jboehle

I just saw this on The Verge as well, I can't imagine this is going to sell well...

Get off my lawn!

Sounds great! Tried my cheapo Monoprice 8323 first, I get a little hiss at 50%+ volume with those, but I'm pretty sure that's coming from my computer or the pupDAC. Listening now with Senn HD650, I'm jamming!

I bet! I was just trying to make sure nothing was getting too hot (beyond operating specs) while the case was on.

After biasing to 125mV at cold startup, then warming up with the case lid on, ch 1 settled to 104mV and ch2 settled to 100mV. I turned down ch 1 to 100mV. Amp heatsinks range from 45-50 deg C. I am using the 2" heatsinks. The PSU TO-3 cases are at 55 deg C, PSU Opamps at around 50 deg C, amp Opamps at around 35C. Looks to be running good! Now to hook up a source & some 'phones!

Prometheus OST.

Thanks for posting this. Of course I had heard some of their work in the past but did not know who the artists were. I'm really loving this! Also been listening to some of Rodrigo y Gabriela's older stuff, it's all great.

Got the replacement 1N914 diodes installed, DC offset is now less than 1mV DC in both channels with opamps in. Letting it warm up now. Gotta get back to work, will listen to it later. Really hope that's all it was!

GrindingThud, thanks for the suggestion! I pulled the 1N914 diodes out of both channels so I could compare the good to the bad. Both of the channel 1 diodes were bad as you suggested. Both of my channel 2 ones were still good. Luckily my local Rat Shack stocks 1N914 diodes (at least according to the website), so I will be picking some up first thing tomorrow. Thank you so much!

It won't be fun but I guess that's the only way to go. What all should I replace? Just the stuff in the red box? Or should I replace any of the stuff in the yellow or blue boxes as well?

Looks like spritzer has some of the original style psu boards still available:

Okay, I left the amp on for 15 min or so, adjusted the bias on channel 1 to 99mV. I believe I read earlier in the thread that the bias should be 125mv cold, or 90-100mV after warmed up. To get channel 1 to 99mV, I had to turn the bias pot all the way down (counterclockwise), so that was as low as I could get it. On channel 2, I was able to adjust the bias down to 95mV without reaching the limit of the bias pot. This gives me pretty much the same DC offset in each channel, without opamps installed, as I posted last night. Channel 1 = 6mV, channel 2 = 56mV DC offset. I also get the same results as last night if I turn the amp off, install the opamps, turn it back on, then my DC offsets are channel 1 = 60mV, channel 2 = 0.4mV. Are there any measurements I can make to verify each output transistor is working correctly?

So, got the Brown Dog adapters in today for the OPA445AU chips I have. In channel 2 of the amp (the one I did not short out the opamp on), I get 57mV DC offset w/o the OPA445, and 0.3mV DC offset with the opamp installed. Does that seem reasonable? Does the servo usually do that well? In channel 1 of the amp (the one I did short out the opamp on), I get 5.9mV DC offset w/o the OPA445. But I get 64mV DC offset with the opamp installed. That doesn't seem right, does it? That the DC offset would go UP 10x with the opamp installed? Any ideas what else I may have damaged in channel 1? Should I replace the output transistors? Or is this expected behavior when the channel starts with a low DC offset to begin with? I feel like I'm so close to getting this working!

Holy crap, I bet that was scary!

Well I guess it would help if I was using opamps rated for the voltage they are seeing. Next time I need to read the spec sheets a little more carefully. The OPA227P is only rated for +/-18V. Who knows if that is causing an issue as well or not. I have some OPA445AU but ran out of SOIC to DIP converters, so I won't be able to do anything more with this until next week....sigh.