Dan Armstrong - Purple Peaker

[chrishopkins]

Hi Guys,

I'm looking around for a suitable circuit for my first project and I've stumbled upon the modified version Dan Armstrong's Purple Peaker on the   Homewrecker site.

What I want to do is to add several additional RC stages to provide more options.

Does anybody know the formula for determining the capacitor values for a particular frequency?  The text accompanying the schematic states that the frequencies for the lo section is 159 Hz and 3.18 kHz for the high.

Any help on this would be most appreciated - i'd like to combine this with an Orange Squeezer at some time.

Thanks

Chris

[B Tremblay]

Check out Mr. EQ.  We used the Purple Peaker as our primary inspiration for the circuit.

However, you may want to choose a simpler circuit for your first build.

Regarding tuning of the Purple Peaker filters, the standard f=1/(2piRC) formula is used.  R is the resistance in Ohms, C is the capacitance in Farads, and pi is pi.  Looking at half of the LO twin-T filter, we use the values 100,000 for R and .00000001 for C.  Plugging in those values results in a center frequency of 159Hz.  In the other half of the filter, we use 51,000 for R and .000000022 for C.  That results in a center frequency of 141Hz.

You'll notice that between each T of the filter, the R and C values have a (nearly) 2:1 ratio.  The closer they are to exactly 2:1, the narrower the peak.

So, let's say you want to tune the filter a little lower.  We can change 100k to 150k and 51k to 75k, still trying to keep that 2:1 ratio.  If we plug in those values (which the same caps), the center frequency moves down to around 100Hz.

I owe my understanding of the circuit to GFR, who dropped the science on me back in the day:

http://diystompboxes.com/sboxforum/viewtopic.php?t=2069

[chrishopkins]

Thanks for the info - just read the other topic as well.  Definately one for experimentation - i suppose the best thing to do is to experiment with capacitor values until you find the sweet spots.

Maybe I will build the Orange Squeezer first  

[B Tremblay]

Here's some more required reading:

http://geofex.com/Article_Folders/EQs/paramet.htm

http://diystompboxes.com/sboxforum/viewtopic.php?t=25788

[chrishopkins]

Thanks - I'm just reading the Geofex article and I'm trying to work out how the filter sections in the Peaker match in with the examples shown.  I can see that their both T junctions I'm just at loss as to see what the extra components in the Peaker are doing.

So I've extracted the filter section from the schematic and labelled it.



Would I be right in saying that c1/c3 are the first filter and that c2 forms the second?

The earlier description of the filters indicated that each filter boosts at 2 frequencies (hence the peak?).  If I'm being dumb then I apologise in advance.

[B Tremblay]

The left-side network is the LO twin-T filter and the (unlabeled) one on the right is the HI.

One T is formed by R1, R2, and C2.  C1, C3, and R3 form the other T.  It's a little tricky, since we use R1 and C1 to calculate one center frequency, while C2 and R3 are used to determine the other center frequency.  Those two Ts combine create a notch filter around 145Hz.  Placing the notch filter in the feedback loop of the gain stage results in a peak.

R4 is a pulldown resistor to avoid popping while switching in the peak.  Without it, there would be no path to ground for the three caps when the peak is off.  They'd fill up and quickly deplete when the peak is activated, causing an audible pop.

[chrishopkins]

Thanks for the info - it's very much appreciated.  I'm going to build a Fuzz Face first then have a go at a Peaker design.

I may even combine the 2 and see if I can come up with something similar to the SD Tweak Fuzz