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OK, you might want to revise the instructions since they are incorrect. As written they currently read:

Sixth, solder diodes D12 and D13 and then solder resistors R5 and R6 followed by capacitors C15, C16, C17, C18, C19, and C20. Now solder green LED D14 and resistor R7 followed by terminal block K4. Connect a transformer or variac to the terminal block K3 again and measure the voltage produced by the left channel of the power supply (at the terminal block K4) as a whole. If you applied 12 Volt AC to the terminal block K3, the power supply should deliver about 17 Volt DC at the terminal block K4.

This error is duplicated in the instructions for both the left and right filament supplies in the Frigate manual.

Yes, this is a typo - it should read 7 VDC not 17 VDC. I uploaded the new manual.

I completed it and it works!!

I have been listening to it all day. I can't hear any hum or hiss even with the volume knob cranked all the way. Thank you so much for creating something that is so satisfying to put together and sounds so good.

I am driving it with the line output from a Meridian Explorer headphone/DAC and listening via Sony MDR-7506 studio monitors (63 ohm impedance). The difference between the Meridian's built-in headphone amp and the Frigate was really surprising to me. Where one might expect the transistor amp to be cold and the tube amp to be rounded and warmed over, I found the opposite. The Frigate has a very precise, clean sound, and the Meridian sounds warmer and more weighty by comparison. The fact that I'm using the same DAC in both cases helps pinpoint the differences. I think maybe the Frigate's sound is actually more accurate than the Meridian's amp.

I am not sure if the bass response/warmth differences I'm hearing is due to an impedance mismatch between my headphones and the Frigate, or if I've just become accustomed to a more bloated sound. I am tempted to try adding a resistor to see how that affects the sound.

I know the Frigate is an OTL amp, and maybe this is heresy or just a terrible idea, but I'm also intrigued by the idea of using a transformer to better match the output. Sowter has some that appear to be designed for this sort of thing. I do not want to second guess your design which is clearly very, very good but I am curious.

Oh, and what kind of fuse should I be looking for?

First, you need to give those 6922 and 6H30PI tubes a break in period of about 100 hours. Once the the tubes and capacitors settle, with proper headphones, Frigate will provide very warm sound.

Second, while Frigate has enough power to run low impedance headphones, it is designed to run a headphone with impedance of 300 Ohm. Frigate was tested with Sennheiser 600, 650, 700 and 800 and really shines with these headphones. The mismatch of impedance between your Sony MDR 7506 and Frigate certainly plays a significant role in the quality of the sound you hear. Borrow Sennheiser 650 and you will feel the difference.

Third, I would not recommend using a transformer to match the impedance of MDR 7506 and Frigate. What you need is a good set of 300 Ohm headphones.

OK, the sound has changed quite a bit since I first turned it on, it's had maybe 20 hours so far so I'll see how it goes before changing anything.

The Sennheisers look good (and expensive), but they all look like open back designs. This amplifier is intended for use at a cubicle at work so I am concerned about annoying coworkers. I don't know anyone to borrow a pair from. Will 250 ohm or 600 ohm headphones work as well, or do you have any recommendations for closed back headphones that will let me get the best out of my Frigate?

ironically one of the goals of this project was to avoid buying new headphones, I looked at the power output but didn't think about impedance mismatches. As the other major goals are to learn new things and build a reference quality headphone system for comparison to future projects, I guess buying new headphones for myself is OK.

However I am also thinking about building these for other people as gifts and my family has at least a half dozen pairs of the Sony monitors so I may still explore some sort of switchable transformer or resistor to help in those circumstances. I understand it won't be the best (I'll have my fancy new headphones at least) but I'd like the Frigate to sound as good as I can make it in those situations. Any advice on that score would be greatly appreciated.

I would recommend Sennheiser 650 as it worth every penny.

To match MDR 7506 with Frigate you need to find an audio output transformer that matches 300 Ohm with 60 Ohm. Have a look what Hammond Manufacturing offer on their website.

Does the Hammond 560G look like it will work?

No, it has wrong input and output impedance. You need a transformer with input impedance 300 Ohm and output impedance 60 Ohm.

OK, I am still learning how to find parts. How about the 101P?

No, it is a telephone industry transformer that will cut off the low frequencies. Also, it is a very low power transformer and you fry it if you connect it to Frigate.

This is is your project and you have to figure out what you want to do and with what parts. Once again, you do need to use any output transformer with Frigate. You need a set of high quality 300 Ohm impedance headphones.