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Messages - Carriage

Here are some samples. The first file is the yamaha thr on its own for reference (usb out). The difference between the other two is the feedback resistor in the first stage. The second gain stage isnt included yet. The tonestack pots are somewhere vaguely pleasing. Guitar is a strat bridge pickup

I think I'm reasonably happy with it at lower gain but at higher gain there's this fizzy distortion on top. I think that may be the op amp distorting (based on some Spice sims) so I'm thinking about putting a unity buffer in front of some other clipping diodes in the front as a boosted/hotter signal is going to make it worse. Not sure yet though. Maybe some filtering would be better but I think I'm struggling with no headroom and then the OP07 isn't rail to rail to make things worse.

It's also unclear how much the Yamaha is helping. Still waiting for the 8302 board.

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So I added the tonemender tonestack to the breadboard and with the gain down in the first stage it sounds like advertised.

With the gain up I still wasn't so happy with it. At very specific gains it sounded okay but past that it sounded kind of like the clean with a fuzz/noise behind it. I don't know if the analysis is leading me to think that but it's not particularly pleasing.

I put the clipping diodes after the tonestack to not much difference. I then removed the ones in the first stage and it's  heaps better to my ear.

There's still a few things I want to fiddle with plus I realised the op amps I've bought aren't designed for single sided supply, though they're working. I'll try and get some sound samples too when I get some time
It wasn't amazing. It's just the first stage into the flat channel on my THR. I'm thinking I perhaps instead play with the tonestack first to get a pleasing enough clean tone and then add the gainy tone on top.

This all assumes the THR's flat channel is going to be similar to the PAM8302. That's still in the post and I need to get my speaker cab back from a friend too.
Parts have arrived from tayda.
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Quote from: joecool85 on July 14, 2022, 12:03:52 PMAlso, I'm not a fan of a Tube Screamer as preamp but if it sounds good to you, there isn't any technical reason not to do it.  As a good starting point, I recommend something more like the Marshall Gov'nor:

The choice of that architecture is semi arbitrary in that lots of people like it as a pedal. I'm actually not a huge fan but that's more because I don't like the frequency shaping they do, but that's a filtering question not the clipping itself as far as I can tell. Guv'nor looks interesting I might try that out too. Really I need to have a play with things on a breadboard to see what I like the sound of. I was just starting with it more as an exercise in amp design at no cost in simulation

Quote from: Loudthud on July 14, 2022, 02:56:00 PMIf you look at the output simulation @Carriage posted in reply #2, you see the top of the wave is just the input signal added to a more of less square wave, so it looks like soft clipping.
Another way to think about this is that when the diodes are conducting the voltage drop across them will be roughly the forward voltage. Given they tie the other op amp input terminal (which should equal the input) and the output, the output will be input + V_f. High gain will make the part of the wave they're not conducting steep.
Quote from: tonyharker on July 14, 2022, 03:06:42 PMJust to clarify the PAM8302 are not made by Adafruit.  They are made in China and available from ALI Express for cents not dollars if you are willing to wait weeks for delivery.
Yeah, I actually ordered a clone off ebay which will take a little while to get here. In the meantime I'll probably use the flat channel on my yamaha THR if my other parts arrive first.
I may have been mistaken about one of the schematics I found, so I've sorted that out
Thanks for the info.

I was under the impression that preamp distortion was typically before the tonestack. Is that incorrect? No headroom/not much room on gain knob before it gets dirty does make sense. Given parts are cheap I might give it a go anyway so I can learn what it sounds like.

I can find a 24VAC 1A supply locally. However, I was looking at 9V for a couple of reasons. First, the power amp is 5V supply, 1.3W (1%THD) at 8ohm and with a input sensitivity of about 300mV so I thought that this is quite a limitation on the headroom anyway. Second, its kind of fun to get something running off the same power supply as the pedals. Third, not that I cant afford to buy a wallwart it's another cost. It was something I was thinking about if I build another amp.

With the Marshall lead 12 I've found two different schematics. One that seems to use op amp distortion and the other clipping LEDs. Is there one you meant in particular or are they both of interest?
I've heard it described as soft clipping but yeah it's not really clipping per se. It does give what ends up looking like crossover with large input signals iirc but relatively smooth for lower inputs.

What makes you say the TS doesn't make a good preamp? Do you have alternative suggestions?

Interestingly the input sensitivity for an 8ohm load is only about 300mV if you want it to stay clean (if I calculated correctly). Apparently that's within the realm of hot humbuckers so it's sort of more putting a pedal in front of it rather than needing a real preamp.

Attached is the output of that stage with varying gain with a 100mV input signal
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Amplifier Discussion / Adafruit PAM8302 Amplifer
July 12, 2022, 04:22:47 AM
I've built a couple of pedals in recent times, an 8" speaker cab plus a ruby amp and have been thinking about building a more complex amp but not too keen on doing mains powered at this stage. From a comment by joecool85 in a honey amp thread I found out about the PAM8302 board by adafruit and have been playing around in LTSpice to build a preamp for it.

Basically it's tubescreamer type clipping but with filter rolloff at around 10Hz and 10kHz. R28 is a gain pot (500k) This is followed by a runoffgroove tonemender tonestack, which I think is a modded Fender/Marshall/Vox. Pots are R7/R8, R9 and R10. Then some clipping diodes to soft limit if there's a big signal coming in and then a 5x boost in case of small/clean signals. R14 is master volume.

I have no idea if it's going to sound good but I'm currently working up a parts order to build a prototype. My current thinking is maybe to breadboard it but maybe build the various modules/stages on stripboard with some sockets for components and screw terminals so I can swap modules out.