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Buffering cv and gate.
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isak



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PostPosted: Fri Dec 28, 2012 5:43 am    Post subject: Buffering cv and gate. Reply with quote  Mark this post and the followings unread

Hi guys.

I'm bulding a synth and now I'm on the midi to cv gate part.
I was wandering if I can send the output cv 1V/oct to 3 vco's and the gate to 3 env's.
Then i understand that if I want to do that i will need to buffer the signals
Of the cv and gate.
Can you advice me on how to do that please?
Please make it as simple as it can be.

Thank you in advance,
Isak E.

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elmegil



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PostPosted: Fri Dec 28, 2012 8:11 am    Post subject: Reply with quote  Mark this post and the followings unread

When I asked about this, many people said that if output and input impedances match the usual expectations, you can just use passive multiples: a set of jacks wired together. (chain the tips, chain the grounds).

If for some reason you suspect you need active multiples, I have used Ray Wilson's CV / Gate buffer circuits:

http://www.musicfromouterspace.com/forums.html?MAINTAB=SYNTHDIY&PROJARG=CVANDGATEDIST/CVANDGATEDIST.html&VPW=1346&VPH=770

I just used a couple instances on my own protoboard, but of course you could order his swiss op-amp knife boards like he describes in the article there.

Not sure this qualifies as the most simple description, although if you use passives it should be easy to set up....
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LFLab



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PostPosted: Fri Dec 28, 2012 8:48 am    Post subject: Reply with quote  Mark this post and the followings unread

I think there's a layout for a quad buffer here somewhere.
The need is depending on the equipment used, my step64 sequencer locks up if the cv is sent to more than one vco, so i'm thinking of building a octal buffer, for both gates and cv.
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JovianPyx



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PostPosted: Fri Dec 28, 2012 8:50 am    Post subject: Reply with quote  Mark this post and the followings unread

Impedance matching is almost a nebulous topic. With audio signals, poor impedance matching will cause at least an attenutation of the signal. That's not horrible, especially if the imedance is flat across the audio spectrum. Usually, what I have read and practiced is at least a 10 to 1 ratio in impedance. That is, the driver should be at least 10 times less impedance than the receiver. The reason for this is current. When the imepdance of the driver is higher than optimal, the current available to the receiver is not optimal. Likewise is true when the receivers impedances are lower than optimal. (remember that parallel resistors reduce resistance). Usually, in an audio circuit, there are capacitors in the audio path. Those capacitors together with the output impedance of the driver create highpass filters. This is causes a loss in bass response.

But that's audio. And the results won't be castrophic if the impedance match is suboptimal, you'll still be able to make music, just maybe not with the punch and drive that you'd like.

With CV, especially pitch CV, there is a different problem - accuracy. When the mismatch is suboptimal, the result is detuning which will show up as pitches that are flat. While you can always crank up the pitch knob to correct this, consider this: You have what is a great patch, everything is in tune and it sounds wonderful - except - you decide you want to add another oscillator or you want to drive a filter's Fc input with pitch CV. The moment you plug in that addtional load, you are lowering the available current to all of the devices attached to pitch CV and the patch might need to be tuned again. If you are doing a gig and try this live, it might degrade your performance. This is the kind of thing that can happen with suboptimal impedance matching.

Receiver impedance should be as high as possible. Thus, input systems using low impedance input opamps should be avoided. LM324 and 741 are examples of "not the best" opamps. TL07x opamps are FET input types and have good high input impedance.

This post does not disagree at all with elmegil, rather it expands on the idea of impedance matching and why one needs to be mindful of it. The general rule would be that it doesn't hurt to buffer. Too much current capacity is far better than too little. Too little can cause problems, one of which I described. Higher current capacity won't, but it will cost a bit more.

Of course, if the synth system will remain small intentionally, fewer active multiples are required, perhaps even none. This is a design decision that you should make considering how your synthesizer might grow in the future. If you intend to plug things into your synth that are not of your design, you may not know much about the impedances of those loads - here again, more current is always better than less.

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bubzy



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PostPosted: Fri Dec 28, 2012 9:26 am    Post subject: Reply with quote  Mark this post and the followings unread

http://electro-music.com/forum/topic-51273.html

i designed this sometime ago but never built one.

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Cynosure
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PostPosted: Fri Dec 28, 2012 12:08 pm    Post subject: Reply with quote  Mark this post and the followings unread

JpvianPyx - You mentioned that the LM324 is not good to use as a buffer, but is that only for audio or for CV too? I thought that the TL07x was not good for CV because it does not output a high enough voltage when powered from +/-5V. Would you have to power the TL07x from a higher voltage in order to make it useful as a CV buffer?
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richardc64



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PostPosted: Fri Dec 28, 2012 12:47 pm    Post subject: Reply with quote  Mark this post and the followings unread

Cheese and Crackers!!!

Any decent op amp with Gain=1 (a non-inverting buffer,) is going to have an output impedance less than 10ohms -- usually less than 1ohm! You could connect 10 or 20 100k inputs (usual "standard" for 1V/oct,) to it and not hear any affect on accuracy.

There's no need for a separate buffer for every 1v/oct input.


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Blue Hell
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PostPosted: Fri Dec 28, 2012 1:04 pm    Post subject: Reply with quote  Mark this post and the followings unread

richardc64 wrote:
Any decent op amp with Gain=1 (a non-inverting buffer,) is going to have an output impedance less than 10ohms[...]


Except when there is that 1k "safety" resistor; when it is not in the feedback loop at least. Most opamps are short circuit safe, so just leave it out, or at least apply the feedback from after it.

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JovianPyx



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PostPosted: Fri Dec 28, 2012 3:26 pm    Post subject: Reply with quote  Mark this post and the followings unread

With regard to LM324 (and 741) I was referring to input impedance of the opamp really. Of course, yes, pretty much any opamp has nice low output impedance.

But there are VCO schematics out there that may use 741 or LM324 or other bipolar input opamps or even LM3900. These are not the best to use for the CV input of a VCO.

The main point I was trying to (long-windedly) make was that it never hurts to buffer stuff. More current drive capacity is a good thing.

As to the TL0x opamps, they are not rail to rail opamps, so it is best that there be PSU headroom. Many modulars use a +/- 10 volt CV range and the opamps are normally powered by at least +/- 12 volts and more commonly +/- 15 volts. It's probably not a great idea to use +/- 5 volt systems unless you are doing special crazy stuff with CMOS.

EDIT ADD: Also, LM324 is a nice opamp because it is rail to rail. Sometimes we are stuck using parts like LM324 because there really isn't anything better. If you need rail to rail, LM324 is a good choice, but it's input impedance is lower than TL0x opamps. In these cases, we just have to make allowances for such and live with it. For example, if the LM324 input impedance is low enough to cause a problem when "too many" LM324 inputs need to be driven - then split the load across more than one buffer.

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elmegil



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PostPosted: Fri Dec 28, 2012 5:54 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for sharing your experience, Scott. I don't know if anyone else might have misinterpreted your earlier post, but I got that you weren't contradicting me before you explicitly said so Very Happy
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PostPosted: Fri Dec 28, 2012 8:21 pm    Post subject: Reply with quote  Mark this post and the followings unread

JovianPyx wrote:
It's probably not a great idea to use +/- 5 volt systems unless you are doing special crazy stuff with CMOS.

Yes, that is me - special and crazy.

Lots of CMOS and now I am also using Atmega328.

I guess I need to start making my circuits powered from both 12V and 5V.
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PostPosted: Fri Dec 28, 2012 8:43 pm    Post subject: Reply with quote  Mark this post and the followings unread

Cynosure wrote:
I guess I need to start making my circuits powered from both 12V and 5V.


For a while maybe Laughing

It is a pondering thing always, about "what if"s, there are no bad answers really, as long as they fit the question Cool

Anyway, by all means, try stuff, find limitations, don't be dogmatic, design Exclamation

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JovianPyx



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PostPosted: Fri Dec 28, 2012 9:41 pm    Post subject: Reply with quote  Mark this post and the followings unread

Blue Hell wrote:
don't be dogmatic


I gotta wonder though, is it OK to be catmatic? Rolling Eyes

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isak



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PostPosted: Sat Dec 29, 2012 12:10 am    Post subject: Reply with quote  Mark this post and the followings unread

Thank you guys.
I found some cool and easy scheme here..
http://www.musicfromouterspace.com/forums.html?MAINTAB=SYNTHDIY&PROJARG=OLDIESBUTGOODIES/VOLTAGEMIXER/voltagedist2.html&CATPARTNO=

After reading what JovianPyx wrote about the opamps I really don't know if using the TL084 is good for this.
found this old post, he used with LF412 for this matter, also they said that the most reccomeneded chip for this matter is TL1013

http://www.electro-music.com/forum/topic-33538.html

EDIT:
Sorry, I meant to say LT1013.

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Last edited by isak on Sat Dec 29, 2012 2:58 am; edited 2 times in total
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diablojoy



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PostPosted: Sat Dec 29, 2012 1:39 am    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
After reading what JovianPyx wrote about the opamps I really don't know if using the TL084 is good for this.
found this old post, he used with LF412 for this matter, also they said that the most reccomeneded chip for this matter is TL1013


yes LF412 or LF442 is a good choice for the lower offset spec's and cost
not sure on TL1013 but i imagine so for the same reason but dearer.
Best i have found is LT1057 but they will cost you $5.00 each so way too pricey for this really.

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PostPosted: Sat Dec 29, 2012 4:07 am    Post subject: Reply with quote  Mark this post and the followings unread

isak wrote:
Thank you guys.
I found some cool and easy scheme here..
http://www.musicfromouterspace.com/forums.html?MAINTAB=SYNTHDIY&PROJARG=OLDIESBUTGOODIES/VOLTAGEMIXER/voltagedist2.html&CATPARTNO=


You don't want to use that to distribute 1v/oct CV because the 100ohm Rs on the outputs will add tracking errors. Besides, it's unnecessary and a waste of hardware.

For the 1v/oct inputs of 3 VCOs, as specified in your original post, all you'll need is ONE non-inverting buffer, with NO output resistor. Even if you eventually add a VCF -- or even a VCF for each VCO, you'll STILL need only ONE buffer.

Also specified in your original post:
Quote:
Please make it as simple as it can be.

That's what I'm trying to do.

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JovianPyx



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PostPosted: Sat Dec 29, 2012 6:12 am    Post subject: Reply with quote  Mark this post and the followings unread

This is a fascinating topic and I will say that I do not know all of the answers. It has been discussed at length as well on the email Synth-DIY forum and what I've taken from those threads it that there is no absolute best way to do this (under all conditions). I will merely attempt to outline some of what I've read there and hopefully I get it right - so please feel free to correct me where I've erred.

The 100 ohm resistors are what I would jokingly call "paranoia parts". They are included because the connection is to an output jack where damned near anything would, could and has been plugged in, both accidentally and intentionally. The resistor attempts to limit the current to a level that allows the opamp to survive, including momentary shorts and the practice of plugging outputs into outputs. As far as I know, many or most modern opamps have some built in output short circuit protection, so the 100 ohm resistor's usefulness is somewhat debatable. We cannot, however, eliminate the possibility of connections to (for example) something that is completely bizzare, like 50 volts from some DIY vacuum tube thing - which may or may not be well designed or well built or functioning normally.

One can correct for the tracking problems that Richard points out, by placing the 100 ohm resistor inside the opamp feedback loop. This protects the output and forces the output to properly copy the input. It does however, reduce the output drive of the opamp.

The attenuator could then be moved to output of that circuit. However, I've also read that an acceptable or even best practice is to not put attentuators on outputs at all, rather that they should be on module inputs. This makes sense to me because it preserves the highest voltage level possible into patch cables which helps to keep the signal to noise ratio higher.

So again, very much really depends on exactly what the intended use would be. If the circuit is to be part of a self contained and unpatchable synth like a PAiA FatMan, then I would not include the 100 ohm paranoia resistors at all. On the other hand, if there is no way to tell what will be plugged in (let's say that we are selling a module to someone else) then the current limiting resistos might be a good idea. We just need to remember what is in there so that we don't expect more from the design than is electrically possible.

This is, as Blue Hell points out, part of the process of design and part of the brain burning fun of it (hee hee). Being dogmatic (or catmatic as the case may be) won't help much because there really isn't a one-size-fits-all solution. We have to consider the conditions under which our musical instrument will be used and whether it will be used by only ourself or others. I know for a fact that there are things that I would never do that others might think is perfectly normal - or artistic.

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oculus



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PostPosted: Wed Jan 02, 2013 2:52 am    Post subject: Reply with quote  Mark this post and the followings unread

i built the MFOS CV and gate buffer. it seems to be working very well.
and its really usefull to have the Voltage offset coarse tune and the fine tune
to change the pitch of all oscillators at once, instead of having to tune them all separately.
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isak



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PostPosted: Wed Jan 02, 2013 4:15 am    Post subject: Reply with quote  Mark this post and the followings unread

That's cool to know.
Did you stripboard it or used the Swiss board he sells?

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oculus



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PostPosted: Wed Jan 02, 2013 6:44 pm    Post subject: Reply with quote  Mark this post and the followings unread

i stripboarded it, like everything i´ve made. too excited to try it out right away ,insted of having to wait for the pcb to ship to iceland hehe. i left out the portamento , and made 5 outputs of gate and 5 outputs of cv.

it´s easy to stripboard it , id say go for it.
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Osal



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PostPosted: Fri Jan 04, 2013 10:22 am    Post subject: Reply with quote  Mark this post and the followings unread

JovianPyx wrote:
As far as I know, many or most modern opamps have some built in output short circuit protection, so the 100 ohm resistor's usefulness is somewhat debatable.

Hello JovianPyx,
I do agree that the pitch CV output must be impedance=0.
However, in a patchable system, the non-inverting op-amp needs a limiting resistor in its output and the negative feedback must be tapped after this resistor in order to achieve impedance=0.
It is true that modern op-amps are short protected, however if we read the small letter we'll see that it has some restrictions.
Let's see the TL072 datasheet.
In the page 6 of the data-sheet says:
Quote:
Duration of output short circuit (see Note 4). . . . . . . Unlimited

In the same page note 4 says:
Quote:
4. The output may be shorted to ground or to either supply. Temperature and/or supply voltages must be limited to ensure that the
dissipation rating is not exceeded.

In the same page also says:
Quote:
Operating virtual junction temperature, TJ . . . . . . 150°C

And:
Quote:
Package thermal impedance, θJA (see Notes 5 and 6): D package (8 pin) . . . . . . 97°C/W

Notes 5 and 6:
Quote:
5. Maximum power dissipation is a function of TJ(max), θJA, and TA. The maximum allowable power dissipation at any allowable
ambient temperature is PD = (TJ(max) − TA)/θJA. Operating at the absolute maximum TJ of 150°C can affect reliability.
6. The package thermal impedance is calculated in accordance with JESD 51-7.

OK, now we can check if the dissipation rating is exceeded in a worst case situation with outputs without limiting resistor:
The worst situation that I suggest to consider is two outputs connected by error, and that one is +13V and the other is -13V for a typical +/-15V power supply.
This is 26V of potential difference.

In page 5 of the datasheet there is a schematic of the TL071. See that there is a 128Ω resistor in series with the output, and maybe the 64Ω resistor sums in series limiting the current when the output is shorted. Resistance values are nominal, so it can vary from one op-amp to another and also depending on the die temperature.
Let's say for this example that Rout=150Ω.

If we connect the two outputs, we have a resistance of 300Ω and a potential difference of 26V, thus Power to dissipate is:
P=V^2/R
P=26^2/300
P=2.25W

We know that the (junction to ambient) thermal resistance is 97°C/W and that the power is 2.25W, so for a 40C of ambient temperature, Junction temperature is:
Tj=Ta+P*Θj-a
Tj=40+2.25*97
Tj=258.25 C

This is much greater than the TL072's junction temperature maximum rating. Which is 150 C.
If we calculate it including the 100Ω limiting resistors, the total resistance would be 500Ω and voltage 26V. It is still insufficient.
I do suggest to use the standard 1K resistors in the output, with the feedback tapped after the resistor. In this way the op-amp is also safe in worst case situations like short the outputs directly to the power rails.
Although that is true that to drive lines is better op-amps that can deliver more current, I don't think that it has importance in our patchable systems , considering that the standard 1K outputs are working successfully until now. I guess.


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isak



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PostPosted: Mon Jan 14, 2013 1:24 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi guys and thank you for all the replays!

ill be doing the "Lite" version midi2cv from the next site.
http://acxsynth.com/midi2cvlite/midi2cvlitefr.htm

this is what I want to do to distibrut the cv, in addition i want to add portamento to every output, i took Ray's CV distributor and mod it to have portamento.
this is my new scheme..
Posted Image, might have been reduced in size. Click Image to view fullscreen.

i took the idea from here..(take a look at the bottom from the right, cv outputs 4 and 5)
Posted Image, might have been reduced in size. Click Image to view fullscreen.

what do you think guys, will it work or i jump over my head with this crazy idea?

cheers,
Isak E.

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elmegil



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PostPosted: Mon Jan 14, 2013 6:15 am    Post subject: Reply with quote  Mark this post and the followings unread

You'll want a buffer op amp stage before that, as Ray does in his schematic.
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isak



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PostPosted: Mon Jan 14, 2013 6:23 am    Post subject: Reply with quote  Mark this post and the followings unread

You think I will need another stage buffer for each channel or only one at the beggining will do?
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elmegil



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PostPosted: Mon Jan 14, 2013 6:27 am    Post subject: Reply with quote  Mark this post and the followings unread

I think it somewhat depends on what op amp you use. And I can't really give you much better guidance than that :/

In my variation on Ray's circuit I drive no more than 2 output stages from a given buffer, but that is probably on the conservative side.
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