Crowther Audio - Hot Cake Overdrive [traced]

[Greg]

Just out of curiosity, at what setting do you have the drive/gain control? Up till now, nobody really much cared for the HC at higher maximum gain, where it turns fuzzy.

modman

If you change R6 (the resistor before the drivepot) to something like 800R - 1K, you'll have less total gain and a more usable pedal IMO.
I also use a smaller pot in order to have less minimum gain, as I find the lower settings aren't a lot of use to me either.
The gain sweep is a lot more useful with these mods i think....

[blanik]

i finally got a sound i love by changing the input cap (C1) to 0.039uF and the output cap (C7) to 0.022uF i think the output cap change made the biggest difference (i'll try putting C1 back to 0.01 and keeping the C7 at 22nF to see if the input has anything to do with the sound i got after i changed the C7)

really sound like a better BD-2 now

Just out of curiosity, at what setting do you have the drive/gain control? Up till now, nobody really much cared for the HC at higher maximum gain, where it turns fuzzy.

about the whole range! lol what i did with this project is a "travel BD-2", meaning, i wanted a small (small hammond box), low voltage, relatively clean booster with mad all out fuzz distortion... the BD-2 really shines with clean to slightly dirty headroom (my modded one at least, D3 and a whole side of the 4 clipping diodes = 3mm LED) so i wanted that kind of overdrive but with the possibility of switching full distortion for those "all out" song parts without having to haul a second pedal...

...soooo what i did was to box the HC in a regular 1290 hammond, wire it true bypass with a mini-switch DPDT, no LED, put a 3PDT (with an hi-brt LED with a 20k... battery should last a long time...lol) but use it to switch between two C50k pots, one set for slightly dirty boost (turns out good on an near clipping tube amp) and the other one set for near full drive... the mini DPDT is mainly there as a safety feature in case of a live IC failure, the challenge was fitting the 4 knobs (3 mains plus extra drive pot), the mini-switch, 9V adaptor and battery... ended up quite cool, i'll post pics later... (i never turn off my BD-2, i just turn down the guitar volume to clean-up, i wanted to keep that feature which is often mentioned as an HC characteristic)

on initial testing i really liked the HC but i was testing it on my P-90 guitar, when i tried it on my humbuckered (Chopper T) the low frequencies where killing the sound... hence all the cap testing i did...

so no matter the drive setting i think the in and out caps keep lows under control (i guess the C7 and 50k pot work as a stable high pass filter cause the pot is placed after the cap and it's other end is grounded, acting as a fixed resistor, effectively cutting lows throughout any pot settings... (my guess, i'm not a big electronics connaisseur...))

[stephenxvp]

My thanks to Torchy for the veroboard and the forum for the intelligent discussion. Even as a complete newb I was able to get it working and to understand a little of the how it werks process.
Since building this pedal it stays on all the time while I am playing, beautiful added thickness and the guitar is very touch sensitive.

A question however: what is the function of the R4 / C2 combination? I am looking to build a second one for my other amp.

[Greg]

My thanks to Torchy for the veroboard and the forum for the intelligent discussion. Even as a complete newb I was able to get it working and to understand a little of the how it werks process.
Since building this pedal it stays on all the time while I am playing, beautiful added thickness and the guitar is very touch sensitive.

A question however: what is the function of the R4 / C2 combination? I am looking to build a second one for my other amp.

If you're looking at the same schematic as I am:-
R2 and R4 form the voltage divider.
C2 is a pwer supply filter.

[stephenxvp]

My apologies if this is a stupid question, but what are they supplying the divided voltage to?

I am looking at the schematic with Torchie's vero layout about page three.

They just sem slightly unnecessary components to a beginner, what are they filtering from the power supply?

[bajaman]

Hi Stephen
First you have to appreciate that op amps are designed for a bipolar or + and - supply rails.
This ensures that the inputs and outputs are close to zero volts.
However when we are only using a +ve 9v DC supply rail, we have to set an artificial mid voltage of approximately +4.5v DC on the inputs, so that the signal can swing evenly between the +9v DC and ground.
Okay - looking at R2 and R4, you will notice that these are disimilar (although they could be identical).
Paul has chosen these values to ensure even clipping on the top and bottom of the signal when amplified.
Using the values 82k and 100k gives us a theoretical centre rail voltage of +4.95v DC (assuming a +9v DC battery voltage) at the junction of R2 and R4. This is known as the half rail or bias voltage, which is applied through the 1M resistor connected to the op amp non inverting input.
The function of the electrolytic capacitor in parallel with R4 is to shunt any signal (AC voltage) to ground.
This gaurantees that the DC voltage at the junction of R2 and R4 remains at +4.95v DC while to the input sound signal it is at 0 volts or ground potential. Due to the op amp's internal construction, the output will also bias up at +4.5 v DC ( do not forget that the 1M bias resistor on the non inverting input will drop some of the +4.95v DC to give a closer +4.5v DC at the actual input terminal.
Cheers
bajaman

[Greg]

Thank you Bajaman... you explained that better than I ever could have.
The Hot Cake runs at 8.7 volts I think due to the Zener, so voltage at the Bias point is around 4.8 volts ?

Can you explain the purpose (and choice of a value) of the 1M from Bias to input as well ?

Cheers

[bajaman]

Can you explain the purpose (and choice of a value) of the 1M from Bias to input as well ?

Sure- the pupose is to provide a high input impedance bias supply to the opamp .
if you use a lower value you will lose bass response (unless you increase the input coupling capacitor by the same ratio as the lower resistor value), but more importantly you will damp the resonant peak in your guitar's pickup, which in the case of a Fender will result in a muddier and flatter top end or less sparkle in the upper harmonic range. Because the opamp serves two purposes (it is a high impedance unity gain buffer when the effect is switched to bypass) I would caution lowering the value of this resistor under 470K, unless you wish to experience "tone sucking"
One other thing - you will have to alter the bias divider value of R2 if you play around with the value of the 1M resistor - you may have to increase this because now the lower value bias feed resistor is not dropping as much voltage to the op amp inverting input.
cheers
bajaman

PS: it would be real nice if the designer - Paul Crowther (or PEC as he is registered here as) was to offer some insight into this circuit

[Greg]

Cool Bajaman - fills in a bit more of the puzzle for me.
I can see that the signal goes through the input cap and then the 1M and 100K to ground - creating a HP filter - right ?
So if the bias point was connected directly to imput, you'd expect to see an input buffer to set the input impedance ?

[stephenxvp]

Mr Baja, heartfelt thanks for the thoroughness and transparency of your explanation, just like a really good overdrive pedal.
Learning does not cease as you age.

So, back to building with no left out bits.

[bajaman]

I can see that the signal goes through the input cap and then the 1M and 100K to ground - creating a HP filter - right ?
So if the bias point was connected directly to imput, you'd expect to see an input buffer to set the input impedance ?

The junction of the three resistors is at 0 volts AC but approx 4.95v DC due to the electrolytic capacitor in parallel with R4.
Electrolytic capacitors block dc voltages (when polarised correctly but allow AC or signal voltages to pass unimpeded.
Yes the input capacitor and the 1M resistor form a high pass filter at the input - the value of the frequency where the input signal is reduced by 3dB is determined by the formula 1/(2 x Pi x R x C), where Pi =3.1412 approx., R is in ohms, C is in Farads.
For example: the rolloff (-3dB point) for a 10nF capacitor and 1M resistor is 1/(2 x 3.1412 x 1000000 x 0.000000010) = 15.9Hz. Change the capacitor to 1nF and the -3dB down point is now 159Hz - less bass response with a smaller input capacitance. What do you think the -3dB down point is with a 100nF capacitor????
cheers
bajaman

[Greg]

For example: the rolloff (-3dB point) for a 10nF capacitor and 1M resistor is 1/(2 x 3.1412 x 1000000 x 0.000000010) = 15.9Hz. Change the capacitor to 1nF and the -3dB down point is now 159Hz - less bass response with a smaller input capacitance. What do you think the -3dB down point is with a 100nF capacitor????
cheers
bajaman

1.6 Hz.
Thanks for the explanation...

[kurtlives]

Can someone explain what the purpose of the zener is?

[TragicTravisty]

d2 filters the supply to its rating, d3 i dont know about.... since only three volts are passing through d3, and its rating is presumably 3 volts, i dont understand what purpose it serves.

[kurtlives]

There is only one diode... A Zener going from the 9V+ to ground? Polarity protection or soemthing?

[bajaman]

polarity protection

yes
bajaman

[kurtlives]

Why not use a cheaper and easier to find 1Nxxxx diode?

[bajaman]

Why not use a cheaper and easier to find 1Nxxxx diode?

Why not ask Paul ?(registered here as PEC)
Perhaps it protects the circuit from overvoltage - for example if someone plugs in an unregulated supply 12 -15 volts, the series resistance and zener will clamp it to 8.6v dc and provide some consistancy to the sound characteristics - JHS suggests that the HC does sound better at a slightly lower than 9.2v supply if I remember a previous comment correctly.
cheers
bajaman

[TragicTravisty]

correct me if im wrong, but a zener was problably used because of its 3.1v rating, so that it would provide no resistance to the current. a regular diode with say a .7 volt rating would provide resistance to the current, right?

[nooneknows]

...You make me want to build one...
I intend to use it with a strat and a fendery amp (a modified peavey c30), what could be the best version in your opinion, 2003 or 1977?
thank you