors to something like 12R and 4.7R
2) 12.5k plate load seems a bit low for 6SN7 (unless you're trying to lower the total THD by compensating for the output stage nonlinearity or something)
3) If you have 8R speakers, the OPT should be 5k:8. If you have 16R speakers, you should change the attenuator resistors to something like 12R and 4.7R
Okay, I can remove C1. I was just told on a different design of mine that it might be useful to limit audio pot scratching. I'll do some more research into loading the 6SN7 stage properly. the speakers I would use are 8 ohms, and the headphones are 32. Should I get the 5k:8 ohm transformers instead?
Yep, 5k:16 OPT with 8R load (the 6.2R+2.2R attenuator is also calculated to have about 8R total impedance with 32R headphones) will reflect 2.5k load to the output tube, which is pretty bad for linearity (though you can probably squeeze another 0.7W or so out of the amp)