More progress on the head phone amp. I've been busy wiring and soldering. The power supplies are providing power, the inputs are wired up, and heat sinks are drilled, tapped, and screwed in place. Absolutely no drama. I adjusted the power supplies for +/-15VDC and that was that. Nothing gets warm either. Hopefully this will turn out good. I'm using silver wire with teflon insulation for most of it.
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OK, It's all wired up now, but I have a problem with it. It plays fine, but there's no bass. I don't understand that. There's no coupling caps in the circuit at all, so no RC filter. The transformers are supposed to be good to 10Hz. They are loaded by 10k ohm pots, just as required by Jensen. The DC offset on the output is about 1.5mV per channel, so I'm not sure that would have anything to do with it. I'll have to do some more investigating to figure this out. I used my old Sennheiser HD540's to listen. 
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This is the schematic for the amp. It's bone simple to me. I added the two small capacitors and in the process deleted a 100k resistor from input to ground. The input is directly connected to the wiper of the 10k pot. There is some scratching when ever I move the volume knob, but I guess that's normal with this arrangement.
Maybe I need to let it run for a while to get all those power supply capacitors formed? I dunno what's up... feeling like I'm
Maybe I need to let it run for a while to get all those power supply capacitors formed? I dunno what's up... feeling like I'm
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I have a theory that I bought the wrong power transformers. I got secondaries with 2x15VAC, so since I don't know the dropout for the regulator, I may not be able to get +/-15VDC out of them. I turned the regulators down to +/-14VDC but haven't had time to test them. I measured about 4 volts across the series power transistors in the regulators after dropping the outputs by 1 volt.
Thanks for the support.
I figured out the problem. It was not the power supplies after all. I wired the potentiometers incorrectly.
Then I found that I had way too much gain. So, I swapped the 20k resistor for a 1k. Still a lot of gain, but I guess the HD580's are fairly sensitive. The amp does sound really good now.Attachments
Yeah, thanks. I was thinking of putting some kind of high value resistor there. I'm just glad I got it working now.
No, I'm sorry I do not. I ordered the boards from Jim's Audio:
eBay - New & used electronics, cars, apparel, collectibles, sporting goods & more at low prices
they are gold plated. Seem nice to me.
I don't use Eagle, but there are PCB layouts in this document:
http://www.ti.com/lit/an/snaa052/snaa052.pdf
It's the manual for the LME49600 evaluation board.
I've found that when using standard digital sources, that you don't really need any gain at all in a headphone preamp and this is with HD650s and HD555s, so with a relatively low and then high impedance pair of cans. Both can blow my brains out with the gain set to unity. I am not surprised that you found you needed to reduce the gain.
Thanks. I have a solution for my particular case. I can replace the 1:1 input transformers with 4:1 step down. It's a shame to throw away voltage, but I see no other good choice in my case.
I think the 100K is there for safety , if the input goes open for any reason, the output of the LME49600 goes to the rail and then you would need the protection. This is for the original Ti design.
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