Anderton's Frequency Booster

[petersenengineering]

Hi Guys,

Recently I thought i'd have a go at an Anderton frequency booster so i can boost frequencies around the 1.4kHz band. I got the main bit of the schematic from here:
http://www.muzique.com/news/frequency-booster/

I then added a buffer infront of this as suggested and made a couple of other changes and ended up with this schematic:


I made up the prototype and it doesn't perform as I intended... it sounds like quite an awesome 8bit fuzz and the potentiometer doesn't do anything at all! Currently I'm not trying to recreate the sounds of a ZX Spectrum, so I'd like to find out what I've messed up on.

I'm fairly inexperienced when it comes to deciding the values of components, so i'm guessing it is a nooby mistake, but can anyone help me out here?

[amz-fx]

You buffer needs a little modding.

Change R1 to 1M and add a new 1M resistor from pin 3 of IC1a to Vcc. The pin 3 of IC1a is then biased at 1/2 Vcc which should make the buffer work correctly.

regards, Jack

[petersenengineering]

Thanks for the help Jack, I made the changes and it works great now!

Cheers
Chaz

[bool]

For a djenty boost?

AMZ instructions are correct, but upping the bias resistors from 1M to something like 3-something megs (fe 3.3Meg) could be even better for increased input impedance (only in case you use passive PUs).

[petersenengineering]

Yep that's right bool, I've got a Boss SP-1 clone that I made that I use for that djentyness at the moment, but I wanna use that for a different frequency for my clean tone and I don't have room for two on my board, so I'm fitting this into a 1590A so I can free up the spectrum clone.

[Mark Hammer]

In theory, one could cascade several resonant booster stages.  So, with the added input buffer stage, you go into a resonant booster, as shown, but go into a second resonant booster stage after that.

Here's where it could get interesting.  The resonant boost is engaged when that 10k ground-leg resistor is tied to Vref.  A DPDT stomp could be used in reciprocal fashion to connect the ground leg of one of those stages when it lifts the ground leg of the other.  In other words, you could use a stomp-switch to select between two different resonant boosts.

[petersenengineering]

That sounds very interesting to try out mark, i think i'll breadboard another one up and give it a try

[bool]

Here's a simpler, single-opamp boost circuit tuned to djenty ranges to try out.

You'll have to look at a different circuit (ROGs thunderbird) for the snippet, but I'll provide the correct component values. http://www.runoffgroove.com/thunderbird.html

See the circuit around the U3a, it's called a "bottom" boost there.

So change both filter caps to 4,7nF.
Pot stays 50K or 47K.
Change the 22K resistor to 10K.
Change the 47K resistor (inline with the pot) to 10K or 8,2K (will change the fc slightly)

Use the same biasing as you used in your buffer (so discard what ROG used at the U3a).


This should give an up to close to 12dB boost at the right frequency.

This is called a "bridged-t" filter. Used as a boost here.


Of course you could just bypass the pot with a stomp switch (or arrange some switching action along the lines what Mark recommended) to convert the circuit from a "boost" to a "buffer".

[petersenengineering]

That sounds awesome, i'll give that one a go too!