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October 05, 2022, 04:49:33 AM

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#91
Amplifier Discussion / Re: Adafruit PAM8302 Amplifer
Last post by Carriage - July 14, 2022, 06:27:55 PM
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: https://www.electrosmash.com/marshall-guvnor-analysis

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.
#92
Amplifier Discussion / Re: Adafruit PAM8302 Amplifer
Last post by tonyharker - July 14, 2022, 03:06:42 PM
Just 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.
#93
Amplifier Discussion / Re: Adafruit PAM8302 Amplifer
Last post by Loudthud - July 14, 2022, 02:56:00 PM
Quote from: phatt on July 14, 2022, 07:51:00 AMHi Loudthudman,
Just to clarify, D9&D10 are surely a Limiter?  ??? 
I can't see how they could work to impart any kind of Xover dist
on the signal. They would need to be back to back in series
with the signal to cause Xover. Maybe that is what was meant?

You know how the equation for gain of non-inverting opamp is 1+(Rf/Ri). Well that "1+" means the input signal is added to whatever distortion the diodes create. If 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. If the input signal gets bigger, the top and bottom of the output just get bigger. This is not the kind of "dead zone" crossover distortion you are used to seeing, it's kind of the opposite where the area near zero crossing is expanded. Run a Clean Boost pedal into a TS and it just sounds louder and cleaner than it did without the boost, but with a gravely undertone. Not what I want my guitar to sound like.

#94
Schematics and Layouts / Re: Looking for the tiny Orang...
Last post by joecool85 - July 14, 2022, 12:14:42 PM
Quote from: smadin on January 18, 2022, 07:54:56 PMThe great thing about DIY practice amps that run off 9V power is that there are so many to choose from! (The other great things are that they're inexpensive and usually fun and easy to build, of course.)

Off the top of my head:
  • The aforementioned Honey, of course, which I had a blast building. (and I'd definitely be interested in a Honey II with a different IC!)
  • The Noisy Cricket Cabell mentioned.
  • I got started with runoffgroove.com's Ruby, and their Little Gem mkI and mkII are also worth a look. They have some mod suggestions for their designs, as well.
  • Bruce Zinky's Smokey is as simple as it gets, it even omits the Boucherot cell from the LM386 datasheet's "minimal" schematic. (The Noisy Cricket page I linked also shows a couple of simple Smokey mods.)
  • At the other end of the spectrum, Electrosmash's 1wamp crams in all kinds of features. Looks like kits/PCBs aren't currently available, but full schematics, KiCad files, etc., are hosted on their forum.

I'm going to go ahead and revive this thread because I'm interested in a comparison between the Honey amp and the 1wamp.  Could you share thoughts on this?  The Honey amp has very similar circuitry, including the same lm386, transistor preamp, and BMP tone control.  The Honey's tone control is dialed in with different values though, mimicking the frequency response of a Vox AC30 top boost channel.  And of course the Honey uses the preamp for primary distortion rather than the LM386, allowing for use of master volume control while using high gain.
#95
Amplifier Discussion / Re: Adafruit PAM8302 Amplifer
Last post by joecool85 - July 14, 2022, 12:03:52 PM
I'm glad someone else is enjoying playing with the PAM8302a boards from Adafruit - they're cheap, decent quality, and provide 1.25w on 8ohm speaker or 2w on 4ohm speaker, which is pretty great.  And real easy on batteries because it's class d.

At one point I had thought about making a class d version of the Honey Amp using this board or similar.  This idea has been put to the back burner indefinitely, but I do have plans for a larger class d amp in the works.

Also, 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: https://www.electrosmash.com/marshall-guvnor-analysis
#96
Amplifier Discussion / Re: Adafruit PAM8302 Amplifer
Last post by phatt - July 14, 2022, 07:51:00 AM
Quote from: Loudthud on July 12, 2022, 04:46:59 AMThe diodes D9 and D10 really don't clip the signal, they just add what looks like crossover distortion. The Tube Screamer really doesn't make a good preamp.
Hi Loudthudman,
Just to clarify, D9&D10 are surely a Limiter?  ??? 
I can't see how they could work to impart any kind of Xover dist
on the signal. They would need to be back to back in series
with the signal to cause Xover. Maybe that is what was meant?

@ Carriage,
As for smart design the tube Screamer is actually good design practice. It has a Buffer stage in front of the first opamp.Early ones were just a BJT follower but later ones used an opamp stage.
They knew quite well that *NOISE is the enemy of high gain circuits* and that is why they used it.

I'll use your Schematic as reference, If R1 is 1Meg and R28 is 500k then you will pay a noise penalty, at high volume it can become unusable.

Any opamp with HiZ input and high gain (such as yours) will be noise prone.
The TS input buffer takes care of the HiZ input and then the gain stage sees a much lower input sensitivity.
The Equivalent R1 in a TS circuit is usually 10k. Not as much Gain but far less noise and a far more usable range of Distortion.

If you care to note, look at many of the so called bootweak guru designs of similar circuitry and they remove the buffer simply to get more gain, dirt or grit. Yes of course it's MORE BUT the noise floor will make that extra dirt unuseable,,
meantime the xperts tell you you need a noise gate.

In reality it's just a poor understanding of the art of smart design. Yes in some circumstances N-gates might help but logic suggests, tiss far better to design out problems rather than bandaid the flaws.
Most of the pro circuit use a hiZ buffer front end if you look around.

As to distortion before tone, that can go either way. With these guitar audio paths it can often come down to personal taste. experiment and find what works for you.

Note;
If your power amp is only 1 Watt then you will have little clean head room for any decent clean sound. Which is likely what Loudthud was eluding to.
Plus; I urge anyone wishing to build stuff, go get a Bread board and test it all first. You will learn a heck of a lot more than just poking resistors into a pcb. That's a 90% chance you will be building land fill.
Keep going,, be warned this stuff is very addictive. ;)
HIH,, Phil.

#97
Amplifier Discussion / Re: Adafruit PAM8302 Amplifer
Last post by Tassieviking - July 14, 2022, 06:35:45 AM
I have seen at least 3 different Marshall 12 watt amp schematics, one has what looks like clipping diodes.
And then there are the 12 watt Reverb models as well to consider.
#98
Amplifier Discussion / Re: Adafruit PAM8302 Amplifer
Last post by Carriage - July 14, 2022, 01:21:58 AM
I may have been mistaken about one of the schematics I found, so I've sorted that out
#99
Amplifier Discussion / Re: Adafruit PAM8302 Amplifer
Last post by Carriage - July 13, 2022, 07:25:05 AM
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?
#100
Amplifier Discussion / Re: Adafruit PAM8302 Amplifer
Last post by Loudthud - July 13, 2022, 05:25:57 AM
The Tube Screamer doesn't make a good preamp because you almost can't turn the gain down far enough to get a clean signal. When you do, the gain is less than one and you have no headroom. Most of the time you are feeding a distorted signal through a tone stack. What's that supposed to sound like ?

The first thing you need to do is get away from the 9V power supply. Can you find any kind of wall wort AC transformer in your country ? Something like 16 or 24 VAC ?

Variable gain solves a lot of problems in the first stage. Look at the Marshall Lead 12.