The Anderton FreqBooster

[tcio]

This is from the "MXR 1970 6 band EQ" topic but it kind of turned into more of an Anterton FreqBoost topic so I wanted to start a new topic so the "MXR" thing didn't throw off people who may be interested in the Anderton FreqBoost .

Note that you can easily and painlessly cascade boost stages with the Freq Booster.  Stick a 3-position toggle for frequency-selection in each of 3 stages and you've got a lot of flexibility there for little cost and a small footprint.

Need help with this. I don't understand what the 3 stages are and where I would connect the switch. Is this referring to the "ins" and Outs" on the IC? Mark or anyone out there......any help would be more than welcome 

As far I understand there are only two caps that control the frequency. I would be grateful for any kind of help on this one. It sounds super simple but it doesn't take much to confuse me. I want to slap it together and play out with it this weekend but would like to try the "3 Stage frequency-selection" thingy first before soldering.

[Mark Hammer]

OK.  Here is the basic circuit, courtesy of Jack Orman.  The original has a buffer stage in front and a volume pot on the end, but we'll ignore that for the moment.

The op-amp (which doesn't really have to be a FET op-amp), is set up as a non-inverting stage.  So, the gain of the op-amp is set by the 10k feedback resistor between output and the '-' pin, and the so-called "ground leg", which is the 10k resistor between the inverting ('-') pin and ground.  The smaller the ground-leg resistance, in comparison to the feedback resistance, the greater the gain.  When the on/off switch is closed, the gain is (10k+10k)/10k = 2.  When the switch is open, the gain is (infinity+10k)/10k = 1.

With a gain of 1, the "boost" control does nothing.  When the gain is turned "on" (and here I think one might achieve greater boost is the 10k ground leg ius made a little smaller, like 8k2), the feedback that comes through the boost control applies that gain selectively, according to the range selected by the cap values.  The extent to which the boost is applied selectively vs generally is a function of the setting of the 10k boost pot and the 5k6 resistance in series with it.  Reduce either of those and you start to move in the direction of a self-resonating circuit.

There is nothing, really, that prevents one from taking the output side of the op-amp, and tacking it directly onto the input of a 2nd identical stage.  If one did that, you'd need to leave the 2u2 cap out or else you'd be obliged to re-establish the Vref for the next stage.  The 2u2 cap normally blocks any DC the audio signal may be riding on so that it is not fed to the next pedal or amp input.  You certainly want it at the final output of the pedal, but not between stages since it complicates unnecessarily.

Now, the drawing shows some selected cap values and their corresponding resonant point.  Imagine that C1 was actually two caps in series, and C2 was actually two similar caps in series.  Let's say .22uf and .15uf.  Now let's say that yo had a DPDT center-off toggle connected to these two pairs of caps such that the middle lug of each set of contacts went to the junction of a .22/.15 pair, and the outside lugs went to the far side of the .22 and the far side of the .15.  When the two caps are in series, their effective value is .089uf.  Shunt the .15 side and the value is now .22.  Shunt the .22 side and the effective value is .15.  So, two pairs of caps and a toggle gets us an estimated resonant frequency range of roughly 90hz, 130hz, and 220hz.  If those cap pairs were .068uf and .047uf, those frequencies would be roughly 290hz, 425hz, and 720hz.  And so on.

The connection from the 10k ground-leg resistor to ground can be lifted independently for each stage and they will still function as unity-gain stages with no selective boost.  The one thing I might caution you about is that there needs to be a 2u2 cap on the final output, along with a 10k volume pot.  The other thing I'm a little concerned with is the cumulative gain aspect.  Keep in mind that gain is multiplicative.  So the final gain is equal to the gain of the first stage times the gain of the next one times the gain of the next one, etc.  For that reason, you want to have the stages that might amplify hiss near the end.  As well, while I have recommended to skip the 2u2 DC-blocking cap until the very end, you still need to insert a 4.5v "floating ground" at some point, and that point is the input stage that is omitted from Jack's drawing.

Switching such a beast can be done in numerous ways.  I suppose the most sensible is 2 toggles and a pot for each stage (on/off, range select, boost amount), plus a final volume control and master bypass.    That easily fits in a 1590BB.  If one's goal was more economical and specific, then something like boost controls for two selected bands (implying two booster stages), plus a master volume could easily fit in a 1590B with a master bypass stompswitch.

Starting to get the idea?

[rubix cube]

Isn't the 4.5V "floating ground" for 9V use?  The picture shows a +-12V supply.  Sometimes these things make me doubt that I am learning anything!

[Mark Hammer]

Isn't the 4.5V "floating ground" for 9V use?  The picture shows a +-12V supply.  Sometimes these things make me doubt that I am learning anything!

You are correct!!  The original IS a +9v circuit with floating ground.  Jack's redrawing indicates a bipolar supply and omits the input stage.  Had I done a search, rather than relied on my memory of where I had seen a posted schem previously, I would have stumbled upon this delightful and useful little drawing from Paul Marossy which HAS all the things missing from Jack's drawing that a beginner would need to know:  http://www.diyguitarist.com/PDF_Files/FreqBooster.pdf

Sorry, guys, and thanks Paul!

[tcio]

Thanks so much Mark! I really appreciate your time and patience. You are right, this circuit is very small and simple. It should be able to fit in about any enclosure even with mods. I am going to get on this today thanks to your help. Thank you also Rubix for your input.

[tcio]

I seem to still be in need of some help here. I have found that my amp has a sensitive input or something. So far a lot of boosters, even some store bought ones seem to just push the input of my amp over the edge but without necessarily getting real loud. I have had this problem with the MXR Micro amp both DIY and store bought. Also when I used a ToneBone amp switcher it had a boost on it and whenever I turned that on my amp would make that sizzling sound also whenever I hit the strings real hard. I have asked about this on a different topic and we basically came to the conclusion to put more resistance in front of the out put pot or just turn it down until it stops doing it.

I am currently having this problem with the Anderton FB. I have been doing the "more resistance on the output" thing and it has worked really well while driving one of my other foot pedals but by itself the output is half or even less than all of my other foot pedals. I would like to use it on its own for a nice tube amp boost just for certain songs but the volume level it puts out is too incompatible (low) for a live application plus sounds real weak compared to my other pedals and if I turn it up my amp starts making those freaky hair raising sizzly clipping noises. It just doesn't make sense to me but I am still quite new at a lot of this. I thought there must be some way to get a louder output level without getting that sizzling sound and clipping my other components (FX unit, Parametric EQ).  I just cannot figure out how it can clip things so easy when I hit the strings real hard yet my other booster (MXR 6 band EQ) gets twice as loud and doesn't clip a thing. Can anyone guide me to what I can do to get the Anderton FB louder without getting that freaky sizzly clipping sound?

[J-Bones]

Found this while searching and decided to resurrect it.   

I'm having the same problem as tcio, the output is WAYYY low while running through this circuit.  I'm going off of the same schematic that Mark posted, http://www.diyguitarist.com/PDF_Files/FreqBooster.pdf.  I'm fairly new to building and modding and this is my first scratch build.  I noticed that in the circuit layout there is a 1M resistor that doesn't appear in the schematic.  I did put that in my circuit.  Could that be the reason?

I'm also wanting to run this with a 3PDT switch for true bypass with an LED indicator, so what should be done with the two points where the SPST switch are in the diagram?

Thanks.

[Paul Marossy]

Isn't the 4.5V "floating ground" for 9V use?  The picture shows a +-12V supply.  Sometimes these things make me doubt that I am learning anything!

You are correct!!  The original IS a +9v circuit with floating ground.  Jack's redrawing indicates a bipolar supply and omits the input stage.  Had I done a search, rather than relied on my memory of where I had seen a posted schem previously, I would have stumbled upon this delightful and useful little drawing from Paul Marossy which HAS all the things missing from Jack's drawing that a beginner would need to know:  http://www.diyguitarist.com/PDF_Files/FreqBooster.pdf

Sorry, guys, and thanks Paul!

Glad I can be of some use around here once in a while. 

Found this while searching and decided to resurrect it.  

I'm having the same problem as tcio, the output is WAYYY low while running through this circuit.  I'm going off of the same schematic that Mark posted, http://www.diyguitarist.com/PDF_Files/FreqBooster.pdf.  I'm fairly new to building and modding and this is my first scratch build.  I noticed that in the circuit layout there is a 1M resistor that doesn't appear in the schematic.  I did put that in my circuit.  Could that be the reason?

I'm also wanting to run this with a 3PDT switch for true bypass with an LED indicator, so what should be done with the two points where the SPST switch are in the diagram?

Thanks.

If you are referring to the 1M resistor to ground on the input, that's just a pull down resistor.

[J-Bones]

OK, Thanks Paul.   

I'm going to try to post some voltage measurements later tonight after I get back from rehearsal.  Probably something simple in there that I can't see...

[J-Bones]

Ok, just to recap, I'm going off of this schematic and layout: http://www.diyguitarist.com/PDF_Files/FreqBooster.pdf.  I perfed it following the layout with no parts substitutions.  As I said before, I want to run this with a 3PDT for true bypass with an LED indicator, so I am in question about what to do at the two points where the SPST switch is in this schem/layout, or should I lay things out a little differently for that?.  The IC IS inserted in the socket and the two electrolytic caps are oriented correctly. 

Like tcio, I'm getting signal through the circuit, but the output is really weak, about half as loud as when it's bypassed, even with the Level control all the way up.  It also seems like the Boost isn't really boosting the mids very much at all.

Here are some measurements:

Battery    = 9V
Circuit V+ = 9V
Circuit V- = 0V

IC (NE5532)
Pin 1 = 7.2V
Pin 2 = 7.2V
Pin 3 = 0.6V
Pin 4 = 0V
Pin 5 = 4.8V
Pin 6 = 4.2V
Pin 7 = 8V
Pin 8 = 8.8V

If I need to measure anything else, let me know. 

Thanks in advance for your help.

Jeremy

[Frances Rhodes]

hey y'all

i have a few questions about this...
first question about the shape of the frequency response of this filter: i guess it is a bandpass filter so how wide is the band width?
and then, can i use this without a buffered input or do i need to add an other op amp section before the circuit shown?

regards

[GibsonGM]

hey y'all

i have a few questions about this...
first question about the shape of the frequency response of this filter: i guess it is a bandpass filter so how wide is the band width?
and then, can i use this without a buffered input or do i need to add an other op amp section before the circuit shown?

regards

Hi Frances - you don't need anything more than the parts as shown in the schematic - BUT, I've had the best luck with this particular version:  http://www.diyguitarist.com/PDF_Files/FreqBooster.pdf

It uses a dual opamp and so is buffered at the input.   The 1 opamp version never worked for me (squealed a lot), don't know why.


You could try simming this in LT spice to see the bandwidth.....to me, the midrange one seems to be similar to the width of a wah, maybe a bit sharper.  Well-suited to guitar, IMHO!  I've built a few of these in different ranges.  They go from subtle to total-tone-changing...

A good project, well worth putting together! 

[Frances Rhodes]

ok great, thanks!

do you know if "detuning" the circuit by choosing 2 "different" values for C1 and C2 would make the band width bigger?
like, i don't mean 2 completely different values but not exactly the same ones.
i want to use this as a bass lift circuit and i'm trying to figure out which would be best between this and a baxandall (but i'm having a hard time finding out how to calculate the cutoff frequencies with this one, and university is farther and farther everyday!!)

[GibsonGM]

Sometimes it's not easy, is it, Frances??  He he. 

I would sim this up in LT Spice if I were you.   I did, and for .22u caps, which center the frequency at about 90Hz, you get a "3dB bandwidth" of about 35Hz.  You could also just build it, and see if you like it - if not, turn it into some kind of swept filter, or use it for special occasions!    Change the caps out to change the range, or use a rotary switch as you've seen discussed.   

IOW, if we take the output of this with boost at .5, and volume at .5, and our center frequency is 90Hz, boosted to 9dB.....then your 6dB points lie at 74Hz and 110 Hz.   This is pretty narrow.   BUT - to my ear, this is exactly where guitar's require boosting in the low end.   That does NOT sound like a wah (sorry if that was misleading...the mid "wah" thing I mentioned = more like Tom Scholz of Boston's tone).

You CAN play with the cap values, but unbalancing this will tend to give really unpredictable results. 

A Baxandall will be more broad, and isn't that great for guitar....I'd try another, if you don't like the Anderton thang (you can get Duncan's TSC to get an idea of what each type can do).   

[Frances Rhodes]

hehe no, sometimes it is not, but i'm no quitter!

ok, i remember using spice in some ancient times when i had no idea i would someday be trying to build effects, i'll try to download it and mess with the circuit a little and see if something good comes out.
thanks for the precious advice!

[GibsonGM]

YW.  I'd get "Duncan's Tone Stack Calculator" off his site, and download LT Spice and get started with that <search engine>.

It's not that hard to learn, and there are many example circuits to get you started.    Once you're up and running, you can sim almost ANYTHING, and mostly the results are accurate!   I think both of them are pretty indispensible.