Joe Davisson's new Guitar amp

[Joe]

Pictures of the build, which is the result of a complete lack of planning:

http://www.diystompboxes.com/analogalchemy/tmp/amp3.jpg
http://www.diystompboxes.com/analogalchemy/tmp/amp4.jpg

I didn't use a feedback loop. A guitar is enough to make it distort just slightly when playing hard, but does so in a crunchy kinda way (the point of using that particular input stage, it can't really hard-saturate).

I estimated 28W, and it sounds about that volume when maxed. The preamp is going to be a 4-stage Vulcan type circuit with 3-band + presence.

[Joe]

I settled on a 10k volume, but I agree it's not the best situation. The preamp will have a buffer before the tone stack, hopefully that will help.

Owell, it's a cheapskate amp but was a good learning experience.

[joecool85]

I still think it's a really cool thing.  As much as I like chip amps, discrete circuits are just so damn cool.

[teemuk]

Getting the thing to work with such long wiring tells something about the stability of the design. I like the way you mounted the emitter resistors.

[Joe]

Went back to the drawing board (not complete):
http://www.diystompboxes.com/analogalchemy/tmp/amp5.gif

Basically a giant opamp. Probably needs some capacitors here and there. Automatically adjusts for rail voltage, just need to adjust the Vbe multiplier. Thoughts and ideas welcome.

[teemuk]

Current mirror to LTP (depends on how well balanced the LTP is already), Zobel network to output, VI limiter? I don't see a limit for DC gain in the feedback path?

No high frequency compensation? Dividing this for both VAS (Miller cap) and the feedback loop yields better results than using the Miller cap only. Do not use too high value in the loop or you get oscillation.

Well that's about it. This reminds me some Rickenbacker amps - they also used an inverting LTP configuration.

I see you run this on a simulator: Replace those constant DC supplies with a sim of a real supply (at least for the part of rectifying, filtering and DC resistance of the source). Replace the constant load resistance with a Thiele-Small model of a speaker. You will get a lot more realistic results by doing that.

[Joe]

few changes, but not done:
http://www.diystompboxes.com/analogalchemy/tmp/amp6.jpg

The fuse is there as a reminder that it needs output protection of some sort. Will probably put this on the back burner for awhile.

[teemuk]

I assume the correct link is:
http://www.diystompboxes.com/analogalchemy/tmp/amp6.gif

Edit: Looking good. C8 seems like an unneccessary component as it's in CCS. You can't utilize Miller effect there so the value should be quite large to have any effect on high frequency roll off. I'd leave slots for C5 and C6 on PCB but for starters would try to get rid of them. As the rail voltage is pretty high you're really pushing the limits with just one pair of output devices.

For anyone interested, a very good information source concerning VI limiter design is found from US patent 7,061,740, System and method for protecting an audio amplifier output stage power transistor, Eric Mendenhall.

[Joe]

http://www.diystompboxes.com/analogalchemy/tmp/amp7.gif

I planned on doubling up the outputs but it's shown now. I raised the 100 ohms back to 470 ohms because of the higher rails. The output centers up better (at least on the simulator) this way.

I decided to build it (nicely this time, heh). Should get about 100W, give or take depending on what values are used for the 470 ohm resistors.

Thanks for the correction and other information. Will check into adding a limiter to it.

[Joe]

Built and tested this:

http://www.diystompboxes.com/analogalchemy/tmp/newamp.gif
http://www.diystompboxes.com/analogalchemy/tmp/clip.gif

I have to rebuild it for several reasons, but the initial test was a thumbs up. This amp sounds great (check out the soft-clipping on the simulation).

The 2nd stage is prone to thermal runaway so requires good heatsinking. The output is centered with the 10k trimmer, and the quiescent current with the 1k trimmer, like most other amps. The part running off the bottom is the short-circuit protection circuit. Negative feedback is through the 470k resistors.

Edit:
I don't have a scope so I can't do much more testing than that. 4 ohms didn't really work in my proto (hum/oscillation), but I mounted the transistors/board pretty close to the fuse/power transformer. The heatsinking was probably inadequate anyhow. I also think better transistor choices can be made than the relics I was using.

[XinTX]

Is it just me?  None of the links work. 

[joecool85]

Nope, its not just you, it seems all the links are down.

[J M Fahey]

Hi friends. I got interested/curious about this amp, after seeing that Joe is back at the drawing board, but none of the 15 or 20 links works. Can anybody re-post or re-link them?
Thanks.

[sergscar]

Hi guys !,
I've browsed the entire site, and seems to be the directory (and the files) have been removed from the server.

At the present stage, the only possible solution should be that some of our friends post the files/schema in some place, and link it here. Obviously, if someone copied/downloaded them !

Rgds,
Sergscar

[Joe]

Sorry guys, my computer was ruined in a flood, so I can't replace the images. The good news is that they probably weren't worth looking at anyway