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Old 20th August 2007, 11:14 PM   #1
Eclair is offline Eclair  United States
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Default Headphone amp based on 45?

I've recently started spending a lot of time with headphones and was thinking about having a custom amplifier built for them. Most commercial headphone amplifiers are basically prosaic preamps, so I was thinking of acquiring something more interesting. Has anyone here built a headphone amp based on the 45 before? I'm probably looking for something that'll drive headphones with impedances around 32 to 50 ohms or so. I know the 45 provides more power than I would need, but darn, the 45 is nice. Well of course, other tubes might be nice (71A, 01A, or something) as well? Hmm...
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Old 20th August 2007, 11:30 PM   #2
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Hi,

45 makes a spectacular headphone amp.

Checkout http://www.headwize.com/

Cheers!
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Old 21st August 2007, 12:50 AM   #3
Eclair is offline Eclair  United States
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Yes, I've been a member of HeadWize for a while. Unfortunately there isn't a 45 or particularly similar design on the site as far as I know.

As far as all I know about 45 headphone amplifiers, I believe Craig Uthus of Moth Audio built a few commercial 2A3/45 headphone amplifiers in the past.
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Old 21st August 2007, 01:11 AM   #4
kevinkr is offline kevinkr  United States
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I used my 45 SE amplifier as a headphone amplifier until quite recently when I built a dedicated headphone amplifier around 5842 and 71A. I used a little attenuation because I could hear a little 120Hz buzz from the AC heated 45 filaments. (DC fully regulated)

Pick a low noise, high transconductance triode like the 5842 or triode connected D3A if you need more gain. Use regulated and well filtered dc on the 45 filament (constant current if you can)

My design uses unregulated B+ and bias supplies, 5842 with red led + diode bias, choke loading capacitively coupled to a 71A operated with fixed bias at ~20mA into 5K:32/8 ohm custom made output transformers. I drive my 32 ohm headphones directly and the design is silent - no hiss, hum or buzz. To avoid magnetic coupling between the output transformers, driver load chokes and the power transformer I have built it on two chassis.

The noise floor of the headphone amplifier appears to be significantly below the noise floor of my Amber 3501A audio analyzer, and is in territory where I wouldn't trust my computer sound card either.

I am pleased overall with the sound, and plan to invest in some really good Grado headphones in the near future to go with it. It was originally intended to be cheap, and destined for my office, but I overkilled on this design so it stays home. Next version will be very cheap and cheerful.

In short the 45 will make a great headphone amplifier output tube, the increased headroom will only result in even better linearity at normal listening levels. Just be careful to minimize noise contributors, keep currents toward the high end of the range, choose a driver as I discussed above and make sure the B+ feeding it is absolutely clean - a few mV or less of ripple is reasonable and achievable with a simple CRCLC type filter. Choke loading and parafeed are your friend in this regard, providing very good PSRR.

Splurge on the best output iron you can afford, depending on your performance target. (That's how mine ended up so far over budget, but no regrets )

Jack Elliano of Electra-Print wound the output transformers for my amplifier to my specification, and I am very pleased with the result.
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Old 21st August 2007, 01:39 AM   #5
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Hi Kevin,

Quote:
Originally posted by kevinkr
... because I could hear a little 120Hz buzz from the AC heated 45 filaments. (DC fully regulated)
Psssst, use a CCS for the fils
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Old 21st August 2007, 03:44 AM   #6
Eclair is offline Eclair  United States
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Wow, that sounds simply spectacular. Thanks so much for the help.

Hmm, a 5842 to 45 or 71A sure sounds like a great idea. Haha, building the thing will probably prove quite tricky for a novice like me. By the way, do you think it would be possible to build such an amp for under a grand initially? (Use cheap irons now and then get good irons later as my budget increases.)
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Old 21st August 2007, 04:09 AM   #7
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Quote:
Originally posted by Eclair
Wow, that sounds simply spectacular. Thanks so much for the help.

Hmm, a 5842 to 45 or 71A sure sounds like a great idea. Haha, building the thing will probably prove quite tricky for a novice like me. By the way, do you think it would be possible to build such an amp for under a grand initially? (Use cheap irons now and then get good irons later as my budget increases.)
My sense, especially if you are not experienced, is that you will spend a lot more than that. My first big project, which was nowhere as complicated as a 45, involved throwing out as many parts as I used ... more, and cost a small fortune.

My inclination would be to start with a cheap tube with the same basic topology, and then convert to the expensive tube and expensive iron at the same time. The iron and topology, after all, probably are more important to the overall sound quality than the tube.

A good tube to start with for experimenting is the 6W6GT. It sounds very good and is similar to the 45 as far as power output, and is very cheap. Indeed, you may like it enough that you won't bother moving on to the 45 for a while.

I agree on the 5842. Also, look into the 6688 triode strapped. It is electrically about the same, cheaper, and I think sounds better. All of these high Gm tubes can oscillate easily, so some real care in layout is in order.

For topology, I'd look into direct coupling. With headphones, the level of detail is much higher with phones which means that you'll need to be spending a ton on coupling caps to even approach direct coupling in performance. The DRD circuit, or a parafeed version, would be my preference.

Finally, I'll agree with Kevin on the custom Electra-Print iron. I use a custom would 5K:32 transformer with a simple 7788/E810F spud amp for phones and it is very good (the pic has a 5842, but I've settled on the more powerful 7788). Sowter also makes headphone iron.
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Old 22nd August 2007, 12:21 AM   #8
Eclair is offline Eclair  United States
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Ah ha, great advice! Perhaps I should give the 6W6GT a go initially. I guess I should probably start from the bottom and gradually go up the ladder. Direct reactance drive and parafeed sound like great choices as well.

As a side question, what about RPower's 71A direct coupled design?
(Schematic attached.)
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File Type: jpg dc71a.jpg (19.8 KB, 568 views)
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Old 22nd August 2007, 09:20 AM   #9
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Oh, I've got another quick question as well. Is it a bad idea to connect headphones with an impedance of 32 ohms to 8 ohm outputs? dsavitsk, you mentioned that you were using a custom transformer with 32 ohm outputs. Does that transformer allow the amplifier to work with headphones of slightly higher impedances (50 ohms for example)? If so, about how high do you think it can go before the performance starts to degrade?
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Old 22nd August 2007, 04:31 PM   #10
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by Eclair
Ah ha, great advice! Perhaps I should give the 6W6GT a go initially. I guess I should probably start from the bottom and gradually go up the ladder. Direct reactance drive and parafeed sound like great choices as well.

As a side question, what about RPower's 71A direct coupled design?
(Schematic attached.)
Looks like a reasonable design except for the 1000uF cathode bypass cap. This will have to be of exceptional quality not to audibly degrade the sound quality, and bypassed with multiple smaller caps. (I made this mistake in one of my own designs, and ultimately found a moderately expensive BG electrolytic with several smaller BG in parallel to be acceptable.) You'd be better off with an LED chosen to provide the correct operating point. You can tweak the bias voltage to get the right operating point for both stages as they are interdependent being dc coupled.


Quote:
Originally posted by Eclair
Oh, I've got another quick question as well. Is it a bad idea to connect headphones with an impedance of 32 ohms to 8 ohm outputs? dsavitsk, you mentioned that you were using a custom transformer with 32 ohm outputs. Does that transformer allow the amplifier to work with headphones of slightly higher impedances (50 ohms for example)? If so, about how high do you think it can go before the performance starts to degrade?
I also have 32 ohm taps on my amplifier which I use to drive 32 - 600 ohm phones without any problem so far. The 8 ohm tap can be used to drive 8 - 32 ohm headphones (or higher) without any problems at some loss in available voltage swing, which with most headphones is not going to be an issue.
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