Analog "bitcrusher" circuit: schematic questions and "improvements"

[NoahMeurer]

Hello everyone!
First things first: I'm new here and I really like your forum.
I'm fiddling around with synths for quite some time now and wanted to add a little bit extra to my setups by building my own pedal. I really enjoy the sound of a bitcrusher and I found ExpAnonColin's circuit for an analog bitcrusher (https://www.diystompboxes.com/smfforum/index.php?topic=48809.0).

It's a sample+hold driven by an opAmp-clock. I ordered all the parts and started prototyping on my breadboard. After long nights I just couldn't get the clock to work (X3 and X4) and decided to replace it by a NE555 with a >50Hz->20kHz frequency. Which works and sounds great but there is a *very* dominant (which I believe is switching noise) sound of the NE555 audible. It's giving me headaches for two weeks now, I asked everyone I know about it but no one seems to be able to help me. So you are quite my last resort!



Things I tried:
1) Decoupling the NE555 by adding a capacitor directly next to it's Vcc and ground rails: This did remove the noisy 9V and Vb rail, however the noise/annoying sound is still there.
2) Increasing the first opAmps gain by 5 and the other ones by 1/5. ExpAnonColin recommended that in his inital post if he would ever sell this commercially. To achieve this I just added 10k resistors and 2k resistors according to resources I found online (https://circuitdigest.com/calculators/op-amp-gain-calculator). However, this didn't change a thing.
3) Giving the NE555 Vb's supply voltage (~4.45V) instead of the 9V power rail: This makes the NE555 go crazy since it's below it's supply voltage.
4) Routing the NE555's output through a voltage divider: this didn't change a thing as well.

A friend of mine suggested me to filter the output but since the frequency varies quiet a lot this isn't possible.

I assume that I did 2) the wrong way, however I do not really understand what he meant in his post. I would ask him face to face but he isn't online since last year and I hope you guys can help me.

If I forgot to share any details with you I'll be more than happy to add them.
Thanks a lot for your help in advance.

Best regards
Noah Meurer

[11-90-an]

Welcome to the forum...

Tried and tested by about everyone, (including you!), the NE555 simply isn't useful for clocks/lfos for audio processing, etc. Very audible thump with lfos and whine with clocks. What most people use is the CMOS 555, simply a CMOS version of the NE555, and (apparently) doesn't have much audible thump/whine! 

Also, you should check out Parasit Studio's Sonic Reducer, which is an improved version of Colin's bitcrusher...

https://www.parasitstudio.se/sonicreducer.html
https://www.parasitstudio.se/uploads/2/4/4/9/2449159/sonic_reducer_pcb.pdf

Give the opamp clock another go? 

[iainpunk]

i echo the opamp clock.
you can even make it simple with a single opamp like this:

i hope you can figure out yourself how to power the opamp 

cheers, Iain

[PRR]

Welcome.

I'm not awake enough to ponder the '555; that looks like an odd setup.

But I see you are grossly over-driving that poor JFET. Add a diode.

[NoahMeurer]

Thank you for your answers! I'm really looking forward to test all your solutions and improvements tomorrow. I'm going to report back to you after prototyping

Also, you should check out Parasit Studio's Sonic Reducer, which is an improved version of Colin's bitcrusher...
Give the opamp clock another go? 

I'm going to give the opamp clock another go and will try Iain's clock which looks very simple!

But I see you are grossly over-driving that poor JFET. Add a diode.

Oh I had no idea! To be honest I'm learning electronics besides my high school standard education. What exactly does over-driving mean and why does a diode help? I thought that those only block voltage in one direction. I'm eager to find out!

Edit - Regarding my 555 circuit which will be removed/changed anyway: It's a fixed duty-cycle circuit. R8 only sets a min-value to the frequency and R9+R10 scale the 50k pot down to I think 46k which prevents the NE555 from generating too low frequencies. Here's the resource for the 50%-duty-cycle-thing: https://www.electronics-tutorials.ws/waveforms/555_oscillator.html

Kind regards from Europe
Noah

[PRR]

The JFET's two outer legs (Drain and Source) are biased at +4.5V. The '555 output swings zero to near +9V (7.5V?). The JFET's Gate should not normally ever rise above either other leg. So current from '555 Out is just flooding the JFET and gushing out of the box into the cellar. (No wait, that was MY plumbing fun for the day.) With '555 thumping along, great thumps of voltage overwhelm the JFET and thus your amplifier.

The diode will block that. In a few months you will ask "how does this work at all??", back-to-back diodes. Thing is the JFET gate diode is LOW leakage, the 1N914/1N4148/1N4007 diode's leakage 'will' pull up the naked Gate, but not the audio path.

[garcho]

...gushing out of the box into the cellar... was MY plumbing fun for the day.

Oh no, sorry to hear! I hope you got it all worked out.

A couple years ago I was in the kitchen, chopping onion. I heard an odd sound, set the knife down, and listened. It was almost like someone was playing a snare drum. I walked into the other room, following the sound, and there were my drums, and sure enough, the snare, being played, by drops of water falling from the ceiling, directly below the 2nd floor shower. Luckily, I discovered it quickly enough that there was no water damage (visible anyway) to the drums, but it did take a while to retile the shower. The previous owners had left all kinds of evil easter eggs hiding for me in the floor and walls.

[anotherjim]

The USN are now fitting snare drums in all compartments of their active submarines.

[NoahMeurer]

Hello there,

I'm done with prototyping so far and I've tried various things:

1) Replacing the NE555 by @iainpunk 's opamp clock. I checked the clocks output with an oscope and it didn't do a thing. I used a 100nF cap (which has 104 code, doesn't it?) and a 20kPot. When I connected the output to the JFET Gate it made a very loud screeching noise and it almost wrecked my amp. I've used a LM358P as opamp.
2) Adding a Diode as @PRR suggested. When I added a 1N914 diode like shown in his post, there is no sound at all or (when reversing the polarity) no bitcrushing at all.

Perhaps I'm using the wrong opamp. Any suggestions?

Kind regards
Noah

[iainpunk]

if you probe the voltage on the capacitor, how does that look like? should be a saw tooth. the opamp's output is 'high level' all the time, except for tiny 'low level' pulses that discharge the capacitor.

wow, sorry im an idiot, i just realise that i flunked the porlarity if the oscilator! sorry!


cheers, Iain

[NoahMeurer]

Rebuilt the circuit but sadly it didn't work out for me, as well.
Here is a picture of my breadboard if you want to take a look. Sorry for the sketchy build but I dont have any normal jumpers so I use dupont cables.

I'm pretty confident that this is the circuit as you posted it.
I checked the LM358P's Vcc and GND, Vcc recieves 9V and GND... well 0V. I took a look at the signal at the cap, it only rises when I turn the pot. The output of the opamp is just a pretty stable voltage.
Hopefully I didn't miss a thing.

Best regards
Noah

[ElectricDruid]

The power looks wrong. Pin 8 seems to go to 4.5V, not 9V.

You've also got the cap and the pot going *up* to something with red wires in. That all shouts "+9V" at me, when it should be ground/0V/black. It does have a blue line on that row though, so perhaps you've got +9 and 0V both top and bottom of the breadboard?

Is the other end of the pot connected? The two pins that are connected by a wire don't seem to have any other connection, but maybe it's hidden.

[NoahMeurer]

Thank you for your answer!

To clear things up a bit:
Pin 8 (referring to this diagram: https://www.jsumo.com/lm358-ic-opamp-gp-11mhz-soic-8-1024-44-B.png) is connected to 9V, the yellow jumper at 42A.
The opamps ground is connected via a barely visible jumper at 39J. Both + rails do have 4,5V (Vb) and both - rails do have a connection to ground.
Looking at the potentiometer, the center and the right pin are connected via another jumper and from the center pin there is the 1k resistor, going to the 0.1uF cap and the yellow jumper to the inverting input of the opamp. The left pin of the pot is connected to the - rail via the red/orange jumper. The other pair of orange jumpers just connects both - rails.

Needless to say, I didn't pay attention to the color of my cables. (I am really sorry)

[ElectricDruid]

Ok, thanks. So you've definitely got 9V across pins 4 and 8 on the op-amp? As long as you have, the power's good.

In which case, we need to be looking for something else. There has to be something we're missing.

[NoahMeurer]

Yeah there definetly is power across 4 and 8. I double-checked with a multimeter to make sure.

I don't know a lot about opamps but perhaps I need a certain type of opamp for this to work? I do have a NE5532 laying around. Perhaps I could try that one if it matters.

[NoahMeurer]

Back again after some more prototyping.

Positive stuff first: Iain's clock works. I've seen a saw wave at the cap. So far so good.
Negative stuff: Now my Vb rail does feature the same pulse frequency as the signal output to the JFET. Also, there is no more sound being fed through. Vb is at ~1V and then spikes to 3.5V for a brief moment.

There is also something weird going on: The clock didn't work until I started measuring with my oscope which somehow "triggered" the circuit. At this time I measured at Vb. This is really strange.

I feel like I'm almost done and there's only a minor thing keeping me away from finishing this built.

Best regards
Noah

EDIT: Making the voltage divider for Vb opAmp-buffered got lost of the spikes. Only a small "regular" noise is visible, but that shouldnt bother me
EDIT 2: The reason the output isn't going to the jack is because it "stops" at the capacitor which scales down the voltage for the jack...

[iainpunk]

im glad the clock works, and i haven't been a complete waste of time here... haha

at which capacitor does the signal stop? an output cap should at-least be 100n, (unless you want to use it to make the sound shrill), it looks like you use a small ceramic, and i doubt its actually 100n or bigger.

also, dupont style cables are the gold standard for breadboarding IMHO

cheers, Iain

[NoahMeurer]

The cap is the far right one in the picture and it's C2 on my schematic, a 100nF one. After I noticed the spikes of the clock on my Vb rail, I buffered the Vb rail with an opamp. Could this be the reason why it's not putting out sound?

I tried to use an unbuffered voltage divider for colin's sample+hold circuit and the buffered voltage for the opamp clock but that didn't make a difference as well.

When measuring 16C, you can clearly see the crushed up sound,  but after the cap, at 14C, there is just one stable voltage.

Regards, Noah

[iainpunk]

After I noticed the spikes of the clock on my Vb rail, I buffered the Vb rail with an opamp. Could this be the reason why it's not putting out sound?

no, that's not possible AFAIK.

what happens if you take out the resistor to ground? have you double checked if the resistor is the right value?
the cap might be bad, have you tried another cap of the same value? also, due to the microphonic nature of ceramic caps, they aren't ideal for audio circuits. i recommend getting foil or polyester caps., they are a bit more expensive, but save a lot of headache.

cheers, Iain

[NoahMeurer]

So I checked everything again this morning.

The resistors are all the right value - I didn't change them at any time.
The caps still work but I'll order some foil caps for the long term. I was finally able to find the problem: before the last cap, there is my signal with a base voltage of ~4 Volts and on top of that is my signal. The last cap and the 1M resistor are cutting off the 4 Volts and leave me with my signal. However, when I'm now connecting my amp to the circuit, the output voltage collapses. This was a problem that didn't appear on my previous revision. (Could that be tied to the now opamp-buffered voltage? The resistor is only going to ground.)
I tried buffering the signal with a LM358 opamp which produced noise and then got really warm (all of a sudden it immediatley drew 50mA). I also tried a NE5532 low noise opamp but that made everything more worse.

Now I'm really banging my head against a wall. Why is everything so damn complicated? I mean it's not a difficult circuit...

Regards, Noah