Low pass preamp for bass

[Domin]

Is there anyone who tried this circuit? I would like to add volume control on output and maybe some gain control on first opamp stage...

[ElectricDruid]

Nope, haven't tried it, but your additions seem easy enough to do. The first stage is a non-inverting op-amp, so you'd just add a couple of resistors, something like this:



For the volume control, you could replace the current 220K to ground at the output with a pot instead, and then take the output from the pot's wiper.

HTH

[Domin]

I was thinking about same thing as you, i try to make pcb layout for it...what is difference between connecting R8 on your example to ground and vref?

[fryingpan]

For single supply circuits, you really want to reference everything to half supply. Which then goes to ground. You can either do this or shunt it to ground through a capacitor. But you can't just shunt it to ground, you'd have actual DC current flowing through the feedback essentially.

[ElectricDruid]

I was thinking about same thing as you, i try to make pcb layout for it...what is difference between connecting R8 on your example to ground and vref?

+1 what fryingpan said.

The difference is you need to add a capacitor if you're going to connect it to ground. The input bias resistor (R1/220K on your schematic, R6/2M2 on mine) sets the DC bias point for that op-amp at Vref, so if you then tie the feedback network to ground, you've got a big DC difference, and the amplifier will think that's an input and try to amplify it. However, if you put a cap in series, it blocks the DC and it'll work.

It would be good practice to add *two* caps to that stage. There'll be one from R8 down to ground which also acts as a highpass filter, so can be sized up to help cut mains hum and subsonic thumps. The other one would go in parallel with R7 and sets a lowpass cutoff to limit the high frequencies (well, at least roll-off the gain for high frequencies to only x1). That should be sized for the highest audio frequency of interest. For Bass, that could probably be pretty low - maybe 5-8KHz, an octave lower than I'd usually go for guitar?

With moderate gains, these parts aren't critical, but as gain goes up, it becomes more and more important to make sure that amp stages are only amplifying stuff you *want* amplified, and not loads of other rubbish that you don't (subsonics, ultrasonics, radio frequencies, etc etc).

[Domin]

For single supply circuits, you really want to reference everything to half supply. Which then goes to ground. You can either do this or shunt it to ground through a capacitor. But you can't just shunt it to ground, you'd have actual DC current flowing through the feedback essentially.

Then why almost every opamp circuit in guitar pedals is connected to ground? Like DOD 250 etc? I think that connecting to vref i found in all Bajaman's designs but not in common circuits...

[Domin]

I was thinking about same thing as you, i try to make pcb layout for it...what is difference between connecting R8 on your example to ground and vref?

+1 what fryingpan said.

The difference is you need to add a capacitor if you're going to connect it to ground. The input bias resistor (R1/220K on your schematic, R6/2M2 on mine) sets the DC bias point for that op-amp at Vref, so if you then tie the feedback network to ground, you've got a big DC difference, and the amplifier will think that's an input and try to amplify it. However, if you put a cap in series, it blocks the DC and it'll work.

It would be good practice to add *two* caps to that stage. There'll be one from R8 down to ground which also acts as a highpass filter, so can be sized up to help cut mains hum and subsonic thumps. The other one would go in parallel with R7 and sets a lowpass cutoff to limit the high frequencies (well, at least roll-off the gain for high frequencies to only x1). That should be sized for the highest audio frequency of interest. For Bass, that could probably be pretty low - maybe 5-8KHz, an octave lower than I'd usually go for guitar?

With moderate gains, these parts aren't critical, but as gain goes up, it becomes more and more important to make sure that amp stages are only amplifying stuff you *want* amplified, and not loads of other rubbish that you don't (subsonics, ultrasonics, radio frequencies, etc etc).

What do you think is better way to do it? Connecting to vref or to ground with 2 caps creating a filter?

[PRR]

why almost every opamp circuit in guitar pedals is connected to ground? Like DOD 250 etc? I think that connecting to vref i found in all Bajaman's designs but not in common circuits...

[fryingpan]

I was thinking about same thing as you, i try to make pcb layout for it...what is difference between connecting R8 on your example to ground and vref?

+1 what fryingpan said.

The difference is you need to add a capacitor if you're going to connect it to ground. The input bias resistor (R1/220K on your schematic, R6/2M2 on mine) sets the DC bias point for that op-amp at Vref, so if you then tie the feedback network to ground, you've got a big DC difference, and the amplifier will think that's an input and try to amplify it. However, if you put a cap in series, it blocks the DC and it'll work.

It would be good practice to add *two* caps to that stage. There'll be one from R8 down to ground which also acts as a highpass filter, so can be sized up to help cut mains hum and subsonic thumps. The other one would go in parallel with R7 and sets a lowpass cutoff to limit the high frequencies (well, at least roll-off the gain for high frequencies to only x1). That should be sized for the highest audio frequency of interest. For Bass, that could probably be pretty low - maybe 5-8KHz, an octave lower than I'd usually go for guitar?

With moderate gains, these parts aren't critical, but as gain goes up, it becomes more and more important to make sure that amp stages are only amplifying stuff you *want* amplified, and not loads of other rubbish that you don't (subsonics, ultrasonics, radio frequencies, etc etc).

What do you think is better way to do it? Connecting to vref or to ground with 2 caps creating a filter?

You are still creating a filter if you reference everything to Vref, because you are going to have a shunt capacitor in parallel with the voltage divider.

A funny thing happened to one of my designs: if I just set the volume to zero (after a gain stage) according to simulations I'm actually still getting the signal, 30dB down, and heavily filtered. That's because the pot is referenced to Vref, and the voltage divider + capacitor are too high an impedance for the signal to be properly shunted to ground, so some of the signal bleeds back into the circuit essentially. (It's not a real issue, I chose to reference the pot to Vref so that I could use one fewer resistor, not good practice but it should work for an overdrive pedal).

Best practice would be:

- use Vref only to set half supply voltage, it's not a true "ground" for the signal;
- shunt signal, whenever signal is actually meant to be flowing through the node, to ground through a capacitor to maintain the DC "bias" point.

Note that bias setting resistors will still have signal flowing through them, so you do need to shunt the signal through ground (through a capacitor next to the voltage divider).

[antonis]

Any designer using VRef for purposes other than its existence (BIAS...!!) is illusively relied upon absolutely no DC offset between various stages..

e.g.
In an ideal world DOD250 design, C3 & C4 could be omitted and clipping diodes lower end could be connected to Vref..

[fryingpan]

Any designer using VRef for purposes other than its existence (BIAS...!!) is illusively relied upon absolutely no DC offset between various stages..

e.g.
In an ideal world DOD250 design, C3 & C4 could be omitted and clipping diodes lower end could be connected to Vref..

DC offset is often inconsequential. The opamp I'm employing states 0.3mV typical, 3mV maximum. Could be lower, but it's not too bad.

[ElectricDruid]

What do you think is better way to do it? Connecting to vref or to ground with 2 caps creating a filter?

I think the better way to do it is to take it to ground with the cap. Like PRR said, you should only use the Vref bias supply for bias. Generally, the *less* stuff you have connected to Vref, the better.
Secondly, limiting the gain of the op-amp to only the bandwidth of interest is good practice, and the caps help with that.

[antonis]

DC offset is often inconsequential. The opamp I'm employing states 0.3mV typical, 3mV maximum. Could be lower, but it's not too bad.

I'm talking about voltage drop across bias resistor..
500nA (max) through 470k resistor -> 235mV..

[fryingpan]

If you were to be very careful about offset voltage and current, you would have to fill the board with (very large) capacitors. If Rod Elliot is fine with leaving them out, who am I (a hobbyist) to object?

[Domin]

Somehow i made this layout, it doesnt have values yet, still thinking about better design...i would like to have something more profesionnal

[Domin]

Somehow i made this layout, it doesnt have values yet, still thinking about better design...i would like to have something more profesionnal

After fixing errors and adding values

[sergiomr706]

I havent build It, but googleing the schem info shows thatthe designer made a video too in post 83 https://www.talkbass.com/threads/a-different-kind-of-low-pass-filter.1146653/page-5
Sounds good to me in his own

[Domin]

I havent build It, but googleing the schem info shows thatthe designer made a video too in post 83 https://www.talkbass.com/threads/a-different-kind-of-low-pass-filter.1146653/page-5
Sounds good to me in his own

Thats where i got schematic from

[Domin]

Is there anyone who can make better layout with onboard pots? Or some better placement for wiring them?

[ElectricDruid]

Is there anyone who can make better layout with onboard pots? Or some better placement for wiring them?

We *could*, but then you'd never get any practice, and you'd never improve.

Start by placing the pots where you want them on the board, then put the IC on, then do the power, and only then route the other stuff.