A "Darling" Headphone Amp?

sweet & juicy SE amp for 1626 Darlings and the 6L6 family

A "Darling" Headphone Amp?

Postby Thermion » Sat Feb 04, 2012 10:16 am

Last winter I bought a pair of Grado SR225 headphones at a "going out of business" sale at a local audio store. I bought them to use with an effects processor so I could practice guitar without disturbing my wife. They work well for this purpose. But I got to thinking it would be a good tube project to build a headphone amp for them. I also wanted to see if I could use some of my 1626 tubes for the project. A nice triode in a ST bottle seemed like just the ticket. So I went to Google and started looking for ideas. I came across this paper by Lloyd Peppard at Mapletree Audio Design.
http://hollowstate.netfirms.com/earplusdesign.pdf

In this paper he discusses his design philosophy for the MAD Ear+ headphone amp. He uses a 5751 GC directly coupled to a 12B4A in parafeed mode. The output transformer he uses is the Hammond 119DA. This is an impedance matching transformer designed for the Collins radio crowd. It is meant to match the 600 ohm output of the Collins radios to modern 4 and 8 ohm speakers. It is rated at 12 watts from 30 to 20khz. The Grado headphones are 32 ohm. With this load connected across the 8 ohm tap, the reflected primary impedance is 2400 ohm and the turns ratio is 8.66.

I decided to base my headphone amplifier on his design using a 12SL7 and 1626 and the Hammond transformer. So I did some TubeCAD simulations and built a prototype of the circuit for testing. Here are a couple of pictures of the setup. This one shows the whole setup on my bench. Tek Oscope, Boonton, HP multimeter and Heathkit high voltage power supply.
Image
And this one is a close up of the circuit prototype.
Image
After trying out some different component values I settled on the same values that Peppard used. I guess he had it tweaked pretty good. The only changes I made (other then the tubes) was to increase the B+ from 200 volts to 260 volts. This results in more current through the 1626 (20ma versus 15ma) resulting in a little more power out. And I used a 10 uF film and foil coupling cap instead of a 30uf electrolytic.

With this arrangement, the prototype performs very well. My 1% THD at 1khz output was 52 miliwatts. At this level the frequency response and THD curves were very flat. I was down 0.25 db at 20 hz with 1.4%THD and was down 0.56 db at 20 khz with 0.38% THD. This level was achieved with a 370 mv input and overall gain was 11db so this amp could be driven by computers, phones, ipods, etc.

The Grado SR225's have an impedance of 32 ohms and a sensitivity of 98 db with 1 miliwatt input. So the 50 miliwatt output represents about 115 db in the ear. That should be plenty for most everyone.

I am planning to build this amp using Shannon's Clementine board. It seems perfect for this application. The board is already partially stuffed. I'm ordering the rest of what I need today including a Hammond P-T269EX power transformer. The Hammond 119DA are available from Antique Electronic Supply for $26.20. I got mine from Mouser where they are closer to $30. Either way, a pretty good deal. I'm hoping to have the amp completed in 2-3 weeks.

JT
Thermion
 
Posts: 181
Joined: Mon Jul 07, 2003 6:04 pm
Location: Fisher, IL

Re: A "Darling" Headphone Amp?

Postby Thermion » Tue Apr 13, 2021 6:02 am

Geo,

I posted to this thread to being it up to the top. This is the prototyping method I was describing in the PM.

BTW, I completed the project and it is an amazing headphone amp for Grado 32 ohm headphones.

Jonathan
Thermion
 
Posts: 181
Joined: Mon Jul 07, 2003 6:04 pm
Location: Fisher, IL


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