In this final build, when it comes to simple booster effects, I am going to use Op-Amps. An op-amp is an integrated circuit (a chip) containing several transistors inside of it to achieve a very large gain. But it does so in a way that makes amplifier design very consistent and predictable.
In this post I’ll actually show two circuits. One is a very dependable and common pedal – MXR Micro Amp. The other is a circuit I designed myself, and it should give pretty similar output as MXR Micro Amp, but should be a slightly improved version (in my opinion).
In this post I’ll get to know the circuits. In the follow-up posts I’ll do some SPICE analysis, do some breadboarding and then prepare the enclosure and complete the pedal on a protoboard. This one got too long so I thought to split this into several and publish them as they get ready.
Getting to Know the Effects
MXR Micro Amp
I got the schematic for MXR Micro Amp from Electro Smash. The article is analyzing the pedal in detail, you should check it out, it is great. Here’s the schematic:
I did a slight adaptation to it. The original schematic uses TL061. That’s a single op-amp chip, I didn’t have it handy so I used TL072.
TL072 it is very, very commonly used dual op-amp chip in pedal builds (and all sorts of other audio equipment). It is a dual op-amp and it has a different pin-out so there’s a bit of a difference, it is not a drop in replacement. Other than that, it should be slightly less noisy than TL061 and it uses slightly more current (so maybe battery won’t last as long if you use battery) but other than that, it is pretty much the same for what I’m going to use it for.
I have a different version of the pedal it seems, mine uses LM741. TL061 and LM741 have the same pin-out, so they can be used interchangeably here. That is a great thing with op-amps, as long as you have the correct pinout, almost any will work.
Anyway, let’s go through the schematic.
The amplifier encircled in red is a non-inverting amplifier. This is a variable gain amplifier, and the gain is controlled by a reverse-log (reverse-audio) pot – R5. The reverse-audio taper is slightly un-common and typically used in variable gain circuits as here. See how this taper looks like in my post on pots.
Gain of the amplifier is G=1 + R4/(R6+R5), so maximum gain is when the pot is all the way clockwise (the way the pot is wired this is 0 ohms). This is G = 1 + (56K/2.7K) = 21.74 or about 26dB. Minimum is when the R5 is at maximum value (all the way counter clockwise), and it is G = 1 + (56K/502.7K) = 1.11 or nearly 1dB. So the gain range is from about 1dB to 26dB.
I conveniently ignored capacitors in the feedback loop in the above calculation. C3 is there to allow only AC (guitar) signal to be amplified but it also forms a high pass filter with R5 & R6 which might affect the response, but here, cut-off frequency is 12Hz so it does not affect audible signal. C2 helps with stability (to prevent oscillations at high frequencies) and with R4 forms low pass filter with cut-off frequency of about 60KHz. With all this said – the amp has flat response between 12Hz-60KHz.
R2 is there to bring reference voltage to non-inverting input and still keep input impendence high. Since single supply is used, input and output are lifted to middle of the supply voltage: 4.5V. This gives the biggest headroom of nearly full 9V peak-to-peak (depending on the op-amp chosen … it turns out that for TL072 and TL061 this is closer to 6Vpp).
Finally – R3, I’m not sure, it’s probably there for stability and/or possibly DC error. The pedal has been around since 70’s, I’m not sure how much the design changed, 741-type op-amp has been around since 60’s. Not sure if it is still needed.
Reference voltage is achieved by way of a simple voltage divider using R7 and R8, C4 is there to filter out any ripple coming from power supply. This is necessary for single supply op-amp circuit otherwise negative portion of the input signal would be clipped. R7 and R8 have the same value so reference voltage is half of the power supply voltage, so 4.5V.
This is standard stuff here, C1 and C5 are coupling capacitors that are blocking DC component of signal coming in (via C1) and removing that 4.5V DC introduced via reference voltage (via C5). C1 forms a high pass filter with R2 and C5 with R9 and R10, but they do not affect frequency response since cut-off frequency is very low.
R1 and R10 are draining resistors for C1 and C5. When the pedal is disengaged and if input and output are left floating, due to leakage, voltage can build up on C1 and C5 that would lead to loud thumps when pedal gets engaged. R1 and R10 are draining capacitors to help with the thump.
R9 is there to help with stability and help preventing oscillation driving capacitive load (guitar cable may produce about 100pF capacitance per meter).
That’s it as far as MXR Micro Amp is concerned. On one hand, the circuit looks intimidating compared to the ones using transistors I did before, it definitely has way more components used then the other circuits. On the other hand, with op-amps I can get more consistent results, and it is easier to predict how they are going to behave, even using different op-amps should not be a big deal.
One thing that I found strange is that in this circuit I could not see decoupling capacitors for the op-amp. I actually have the pedal at home and I could not find it. I’ll get to that in the next section.
I called my design … wait for it … Thunder. OK, slightly uninspiring name, but still, it should be a great circuit (oh wait, I already revealed it in the heading … doh).
I got most of the inspiration from Douglas Self’s book – Small Signal Audio Design 3rd Edition. I pretty much picked up things I liked from various circuits from the book while I was reading it and adapted them into a guitar pedal.
It is similar to the MXR pedal even though it might not look like it. I’ll go through the parts and just reflect on differences and similarities between the circuits as I go.
This is also a non-inverting amplifier, but with a difference. Gain control is in a different place in the feedback loop. The pot (RV1) is with linear taper, and R4 resistor is added here to improve the gain law. This makes gain equation a bit different but still easy to calculate for min and max positions of the pot.
Max gain, when the pot is all the way clockwise, is equal to: G = 1 + (RV1max/R5) = 1 + (10K/680) and this is 15.7 or about 24dB. Essentially, in this position, R4 is shorted and taken out of equation.
When the pot is all the way counter clockwise, RV1 shorts pins 1 and 2 of the op-amp so resistance is 0 ohms. Minimum gain, thus is equal to: G = 1, this is about 0dB. So the gain range is 0dB to 24dB.
C4 has the same purpose as C3 in micro amp, to only allow AC signal to be boosted. The values were chosen so they do not affect the frequency response (10Hz cut-off). R3 has the same purpose as R2 in micro amp, to bring reference voltage to non-inverting input of the op-amp.
Some notes here are in order. The gain here is less than Micro Amp, but there’s still plenty of gain. Lowering R5 to 470 ohms would bring gain to about 27dB if that is really needed. Resistor values are lower, so the circuit should be less noisy than Micro Amp.
C5 there is used for stability at high frequencies. Forming a useful low-pass filter using RV1 and C5 is slightly more difficult than in Micro Amp. RV1 is variable so the frequency response varies depending on the pot position, whereas Micro Amp has fixed R4. Definitely one of the trad-offs. Nonetheless, C5 does not affect frequency response.
Reference voltage is the same as for Micro Amp, R8 and R9, form a voltage divider and C6 is there to filter out any power supply ripple. R8 and R9 have the same value so reference voltage is half of the power supply voltage: 4.5V.
Standard stuff again, C2 and C3 are coupling capacitors, same discussion as for Micro Amp, they do not affect frequency response.
R2 and R7 are draining resistors for C2 and C3, same purpose as in Micro Amp described above.
RF Filter & Stability
R6 is there to help with stability and help preventing oscillation driving capacitive load, same as in Micro Amp.
One difference here is that I added RF filter (low-pass filter) on the input formed by R1 and C1. Cut-off frequency for it would be around 16MHz, but in reality, because if either there is a guitar directly plugged into the effect, or there is a preceding pedal, they would have their own output resistance added to the equation so the cut-off frequency would be lower.
Anyway, this filter should prevent radio frequency interference even reaching the op-amp and should lower the noise and improve op-amp performance.
Op amps, as most integrated circuits, suffer performance degradation of some type if there is a ripple and/or noise on power supply pins. To remove high frequency noise usually a 100nF cap is used like I did with C7. It should be ceramic capacitor and placed as close as possible to the chip. If there’s more issues with high frequency noise, another smaller cap could be used to help, but at this stage I might be just overthinking it.
Some extra reading
Anyway, there’s a lot of useful resources online. Texas Instruments has some great free e-books: The Signal E-Book and Analog Pocket Reference Guide, also Analog Engineer’s circuit cookbooks. Analog also has free e-book: Op Amp Applications Handbook. Or get the book I mentioned earlier by Douglas Self: Small Signal Audio Design.