Hotcake Circuit Analysis?

[thehallofshields]

I've seen plenty of breakdowns of what's happening inside the classic distortion circuits; the BMP, FuzzFace, TS, Dist+, and even ultra basic circuits like the Elektra. Now I'm curious about the Crowther Hotcake. If anyone can point to any literature on the circuit itself, I'd much appreciate it.

Here are few of the specific questions I have:

1.) Why does clipping a TL07x opamp sound so good? (Treble Filtering? Slew Rate?)
2.) What is the headroom?
3.) Does the bias create asymmetrical clipping?
4.) What is the center frequency of the Presence Control?
5.) What are the best bypass options for this circuit? (Tips for low-noise layout, wiring and switching?)

[PRR]

Schematic or we can't talk.

Maybe? http://4.bp.blogspot.com/-pgDMzXAv3Pw/TxxcF2JgeNI/AAAAAAAAAqg/zYABLMqgeXA/s1600/Crowther_Hotcake-2003.gif

> Why does clipping a TL07x opamp sound so good?

Naked '07x is harsh. (Note that a '741 was also used.) There's heavy response bending, which must take some curse off. The Presence knob does not have a "center", it is more/less highs. Not inclined to work out the frequency response. Breadboarding with 2:1 changes of each R and C will tell you more than theory will.

[thehallofshields]

How about we just go off the Flapjack Clone for simplicity:

[Kipper4]

Schematic pls

Sent from my SM-G920F using Tapatalk

[antonis]

1.) Why does clipping a TL07x opamp sound so good? (Treble Filtering? Slew Rate?)
2.) What is the headroom?
3.) Does the bias create asymmetrical clipping?
4.) What is the center frequency of the Presence Control?
5.) What are the best bypass options for this circuit? (Tips for low-noise layout, wiring and switching?)

1. Paul answered..

2. It depends on specific IC..
(i.e. for 741 is 2V lower than PS..)

3. As above..
(many ICs don't swing exactly the same "off-set" from their PS so in some cases we have to "balance" their possitive to negative PS headroom "deviations"..)

4. It depends on Presence + Level pots settings + OUT load..

5. The all time classic DPDT switch (or 3PDT if you wish an indicating LED..) with a 1M (or higher) shunt resistor before the input cap..

[thehallofshields]

5. The all time classic DPDT switch (or 3PDT if you wish an indicating LED..) with a 1M (or higher) shunt resistor before the input cap..

Thanks for the replies. 

I've got the circuit breadboarded now, when I use my 3PDT bypass box I get whining type oscillation when the effect is bypassed. Now, I've got a lot of wire between the breadboard and the jacks, but since the original circuit designer warned against using 3PDT in this design, I'm wary to box it up.

[thehallofshields]

Another interesting finding is that the gain increases with increases in C6.

With C6 at 86n as in the design, I get a some nasty fizz from the OpAmp. When I pull it, I barely get any clipping. Not what I expected.

[dschwartz]

Funny how the schem looks like they wanted to use a bmp tone control but maybe they got it wrong and was a happy mistake

[thehallofshields]

Naked '07x is harsh. (Note that a '741 was also used.) There's heavy response bending, which must take some curse off.

Is it just a 2nd Order LPF? [IC1a, C4] and [R9, C5]

Is the circuit just dumping the OpAmp fizz to ground or is there some other magic loading happening there?

[Ben Lyman]

...whining type oscillation when the effect is bypassed...

Yep, been there too. Try my favorite switch wiring, lower right pic:
"circuit input and output grounded"

[reddesert]

There are some threads about the Hotcake on the other forum (FSB). You can look for comments by user bajaman on the circuit. One of his remarks is that the feedback network, R8-C6-R10 & R7, is frequency dependent.  Additionally, apparently early versions of the Hotcake, instead of the Presence pot, had a switch between the two extreme positions.

Oscillation is often reported in Hotcake builds, possibly because there's a lot of gain available, although you don't have to turn it all the way up (it's not a fuzz pedal). There's even a letter from Paul Crowther, the designer, about bypass and oscillation quoted in one of the FSB threads.

[antonis]

Another interesting finding is that the gain increases with increases in C6.
With C6 at 86n as in the design, I get a some nasty fizz from the OpAmp. When I pull it, I barely get any clipping. Not what I expected.

But you should expect it...

Inreasing C6 results in let more bass to pass through it so more bass gain - it creates (with R10) a HPF in NFB loop (as it's wired via R8) of about 1942Hz corner frequency..
(i.e. 19kHz don't actually mind about C6 value but 190Hz DO mind a lot..) 

When you pull out C6 you turn your amp's specific section in a unity gain one..!!
(R10 is set off NFB loop so there isn't any voltage dividing from output to inverting input - although the specific loop wiring needs more complex analysis..)

[thehallofshields]

I found a blog / article that talks about the fizzy decay (glad that it's not just me!) issues, as well as the behavior of the 'Bluesberry' diode mod.

http://atomiumamps.tumblr.com/post/104789328041/heres-the-aforementioned-hotcake-all-boxed-up

[Plexi]

Built this one last week.
It got me amazed how good it sounds: love the distortion it produce.

I've built the latest version, with presence pot.
Used TL081 instead the '071.

Get some weird oscillation when the guitar volume pot is at 7/8 (80% of the sweep).
What could be?

[thehallofshields]

I was also very impressed by the distortion character. I tried it with all of my different Sansamp pedals and have to agree with the conventional knowledge that it sounds perfect with a slightly dirty Vox amp.