Double Bass Building 3-band Parametric EQ with HiZ

[Moofmilker15]

Hello there! I'm a long time reader, first time poster. My DB setup currently has Yamahiko piezos run in stereo with separate EQs, and I am toying with a circuit that promises to be fully parametric. Link Removed

Correct me if I'm wrong, but I think the input impedance of this circuit is determined by R2, which is 1M ohm. If I want it to play nicer with piezo pickups, should I raise it to 10M? Is there a capacitor value change that will need to happen too? How would I calculate that? Other considerations?

Any help will be appreciated, as I am just beginning to dive into such things.

 

[lowfreqgeek]

The combination of R2 in parallel with R4 sets the AC input-Z because node VR is very low Z. With the current values, all frequencies above the C2/R4 cutoff are essentially dominated by 100k instead of the 10Meg. In other words, if you change R2 to 10Meg and leave R4 at 100k, the input-Z for frequencies greater than 1.6Hz will be 100k||10Meg, which is essentially 99k ohms. You could replace R2 and R4 both with a 10Meg resistor, which would make the input-Z 5Meg, or even go all the way to a 20Meg for those. The C2/R4 combo sets the low corner frequency (fc), but you could lower C2 to raise the low corner using the equation fc = 1/(2*pi*R4*C2).

I would use the calculator linked in the instructions and lower the low frequency control quite a bit. 140Hz at the minimum setting is high for bass.

Looks like a fun project. I have some other minor concerns, but nothing that would make the circut unusable apart from the input-Z thing.

 

[Jeff Kissell]

~5Mohm input would be pretty ideal for Yamahikos IMHO.

 

[Moofmilker15]

Thanks for your help! That all makes sense. I'll start by them both 10Mohm, netting a 5Mohm impedance.

And good to know that C2/R4 function as a high pass filter. My current EQ cuts all bass below 80hz on the G side pickup, so I will definitely make use of that.

I'll play around with the different filter values and try to tailor each to the E side or the G side pickup.

The vendor states using IC TL072P (or better). Doesn't say what better might be. Any thoughts what to try?

 

[DoubleMIDI]

Something with less noise and a low current and supply voltage.
Hard to find, SMD and/or expensive.
Have a look at the schematics of Francis Deck‘s HPFpre version 3 and the information about it.

 

[dreamadream99]

I feel like I just finished addition and subtraction, and was handed a test on calculus.

Here is the waterline:___________

Here’s my head: 0

 

[lowfreqgeek]

Thanks for your help! That all makes sense. I'll start by them both 10Mohm, netting a 5Mohm impedance.

And good to know that C2/R4 function as a high pass filter. My current EQ cuts all bass below 80hz on the G side pickup, so I will definitely make use of that.

I'll play around with the different filter values and try to tailor each to the E side or the G side pickup.

The vendor states using IC TL072P (or better). Doesn't say what better might be. Any thoughts what to try?

Another option is to just leave R2 off completely, which then makes the input-Z equal to R4.

As to alternatives to the TL072, there are many. The TL072 is quite old, so you can get lower noise, lower quiescent current, faster slew rate opamps from just about everyone. You may have difficulty finding one in a DIP package, though. I haven't used DIP for decades. There are plenty of smaller surface mounts available, and you can always get a DIP adapter for just about any surface mount variety. One that I am considering for an audio project is the OPA1662, but I don't know if it's in a DIP. Use the Dikigey search function and you can sort through all kinds of things. To start the project, there's nothing wrong with the TL072 - especially if you use DIP sockets so you can swap them out later.

 

[TroyK]

~5Mohm input would be pretty ideal for Yamahikos IMHO.

I was going to say 4.7 seems perfect for Yamihiko. If you build a parametric eq with a variable HPF and 4.7-5Mohm input impedance, please count me in!