Crowther Audio - Hot Cake Overdrive

Greg · Post by **Greg** » 03 Jan 2008, 02:54

Thanks Bajaman.
That gives me a better understanding, but I guess I need to do some more study on filters...

bajaman · Post by **bajaman** » 03 Jan 2008, 10:03

this may help Xc = 1/(2 x pi x f x C)

where:
Xc is the reactance or equivalent resistance of the capacitor at the frequency f in cycles per second (Hz), and C is the capacitance in farads.
pi = 3.1416 (approximately)

for example: at 100Hz, the equivalent resistance value (impedance) of a 100n capacitor will be 15915.49 ohms ( 16k approx).
at 5kHz the same capacitor will be 50 times smaller or 318 ohms approximately.

Once you know a capacitor's impedance at a specific frequency, you can determine how much it will attenuate or boost the signal level in conjunction with a fixed resistor.
cheers
bajaman

analogguru · Post by **analogguru** » 03 Jan 2008, 10:07

Here is a table for the most values:

http://analogguru.an.ohost.de/techstuff/Reactance.htm

analogguru

Post by **modman** » 03 Feb 2008, 23:34

bajaman wrote:
Maybe someone could talk me through it (10k, 82n and 1k) ??
Normally the 100k feedback resitor would be connected between the output terminal and the inverting input terminal of the opamp.
Varying the resistance between the inverting input and ground ( via the series capacitor to block dc bias and set low frequency gain structure), will increase or decrease the op amp stage gain in a high pass fashion.
Okay - here is the trick
We connect a series chain of 10k, 1k, and 82n from the op amp output to ground. This combination provides a variable frequency reactive filter on the output of the op amp.
At low frequencies - say 100hz, the 82n capacitor's impedance will be 19.4k ohms.
At high frequencies - say 10khz, it's impedance will be 194 ohms!!!
So - at high frequencies only 10% of the op amp's output signal voltage appears at the 100k feedback resistor, but at low frequencies 67% of the op amp's signal voltage appears at this point.
Because less feedback of signal is available at high frequencies, as the gain is increased the op amp goes into slew rate induced distortion a lot quicker at high frequencies than at low frequencies - this accounts for the clarity in the low end response when driven hard - the fundamental remains reasonably clean compared to the distorted top end response.
cheers
bajaman

Today I tried the 8k2 -150nF -560R suggestion for R8 - C6 - R10 which indeed might the XLF version, and it brings in more bass distortion earlier. But a lot depends on the action of the drive pot -- and as I didn't have the 50kC pot, I used a 100kB with 100k across the wiper/outer lug which should give me as perfect an antilog 50k (relative to the linearity).

This didn't give me a taper useful for gradually dialing in gain -- I finally settled for 20kB by ear. To my surprise this value is mentioned somewhere in this thread.

I then used 12k - 68nF -1k3 as an inituitive extrapolation, and this at t least sounded more bluesy to me. Berry or not, it also permits settings in which you put the overdrive on the bottom strings (G-B-E) only. I did use 27nF instead of 22nF for C5. Drive maxxed out still produces that fuzzy, trebledead tone that might be useful for some leads, but at lower drive settings it's your perfect modern rangemaster. Or what people think a treble booster does,

Distortion for the rhythm guitarist with great "ka ching". I guess you can tell I like this circuit

more later!

bajaman · Post by **bajaman** » 04 Feb 2008, 01:59

Hi Modman - yes, there are a lot of hidden sounds in the hot cake, which can be found by variations of a few component values

.
try different values for the capacitor in series with the gain control too!!!

cheers
bajaman

phantasm · Post by **phantasm** » 11 Mar 2008, 18:24

Does anyone have any idea what the differences in this pedal are for the "bass version?"

bajaman · Post by **bajaman** » 11 Mar 2008, 20:18

Does anyone have any idea what the differences in this pedal are for the "bass version?"

Now that the designer is here on this board (PEC) - it would be far better for him to comment on this
cheers
bajaman

Deric · Post by **Deric** » 24 Mar 2008, 04:46

Built this today...

...from this layout I did some time ago...

Sounds very good....except, there is a very subtle gatey sound to the decay. This only happens in a small part of the gain pot rotation - just above halfway. If you turn the gain up or down it goes away. Tried an OPA134 and, if anything, it was worse. Double checked the layout and parts values in the pedal and all seems good.

Is this normal? Maybe some fresh eyes will see something I'm missing.

bajaman · Post by **bajaman** » 24 Mar 2008, 10:41

sounds like a parasitic oscillation problem - try placing a 22pf capacitor in parallel with the 100k feedback resistor - that should shut it up!!!
cheers
bajaman

Deric · Post by **Deric** » 25 Mar 2008, 05:45

Thanks for the suggestion. Tried this tonight with no noticable change. Overall it sounds very good - it just has what I would call an "ugly" decay. I noticed tonight that this really happens all thru the range of the gain pot, it's just easier to notice at certain settings. At the very end of the note decay, it gets slightly splatty or gated sounding - this is very subtle - but it's definately NOT a smooth decay.

It also seems VERY compressed - not sure if this is normal or not.

IC voltages - Battery = 9.22v
pin 2 = 4.75v
pin 3 = 4.34v
pin 6 = 4.75v
pin 7 = 8.7v

blanik · Post by **blanik** » 25 Mar 2008, 08:19

about the farting decay, i noticed the same thing as deric... overall, i really like this pedal, it reminds me a lot of the BD-2 but livelier, i only need to tame the lower frequencies in it, where the BD-2 sounds a tad thin (but has better decay and cleans up better), the HC sound too bassy, i'm not sure if i should lower the input cap or output cap?

if i could just tighten the bass it would be a killer!

Post by **modman** » 25 Mar 2008, 09:44

blanik wrote:about the farting decay, i noticed the same thing as deric... overall, i really like this pedal, it reminds me a lot of the BD-2 but livelier, i only need to tame the lower frequencies in it, where the BD-2 sounds a tad thin (but has better decay and cleans up better), the HC sound too bassy, i'm not sure if i should lower the input cap or output cap?

if i could just tighten the bass it would be a killer!

Hi Blanik welcome,

The HC doesn't handle the max gain well, but you should be able to tame the bass with the gain control. Else you can tingle with the selective feedback circuit like I did (at Bajaman's suggestion) you should be able to tame the bass even more.

modman wrote:
bajaman wrote:
Maybe someone could talk me through it (10k, 82n and 1k) ??
Normally the 100k feedback resitor would be connected between the output terminal and the inverting input terminal of the opamp.
Varying the resistance between the inverting input and ground ( via the series capacitor to block dc bias and set low frequency gain structure), will increase or decrease the op amp stage gain in a high pass fashion.
Okay - here is the trick
We connect a series chain of 10k, 1k, and 82n from the op amp output to ground. This combination provides a variable frequency reactive filter on the output of the op amp.
At low frequencies - say 100hz, the 82n capacitor's impedance will be 19.4k ohms.
At high frequencies - say 10khz, it's impedance will be 194 ohms!!!
So - at high frequencies only 10% of the op amp's output signal voltage appears at the 100k feedback resistor, but at low frequencies 67% of the op amp's signal voltage appears at this point.
Because less feedback of signal is available at high frequencies, as the gain is increased the op amp goes into slew rate induced distortion a lot quicker at high frequencies than at low frequencies - this accounts for the clarity in the low end response when driven hard - the fundamental remains reasonably clean compared to the distorted top end response.
cheers
bajaman
Today I tried the 8k2 -150nF -560R suggestion for R8 - C6 - R10 which indeed might the XLF version, and it brings in more bass distortion earlier. But a lot depends on the action of the drive pot -- and as I didn't have the 50kC pot, I used a 100kB with 100k across the wiper/outer lug which should give me as perfect an antilog 50k (relative to the linearity).

This didn't give me a taper useful for gradually dialing in gain -- I finally settled for 20kB by ear. To my surprise this value is mentioned somewhere in this thread.

I then used 12k - 68nF -1k3 as an inituitive extrapolation, and this at t least sounded more bluesy to me. Berry or not, it also permits settings in which you put the overdrive on the bottom strings (G-B-E) only. I did use 27nF instead of 22nF for C5. Drive maxxed out still produces that fuzzy, trebledead tone that might be useful for some leads, but at lower drive settings it's your perfect modern rangemaster. Or what people think a treble booster does.

@Deric:
Only built this from Torchy's layout, an do not know for sure whether that layout was verified. Do report back whatever your findings are.

modman

blanik · Post by **blanik** » 26 Mar 2008, 04:01

hi, modman, thanks for the tip, i tried it and it was really interesting to experiment with 10k, 1k, 82n to clean up the lower frequencies but it's not what i was looking for, even clean the bass frequencies are "boosted" and sloppy, i'm really trying to get it closer to the BD-2 by thightening the bass (throughout the drive settings) by cranking the amp in a live setting, the bass will come through but thighter, i guess i should try to set a cap (input C1 or C7 on Torchys layout) to get a cut at about 100Hz (how can i find where the BD-2 cuts off?)

r.

bajaman · Post by **bajaman** » 26 Mar 2008, 05:18

Try reducing the electrolytic capacitor in series with the drive potentiometer - try half the value - that should tighten the bass for you - leave the input coupling capacitor as it is though!!
cheers
bajaman

blanik · Post by **blanik** » 26 Mar 2008, 06:26

thanks bajaman, i'll do that first thing when i get up tomorrow! (right now my neighbors would kill me if i'd do that...

)

i'll try a 4.7uF

blanik · Post by **blanik** » 26 Mar 2008, 15:06

i tried it and the C5 to 27n instead of 22n (like suggested by to no avail, it still has too much bass freq like suggested by modman (to get in rangemaster territory) i'm wondering about the relation between C4 and C6

blanik · Post by **blanik** » 26 Mar 2008, 16:33

i put the values back to stock but changed the input cap from 0.01 to 0.0036, neat improvement, still a little muddy, i'll also shrink C7 to a lower value (output cap)

what part of the circuit cuts the high frequencies?

bajaman · Post by **bajaman** » 26 Mar 2008, 19:10

what part of the circuit cuts the high frequencies?

try increasing the 470pf capacitor - try double the value.
cheers
bajaman

blanik · Post by **blanik** » 27 Mar 2008, 19:42

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

Post by **modman** » 27 Mar 2008, 23:32

blanik wrote: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.

Crowther Audio - Hot Cake Overdrive [traced]