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I saw some speculation that the Caline White Heat was a DOD Overdrive Preamp 250, so I found a schematic and compared it to the circuit board, but it I see no potential for it to be a DOD 250, most especially because there is only one diode on the entire circuit board. All of the schematics I've looked at to date involved at least two diodes for clipping, and another one or two in the power supply section. In this pedal it looks like the one and only diode is in power supply section. I'm hoping maybe someone has a hunch as to what circuit it's based on, and can maybe use the pics to confirm or deny.

I bought one and traced it out, seems like a unique circuit. I'll post a schematic when I've got a decent one made.

Here's a schematic, I'm not sure if it's unique or a copy of something. Either way it's pretty basic.

Are you sure R2 is 47k and not 470k? I guess it's just relying on the op amp to distort for any dirt. I'd experiment with other op amps since it's already socketed, maybe a LM1458

Yeah I measured them all.

The op amp is not socketed in my pedal.

It's a Fuzz/Boost sort of.

what's the purpose of the buffer U1B ?

nooneknows wrote: ↑28 Nov 2020, 18:41 what's the purpose of the buffer U1B ?

Voltage source with a low output resistance to generate the reference voltage.
Does this have an advantage over the voltage generation with the conventional voltage divider?
The AC signal voltage is short-circuited by the electrolytic capacitor with the high capacity in parallel to the second resistor of the voltage divider and thus connected to the virtual ground.
Also the bias direct currents of the OP-amps are so low that the internal resistance of the voltage generation with the conventional voltage divider does not have any disadvantage in my opinion.

I bet the double opamp was cheaper then a single and they thought 'what could we do with the other?' , although I would had use it as output buffer indeed.

nooneknows wrote: ↑28 Nov 2020, 18:41 what's the purpose of the buffer U1B ?

They did that to reduce PSU noise.

CheapPedalCollector wrote: ↑29 Nov 2020, 12:54 They did that to reduce PSU noise.

ok, but how? I don't understand the purpose of adapting the impedance, a buffer doesn't reduce hum by itself as far as I know

There's lots of examples on using an op-amp to buffer a voltage divider, I can't remember the particulars of doing so other than increased current and noise rejection as I'm pretty sure it acts as a low pass filter.

For some reason i like these caline boxes...

mirosol wrote: ↑13 Dec 2022, 15:56 For some reason i like these caline boxes...

I do too mostly, so far only 2 duds out of about 50. Slowly been buying them all. A lot of the newer ones are just reboxes/repaints of the original series though.

Thanks for the tagboard.

The use of the op-amp for the creation of a low impedance midrail is a well accepted practice in audio engineering. The lowered impedance on the midrail means that there will be very little audio impressed on to that bias line. It also means that you can use relatively high value bias resistors on the non-inverting input of the bias generating op-amp, and a fairly low value capacitor from that input to ground. The midrail will also be quieter than the junction of two high value resistors.

My only issue(s) with this design is that I would have made R2 a larger value, to raise the input impedance to prevent loading on either the previous effect or the guitar pickup. I've also been wary of exposing op-amp FET inputs to the outside world - they can be more susceptible to static than a single bipolar transistor, which is why there's a transistor buffer on (for example) the Tubescreamer. Also, I would also have made C5 larger - the -3dB point will be 60Hz as designed, so basses wouldn't sound good through this, and the turnover is too high to prevent hum getting through..... It's going to colour your sound slightly.

My only issue(s) with this design is that I would have made R2 a larger value, to raise the input impedance to prevent loading on either the previous effect or the guitar pickup. I've also been wary of exposing op-amp FET inputs to the outside world - they can be more susceptible to static than a single bipolar transistor, which is why there's a transistor buffer on (for example) the Tubescreamer. Also, I would also have made C5 larger - the -3dB point will be 60Hz as designed, so basses wouldn't sound good through this, and the turnover is too high to prevent hum getting through..... It's going to colour your sound slightly.

Hello Mictester!
I agree with you.
For myself, I would increase the value of R2 to 470k and change the value of C5 to 47n for guitar and 100n for bass application.

Manfred wrote: ↑15 Dec 2022, 14:38

My only issue(s) with this design is that I would have made R2 a larger value, to raise the input impedance to prevent loading on either the previous effect or the guitar pickup. I've also been wary of exposing op-amp FET inputs to the outside world - they can be more susceptible to static than a single bipolar transistor, which is why there's a transistor buffer on (for example) the Tubescreamer. Also, I would also have made C5 larger - the -3dB point will be 60Hz as designed, so basses wouldn't sound good through this, and the turnover is too high to prevent hum getting through..... It's going to colour your sound slightly.

Hello Mictester!
I agree with you.
For myself, I would increase the value of R2 to 470k and change the value of C5 to 47n for guitar and 100n for bass application.

Yes Manfred but that will raise the noise gremlin.
With high gain circuits the enemy is Noise.
There are many boatleak circuits around just like this one and they are just noise making crap.
The TS and similar circuits use a HiZ bufferd front end for a VERY Good Reason!!!
I think *mictester* has commented on this very issue many many times.

With the circuit posted,
If R2 and R6 are high value then you WILL certainly have higher gain BUT at the cost of higher noise.

Just by adding a simple BJT buffer stage (like TS9) then R2 can be as low as 10k while R6 can remain at 1Meg. Noise is then dramatically reduced and you still have more dirt that is likely ever needed.

Except for some rare situations tiss wise to buffer everything.
If you care to research schematics of some top end gear you will see most use a buffered input stage at the front end,
Often just a HiZ opamp follower stage.

This Caline design has got the best bias circuit around but U1B would be better utilized as the buffer stage front end.

I don't even bother with these kind of circuits as I know the outcome will be frustration.
Phil.

Hi Phatt
Thank you for your advice and I must say that I did not focus on the noise.
The R1 with the value of 2M2 resistance is also a higher noise source.
AC-wise, R2 is in parallel with R1.
Now I ask myself the question whether the noise voltage by the lower value of R2 as load of R1 lets the noise voltage drops or whether the noise voltages add up.
This would have to be tested in the real circuit.
The buffer before the circuit is of course a good idea.

Glad to hear it helps you Manfred,
The sensitivity of the input is the issue.
Remove R1 as it's likely irrelevant,, R2 is the dominant load.
Try looking at it this way.
As *mictester* noted the input Z is way too low to be useful for guitar.

OK you change R2 to 1Meg and you raise the input sensitivity,, BUT as R6 is already 1meg then you have a very sensitive input with high gain. ( a recipe for noise) Yes a much better interface for the PU's but now you have also made the input very noise prone.
The Hiz Buffer resolves this problem by giving you a sensitive HiZ input with NO Gain and a low Z output Then the opamp input only needs to be low input Z and now you can crack the gain with far less noise.

Anyone can test this out on a breadboard to see/hear just how well this simple trick works.

I'm only an amateur hobby geek and hopeless at all the maths needed to explain intricate details but I've built enough land fill to learn that Noise is a huge problem for a lot of these hyped up dirt circuits.
There is some very good explanations and info on how to resolve noise problems in the book the Art of Electronics.

Even a high gain well designed basic 741 opamp dirt circuit would likely beat a crap design using the most Exotic opamps when testing noise issues.
As it's not the quality of Actives in a circuit it's how well you design the passives that run it.
Same goes for the gold plated dust on NOS valve gear.
It's not the Valve,, it's the design of the passives that make it work.
If more time was spent learning R/C maths than chasing the best opamp or Nos parts we would all learn so much more and spend a lot less money chasing our tails. Rant over
Others here are far more qualified to explain the maths for working out the complexities.,, heck I'm just a muso with a hot soldering iron who learned enough to build a few successful circuits.
Phil.

Thanks phatt, excellent info.

I think the pulldown resistor in the input is just to stop the switch from clicking from capacitive buildup of voltage since the LED is also going through it, or maybe backwards charge potential on the input cap, I remember popping being some issue with it.

I'll try building the circuit with a buffer transistor and see how much less noise it makes, should be able to lower R2/R6 also to 4.7k and 100k to retain the same gain but less noise yes?

I really need to buy a copy of art of electronics...

CheapPedalCollector wrote: ↑17 Dec 2022, 06:32 Thanks phatt, excellent info.

I think the pulldown resistor in the input is just to stop the switch from clicking from capacitive buildup of voltage since the LED is also going through it, or maybe backwards charge potential on the input cap, I remember popping being some issue with it.

I'll try building the circuit with a buffer transistor and see how much less noise it makes, should be able to lower R2/R6 also to 4.7k and 100k to retain the same gain but less noise yes?

Thanks for trying Phat's recommendation and further modification.
What about the value of C5, do you keep it or adjust it?
Regarding the noise, I was looking for the behavior when resistors are connected in parallel.
I found that the total resistance of parallel circuit is taken for noise value calculation.
I also found that with mixed circuits where coils and capacitors are added, only the so-called real part, i.e. the DC value components of these are included in the calculation.
A low-pass filter also reduces the noise power because higher frequency components in the noise are attenuated.
In the case of the original circuit only R2 has an effect regarding the noise because R1 with the high value of 2.2 megohms can be neglected.