Is this a feasible distortion with blend?

[nchauhan890]

I'm trying to design a simple circuit that I can build on perfboard and will fit into a 1590A enclosure and I've found this distortion circuit which is a variant of the Electra Distortion or Bazz Fuss: http://diy.smallbearelec.com/HowTos/BreadboardUrsaMinor/BreadboardUrsaMinor.htm. I want to add a clean blend so I can dial in some of the clean bass tone (and to give me more of a challenge for my 1st 1590A build). It's for use primarily with a bass, by the way, but it doesn't need to be a perfect circuit by any means.

I've considered a few different (simple) ways of adding a clean blend such as incorporating a split and blend circuit or the runoffgroove splitter-blend, but these were too large for the enclosure, or at least my perfboard layout designing skills. This led me to attempt to stick together 2 circuits: the first is the one I mentioned above and the second is a very simple buffer from AMZ effects (http://www.muzique.com/lab/buffers.htm, the 3rd circuit shown).

This is what I have drawn up below... does anyone think this would work?



Thanks for any help on this, even if you aren't sure.

[11-90-an]

as far as my experience goes, if the mix pot is set to about 12 o clock, you wouldnt really hear anything because both signal would have to go through 50k ohms.. im not really sure but i think that's what happens...

[marcelomd]

Test it =)
I think it will work as is, but maybe at a low volume.

I'd add at least a buffer (maybe with a little gain) after the blend.

I like to use this circuit, or some variation. Not too many parts and works well:

[antonis]

as far as my experience goes, if the mix pot is set to about 12 o clock, you wouldnt really hear anything because both signal would have to go through 50k ohms..

Well not exactly 'cause it also counts individual signal's amplitude..

But, in general, specific configuration doesn't help anywhere..
In case you wish to use Q2 circuitry as a buffer, place it before Q1..

[duck_arse]

you will need a "C1" to feed Q1 and another to feed Q2, as they will fight bias if joined as shown.

[nchauhan890]

I'd add at least a buffer (maybe with a little gain) after the blend.

I like to use this circuit, or some variation. Not too many parts and works well:

I did consider something like this but I'm still constricted by the enclosure size - perhaps I can just add another buffer after the blend pot.

you will need a "C1" to feed Q1 and another to feed Q2, as they will fight bias if joined as shown.

Thanks, this should be easy enough to add to the layout.

The question that is raised for me is if I add a buffer after the blend pot, is it necessary for the clean signal to go through a buffer (or perhaps a boost?) and another buffer? That seems redundant to me, but I've never really experimented.

[marcelomd]

Ah, couple of things.

- As always, with distortion, you need to pay attention to relative phase of the signals. In your case, the top part inverts the phase and the bottom part doesn't. It will not sound the way you expect because one signal will cancel parts of the other.

- In this configuration you'll need a buffer for the clean side, otherwise you are connecting Q2's input and output via the blend pot. It won't work, I've tried =)

A simple buffer after the blend pot may work well enough.

What does you enclosure look like? One dual opamp takes less space than two buffers, that's why I suggested that.

[nchauhan890]

the top part inverts the phase and the bottom part doesn't. It will not sound the way you expect because one signal will cancel parts of the other.

A simple buffer after the blend pot may work well enough.
One dual opamp takes less space than two buffers, that's why I suggested that.

The 1590A is what I want to fit it in whichever gives me space for about a 13x16 (perfboard pads) layout, where 13 is probably the max width (~ 33mm). I think even just another buffer instead of the dual op amp is okay for me at this level - is there a way to either keep the normal phase of the Q1 circuit or invert the Q2 circuit? I didn't realise one would invert it.

And with the image you posted before with the dual op-amp, would this have the phase problem?

[marcelomd]

The 1590A is what I want to fit it in whichever gives me space for about a 13x16 (perfboard pads) layout, where 13 is probably the max width (~ 33mm). I think even just another buffer instead of the dual op amp is okay for me at this level - is there a way to either keep the normal phase of the Q1 circuit or invert the Q2 circuit? I didn't realise one would invert it.

And with the image you posted before with the dual op-amp, would this have the phase problem?

You can't not invert phase with Q1. That's a consequence of the circuit.

Simple JFET buffer/inverter (I'm not sure about the values tough):


Hmmm with the circuit I provided, you still have to invert something. Sorry, my bad.

So to keep components to a minimum:
- Change the clean buffer to an inverting one. This is one more resistor;
- Add an inverting buffer after the blend knob. 4 resistors and a transistor.

Sounds doable. I THINK 1590As are deep enough to fit two stacked boards if you need the extra space..

[nchauhan890]

So to keep components to a minimum:
- Change the clean buffer to an inverting one. This is one more resistor;
- Add an inverting buffer after the blend knob. 4 resistors and a transistor.

Thank you very much for this; it's extremely insightful! I think in this case my revert to using an op-amp as a buffer, something like this:



I think a TL072 (is that the dual one?) would be okay to have an inverting buffer in "parallel" to the Q1 circuit and then another after the blend. Would you agree that this is feasible? (Which after all is the aim of this thread!)

I suppose I could even add a slight boost on the output buffer instead by altering the resistor values on the op-amp <– that's about all I know about theory here!

[FiveseveN]

For what it's worth, I needed something similar recently so I came up with this. Use your antiparallel diodes of choice for D1 and you'll probably want a bigger value for C4.

[marcelomd]

So to keep components to a minimum:
- Change the clean buffer to an inverting one. This is one more resistor;
- Add an inverting buffer after the blend knob. 4 resistors and a transistor.

Thank you very much for this; it's extremely insightful! I think in this case my revert to using an op-amp as a buffer, something like this:



I think a TL072 (is that the dual one?) would be okay to have an inverting buffer in "parallel" to the Q1 circuit and then another after the blend. Would you agree that this is feasible? (Which after all is the aim of this thread!)

I suppose I could even add a slight boost on the output buffer instead by altering the resistor values on the op-amp <– that's about all I know about theory here!

Could work. I've never used an inverting opamp as the first stage, but I can't think why it wouldn't work.

So you connect the input to both Q1 and to the first half of the opamp, configured as an inverting buffer, and then you send both to the second half of the opamp. There you can either use a blend pot like your original design, or as a "panner", like the first snippet I posted. Both work.

Just prototype it somehow before commiting to the build =)

[nchauhan890]

So you connect the input to both Q1 and to the first half of the opamp, configured as an inverting buffer, and then you send both to the second half of the opamp. There you can either use a blend pot like your original design, or as a "panner", like the first snippet I posted. Both work.
Just prototype it somehow before commiting to the build =)

Cheers for that. I will of course test it on a breadboard beforehand although I have to admit I still have 2 current projects so it's looking like a late July/August time. I'll definitely draw up a schematic for this and perhaps a layout for others to use - I'm sure I'm not the only bassist with a need for a clean blend.

And one last question from me: what does that 5pF capacitor do in the feedback to the op-amp in the previous inverting buffer I showed you? (And will that value be okay for bass frequencies?)

[antonis]

what does that 5pF capacitor do in the feedback to the op-amp in the previous inverting buffer I showed you? (And will that value be okay for bass frequencies?)

It forms (together with 1M feedback resistor) a Low-pass filter of cut-off frequency at about 32kHz.. ( f = 0.159/(1M x 5pF) )
Doubling its value (10pF) should resut into 16kHz cut-off frequency which should be OK for Bass guitar..
(many guitar distortion stages of the above configuration utilize 16kHz LPF cut-off frequency..)

The reverse (High-pass fliter) stands for 100nF/1M input cap/resistor..
Its cut-off frequecy is at about 1.6Hz..

[nchauhan890]

It forms (together with 1M feedback resistor) a Low-pass filter of cut-off frequency at about 32kHz.. ( f = 0.159/(1M x 5pF) )
Doubling its value (10pF) should result in 16kHz cut-off frequency which should be OK for Bass guitar...
The reverse (High-pass filter) stands for 100nF/1M input cap/resistor...
Its cut-off frequency is at about 1.6Hz..

Thank you - I don't think those values will need changing then.

If anyone is still here, I've drawn up a revised schematic today (and managed to make a pretty tiny 12*10 layout). Please excuse the fact that it's on paper - but I find it easier to plan on. The only part I've "removed" is what would have been the 0.1uF input capacitor on the 2nd inverting op-amp stage because I figure that the 2 capacitors from both of the circuits being blended should do the trick. Please also note that the layout includes both the trimpot and the resistor in series for the DC going to the distortion circuit, but the schematic combines them into a single component for ease (and reaching the top of the page!).



The values of the components are the same as the original "ursa minor" schematic from smallbearelec and the 2 inverting op-amp buffer stages are this one from AMZ effects, combined into a single TL072 dual op-amp IC.

[Marcos - Munky]

After the output cap of both distorted and clean parts, add a mixing resistor. Something between 1K and 4.7K will do the job. I can't recall the reason rn, but I know for sure the signal woudn't blend correctly without them. Also, keep in mind your distorted part will be way louder than the clean part, because you hard setted the distorted part to full volume. I'd add a trimpot or a pot to adjust the volume of the distorted part (you can fit up to 4 16mm pots in a 1590A).

[Nitefly182]

In a circuit like this a blend control doesn't make much sense when you could have your fuzz volume control and a clean volume control. Especially if you get a little bit of gain going in that clean side and change that output buffer to an active mixer. Otherwise a unity blend limitation hobbles the pedal. You have all the bits there. They just need to be reconfigured slightly for better functionality.

[nchauhan890]

keep in mind your distorted part will be way louder than the clean part, because you hard setted the distorted part to full volume. I'd add a trimpot or a pot to adjust the volume

Especially if you get a little bit of gain going in that clean side and change that output buffer to an active mixer. Otherwise a unity blend limitation hobbles the pedal. You have all the bits there. They just need to be reconfigured slightly for better functionality.

Is adding some gain to this as simple as doubling (or more) the feedback resistors in the op-amps? For simplicity's sake I'd prefer to keep the number of pots at 2 where I could potentially leave off the distortion pot and add 2 volume pots.

I like the sound of the active mixer idea. I'll have a deeper look into it, but as you say, the parts are there, so... is there a way to convert this into a mixer? I would guess that I need to add a bit of gain to the clean inverting op amp stage and a little bit to make up the signal after mixing.

[antonis]

After the output cap of both distorted and clean parts, add a mixing resistor. Something between 1K and 4.7K will do the job. I can't recall the reason rn, but I know for sure the signal woudn't blend correctly without them.

It's just a matter of impedances "seen" by the signal sitting on blend pot wiper..
(mixing buffer input impedance and each previous stage output impedance..)

The issue here is rather complicated 'cause we deal with signals coming out of two voltage sources (Fuzz Collector & inverting buffer out) of
different amplitudes AND different impedances..

[Nitefly182]

keep in mind your distorted part will be way louder than the clean part, because you hard setted the distorted part to full volume. I'd add a trimpot or a pot to adjust the volume

Especially if you get a little bit of gain going in that clean side and change that output buffer to an active mixer. Otherwise a unity blend limitation hobbles the pedal. You have all the bits there. They just need to be reconfigured slightly for better functionality.

Is adding some gain to this as simple as doubling (or more) the feedback resistors in the op-amps? For simplicity's sake I'd prefer to keep the number of pots at 2 where I could potentially leave off the distortion pot and add 2 volume pots.

I like the sound of the active mixer idea. I'll have a deeper look into it, but as you say, the parts are there, so... is there a way to convert this into a mixer? I would guess that I need to add a bit of gain to the clean inverting op amp stage and a little bit to make up the signal after mixing.

Its definitely possible to do what you're trying to do in a 1590A with only two pots but the reasons for the limitations on this build seem kind of arbitrary. Especially with bass there is an advantage to having gain and volume controls as max gain isn't necessarily what will sound best.

On the clean side you would just make that buffer into an inverting amplifier stage (to flip the phase to match the fuzz side) and then bring the two signals together with some mixing resistors into the output buffer. Google should have some good results for active mixer circuits.