This effect is not what I would call "Diode hash". It's not any fault with the diodes. The circuit is doing exactly what the math says it will do. To get rid of the "1+" effect, use the inverting form of feedback. See attached. The one disadvantage is the low input impedance, so you really need an input buffer like the TS pedal.
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Where can I find out more about the EU's Right to Repair regulations ?

Do you have an oscilloscope and some kind of signal generator ? Do you have a smartphone ? Build the first stage of a Tube Screamer and run a signal into it. Turn the pot all the way down. The gain will be one. Turn the pot up slowly and watch the output on the scope. When it starts to look like the simulation above, turn up the generator amplitude.

Don't have a scope ? Get one.

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.

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.

The 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.

First of all, all the pots go to 11. Here's the lineup:

Gain: A500K
Bass: A1M
Middle: B25K
Treble: B250K
Presence: C25K
Master: B25K wiper goes straight to the Send jack.

The Soldano SLO Mini 30 runs off of a 24V 2A switchmode brick like you might find on a laptop computer. Inside there are three PCBs. One runs across the back where the input DC jack, the effects loop jacks and the speaker jacks are mounted. There are a couple of diodes and an LM7809 TO-220 Voltage regulator without a heatsink. All connections are made with flying leads through 2 or 3 pin connectors.

The main PCB is through-hole technology with the input jack, the pots and the two mini-toggle switches soldered to it. The power switch is mounted to the front panel with leads back to the back PCB. The Class D power amp is a small PCB mounted to the main PCB with standoffs. The power amp PCB is all surface mount components with leads soldered to it. The power amp chip is TPA3106D1, there is also a quad TLO74 opamp and several small transistors and surface mount diodes. The power amp PCB is branded PM30 (SYNERGY)_V03. The preamp has three TLO72 opamps and a chip, TC1044S that converts +9V to -9V to run the opamps.

The Effects loop Send gets it's signal from the preamp output, the Return goes straight to the input of the Power Amp Board. There is roughly 12V on both sides of the Speaker output with no signal. The speaker output is rated for 8 or 16 Ohm loads only.

Just got a msg from my local shop. The amp I ordered has arrived and the main guy there, who is a diehard tube amp guy, says it is amazing and he has ordered one for himself. You can't get a bigger endorsement than that.
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I was thinking it's really a great idea for novice amp builders because they don't have to deal with Mains wiring. With one of those switchmode supplies that are common now, they don't even have to worry about Mains Voltage. They work on anything from 90 to 250 VAC. Just get the power cord that will work in your country.

Interesting that it uses an external power supply. The jack on the back looks like the same one used on pedals.

Check for DC on the speaker output and DC Voltages around the preamp. Something must have changed.

Quote from: Enzo on April 11, 2022, 12:54:41 PMIt is like an electrical appliance in your home.  If double insulated, no need for earth.   Just my opinion.

All you need is a double insulated guitar and you're set. Without the safety ground, you can get hum and mild shocks when you touch microphones or other guitar players

The clipping LEDs make the amp a little more bedroom friendly. When operated with the Master Volume turned down so as not to allow the power amp chip to clip, the LEDs give the effect of more preamp gain and allow the Master Volume to be turned up a little farther past the point where the volume jumps from nothing to too loud.

A low cost amp such as this is probably not capable of driving an additional speaker. The power transformer and power output chip are carefully chosen for low cost. It may be possible to drive two speakers, but they would need to be twice the impedance of the stock speaker.