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Old 23rd May 2014, 01:46 AM   #1
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Default 12BH7 Headphone Amp

Hi All,



So, I've done about a dozen tube instrument amp projects and I'm thinking it's time to branch out and try something in the hifi type world. Having very little experience trying minimize tube distortion I figured I'd start small and make myself a little tube headphone amp... maybe even spring for a new set of headphones too once I get done with it.

I like transformers, and I don't mind paying for a decent pair, so I figured I'd do a small SE design using an off-the-shelf OT - Sowter and Lundhal are the two makers I've looked at, the latter of which is pushing it a bit budget wise. I've been googling the subject of transformer-coupled SE headphone amps and I'm surprised how little is out there. Maybe I'm not looking hard enough or I'm searching the wrong terms... everything seems to be OTL designs.

So I thought I'd finally learn to plot a load line and try to put a plan together myself. I'm afraid I'm a bit uninitiated outside of the 12*** world so I picked the 12BH7 as a starting point since it can handle some current and it seems liked, at least in voltage gain stages. And I can get them reliably.

So, I plotted a load line (Basically my first one!) based on the "typical" (datasheet) characteristics of 11.5mA@250V - it worked out to a 22k load impedance and about .5W of output power:

Click the image to open in full size.[/URL]

I went by this http://www.blueguitar.org/new/text/t...nes_Series.pdf instruction. Sorry for using pencil...

I'm thinking I should have a bit more headroom so I figured I'd parallel both halves of the tube and use 1/2 the impedance to get about 1W output. Sowter sells the 8988 OT which is 10k:600 (And down) and is good for 2W @ 50hz. Seems like a good match and a reasonable starting point.

And it looks like an incoming swing of about 20Vp-p at the grid yields about 280Vp-p at the anode, voltage gain of 14.... I'll need a driver stage to get to that 20V though. Looking for suggestions on that part...

So am I way off or off to a decent start?

Regards,
Brian
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Old 23rd May 2014, 11:39 AM   #2
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Check out my headphones amp design here:

Improved Tube Headphones Amp

Cheers

Ian
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Old 23rd May 2014, 10:07 PM   #3
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Ian,


You know what? I just might. Not sure why I'm so in love with the idea of an SE design, but without a distortion analyzer I'm probably not well equipped to really break new ground. A proven design would be a smart choice and the transformers are even a bit cheaper.

Does the SRPP stage require an trimming or would I get good performance just dropping in the same values you used? Do the top and bottom halves of the EC99 require matching? I'm thinking not, but the pairs for both channels probably do (including the 12AX7).

Do you have any feedback on the plots I posted? I'd still like to figure out if I'm grasping the load line concept.



Brian
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Old 24th May 2014, 06:26 PM   #4
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Hi Brian,

All topologies are a compromise of some sort. The SE has its own problems, in my view the biggest being the need to have dc flowing in the output transformer. This means it must be gapped which lowers its inductance reulting is poorer bass response. Fine for guitar amps but not for headphones. of course you can get one with a decent inductance but it will be very big and expensive.

No special trimming is required for the SRPP. it is a meaty stage that will have a good go at driving almost anything. Its drawback is its relatively high distortion which is reduced 10 fold in my design by the negative feedback. For the same reason there is no need to do any tube matching. The gain is determined by passive components not the tubes themselves.

For an SE design, the dc loadline is almost vertical because the dc resistance of the transformer is very low. So you can start by drawing a vertical line at the chosen HT voltage. Your operating point will be somewhere on this line. You then need to draw an ac load line with a slope equal to the load reflected back to the primary of the output transformer. You ten side this ac load line up and down untill you find a suitabgle intersection with the dc load line that llies within the dissipation of the tube.

The Valve Wizard explains it much better than I can:

The Valve Wizard -Single Ended

Cheers

Ian
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Old 24th May 2014, 09:14 PM   #5
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Quote:
Originally Posted by BJosephs View Post
I like transformers, and I don't mind paying for a decent pair
Me too. I think you are searching in the wrong places.

ecp audio diy
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Quote:
Originally Posted by BJosephs View Post
So, I plotted a load line (Basically my first one!) based on the "typical" (datasheet) characteristics of 11.5mA@250V - it worked out to a 22k load impedance and about .5W of output power:
Most headphones are very efficient and don't need much power. A little headroom is not a bad thing, but no reason to shoot for a full watt. Sennheiser HD800s, for instance, are 102dB per Vrms. That works out to ~3mW.

12BH7 sections in || with a 10K load seems like a pretty decent option to me. And, finding transformers with a 22K impedance that work well will be a challenge.
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Old 25th May 2014, 01:41 AM   #6
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Thanks for the links, dsavitsk, I'll check them out.


The mention of ECC99 prompted me to repeat my load line exercise this morning. I'm a bit confused about how the numbers work out… Should I be plotting load lines for the transformer load (let's say 10k in this case) and one tube, then expecting double power w/ two tubes into the same load; or plotting the load line with 2xLoad when I know I would intend to parallel two sections?


Brian
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Old 25th May 2014, 03:32 AM   #7
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...And on the subject of an AC coupled output driven by an SRPP, both Jensen and Cinemag (transformer companies I've used before for DIY studio stuff) have 10k:600 output transformers. The Jensen is $140 (About as much as the Sowters) but the Cinemag looks to be less than $100:

http://cinemag.biz/output/PDF/CM-9589.pdf

And it looks to be able to handle +23dbu @ 30hz into 600 ohms with a 2.4k driving source and ~.005%THD… If my math is right that works out to about 11V and 200mW? Maybe an option? Distortion is better with lower source impedances and the ECC99 SRPP works out to about 1400 ohms (before accounting for the feedback). So, maybe a decent choice for a local option?

I should have mentioned that my best set of headphone are beyerdynamic DT 700 600ohm, so it's not a bad call to plan around 600Ohm only right now.


Brian

Last edited by BJosephs; 25th May 2014 at 03:42 AM.
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Old 25th May 2014, 07:14 AM   #8
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Math looks right. If you are driving 600R phones, then there are lots of good options. Cinemags are great. And they'll butt stack them so you can use them single feed -- though I don't see a spec for how much DC is OK.

If you are thinking parafeed, however, you can get cheapo Edcors to experiment with, and then buy good stuff later.

Do keep in mind that there aren't really [m]any great 600R phones out there, so you might want to plan for more future options.
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Old 25th May 2014, 11:02 AM   #9
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If you want excellent quality 10K:600 output transformers at a reasonable price then I would suggest the Carhnill VTB2291 for a capacitively coupled output. I think they also do a gapped version if you want an SE version.

If you are driving 600 ohm phones then you do not really need something as beefy as the ECC99. A regulat 6922 (industril version of the 6DJ8/ECC88) in an SRPP will be fine.

After developing the headphones amp, I applied the same basic principles to a mic pre which uses a 6922 SRPP output stage and a 12AX7 input stage modified to allow the gain to be varied from 6 to 40dB. It is used as the basis of my EZTubeMixer project at groupdiy.com. It used the VTB2291 output transformer wired 2K4:600 and will happily ouptut +20dBm into a 600 ohm load. Schematic is here:

http://www.ianbell.ukfsn.org/EzTubeM...rdCCTsht3.jpeg

Cheers

Ian
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Old 25th May 2014, 03:57 PM   #10
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Quote:
Math looks right. If you are driving 600R phones, then there are lots of good options. Cinemags are great. And they'll butt stack them so you can use them single feed -- though I don't see a spec for how much DC is OK.
OK, so there are no tricks in this. Any 10k:600 with enough output and low distortion would all technically 'work'. Thanks.

Quote:
Do keep in mind that there aren't really [m]any great 600R phones out there, so you might want to plan for more future options.
Good advise. Future proofing is worth consideration… plus I spent a hour yesterday drawing up a switching arrangement for a 3 or 4 winding secondary like Sowter uses.

Quote:
After developing the headphones amp, I applied the same basic principles to a mic pre which uses a 6922 SRPP output stage and a 12AX7 input stage modified to allow the gain to be varied from 6 to 40dB. It is used as the basis of my EZTubeMixer project at groupdiy.com. It used the VTB2291 output transformer wired 2K4:600 and will happily ouptut +20dBm into a 600 ohm load. Schematic is here:
Ian, I've seen your project page. I have to say you only make these things look "EZ"! I noticed you dropped the grid stoppers from Pete Millet's design, did you find them to be unnecessary? Also it looks like you power the driver and SRPP stages from the same PS node rather than decouple with a resistor and cap. Did you use the same 470uF that PM used?

I'm going to pull the trigger on parts before the weekend is over.

Thanks,
Brian
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