No sound at all on double-checked superbasic distortion circuit... - Page 1
Forum section: electronics / projects

author: Vicc
author_rank: Newbie
timestamp: May 24, 2020, 09:32:50 pm
content: Hiya all,Decided to build my very first distortion pedal and found this easy-to-follow DIY guide with schematic and all (attached here); the circuit needs a 2N3904 transistor, two104 capacitorsand three 1/4-watt resistors (470R, 68K and 2.2M). As I couldn't find all the exact same components, I bought a2N4001instead as well as1/2-watt resistors. I'm in the breadboarding phase (see attached photo) and I've double checked everything from properly identifying the CBE terminals to continuity issues using my multimeter, battery charge if full, guitar/amplifier connections, settings, etc. OK. I'm assuming the problem lies with those 1/2-watt resistors although I don't understand the reason why there's no sound at all when signal goes through the circuit...Any help would be greatly appreciated.

author: moffy
author_rank: Super Contributor
timestamp: May 24, 2020, 10:35:30 pm
content: Weird. The only datasheet for the 2N4001 I can find is a TO-5 metal can. Which is not something you see much these days. Where did you get it and what does the printing on it say?

author: rs20
author_rank: Super Contributor
timestamp: May 24, 2020, 11:45:17 pm
content: Do you have an oscilloscope? Just doing the basic binary search technique (i.e. find the actual component or breadboard connection for which the signal is present before, but missing after) should isolate this problem down to one or two faulty components or connections. Doing this is much less frustrating that continually re-checking and re-plugging every little component in your long chain from guitar to amplifier. Just figure out which component/connection in the system is the point the signal dies.Remember that breadboards can be very crappy; so watch out for that. For example, if the signal is present on the collector of the transistor, but not on the left hand side of the output capacitor, then you have a crappy breadboard connection.

author: Vicc
author_rank: Newbie
timestamp: May 25, 2020, 12:04:02 am
content: Quote from: rs20 on May 24, 2020, 11:45:17 pmI'd have to buy one, but I get your point. Thing is, I've already checked my entire breadboard for continuity issues and there was no problem...

author: John B
author_rank: Frequent Contributor
timestamp: May 25, 2020, 12:12:18 am
content: What voltage do you get at the base in circuit? The 10M input impedance of the meter will change the result, but I wonder if the base is properly biased at all. A resistor divider base bias should give you a more predictable voltage at the base.

author: rs20
author_rank: Super Contributor
timestamp: May 25, 2020, 12:25:51 am
content: OK, since we don't have an oscilloscope available, we'll need to improvise a bit. Pull out the red wire corresponding to "OUT" on the schematic, and connect it to one end of a brand new "104" capacitor. The other end of this capacitor is now your "probe". You can poke this into various points in your circuit to see how far the signal gets (beware loud crackling sounds in you brush the 9V rail, etc etc).- So if you poke it in right at the "IN" point, and you get no sound, then you know that the problem is not in your circuit, because you've completely bypassed your circuit and there's still no signal coming through (so, maybe your amplifier has a low input impedance that doesn't interact well with a 0.1uF series capacitor).- If you poke it in at the "OUT" point, and you get sound, then everything's working after all.- OTHERWISE (i.e., if poking it in at the "IN" point gives sound and poking it at the "OUT" point doesn't given sound), you're ready to start actually debugging this problem constructively rather than randomly checking random things in the circuit. Poke it everywhere along the circuit (basically, B, C and E of the transistor), see where the signal dies.Quote from: John B on May 25, 2020, 12:12:18 amThis is also good advice; definitely worth checking DC voltages all over the circuit. It's worth pointing out though, that this is a distortion pedal, so I suspect the strong distortion that comes from "improper bias" is very much the whole point of this circuit. Of course, your point may be that it might be so badly biased that the signal is just dying rather than getting distorted, which is fair.

author: Vicc
author_rank: Newbie
timestamp: May 25, 2020, 12:41:24 am
content: Quote from: John B on May 25, 2020, 12:12:18 amMy meter's lowest voltage value is 200M where I get -57.5 touching base and collector...

author: rs20
author_rank: Super Contributor
timestamp: May 25, 2020, 12:52:41 am
content: Quote from: Vicc on May 25, 2020, 12:41:24 amThis makes absolutely no sense (200 megavolts!?). Please restate while being more specific (what mode was the multimeter in, what digits did it read out, where was the red probe, where was the black probe)

author: Vicc
author_rank: Newbie
timestamp: May 25, 2020, 01:03:03 am
content: Quote from: rs20 on May 25, 2020, 12:52:41 amI meant 200m (milivolts) DC voltage mode. Red probe at base, black probe at collector terminal. Now it read -58

author: John B
author_rank: Frequent Contributor
timestamp: May 25, 2020, 02:01:01 am
content: So there's maybe ~30nA flowing through the base. What's the Vbe voltage, ie red probe on the base and black probe on the emitter? The base may not even be forward biased.

author: John B
author_rank: Frequent Contributor
timestamp: May 25, 2020, 02:04:30 am
content: Probably a good time to think about what you want or expect the circuit to do? I gather it's a distortion circuit, but do you have even a visual idea of what the output waveform should be? Does it essentially clip one half of the waveform?The way that it is currently biased leaves a lot of variables that would affect the output signal significantly. Resistor tolerances, transistor gain and Vbe due to temp etc..

author: bob91343
author_rank: Super Contributor
timestamp: May 25, 2020, 06:39:36 am
content: I suspect the transistor is saturated. With 68k collector load from 9V we would like a collector current of perhaps 60 microamperes. If B = 100 that means base current of around 600 nA. Through 2.2 Meg should drop a couple of volts which puts the emitter at 3 mA way too much.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: May 25, 2020, 07:08:42 am
content: Just running some quick numbers:You mention that you have about 58mV drop over the 2.2 Mohm bias resistor. That translates to a base current in the order of 26 nA. Assuming a ballpark gain of 100, that gives a collector current of about 2.6 uA.If we then figure that 2.6 uA has to flow through that 68kOhm collector resistor, we find that you have a voltage frop of just 176 mV drop, so your collector is at a ballpark of 8.8 V wrt ground. Emitter degeneration resistor drops about 1.2 mV. Seems a bit odd to have your output at DC be so close to your rail.But on second thought, that might be the idea of the circuit. You need your input signal to be so large it pulls down that base till it closes off, and so you get very non-linear behavior. Kinda makes me wonder why they even bothered with that emitter degeneration resistor to be honest.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 25, 2020, 09:02:29 am
content: Try a voltage divider for base bias. This may give better results than the "suicide bias" shown.May also want to use lower resistors, so the output current is a bit higher. That'll limit performance if the load is heavy. Likewise using larger capacitors (say 1 or 10 uF electrolytic, positive terminal towards the circuit) will maintain low frequency response when that is the case.I would suggest:R2 = 10kR1 = 220kAdd resistor from base to GND, 33k.This should amplify cleanly, and shouldn't be disturbed much by reading with a voltmeter. You shouldn't see much if any distortion, at least until it's really kicking (>3Vpk output voltage swing). Once it's verified working, you can change the added resistor and see how that affects the sound.2N4401 is fine, 2N4001 I assume was just a typo. Mind that if you got them from unverified sellers, you might have, who knows what, floor sweepings, instead of actual transistors in there. If they're substituted with general purpose Chinese types, it should still work, this is a very noncritical circuit; the problem really is if whether they work at all.May be worth setting up some basic test circuits to determine what the transistor is, BJT or MOS, N or P, EBC or what. A voltmeter and resistors should be fine for this.Tim

author: MarkF
author_rank: Super Contributor
timestamp: May 25, 2020, 11:21:52 am
content: As Tim suggested, look at a Common Emitter Amplifier instead:https://www.electronics-tutorials.ws/amplifier/amp_2.html

author: bob91343
author_rank: Super Contributor
timestamp: May 26, 2020, 12:03:20 am
content: I don't think a well designed circuit is intended. Whoever invented this probably cobbled together the parts he had and then published it without regard for parameter spread or temperature stability.With that in mind, perhaps adjusting the 2.2 Meg resistor over a wide range might locate a satisfactory operating point with, of course, the desired distortion. Try values from around 100k up to 10 megohms to see what works. You want about a tenth of a volt on the emitter.

author: John B
author_rank: Frequent Contributor
timestamp: May 26, 2020, 01:22:16 am
content: But this goes back to what the circuit is actually supposed to do. What is the input voltage range? What should the output look like? Is it supposed to add some 2nd order distortion "overdrive" or is it a total tone crushing circuit?

author: bob91343
author_rank: Super Contributor
timestamp: May 26, 2020, 03:50:42 am
content: As I said, the chances are that it wasn't designed. It was just cobbled together and fussed with until it worked. That kind of circuit doesn't lend itself to precision design. And it's likely that it doesn't keep working as it warms up.If you want distortion, you need to decide what sort of response to look for. Usually it's clipping that people want. So a signal needs amplification to the point where it can be clipped by, say, some diodes with a reverse bias. And you need gain controls on both input and output in order to have decent control over the setup.

author: SiliconWizard
author_rank: Super Contributor
timestamp: May 26, 2020, 02:33:45 pm
content: Quote from: John B on May 26, 2020, 01:22:16 amAssuming biasing is properly adjusted - which likely will need manual trimming here - it won't be anything more than some kind of crude "fuzz" distortion - the typical hard clipping, which sounds really awful, but was popular at some point. You may like it. Playing with biasing will also allow to get somewhat asymetric clipping, which may introduce interesting variations in the distortion effect, but again don't expect much from it.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: May 26, 2020, 03:20:36 pm
content: Quote from: SiliconWizard on May 26, 2020, 02:33:45 pmI imagine this circuit came out of some 'playful simple pedal circuits' type book. I suspect nobody makes it expecting the most amazing tone ever, but it is just fun to play around with.

author: Vicc
author_rank: Newbie
timestamp: May 29, 2020, 11:04:40 pm
content: I want to thank everyone for your input. Adhering to the forum rules, I didn't state that I'm a total newbie but you guys won't believe it even if I told you so... THIS CIRCUIT was used by probably the most innovative american musical instrument manufacturer of the 70's:Electra Guitarshttps://www.electraguitar.com/pages/historyI replaced the 2.2MΩ with a 470R though, still nothing. But when I changed that back and then decided to use a 470Ω as the R2 resistor, I can now hear sound through the speaker, almost imperceptible even if I crank up my amp's volume from 10 to 11 and that's definitely no distortion at all! The only logical explanation, at least for me, lies in those 1/2-watt resistors I'm using, so I'll post any update as long as I acquire some of those pesky 0.25-watt resistors...

author: bob91343
author_rank: Super Contributor
timestamp: May 30, 2020, 06:43:32 pm
content: The power rating of the resistors won't matter. Your change from 2.2 meg to 470 Ohms is too radical. Try 1 meg and other values in that general range.

author: John B
author_rank: Frequent Contributor
timestamp: May 31, 2020, 03:15:29 am
content: Changing the 68k to a 470R drastically lowers the output impedance. Did you specify what load on the output this circuit was driving? Maybe the 68k produced no sound because your output is quite loaded.

author: Vicc
author_rank: Newbie
timestamp: May 31, 2020, 04:24:45 pm
content: Specifying a load? Sorry John B, I don't understand.Using the 470Ω as the R2 resistor, Vbe voltage is 0.49 when meter is on 20V DC mode (red probe on the base and black probe on the emitter)...

author: John B
author_rank: Frequent Contributor
timestamp: May 31, 2020, 09:27:06 pm
content: As in, what did you connect the output of the circuit into?

author: Vicc
author_rank: Newbie
timestamp: June 01, 2020, 02:44:50 pm
content: Alright, the breadboard's "lower-left bus" is connected to the audio jack that goes into my Orange Crush Mini amp combo w/3-watt speaker. Orange Amplification does not provide a schematic for such amplifier circuit...

author: Vicc
author_rank: Newbie
timestamp: June 02, 2020, 02:56:12 am
content: I found a manual for an earlier, very similar mini amp model and it states that the rated input impedance is:470 Kohmshttps://www.zikinf.com/manuels/orange-micro-crush-manuel-utilisateur-en-28163.pdf