Blend Circuit on BB preamp

[lukatosh]

Hi Everyone!

i'm writing from chile!... First of all, please excuse my bad english...i'll do my best!

Well, i'm trying to mix the blend portion of the sparkle drive with the BB Preamp.

i've breadboarded the circuit and it works... at least the blend portion works pretty well!...however it sounds to loud with the level at 12:00 even with the blend all the way to the clean side it's too loud that my amp get saturated. (The same thing happens with the blend knob all the way to the overdrive side)

i've reduced the input of both signals (clean an distorted) comming into the final mixer (IC3B) increasing the values of R22 and R23 (both of them are trimpots) to reduce their gain... But i was wondering that is there another way to reduce de output level without changing the original values of those resistors, 100k and 13k respectively.

What is the pourpose of the 150R and 100k at the end of the circuit?... i've looked at some discussion about circuits that has low resistance values at the output and some people say that they help to reduce output... also i've seen in the *other* forum that a resistor in series with the output cap will reduce overall output but in this case the gain is set by de 100k on the feedback loop of IC3...

and what about the 100k(R25) resistor?... takes out (reduce) some signal?

Well i've rally like to understand this thing... i Think that the trimpot trick will solve the issue... or at least give me resistor values and then just change the 100k-B level pot... Don't know wich is the best way to reach the goal without altering the circuit behavior...

Another problem that i've found is that when I lower the level potentiometer at minimum you can still hear the guitar sound coming out the amp (or the breadboarded effect in this case) ... there is no complete muting. but i would like to look carefully if i made some mistake on the breadboard.

Thanks so much for your help!

Here is the schematic!

[phatt]

Hi lukatosh,
You need some series resistance in front of pin 6 at IC3B, try 10k~50k see what happens,, otherwise the signal gets too hot (Big)
Yes " level B" can't mute the signal as it's setup as a gain trim Not a volume. For a better volume control just make R25 a pot.
R24 is just a bit of protection for the output of IC3B.
HTH, Phil.

[teemuk]

Yes, R24 is just a crude current limiter for currents both in and out. Value is chosen "low-ish" because it dictates the overall output Z (and hence may affect frequency response) but "high-ish" enough to actually provide decent protection. One function is that when the output is short-circuited the opamp still sees a minimum load of 150 ohms. Bit of extra safety in addition to internal current limiters of OpAmps. Another function is that the OpAmp never sees a fully capacitive load, which could lead to instability. Additionally R24 provides 150-ohm resistance for currents from the output circuit, which could originate from, say, static electricity in coords. If you really wanted to improve this further you'd add two shunt diode clippers, referenced to supply voltages, to the OpAmp pin 7 as well. That would be protection from static -voltage- pikes.

C13 and R25 is just generic AC coupling isolating potential DC levels from each other. C13 for actual coupling, R25 to provide voltage reference for the other capacitor terminal so that it doesn't "float" and make an annoying "pop" whenever you plug in the effect. R25 is generally chosen so high in resistance that it becomes a virtually meaningless load in parallel to input impedance of the device this connects to. C13 is commonly dimensioned ample enough to overcome excessive low cut of the RC filter. Overall, all these components will interact with the input circuit of the following unit so it's beneficial to dimension them "transparent" enough. For example, what's attenuation of the 150R/100K resistive divider...? Not much to care about. Similarly cut-off frequency of the 2.2uF/100K hi-pass filter is around 0.7 Hz.

[snofla1900]

around 0.7 Hz.

You mean KHz I suppose ?

[deltafred]

around 0.7 Hz.

You mean KHz I suppose ?

Nope, Hz.

You can check it with an online RC filter calculator. e.g. http://www.muzique.com/schem/filter.htm

[DesertCruiser]

Here is a mixer that i have planned to use in another project, i don't know if this actually works satisfactory, it's only been simulated yet but you can try it out if you want. Also the values shown probably has to be tinkered with in order to achieve somewhat balanced levels between the dry and wet signals when the balance pot i set to 50%.
I assume that your present mixer circuit uses a dual pot as the blend control. This one uses a mono pot (10k lin) with the wiper grounded as far as AC is concerned. This pot in addition with R2 and R3 forms two variable counteractive voltage dividers and the output of theese are summed by R4/5. The summed signal is then sent via the volume pot (100k log) to the non inverting OP amp. Beacuse the lower lug of the volume pot is connected to the VCC/2 supply you can get zero gain if the wiper is set to the minimum, which you can't do with your inverting summing OP configuration that has a minimum gain of -1. Or simply 1 (unity), the minus sign tells us that the amp is inverting, not that it is damping the signal.
Beacuse the two voltage dividers will attenuate the signals we will have to have some sort of make-up gain. This is provided by the output amplifier, and the gain is determined by the R6 and R7 ratio. If you need more gain simply increase R7 or decrease R6.
If the wet signal is stronger than the dry signal, try to increase the value of R2. You can put a trim pot here, maybe 10 - 50k, and set it to a value that gives you suitable amounts of blend.
The voltage follower in the bottom left provides a low impedance output to the blend circuit as well as isolating the output of the distortion circuit from its input, so that oscillation is prevented. R1 provides the quiscent voltage to both the voltage follower and the clipping stage of the distortion circuit. Make sure that you don't use a coupling capacitor here, so that you have VCC/2 DC coupled via R1 to the distortion OP amp input pin.
The obvious flaw about this design is that the blend response is linear rather than logarithmic, but i don't know how much that affects the feel of a guitar pedal