12AU7/12BH7 Headphone Amp

the thermionic watercooler

Postby WA4SWJ » Fri May 11, 2007 6:58 pm

A couple of more comments. I'm doing this simulation at 5 kHz. Also if the plate resistor on the first stage is increased to 43 K the gain goes to about 10 and the distortion decreases to just over .1%. The AC Analysis looks pretty good. I'll post that too. I have to get through the thumbnail stuff to post it.
Last edited by WA4SWJ on Fri May 11, 2007 8:13 pm, edited 1 time in total.
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Postby TomMcNally » Fri May 11, 2007 7:27 pm

Thanks for experimenting with this Ed ...

I just messed with the amp a little on the bench, and it really
gets distorted fast. If I'm coming right off the 320 volt supply
to the first stage, what do you think I should use for a plate
resistor ? I started with 47K. The output stage has 22K.

Next wave of tests I'll put tone though it and take some
mesurements. It does sound clean at lower levels,
again this is with 600 ohm AKG headphones.

... tom
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Postby erichayes » Fri May 11, 2007 7:33 pm

What you're seeing makes sense, Ed. Never having heard of a "symmetrical" WCF, my first perusal of the circuit last night set off alarms. With the values given for the two plate resistors, there was a train wreck waiting to happen. Either the first stage resistor had to be a lot larger to jack the gain up, or the second resistor had to be a lot smaller--which didn't make sense, instinctively.

With the numbers you'rre coming up with, I can see much more clearly what the function of that second plate resistor is, although I think it defeats the reason for using a CF in the first place: low output impedance.
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Postby WA4SWJ » Fri May 11, 2007 8:08 pm

OK Guys,

I've posted the best of the Multisim SPICE runs I did. Interesting amp. Here's the circuit simulation with about 1 volt pk-pk on the grid at 5 kHz input:

http://diytube.com/phpBB2/album_pic.php?pic_id=22

Sorry for the blurry picture. I had to shrink it to get it to upload. Maybe there's a better way. Note the circuit changes that were made. With this arrangement it has a gain of about 10. It's kind of hard to see the 'scope traces. Again - sorry 'bout that.

Channel A is connected to the input grid (that's the small trace) and Channel B is connected across the output resistor as is the distortion analyzer. Note the volts per division on each channel. The analyzer is showing about .065% THD. Note that I used 43K as the plate resistor and 6SN7's as the tubes.

Here's the AC Analysis plot:

http://diytube.com/phpBB2/album_pic.php?pic_id=23

Frequency response in the top trace looks pretty good. Starts to roll off on the low end at about 10 Hz.

Obviously this is a low gain amp so it might not be too good for high volumes or hard to drive loads. Hope this analysis is interesting for you guys. Removing that 37.4K resistor on the plate of the third tube did the trick Eric.

I've also done an analysis on Shannon's ST-70 driver and Tom, on the 832 amp driver. A pic of my 832 amp is at left. Maybe I can post those if you want to see them. I don't want to hijack the thread here. Anyway, the 832 driver is very good indeed. That's probably why you say it's your best sounding amp.

Now all I need is some good models for ST-70 output transformers or some Hammonds and we'll be all set!

Regards,
Ed Long
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Postby sorenj07 » Fri May 11, 2007 8:09 pm

Wow, thanks for the in-depth analysis! Maybe there is still hope for my 63 ohm Sony MDR-V6's after all. One TubeCAD-related question - how much should I read into the "Vo max" (voltage swing) tab? Here's an example of what I mean:

Image

I chose the 14K anode resistor so that there was a bit of headroom. Output impedance and headroom both go down, so I picked kind of a middle ground.

The WCF looks pretty legit but I'm bothered by that 0-.18V swing. My headphones aren't 32 ohms but I'd like reasonable performance down to impedances that low. In fact, raising the load resistance to 63 just doubles the swing to 0-.36V. Huh? Would this circuit really feed DC into headphones?
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Postby TomMcNally » Fri May 11, 2007 8:58 pm

OK guys ... here is a quick recap of my real world analysis ...

The frequency response and distortion into my Boonton analyzer
is about what Ed found ... .09% distortion at 100/1000/10000
with 1/2 volt input and no load ... slap a 500 ohm load on it
and it goes to about 20% distortion. This is with 47 mfd caps
on the output. I also removed the cathode resistor on the
first stage of the CF so it looks like a standard circuit.
That didn't seem to affect anything (listening)

I think I'll put RCA's on the output and use it as a preamp !
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Postby sorenj07 » Fri May 11, 2007 11:06 pm

Wow. I definitely want a headphone amp, though, so I'll keep tweaking this thing. Either that or I Craigslist this thing for a hopefully decent profit to re-start.
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Postby sorenj07 » Wed May 16, 2007 6:01 pm

I keep forgetting to mention this. Have you tried applying any NFB? From what I gather, it's not a fix-all, but it CAN lower output impedance in addition to improving linearity... that's why I drew in a spot for a NFB resistor. I'd probably just use trial and error for a decent value.
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Postby sorenj07 » Thu Jun 14, 2007 6:28 pm

Hokay. Major update. After a boatload of hours inhaling sweet sweet solder/lead fumes, I came up with something. Not too unexpectedly, it makes almost ZILCH sound. But, it looks kind of cool while doing it :)

I'll post full schematics as soon as I write them out nicely. In the meantime, it's a White CF per channel driven by a single triode grounded-cathode. All tubes are 7AU7's. Why 7AU7's? they're cheap. Why two filament transformers? I had them lying around when I thought I needed them for monoblocks. The big cap in the middle is also used, if you recall my first somewhat awkward 6SN7 preamp, that was one of the three. Also, please don't laugh TOO hard at my very "special" looking homemade cap clamps for the output couplers...

Details of the White CF:

Top anode resistor = 14K 2W. bottom cathode resistor is 680 ohms bypassed with a 220uF 16V Blackgate and a 2.2uF 63V WIMA. There is a .1uF 630V Solen between the top anode and the bottom grid, and a 1M resistor between the bottom grid and ground.

The output stages branch off from the top-cathode bottom-anode connection. One of them is a simple 4.7uF/47K to the RCA jacks. The other goes to the chassis-mounted caps. 220uF 105C 450V bypassed with 2.2uF 630V Solens, to the 1/4" stereo jack. I have two adjustable feedback pots connected by a 1K resistor to the outputs. They're allegedly 50K linear but I never managed to measure anything higher than 25K from them.

In short - what in God's name do I do now to get this thing working? I'm thinking of subbing in some 6N23P's (they sim better in TubeCAD.) I'll also mess with the cathode resistor and bypass caps, because TubeCAD told me to use a value of less than 32uF...? This thing will be a bit of work especially since it's the highest-density soldering I've ever done.

Some pics:

Outside
Image Image
Inside
Image

(and yes, that is Deep Dish's classic "Penetrate Deeper" album. Cheesy deep house at it's best Yellow_Light_Colorz_PDT_06

If anyone wants detail pictures, let me know..
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Postby TomMcNally » Thu Jun 14, 2007 6:49 pm

Hi Soren -

Nice job anyway ... you'll get it working. I haven't messed with
mine for a few weeks, I need to get motivated and build some
amps.

... tom
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Postby sorenj07 » Thu Jun 14, 2007 11:04 pm

Thanks! I'll draw up the schematic right now. I also ordered some oscilloscope probes off that website so I can do some sine-and square-wave testing. I checked the B+ a minute ago. A steady 297V (I'd simulated 320) for main B+ and only around 230V instead of 260 for secondary B+. Not terrible but I can't figure out why I'm getting such poor results.. Is it something fundamentally wrong with the White CF?

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Postby TomMcNally » Fri Jun 15, 2007 9:53 am

I just wonder if TubeCad comes up with some bogus circuits ?
I bought both TubeCad and SECad to mess with, but haven't
done much with them yet. I have a big pile of my fav Hammond
chassis on the bench, just need some ambition !
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Postby sorenj07 » Fri Jun 15, 2007 11:08 am

Ok, according to people over at diyAudio, the CF CANNOT work with such a high anode resistor. it's supposed to be the inverse of the transconductance of the tube so I assume a value of 470 or so should work? (1 / 2200 microhmos = 1/.0022 = 454).
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Postby TomMcNally » Fri Jun 15, 2007 12:45 pm

Have you seen this headphone amp schematic ?

Image

(here's a link in case angelfire blocks a direct link to the pic)
http://www.angelfire.com/electronic/funwithtubes/Build_Headphone_Amp-2.html

He's using 270K ...
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Postby sorenj07 » Fri Jun 15, 2007 1:05 pm

Well, it DOES look like a real White CF, so that flummoxes me. I changed my WCF's cathode resistor to 280 ohms to suck a bit more current. Plate resistor is now 1K 1W. I also cut the 1M between the bottom grid and ground, and both NFB connections. Unfortunately I'm still getting the same as before - really loud hum, and only the tiniest faintest bit of music when plugged into my EL34 Williamson with gain all the way up, from either output. Nothing through 32 and 63 ohm headphones on the headphone out.


STUPIDITY EDIT

I wired the input to the potentiometer wrong. It don't get much stupider than that. As soon as I fix, I anticipate at least SOME music.
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