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 Forum index » DIY Hardware and Software » Thomas Henry designs
XR-2206 VCO problems
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elmegil



Joined: Mar 20, 2012
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PostPosted: Mon Feb 17, 2014 6:01 pm    Post subject: Reply with quote  Mark this post and the followings unread

A semi-common error that has bitten me and some friends is to wire two leads of a pot backwards, when it's supposed to be a voltage divider. It's a fast way to short two rails together as soon as you turn the pot to one end or the other. You might want to check that.
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John.R.Strohm



Joined: Feb 09, 2014
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Location: Huntsville, AL, USA

PostPosted: Thu Feb 20, 2014 8:56 pm    Post subject: Reply with quote  Mark this post and the followings unread

You said you were frying the TL074.

I looked at the schematic. The only thing that stood out to me was on page 2, lower right quadrant, the square wave output using IC3d, 1/4 of the TL074.

The intention seems to be to make a sort-of comparator, comparing the XR-2206 output on pin 11 to +7.5V. The XR2206 datasheet shows pin 11 to be an open collector output. It is pulled to +15V through R11 (4.7K), and grounded at the VCO rate.

I've never seen a circuit that closed the feedback loop around an op amp to the noninverting input, and that's what looks strange to me.

You might try breadboarding that part of the circuit, and see how it behaves by itself.
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jpmaus



Joined: Jan 16, 2014
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PostPosted: Thu Feb 20, 2014 11:06 pm    Post subject: Reply with quote  Mark this post and the followings unread

John.R.Strohm wrote:
You said you were frying the TL074.

I looked at the schematic. The only thing that stood out to me was on page 2, lower right quadrant, the square wave output using IC3d, 1/4 of the TL074.

The intention seems to be to make a sort-of comparator, comparing the XR-2206 output on pin 11 to +7.5V. The XR2206 datasheet shows pin 11 to be an open collector output. It is pulled to +15V through R11 (4.7K), and grounded at the VCO rate.

I've never seen a circuit that closed the feedback loop around an op amp to the noninverting input, and that's what looks strange to me.

You might try breadboarding that part of the circuit, and see how it behaves by itself.


Thanks allot John... I'm a newbie here, and so I don't completely understand the mechanism involved with the ICs. I'm just trying to "follow the instructions," without really understanding how anything actually works, "Chinese Room" style. If I was going to breadboard anything, what I'd really like is another circuit for a simple 1V/Oct XR2206 VCO. No bells. No whistles. So long as it has 1V/Oct pitch control over the XR2206's VCO. Anything like that laying around? Anyways, thanks again, and I'll read up some more on comparators, non-inverting inputs, etc.
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Thomas_Henry



Joined: Jul 24, 2009
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PostPosted: Thu Feb 20, 2014 11:13 pm    Post subject: Reply with quote  Mark this post and the followings unread

John.R.Strohm wrote:

I've never seen a circuit that closed the feedback loop around an op amp to the noninverting input, and that's what looks strange to me.


That arrangement is just a comparator with hysteresis, a standard affair from the Electronotes days. Very little current will flow.

I'd be looking for something related to the power rails, not the signal path.

Thomas Henry
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John.R.Strohm



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PostPosted: Fri Feb 21, 2014 1:49 pm    Post subject: Reply with quote  Mark this post and the followings unread

Really stupid question.

Is the PCB artwork on birthofasynth.com a correct picture of the foil side, or is it intended to be flipped?

Reason I ask: the foil side photo posted earlier appears to be a mirror image of the PCB artwork.

If the artwork is intended to be flipped in the board manufacturing, then the foil photo would be correct. If the artwork is what the foil side of the board is supposed to look like, then the boards the guy made are reversed, which would put the power rails on interesting pins of the ICs.

With all five (?) of the guy's boards blowing up, it seems like whatever is wrong has to be something wrong with the board, as opposed to the components or the assembly process.
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jpmaus



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PostPosted: Fri Feb 21, 2014 7:00 pm    Post subject: Reply with quote  Mark this post and the followings unread

John.R.Strohm wrote:
Really stupid question.

Is the PCB artwork on birthofasynth.com a correct picture of the foil side, or is it intended to be flipped?

Reason I ask: the foil side photo posted earlier appears to be a mirror image of the PCB artwork.

If the artwork is intended to be flipped in the board manufacturing, then the foil photo would be correct. If the artwork is what the foil side of the board is supposed to look like, then the boards the guy made are reversed, which would put the power rails on interesting pins of the ICs.

With all five (?) of the guy's boards blowing up, it seems like whatever is wrong has to be something wrong with the board, as opposed to the components or the assembly process.


This was the /first/ lesson I learned about etching PCBs... That is, I /did/ etch the first boards backwards in the way you mention here. In those cases, I tried soldering on the copper side (not fun), and I also tried what I suspect would be amusing to many of you, viz. turning the ICs inside-out (not easy, and I destroyed a few dollars worth of them doing it). In any case, I then etched some new ones correctly (I double checked the foil 'chirality' against the circuit diagram and everything appeared kosher to my newbie eyes). But still... I'm running into problems.

If it is the power rails, I have no idea how to check them. There appear to be no visible shorts, and so far as I know how to test for shorts with a multimeter, things seem to be fine as well. I was hoping someone with expert eyes could just look at the two photos I provided and /see/ what I'm doing wrong. Some particular joint I should triple check? Some misunderstood jumper? Some upside down IC or backward diode? Something else? I agree with you that it must be I am reading something wrong in the diagram. In any case, if any of you all help me sort this out, I promise I'll make a new post: "Building the XR-VCO: A Guide for Newbies," in which I will painstakingly document all the errors to avoid, which things go where and in which orientations, etc. etc.
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John.R.Strohm



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PostPosted: Fri Feb 21, 2014 11:07 pm    Post subject: Reply with quote  Mark this post and the followings unread

Here's a suggestion, that might or might not work.

Remove the ICs from the board. Apply power. Connect the common lead of your DVM (or the ground lead of your scope probe) to the ground pin of your split +/- 15V supply. Probe the IC power pins, on the socket or on the board, and verify that you have the correct voltages on the correct pins. You need to pay attention to the intended IC orientation here, since it is not marked on the board.

IC1 (XR2206) pin 4 should be +15V, pin 12 should be ground.

IC2 (LF442) pin 4 should be -15V, pin 8 should be +15V.

IC3 (TL074) pin 4 should be +15V, pin 11 should be -15V. [EDITED TO CORRECT VOLTAGES - I CAN'T READ SOME NIGHTS]

Beyond that, I'd have to have the board on the bench in front of me, where I could start scoping things.

Editorial comment: Industry practice is to mark pin 1 of the ICs on the foil, usually with a different-shaped pad, but occasionally with a small arrowhead marker (that doesn't connect to anything) to ensure that board chirality and IC orientation errors are immediately obvious.

Last edited by John.R.Strohm on Fri Feb 21, 2014 11:39 pm; edited 1 time in total
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jpmaus



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PostPosted: Fri Feb 21, 2014 11:24 pm    Post subject: Reply with quote  Mark this post and the followings unread

John.R.Strohm wrote:
Here's a suggestion, that might or might not work.

Remove the ICs from the board. Apply power. Connect the common lead of your DVM (or the ground lead of your scope probe) to the ground pin of your split +/- 15V supply. Probe the IC power pins, on the socket or on the board, and verify that you have the correct voltages on the correct pins. You need to pay attention to the intended IC orientation here, since it is not marked on the board.

IC1 (XR2206) pin 4 should be +15V, pin 12 should be ground.

IC2 (LF442) pin 4 should be -15V, pin 8 should be +15V.

IC3 (TL074) pin 4 should be -15V, pin 11 should be +15V.

Beyond that, I'd have to have the board on the bench in front of me, where I could start scoping things.

Editorial comment: Industry practice is to mark pin 1 of the ICs on the foil, usually with a different-shaped pad, but occasionally with a small arrowhead marker (that doesn't connect to anything) to ensure that board chirality and IC orientation errors are immediately obvious.


The circuit diagram /explicitly says/ (TL074) pin 11 -15V and pin 4 +15V. The datasheet looks like that's right: http://www.ti.com/lit/ds/symlink/tl074.pdf. And the multimeter readings I'm getting all check out... I'll have to play with it some more...
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John.R.Strohm



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PostPosted: Fri Feb 21, 2014 11:41 pm    Post subject: Reply with quote  Mark this post and the followings unread

jpmaus wrote:
The circuit diagram /explicitly says/ (TL074) pin 11 -15V and pin 4 +15V. The datasheet looks like that's right: http://www.ti.com/lit/ds/symlink/tl074.pdf. And the multimeter readings I'm getting all check out... I'll have to play with it some more...
You're right. I can't read some nights.

Mea culpa.

I've edited the post and corrected that line, and I'm going to bed before I do anything else that stupid.
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Thomas_Henry



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PostPosted: Fri Feb 21, 2014 11:51 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hello all,

I'm not sure what to make of all this.

First off, I was using the VCO just last night. Rest assured--it works. I'm the guy who designed the PCB artwork shown on Scott's Web site and also etched the board used in the unit I was playing. Unless I've been struck in the head with a blunt instrument, the schematic is correct and the PCB artwork is correct. But a number of other people have built and are using the unit, so perhaps they can weigh in with some comments.

As for the power rails, again, I don't know what to make of the comments above. The schematic is correct, and so is the PCB layout. As for the data sheet, it indeed shows pin 4 as +15V and pin 11 as -15V, exactly as shown in the schematic on Scott's Web site. There are no discrepancies between the data sheet and the XR circuit, but I can't account for John R. Strohm's comment; I believe he's got the power pins reversed in his comments.

So again, I really do believe the docs are okay, unless some other XR user can chime in otherwise.

Thomas Henry
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John.R.Strohm



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PostPosted: Sat Feb 22, 2014 12:03 am    Post subject: Reply with quote  Mark this post and the followings unread

Thomas, it is almost 1 AM here, I'm tired, and I misread the TL074 supply voltage pin callouts on the schematic. My fault.

I corrected the post when the error was brought to my attention.
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Thomas_Henry



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PostPosted: Mon Feb 24, 2014 3:12 pm    Post subject: Reply with quote  Mark this post and the followings unread

How about putting the multimeter in current mode (amperage) in series with the positive supply, all the chips removed. Keep it on a high scale until you see what's happening.

Now add the chips back in, one by one, leaving the XR chip for last. Then repeat with the negative rail.

I don't have the data in front of me, and it's too messy to rip open my large rackmount synth case to check from the studio, but the power supply currents ought to be somewhere around 50 mA, as a quick guesstimate.

Thomas Henry
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jpmaus



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PostPosted: Wed Feb 26, 2014 4:59 am    Post subject: Reply with quote  Mark this post and the followings unread

I just etched four of Bugband's XR-VCOs and I'm running into the same problems. As I said, I'm an absolute newbie, so the problem here is probably something /really stupid/. Someone suggested it might be the power rails, but I'm more and more convinced it has something to do with my power supply. I'm using one of these cheapos:

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

On my multimeter, red to red / black to black gives +15V, and black to red / red to black gives -15V. I thought that was all I needed, but then maybe I don't understand "split" power supplies? Now... I have /two/ of those cheapo power supplies, and so I followed this guy's video and tried powering it that way: +15V from one -15V from the other, using "common" as GND, not using common as GND. No luck. I also bread-boarded Thomas' own Power Supply design. But maybe the VCO was already broken by that point, although I tried replacing all the chips...

I was sorely mistaken when I thought this would be as easy as soldering together one of those "components and all" DIY kits available online. Either way, it is definitely me: TH's PCB, Bugband's PCB. Same problems. I just hope that whenever magic smoke gets around to sending me the four El Cerito's I ordered out of desperation here, the same thing doesn't happen!
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elmegil



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PostPosted: Wed Feb 26, 2014 6:11 am    Post subject: Reply with quote  Mark this post and the followings unread

I can't see what that supply is. Model number?

So the first supply you should connect red to the +rail, black to the ground rail. The second supply you should connect red to the ground rail and black to the -rail. I'm not following your description of common, so maybe that's already what you're doing....
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jpmaus



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PostPosted: Wed Feb 26, 2014 3:02 pm    Post subject: Reply with quote  Mark this post and the followings unread

elmegil wrote:
I can't see what that supply is. Model number?

So the first supply you should connect red to the +rail, black to the ground rail. The second supply you should connect red to the ground rail and black to the -rail. I'm not following your description of common, so maybe that's already what you're doing....


Thanks Elmegil. I followed your instructions, and everything seems as it should be in relation to the load, IC temps, etc. But still no sound. I tried switching out all the ICs, so maybe a diode or a capacitor or the 2N3904 are fried from all my foolishness.

Let me ask you something, could you maybe point me towards a simple XR-2206 circuit that enables 1V/OCT? After two months of soldering and etching, I just want to play with some sounds. The whole reason I spent so much time on the XR-VCO is because I breadboarded an XR-2206 test circuit once and it just sounded awesome. But I couldn't figure out how to enable 1V/Oct, and so I've been trying to build the XR-VCO ever since... I've etched and soldered four of TH's PCB now, and one of bugband's... But I give up. Maybe better to start simple... Wait the El Cerito boards... Or something...
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elmegil



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PostPosted: Wed Feb 26, 2014 5:06 pm    Post subject: Reply with quote  Mark this post and the followings unread

Expo converters aren't terribly complicated with regard to parts, but they're not the easiest thing to understand or explain. There are some folk around here who could do a good job, so I'll pass. I wish Aaron Lanternman's lectures were still up on BlipTV, he does a good job and I could link to them, but sadly they've been down for some months now.

The main thing is that the expo converter has to be set up so that it precisely drive the Oscillator it's connected to. So if I were working with an XR-2206, and wasn't able to derive my own values, I'd use Thomas' expo converter.

Why not build Thomas' circuit on your breadboard at least to get a handle on things?
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jpmaus



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PostPosted: Wed Feb 26, 2014 5:21 pm    Post subject: Reply with quote  Mark this post and the followings unread

elmegil wrote:
Expo converters aren't terribly complicated with regard to parts, but they're not the easiest thing to understand or explain. There are some folk around here who could do a good job, so I'll pass. I wish Aaron Lanternman's lectures were still up on BlipTV, he does a good job and I could link to them, but sadly they've been down for some months now.

The main thing is that the expo converter has to be set up so that it precisely drive the Oscillator it's connected to. So if I were working with an XR-2206, and wasn't able to derive my own values, I'd use Thomas' expo converter.

Why not build Thomas' circuit on your breadboard at least to get a handle on things?


Good call. Thanks again. I should've breadboarded the circuit before I did anything else, I just figured using the PCB would save me the mess of flimsy jumper wires and the rest. But I'll go ahead and breadboard the circuit next.
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jpmaus



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PostPosted: Sun Mar 09, 2014 3:58 am    Post subject: Reply with quote  Mark this post and the followings unread

elmegil wrote:
I can't see what that supply is. Model number?

So the first supply you should connect red to the +rail, black to the ground rail. The second supply you should connect red to the ground rail and black to the -rail. I'm not following your description of common, so maybe that's already what you're doing....


I ruined a handful of ICs because I did not understand that +15V and -15V on a circuit diagram are not the same as the "positive" and "negative" terminals of a 15V DC Power Supply. In other words, I did not understand the concept of a "split" power supply. I admit it here for any lurkers who might be making the same (embarrassing) newbie mistake I did. Thanks for the help with this Elmegil!

The good news is, I now have four of Bugbrand's PCBs soldered and oscillating. (I wish I would have figured this out with the Birth-of-a-Synth PCBs I etched, because, IMHO, they just look way cooler!).

The bad news is, the VCOs are unstable. There is no RF noise, but the frequency is all wobbly: it suddenly/gradually wobbles up/down.

I am assuming that as with my earlier problem, this is being caused by something really basic, so I hope someone can help me out (I've checked the threads, and while there is discussion of RF noise, I can't find anyone discussing this particular problem).

FYI, the components/values are all exactly as specified. The pot wiring is kosher. Another weird thing I noticed is that when I place a square of that foamy stuff ICs are often packaged/shipped in underneath the PCB, and then sort of press on it, it makes the VCO go looney. One of them won't even oscillate w/o that stuff underneath. The same isn't the case with cardboard, plastic, or anything else. I don't know if that means anything... Just trying to throw it all out there. Thanks.
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roglok



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PostPosted: Sun Mar 09, 2014 4:15 am    Post subject: Reply with quote  Mark this post and the followings unread

jpmaus wrote:
Another weird thing I noticed is that when I place a square of that foamy stuff ICs are often packaged/shipped in underneath the PCB, and then sort of press on it, it makes the VCO go looney. One of them won't even oscillate w/o that stuff underneath. The same isn't the case with cardboard, plastic, or anything else. I don't know if that means anything... Just trying to throw it all out there. Thanks.


that foamy stuff is conductive! no wonder it does weird stuff to your circuit... are you sure your power connections are correct now? did you also connect ground, or just +V and -V?
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elmegil



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PostPosted: Sun Mar 09, 2014 7:08 am    Post subject: Reply with quote  Mark this post and the followings unread

jpmaus wrote:
elmegil wrote:
I can't see what that supply is. Model number?

So the first supply you should connect red to the +rail, black to the ground rail. The second supply you should connect red to the ground rail and black to the -rail. I'm not following your description of common, so maybe that's already what you're doing....


I ruined a handful of ICs because I did not understand that +15V and -15V on a circuit diagram are not the same as the "positive" and "negative" terminals of a 15V DC Power Supply. In other words, I did not understand the concept of a "split" power supply. I admit it here for any lurkers who might be making the same (embarrassing) newbie mistake I did. Thanks for the help with this Elmegil!


You're quite welcome.


As roglok pointed out, one thing that can cause the behavior your describe is bad grounds. If you don't have them connected, that's bad, but even if you do, you could have cold solder joints that will cause you all manner of grief. This happens to everyone too--I have a build I really have to redo entirely because I can't find the bad solder joints without ripping it apart and starting over.

Some indications of bad joints: look at the solder closely, if it's "dull" looking rather than shiny, or if there are cracks, or if you look at the top of a wire connection and the insulation looks like it goes INTO the joint (melted right up against it), you've likely got a cold solder there, and need to redo it. If there's no insulation or other foreign matter there, you ought to be able to reheat the joint and get it to be OK. If there's insulation you have to pull it apart and start over. At least that's my experience.

BTW I mention cold solders in context of ground, that's a common place for problems (because it's harder with some irons to heat a ground connection if there's a big ground plane to dissipate the heat) but by no means is it the only place to look for these kinds of problems.
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jpmaus



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PostPosted: Mon Mar 10, 2014 8:13 am    Post subject: Reply with quote  Mark this post and the followings unread

elmegil wrote:


You're quite welcome.


As roglok pointed out, one thing that can cause the behavior your describe is bad grounds. If you don't have them connected, that's bad, but even if you do, you could have cold solder joints that will cause you all manner of grief. This happens to everyone too--I have a build I really have to redo entirely because I can't find the bad solder joints without ripping it apart and starting over.

Some indications of bad joints: look at the solder closely, if it's "dull" looking rather than shiny, or if there are cracks, or if you look at the top of a wire connection and the insulation looks like it goes INTO the joint (melted right up against it), you've likely got a cold solder there, and need to redo it. If there's no insulation or other foreign matter there, you ought to be able to reheat the joint and get it to be OK. If there's insulation you have to pull it apart and start over. At least that's my experience.


Thanks again, Elmegil. I resoldered any suspicious joints (which, I wouldn't have even tried w/o your suggestion, because I figured if the pin was fused to the solder then it must be making a good connection), and that seems to have taken care of the problem.

A word to any lurkers with a wobbly or squirrely XR-VCO, (keywords), for an XR-VCO that does not have a stable frequency, an XR-VCO that is unsteady, that rises and falls, etc., etc. double checking and resoldering any joints that look even slightly suspicious will likely take care your problem.

Next step, calibrating my 1V/Oct CV, wish me luck! For both R1 and R42, the 100R and 100M trimpots I used were not multiturn, I hope that single-turn trimpots will be adequate to the task. Thanks again.
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elmegil



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PostPosted: Mon Mar 10, 2014 8:46 pm    Post subject: Reply with quote  Mark this post and the followings unread

Glad that it worked. This is actually true for just about any VCO.... I can't imagine there's anything special about the XR-2206 that makes it more susceptible....
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ungleichklang



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PostPosted: Tue Jul 29, 2014 3:39 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi all .. I have the board from Magic Smoke an run into the same problem ... all noise ...

so it seems to be the XR-2206, right?? ..
Can someone point me to some good sellers in europe??

I also used some matched BC547s instead of the LM394 (like Yusynth does for his VCO) ..
are there any distributors in europe that provide those LM394 for an affordable price too??

Thanks in advance

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elmegil



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PostPosted: Tue Jul 29, 2014 9:04 pm    Post subject: Reply with quote  Mark this post and the followings unread

Matched BC547's should not cause noise problems, I don't see any reason to suspect them in your issue. Save your money on the 394's....
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jpmaus



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PostPosted: Wed Jul 30, 2014 4:34 am    Post subject: Reply with quote  Mark this post and the followings unread

Just an update on my end, and a reminder that with the XR-VCO it isn't /always/ about problems. Here's a photo of my finished quad XR-VCO (above my finished quad SN-Voice):

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

And here is a little sound sample, each of the three voices is one of the XR-VCO's waveshapes (i.e., sine, triangle, square). It is not the best recording, and certainly not a very good demonstration of what's possible with all the parameters, but hopefully it is inspiration to keep going.

The XR chips are pretty fragile, and once they're fried they're fried. I went through a couple of them before I had everything safely working. You'll probably want to make sure you have a few extras if you're going to be building one of these things. Also, I found it handy to have a little breadboard test circuit (from the XR datasheet) ready in order to make sure the chips were still good as I was troubleshooting.


XR-VCO.mp3
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 Filename:  XR-VCO.mp3
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