Behringer System 55

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ishi
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Re: Behringer System 55

Post by ishi » Sat Jul 04, 2020 3:23 pm

Ebotronix wrote:
Sat Jul 04, 2020 2:36 pm
I don't get warm with the 921 b, it's always out of tune....the 921 VC is much better
I like the sound of the Behringer 921b pretty much, but tuning seems indeed to drift after some time. I've got the preliminary impression all 921b's drift somewhat together. Will investigate further and compare to MosLab.

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Re: Behringer System 55

Post by galanter2 » Sat Jul 04, 2020 3:40 pm

ishi wrote:
Sat Jul 04, 2020 3:19 pm
galanter2 wrote:
Thu Jul 02, 2020 10:18 pm
Hi, the first pair of 914 fixed filter bank images are fine, but the second pair of image links seem to be broken.
At the moment there is only one pair of 914 FB images.. will post some more.
Cool! Thanks!

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Re: Behringer System 55

Post by Ebotronix » Sat Jul 04, 2020 3:45 pm

ishi wrote:
Sat Jul 04, 2020 3:23 pm
Ebotronix wrote:
Sat Jul 04, 2020 2:36 pm
I don't get warm with the 921 b, it's always out of tune....the 921 VC is much better
I like the sound of the Behringer 921b pretty much, but tuning seems indeed to drift after some time. I've got the preliminary impression all 921b's drift somewhat together. Will investigate further and compare to MosLab.
it was never in tune ,always drifting....
the 921 VC is much better
Last edited by Ebotronix on Sun Jul 05, 2020 12:21 am, edited 1 time in total.

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Re: Behringer System 55

Post by SkyWriter » Sat Jul 04, 2020 8:31 pm

My old pocket 4-ch scope worked best on envelopes. After confirming the shelving frequency tracking with pot rotation on the 923, I focused measurements on msec timing on the 911 and 911A modules. I found that below 4ms there was also discontinuity and tracking errors until the end of travel similar to the 923 on both modules. I have two more 911's arriving tomorrow. I expect identical results.

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Re: Behringer System 55

Post by KSS » Sat Jul 04, 2020 8:42 pm

Reality of todays cheap pots. Mechanical travel significantly exceeds electrical travel. And it's getting worse as time goes on. Hard to imagine that a few degrees less resistive ink printing saves enough money to be happening, but that's the reality of capitalism.

Thanks for the notes.

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Re: Behringer System 55

Post by SkyWriter » Sat Jul 04, 2020 9:40 pm

Yeah, we pay a price in the end. No surprise.

I am very partial to that part of the attack, and would definitely modify a few to get there.

So, the 911 has a 100 ohm in series. We might be able to reduce the series resistance and bring the CCW end into spec - I'll gladly sacrifice the 10sec end. The 911A, has calibration trim pots on the back. They may also ameliorate discontinuities in the usable range low end.

There will still be A glitch at the end of travel, but it'll be below what the panel timing indicates, just not where the panel puts it. That's acceptable.

Any other's? I expect the mixer CP3A-M* may also have an issue. 995 too. I'll check those out tomorrow.

So, yeah, I misread that reversible-attenuator. Didn't consider the use cases until I saw them on the scope with the 921. The difference between having and wanting strikes again. :-) the 992 schematic was immediately obvious in a way the panel never was.

*-which btw has a very nice clean/distortion response.
Last edited by SkyWriter on Mon Jul 06, 2020 7:32 am, edited 1 time in total.

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Re: Behringer System 55

Post by betovelandia » Sat Jul 04, 2020 11:12 pm

I can post any pictures you need of the 914 904A & 923 just let me know what you need to see.

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Re: Behringer System 55

Post by SkyWriter » Sun Jul 05, 2020 11:03 am

betovelandia wrote:
Sat Jul 04, 2020 11:12 pm
I can post any pictures you need of the 914 904A & 923 just let me know what you need to see.
I'm interested in the component side. But, I can wait until I have one of my own. Way too much to do lately, before I think about that mod. Thanks beto!

My 2nd 921, and two more 911's arrived just now. Time to get to work :-)

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Re: Behringer System 55

Post by SkyWriter » Sun Jul 05, 2020 3:17 pm

I haven't cracked open a 921 yet, so maybe there's a calibration for this "Tri/Sin Offset?".

On both 921's I see the both Tri and Sin outs have an asymmetry. It is only in the LFO range. Audio is clean.

N.B. This is the inverted output in images.
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Re: Behringer System 55

Post by galanter2 » Sun Jul 05, 2020 5:40 pm

accidentally replied to a very old post...

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Re: Behringer System 55

Post by BowserB61 » Mon Jul 06, 2020 3:34 pm

SkyWriter wrote:
Sun Jul 05, 2020 3:17 pm
I haven't cracked open a 921 yet, so maybe there's a calibration for this "Tri/Sin Offset?".

On both 921's I see the both Tri and Sin outs have an asymmetry. It is only in the LFO range. Audio is clean.

N.B. This is the inverted output in images.
Thanks for posting these. I noticed that hiccup on my 921 the other day, but my scope is broken so I wasn’t able to see it visually. Can you confirm my suspicion that, by about the top of the 16’ LFO range, the glitch disappears? Certainly in the audio range there aren’t any extra harmonics in the sine output that you would expect to hear if the glitch was still there.

I’m not terribly optimistic that the available adjustments on the back are going to help, but definitely keep us updated if you find anything. Meanwhile, I think I’m just going to open a ticket with Behringer and see if they have anything to say about it.

I have 3 921Bs and none of them have this issue in the LFO range. I don’t seem to have any serious drift issues with them like Ebotronix mentioned either. Certainly nothing beyond the warmup drift that I was already used to on the Model D.

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Re: Behringer System 55

Post by SkyWriter » Mon Jul 06, 2020 7:50 pm

@bowserB61, the glitch is only present on the two waveforms when the SUB/AUDIO switch is in SUB position. No other switch or knob position matters. There are two trimmers on the back H ADJ and SCALE. There is also an edge trimmer WIDTH ADJUST. This is without taking it apart yet. I expect some amount of pre-adjustment, but it doesn't seem to be complete. I'll dig deeper soon.

There are several candidates on the canonical schematics, haven't checked how accurate they are. Chits too small for my eyesight to buzz out, or identify without a microscope; the cheap oscilloscope's are bad enough.
29689A51-131C-47FD-9473-DAC9A56455C9.jpeg
A7F1CF9B-11AF-45FD-849A-62D0A71E44C7.jpeg
I Identified these trimmers from the system 15 schematics before taking it apart:
SCALE *-pictured above
H ADJ *-pictured above
BOTTOM END
SINE SHAPE
SINE OFFSET
TRI OFFSET
RECT WIDTH *-probable... we'll see!

I'll have to pop the panel to find the rest.
Anyone know a good source of readable schematics online? The most immediate one is of poor quality.
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Re: Behringer System 55

Post by SkyWriter » Tue Jul 07, 2020 8:20 am

Opened one up this morning. Plenty of physical trimmers:
HI ADJ - rear
SCALE - rear
WIDTH ADJUST
RANGE ADJUST
FREQ ADJ
SINE SYM
TRI ADJ
SINE SHAPE
CLAMP ADJ

So, nine trimmers and four pots. The text doesn't always match up between silk screen and these schematics. Any guesses?*

Schematic trimmers:
SCALE
HI ADJ
TOP END
BOTTOM END
TRI OFFSET
SINE OFFSET
SINE SYM
SINE SHAPE
RECT WIDTH
56514C9E-1A75-4A31-A99C-11FA0901924E.jpeg
D22FAE3C-FB34-476F-BD65-42F0DE0EAEFF.jpeg
*-different schematics? What's the lineage between system 15 and behringer? Yeah... these are not the schematics we're looking for.
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Re: Behringer System 55

Post by SkyWriter » Tue Jul 07, 2020 9:17 am

I was able to improve the LFO waveshapes, but it introduced a small DC offset in the outputs.
The TRI ADJ will shape the positive peak of the TRI.
SINE SYM will shape the top of the sine.
SINE SHAPE will shape the bottom of the sine.

However, these settings are frequency dependent. They seem to be set for audio range symmetry and offset.

For me, the LF is the most important shape for glitches, the high-end will just be some extra harmonics. Anyone would see an audio glitch in a DAW, but an LFO... not likely.

You can see the TRI now has a a negative DC offset*, and the SINE is positive, but they look pretty good now.
2E537EA2-1298-4426-9CBC-E78F617DC6ED.jpeg
F8C0DD64-5390-45F5-BFC8-75B6FF0EDD85.jpeg
Warning, everything is SMALL. Get a very small screw driver 1.4mm and use it very lightly on the trimmers. Use a #0 phillips for the rear screws - and push the tool is tight. The lock washers keep the screw surprisingly tight, easy to strip it.

*-thanks goodness for three inputs to null the offset :-) just need a +/- 6V source and attenuators, like a CP35 :-) it all works!
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Re: Behringer System 55

Post by SkyWriter » Tue Jul 07, 2020 9:52 am

Ok, forget the schematic. There are Coolaudio chips on this board. Hmm....


V13700M
V411

Some LM3046's, LM337... so, completely different. Oh well.

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Re: Behringer System 55

Post by BowserB61 » Tue Jul 07, 2020 9:55 am

Yeah, I took mine apart last night and looked at that same schematic. Looks like I’m already a day late and a dollar short with what I learned though.
Shame that it can’t be adjusted correctly without some other artifacts. Do you have a scope image of how far off it is in the audio range?

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Re: Behringer System 55

Post by SkyWriter » Tue Jul 07, 2020 11:29 am

BowserB61 wrote:
Tue Jul 07, 2020 9:55 am
Yeah, I took mine apart last night and looked at that same schematic. Looks like I’m already a day late and a dollar short with what I learned though.
Shame that it can’t be adjusted correctly without some other artifacts. Do you have a scope image of how far off it is in the audio range?
Yeah, I'll get one. It's not bad, nothing you can't shave off with a filter, or chalk up to quirky.

My expectation on a module with a redesign and their own parts, is that there shouldn't be any quirks to speak of. So far we have tuning issues and waveform distortion tight out of the box. Hoped for better.... welp, finish the work before starting the complaining, that's the way forward.

Ok, just one: I wonder if the 960 will be their little sequencer in sheeps clothing. LOL!

update: I'm going to get a better scope to make further measurements.
Last edited by SkyWriter on Tue Jul 07, 2020 1:52 pm, edited 1 time in total.

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Re: Behringer System 55

Post by KSS » Tue Jul 07, 2020 11:54 am

Some of those make sense. The 3046 is supposed to be there. The 13700 probably doing differential duty in the sine shaper and pulse. The 411 handling sync and maybe core reset. 337 also candidate for sync or pulse duty.

From what i've seen, Behringer seem to clone the clones rather than the originals. I'd look at old Mos-lab, AION moon and COTK for clues to what they've done. Also the MME euro minimoog clone. I'd bet these have more in common with one or more of those than a moog 921.

They picked a bad place to mess up on this B55 though. moog oscs are a core of their modular sound. Sometimes more than the filter.

Edit: On a positive note, at least they got some solder up topside in the photo you posted. Still not very good, but is an improvement over what the earlier module photos showed!

But that VCO range has got to get better, or this whole B55 project starts to lose luster in a big way.

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Re: Behringer System 55

Post by SkyWriter » Tue Jul 07, 2020 1:25 pm

Agreed. But if it's only a bit of a tweak to get them up, then that's fine. Back then, I was de-skewing minicomputer backplanes. This is easy!

Looking over the construction and schematics of the canonical 921, it was merely a coincidence they had nine trimmers. Two, maybe three are the same, the rest are something else entirely. There really should be an offset for sine and tri, but room is tight. They're working right out to the edge of the card, but it's apparently conventional design rules. So, here's cost reduction again. Haven't checked if these precision trimmers on back yet (HI ADJ, SCALE). If ya can't tune it...

I really need a CV filter next. Any-hoo... back to work!

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Re: Behringer System 55

Post by BowserB61 » Tue Jul 07, 2020 1:48 pm

To add my own data point on the tuning issues, I am 100% satisfied with the tuning on all of my oscillators. I have 3 921Bs and one 921 and haven’t seen any issues with tracking or drift on any of them.

I tested by daisy-chaining all 4 oscillators off of the 921A CV output, tuned them all individually on the 8’ setting to A440 with a guitar tuner, and checked by ear against my Hammond XK-3 with only the 8’ drawbar pulled (digitally in-tune and approximately sine wave with the Leslie bypassed). I checked tracking at all footages against the Hammond over the 8 audible octaves that I could check and didn’t hear any difference at the extremities or any inconsistencies in between. I left it on for an hour and checked every 5-10 minutes to see if each oscillator was still in tune at A440. They all went sharp a few cents as it warmed up, but they were so consistent that adjusting the 921A usually brought all 4 oscillators back into tune. Occasionally the 921 would go a few cents sharper than the 921Bs, but it was such a small difference that it could be fixed by just tapping on the tuning knob.

Given the horror stories I’ve heard about the drift on the original Moogs (mostly about 901s, to be fair), I was expecting a lot worse. I could do a shootout at some point, but I suspect these are more stable than my Behringer Model D.

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Re: Behringer System 55

Post by SkyWriter » Tue Jul 07, 2020 2:05 pm

@bowserb61, that's why I'm not disappointed from an authenticity viewpoint. Really, what I want is system 55 front panel workflow, reliable performance, and modules that sound good. Not magical. Great is preferable, but JustGoodIsEnough(tm). So far, I'm good to keep going on with my build plan. I had scaled it back in my head for a second there!

I like a massive analog oscillator bank. That's got Authority!

So, thank you for that tuning report! I'm cheered once more :-)

Ps: just ordered a DSC-5300. Looks capable - you can tell, the manufacturer has a real website, manual, and you can't get one on amazon. :roll: my HP gear is in the basement shop. Too big and noisy for upstairs work :-) oh good, real USB port. No more 'scope camera' shots.

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Re: Behringer System 55

Post by KSS » Wed Jul 08, 2020 3:09 am

@BowserB61
I'm also buoyed by your report. Thank you. That suggests its a setup issue more than a design problem.

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Re: Behringer System 55

Post by lorez22 » Wed Jul 08, 2020 3:48 pm

I'm Struggling to get that fat Moog Sound with the modules I've got so far 3x 921, low pass filter, VCA and mixer...any hints

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Re: Behringer System 55

Post by SkyWriter » Thu Jul 09, 2020 7:43 pm

Drove two 921's (one tweaked waveforms, one not), and a Plaits module from a Roland A-01 CV out, perfectly in tune across whole range, right from power up with out-of-the-box calibration. After 1/2 an hour, had to tune one up a little. The case has several mutable CPU's in it - so, it warmed up a bit. I'll make a measurement this weekend, scopes coming tomorrow. Too bad about the schematics, but if I don't have to fix anything, that's preferable - I can spend my time on these non-color-blind friendly mutable modules. Thank goodness for open source vendors.

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Re: Behringer System 55

Post by SkyWriter » Thu Jul 09, 2020 7:56 pm

lorez22 wrote:
Wed Jul 08, 2020 3:48 pm
I'm Struggling to get that fat Moog Sound with the modules I've got so far 3x 921, low pass filter, VCA and mixer...any hints
I've never understood what the 'Fat Moog Sound' phenomenon is all about. But, from experimenting with just voicing band limited sawtooth waveforms In Plaits code, even with ostensibly equivalent looking waveforms, sawtooth's can sound dramatically different. The thing about analogs, non-linearities - such as ramp slope - can affect perceived sound.

Having said all that, these are not the same circuits as the OG modules, and may have a characteristic waveform that differs unremarkably in every way from the canonical "Moog Sound" waveform, but lack that quintessential aspect folks hunt for.

Push the loudness button. :-)

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