I found the caps and headphone jack at Newark and got them this am. Put in the last 6 caps and the headphone jack...funny thing I was so much in a hurry to play with this
Millett high gain amp I soldered the caps and thought I soldered the headphone jack, which was a nice tight fit into the pcb holes but didnt!
As I was trying out headphones the jack cam out!! Ha!
So much for being careful!
Back downstairs to the shop and soldered that puppy in....
The op amp used in Pete Millett's design is a OPA552 which are optimized for gains of 5X or higher. The amp is set up with a 11x gain.
I dont ususally like to use anymore gain than is necessary to drive headphones but this was an experiment to see how this amp performs.
WIth low impedance cans like 35 ohms you cant hardly touch the volume control to get a really loud level in these cans!! With 600 ohm T1's the volume congtrol is about 9 oclock...definitely high gain here..
I immediatley went back and soldered some 1K resistors in parallel with the 1K's in the op amp circuit cutting the gain down to 6. I have more range of control and it sounds better to me.
THe 600 ohm cans are now in the 9-11 oclock range...still could stand to lower it more, BUT the op amps are designed to run at these high gains... Pete Millet has stated to run at a lower gain you should use a OPA 551 op amp but doesnt see any real difference in perf between the two.
This is a very inexpensive amp, the parts like the pot is really "cheap" or should I say "inexpensive" and you get what you pay for....I will probably upgrade this to a better alps pot in the future. Its a bit grainy in one position, but its so far away from what the gain allows me to use its not a real issue.
You could also use better caps in certain spots but again probably not going to matter that much..
So how does it sound or not sound?
This amp really liies high impedance headphones..the 600 ohm T1's sound wonderful....open, spacious great soundstage, the sound is slightly warm IMO...Linda Ronstadt's Round Midnight is a melt in your mouth experience!!
One thing with the lower gain the background circuit noise is almost in audible compared to the 11X original gain setup..this is a good thing.
IMO this is a very good pairing for the T1's. Just as good as anything else I have in house at the moment..its amazing how simple can be very good if designed well...and Pete Millett has a nice entry level amp here.
Here is a picture compared to the new Burson FUN amp I recently was overwhelmed at how good it is:
The Butte has about $45 in parts, $15 for a 12VAC 1 Amp adapter....and you can get a case for $40 from Pete Millet that has all the holes drilled out and labled! So for about a hundred bucks your home free...yes there are so many other options like a Schitt Magni 3 ( B stock for $89)....but you dont get the DIY buzz I like so much...
To me its so neat to see the pcb and the parts right there looking at you..lol.
Ok so whats next to build!!
Oh heres the specs for the Butte:
Max output leven into 150 ohms @ 1kHz: 7.2V RMS / 20V P-P / 340mW
Max output level into 33 ohms @ 1kHz: 5.0V RMS / 14V P-P / 750mW
Peak output current: 200mA
Output impedance: 3 ohms
THD+N 1V 1kHz 150 ohms: 0.003%
THD+N 1V 1kHz 33 ohms: 0.005%
FR: 20-20kHz +/- 0.1dB
FR: -3dB DC - 240kHz
Noise 400Hz - 30kHz: 30uV
Max gain 1kHz: 20dB
Dimensions: 4.3" W x 5.3" D x 2" H
Read more at https://www.innerfidelity.com/content/diy-beauty-pete-milletts-butte-headphone-amp#ym8zPsihIm2vJb56.99