Breadboard A Vintage IC Distortion

2010 By Small Bear Electronics LLC

My How-To on Breadboarding A Silicon Fuzz Face got a very good response, and some of the people who liked that piece asked me to do something similar for an IC-based distortion--maybe similar to an MXR Distortion + or a BOSS DS-1. Sounded like a fine idea!

If you have never used a solderless breadboard, please refer to the intro article before you continue on here. That How-To covers a lot of basic information and techniques; I presume that you have been through it, have done the demos, understand how the tool works and have started learning to use your multimeter. If you are already familiar with breadboarding, note that the level of detail in this article is meant to guide complete beginners, so please be a little patient.

For a demo at this level, I thought the MXR Distortion + a little too easy. But the BOSS DS-1, even with all its switching circuitry stripped out, contains more components than I thought a beginner would be comfortable with setting up. So I came up with a variant that uses an FET (Field Effect Transistor) input stage to reduce the complexity. I'm sure you will like the result, and you will learn enough in setting it up to breadboard lots of other distortions and try out mods.

Here's the basic circuit that we will set up:

I have purposely not included jacks, stomp switch or power switching in order to keep it simple; we'll add those later. This schem is a lot more complex than the LED demo, but don't let it scare you; it breaks down easily to separate functional blocks, and I'll walk you through it slowly. We'll stop at key points and do some tests and demos to make sure things are working and learn what the various sections do.

Here are few rules that will help with interpreting the schem:

If you have already breadboarded the silicon Fuzz Face, you'll immediately notice that the power section of this circuit is much more complex. Let's set that up first and then we'll look at what it does and why it is needed.

Trim the leads of a 100 mf. electrolytic capacitor, following the convention of leaving 1/4" to 5/16" of lead length for insertion in the board. Electrolytic caps are most often polarized; on radial-case types like this one, a black band marks the negative side (Fig. 18.)

You may want to follow my directions and layout this time around; once you have the idea, you're free to do whatever works. Similarly, I am working from left to right on the board, but that's not engraved in stone, either. Plug in the cap with the negative side to the negative power bus (Fig. 19.) You don't have to use exactly the same locations I do. However, if following my footsteps (bear tracks?) helps to build your confidence and avoid mistakes, go for it. Fig. 20 shows the connections you have made.

Now set up the voltage divider network R1, R2, R3. It's probably easiest to start with R3, 11K, color code Brown, Brown, Orange, Gold. Clip its leads and form them to span the distance from the negative power supply bus to the first hole in the sixth column. On this breadboard, that is index J57. Follow with R2 and R1 (Fig. 21.)  and Fig. 22 shows where we are at. Got the idea?

Finish this section by adding C2, a 10 mf. radial electrolytic. This goes from the junction of R1 and R2 to the negative supply bus, so create a bare wire jumper spanning three holes and form the leads of the capacitor to span the distance to the bus (Fig. 23.) Observe polarity when inserting the capacitor! The junction we have just created (Fig. 24.) is Vref in the schematic.

Just a note:  Because this is a first project for many people, I have been very prescriptive about exactly how to locate components and jumpers. Also for the sake of clarity, the physical layout is very precise and all wiring is squared off. As you do your own experimenting, this need not be the case if it isn't convenient or you need to get something done quickly. Breadboarding as I show it here makes documenting your work much easier and mistakes more clearly evident, so it's worth learning this way from the start.

The power distribution section is done. For a look at what it does, fire up your multimeter and connect the battery. Set the meter to measure DC volts in a low range, and connect the negative side to the Vref point. Use a short length of wire to help with making the connection. Now measure the voltage at the positive supply bus (Fig 25.)

We are seeing about half of the battery voltage, and a look at the schematic tells us why. Resistor R1 is 22K, and the series combination of R2 + R3 is the same value. In the way Ohm's Law works, the battery "drops" a proportionate 50% of its voltage across each of two equal value resistances. If you now shift the positive meter lead to the negative power supply bus, you will see exactly the same voltage, but the meter will read it as Negative 4.xx volts (Fig. 26.)

Why do we need to do this? Again, look at the schematic. Like most op-amp chips, the ones we use want to see two supply voltages--one positive and one negative--with respect to a reference point. We actually could provide V+ and V- by using two batteries, but that gets awkward for a pedal. So this circuit dodge (which you will see in thousands of pedal schematics) is used to, essentially, fool the chip into seeing what it must in order to work.

Now let's talk about the Junction Field-Effect Transistor (JFET) stage, which comprises Q1 and its associated parts.

As I mentioned earlier, this input stage differs from most BOSS designs. Most BOSS pedals, including the DS-1, use two NPN transistors at the input. The first is connected as an emitter follower, and the second as a common-emitter amplifier. The emitter follower presents a high input resistance for a guitar, and the second stage boosts the signal enough to drive whatever follows. While BOSS has good technical reasons for doing things this way, my priority was getting a similar result with fewer components. The JFET stage (which I actually lifted from the BOSS FA-1) does a nice job of this.

I promised you a "vintage" design, so, if you have my kit, you get to do your first experiments using the 2SK246-GR JFET, which was the OEM part in the FA-1. If you don't have the kit, no worries; many common and inexpensive JFETs will work fine in this stage; I tried a J201 with good results. The only thing you must be aware of is that not all devices have the same pinout. I show in the schematic the pinouts for the devices I used; if you are using something else, check an on-line reference for the pinout and change the breadboard connections accordingly if necessary. I will continue with setup directions using the 2SK246-GR and then include a photo that shows the layout using the J201.

Install the transistor with its Gate (middle lead) in the seventh column, second row. On my breadboard, this happened to be index B5. The flat side faces the power supply bus. Then add input capacitor C3 (.01 mf., marked "103") and input resistor R4, 10K, color code Brown, Black, Orange, Gold. For this layout, the capacitor leads should be formed to span four holes. Fig. 27 shows the layout, and Fig. 28 shows the connections you have made.

Now add resistor R5, 3.3 Meg, color code Orange, Orange, Green, Gold, and a jumper from one side of it to the Bias point in the power section. You will need to use one two-hole jumper to do this. See Fig. 29 and Fig. 30.

Now add resistor R7, 4.7K, color code Yellow, Violet, Red, Gold, (needs to span 5 holes)  and the 22 mf. electrolytic capacitor C6. Fig. 31 shows where the jumpers go.

A trimpot is a potentiometer that is specifically made to be mounted on a board and adjusted with a screwdriver when the exact resistor value needed is not certain. R6, a 25K cermet (metal film) type, is used to adjust the Drain current through the JFET. It has three terminals like any regular pot, but we only need to connect to one of the end terminals (a or b in Fig. 33) and the wiper (c). Fig. 34 shows the jumpers installed.

To make absolutely sure that the Drain of the JFET will see a variable resistance, insert a couple of short leads as shown in Fig. 35. (One goes to V+ and the other to the left-hand contact of the trimpot.) Set your meter to measure resistance, and watch the resistance change when you take a screwdriver to the trimpot. If nothing happens, check out how you have plugged in and connected the trimpot.

 

Capacitor C5, .47 mf., will usually be marked 474K. Add this as shown in Fig. 36 and Fig 37. The jumper that runs in the channel past the trimpot should be insulated to avoid the possibility of a short circuit.

Fig. 38 shows the layout using a J201. The circuit does not change at all--only the layout does to accommodate the different pin arrangement. To get oriented, start by placing the transistor with its middle lead in the eighth hole, second row, with its flat side facing away from the power busses. That should help you figure out what else has to be moved. I have tested this arrangement and confirm that it works. As I mentioned earlier, it will also work with many other JFETs that have the same pinout.

The preamp/buffer is finished, and we can run an initial test before adding the driver amplifier. The pics that follow show the preamp using the 2SK246-GR, but the J201 version will work equally well.

For testing purposes, we want to install input and output jacks as shown in Fig. 39 and Fig. 40.

The schematic shows mono jacks. This type has two contacts: tip and sleeve. Figure 41 shows the relationship between the schematic symbol and the physical item. Figure 42 does the same thing for a stereo jack, which has three contacts: tip, ring and sleeve. The stereo jack shown is a Switchcraft #12B; note that the arrangement of the contacts may be different on other makes.

In a typical pedal build, we would use a stereo jack for the input and connect the ring contact to switch battery power. To keep things simple here, we use only the tip and sleeve. Connect a short length of shielded cable to each jack, making sure that the shield is soldered to the sleeve. (Ordinary insulated wire will work, but shielded cable reduces some of the noise inherent in an open layout like this.) Add bare wire terminations to the ends that go to the breadboard just as you did for the battery snap.

The shields are plugged to the ground bus. Input tip goes to the 10K resistor, output tip to capacitor C5.

Are you ready? Connect up your guitar and an amplifier. Connect your multimeter set to measure DC voltage, negative side to V- and positive side to the Drain of the JFET. Set the trimpot to about the middle of its range, and and connect a 9-volt battery or power supply. Figure 43 shows the setup, and what I saw.

If the trimpot is set anywhere near its midpoint, you should hear the guitar clearly, and the voltage should be somewhere between 3.5 and 5.5 volts. Tweak the trimpot so that the voltage is around 4.5 volts. You should be getting a clean volume boost. Want to prove it? Move the lead from the output jack tip off its connection to C5 and to a hole in an unused column. Then connect a short lead from that column back to the tip of the input jack. Play, and switch briefly between the unboosted guitar and the output. (The input resistance is so high that this preamp doesn't suck tone, so don't be concerned that the circuit is not fully bypassed.) Hear the boost?

If you don't hear the boost or you can't properly set the voltage, Check Your Wiring. I have proceeded exactly as shown here, so you should be duplicating my results when everything is correct.

I found that I got slightly more boost from the preamp with the Drain voltage set to about 5.3 volts. If you are with me so far, let's set up a driver stage.

I have tried two old-stock, single op-amps and one modern part in this build with good--and similar--results. The HA1457W was used in the BOSS FA-1, and the TA7136P was used in the oldest DS-1s. The kit can be ordered with either one, and the chips are both available separately. Since many modern clones of the DS-1 are based on a dual-in-line chip, every kit also includes a TL071. If you are using the HA1457W, proceed below. If you are using the TA7136P, click here. Notes on the TL071 start here.

Setting Up The Driver Stage - HA1457W

The package style of this chip is called single-in-line, or SIL. This version has eight pin positions, but there is no pin in position 2. Plug in the chip, and make the connection from the output of the preamp to the input, pin 6, as shown in Fig. 45 and Fig. 46.

Add resistor R8, 100K, color code Brown, Black, Yellow, Gold, and run one end of it back to the Vref point that we created earlier. Add the 100 pf. capacitor C8 (code 101) between pin 3 and pin 5. See Fig. 47 and Fig. 48.

Power the chip: V- to pin 4 and V+ to pin 8. Run a jumper from the output, pin 1, past pin 8. Add resistor R9, 100K, color code Brown, Black, Yellow, Gold. One end goes to pin 7 of the chip, and it should span five holes.  See Fig. 49 and Fig. 50.

Potentiometer R11 (500K reverse audio taper, which is what the "C" indicates) together with Resistors R9 and R10, set the gain of this stage. Prepare the potentiometer for connecting to the breadboard by soldering leads to it. A few inches of insulated breadboarding wire to each terminal is fine. By convention, we refer to the terminals of the pot as ccw (counter-clockwise), W (wiper, for the moving contact) and cw (clockwise.) The connections are:

Add resistor R10, 4.7K, color code Yellow, Violet, Red, Gold, and electrolytic capacitor C7. Observe polarity of the cap! See Fig. 51 and 52.

From the column that joins pin 1 of the chip and the CCW side of the potentiometer, add a jumper that spans six holes. One side of resistor R12, 2.2K, color code Red, Red, Red, Gold, connects here. Add capacitor C9, .047 mf., code 473K. The ground bus on my breadboard is split, so I had to connect the two halves with a jumper.

I based this design partly on the DS-1, which has a small capacitor (shown in dotted lines) from the wiper of the gain pot to the IC output. In various schems, I have seen values for it from 100 to 250 pf. I don't hear a clear difference with or without it--YMMV.

Now re-connect the output jack. See Fig. 53 and Fig. 54.

We are ready for another test.

Setting Up The Driver Stage - TA7136P

The package style of this chip is called single-in-line, or SIL. This version has seven pin positions. Plug in the chip, and make the connection from the output of the preamp to the input, pin 2, as shown in Fig. 55 and Fig. 56.

Add resistor R8, 100K, color code Black, Brown Yellow, Gold, and run one end of it back to the Vref point that we created earlier. Add the 150 pf. capacitor C8 (code 151) between pin 1 and pin 6. See Fig. 57 and Fig. 58.

Power the chip: V- to pin 4 and V+ to pin 7. Run a jumper from the output, pin 6, to the first hole past pin 7. Add resistor R9, 100K, color code Black, Brown Yellow, Gold. One end goes to pin 3 of the chip, and it should span eight holes.  See Fig. 59 and Fig. 60.

Potentiometer R11 (500K reverse audio taper, which is what the "C" indicates) together with Resistors R9 and R10, set the gain of this stage. Prepare the potentiometer for connecting to the breadboard by soldering leads to it. A few inches of insulated breadboarding wire to each terminal is fine. By convention, we refer to the terminals of the pot as ccw (counter-clockwise), W (wiper, for the moving contact) and cw (clockwise.) The connections are:

The ground bus on my breadboard is split, so I had to connect the two halves with a jumper. Add resistor R10, 4.7K, color code Yellow, Violet, Red, Gold, and electrolytic capacitor C7. Observe polarity of the cap! See Fig. 61 and 62.

We are almost done with the driver stage. Add resistor R12, 27K, color code Red, Violet, Orange, Gold, and the jumper that links it to V+. From the column that joins pin 6 of the chip and the CCW side of the potentiometer, add a jumper that spans six holes. One side of resistor R12, 2.2K, color code Red, Red, Red, Gold, connects here. Add capacitor C9, .047 mf., code 473K. Now re-connect the output jack. See Fig. 63 and Fig. 64.

Testing The Driver Stage

Connect your guitar and amplifier. Set the amplifier volume very low and start with potentiometer R11 fully counterclockwise. Connect the battery.

Notice how loud the guitar is even with R11 fully counterclockwise and the amplifier low. Turn R11 a little bit and the circuit will break into feedback. If you are with me to this point, we are ready to add some distortion. If you don't hear LOUD guitar, check your wiring.

Distortion - Adding a Diode Clipping Stage

If the circuit is working to this point, disconnect the battery and the output jack temporarily. Insert two1N914 diodes connected back-to-back (the bar side on the case is the bar side in the schematic) and a 50K audio potentiometer to control the output level. Return the diodes and the CCW side of the potentiometer to Vref. Fig. 65 shows what this looks like when added to the build with the TA7136P, but the idea is the same with the HA1457W. Fig. 66 will help you wire it with either chip.

With both controls fully counterclockwise, re-connect the battery and the output jack.

With your amplifier at a moderate playing level, rotate the level pot till you hear fairly loud guitar. Now bring up R11. Got distortion?

CONGRATULATIONS!

I don't discuss much theory in these tutorials, because the "Why" is covered extensively in references done by people who know far more than I do. But I'll make an exception for the question that I know just occurred to you: How come it takes so many components to get to this point, and only two diodes to create the distortion?

The answer is that the stages prior to the diodes are needed to boost the signal from the guitar pickup (10-20 millivolts) to a level sufficient to make a diode conduct. A diode conducts in one direction only, and it won't even do that unless the voltage across it exceeds its "forward conduction voltage." This is about 200 mV for germanium diodes and about 600 mV for most silicon diodes. Notice from the schematic that the diodes are connected directly across the signal output. So when one half-cycle of the guitar signal exceeds the forward conduction voltage, the diode conducts any part of the signal greater than that voltage directly to Vref (which is effectively ground.) The same thing happens with the opposite half cycle. Your ear perceives as distorted the "clipped-off" signal that results. Advancing R11 raises the signal level into the diodes, which increases the amount of signal that is clipped and so the distortion.

OK, Now What?

Well, I did say that you could use for the driver stage a common, inexpensive, readily-available op-amp rather than one of these exotic, long-obsolete thingies. It took me about five minutes to pull out the driver IC and replace it with a TL071 as shown in Fig. 67 and Fig. 68. The TL071 comes in an eight-pin dual-in-line package, usually called a DIP. It is a little simpler to wire than the older chips because, like most modern op-amps, it doesn't require an external small capacitor for frequency compensation; that's on the chip. I'm not going to walk you through this setup, because you should be able to lay it out on your own if you have gotten to this point successfully. I have tested what I did and confirm that it works with the basic diode clipping stage and level pot.

What To Try Next

I hope you enjoy coming up with a version of this build that you want to commit to solder. I'll be doing that myself in a future article. Following is a list of parts needed to do everything shown above, including the LED demo in the intro article.

Quantity Description SBE Stock List SKU
  Resistors - All 1/4-watt 5% Carbon Film  
1 10K 0900, 0901, etc.
1 22K  
2 11K  
1 3.3 Meg  
2 4.7K  
2 100K  
1 2.2K  
2 6.8K  
     
  Potentiometers and Trimmers  
1 500K Reverse Audio 1007
1 50K Audio 1005A
1 20K W Taper 1007
1 25K Cermet Trimmer 1015
     
  Capacitors  
1 .01 mf. 50 Volt Polyester Film 1101B or 1150
1 .47 mf. 50 Volt Polyester Film 1105
1 .047 mf. 50 Volt Polyester Film 1101B or 1150
1 .022 mf. 50 Volt Polyester Film  
1 100 pf. Polyester Film 1100
1 150 pf. Polyester Film 1100
1 220 pf. Polyester Film 1100
1 .1 mf. 50 Volt Polyester Film 1101B or 1150
     
1 1 mf. 16 Volt Radial Electrolytic 1400
1 10 mf. 16 Volt Radial Electrolytic  
1 22 mf. 16 Volt Radial Electrolytic  
1 100 mf. 16 Volt Radial Electrolytic  
     
  ICs, Transistors and Diodes  
1 2SK246 2106A
1 J201 2111
1 HA1457W or TA7136P 1506A or 1524A
1 TL071 1528
1 LED 5mm High-Brightness Red 2302
2 1N914 2208
1 Germanium Diode 1N34a or similar 2209
     
  Wire and Tubing  
  Bare Tinned Copper Wire, #22 or #24 0509
  Insulated Tinned Copper Wire, #22 or #24 0508M
  Shielded Cable 0510
  1/16" heat shrink 0500
     
  Jacks, Fittings  
1 Mono Jack, Switchcraft #11 0600
1 Stereo Jack, Switchcraft #12B 0602
1 9-Volt Battery Snap 0619
     
  Tools  
1 Breadboard or Breadboard Strip 2700, 2700A, 2700B
1 Multimeter 2701, 2701A