I'm wondering if anyone has had any success building an active version of the BMP tone control. I've read the section in Teemuk's book about active tone controls but I'm still not confident that what I have will work (I have not yet breadboarded it). I've attached a rough schematic of my idea. Do you guys think this will work? There's a DC path from the output back to the inverting input, so the bias offset can make its way back. Adjusting the tone pot shouldn't affect the overall gain of the opamp since there's no DC path to ground in the feedback loop.
Finally... Can anyone point me to a tutorial for simulating these circuits in LT Spice? I know how to produce a simple simulation to measure the gain and clipping characteristics but I'm not sure how to simulate the frequency response curve.
In Ltspice,
Open the properties window of the input voltage source and insert an appropriate value to the AC amplitude field (e.g. 1V is usually a pretty good value).
Edit the simulation command and select AC Analysis. Number of points per octave is the "resolution" of the simulation (25 points is usually a good basic value for that) and start and stop frequencies should be self-explanatory (the frequency band to plot, e.g. 20 - 20k). Type of sweep is initially set to octave, which is a pretty good value because it creates a logarithmic scale for frequency in the x-axis.
Run the simulation and plot the node(s) of interest.
Profit.
Thanks! I'll give it a try. So, any thoughts on the proposed circuit?
I think I found a fatal flaw... I was wrong, there is a DC path to ground so adjusting the tone pot will affect the overall gain since the blend pot combined with the 22K resistor to ground will act as a voltage divider... hmm.. bummer. I'll still try simulating the circuit though.
Profit?? What's that? I thought all of this was just somewhere for me to sink my unused money ;)
Hi Armstrom. Don´t worry, the "flaw" , if any, is microscopic. You gan keep the D.C. gain as unity by grounding the 22K resistor through an electrolytic; 1 to 10 uF will do. It will still be a somewhat scratchy pot because there´s some DC through it. Please test as Teemu said and post your results. It will be interesting to see the "mirror image" of that classic tone control. Thanks.
:'( Unfortunately, your circuit won't work as is.
If you take a look at the active circuits shown in my book you'll see that the concept of your "conversion" is not the same as mine. Here's an interpretation as how it would go according to the conversion principle presented in my book. [see attachment]
The main ideas (in my book's conversion method) are:
-inverting amp
-the ground reference(s) of the formerly passive circuit become the negative feedback loop's inputs
-amp's input is the output of the tone control
Just compare this to the standard, passive BM circuit found from e.g. Tonestack Calculator and you'll see what I just did.
Yeah... I noticed that soon after I posted. The circuit I ended up simulating is attached along with the frequency response curves for "full" bass (90%), full treble and the mid position. The response seems a bit "extreme" with a great deal of boost (12-16dB) at the extremes but no gain at the mid setting. I adjusted some of the RC filter values to get the mid scoop around 1kHz... I'll probably have to play with the filters some more to get a response curve I like.
I wasn't able to find a POT in the LTSpice library... As you can see I'm just using two resistors and adjusting the values to simulate adjusting the pot wiper.. Is there an easier way to do this? Maybe even have the simulation adjust it for me?