Joe Davisson's new Guitar amp

[Joe]

A little update on the amp project:

Have been doing a lot of simulation work, and it looks like I can make the "diode compression" trick work with the Depalma amplifier. This will be a very easy build (~40 parts), but I'm pretty sure it needs to be capacitor-coupled because of possible DC offset problems.

If the extra peak compression is worth bothering with (the Depalma amp already soft-clips) then it's very easy to make this switchable, or even variable.

I've made a cheap 12" combo test rig & chassis for the project already, just need to build the thing when I get some parts in.

Sorry again about the files, will keep them available in the future.

[joecool85]

Keep us updated, I'm pretty stoked about this little guy.

[Joe]

This is the latest circuit, which "probably" will bias itself alright. Went back to the basic circuit, which is probably good enough for government work.

It looks like a strange amplifier, but in reality it's not that much different than anything else. The 1uF/4.7K is a positive-feedback network which increases gain.

Different supply voltages are easily accomodated by changing both 22K resistors to be around the value of the supply voltage. So a 50-0-50 volt transformer would use 47K, etc.

[J M Fahey]

Hi Joe.
What would the "diode compression trick" be?
Thanks.

[Joe]

This is how "diode compression" is added to the amp. It does a similar thing to this pedal:
http://www.diystompboxes.com/analogalchemy/sch/vulcan.html

The first waveform shows the amp with the diodes disabled (shorted with a jumper) being heavily overdriven at the input. The second picture shows the effect of the diodes. The amount of peak compression depends on the value of the 680K resistor, which in this situation can be from 470K - 1M.

The diodes are biased so that they aren't quite on, but not quite off either. In this situation, the diodes resist signal peaks as they rise, producing the more curved output.

[J M Fahey]

OK Joe, thanks.

[Joe]

Posted a better copy:
http://www.diystompboxes.com/analogalchemy/sch/diodepwr.html

[Joe]

Ok, I'm getting ready to make this, but don't really know where to get a 50VCT transformer. Would a 50VA transformer be enough for 25 audio watts? Thanks.

[joecool85]

Ok, I'm getting ready to make this, but don't really know where to get a 50VCT transformer. Would a 50VA transformer be enough for 25 audio watts? Thanks.

50VA would be enough, but it's always better to have more than not enough.  I'd try this one:

48VCT, 3amp (144VA) - $28
http://www.parts-express.com/pe/showdetl.cfm?Partnumber=120-225

[Joe]

Thats great, thanks!

[joecool85]

Thats great, thanks!

Any time, Joe.  Keep us posted, I love this little amp of yours.

[Joe]

Did some work on the chassis today:
http://www.diystompboxes.com/analogalchemy/sch/dc_wip.html

I found some huge extrusions awhile back, which I cut the heatsink out of. That gets bolted to the chassis with heatsink compound between, sorta making the whole chassis part of the heatsink.

[Brymus]

Hey Joe looking good!
Thanks for sharing ,I am looking forward to seeing the finished build.

[joecool85]

Did some work on the chassis today:
http://www.diystompboxes.com/analogalchemy/sch/dc_wip.html

I found some huge extrusions awhile back, which I cut the heatsink out of. That gets bolted to the chassis with heatsink compound between, sorta making the whole chassis part of the heatsink.

Looking awesome, as always!  Decided against the 48VCT I posted?

[Joe]

Thought I would try it this way, since the transformers are from RS, and only $10 a piece. Also, I think it will bump the output some.