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Old 27th August 2005, 12:51 PM   #1
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Default Ideas for using a pair of ECL82 in a headphone amp.

Hi there,
I have a few nice ECL82 and don't really need a flea powered SE amp, so I was thinking of using them as a headphone amp. I have done some searching but can't find any schematics for them in this application, though I have seen them mentioned as suitable in this role.
My original idea was to just take the output off the cathode, but this just wouldn't provide enough current for a lot of headphones.
Has anyone tried this and what were there results?

Thanks
Shoog
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Old 27th August 2005, 04:49 PM   #2
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Default Re: Ideas for using a pair of ECL82 in a headphone amp.

Hi Shoog,

Quote:
I have a few nice ECL82 and don't really need a flea powered SE amp, so I was thinking of using them as a headphone amp. I have done some searching but can't find any schematics for them in this application, though I have seen them mentioned as suitable in this role.
Well, even when triode strapping the output pentode of ECL82, it is not particulary linear, see this link.

Quote:
My original idea was to just take the output off the cathode, but this just wouldn't provide enough current for a lot of headphones.
My best idea would be to build a "real" SE amp with it, output section triode strapped, Po about 1-1,5 watts. Use a stereo headphone jack with an integrated DTDP switch to cut off the "speaker output" and insert a 10 ohms dummy load across the secondary, to which the headphone is paralleled. You can drive any headphone even with extreme low impedance with this setup.

To get the picture, take a look at the schematic and description of my own headphone amp:

http://www.tubes.mynetcologne.de/roe...ematics_e.html

You can find an ECL82PSET on my homepage, too.

Tom
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Old 27th August 2005, 06:19 PM   #3
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I was coming round to thinking that that might be the sensible approach, but it means I have to track down some suitable transformers - which is always the tricky bit.

Shoog
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Old 28th August 2005, 04:18 PM   #4
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The ECL82 pentode triode-connected is perhaps not as linear as a 45 or whatever one might compare against. However, it's not bad. I've used it for a SE transformer-coupled amp with partial (plate) feedback and I get good sound for headphones too with a voltage divider off the 16-ohm tap (not ideal, but HPs need very litte power, so the sound is good and loud enough!). So I second the OPT suggestion.
The major issue, IME, with HP amps is noise more than distortion. Whether OPT or OTL, you'll get enough power/low enough distortion. With OTL, 300-ohm HPs could be direct connected, while 32-ohm ones might need a 270 ohm (or so) series resistor on the output to keep noise/sensitivity down. (Damping factor goes out the window, but with HPs is usually not a problem - if found to be, a 15-ohm resistor i parallel to the 32-ohm HPs raises DF to about 2, which solves that "problem".)
My OTL HP experiment was with EL86 rather than ECL82, but it's not so different. HT noise and heater hum are the major issues, plus avoiding ground loops (hum/buzz). Active regulation or heavy filtering solves the first two.
With ECL82 in triode mode and OTL, the criterion is linear current swing; you'll only need a few volts swing, so that's not a problem. 100Vpk and 30-40mA idle current on the ECL82 looks pretty linear to me (loadline is almost vertical), and should be sufficient for most HPs. You could use two pentode sections, upper triode connected, lower as pentode current sink. Or both in triode mode in White CF connection. Or just one tube with a silicon CCS. (Broskie's Aikido tricks might come in handy to get the noise down with WCF.) Gain may be sufficient without an input stage, just the CF. The ECL82 triode section has high mu, which you don't need/want, but it might be used as an input/feedback return stage, esp. for lo-Z HPs. (You prob. don't want a closed-loop gain of more than 5 or so.)
I can't upload a schematic right now, but if you're interested in a ECL82 long-tailed pair input/white CF output OTL HP amp schematic, send an email. I have both a direct-coupled version with +/- PS and a cap-coupled with single supply. Untried in reality as yet with ECL82, but same circuits work well with 5751/EL86, so shouldn't be a problem.

morgamp1 'at' yahoo 'dot' se

Morgan
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Old 28th August 2005, 08:23 PM   #5
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Default ecl82 / 6bm8 headphone amp

Hello everyone,

I have built several SE and PP headphone amps with this tube and the one I like the most (and the one still in active service) is a design coming from Japan, you can see it here:
http://www.za.ztv.ne.jp/kygbncjy/tubeamp/tubeamp.html.
I have used the 6bm8pp design as a headphone amp, a very good design - in my humble opinion, and a very good sound. This is my main headphone amp - very recommended.

While you are in that site you can take a look at the other designs I think you will find many nice ideas.

Good Luck.

Glass_painter
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Old 29th August 2005, 07:58 PM   #6
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Thanks for the detailed reply - they have touched on a few of my thoughts.

"Or both in triode mode in White CF connection. Or just one tube with a silicon CCS. (Broskie's Aikido tricks might come in handy to get the noise down with WCF.)"

I was looking at a white cathode follower using ECC88's on the headwize site. I was thinking of something similar but using some PC84's I have for one of the output valves, along with the ECL82's.. Seems as though this requires careful optimisation to get a satisfactory output power and minimum distortion. I'am not certain I'am up to the challenge.
I have built a MU stage SE amp with TT21's as the top current source, and 807's as the bottom output valve. I am happy with the result and don't see the limitations of not having a truly constant output impedance (though I do use simple to drive full range speakers). I burn a wopping 80V on current sensing resistors to make it approach a true CCS. I was thinking about using this topology as the basis of my headphone amp, and since these have fairly flat loads it might be satisfactory. The advantage of this circuit over the WCF is that it kills power supply noise dead with minimum power supply filtering. Any thoughts.

Another thing, I don't have any headphones at the moment so I can choose the headphones to suit the project.

Shoog
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Old 29th August 2005, 09:27 PM   #7
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Hi Tom,

Do you think your EL34SET with the Hammond 125ESE would be a good headphones amp?

Quote:
Use a stereo headphone jack with an integrated DTDP switch to cut off the "speaker output" and insert a 10 ohms dummy load across the secondary, to which the headphone is paralleled. You can drive any headphone even with extreme low impedance with this setup.
I am going to try this with my JE Labs 300B. My headphones are Sennheiser HD650 (600Ohm).
What output of the OT (Hammond 1627SE) should I use? 4-8-or 16 Ohm?

Thanks

Manuel
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Old 30th August 2005, 03:53 AM   #8
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> just take the output off the cathode, but this just wouldn't provide enough current for a lot of headphones.

It can give current comparable to a common chip op-amp, and many people are happy with such things as headphone amps, especially for loads >>32Ω.

The cathode output resistance is ~140Ω, which means THD will be quite high when working hard in 32Ω, and the low-THD load should be well over 300Ω, higher than most phones.

Adding a transformer solves both problems. Use a 3K:8Ω or 20:1 tranny, output current is 20 times higher which is more than ample for any headphone; load is sufficiently high to get into low-THD zone.

A third feature is: you need less B+ power, so filtering is easier; and the ~20:1 transformer gives ~20:1 reduction in ripple at the load.

The basic problem is that vacuum conducts juice a million times worse than metal. Even if you flatten the vacuum tube structure (wide cathode, short plate spacing) and stretch the metal to long fine wire, a tube has 1K-10K resistance, a motor has 10Ω-100Ω resistance. Even a 6c33 is not the most efficient thing driving 32Ω. 300Ω phones have fine fragile wire, you don't want to go higher. Tubes need transformers to drive motors well.

The cute thing about headphones is we can do terribly wasteful things and not go broke. Instead of a good DC-rated transformer, resistance-load the tube and use a cheap core. Let's see.... 200V supply, 200V G2, 30mA, 100V on the plate, 100V/30mA= 3K 5W plate resistor. Transformer sees ~1K source, so it does not need a lot of Henries to go to deep bass. A 230VAC:12VAC power transformer and 10uFd coupling cap will rumble fine. A split-bobbin power transformer will have heaps of leakage inductance and poor treble, but a one-bobbin transformer may be OK, especially in the higher-Z loads. Or use some small core that would melt at 30mA DC but can handle milliWatts of DC-free audio.
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Old 30th August 2005, 01:26 PM   #9
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Hi Manuel,

Quote:
Do you think your EL34SET with the Hammond 125ESE would be a good headphones amp?
Frankly, I would consider it complete overkill and not suited very well. Even the 1 Watt headphone PP amp I built http://www.tubes.mynetcologne.de/roe...pcl84pp_e.html still is overkill, but it works excellently. If asked, I always would suggest and favour a "real" very small mini triode PP class A amp instead of an SE one. Too many objective advantages that cannot be ignored.

Quote:
I am going to try this with my JE Labs 300B. My headphones are Sennheiser HD650 (600Ohm).
What output of the OT (Hammond 1627SE) should I use? 4-8-or 16 Ohm?
If you really want to do this, attach a nominal dummy load at the accordingly specified secondary tap. If in doubt, use 10 ohms dummy load at the 8 ohms tap and strap the headphone in parallel to the dummy resistor. Very low Z headphones like my 50 Ohms Sennheiser will lead to an effective load of about 8 ohms for the amp, while for high Z headphones (like yours) the mismatch is slightly less than 2 ohms in this situation and safely can be ignored. Any speaker is much worse as a load than this dummy-res/headphone combo.

Watch out with the Pd the dummy res must be capable to dissipate.

Watch out for your ears and headphone, since some milli-Watts will give extreme soundpreassure levels already.

Tom
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Old 30th August 2005, 07:20 PM   #10
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"The cute thing about headphones is we can do terribly wasteful things and not go broke. Instead of a good DC-rated transformer, resistance-load the tube and use a cheap core. Let's see.... 200V supply, 200V G2, 30mA, 100V on the plate, 100V/30mA= 3K 5W plate resistor. Transformer sees ~1K source, so it does not need a lot of Henries to go to deep bass. A 230VAC:12VAC power transformer and 10uFd coupling cap will rumble fine. A split-bobbin power transformer will have heaps of leakage inductance and poor treble, but a one-bobbin transformer may be OK, especially in the higher-Z loads. Or use some small core that would melt at 30mA DC but can handle milliWatts of DC-free audio."

Hi there PRR,
Thats just the sort of thing I have been thinking of. I like the idea of only needing two tubes and a rectifier. Using mains transformers has worked for me on my Parafeed 807 amp so I don't need to be convinced that this can work satisfactorily. I have access to some nice small potted 12V toroidals which should do an excellent job.
The only thing I am not entirely certain about is using just a 3K load resistor. A constant current source would allow the ECL82 to perform to its best (as poor as that is).
The only problem is finding a suitable pentode/triode for the CCS. It would need a DC resistance of about 3K so that it consumed about 100V. I was looking at the PL84 in this role, but it doesn't look to have a happy operating point at about 30mA and 100V. I was also looking at the 6aq5 (a mini 6V6) but again it wants to see more voltage on its anode. I really want to use some of the valves I already have, so exotics are not ideal.

Do you think I would see a significant increase in performance using the CCS rather than the 3K resistor?

Thanks for the great input.

Shoog
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