Active EQ questions

[MikeH]

So I just finished building my first tube amp, a fender champ.  Great little amp, but obviously it lacks tone control, so versatility is not exactly a strong suit.  So I was thinking of building an active eq for it, but I didn't want to go full on graphic eq, just because they're a pain in the ass to build.  For obvious reasons, I was thinking fender-style tone stack.  And then I'd need some sort of gain adjustment to compensate for signal loss, right?  So I was thinking I'd put some kind of clean boost in line like an MXR microamp, because it has plenty of headroom and is fairly transparent.  Which brings up the first question:

Would the gain stage work best before or after the tonestack?

Thinking about this I realized I have gobs and gobs of dual opamps on hand, so why not work up 2 gain stages, a pregain and a post gain, both micro amp style, off the same chip, and tweaked to put out about 1/2 the usual output, just to get a more useful range of  control out of each, because obviously with 2 micro amps anything past 2 on both would push the whole thing into distortion.  Which I'd still want the ability to do, just not that easily.  So...

Would a pre-gain and a post-gain be useful in this application?  Are there advantages to being able to hit the eq either hard or softly and still be able to maintain the same output level?

And I figured with the extra gain available I might throw in a bypass-able varitone or something, maybe a passive knotch control a la Anderton... etc. 

[Minion]

Well why not try this active EQ circuit...




It boosts and Cuts and doesn"t need any makeup gain (I don"t think you need Makeup gain with an active EQ only a Passive EQ) ...

I would also add an Inverting buffer before the Curcuit to maintain Phaze....

I use this exact Curcuit in my SS Guitar amp and it works very well.....

Cheers

[stm]

I suggest you better go with a guitar amp tonestack design and a gain recovery stage.

That three-band tone control is a classic in several op-amp application notes, however bands interact too much and frequencies are not optimal for guitar use.  First example:  the mid band acts around 1 kHz, where for guitar use you get best results between 300 and 800 Hz.  Second Example:  the high band acts more like a Presence control since t control the very high frequencies (around 10 KHz and above).

I built it myself and it did work, but wasn't very effective for guitar use and above all it had an enormous amount of hiss when the Treble control was raised--not the type of phenomena you want before any gain stage.

[Minion]

The center frequencies for each band can easilly be changed by altering the Resistor and Cap Values....

This circuit in my Guitar amp is Dead quiet so Maybe you did something wrong when you did it ,and I notice that the frequencies Interact in each band MUCH more with a Passive Tone stack ,Pluss with a passive tone stack you can only Cut not boost...This is a reason most all EQ"s use active Tone controlls ,I don"t see why in this day and age poeple still use passive tone controlls ,To me they sound Bad and are really no easier to implement that an Active tone controll.....

I recently put an active tone controll in my Bass guitar (Only useing 2 bands with altered Cap resistor values to suit bass) and scrapped the Passive one that it came with and the sound it Much better with heaps more versitility......to each his own I guess....

Cheers

[tranceracer]

I would also add an Inverting buffer before the Curcuit to maintain Phaze....

Sorry if this is a newbie question, What does the phase inverter buffer schematic ckt look like?

[MikeH]

Thanks for the responses guys.  Although it is true that a passive tonestack can only remove frequencies, and not boost them, if it is followed by a variable gain stage, why can't you boost frequencies?  If you pull away the highs and lows, then boost the remaining signal, haven't you boosted the mids?  Is it a "there's more than one way to skin a cat" sort of thing, or am I talking about a completely different concept all together?

[tcio]

Well why not try this active EQ circuit...




It boosts and Cuts and doesn"t need any makeup gain (I don"t think you need Makeup gain with an active EQ only a Passive EQ) ...

I would also add an Inverting buffer before the Curcuit to maintain Phaze....

I use this exact Curcuit in my SS Guitar amp and it works very well.....

Cheers

I am Interested in this tone control for future foot pedal builds. I am somewhat of a noobie and so far have been unsuccessful at every EQ I have tried to build so far. Also, every EQ I have tried to build so far didn't specify the voltage to be applied so I just stayed with 9v. Can anyone tell me how to figure this out if the voltage is not given?

[Rob Strand]

There is a lot of interaction on the three band active equalizer.   You can change the frequencies but it's not a simple matter, you get all sorts of funny interactions that simply aren't present with a separate mid section.  The interactions are reduced when you make the bass frequency low and the treble frequency high - ie away from the mid frequency, but you don't always want that.  I agree with what stm said about the frequencies.  A few years back I worked out a three-band  preamp for bass and I made some changes to the circuit which gave better guitar frequencies and minimized interactions.

It is not true that a passive eq circuit can only cut frequencies, it's a misleading spin on words.  The passive James circuit (some people call this Baxandall but it's not, Baxandall is the active version) provides bass boost and cut.  In order to do this it provides an overall attenuation and the frequencies which are boosted are those which are not attenuated.  When you add a recovery stage the flat-setting gain is restored and the circuit as a whole behaves as it would if it could boost and cut.

Like stm said, you should start with the usual fender or marshall stack.

[tcio]

Thanks for responding. I have done a few tone stacks but always lost the guts and output level so never used any of them on my own designs. I just recently found out on this forum about adding an after stage gain to bring the level back up but haven't looked into it yet. I'm not sure whether that is just a buffer or what. So what you are basically saying is that the passive tone stacks as opposed to active are more flexible when it comes to tweaking changing values, down scaling, and etc?