Evolution of a tube headphone amplifier

BuzzK

AK Member
Subscriber
Back around 2003, I had a musician friend ask if I could build a tube-based headphone amp (HPA).

I scrounged around my junk box and came up with a chassis; two Wollensak single-ended output transformers; a power transformer; and a choke and most of the other bits.

Fifteen years ago I was pretty green on actual circuit design. I bought the TubeCAD CDs and designed an amp based around 6SN7 amplifiers with 6V6. I didn't do a very good job, either with the design or the layout. As you can tell from the pics, this was a real Rube Goldberg project.

I thought it might be interesting to revisit this project and update it, both in design and layout, in "installments" on AK. Dave Gillespie really helped out, basically redesigning the circuit.

Would anyone be interested?
 

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Buzz- You built a small power amp. for your phones. i never saw the point to that, all that extra noise potential. I built a standard cathode follower design that uses a 12AU7 up front as a voltage amp and 12B4 triodes as the output tubes. No output transformers needed. It was designed to run my Senn. HD580s @ 300 Ohms and works beautifully.
Plus, if Dave redesigned it already , what more do you want to do?
PS - Rube Goldberg never built any of his nutty but clever contraptions.
 
After having a pair of headphones BLOW UP while on my head due to capacitor failure in a tube headphone amp- I think I would WANT output transformers.

Even if they were good line output transformers (not speaker transformers)- something to positiviely isolate me and my headphones from high voltage, would be a requirement for me.

PS: If the best line stage in the world (IMHO- the VAC Statement Line Stage) uses transformers in and out- I think we can find something that will work exceptionally well for an output transformer for headphones...

Regards,
Gordon.
 
I'd think line trafos fed off the cathode of the output tube wouldn't be too bad a match. Fairly low impedance source at that point, and with a low winding ratio plus low power you shouldn't have bandwidth problems.
 
I'd think line trafos fed off the cathode of the output tube wouldn't be too bad a match. Fairly low impedance source at that point, and with a low winding ratio plus low power you shouldn't have bandwidth problems.

That's what I'm thinking. Even cathode followers do a better job of swinging high voltage, than swinging high current. A, say, 10:1 or 5:1 step down ratio would decrease output impedance by a factor of 100 or 25, respectively. That's a lot better damping and dynamic "freedom" for the output tube, in terms of its ability to drive the load.

And, you could, in theory, just use the primary of the OPT as your cathode load on the CF, and bias it just like a "unity coupled" stage (fixed bias). Or, do it more "conventionally", with a resistive load and a coupling cap- which would effectively be a cathode follower parafeed...

Regards,
Gordon.
 
Buzz- You built a small power amp. for your phones. i never saw the point to that, all that extra noise potential. I built a standard cathode follower design that uses a 12AU7 up front as a voltage amp and 12B4 triodes as the output tubes. No output transformers needed. It was designed to run my Senn. HD580s @ 300 Ohms and works beautifully.
Plus, if Dave redesigned it already , what more do you want to do?
PS - Rube Goldberg never built any of his nutty but clever contraptions.

I've built a couple of other HPAs . . . an OTL one that uses 6N1Ps, and a hybrid that uses tubes and MOSFETS. Now I want to use an OPT version for comparison (I think the other two don't employ global NFB).

The frequency response of these small trannies is good at low levels. I'll include some of Dave's measurements later.

I gave Dave the one that he redesigned. I wanted to make one for myself, using his improvements (and mine). :)
 
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Project Phase 1 - Parts: What do I have vs. what do I need?

The heart of this project is a pair of small single-ended output transformers from an old Wollensak reel-to-reel. They have a decent frequency response according to Wollensak. I'm going to need a power transformer, enclsoure, tubes, etc.

The next step was to find a decent enclosure.

I had ordered some nice HPA enclosures from China via eBay in the past, and I thought I was ordering a similar unit. Except this one cost less and didn't have all of the hardware that the others did (e.g. input jacks).

The first pic below is what the vendor initially showed on his ad. What I received looked like the 2nd pic (I don't think this 2nd pic was included with the ad when I initially bought it. I'll explain).

The faceplate of what I received was totally unacceptable . . . the holes did not line up, and there was some strange brand name printed on it. I convinced the seller to send me a blank plate, which he did. I had that part machined to my specs.
 

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A local fabrication house machined the front panel for me. They were nice enough to install some press-fit anchors for the power switch.

A nearby trophy shop laid out the artwork and etched it on both the front and back panels, and then applied paint, for $120. (Kinda pricey for the folks who like to use baking pans for chassis, but as Nana always said, "any job worth doing is worth doing right").

Also shown are the trannies. I found a couple of nifty transformer covers for cheap on eBay. They fit perfectly.

Now I've got to get a suitable power transformer.
 

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Any voice coil headphone, regardless of brand, will experience catastrophic failure when DC is dumped into the voice coil.

Fine wire + High current = Meltdown

This is why DC speaker protection devices are used. Within a milliSecond of DC appearing the circuit is interrupted and meltdown is avoided.
 
Errant Schematic

Attached is a .pdf of the original HPA circuit. I have included it for reference if only to contrast with Dave's improvements (later on).

After look at Edcor, ebay, etc., I decided to order a Hammond 269BX power transformer. It includes a 2A 6.3V winding. I'll probably sub 6AQ5s for the 6V6GTs for space. Each draws 450 mA of filament current.

I'll wait for the power tranny before I start drilling, just to be sure of the layout.

Is there a 9 pin electrical equivalent to the 6SL7? I know some folks use a 12AT7 but it is hard to find quiet ones.

Buzz
 

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Errant Schematic

Is there a 9 pin electrical equivalent to the 6SL7? I know some folks use a 12AT7 but it is hard to find quiet ones.

Buzz

Buzz,

Traditionally, 12AX7 is considered a 9pin equivalent of 6SL7. There are some minor differences, but for the most part, they can be discarded...

Also, thank you for starting the thread on tube and opt based headphone amps. I love those and been experimenting with them for a while now. To me, the HPAs have a very distinct set of requirements
  1. Schematics requirements
    1. Topology selection - we are not going after power here, but quality of the output signal. HPA is where the "differences" between an SE and PP are noticed in the most prominent way, and ultimately, the topology is a users call (IMO). At first glance, it might seem as though SE/SEUL might be the right choice, but not necessarily. A low power PP amp could (and does) produce spectacular sound into headphones as well. Which brings me the next point.
    2. Driver and output selection - tubes known to have low internal noise, minimum microphonics are very good candidates here. IMO, the trick is to choose the output tube that operates produces low output power, while operating within its optimal operating parameters. Why use a 6L6 if only 1/50 of its power is needed? Why deal with voltage dividers in the signal path? And, while the choice of the driver tube is driven by the output tube, there is still a lot of options, as well as driver topologies. Another parameter that often gets overlooked is the heater voltages of the tubes. The are very distinctly noticed in HPAs. And finally, DC or AC heaters?!?!?
  2. Components requirements
    1. I am not a snob, when it comes to the component selection, and happily use common/generic Chinese made components almost everywhere, but for the HPAs, I choose my components more carefully. The biggest (IMO) challenge is the OPT. How does one match impedance of headphones that range from 32 Ohms to 600 Ohms? Different secondary windings for each impedance vs 32 -120-300 taps? More over, it turns out (at least as I discovered) that bigger iron is not always the best choice for an HPA...
  3. Assembly requirements - proper wiring and soldering are the key. Actually, the HPAs is where I would argue the PCB is not only appropriate, but preferred as well... A tube amp can be made compact and have a small footprint without any impact on the quality of the sound (despite the claims to the contrary), and HPA has a lot of room for experimentation..
Anyway, I am looking forward to you next postings...

Cheers, Paul.
 
The Devil is in the Details.

But so is the glory.

Part of the fun -- as well as a challenge -- of building from scratch is to try to make the piece aesthetically pleasing -- to give it a professional look.

So I endeavor to pay attention to detail, and think though each step of the way. I apologize if I am too wordy or bore you with my OCD.

I had an ALPs 50K audio taper pot in stock, but I ordered a smaller control from DigiKey. The front panel is about 1/4" thick, and I didn't realize that it was too thick for most potentiometers without countersinking the hole. I didn't want to send the decorated part back to the machine shop, so I decided to try to drill it out. I knew that it wouldn't be as pretty as milling it, but it wasn't going to be visible once the knob was installed. So I drilled a hole deep enough for the retaining nut. It ain't pretty.

The knob that came with the chassis kit was way too big, and it lacked any kind of position indicator. So I dug into the junk box and pulled out a brushed aluminum knob that was smaller. Perhaps not the optimal size, but better.

I had to cut the shaft of the pot to fit the knob, so I did it with a vice and hacksaw. The pictures below show the process and the result.
 

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Emery paper wrapped around a dowel can smooth that down. Or one of the fine-grit conical aluminum-oxide grinding points to smooth out the hole. Not used in a drill, just rotated by hand. This can also be used to generate a flat bottom, sort of like a Forstner bit for metal, but I would never do this with a power tool as it will walk and eat into the sides if everything is not perfectly clamped and aligned. Always test crazy ideas out on scratch pieces and then have that "well, glad I didn't do this on anything which matters" epiphany.
 
Good points -- thanks for the technique ideas.

I did try it on the scrap front panel before I did it on this one. And yes, there was some walking. :)
 
Here's the revised schematic (of just the amp section. Power supply is a work in progress). I chose 6AQ5As instead of 6V6s for the smaller footprint.
 

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