Low Gain, neutral dist/OD

[Fancy Lime]

Impedance is too complex a concept to give generalized answers that are still useful in a concrete application. It does indeed have to do with resistance, reactance and phase. Fortunately, for our little diystompbox problems  we usually only need to concern ourselves with the sum of resistance and reactance. Resistance is the (frequency independent) resistance to a DC or AC signal (well, duh!), reactance is the frequency dependent resistance to an AC signal. Since audio is always AC, the sum of the two is also simply a frequency dependent resistance, for all we care (most of the time).

Output impedance of a circuit is the frequency dependent resistance that follows the last active component of the circuit in series until the output jack. In a real-world circuit that includes the output impedance of the last active device itself but this is often (not always!) small enough to be ignored.

Input resistance is the frequency dependent resistance to "ground" before the first active component. Beware, though, that "ground" in this case means "any and all reference voltages", including things like the virtual ground point of an inverter input. It also contains contributions from the active device itself that depend on the circuit topology and the device used. Luckily, there are equations for all reasonably common topologies and many uncommon or downright bizarre topologies to be found on the great interwebs. And most of them can be simplified dramatically without introducing too much additional errors. For example, for a non-inverting opamp, you can say "the input impedance is the total DC resistance to ground before the input", and that is wrong bit it is "correct enough" for most things where we care about impedance.

The "total impedance of a circuit" does not generally exist, especially for active circuits. So hooking a multimeter to both ends of a stompbox is not going to tell you anything about the impedance.

But for your design problem, I think the whole impedance discussion will not be super useful without much, much more other background. And learning theory is not as much fun as building a working pedal for most people, or so I'm told.

So my recommendation to you would be to try the following: You like the Red Llama (or your modded version of it), right? Take that circuit up to and including C6. Leave out the Vol pot. Add another inverter stage with a 100k pot wired as a series resistor before the inverter input and a 100k resistor in the negative feedback loop. This is now a buffered Volume control. Next add another stage with one more inverter, which you use as a 2-band Baxandall tone stack. This is the most useful tone stack topology in my personal opinion. Very flexible and easy to use. Banxanwhat? This:
http://www.learnabout-electronics.org/Downloads/NegativeFeedbackTone.pdf
I hope I find the time to draw the whole thing for you tomorrow, but I am a bit short on time this weekend, so it may be a few more days. Unless someone beats me to it. I've been meaning to build something like that anyway, just did not find the time.

Hope that helps,
Andy

[PRR]

> Is the impedance the resistance between the first and last component of a circuit when Measured with a multimeter?

It isn't, but would be, IF you replaced the DC battery in the meter with an audio source (variable frequency because impedance often varies with frequency) and replaced the DC meter with an AC/Audio meter.

There have been audio impedance testers with 50Hz/1KHz/5kHz to cover 10-1k Ohms (the most likely values in large PA systems). Here's a $150 job which only tests 1kHz.
https://www.amazon.com/d/Multi-Testers/Tenma-72-6948-Audio-Impedance-Meter/B008BW9MI0
$700 with more settings (but does not sense high ohms):
https://www.amazon.com/Gold-Line-ZM-1P-Impedance-Protection/dp/B0006TZ4UG

Generally you rig a signal generator, variable resistor, and audio voltmeter and do math.

A super-simple trick for guitar levels is to put a 0-500k pot in series with an input. Trim the pot up from zero until it is "half loud", then measure the pot. But what happens on many guitar inputs is the bass and treble don't fade-out equally-- which matters more? It Depends.

[Fancy Lime]

Hi folks!

So here is the "Baxandllama" I've been promising, finally:



The inverters should of course be CD4069UBE or CD4049, I just did't have the right symbol at hand. The EQ gives you + or - 12dB at 100Hz and 6kHz. This is not tested, not even on the breadboard, just simulated. So take this as a starting point and tune from there by ear. Best to start by playing around with the values of C7 and C8. Also, it may be wise to add Capacitors in the 500p to 1n range in parallel with R6 and R12 to cut the hiss. The first 2 inverters are just the standard Llama values, so replace that with your favorite values.

Cheers,
Andy

[Killthepopular]

If I'm using a cd4049 how can I tell which pin is which on the baxandllama? The Red LLama schematic I used had the pins numbered 1-16, which I understand.

I'll be intrigued to see what it sounds like. I have a sneaking suspicion that it might actually sound quite different from the red llama.

Also, where does the 9V enter the circuit? I can't see it on here.

And does it matter that there's only 2 points connected to ground? I'm just used to there being more ground connections in these circuits is all...

[Fancy Lime]

No time right now, so real quick: google "cd4049" and look at the datasheet. It contains the info you are searching. I'll try to be more helpful soon.
Andy

[Kipper4]

Not the data sheet but a pinout diagram. Which should clear up a few questions.
Pin1 Vdd will be your plus rail voltage.
Pin8 Vss will be ground
Like I say I haven't googled the sheet.

The plus rail and ground will be common to all the inverters in the package.

The diagram is handy in that it gives you a sense of the inputs and outputs.

Not sure where you're up to and no offence but if you were looking at an inverter circuit like this when your used to seeing maybe tube screamers. You'd wonder where's all the non inverting inputs and how are they biasing the chip.

So the circuit is an gain stage. That's gonna distort like mad if dimed.

LPF

Another gain stage.

Active tone stack.

Hope I'm not interfering.

[Killthepopular]

FWIW, here's a way to add a control to the DOD 250.

What does C5 NF mean?

[bluebunny]

What does C5 NF mean?

On the rare occasions I've seen this, it's meant "not fitted".  So ignore it.  Not sure of the value of this notation, but perhaps there were variations of this circuit where this particular cap was indeed present?

[Killthepopular]

Currently fooling around with a 250, just a standard schematic, not the tone control one. Comparing it to the vox distortion I'm struck by 2 things. 1, it seems kinda lower gain to me. 2, I'm shocked by how much it just sounds like a treble booster. When I turn up to max gain it's like the overall sound doesn't get that much louder but the trebles seem to get boosted. It's possible that I've messed up the schematic a bit but I have heard that 250s are kinda like treble boosters so I'm guessing this is right.



What the function of C5? With the gain set to minimum the tone is really dull and really bright when set to high. Increasing C5 makes the tone duller which is good for high gain but it makes the low gain sounds even more unusable.

Why the big difference between high and low gain tones? Is the gain pot partially acting as a high pass filter?

[Killthepopular]

How do I add a high pass control to this circuit? I've done the low pass thing. The signal goes through a pot and then past a cap that goes to ground. Simple. 2 Components. But when I try to do the same thing with a high pass filter I just create a sort of volume control. The signal goes through a cap this time and then past a pot that's connected to ground but as I turn the pot I end up just sending the entire signal to ground.
I can't get my head around how it's supposed to be wired. How do you wire up a high pass control? Is it not possible to do it without a bunch of extra components? I understand how a HPF should be wired, I just don't understand how you can subsitute a pot for the resistor without just turning it into a volume control.
I'm just playing around here, experimenting. I might try a big muff style tone stack instead.

[GGBB]

How do I add a high pass control to this circuit? I've done the low pass thing. The signal goes through a pot and then past a cap that goes to ground. Simple. 2 Components. But when I try to do the same thing with a high pass filter I just create a sort of volume control. The signal goes through a cap this time and then past a pot that's connected to ground but as I turn the pot I end up just sending the entire signal to ground.
I can't get my head around how it's supposed to be wired. How do you wire up a high pass control? Is it not possible to do it without a bunch of extra components? I understand how a HPF should be wired, I just don't understand how you can subsitute a pot for the resistor without just turning it into a volume control.
I'm just playing around here, experimenting. I might try a big muff style tone stack instead.

Use a 10k pot in place of R9 or R6 - connected at pins 1 and 3. Then add a cap from the wiper to one end. You'll have to experiment with the cap value, and you may want to use a fixed resistor in series with the pot to set the filter limit. See the second image here: http://www.muzique.com/lab/swtc.htm.

[Killthepopular]

Use a 10k pot in place of R9 or R6 - connected at pins 1 and 3. Then add a cap from the wiper to one end. You'll have to experiment with the cap value, and you may want to use a fixed resistor in series with the pot to set the filter limit. See the second image here: http://www.muzique.com/lab/swtc.htm.

I couldn't get that circuit to work but I tried the one below it (3rd picture) and it works great although I can't get enough hi boost from it. How would I increase the amount of hi boost?

The DOD 250 is now the 8th circuit I've tried and it's the first one so far that really feels like THE SOUND. I'm inclined to stop trying out different circuits and instead stick with this one but tinker with it to get it to do what I want.




I've got a 1n4001 vs 2 x 1n34a as my clipping diodes. How do I reduce the minimum amount of gain without reducing the maximum gain? I 've swapped the 500k pot for a 1m. Is that the right way?

I might want to add a post tone control boost as well as the current max output level at low gain is a bit below my guitar at full volume.

[duck_arse]

I'm the worst person on earth to ask/tell about tone controls, but this top-cut is what I put on my recent distortion + build. it works ok, could almost be called a bass control.


the more I look at my diagram, the more I'm sure I've connected it wrong, but that is how it was tested and built.

to get less minimum gain on your dod, you need to match the gain pot value to the feedback resistor value. you will never get less than a gain of 2, tho, I thimk.

[edit :] deleted

[GGBB]

Use a 10k pot in place of R9 or R6 - connected at pins 1 and 3. Then add a cap from the wiper to one end. You'll have to experiment with the cap value, and you may want to use a fixed resistor in series with the pot to set the filter limit. See the second image here: http://www.muzique.com/lab/swtc.htm.

I couldn't get that circuit to work but I tried the one below it (3rd picture) and it works great although I can't get enough hi boost from it. How would I increase the amount of hi boost?

Any one or a combination of: bigger C1, smaller R1, bigger R2, smaller Tone pot.

[Kipper4]

You'd be better off starting a new how  to mod my dod 250 thread of your own.
Meantime heres some reading that will help further your learning about op amp gain and filters

http://www.generalguitargadgets.com/how-to-build-it/technical-help/articles/design-distortion/

[PBE6]

Comparing it to the vox distortion I'm struck by 2 things. 1, it seems kinda lower gain to me. 2, I'm shocked by how much it just sounds like a treble booster. When I turn up to max gain it's like the overall sound doesn't get that much louder but the trebles seem to get boosted.

Why the big difference between high and low gain tones? Is the gain pot partially acting as a high pass filter?

Your observations are all related.  Many non-inverting opamp distortion circuits have two simple filters, a low-cut and a high-cut.  In this case, the combination of C3 and R7+VR1 act as the low-cut filter, with a frequency that varies between 1/(2*pi*C3*R7) = 63 Hz at minimum gain and 1/(2*pi*C3*(R7+VR1)) = 7.2 kHz at maximum gain.  The higher the gain the less bass content there is in the signal, hence the impression of a treble boost.  Coupled with that, less bass content means fewer mid-range and high-end harmonics are formed when clipping the signal and so the character of the distortion is less muddy, hence the impression that it doesn't have as much gain as other fuzzier pedals that retain more bass content.  (As a side note, if you want more bass but like the character of bass-light clipping, you can add a clean bass boost after the clipping stage.)

What the function of C5? With the gain set to minimum the tone is really dull and really bright when set to high. Increasing C5 makes the tone duller which is good for high gain but it makes the low gain sounds even more unusable.

This is the high-cut filter, which is formed by C5 and R8 and has a frequency of 1/(2*pi*C5*R8) = 7.2 kHz, and it has the effect of minimizing the shrill high-end harmonics generated during clipping.  Increasing the value of C5 will lower the frequency of the filter, making things more mellow or more dull depending on your preference.  If you notice, the low-cut and high-cut filters are almost the same when the gain is all the way up which creates a very trebley 7.2 kHz-focused sound.  (Tubescreamers do this too, but the emphasized region is wider and starts much lower, around 720 Hz.)  You may want to play around with the filters to fine tune where this emphasis will be when the gain is cranked.

[Fancy Lime]

A few answers about the Baxandallama, finally:

@Killthepolular:
With the tone controls on 12 o'clock, the Baxandallama should sound very similar to the normal Llama. The 3rd and 4th stage do not perform any tone shaping unless the knobs are turned. At high volume settings these stages may clip but that will make very little difference after the first two massively clipping stages. Anyway, it is worth tinkering with the first two stages to reduce gain (as we discussed in this thread earlier).
CMOS inverters need no referencing to ground or any other reference voltage (which may in turn be connected to ground) because they are biased via the resistor between the output and input of each inverter (R3 + Gain, R4, R6, R12). This type of biasing is known as "virtual ground" biasing because the inverter itself just makes it's own "ground" right in the middle of V+ and V- (where V- is usually the "external ground"). Try to think about what happens when the input of the inverter swings high and what happens when it swings low. What will the output of the inverter do? And if input and output are DC coupled via a resistor, what does that mean for the DC voltage at the input when there is no signal coming in?

@Kipper4
Hi Rich, well, the point of having the volume control setup the way it is, incorporated in stage 3, which has a gin between 0 and 1, is to be able to use an active one stack without overloading it completely. Unless you want to. In fact, having a boosted bass band clip a CMOS inverter gently, sounds rather pleasant. But if you do that, you get almost 9V of signal swing at the output of this thing. And if that is something the user wants is, uhm, questionable. But a second volume control (master) at the end would fix that.

Cheers,
Andy

[Killthepopular]

I had a listen back to all the clips I've recorded of the 8 circuits I've tried and now I'm thinking that the electra is my favourite. Here's a summary of the 8 circuits:

Electra: Surprisingly good. I like the compression. Generally pleasant, smooth, well balanced tone. Seems to have a good gain range and can do low and high gain stuff equally well.
Llama: Very good at that Tubey, fendery sound of making everything sound just slightly overdriven all the time. It's a bit too transparent though, or too dynamic. It lets some harshness or twanginess of the guitar leak through where a more aggressive clipping circuit would turn it all to mush and make things easier on the ear. I also get a bit frustrated when i turn up the gain and don't get much more out of it.
Vox distortion: rubbish at low gain. Sounds badass but can get a bit harsh.
Peppermill: sounds outright bad but can be surprisingly good at low gain stuff.
Barbershop: Generally sounds decent but it's not one of my favourites.
Eighteen: Mostly sounds good but it can be a bit nasty. Not great at low gain.
Dod250: Rubbish at low gain. I don't like the way it's really thin and bright at max gain and totally muffled at minimum gain. Would require me to make a really good tone circuit to go with it and I'd need to fiddle with it every time i touched the gain knob.

I'm starting to get a sense of the difference between "feel" and "tone". The dod250 makes me feel like a rockstar. It's so awesome to play but when i listen objectively i find it to be very poorly balanced and unnatural. The llama on the other hand is one of the better sounding circuits but I find it a bit boring to play for some reason.

I ended up buying parts for several more circuits I wanna try. There were a few circuits that just had way too much praise on The Gear Page so I felt like i had to try them out. I'll make the following circuits soon: Son of screamer. Baxandllama. Rat. Timmy. OCD. Barber LTD silver.

I also got the parts for an ep booster which i'll try out once I've got my main dirt circuit sorted.

[Mark Hammer]

Dod250: Rubbish at low gain. I don't like the way it's really thin and bright at max gain and totally muffled at minimum gain. Would require me to make a really good tone circuit to go with it and I'd need to fiddle with it every time i touched the gain knob.

I'm starting to get a sense of the difference between "feel" and "tone". The dod250 makes me feel like a rockstar. It's so awesome to play but when i listen objectively i find it to be very poorly balanced and unnatural. The llama on the other hand is one of the better sounding circuits but I find it a bit boring to play for some reason.

The 250 and its sibling, the MXR Distortion+, use changes to the ground leg resistance to alter gain, rather than the feedback resistance, as the Tube Screamer does.  You can do the math but dropping the ground leg resistance to increment gain always moves the bass rolloff upwards, while increasing the feedback resistance to increment gain always moves the treble rolloff downwards.  Whether either of those gain-changes results in audible changes in tone depends on the value of the capacitors used in each respective arrangement.  The ground-leg cap used in the 250 is 47nf (.047uf).  At min gain the rolloff is inaudible, but at max gain the bass rolloff begins at 720hz, resulting in the thin sound.  And in the absence of any useful treble-cut anywhere else in the circuit, that gets really shrill.

To "fix" it, replace the 47nf cap with 220nf, stick a 68-100pf cap in parallel with the 1meg feedback resistor, and replace the 1000pf cap in parallel with the clipping diodes with a 2700-3300pf cap.  Those changes will sand down the sharp edges more, and deliver more oomph at higher gain.  One of the side-effects of retaining more bass in the "drive" stage, prior to the clipping diodes, will be that overdrive is achieved a little more easily, since retaining bass will increase overall amplitude.

[Fancy Lime]

Dod250: Rubbish at low gain. I don't like the way it's really thin and bright at max gain and totally muffled at minimum gain. Would require me to make a really good tone circuit to go with it and I'd need to fiddle with it every time i touched the gain knob.

I'm starting to get a sense of the difference between "feel" and "tone". The dod250 makes me feel like a rockstar. It's so awesome to play but when i listen objectively i find it to be very poorly balanced and unnatural. The llama on the other hand is one of the better sounding circuits but I find it a bit boring to play for some reason.

The 250 and its sibling, the MXR Distortion+, use changes to the ground leg resistance to alter gain, rather than the feedback resistance, as the Tube Screamer does.  You can do the math but dropping the ground leg resistance to increment gain always moves the bass rolloff upwards, while increasing the feedback resistance to increment gain always moves the treble rolloff downwards.  Whether either of those gain-changes results in audible changes in tone depends on the value of the capacitors used in each respective arrangement.  The ground-leg cap used in the 250 is 47nf (.047uf).  At min gain the rolloff is inaudible, but at max gain the bass rolloff begins at 720hz, resulting in the thin sound.  And in the absence of any useful treble-cut anywhere else in the circuit, that gets really shrill.

To "fix" it, replace the 47nf cap with 220nf, stick a 68-100pf cap in parallel with the 1meg feedback resistor, and replace the 1000pf cap in parallel with the clipping diodes with a 2700-3300pf cap.  Those changes will sand down the sharp edges more, and deliver more oomph at higher gain.  One of the side-effects of retaining more bass in the "drive" stage, prior to the clipping diodes, will be that overdrive is achieved a little more easily, since retaining bass will increase overall amplitude.

The "treble boosting" effect of a pot in the ground leg and the "bass boosting" effect of a pot in the feedback leg of a non-inverting opamp can also be used deliberately for extra flexibility. I just happen to have an overdrive circuit on the breadboard right now that does exactly that. A 500k pot in the feedback loop provides the overall gain and has a 1n cap in parallel. There are two ground legs. One is just a 2k2 resistor and a 3u3 cap for full range response down to 21Hz. In parallel with that is a 470R, 100n leg with a 5k pot in series. This pot now acts like a variable treble booster or a Presence pot. Very useful and flexible combination, definitely worth the extra pot in my oppinion.

The thing on my bradboard is a kind of multi-drive. Very simple design but goes from clean boost through blues-driver territory to high gain metal distortion or fuzz, depending on just a vew options. A few of the possible incarnations may be of interest to you. My favourite low gain, natural overdrive variant relies on opamp clipping from a NJM2068. Yes, I am an absolute NJM2068 fanboy, and I don't care who knows it. These are a bit hard to come by though. I highly recommend you get some if you can, their clipping is by far my favourite opamp clipping. Very Blues Driver like, to my ears at least. A NE5532 works too and sounds amazing with bass in this circuit but not quite as good as the NJM2068 with guitar. However, MOSFETs as clipping diodes make the NE5532 variant into a great tubey drive. Enough teasing, I hope I'll get the drawings of the circuit done tomorrow or on the weekend at the latest. If i don't get the full shebang with all options done, I'll try to at least get the low gain bluesy thing (which is the simplest variant anyway) out the door.

Cheers,
Andy