Apheared
You call that an amp? THIS is an amp!
- Joined
- Jun 20, 2001
- Posts
- 377
- Likes
- 10
(God only knows what the actual count is)
It's 2am, this is gonna ramble a bit.
A few people have stuck hansen's pcb in the ET version of the HML-9VB PacTec case, to fit 2 9Vs in there. That's a killer idea, but the hansen board still takes up as much "floor space" and limits what caps, jacks, and pot you can use.
I decided to make an HD600 portable amp. Not that I really like HD600s or plan on walking around with them, but hey it's a project.
To be different, instead of laying the board flat, I made it a vertical board... look at all the room. And since there's no "bottom" to the board, I can stuff both sides.
One chip I really like the sound of on HD600s is Analog's AD823. It's a beautifully designed chip, although 15mA isn't much drive for our needs. It's a true single supply opamp (although I didn't use it that way) and can swing like the dickens because of it.
I was going to buffer the output with a low power discrete stage... 2N3904/06s or BC549/559s, or BC317/337s... but ppl graciously sent me some 100mA buffers from Elantec to try, the 2001s (baby brother to the 2008s I love)
The best thing about all this: battery! The 823 is 6.5mA quiescent for both channels, the 2001s are 1.3mA each quiescent, and a TLE2426 isn't even worth mentioning. Even with a 1mA CRD per channel, the total is under 12mA! Compared to just one dual OPA132/4 and a resistor voltage divider... sweet.
Driving HD600s I doubt you can get the drain to 15mA. You can try.
An added bonus of the 100mA buffers is it will drive lower ohm phones, although say goodbye to your 9Vs pretty fast... an average rockin volume on Grados is 50mA.
I wanted a gain around 12 and the parallel of the feedback to be 300. 332 and 4k7 ohm worked close, 308 ohm parallel and gain x14. Add a 301 ohm +input resistor and the offset for both channels is under 0.8mV - It's not bipolar input and this is really overkill, but why not?
The 2426 doesn't really need the 470uF Cerafines, it's output is under 1/100th of an ohm to like 1k, and 20k is still like 0.5 ohm. For just driving HD600s, you can completely omit the reserve caps. I'm serious, try it. You can't hear the difference (but you can measure it, more on this below) The 470s/1s are for "cheater current" driving lower ohm loads. There is a 22uF stablizer cap in front of the 2426, again just because.
Since this is for HD600s that I will probably not use portable that much, I added a power jack for an external 24V filtered wallwart. Both the batteries and AC in go thru 4001s in a diode OR gate config. (only the higher voltage will get passed). To add more safety to the thing, the AC jack is switched and the battery snap ground goes here. When the plug is inserted it lifts the battery ground, so diode gate or not - nothing is flowing to/from batteries.
The silliest part - a huge Noble AP series potentiometer is in there! heh. Took over half the faceplate space. Someone recently posted pics of a cmoy he built for $20... well, this one ain't. The pot was that, alone.
One thing re: addiction... I have gotten a little obsessed with this, as most of you know.
So here I am with a scope trying to find the perfect output resistance to stop the tiny ring/overshoot. But scopes are dangerous. What looks like an insanely large overshoot on 10mV/div isn't audible. And with resistors on the opamp outputs, it adds distortion and shifts the phase. By MINISCULE amounts, but there ya go. I could visually see the tone generator's pulses looking different... from overshoot and a little ring, to mostly flat, to softened corners... depending on the resistance used. In this case with HD600s as load, the closest to perfection was 66 ohms. 61.9 went in, and that's what it's stayin at. But truthfully, I can't hear a difference. But I can measure it. Damn measurement tools.
All in all, it sounds great! It's a cooler chip, and on Grados the treble might be too agressive... but I didn't make it for Grados.
edit: duh, photos... ok so it's 2am did I say that?
It's 2am, this is gonna ramble a bit.
A few people have stuck hansen's pcb in the ET version of the HML-9VB PacTec case, to fit 2 9Vs in there. That's a killer idea, but the hansen board still takes up as much "floor space" and limits what caps, jacks, and pot you can use.
I decided to make an HD600 portable amp. Not that I really like HD600s or plan on walking around with them, but hey it's a project.
One chip I really like the sound of on HD600s is Analog's AD823. It's a beautifully designed chip, although 15mA isn't much drive for our needs. It's a true single supply opamp (although I didn't use it that way) and can swing like the dickens because of it.
I was going to buffer the output with a low power discrete stage... 2N3904/06s or BC549/559s, or BC317/337s... but ppl graciously sent me some 100mA buffers from Elantec to try, the 2001s (baby brother to the 2008s I love)
The best thing about all this: battery! The 823 is 6.5mA quiescent for both channels, the 2001s are 1.3mA each quiescent, and a TLE2426 isn't even worth mentioning. Even with a 1mA CRD per channel, the total is under 12mA! Compared to just one dual OPA132/4 and a resistor voltage divider... sweet.
Driving HD600s I doubt you can get the drain to 15mA. You can try.
I wanted a gain around 12 and the parallel of the feedback to be 300. 332 and 4k7 ohm worked close, 308 ohm parallel and gain x14. Add a 301 ohm +input resistor and the offset for both channels is under 0.8mV - It's not bipolar input and this is really overkill, but why not?
The 2426 doesn't really need the 470uF Cerafines, it's output is under 1/100th of an ohm to like 1k, and 20k is still like 0.5 ohm. For just driving HD600s, you can completely omit the reserve caps. I'm serious, try it. You can't hear the difference (but you can measure it, more on this below) The 470s/1s are for "cheater current" driving lower ohm loads. There is a 22uF stablizer cap in front of the 2426, again just because.
Since this is for HD600s that I will probably not use portable that much, I added a power jack for an external 24V filtered wallwart. Both the batteries and AC in go thru 4001s in a diode OR gate config. (only the higher voltage will get passed). To add more safety to the thing, the AC jack is switched and the battery snap ground goes here. When the plug is inserted it lifts the battery ground, so diode gate or not - nothing is flowing to/from batteries.
The silliest part - a huge Noble AP series potentiometer is in there! heh. Took over half the faceplate space. Someone recently posted pics of a cmoy he built for $20... well, this one ain't. The pot was that, alone.
One thing re: addiction... I have gotten a little obsessed with this, as most of you know.
All in all, it sounds great! It's a cooler chip, and on Grados the treble might be too agressive... but I didn't make it for Grados.
edit: duh, photos... ok so it's 2am did I say that?