Thomas Henry XR-2206 VCO Page is Up

[bugbrand]

[para]

thanks tom i'll give it a shot


steven

[numbernone]

Going to be finalizing my XR's this weekend (FINALLY), my own squashed version of the Bugbrand layout. A couple questions about the LED for those that have implemented it. Did the CGS style driver work? I have a bunch of those PCBs from Ken that he insulated a ring mod with for shipping (!!!). Also, where was it tapped from?? Straight from the triangle output?? Thanks to all.

[para]

"Did the CGS style driver work?" yes very well

"Straight from the triangle output?? " exactly


steven

[bugbrand]

Or if you can lay your hands on BiColour LEDs you could try the little circuit I mentioned on the Mankato thread - I'm sure that'd work and give you bipolar light - the LED drive mentioned above only indicates the positive voltages..

[numbernone]

Finished the first of 3 XRs late last night, only leaving enough time before bed to listen to its insanely strange behavior, debugging starts tonite. I am hoping maybe somebody could give me some things to think about while I sit about at work. Here are some symptoms:

Tune pot: The last 1/4 turn counterclockwise send the pitch BACK UP!?!? I have wired pots backwards before but never like this.
Fine pot: Seems fine just like its name
FM inputs: I have attenuators (100k) attached, with the pots fully CCW, the osc tone goes strait to a high pitch, unaffected by the tune or fine pots. Rotating them CW changes the pitch considerably and allows for the regular pots to adjust it. Note this is with no incoming CV.
Shape: Too early to tell whats going on here
Shape switch: Hard to tell as I have not trimmed the sine at all.
Skew: Appears to work ALMOST. definitely warping the wave, and increasing pitch. BUT fully CCW is like a volume control taking it out completely.
Sine Tri volume is lower, and I will look into the 20k to 22k change.

Please forgive the long post, and any implied idiocy!! My only test gear is a good DMM and my ears, but I am looking forward to this as pretty much everything I have built so far has worked properly from power up. I double triple check each component as it goes onto the board, have checked the PCB for flaws, and determined that my board conforms to the Bugbrand layout mulitple times, including the diode fix.

Chips are not getting noticeably hot, and all are properly seated.

I guess that is all for now, it really kills me trudging thru the 8 hour american workday while my babies sit on the bench crying for attention!!

Oh yeah, my PSU is trimmed to +/- 15.00v exactly and behaving beautifully.

[bugbrand]

I'd have a try at trimming first -- darn, that may be tricky without an o'scope -> there's the offset control that's important for the sin/tri.. Sin shape shouldn't be too important to start with (..I reckon?!)

You could always check that the voltages off the pots are acting correctly - eg, check the tune pot is going between - and + 15v volts before its summing resistor.

That is strange behaviour hohum.. especially the mod CV inputs.. You could always try disconnecting these? At least checking voltages on them?

Good luck!

[numbernone]

Ok the mod inputs are sorted...look to hard and you can miss the obvious sometimes! Not sure if I have the switch wired correctly, there is definitely a difference to my ears but not all that severe. Mostly down to the trimming now. It has become quite apparent that I need scope from this point forward. Probably finish the other 2 tonite and start investigating the scope market.

Big thanks to Tom for his excellent layout and assistance!!

[Fernando]

you can find cheap scopes on ebay

two channels, 20 MHz is great. Z input useful too.

you will enjoy it when playing too. Very often monitoring waveforms is needed (to achieve the right control shape, see audio waveforms, filter behaviour, etc)

[bugbrand]

Yeah, absolutely - its so useful to see what's going on for waveform trimming --- especially in this case with the sine shaping / switch -> if its not set right then you'll not hear so much difference from tri!

[numbernone]

The scope hunt continues... for the moment I took a shot at trimming the triangle by throwing it into LFO mode and modding another one of the oscillators. doing it by ear its so-so, good enough to mess about with until I can get a decent scope.

But let me say these are great oscillators! Mucking about with a few cables and setting them furiously against each other is real treat. Kudos to Thomas and Tom.

[macumbista]

Hey Folks...

I just finished the first of four dual XR-VCO modules (yes that's right, eight VCOs in total is the goal...done in pairs so I have an LFO and a VCO in each module). I used the Bugbrand layout--thx Tom--, etched my own PCBs and have been stuffing parts and doing cables for days now!

All in all it's quite a cool circuit.

But I've got one problem I'd like to ask about before I finalize the others: the main tuning pot seems more than a bit too wide.

The bottom third is subsonic, the middle third is within hearing range and the upper third is ultrasonic. The fine tune, on the other hand, is quite subtle--as it should be. But I'd prefer that most of the main pot was audible, so what can I do?

Here's what I haven't done yet:

* I haven't tuned the VCO. Since I do generative patches, having a master 1V/Oct input isn't really my main interest.
* I haven't changed the timing caps (C7 and C7x) from 47nF for the high range and 4.7 uF for the low range (incidentally, the LFO range is quite wide as well, but perhaps more useful because of that).
* I haven't calibrated the Sine/Tri offset nor the Sine shape. I'm waiting to borrow a friend's scope for that, but I can't see how that would affect frequency response.

Also, I used an LF412CP in place of the LF442, since that's what was available locally and was the "suggested replacement". Don't ask me what the difference is, I have no idea! I'm also using the LM394 TO matched transistor pairs (from Reichelt.de) and 2K tempcos (from Magic Smoke).

I'm hoping that the first point isn't the main problem, since it's a rather time-consuming process However, I'm all ears on this one, and if doing a proper tuning will give me a wider range on the pot I'm willing to go for it!

best!
D.

[Uncle Krunkus]

Where can I see the schem?
I think adding a trimmer on either side of the pot would give you the ability to "set it's boundaries", but I'd like to see the schem first before you try it.

[macumbista]

Tom's schemo and layout are here:

http://www.bugbrand.co.uk/docs/TH_XR2206VCO_BugPCB.PDF

[macumbista]

OK, one thing I noticed is that the fine tune pot has a 3M3 resistor after it (R46), while all the other inputs have 100K. If I wanted to reduce the range of the main tune pot, I could increase the value of the resistor after that pot (R28), right?

So rather than trial and error, is there any predictable way to find a good range? How would that be calculated? Or should I just throw a 500K or 1M trimmer in there? Would be nice to find the same value/range for all 8 oscillators I've made though...

[macumbista]

After some mathematical discussions on the SDIY list, we came to the following:

When R28 is 100K, then the tune pot has a range of 30 octaves (!!!!!). Since this board has an LFO switch, that's not very useful here.

If R28 is 300K, then the tune pot has a range of 10 octaves, which is bit more in line with what I expected.

If one wanted to shift the range up or down instead of constrain it, then trimmers on either end of the pot would do the trick.

Future builders may want to take note of that.

[Thomas_Henry]

Hi all,

It seems we go through this every six months or so.

My designs follow most of the Electronotes standards. Bernie was astute enough to plan ahead and come up with interfaces that are truly universal. I don't think his electrical standards can be improved upon.

In the case of a VCO like this, the Coarse, Fine and Exponential FM inputs are designed to accommodate any combination of control voltages: positive, negative, 0 to +10, 0 to +5, 0 to -5, 0 to -10, -5 to +5, -10 to +10, mixtures, etc.

By all means, feel free to change controller values for specialized standalone applications, but be aware that you're losing something kind of neat---a circuit that will do it's job without kluges, add-ons, extra modules, etc. in any patch on any synthesizer. The 100k and 3.3M resistors are features, not impediments thrown in without thought.

And for what it's worth, my design didn't specify an LFO switch, an idea I've never liked for several accuracy/tracking reasons.

Thomas Henry

[Uncle Krunkus]

If the standard design goes subsonic, there would be no reason to add an LFO switch anyway hey?
Maybe macumbista, like Thomas has pointed out, you need to re-assess how you use the unit rather than how it's built.
I'm really no expert, (I haven't even begun the process of building a modular, but with my new job, I think I might take it on!) but what Thomas has said makes heaps of sense to me.

[macumbista]

Hi Thomas,

and thanks for contributing your thoughts to the discussion! I'm certainly not questioning the wisdom of your design, I find it elegantly simple and very functional. For me, one of the things that makes DIY interesting is that it's not a "black box", and that one can customize things and make them your own rather than "off the shelf".

My own expectations are driven by two things:

1) Time I spent sitting in front of the Buchla 200 system at EMS in Stockholm, specifically the 259 Complex Waveform Generators. (And also using my Cwejman VCO-2RM, which I always thought I needed 4 of until I built the first dual XRVCO...)

2) The fact that I do generative synthesis rather than control via keyboard or other note-based system.

The first is a "user interface" issue more than most of what goes on under the hood. I liked the panel layout and the basic functionality, and used that to influence how I set up the XRVCOs. An LFO switch was useful there, as was a main tuning pot that went across the audible range.

The second consideration is specific for my implementation, and I realize it negates quite a bit of the engineering done on the tracking etc. In spite of that, I found the XRVCO highly suitable for my needs. It's very easy to build, so I could make 8 of them in a few weeks, I really enjoy the shaping function and I appreciate being able to mix lin and exp CVs! Even if I don't use all the functionality intended, I think it's fantastic that it's there and that it didn't cost me much more time

You can see two of the dual XRVCO modules in the upper rack here:



Best wishes!
Derek

[macumbista]

Going against Thomas' (and Bernie's) better judgment, I used a 30K resistor at the bottom of a 50K lin main Frequency pot and a 20K at the top, giving a total of 100K for that pot. I had no use for a Freq pot that was ultrasonic for the last quarter of the turn!

This setup gives me a range pretty much between the thresholds of audibility (with a bit more in the subsonic range) within one turn of the knob, which is what I am going for. The range of the VCO feels much better now, much more in line with the Buchla I'm (clumsily) trying to emulate. There is a low frequency range on the Bugbrand PCB which I use for slower LFOs, which obviously wouldn't need precise tracking.

Take this info FWIW to you and your own purposes, as Thomas clearly indicates above.

I also installed some CGS-style LED drivers off the triangle output, which worked after some fiddling... currently 100K input resistor, using diode in place of second input resistor (between input and ground) and some ultrabright LEDs with a big current limiter to keep them from detuning the oscillator.

[robboten]

Trying to understand the AM function on this IC. And maybe make a mod for the nice XR-VCO.

From the datasheet I see that an AM input at VCC/2 means zero amplitude and VCC-4 (?) AM means full amplitude out. When disconnected from the old skew, sync and Sqr circuit and fed with external DC voltage (at AMSI pin) I see that this is true. When I go past VCC/2 the phase inverts as stated. But at about 10V to 12V (VCC) amplitude goes down again very fast. That would be ok if it wasn't for the phase inverted amplitude being several volts (like 5Vpp) lower than the non-inverted output. Am trying to understand this. Is it because of the XR itself or because of surrounding circuits? Seems this also makes the skew function work less than intended on 12V supplies (doesn't skew all the way to ramp from tri).

Any insights are welcome!

[JimRead]

Hello all,

I'm awaiting parts for my VCO-1 board, I found this thread from looking at Scott's site.
I also checked up on the XR2206 and ordered some through AliExpress. I did wonder about them and found a mention of Jameco, they'd got some, also the Chinese ones with the small 'XR' on them, so I thought well that's probably OK then. Further to that lots of signal generators using the chip on AE and reviews led me to think that at supersonic levels they were not so good, I assume from that the audio range signals are OK.

Onwards

Two things first of all the 394 replaced by two matched 2N3904's


Parts placement left board right

And

Reading Scott's notes about D2


Parts placement left board right.

I will print the board on paper and check it with a chip socket and then post it here.

Cheers

[Grumble]

Every link in this thread is dead or 404, or is it me?

[JimRead]

Hello again,

This is the modified board the alteration to the LM394 socket to take two 2N3904's can be partly seen on the lower left


The board size is perfect as you can see from the socket.

Cheers

[JimRead]