DOD280 caps question

[lazerphea]

Hello!
After being unproductive for almost two years, I started to build stompboxes again, mainly pushed by some friends who want some new cost-effective pedals
I'm now building a couple of DOD280, using the Tonepad layout: the first time I built it I've been able to gather all the exact components but unfortunately, this time, I couldn't find the 0.05uf caps, so I used 0.047uf instead.
To me, there's no audible difference, but I would like to understand how this value change affects the signal, and here I'm asking for some help.

I'm not very good at circuit analysis, but I tried and this is what I end up with:
* starting from the output pot of the circuit, the 0.05uf cap in series with it is a coupling cap, also functioning together with the pot as a high pass filter with corner frequency of 14.5Hz
* the two 0.05uf caps in parallel with the two 220K resistors connected at the bases of the trannies contribute to realize two hi-pass filters (14.5Hz corner freq again),
* the arrangement of the last 0.05uf cap, in series with the 22K resistor to ground and the 500K pot reminds me of the ts808 clipping stage (well, without clipping... : it controls which frequencies should be amplified by the LM358 and how much

Is this analysis correct? Am I nuts?
I'll wait for your response
cheers!
Paolo

[lazerphea]

I'd appreciate just a nod or a shake of your head...

[slacker]

Yes, that's all pretty much correct.
Think you got your maths wrong with the first one, the corner frequency is about 31Hz.
For the transistor ones I think you have to take into account the impedance of the transistors as well as the 220k resistors to calculate corner frequency. With the variations you will get with the transistors, the LED and the LDR, a slight difference in the value of the caps won't make any difference.

[lazerphea]

Yes, that's all pretty much correct.
Think you got your maths wrong with the first one, the corner frequency is about 31Hz.
For the transistor ones I think you have to take into account the impedance of the transistors as well as the 220k resistors to calculate corner frequency. With the variations you will get with the transistors, the LED and the LDR, a slight difference in the value of the caps won't make any difference.

Thanks a lot slacker!
I suspected the transistors could contribute in some mysterious ways, but I'm still not brave enough to take into account their effect!!

[amptramp]

Quite some time ago, manufacturers and designers agreed that the values of components should be a fixed percentage from the next lower value, so you got a logarithmic progression in values for components that goes as follows for 5% and 10% tolerance:

10, 11, 12, 13, 15, 16, 18, 20, 22, 24, 27, 30, 33, 36, 39, 43, 47, 51, 56, 62, 68, 75, 82, 91

Capacitors are not high-precision devices (<10% tolerance) unless you are willing to pay a lot of money, so designers use this logarithmic progression of values.  There should be no noticeable difference between a 0.05 uF and a 0.047 uF capacitor.

[lazerphea]

Quite some time ago, manufacturers and designers agreed that the values of components should be a fixed percentage from the next lower value, so you got a logarithmic progression in values for components that goes as follows for 5% and 10% tolerance:

10, 11, 12, 13, 15, 16, 18, 20, 22, 24, 27, 30, 33, 36, 39, 43, 47, 51, 56, 62, 68, 75, 82, 91

Capacitors are not high-precision devices (<10% tolerance) unless you are willing to pay a lot of money, so designers use this logarithmic progression of values.  There should be no noticeable difference between a 0.05 uF and a 0.047 uF capacitor.

Ron, thanks a lot for this piece of information! I always thought caps had 5% tolerance!!

[PRR]

> I'd appreciate just a nod or a shake of your head...



> couldn't find the 0.05uf caps, so I used 0.047uf instead.

Nearest-value is usually no-difference in audio circuits.

More precisely, the cap and load (100K plus the external load, which is usually much higher) define a bass-cut. 0.050/0.047 is about 6% difference. From one note to the next (a semitone) is also about 6%. However simple bass-cuts don't "Cut-Off" at a certain note, they fade over a range of notes, most of an octave. So if you move the bass-cut one semitone, not much difference.

> from the output pot of the circuit, the 0.05uf cap in series with it is a coupling cap, also functioning together with the pot as a high pass filter with corner frequency of 14.5Hz

Assuming 100K load, I get 31.8Hz.
1 / (0.05EE-6 * 100,000 * 6.28) = 32

Assuming another 100K connected to the jack, 64Hz.

Guitar goes to 82Hz. So hardly any effect. So really little difference if the truth is 31.8Hz or 33.8Hz.

And all +/- cap tolerance which can be as bad as +100%/-50% (for old large caps) to +/-20% (most historic film caps) or 10% (many modern caps); or as good as you want to special-order and pay for.

> 0.05uf caps in parallel with the two 220K resistors connected at the bases of the trannies

They are in series.

And the true load is the transistor base impedance, which ranges from near infinity to under 30 ohms. However as the signal slams the transistor toward 30 ohms, the LED/LDR is throttling-back the signal. This is an Advanced Topic in signal processing. Suffice to assume that if 0.05u worked acceptably, 0.047u won't suck either.

> last 0.05uf cap, in series with the 22K resistor

This is a bass-cut that varies from subtle at low gain (6Hz) to massive at max gain (144Hz). Again we hardly care between 144Hz or 154Hz. If we decide we want FULL bass at max gain (tends to be brutal) then we are not looking 0.050u/0.047u changes, we'd try 0.1u or 0.2u (now called 0.22u).

[lazerphea]

> I'd appreciate just a nod or a shake of your head...



> couldn't find the 0.05uf caps, so I used 0.047uf instead.

Nearest-value is usually no-difference in audio circuits.

More precisely, the cap and load (100K plus the external load, which is usually much higher) define a bass-cut. 0.050/0.047 is about 6% difference. From one note to the next (a semitone) is also about 6%. However simple bass-cuts don't "Cut-Off" at a certain note, they fade over a range of notes, most of an octave. So if you move the bass-cut one semitone, not much difference.

> from the output pot of the circuit, the 0.05uf cap in series with it is a coupling cap, also functioning together with the pot as a high pass filter with corner frequency of 14.5Hz

Assuming 100K load, I get 31.8Hz.
1 / (0.05EE-6 * 100,000 * 6.28) = 32

Assuming another 100K connected to the jack, 64Hz.

Guitar goes to 82Hz. So hardly any effect. So really little difference if the truth is 31.8Hz or 33.8Hz.

And all +/- cap tolerance which can be as bad as +100%/-50% (for old large caps) to +/-20% (most historic film caps) or 10% (many modern caps); or as good as you want to special-order and pay for.

> 0.05uf caps in parallel with the two 220K resistors connected at the bases of the trannies

They are in series.

And the true load is the transistor base impedance, which ranges from near infinity to under 30 ohms. However as the signal slams the transistor toward 30 ohms, the LED/LDR is throttling-back the signal. This is an Advanced Topic in signal processing. Suffice to assume that if 0.05u worked acceptably, 0.047u won't suck either.

> last 0.05uf cap, in series with the 22K resistor

This is a bass-cut that varies from subtle at low gain (6Hz) to massive at max gain (144Hz). Again we hardly care between 144Hz or 154Hz. If we decide we want FULL bass at max gain (tends to be brutal) then we are not looking 0.050u/0.047u changes, we'd try 0.1u or 0.2u (now called 0.22u).

Wow Paul, thanks a lot for the in-depth analysis! This makes me realize how many variables I left out my naive one