State variable filter preamp

[garfo]

Hello, I've been trying to gather information on this Alembic style preamp I've built.
It works, but, I wanted to get some expertise opinion or suggestions for three points that are lacking on this project.
First: the filter resonance goes from 300 hz to 4.8k approx.I wanted to get it to go a bit lower, perhaps 100/150 hz.

Second: from what I've gathered, this circuit consumes a bit more power than usual, and I'm worried since I will be using two of these on my bass with a single battery.Is there anything it can be done besides choosing a low power IC?

Third:On the schematic, there is a final Volume(10k)pot.I've made the circuit without that pot, just wired it straight to the output jack and it works great.But, doesn't the schematic say that there should be a 10k resistor always connecting from output to ground!?
Could anyone take a look on what could be done here?

Thanks millions
Schematic: http://forum.musikding.de/cpg/displayimage.php?pid=871&fullsize=1

[RobA]

One thing you could do to reduce power consumption is replace the Vbias circuit with a TLE2426. Since you have two units involved, you could run them both from the same TLE2426.

Going to a low power op-amp could save a ton of current.

I haven't looked at the specifics of this filter, but typically there are some tradeoffs made in the frequency range it can do. You could probably adjust some values to lower the minimum freq, but you'd probably end up either losing the max. freq. or having a lower, broader peak that didn't work all that well.

On the output, are you passively summing the two filter sections at this point?

[garfo]

Ei RobA.
Well, the ic being uses is a tl074, so I'm not sure How that would work.
Regarding the mixing, I haven't decided yet If I'll do it passively or actively.
But before that, I can't figure If that last supposed 10k res is actually part of the circuit.For now I want to work the circuit individualy.
I might forget the freq range issue.

[RobA]

The last 10k resistor is going to have some influence on the circuit, but mainly in its interaction with the cable and other downstream components. The output from the final op-amp stage of the SVF is a reasonable low-Z buffer, so that part is OK. For me, even if I were going to have a volume pot at the end of the chain, I'd put an output buffer after the volume pot to drive the output and help avoid cable impedance and capacitance issues.

If you are going to use an active summing circuit, you can change the output cap and that resistor to whatever the input of the summing circuit needs and then use the 10µF cap on the output of the summing buffer.

I'm not sure what you mean by you're not sure if it will work with the TL074, but the TLE2426 will work in any circuit where you need a reference voltage at half the supply voltage as long as the total current it needs to source or sink is less than about 20mA. I think you'll be below that for this circuit even if you add in the output buffer and power both of them from the same TLE2426 reference point.

The spec sheet on the TL064 versus the TL074 indicates a pretty big current savings there. It might be worth doing that change if the current draw ends up being too high.

If you do use an active summer and you end up having a leftover op-amp, you could use that to buffer a single Vbias for both filters. You could move to higher value resistors in the divider and save some current there. The savings using this method though will depend on what type of op-amp you are using.

[kothoma]

As the frequency is determined by 1/(2 pi R C) with R=R8=R9 and C=C4=C5 you can halve the frequencies by doubling either the values of C or R. Using C4=C5=22n should shift the frequency range to 136Hz...2.2kHz.

[garfo]

So, regarding that 10k final resistor that shows off as a pot on the schematic, How should it be wired If I use a fixes resistor.should it be in series, or should it go parallel from output to GROUND!?

The last 10k resistor is going to have some influence on the circuit, but mainly in its interaction with the cable and other downstream components. The output from the final op-amp stage of the SVF is a reasonable low-Z buffer, so that part is OK. For me, even if I were going to have a volume pot at the end of the chain, I'd put an output buffer after the volume pot to drive the output and help avoid cable impedance and capacitance issues.

If you are going to use an active summing circuit, you can change the output cap and that resistor to whatever the input of the summing circuit needs and then use the 10µF cap on the output of the summing buffer.

I'm not sure what you mean by you're not sure if it will work with the TL074, but the TLE2426 will work in any circuit where you need a reference voltage at half the supply voltage as long as the total current it needs to source or sink is less than about 20mA. I think you'll be below that for this circuit even if you add in the output buffer and power both of them from the same TLE2426 reference point.

The spec sheet on the TL064 versus the TL074 indicates a pretty big current savings there. It might be worth doing that change if the current draw ends up being too high.

If you do use an active summer and you end up having a leftover op-amp, you could use that to buffer a single Vbias for both filters. You could move to higher value resistors in the divider and save some current there. The savings using this method though will depend on what type of op-amp you are using.

[garfo]

 Is it not possible to shift Only the lower frquencies?

As the frequency is determined by 1/(2 pi R C) with R=R8=R9 and C=C4=C5 you can halve the frequencies by doubling either the values of C or R. Using C4=C5=22n should shift the frequency range to 136Hz...2.2kHz.

[kothoma]

Is it not possible to shift Only the lower frquencies?

As the frequency is determined by 1/(2 pi R C) with R=R8=R9 and C=C4=C5 you can halve the frequencies by doubling either the values of C or R. Using C4=C5=22n should shift the frequency range to 136Hz...2.2kHz.

Oops, need to be make a little correction: R=P2+R8=P3+R9, of course.

Using 50k+3k3 and 10nF as in the schematic:
R=53.3*10^3 and C=10*10^-9 gives 298.6Hz
R=3.3*10^3 and C=10*10^-9 gives 4823Hz

Using 50k+1k5 and 22nF for example:
R=51.1*10^3 and C=22*10^-9 gives 140.5Hz
R=1.5*10^3 and C=22*10^-9 gives 4823Hz

You should get the idea...

But now your freq pot could be a bit harder to dial in as it now sweeps over 5 octaves instead of 4.

[RobA]

So, regarding that 10k final resistor that shows off as a pot on the schematic, How should it be wired If I use a fixes resistor.should it be in series, or should it go parallel from output to GROUND!?

You would put the 10k to ground, but what you want to do with it really depends on what comes after, like active or passive summing. What you want to use there depends on the input to the next stage of the circuit.

[garfo]

Thanks guys.Well, I might Check the frequencies.
I think I'll just use an mn tapper blend pot I already have on my bass.so, each circuit will Go straight to the blend pot, in the end I will use a master volume pot, I'm not Aurélio there will be any need for buffering the output.

[garfo]

Also, the tl064 is not jfet input, or is it!?what other good opamps besides the 064 would work Well and push less power?

[kothoma]

The TL064 has J-FET inputs like the TL074.
Less power consumption, but a bit more noise.

[garfo]

RobA, If I decide to Go all passive after, the 10ufcap and the 10k should be as in the schemattic, right!?
Would I Only remove them if I want to use a buffered summing(blend) and a normal volume pot; or should I Also remove them using a passive blend and then a buffered volume and place them at the buffer output!?

[garfo]

https://docs.google.com/viewer?url=http://diy.musikding.de/wp-content/uploads/2013/06/bufferschalt.pdf

I have that buffered laying around at Home.would it work well after the volume pot on my bass?it is the jfet version.

[RobA]

Let me understand what you want to do now. You have two pickups with each pickup through an independent SVF. Then the SVF output is passively summed and then run to a single volume pot and then into the output buffer. Are you going to have a pickup selector toggle in there somewhere too?