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 Forum index » DIY Hardware and Software » Lunettas - circuits inspired by Stanley Lunetta
Lunetta Mixing
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analog_backlash



Joined: Sep 04, 2012
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PostPosted: Tue Oct 09, 2012 3:19 am    Post subject: Lunetta Mixing
Subject description: Op-Amp mixer questions
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Hello everyone,

Possibly some very dumb questions.

JingleJoe pointed out to me recently, that I was high-pass filtering my square waves. I then realised that the input to my mixer was the culprit. I've been doing a few experiments and I've come up with the circuit that I've shown below. It seems to work better than my original mixer, but I'm still not 100% happy about it. I've looked at other posts, but I haven't found an exact answer to the best way to mix square wave signals (although I've probably missed an obvious post - please tell me if I have). Mosc says here, that there is no problem in using an op-amp, but there is no detail:

http://electro-music.com/forum/topic-23909.html

I've marked 3 points on the circuit, questions are as follows:

(1) My original value of Cm, was 100nF which was hopeless. I've tried a 10uF and this seems better. I know that the high-pass filter -3dB cut-off frequency is given by:

1/(2 x pi x RC)

So, in my example, for 100nF this gives 15.9Hz and 10uF gives 0.159Hz (I think). Would the latter be OK? I've also seen mixers which use diode inputs instead of capacitors - is this a better idea?

(2) I've used a single 100uF capacitor for Co. I have seen other mixer circuits with two capacitors on the output in series, with the negative terminals connected to each other. Does this give better results?

(3) I've created a virtual ground for the non-inverting input of the op-amp, but I'm never quite sure whether the input mixing pots should be connected to this, or to 0V. This is more straightforward with a dual rail supply, as both would go to 0V, but Lunettas (obviously) have a single rail supply. I've tried both alternatives but I haven't been 100% happy with either. Any advice?

Of course, the other alternative is to use a totally passive mixer, but I'm not sure how well this would work.

Thanks for your help (in advance). I'm sorry if these questions seem stupid, but I struggle a bit with electronic design.

Gary


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attdestroyers



Joined: Mar 29, 2012
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PostPosted: Tue Oct 09, 2012 4:20 am    Post subject: Reply with quote  Mark this post and the followings unread

I'm not too sure about too much of the technical stuff either. I used this circuit...

http://cmoslove.blogspot.com/2011/11/build-mixer-for-your-lunetta.html

Works good. Thx Draal
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analog_backlash



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PostPosted: Tue Oct 09, 2012 5:04 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for that attdestroyers. I've been to that website before, but obviously didn't notice that circuit. That'll give me something to work from.

Gary
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richardc64



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PostPosted: Tue Oct 09, 2012 6:18 am    Post subject: Reply with quote  Mark this post and the followings unread

attdestroyers wrote:
I'm not too sure about too much of the technical stuff either. I used this circuit...

http://cmoslove.blogspot.com/2011/11/build-mixer-for-your-lunetta.html

Works good. Thx Draal


That circuit is dual-supply (+/-V) but should work single-supply if pins 3 & 5 are connected 1/2+V (VG) instead of Gnd. An LM358 can substitute for the low-noise TLO72.

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Psyingo



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PostPosted: Tue Oct 09, 2012 6:24 am    Post subject: Reply with quote  Mark this post and the followings unread

richardc64 wrote:
attdestroyers wrote:
I'm not too sure about too much of the technical stuff either. I used this circuit...

http://cmoslove.blogspot.com/2011/11/build-mixer-for-your-lunetta.html

Works good. Thx Draal


That circuit is dual-supply (+/-V) but should work single-supply if pins 3 & 5 are connected 1/2+V (VG) instead of Gnd. An LM358 can substitute for the low-noise TLO72.


it would be dual supply if pin 4 were connected to -V instead of GND, amirite?

... in its original state im not sure it would work.
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analog_backlash



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PostPosted: Tue Oct 09, 2012 7:35 am    Post subject: Possible mixer? Reply with quote  Mark this post and the followings unread

Yes, I've looked at that again and I've never seen a circuit wired like that before and I'm not sure it would work properly now. I would have wired it as richardc64 suggests, with pins 3 and 5 at +V/2 (I called it a virtual ground, but now I'm thinking that is something else). However, this brings back my question 3 again, i.e. should the pot be connected to +V/2 or 0V. Psyingo, you're also right that if pin 4 is connected to -V, it would work, but I want to avoid a dual-rail supply.

Thanks for the input,

Gary
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Psyingo



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PostPosted: Tue Oct 09, 2012 7:39 am    Post subject: Re: Possible mixer? Reply with quote  Mark this post and the followings unread

analog_backlash wrote:
should the pot be connected to +V/2 or 0V.


the pots should be connected to gnd. as i understand it, virtual ground is just a reference level for the opamp to use to 'swing' around.
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analog_backlash



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PostPosted: Tue Oct 09, 2012 7:43 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks again Psyingo, I'll try this out again and see how it goes.
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PHOBoS



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PostPosted: Tue Oct 09, 2012 8:01 am    Post subject: Reply with quote  Mark this post and the followings unread

I would indeed connect the pots to GND/0v, and add the capacitors after the pots, not before. (or maybe both but I don't think that's necessary)
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analog_backlash



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PostPosted: Tue Oct 09, 2012 8:43 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks PHOBoS, I'll try putting the capacitors after the pots and the pots to 0V (that's a combination that I haven't tried yet).

Gary
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richardc64



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PostPosted: Tue Oct 09, 2012 9:26 am    Post subject: Re: Possible mixer? Reply with quote  Mark this post and the followings unread

Yeah, the pot bottoms can be at 0volts -- actual ground.


analog_backlash wrote:
Yes, I've looked at that again and I've never seen a circuit wired like that before and I'm not sure it would work properly now. I would have wired it as richardc64 suggests, with pins 3 and 5 at +V/2 (I called it a virtual ground, but now I'm thinking that is something else). However, this brings back my question 3 again, i.e. should the pot be connected to +V/2 or 0V. Psyingo, you're also right that if pin 4 is connected to -V, it would work, but I want to avoid a dual-rail supply.

Thanks for the input,

Gary

It'll work, single- or dual-supply. And not to nit-pick, but the usual nomenclature for a pseudo-ground created by taking half of a single-supply voltage is Vref or Vr. "Virtual Ground" more specifically refers to the potential at the op amp's inputs. I know: it seems like the same thing, but it isn't.

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analog_backlash



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PostPosted: Tue Oct 09, 2012 12:42 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for that. Yes, I thought there was something wrong with calling +V/2 the virtual ground, so I looked it up. It's actually the connection (in the case of my circuit above) to the inverting input which is the virtual ground (if I've understood it). Pseudo-ground - I'll remember that!

Cheers,

Gary
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attdestroyers



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PostPosted: Tue Oct 09, 2012 6:54 pm    Post subject: Reply with quote  Mark this post and the followings unread

Ok, so why does this circuit work for me then? I've had no problems with it.

Also, if somebody would throw up a corrected schematic of how this circuit should look, I'd like to see it (all the +V/2, Vr, 1/2+V stuff has me confused a little). Also would be curious to build it and compare to the mixer I have.
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analog_backlash



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PostPosted: Wed Oct 10, 2012 2:55 am    Post subject: Reply with quote  Mark this post and the followings unread

Sorry to leave you out of the loop there.

The more usual way of making this type of circuit is to connect the 2 non-inverting pins of the op-amps (i.e. 3 & 5) to a potential which is half-way between + power and ground (Vref as richardc64 says). This is achieved by putting two equal value resistors, R1 and R2 in series between + power and ground. The voltage at the junction of the 2 resistors is given by:

Vref = R1/(R1 + R2) x Vp

where Vp is the power supply voltage (+12V on my circuit - Vp is my nomenclature). So, if we say that R1 = R2 = 10K and Vp = 12V, then:

Vref = 10/(10 + 10) x 12 = 6V

Since both resistors were in the K range, there is no need to put 10000 into the calculation, as it all cancels out.

Now, here's where my knowledge starts to falter, so I tend to do what I've seen on other circuits. For the 2 resistors, I've seen anything from 1K0 to 1M0, so I tend to use something like 10K for both. I do not know what the best values are for a given circuit. Secondly, I've added a capacitor from the R1-R2 junction to ground (I usually use a 10uF for this). Again, I am not totally sure of the explanation for this. I never used to add the capacitor and it seemed to work OK without it, but you very often see that a capacitor is shown in this position. I would be grateful to anyone who knows more about this, why the capacitor is added.

As for the original version of the circuit, it may work, but it didn't 'feel right' to me (and the others). I don't think that it would damage the TL072, but I was a bit cautious about trying it in its original form.

I will experiment with my amended version and see how it goes and then report back.

Gary


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richardc64



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PostPosted: Wed Oct 10, 2012 4:56 am    Post subject: Reply with quote  Mark this post and the followings unread

attdestroyers wrote:
Ok, so why does this circuit work for me then? I've had no problems with it.


Well, this is embarrassing. I don't know why I saw it as being dual-supply. I also don't know why it works, as you claim, without Vref. Maybe because as long as the inputs are rectangular on-offs it doesn't matter that the op amp isn't "centered"?

scratch

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RingMad



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PostPosted: Wed Oct 10, 2012 5:43 am    Post subject: Reply with quote  Mark this post and the followings unread

Earlier this year I built this circuit (the one from Draal's blog) with a 5532 (pin-compatible with the TL072) and it didn't work until I used the Vcc/2 Vref thing. I might have changed the cap and resistor values a bit. I've had and still have lots of problems with op-amps in general, so I figured it was just me and my lack of understanding.

James.
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analog_backlash



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PostPosted: Wed Oct 10, 2012 6:26 am    Post subject: Reply with quote  Mark this post and the followings unread

I thought that I knew a bit about op-amps, but the holes in my knowledge are now showing! I know that it seems more straightforward with a dual-rail supply. It's the Vref that gets me confused on single-rail. Also, I need to think more about the filtering action of my inputs - this was something that I haven't paid that much attention to until now. Thanks again JingleJoe for pointing that out.

Gary
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PHOBoS



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PostPosted: Wed Oct 10, 2012 6:32 am    Post subject: Reply with quote  Mark this post and the followings unread

I noticed that the circuit has a second opamp wired as a voltage follower attached to the virtual GND.
If this is just because it's a spare opamp then you could use the output of that opamp as your actual Virtual GND
(so connect the non-inverting input of the opamp you use to mix to the ouput of the other). For driving one opamp this
probably doesn't make much of a difference, but if your're gonna add some more stuff later on that also need a
virtual GND, it will be a more stable if you do it this way.

edit: just noticed that in the latest schematic it's used to reinvert the signal, which is also a good use Smile

analog_backlash: in the circuit you posted reverse the input caps (if polorized), because the side that's connected to the pot
will be grounded when you turn the level completely down. non-polorized would be better, but we're not talking
about a high-end mixer here so it will work fine.

richardc64 wrote:
..,I also don't know why it works, as you claim, without Vref. Maybe because as long as the inputs are rectangular on-offs it doesn't matter that the op amp isn't "centered"?

scratch

yeah it's indeed a bit mindboggling, maybe you're onto something with the rectangular waves. Or it just sounds ok,. but isn't when you would look at it on a scope Rolling Eyes

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PHOBoS



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PostPosted: Wed Oct 10, 2012 6:42 am    Post subject: Reply with quote  Mark this post and the followings unread

analog_backlash wrote:
I thought that I knew a bit about op-amps, but the holes in my knowledge are now showing! I know that it seems more straightforward with a dual-rail supply. It's the Vref that gets me confused on single-rail. Also, I need to think more about the filtering action of my inputs - this was something that I haven't paid that much attention to until now. Thanks again JingleJoe for pointing that out.

Gary

just view it as a dual supply, the only difference is that you have to create a 1/2v point which is otherwise just your
GND rail. the reason not to attach the pots to this, which you would otherwise do, has to do with stability. everything
you attach to the voltage divider will have an influence on it (a high impedance attached to it will not have
much of an effect that's why it's ok to use it for the opamp). the capacitors take care of the DC offset, which is why
they are placed after the pots.

at least that's my view on it, could be incorrect.

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analog_backlash



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PostPosted: Wed Oct 10, 2012 7:54 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for all of those comments PHOBoS, I think that it has made things much clearer. I'll now try to put it all into practice and see what the results are.

I'm still not absolutely sure how the other circuit could work without the potential divider - that's an interesting one...

Cheers,

Gary
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attdestroyers



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PostPosted: Wed Oct 10, 2012 5:54 pm    Post subject: Reply with quote  Mark this post and the followings unread

This is great stuff... Thank you all so much for the detailed explanations on this. Big time learning for me. Anyway, I plan on building this mixer again and seeing how it compares to the one I've got.

Thanks again.
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analog_backlash



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PostPosted: Sat Oct 20, 2012 7:15 am    Post subject: Reply with quote  Mark this post and the followings unread

I know that it was a long time ago, but I've only just got round to testing this (I've been building the final(?) version of my randomizer, etc.). I have breadboarded the amended circuit (as shown below) and it works beautifully (square waves in - square waves out). The reason I've gone back to my original LM358 design, is that it already exists on a PCB on my Super Stylophone. Rather than produce a brand new circuit, I'm going to do some mods which should solve earlier problems (but I might change the op-amp to a TL072/082).

Thanks for all your help guys.

Gary

P.S. I have also updated the TL082 mixer schematic to show PHOBoS' recommended changes.


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analog_backlash



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PostPosted: Tue Nov 06, 2012 3:46 am    Post subject: TI Document SLOA058 Reply with quote  Mark this post and the followings unread

I have throughout this thread, used two 10K resistors to produce the reference voltage (V+/2) to supply the non-inverting pins of op-amps. I have had some correspondence with RingMad about this and he suggested that these resistor values should be increased to at least 100K. After this, I found the following TI document:

http://www.eng.yale.edu/ee-labs/morse/compo/sloa058.pdf

This indeed confirms that this is the case. I have added this post, as the document gives a very good grounding in using op-amps on a single-rail supply and doesn't fly off into totally impenetrable mathematics. If you haven't already seen this, it's worth taking a look.

I have amended Mixer 2 above (now 2.1) to reflect this, but other diagrams still show the 10K resistors. If you try these out, it's best to use 100K resistors instead.

Gary
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jonasx26



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PostPosted: Fri Nov 16, 2012 10:45 am    Post subject: Reply with quote  Mark this post and the followings unread

Hey! Guess this is my first post. I mostly build guitar effects. But I enjoy fiddling with simple synth stuff as well.

Thought I'd chime in on the single supply, VCC/2 discussion.
What resistor values to use depend on what load the voltage reference sees.
If its high impedance, like direct connections to non-inverting op amp inputs (as in the posted mixer circuit) and/or high value biasing resistors to op amp inputs, a passive divider with reasonable resistor values (10k-100k maybe) will probably work.
R.G. Keen explains it better than me here: http://www.geofex.com/circuits/biasnet.htm

Easiest is to measure the virtual ground with a multimeter and see if it's stable while trying different values. Or even better, look at it with a oscilloscope while changing the load. It should remain steady at all times.

I always buffer the "virtual ground" with a op amp follower. That way you can use high resistor values (up to 1MEG will probably work) for reduced current draw. Which in turn means you can use a smaller capacitor to filter the VCC/2 point.
Most op amps will be able to source/sink enough current (something like 20mAish maybe?) to keep the VCC/2-ground stable.

Check out figure 2, page 4 in the design reference linked previously:
http://www.eng.yale.edu/ee-labs/morse/compo/sloa058.pdf
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analog_backlash



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PostPosted: Fri Nov 16, 2012 2:07 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for the extra information on that jonasx26. I think that I'll also review one of my single rail LFO designs, as it definitely was unstable around the VCC/2 point and at the time, I didn't know why. Looking back at it, I was using very low value resistors in the divider and certainly no op-amp follower, so that probably explains everything!

I am always open to advice like this.

Thanks again,

Gary
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