How do I sum or combine 4 filters into a coherent EQ?

[Jidis]

Thanks Abbey Road!

So I guess regular log is going to be even worse than linear. 

<edit> I just remembered those resistor additions to get a smooth audio/log taper from a linear pot. You don't think you can do something like that to get reverse log from one do you?

I may look into what I need to change on the caps to use different pot values. The project isn't worth seeking out anything hard to get or expensive. If changing caps in order to use different pot values doesn't affect much else, I may even invest in a bag of pots so I can play around with some other EQ circuits.

I'll also look into those models you suggest on the switched shelf, but as you hint that it's sort of a pain, I may not mess with it. I was hoping it could be done just switching a couple passive parts in or out of one stage.

Thanks again (and sorry again if this is off topic).

 

[abbey road d enfer]

Thanks Abbey Road!

So I guess regular log is going to be even worse than linear. 

Definitely.

<edit> I just remembered those resistor additions to get a smooth audio/log taper from a linear pot. You don't think you can do something like that to get reverse log from one do you?

Yes. That's what I did in the days I designed products for a living. I didn't invent it. I stole it from someone who had probably stolen it from someone else. 

I may look into what I need to change on the caps to use different pot values. The project isn't worth seeking out anything hard to get or expensive. If changing caps in order to use different pot values doesn't affect much else, I may even invest in a bag of pots so I can play around with some other EQ circuits.

Too high a value generally increases noise and makes parasitic capacitances becoming a nuisance. Too low a value and the opamps may have a hard time driving the pots. I use successfully 22k linear pots with 5.6k wiper resistor and 56k top resistor for a 1:11 sweep range.

I'll also look into those models you suggest on the switched shelf, but as you hint that it's sort of a pain, I may not mess with it.

I've found that stability is often a problem. Considering the added complexity I chose to make the BW range large enough to act almost as shelving EQ.

 

[abbey road d enfer]

*

 

[Jidis]

Thanks again Abbey Road.

Too high a value generally increases noise and makes parasitic capacitances becoming a nuisance. Too low a value and the opamps may have a hard time driving the pots.

I guess you mean too high/low a value of 'pot', right? If I change that and try to figure out what to change on the caps, I'd probably be dropping down to a 100 or 50k, if they have rev.log on those.

I'm also interested in that resistor mod for linear to rev.log if anybody can give some insight. The info I remember was about making a more accurate volume (log) control using a linear pot and resistors. It may have been at Rod Elliot's site. As I usually home etch my boards, squeezing a couple resistors near the pot should be a cinch if that's an option.

George

PS- loss1234 mentioned having filter books. I'm lacking there and wouldn't mind getting one if you guys have a recommendation. Only thing I've got is that "Active Filter Cookbook", most of which is over my head and aimed at really specific HP/LP non-audio application.

 

[abbey road d enfer]

I guess you mean too high/low a value of 'pot', right?

Correct.

I'm also interested in that resistor mod for linear to rev.log if anybody can give some insight.

Check the attached schemo. Signal comes in at C79 and goes out at C24. Ignore U11 and attached capacitors, they're just range mutlipliers.

PS- loss1234 mentioned having filter books. I'm lacking there and wouldn't mind getting one if you guys have a recommendation. Only thing I've got is that "Active Filter Cookbook", most of which is over my head and aimed at really specific HP/LP non-audio application.

As I wrote earlier on, there's not many serious "audio" books. Study the state-variable filter, which is the basis of 90% of parametrics, and the Wien bridge, which constitutes another 9%, the remaining 1% including phase-shift filters, pseudo-gyrators...

 

[Jidis]

I think you can get them from Audio Maintenance. If not you may just use different value and change the caps accordingly.

Abbey Road (or anyone)- If you were stuck with only a couple specific pot values for playing around with EQ circuits, what do figure would be the most practical to have? I'm thinking of grabbing a bag or two off eBay. Note - I think I'm pretty well stocked on 10k linears (with and without a detent). Also, if the log/reverse log resistor mods produce a usable curve, I guess I'll stick with linear on whatever else I get.

Thanks Again!

 

[abbey road d enfer]

I think you can get them from Audio Maintenance. If not you may just use different value and change the caps accordingly.

Abbey Road (or anyone)- If you were stuck with only a couple specific pot values for playing around with EQ circuits, what do figure would be the most practical to have?

Look at the schemo, they're 22k's.

  I think I'm pretty well stocked on 10k linears (with and without a detent).

There is an issue with 10k. In order to have the correct taper-steering, you need to load them with 2.2k. When the pot is at max, the load applied to the preceding opamp is about 1.7k; many opamps are not in their comfort zone here. You must use opamps with enough output capability such as 5532/34 or 49710.

Also, if the log/reverse log resistor mods produce a usable curve, I guess I'll stick with linear on whatever else I get.

Pls note that they only mention volume control applications; they are not directly applicable to other functions such as boost/cut or BW ("Q") control.

 

[Jidis]

Look at the schemo, they're 22k's.

I was thinking more "in general", and probably something easy to get a cheap bulk of (like 20/50/100k). I hadn't seen a 22k pot show up in most stuff I looked at, but there was a pretty wide variety.

There is an issue with 10k. In order to have the correct taper-steering, you need to load them with 2.2k.

Well the 10's I've got, I was figuring might knock out the boost/cut pots, as I did see that value a lot online. Figure I'd be using them as intended there (linear). The higher valued ones I'm looking to get would be the ones I'd need to fake log & revlog tapers on with added resistors.

Thanks Again!

 

[Jidis]

I'm liking the heck out of this LTSpice thing. 

For some reason I hadn't tried playing with a circuit sim since ten years ago with Electronics Workbench or something. It'll probably make it a ton easier to audition different values to get back my frequencies like you were talking about if I have to compromise on pot values. I've learned enough to do basic frequency sweeps in there, but don't know enough to test for other (possibly more important) effects of the part changes. Guess I'll know when I hear it.

George

PS- Sorry for dragging this off topic again, but I guess the OP has what he needed.

 

[Jidis]

Sorry to keep hounding on this, but I had one quick question regarding that changing of cap values thing from above-

I played around with that in LTSpice and it seemed too easy. Even with something as extreme as taking a 100k pot down to a 10k for the frequency control, I was able to bring back roughly the same curve and center frequency simply by switching a pair of caps.

I'm guessing there's some downside to this that I'm missing??

Thanks 

 

[abbey road d enfer]

Sorry to keep hounding on this, but I had one quick question regarding that changing of cap values thing from above-

I played around with that in LTSpice and it seemed too easy. Even with something as extreme as taking a 100k pot down to a 10k for the frequency control, I was able to bring back roughly the same curve and center frequency simply by switching a pair of caps.

I'm guessing there's some downside to this that I'm missing??

Thanks

Don't sweat too much. 100k is fine. As I said 10k puts some stress on the opamps. Some use 470k. I reckon 1Meg should work too provided the PCB tracks or wires are not too long in order to keep control of stray capacitance. With >100k, the noise current of the opamps may become a problem so JFET types would probably be de rigueur.

 

[Jidis]

Sounds good Abbey Road, thanks!

I was really just wondering if there was a reason to see some of these specific oddball values like that 500k one I hit, or the 22k you posted, if it's just a matter of changing caps. I'll try to keep the values realistic and stick with what they call for when I can.

Haven't learned enough yet to test for some of this stuff in LTSpice or real life, but hopefully I'll get there eventually.

Take Care