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12HL7-triode SE Headphone Amp - I finally built something!
12HL7-triode SE Headphone Amp - I finally built something!
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Old 5th July 2018, 02:54 AM   #1
rongon is offline rongon  United States
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12HL7-triode SE Headphone Amp - I finally built something!
Default 12HL7-triode SE Headphone Amp - I finally built something!

I finally (finally!) finished putting together my first self-designed headphone amp. I think it came out pretty well.

This amp is a pure junkbox project. The goal was to make a useful toy out of parts I've collected over the years. The idea was to NOT buy ANY parts, if at all possible. The amp's built in a discarded Heathkit W5M chassis a friend gave me something like 15 years ago. I also wanted to put to work things I've learned from folks here on diyAudio. Thanks to everyone who gave me advice and instruction along the way. (I know the "strain relief" on the AC cord is laughable. I promise I'll do better next time.)

It was pretty funny how many gremlins were tripping me up today. The first time I brought the amp up on the variac it blew its fuse. Turns out there was a faulty rectifier diode. Replaced that and brought the amp up again. This time the fuse held. Now the right channel tube's heater wasn't lighting up. A little jiggling of the pins in the socket and that problem went away. Weird. I checked for a cold solder joint, but all seemed fine. Oh well. Brought it up again. All seemed to be going well so I connected RCA cables to the inputs and gave it the Bzzzzt Test (touch the central pin in each RCA plug in turn, slowly turn up the volume, listen for the bzzzzt from each channel in turn). Hmmm, no sound from the right channel. Oh no... Continuity checked out OK. All resistances OK, including both OPTs. The output jack? Brought it out, disconnected it. Clip-leaded in a new jack and there it was. Would you believe a defective jack? Man, I'm snake-bit!

Finally, all was looking good. I took the amp off the variac, powered it directly from house AC. All good. Time to listen.

I connected the amp to my PC's soundcard, plugged in a pair of beater headphones, gave it a listen.

Four hours later and I'm still listening. This thing sounds really, really good to me. Clean and clear. Not "warm." Lots of impact. Drums go "thwack." Isaac Stern's violin is really nice. Not 'electronic' sounding at all. Bass has real body. I think it goes low enough to be enjoyable. (I was worried about that.) Overall the amp is 'livelier' sounding than my Objective 2 amp. I don't want to go down the rabbit hole of audiophool jargon blather, so I'll just post some pictures of my fuggly build and the schematic of the basic circuit.

In the schematic there's a B+ supply common to both channels, then the DC splits off into an RC filter for each channel of 560R and 330uF (R2 and C4). Later I hope to replace that RC filter with a simple voltage regulator member Elvee basically designed for me (thanks Elvee!). The idea is to have a couple of pretty 0D3 glow tubes standing on top of the chassis. I rigged it up with the passive supply to get it going.

I was worried about hum from AC heaters. I very carefully followed the suggestions in the "Heater Wiring - The Good the Bad and the Ugly" thread. I'm getting absolutely zero hum of any kind. (I'm so happy about that.) I also followed the suggestions for grounding layout in Merlin Blencouwe's Valve Wizard Grounding article. That worked perfectly too. (I'm elated about that.) I also referred to Merlin's "Designing High Fidelity Tube Preamps" book a lot, which was especially helpful for power supply stuff.

So it's a success. Gain is about 2X, I think. I haven't measured it yet, as I don't have my signal generator and scope fully accessible right now. I need to get to that.

Finally, I need to figure out whether I need damping resistors across the OPT secondaries for using the amp with 300 ohm headphones. The amp sounds excellent to me into my Fostex T50RP headphones (60 ohm impedance). It also sounds clean and clear into my Sennheiser HD650 headphones (300 ohms), but these supposedly 'dark-sounding' cans sound bright and a little bass-shy from this amp. I'm wondering if that's an artifact of putting a higher than optimal 300 ohm load on the nominal 50 ohm secondary. This weekend I'll have some time to clip in some different values of parallel resistors to see what sounds good to me.

But for now, I'm just happy this thing is working well. Thanks again to the diyAudio community.
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Last edited by rongon; 5th July 2018 at 03:37 AM.
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Old 5th July 2018, 03:13 AM   #2
H713 is offline H713  United States
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Looks pretty nice. Reusing an old chassis always makes things harder to get a clean look, but still not bad. My first build is way uglier on the inside. Definitely would be cool to do a regulated power supply.

Also, congrats on your (I assume) first project, and you did not make the same mistake I did with my first preamp project. You chose a large enough chassis, and you didn't try to hot-glue in your filter caps in (yeah, hot glue + heat from tubes = not glued anymore).

Also, I've found that Crestron devices have pretty nice chassis.
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Old 5th July 2018, 03:36 AM   #3
rongon is offline rongon  United States
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12HL7-triode SE Headphone Amp - I finally built something!
Thanks. It's actually not my first project at all. Just the first successful from-scratch build in a few years. The last thing I built, a push-pull power amp, didn't work out. It hummed. I was traumatized, so was very careful to do this one more properly. It seems OK.

Last edited by rongon; 5th July 2018 at 03:52 AM.
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Old 5th July 2018, 05:13 AM   #4
H713 is offline H713  United States
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With my preamp project, I chased lots of hum issues. At the time, I didn't know about grounding the center tap of a heater winding. I figured out that grounding the heaters through a .1uF capacitor killed much of the noise. And that was a DC heater, so it was equally confusing.

Also confusing was the chasing down a massive hum that was ultimately due to a bad pot.

Anyhow, 12HL7... not a tube we see every day. What made you choose it?
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Old 5th July 2018, 10:12 AM   #5
bondini is offline bondini  Australia
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Default 12HL7 is a fine device - well done!

Nice project - thanks for posting and happy listening! I will be interested to hear from you as you play with this amp!
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Old 5th July 2018, 02:20 PM   #6
rongon is offline rongon  United States
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12HL7-triode SE Headphone Amp - I finally built something!
Quote:
Originally Posted by H713 View Post
With my preamp project, I chased lots of hum issues. At the time, I didn't know about grounding the center tap of a heater winding. I figured out that grounding the heaters through a .1uF capacitor killed much of the noise. And that was a DC heater, so it was equally confusing.

Also confusing was the chasing down a massive hum that was ultimately due to a bad pot.

Anyhow, 12HL7... not a tube we see every day. What made you choose it?
Having lots of experience built up over many builds is a huge asset. Unfortunately, that takes a lot of time and effort. Since I'm not properly set up for metalwork, I find it hard to stay motivated to start projects. Layout is a challenge for me, as is grounding. I think I'm getting better at it. But I need to build more stuff, learn what doesn't work and how to make the right compromises.

I read about 12HL7 here on diyAudio and found a sleeve of them for cheap. They have high transconductance so low plate resistance when wired triode, with a fairly hi mu of ca. 30. That makes it a good candidate for a 'spud' headphone amp. I'd use something fancy like a D3a or 7788, but this was a kind of 'test build' using $15 Edcor OPTs, so I didn't want to throw expensive parts at it. Definitely a junkbox project.

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Old 5th July 2018, 04:35 PM   #7
H713 is offline H713  United States
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Quote:
Originally Posted by rongon View Post
I find it hard to stay motivated to start projects.

--

See, I have the opposite problem. I start new projects before I actually finish the previous.

Also, I must say that I like your design quite a lot. Been thinking about a headphone amp for a while... this might be it. How are those Edcor transformers?

Last edited by H713; 5th July 2018 at 04:41 PM.
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Old 5th July 2018, 07:49 PM   #8
rongon is offline rongon  United States
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12HL7-triode SE Headphone Amp - I finally built something!
Quote:
Originally Posted by H713 View Post
See, I have the opposite problem. I start new projects before I actually finish the previous.
Yeah, I know how that is. I went through a period of thinking I could make a large pair of efficient speakers with horns. Now I have several large speaker cabinets, waveguides and a bunch of boxed up speaker drivers cluttering up my closets. I bought a pair of JBL LSR-305 on a whim, and now I have less interest in speaker building. I feel a speaker parts sell-off coming on...

Quote:
Originally Posted by H713 View Post
Also, I must say that I like your design quite a lot. Been thinking about a headphone amp for a while... this might be it.
Thanks! I think it's pretty good, and certainly is easy enough to put together.

So far I like it best matched up with my Fostex T50RP headphones. That makes sense, since those are 50 ohms impedance with flat impedance curve, being that they're planar magnetic.

Last night I listened a bit with my Sennheiser HD650 cans hooked up. I didn't like that as much. They were kind of 'thin' sounding driven by this amp. Very clean, but the bass is somewhat attenuated. That's probably because of the impedance mismatch between the OPT 50 ohm secondary and the high and variable impedance of the HD650s (varies from 310 to about 500 ohms). I'm going to try a suggestion from famousmockingbird to place a 100 ohm resistor across the OPT secondary to smooth the load seen by the output tube through the OPT. Perhaps that will allow the HD650s to play better with this amp.

Today's the first day I've had to listen to some 32 ohm headphones with the amp. They work just fine. Very nicely, in fact. Only one complaint. What I'm finding is that the tubes are a bit microphonic, which is likely to be a problem if using efficient 32 ohm cans (like Grado). With the volume set for normal listening, if I mute playback and tap the top deck of the chassis, I clearly hear a tinkling and ringing noise. It dies down quickly, unlike some other RF pentodes I've tried triode-strapped (6J52P anyone?). However, with less efficient cans that require turning up the volume control, the microphonic noises are now low enough in level that I can easily ignore them. I know this would drive some people crazy, though.

Another thing is that channel balance is dependent on tube characteristics, since there's no negative feedback employed. I'm finding that channel balance is shifting slowly back and forth between left and right as the amp plays over the course of several hours. I suppose the tubes will settle in at some point. Tube matching mandatory, so buy extras to sort through.

Quote:
Originally Posted by H713 View Post
How are those Edcor transformers?
Surprisingly good, I think! However, the primary doesn't have high enough inductance to get great bass response. It's good enough, but I can tell right away that my Objective 2 amp goes deeper in the low end, and with better control (probably due to much lower output impedance). However, I do like that tube 'liveliness' to the midrange reproduction, and the extra-smooth highs. Vocals, saxophones, and orchestral strings and percussion sound great through triodes and transformers.

Quick switch back and forth, Ugly Beastie vs. Objective 2 -- the tube amp sounds 'warmer' for sure, the solid-state amp goes deeper in the bass, for sure. It's really hard for me to choose which I prefer overall.

An interesting experiment might be to use a higher gain pentode like 6J52P (or the pricey D3a) and employ it as a plate follower (apply plate-to-grid negative feedback). Only about -6dB of NFB would be available, but that might be enough to tame the 6J52P's microphonic tendencies and lower its plate resistance enough to get better bass out of the OPT. The result could be a more versatile design, able to successfully drive a wider range of headphones.

Anyway, thanks for the encouragement.
--

Last edited by rongon; 5th July 2018 at 07:52 PM.
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Old 5th July 2018, 08:14 PM   #9
H713 is offline H713  United States
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All my headphones are 32 ohm, and I've been thinking of playing around with a 6J7 design, maybe triode-strapped. Might be considering the 12HL7 now too though. I've got some thinking to do, but I better do it quick because the roommate might not like my rather large speakers and big tube amp. I mean, who doesn't want a 50 pound heater in their room that makes a whopping 30WPC?

Also, I'm really not an expert, but wouldn't 600 ohm headphones work better with an OTL amp?
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Old 5th July 2018, 08:37 PM   #10
gofar99 is offline gofar99  United States
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Hi, Good show, I will pass along a suggestion I learned the hard way some years back. Using LEDs is a fine way to get bias....but to get more current capability using them in parallel can be an issue. The voltage drop across each diode will not be exactly the same. The result is that the one with the lowest drop will pass the most current. In some cases nearly all of it. This can result in failure do to excess dissipation. The solution is to put a small value resistor in series with each diode. They will then share the current. The larger the value of the resistor the more equal will be the sharing. If you make it too large though they will will alter the bias setting that you wanted.
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