At the moment I am writing these words, I am very excited to know that you are one of the first people to assemble the kits we have designed, thank you very much for your trust and your perseverance!
We have had this project on our hearts for 3 years and since only 6 months we have been able to invest enough time and resources to make it happen. I hope you really enjoy this experience and that you will be able to present your creation very soon. And, that we will have the opportunity to meet during a masterclass!
vital information to assemble your ego driver kit
If you are familiar with Do It Yourself with your effect pedals, you will find everything you need in this downloadable document.
If you’re just starting out, the rest of the article is really made for you, so hang in there! Download this document anyway, you will need it to move forward and understand the rest of the events. Then you’ll see it’ s very useful.
leave your soldering iron on for 2 minutes before we start
The assembly of a FX Teacher kit has nothing to do with what you have already found on the net. Indeed, we have developed our own method to assemble your PCB, step by step, and to constantly check its proper functioning! And yes, we are committed to the infallible, but you also have some work to do so.
So, in order for you to understand all the tricks we’re going to ask you to do, I suggest you to start by devouring all of our tutorials, if you haven’t already done so.
Here they are, in order:
now we can start!
Congratulations you are finally starting to enjoy the joys of assembling an effect pedal! Your head is full of super condensed information, we’ll decompile all this with a bit of exercise!
We will also do everything we can to make sure you can get your kit up and working.
We warm up the soldering iron and let’s go!
Confessions: The neurons will also continue to heat up, sorry. But you’ll get out of it taller, I promise!
the power supply stage
The power supply presented is not the same as for a Tube Screamer, for more headroom and a better filtering, we have added our personal touch! The power supply is first filtered by R22 and E4 and then when entering the TC1044 chip, it will create the -9V supply. The advantage is therefore to have +9 – (-9) = 18V of headroom. Then, we can feed our active circuits symmetrically!
The usual method is unfortunately to center the signal on 4.5V to be between ground and +9V. It sounds much better with our method you’ll see!
Here is a short list of points to be considered before soldering. All the tricks are in the blog articles mentioned above.
- Do not burn the SMT LED when soldering it. It’s made of plastic, it melts!
- Wires +9V and GND must be inserted through the back of the PCB, on the solder side.
- Insert the electrolytic capacitors with the correct angle.
- Push the integrated circuit in the right side.
|Name of the component||Value||Type|
|LED+||LED CREE||Surface mounted LED|
|E2, E3||10µF||Electrolytic capacitor|
|IC2||Socket DIP8||Integrated circuit socket|
Here’s what you should get after this step!
Let’s plug in the multimeter and try to find more or less the same values (at 10% ready). I use a power supply that provides 9.35V so it influences the rest of the measurements. Start by discovering your power supply by measuring TP1. The Test Points (TP) are available at the end of the PDF file you downloaded. When assembling step by step, these values differ from the table in the document because we have not yet assembled everything. Once everything is assembled, the circuits consume current and the resistors cause some voltage drop! Hence the slight differences between the final table and the table that is being defined.
|Test point||Measured voltage|
2. the input buffer
The first block that interacts with the signal is a transistor buffer stage. Here R1 is placed to unload any line capacity and avoid a “pop” at ignition. F1 and R2 form a high-pass filter that eliminates any continuous signal. The transistor is mounted in a common collector which allows a gain close to 1 in voltage and a significant gain in current. Its input impedance is highly dependent to R3. So we have a very interesting buffer. It is the same as the original Tube Screamer.
- Put the transistor on the right side
- Put the resistors in the right order so that you don’t have to force to insert the last one
|Name of the component||Value||Type|
The important voltage here is TP4 which is the polarization voltage of the transistor emitter.
|Test point||Measured voltage|
In order to connect easily to the FX Teacher tester, we solder the following wires:
- A white wire between R1 and F1: this will be the entry of our circuit
- A green wire between R5 and F2: this will be the output of our circuit
- Two black wires, one on G1 and one on G2, they will be the ground of the tester
Now, the PCB is connected to the sound card using the FX Teacher method.
The tester is in place, we’re ready to go. We’re going to create two stimuli:
- Sine of 1kHz to measure the gain of the circuit. Also, it will be possible to control the saturation rate of the system and notice the appearance of undesirable noise.
- Frequency sweep made of sine waves from 20Hz to 20kHz, in order to know the filter gain on each frequency!
The expected result for this stage, with or without the effect, is only a 1kHz line with no harmonics (2kHz, 3kHz…). The spectrum must be completely flat because there is no amplification and we must preserve our signal.
We now generate our “sweep” with an amplitude close to -34dBFs, because it is an amplitude close to a guitar level.
After measurement, we notice that all frequencies come out with more or less the same amplitude.
To summarize, our input buffer stage is completely flat in the bandwidth (it does not filter any useful frequencies) and does not saturate when you inject a sinus.
Perfect! Perfect! He does his job very well. Let’s move on!
3. the gain stage
This first part of the gain block is made up of an operational amplifier or “OPA”.
This setup is called a non-inverting amplifier stage, the gain in the bandwidth is theoretically calculated by the 1+(R18+GAIN)/R12 ratio.
But all this depends on the filters created by R18+GAIN // C1 and R12 – F5.
Then, R11 and F6 are used as a low-pass filter to accentuate the mids and cut the highs that are a little too strident ! With a cut-off frequency at -3dB of 7kHz.
- Set the potentiometer on the right side of the PCB.
- Put the IC in the right way.
|IC1||Socket DIP8||IC socket|
Here the two important voltages to be checked are those of the integrated circuit power pins
|Test point||Measured voltage|
In terms of connections, the input is always the same because the stages are cascaded. And, we are adding one stage after the other so the input will not change until we have finished assembling the kit.
On the other hand, the output will regularly move according to our progress in the assembly of the kit. So here we connect the green wire between R11 and F6, as shown in the picture.
To understand how the gain stage works, we’ll start by putting our gain knob at maximum! Attention it will be necessary to lower the input level of the sound card until it is no longer saturated (when the vumeter is red it is necessary to lower until it is green).
We thus obtain a beautiful bump between 400Hz and 2kHz. That’s exactly what you expect from a Tube Screamer! Everything else is attenuated. Except for the 50Hz that annoys us but that we won’t have once the pedal is finished!
To know the gain (approximate and in the bandwidth) of the buffer + amp stage, we start by sending a 1kHz sinus when the effect is connected. We lower the input of the sound card so that it does not overload. For example, we note -7dB with our sound card setting.
Then, a jack is connected between the input and the output of the sound card, without touching the input level potentiometer.
This signal is injected again without modifying anything on the sound card, to obtain in this case, -41dB.
For the calculation of the gain: Gain = -7 – (-41) = 34dB.
It’s already huge!
Pretty simple method, right? If you want to know the gain for each frequency you repeat the experiment but with a sweep rather than a pure sinus.
4. clipping and voicing stage
At this point if you listen to your gain stage, you might be a little disappointed! Its harmonic content is not remarkable, it will only try to saturate the input of your guitar amp.
We will therefore add clipping diodes, in order to cut the peaks of the sinusoidal signal and finally find them almost square!
Each diode cuts more or less high in amplitude according to its threshold voltage.
Regarding the voicing, you remember that the total gain of the amplification stage depends on 1+(R18+GAIN)/R12 (in theory, in the bandwidth).
Here we will modify R12 by switching to R10 and the BASS trimpot with the voicing switch.
Also, the bandwidth will be modified by choosing F4 at 1uF instead of F5 at 47nF. So we’re going to measure all this and discover our new spectral patterns for each of the possibilities!
- Solder the switches on the right side, solder side
- Choose the right switch at the right place (ON-ON at the middle, so 2 positions and ON-OFF-ON at the bottom left, so 3 positions)
- Mount the terminals in the right direction, with the holes facing upwards
- Put the diodes in the right direction, look at the ring carefully
- Set the trimpots at 3/4
|VOICING||SPDT ON-OFF-ON||Commutateur à levier|
|CLIPPING||SPDT ON-ON||Toggle switch|
Take the #5 bag and screw your favorite components on the terminals! Don’t worry, there’s no direction to connect.
In the team we prefer these:
- 1N34 + 2* 2N7000 on the diodes terminal
- 22nF on the caps terminal
For these measurements we use the same connection wires as the previous stage because we are located at the PCB input and at the output of the gain stage. The clipping is connected in parallel with the gain stage. So no physical modifications required.
- We measure the sweep and the 1kHz signal in bypass (jack connected in the sound card without the board):
2. Pedal connected this time and all switches switched up. We can start the measurements.
The frequency sweep indicates that the bandwidth is now between 50Hz and 1.5kHZ. This is normal, when the voicing is at the top, it activates the branch that amplifies the basses!
The pure sine at 1kHz create harmonics. We notice that the 1st odd harmonic (3kHz) has a greater amplitude than the even harmonics (2kHz and 4kHz). This is due to clipping distortion from the diodes which has this very particular characteristic and opposite to valves! As we might be led to believe. It is mainly the valves of our preamplifiers that will saturate following the arrival of large amplitudes in the mediums and thus finally create our even harmonics.
3. Clip on top and voicing in the center. Same configuration as a classic Tube Screamer.
The sweep shows us a bandwidth between about 300Hz and 1.2kHz. So that’s what we love and expect from a Tube Screamer!
4. Voicing at the bottom and clipping at the bottom. We are now on the 22nF cap and on the diode pair of 1N34 and 2N7000 screwed on their respective terminals.
The capacitor being smaller than the original one, 22nF versus 47nF, we expect to operate a bit more in the high frequencies. And, this is the case with a bandwidth that goes from 450Hz to 1.5kHz. However, the difference remains less significant than when you boost the bass.
In terms of the differences in the harmonics of the sine at 1kHz, it is too small to notice it with this method. There you have to play and listen, ears are also very often excellent tools!
6. Tone stack
We start the Tone floor, a very important stage that allows us to find more or less highs at the output!
First of all, you should know that we didn’t have a “G” type potentiometer. This “G” curve of potentiometers allows a very musical logarithmic progression at the mid-range level.
We therefore faked it, by putting R19 and R20 in parallel of our B-type potentiometer, which offers a very similar result.
To understand this layout, it is necessary to go to the 2 extremes of the Tone pot. Here are the equivalent diagrams when it is at the maximum and then at the minimum:
When the pot is at full throttle, the gain created by the OPA depends on R16, R15 and F7. Knowing that R15 and F7 cut at 3kHz and that R16 is larger than R15, we amplify the high frequencies well.
When the pot is at minimum, the gain is very close to the unit because it is equal to 1+R16/R1 or about 1.2. While at the input, the couple F7 and R15 form a beautiful low pass filter at 3kHz.
We can better understand why aficionados are thrilled by a Tube Screamer Tone, we can gradually switch from a filter that cuts the high frequencies to a filter that amplifies them!
- Assemble the potentiometer on the right side of the board, solder side
- Put the resistance in the right order, follow the BOM
Start by placing the green wire at the end of the new stage, i.e. on the middle tab of the volume pot.
We keep the gain and volume at maximum, the voicing switch in the middle and then the clipping switch at the top.
As we are on the same configuration as before, except for the Tone, we can take for reference in our measurements, the spectrum of the previous step as follows. (the one at the gain output with clipping at the top and voicing in the middle)
With the tone at its maximum, here is what we measure:
With the tone at minimum:
This curve reflects the theoretical performance of the Tone stage, when the tone is at minimum, the bass and low-midrange frequencies are pushed out, while the high frequencies are filtered out.
When the tone is at its maximum, on the opposite, we add trebles and break through the mix. So it’s up to you to find the right compromise that you like the most!
7. the output buffer
And finally the last block! Almost identical to the one at the entrance. Its purpose is to adapt the signal after all the processing that has just been done to it. This way, it will be ready to pass without any problems to the next pedals!
- Solder the transistor on the right flip
- Put the resistors in the right order so as not to have to force the last one in.
The important voltage to measure here is TP8. This is the polarization voltage of the output transistor.
|Test point||Measured voltage|
By putting the same settings as in the previous test and the Tone at noon, we should not change anything and have a similar result at the gain stage:
okay, so what now?
The most difficult part is done! Your board works perfectly, you will be able to transform it so that it fits perfectly into your enclosure and you can finally take it with you from everywhere.
If you have any questions, comments…. Feel free to ask them here in comment. Meanwhile I hope you liked this first kit and that we will find you soon for the next ones!
Stay in the motion and go follow the last 2 articles to finish your pedal!