Serious Blocking Distortion

[Littlewyan]

I've updated my schematics so they are much easier to read now. I've added the dots in and also removed the 1M Pot between the two Pre Amp Transistors as I believe when I had this in the circuit before it simply acted as a volume control.

I've not got round to building the circuit just yet as I've been a bit busy, but tonight I have taken off all the components off the PCB from the old Power Amp so I'm ready to start laying out my circuit.

Now once I've built this, if I want to increase overdrive/distortion within the pre amp, how would I go about it?

[stormbringer]

To increase distortion you could either add more gain stages, amplifying past the headroom voltage avaliable to the transistor, it will then cut the peaks of the waveform creating distorsion, or you could use clipping diodes. There are several ways of doing this, but the simplest form is just 2 facing the opposite direction in parallel shunted to ground. You might wanna add another gain stage after them to keep the signal at the desired level for he power amplifier.

I'm working at a heavy distortion preamp design, which i will upload once it is done, it uses both soft clipping and hard clipping through diodes.

[stormbringer]

You can also use a boost pedal in front, which usually basically works just like adding another gain stage in front of the others. The result depends on how the circuit works. A nicer distorsion requires some tone shaping in my opinion, filtering out some frequences that would make the sound "muddy" for example. It's not easy making a heavy distorsion, although there are some diy versions out there that sounds great. (Madbean upper decker, j0k3rx's black forest) etc.

For less distorsion, but great sound. Take a look at maxon od808 or any of the tube screamers. Might take some work integrating into your preamp, but good way to learn i suppose.

[Littlewyan]

Thanks, I'll see what the amp sounds like once I've built it and go from there. I don't actually like very much overdrive, but then I don't like clean either . If I can't hear my open chords ring out, theres too much overdrive!

[Littlewyan]

I'll probably try both the diodes and the extra gain stage, get an idea of the different sounds both methods make. I quite like the fact that a lot of people slag off the diode method when actually companies such as Marshall were using that method in their valve amps

[stormbringer]

I think you would prefer soft clipping then. (diodes in the feedback loop of a gain stage)

[Littlewyan]

Ok on my amp would that be between the negative input of the TDA2050 and the 22k resistor coming off the output of the chip?

[stormbringer]

The hard clipping diodes could go right after the 22uF cap after the input stage, but would benefit alot from Another stage after, to bring the level back up, as the clipping will lower the signal volume, so i would add a gain pot Before the diodes, and Another stage after.

Soft clipping, i have only seen done with op-amps. cant really say how well they would work or how to be done with a transistor.

Edit:

Found this at GeneralGuitarGadgets, uses both types of clipping.

[Littlewyan]

Right I see, I think I know how to achieve the soft clipping now, however the online schematic website I use is currently down so I can't publish a schematic to show you how I think I can do it 

[Littlewyan]

Site came back up, I've attached an updated schematic. I picked the 1N4148 as a few circuits I looked at used those.

[stormbringer]

Hm.. I dont think using the clipping diodes in the power amp is a good idea. They should be located in the preamp section, preferably followed by another gain stage to bring the level back up again.

Stage 1: the input stage, raise the level.
Stage 2: clipping stage
Stage 3: recovery stage.

With a gain pot before the clipping stage you get controllable distorsion, without the output volume being affected as you know the total voltage after the clipping stage depending on the diodes used. That way its easy calculating the optimal gain for the recovery stage.

[Roly]

May I just chip in here that it is worth experimenting with using different diodes in each half of a clipper to get asymmetric/unbalanced clipping, say a single diode in one direction and a couple of diodes in series in the other.  If you can find some germanium diodes try pairing one with a silicon.  Different colour LED's have different cut-in voltages so something like a red and green back to back is also worth a try.  This imbalance tends to give more even harmonics.

[Littlewyan]

Hmm could i do that with a JFET though? Or would i have to add a bjt stage?

Yer i think i saw a schematic where they mentioned using 2 diodes one way and 1 diode the other to get the extra harmonics.

[Roly]

Ah, Mk4 and Mk5 we have lost the ground return resistor on the Gate of the second FET.

I'm also not sure that this second FET is doing much for you.  While it lowers the impedance at that point it is already pretty low out of the first stage anyway.  What might be more interesting is to put the Gain control back in after the first stage, and change this second FET stage for something like the electra.gif circuit above.

Re: the diodes in the feedback of the TDA2050; if you do that you will limit the output voltage swing to +/-0.7 volts, or pretty much stuff-all, into your speaker.

[Littlewyan]

Right I found a website to help me design the second stage of the Pre Amp, only part I'm not sure about is the 5K Pot as I'm supposed to use a 3K Resistor on the base......