I have one guitar that belts out a huge signal from the neck pickup. I was able to make the first stage just start to distort. To correct this, I have changed R5 to 1K and R3 to 4.7k also I have un- bootstrapped R4 and directly connected to ground. I thought the bootstrapping was a good idea – I must have eaten too many Twinkies that day. C30 and R29 are still optional but could be used to give treble boost or hi/lo gain boost etc.
If you have already obtained the earlier board rev, you need to cut the trace from the emitter of Q1 going to R4 and ground that side of R4 to the ground plane.
I am posted a link to the new board rev here and obsoleting the schematic and layout in the old post. All I did was eliminate this trace and add a couple of wire connection pads to allow making R29 a POT for variable gain. If you have the old board it will work just fine – with the mod described above.
Updated Schematic:
https://circuitsaladdotcom.files.wordpress.com/2013/03/portaamprev1-1-1.gif
Updated Layout:
http://www.fileswap.com/dl/EDZsklrqcT/
Couple other minor notes: C16 may need to be increased to 22pF or so if you have a sluggish crystal – had one out of five that needed the bigger cap. Also with the mods above you may find that C21 should be increased to .22uF or more depending on the range and level of treble boost desired.
Circuit Salad 
[…] https://circuitsalad.com/2013/03/27/more-amp-updates/ […]
My favorite version of the battery amps is the stomp amp – I would look at that – at least the tone stack for that one. I use linear pots for the tone – so center for the pot is not flat frequency response but for guitar the controls at mid works out well for a relatively flat guitar sound. The other pots are as you described in your comment. I’m not sure but you can get Fv-1’s from: http://www.experimentalnoize.com. I bet they will ship to you. I highly recommend the FV-1. Also a Belton brick ( which uses pt2399’s) may be a good choice. Make sure your speaker is 8 ohm or less. At 8 ohms and 14.4V supply …I think you can get about 13 watts out. At 4 ohms you can get the 20 -25 watts. Any of the power amps I used will work(as well as others). My recommendation would be to mod my stomp amp design to have the presense on the driver and use the output power amp that you have available.
The darlington version works okay but I had some headroom issues with certain guitars and a little bit of noise I didn’t like.
I will post an updated link for you!
Hey, thanks for the schematics and an awesome blog! I got a subwoofer casing from an old PC speaker set, and I bought a 6″, 25 W guitar speaker element with your earlier design in mind, but now I’m going to do a hybrid of these two, using the updated preamp circuit up until the C20 (in old version) / C22 (in updated version) and R22 and then the original design from that point on, since my element can’t handle the 60 watts the TDA7396 gives out and I still have access to a TDA7350 circuit (which should be the same as TDA7360, but without the clipping detector). The reverb IC is also a bit hard to find in Finland, so I’m going to replace the reverb part with a PT2399 based reverb design, since that IC is sold in a local shop. Any thoughts? Do you think it would work like that or is there some fundamental flaw in my chain of thought?
And then more importantly, a question about the pots… I read through the three posts about this amp, but I couldn’t figure out whether I should use logarithmic or linear ones. My guess would be that the main volume pot should be logarithmic, since it’s about dB’s and probably the presence and reverb level pots linear, since they just mix two signals together but I’m not sure. Am I correct with my highly scientific analysis? How about the bass and treble pots?
Thanks for your time!
Antti
I can’t find the layout on the fileswap link. i get a Frontier Communications site search engine. I am not sure what to put in the search box. Is this link still good? Looks like this info. was posted a while ago.
Thanks.