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Help with basic limiter/VCA at end of diy synth signal path
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floating-water



Joined: Oct 06, 2020
Posts: 39
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PostPosted: Tue Oct 20, 2020 7:14 am    Post subject: Reply with quote  Mark this post and the followings unread

I did some buffering with some tl072s on the input and out for the reverb, just treated it like a reverb pedal, seems to be working good. I need to get a small speaker for the build; I understand the smaller the ohms the more current it draws? For the tiny wattage of the lm386 are there any specs I should look out for like what does nominal rated power mean? Does it matter?
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PHOBoS



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PostPosted: Tue Oct 20, 2020 7:42 am    Post subject: Reply with quote  Mark this post and the followings unread

floating-water wrote:
I did some buffering with some tl072s on the input and out for the reverb, just treated it like a reverb pedal, seems to be working good. I need to get a small speaker for the build; I understand the smaller the ohms the more current it draws? For the tiny wattage of the lm386 are there any specs I should look out for like what does nominal rated power mean? Does it matter?


yes, a smaller impedance (difference from resistance is that it changes with frequency) will draw more power.
In this case you have to check the datasheet of the LM386 as there are a couple of different versions.
The LM386 wikipedia page actually has them listed: https://en.wikipedia.org/wiki/LM386
I don't think that info is complete though, if I remember correctly the minimum impedance also depends on the supply voltage
and it can be lower with a lower voltage.

nominal power rating of a speaker is generally the continuous power, peak power can often be much higher with large speakers.
For speakers the SPL rating also matters but with the kind of speakers you'll probably be using that info is likely not available.
A higher SPL rating gives you more bang for your buck (or loudness for your watts).

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floating-water



Joined: Oct 06, 2020
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PostPosted: Wed Oct 21, 2020 1:11 am    Post subject:   Reply with quote  Mark this post and the followings unread

I have the 386-1 (of course i do) so would a 4ohm be too little/cause problems? Im running out of budget just want to get the right thing.

I've got this:
https://www.ebay.co.uk/itm/363076889353?ul_noapp=true

or this:
https://uk.rs-online.com/web/p/speaker-drivers/1218637

Going back to the synthesizer, everything's sounding great. I was wondering however, how would I switch between no fuzz-delay and just oscillators? No matter how much resistance I put between the outputs of the oscillators, any more than one osc going through the fuzz is inaudible. I have a main switch in the middle which is three way:

https://www.ebay.co.uk/itm/Guitar-toggle-switch-3-way-chrome-black-or-gold/390322262654?hash=item5ae105127e:g:FhsAAOSwC8pb5wRj

taken from a les paul style gutar. I can't seem to figure out how I could have:

switch in bottom position: off
switch in middle position: just osc's (i've got to this point)
switch in top position: osc with effect

Could I just wire the audio through the switch? What the proper way to do it?
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PHOBoS



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PostPosted: Sun Oct 25, 2020 5:45 am    Post subject: Reply with quote  Mark this post and the followings unread

floating-water wrote:
I have the 386-1 (of course i do) so would a 4ohm be too little/cause problems? Im running out of budget just want to get the right thing.

I've got this:
https://www.ebay.co.uk/itm/363076889353?ul_noapp=true

or this:
https://uk.rs-online.com/web/p/speaker-drivers/1218637


going by the datasheet it looks like you need an 8 ohm speaker > 500mW
So you could put 2 of the ones from ebay in series to get 8 ohms, or get the other one but that seems to be a bit overpowered.
Not that it would break anything but you probably get more sound out of 2 lower powered speakers.



Quote:
I have a main switch in the middle which is three way:
switch in bottom position: off
switch in middle position: just osc's (i've got to this point)
switch in top position: osc with effect


I am not sure if there is anything special about that switch but normally you would connect the 2 signals [just osc's][osc with effect]
to the outer lugs and then take the signal from the center lug. This will move the off position to the center though.

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floating-water



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PostPosted: Sun Oct 25, 2020 2:46 pm    Post subject: Reply with quote  Mark this post and the followings unread

Can't thank you enough for your help mate, ive learnt a ton. You're right about the switch but it will do for the 1st version of this project, there doesn't seem to any sound degradation.
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Ricko



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PostPosted: Wed Oct 28, 2020 4:41 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi.

Five ideas.

First, would adding a volume control get you a long way there? It can be just a single potentiometer.

Second, would a permanent cut in volume get you a long way there? So just a voltage divider (two resistors) at the outpit.

Third, I often put soft clipping on my modules. So this is like the anti-parallel diodes as above, but with blue or purple or green LEDs (highest voltage drops) preceded by a 1k or 2.2k resistor in series. You need the resistor otherwise the LEDs can blow, but getting much light out is not the deal. And the LEDs act like a clipping indicator if they do light up.

Fourth, it is possible to use a vactrol as a kind of passive limiter. I am putting out some boards for that, (fricko "tame") in November.

Fifth, if youbare a builder, consider thus fast limiter: https://sound-au.com/project67.htm
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floating-water



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PostPosted: Wed Nov 11, 2020 11:42 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for your info Ricko, I'm yet to attempt the limiting section just yet as still working on the noise making part; just a continuation of this, if anyone could help, is there a way to stop the oscillators affecting each others pitch, say, when one of them is at their highest on the potentiometer, it will affect the others. They are RA transistor oscillators

Another is how could I build a variable capacitance? would simply putting a variable resistor in the middle of two caps work? The two caps being the threshold of the minimum and maximum capacitance. It's for a kind of 'release' control on the trigger switches for the osc's

And the powering situation is causing some headaches, I've been breadboarding using a 9v batt but I want it to be mains powered, i' wired up a dc jack, did the diode/cap filtering but theres a load of noise. Is it most likely from the psu? Is there any procedures anyone takes for filtering noise in a basic 9v dc in?

Thx
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Ricko



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PostPosted: Thu Nov 12, 2020 12:24 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi. If you have several oscillators, and tuning one affects the others, your circuit has some coupling.

So it could be occuring through the power supply, in which case you add more decoupling capacitors as close each of the oscillators as possible (e.g. 1uF and a 10 nF capacitor from the +ve power to the 0V and probably the same from -ve to 0v if you use it).

Or it could be coupling on their outputs, if you have a simple mixer. Do your ocillators outputs go through op-amp buffers? Buffers are a good way to prevent interaction when outputs are mixed together.

For a variable capacitor, there is such a thing (a varactor, as used in old radios) but no synths have used them for maybe 60 years.

You cannot make a variable capacitor in the same way with two caps and a variable resistor. (Each end of the pot roration might be what you want, but in between it is not a simple response: probably more like a cross fade between the two capacitor values rather than a capacitor diminishing in value.)
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floating-water



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PostPosted: Fri Nov 13, 2020 6:03 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks Rick, ye Im just using the passive resistor mixer atm, how do I go about mixing them with an op amp? I have some tl072s, I understand the inputs and outputs but how does one do any mixing with them? Do they not need dividing with resistors?

Another thing, is there a correct way to wire switches/buttons or 'keys' for the oscillators, do I need pull down resistors? And why are they necessary?

edit: ive hooked up a tl072 'correctly?' with the output and the inverted input connected and the osc going into the non inverted but the out put is basically nothing

Thanks
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Ricko



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PostPosted: Sun Nov 15, 2020 10:34 pm    Post subject: Reply with quote  Mark this post and the followings unread

A good way to apporoach problems is to be systematic.

So lets start with the theory that it is the passive mixing causing the interference. Are they just resistors or are they pots? If they are pots, does turning the knob change the severity of the problem?

If resistors, what value are tou using! You would want at least 10k. To ecperiment, at the passive mixer disconnect all of the oscillators except one (if there was one oscillator that was worst, keep that.) Now see if changing the other oscillators still has an effect. If it does not, then the mixer is probably the culprit.

What people usually do is have an op-amp buffer (eg a non-inverting buffer) at the saw output of each VCO. That isolates it from the effects of parallel circuits.

On the "pull down resistor" I expect the correct term to look up is voltage divider: two resistor to reduce a voltage that is too large. (A pull down resistor may look the same, but the name us used for a dufferent effect: to detetme what the signal is for "open collector" circuits or open switch circuits.)
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floating-water



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PostPosted: Sun Nov 15, 2020 11:23 pm    Post subject: Reply with quote  Mark this post and the followings unread

i'm using 47k resistors on each output from the oscillators. im hoping to use a centre negative 9v supply for this thing just fyi but have only ever tested on a 9v battery for now. don't op amps need a bi-polar supply? what powering situation would you suggest?

also when the oscillators are at their fullest/highest they stop making sound but assuming the current is still being drawn

i'm using either 10k or 50k pots on the oscillator frequency tune
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PHOBoS



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PostPosted: Mon Nov 16, 2020 7:41 am    Post subject: Reply with quote  Mark this post and the followings unread

not necessarily a bi-polar supply but you will need a reference/bias voltage. With a bipolar supply you can use the GND for this
which can also be used by other ciruits to keep everything biased at the same voltage. Since you have a single supply you will
have to create a fake GND or reference voltage. if you only use it for the input(s) of an opamp the current draw is very low and
you can get away with just a voltage divider made with 2 resistors and a capacitor.

so something like this:
Posted Image, might have been reduced in size. Click Image to view fullscreen.

or this:
Posted Image, might have been reduced in size. Click Image to view fullscreen.

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floating-water



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PostPosted: Mon Nov 16, 2020 8:02 am    Post subject: Reply with quote  Mark this post and the followings unread

Right so thats 9v going into the vcc, ground going to ground, output of voltage divider going into the non inverting inputs...

(and thats all the oscillators going into one inverting input on one of the op amps? i thought I'd need one op amp for each oscillator)

...and then the output of the first op amp going into the second op amp.
whats that cap and 1m resistor at the end? is it necessary?


edit: if i read that correctly, whats the limit to how many oscillators I can add to one op amp?

further edit: regarding power supply implementation; if im making a supply for a centre negative psu does the diode have to face a different way? can I just copy a power section from a pedal?
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floating-water



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PostPosted: Mon Nov 16, 2020 8:23 am    Post subject: Reply with quote  Mark this post and the followings unread

ye that worked mate, amazing
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PHOBoS



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PostPosted: Mon Nov 16, 2020 9:21 am    Post subject: Reply with quote  Mark this post and the followings unread

floating-water wrote:
and thats all the oscillators going into one inverting input on one of the op amps? i thought I'd need one op amp for each oscillator

For summing the signals together yes, you only need one opamp to rule them all. However it can sometimes be useful to add a buffer before each input.
This adds a bit of extra isolation between inputs and can take care of some impedance problems when using different sources. For example for my lunetta
synth setup I had to deal with a bunch of different sources with different levels so I added an attenuator for each which I then buffered before going into
the actual mixer.
Posted Image, might have been reduced in size. Click Image to view fullscreen.
note that this one uses a bi-polar supply

Quote:
...and then the output of the first op amp going into the second op amp.

The function of the second opamp is to invert the signal as the first one is actually an inverting summing mixer. So by inverting it again you're back to how it was.
This is not always needed when dealing with audio signals as you can't hear the difference if a signal is inverted. But if for example you split a signal and you send
it through the mixer (maybe with some effects) and then combine it at the end with the original signal it would cause some phase problems and in the worst case it
will completely cancel it out.

Quote:
whats that cap and 1m resistor at the end? is it necessary?

That's for biasing as are the caps on the inputs (which you might not need). Normally a signal is biased around GND in other words it has a positive and negative
voltage (think of a sinewave which fluctuates above and below GND). Because you use a single supply you can't use a negative input voltage and with the AC
coupling capacitors on the inputs it will bias it to the voltage you set on the non-inverting input of the opamp. In this case with 2 equal sized resistor that will be at
1/2 the supply voltage. On the output you need to do the opposite and bring it back to GND which is what the capacitor on the output does. The 1M might not be
needed as there should be a path to GND on whatever input you connect it to but it can prevent some loud plops when you plug it in

Quote:
edit: if i read that correctly, whats the limit to how many oscillators I can add to one op amp?

In theory it's unlimited. In practise however it will get harder to control when you have a lot of input sources. The problem is that it is a summing mixer so it adds
the voltages together and it can only do this untill the output starts to clip against the supply voltage (or whatever the range of the opamp is). So for every source
you add you will have to reduce the level of the other ones. There might also be some other problems since an opamp isn't perfect and you will probably have to
increase some resistor values which will make it more susceptible to noise.

Quote:
further edit: regarding power supply implementation; if im making a supply for a centre negative psu does the diode have to face a different way? can I just copy a power section from a pedal?

only the connector itself will be wired the other way around compared to center positive.

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floating-water



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PostPosted: Tue Nov 17, 2020 1:48 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks so much for that info.

No doubt a stupid question but im on a time constraint, how do you recommend wiring the ground and power rails if you're hand-wiring it, also considering there is a starve section for oscillators. If the starve section just comes off the power rail, but last, will it affect the rest of the power?
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floating-water



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PostPosted: Thu Nov 19, 2020 11:51 am    Post subject: Reply with quote  Mark this post and the followings unread

The oscillators now seem to rise in pitch slowly, is there something I should be aware of with opamp summing?
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PostPosted: Thu Nov 19, 2020 4:51 pm    Post subject: Reply with quote  Mark this post and the followings unread

Most important thing for power distribution is low resistance. Use wires that are thick enough, though you don't need a lot of current so they
don't have to be massive. Also if you have different circuits it's better to wire them to the power rails seperately than daisy chaining them,
so not from one circuit to the other.

floating-water wrote:
The oscillators now seem to rise in pitch slowly, is there something I should be aware of with opamp summing?

That could have to do with temperature or maybe the voltage isn't stable. If they rise in pitch I would guess the voltage might be dropping.
The opamp should not be affecting the pitch in any way unless it is affecting the supply voltage.

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floating-water



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PostPosted: Fri Nov 20, 2020 2:24 am    Post subject: Reply with quote  Mark this post and the followings unread

OK that makes sense for the power distribution, least resistance and separation. What are those things called that you can screw in or layer multiple connection points? I can't find the right thing on ebay. I want to say power terminals but it doesnt come up with what I've seen before.

The pitch issue resolved with proper power supply that finally arrived
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PostPosted: Fri Nov 20, 2020 11:00 am    Post subject: Reply with quote  Mark this post and the followings unread

terminal blocks or screw terminals ?
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floating-water



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PostPosted: Yesterday, at 8:40 am    Post subject: Reply with quote  Mark this post and the followings unread

I think screw terminals, ordered. Another little thing if I were to use coins as conductive keys or switches whats the concept of that. I assume the coin needs to be tied to ground but would your finger also have to be touching ground when completing the circuit? Thanks
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PHOBoS



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PostPosted: Yesterday, at 2:47 pm    Post subject: Reply with quote  Mark this post and the followings unread

For resistive touch control you need to bridge 2 contacts with one side usually connected to GND it can also be made very sensitive where you provide
the GND connection yourself. You can also use capactive touch sensors which you don't actually have to touch so there is no electrical connection with
your body. You can get those very cheap these days and they work pretty well (search for ttp223 touch sensor).

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