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OB-8 "OB-Xa sound" modification
Old 31st March 2018
  #1
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OB-8 "OB-Xa sound" modification

I decided to do the "OB-Xa sound" mod to my OB-8. For those of you not familiar with this mod, I suggest you read this first:
OB-8 mod (OB-Xa sound)

In short, the person that came up with this mod, Mr. Juergen Haible, found that the OB-8 has a 22Hz highpass filter that occurs in the coupling circuit between the VCF and VCA (I guess this was in comparison to the OB-Xa). His original mod removed the highpass in the coupling circuit but it caused the Auto Tune to fail. As an alternative, he reversed the highpass filtering at the voice summing stage, just before the outputs.

The mod is pretty simple. All it requires is the replacement of 4x 100 kOhm resistors with 1 MOhm resistors connected to a small circuit soldered on top consisting of a 56nF capacitor and 120 kOhm resistor. The resistors you replace are R505 and R506 on each voice board. They sit opposite sides, next to the TL082 opamp near the edge of the board above the pan pots. So in total you remove (4) 100 kOhm resistors, and need (4) 1 MOhm resistors, (4) 120 kOhm resistors, and (4) 56nF capacitors.

Attached photos include a screenshot taken from a recording (not from an oscilloscope) showing how much of a difference is made to the lowest C note saw and triangle waves. As for the sound, I am not going to say my OB-8 now sounds like an OB-Xa (no idea on that), but I can certainly tell there is more thump in the lowest frequency and more overall low end in the low-to-mid frequencies. I use EQ on my OB-8 and had to trim back some of the bands particularly for when playing multiple notes.

I'm sure someone can get more creative with the design of the circuit, so please excuse my amateur skills. The purpose here is just to show what you need, how I did it and a visual of the results in the waveforms.
Attached Thumbnails
OB-8 "OB-Xa sound" modification-ob8_xamod1.jpg   OB-8 "OB-Xa sound" modification-ob8_xamod3.jpg   OB-8 "OB-Xa sound" modification-ob8_xamod4.jpg  
Old 31st March 2018
  #2
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Here are some short samples of my OB-8. Completely dry; no processing, EQ, FX. Note that this OB-8 was recently overhauled with all new capacitors, trim pots and calibrated. You can read more about that here: OB-8 Restoration

Most of the sounds are my own, but there are a few Kid Nepro patches in there as well. Enjoy! Any OB-Xa owners, let me know what you think about the OB-8 mod. Just curious. I don't think it really makes it "sound" like the Xa, rather just an OB-8 with more low end, but I don't have personal experience with the Xa.



Here is an example of how you can get crossmod/FM type sounds with the OB-8. It starts with a saw and gradually mixes in some FM using the fast vibrato LFO in the bend/modulation box. I also mix in some of the main LFO later which can get to a pretty high rate as well but that is better served for other modulation. The 2nd LFO can only modulate VCO1/2 frequencies.

Last edited by BM0; 31st March 2018 at 05:26 PM..
Old 31st March 2018
  #3
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Here is an attempt at some percussion sounds. Some EQ was used.

It has been pointed out that the envelopes in the OB polys are slow or not "snappy". In comparison to Rolands, that certainly seems to be true. However, this may mislead people into thinking that the Obies aren't good at all for fast sounds. While they are more known more for brass and pad sounds that have a soft attack, they can also do fast lead and bass sounds pretty well.
Attached Files

BD01.wav (51.8 KB, 2143 views)

SD01.wav (30.8 KB, 2014 views)

CH01.wav (28.5 KB, 1977 views)

OH01.wav (82.7 KB, 2187 views)

CY01.wav (116.6 KB, 2268 views)

Old 31st March 2018 | Show parent
  #4
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Quote:
Originally Posted by BM0 ➡️
It has been pointed out that the envelopes in the OB polys are slow or not "snappy".
Some of this "slow" perception might be due to the fact that the service manual gives the wrong value for the envelope decay calibration value, so this inadvertently made them slower.

When properly calibrated, the OB-8 envelopes are plenty fast/snappy--maybe not Roland jupiter-4 lightning speed--but certainly as fast as anything else from that era.
Old 1st April 2018 | Show parent
  #5
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Quote:
Originally Posted by Sir Ruff ➡️
Some of this "slow" perception might be due to the fact that the service manual gives the wrong value for the envelope decay calibration value, so this inadvertently made them slower.

When properly calibrated, the OB-8 envelopes are plenty fast/snappy--maybe not Roland jupiter-4 lightning speed--but certainly as fast as anything else from that era.
The first time I calibrated, I followed the service notes that specified the envelopes be set to 0.00v. It screwed up the timings pretty bad. Fortunately, I had written down all voltages prior, so I just put them back and adjusted with a stopwatch. Once I found the proper value (-0.256v) I used that. My two voice boards don’t match at that value so I just set the lower board to -0.256v and adjusted the upper to get it as close as possible. The envelopes are analog, so there is always going to be some slight variation between voices, which is desirable. You can’t calibrate the timings per voice anyways.
Old 2nd April 2018
  #6
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Indeed that is an error in the manual. They're supposed to be at -0.256. They get snappy enough there's only a click left, you can do the compression pedal emulation by lowering the volume sustain with 0 attack and decay.

I've heard this also alters the sound favorably, you replace 16 resistors:

https://rubidium.dyndns.org/~magnus/...eim/ECO441.jpg

I'm going to perform it on my instrument. I did the Juergen Haible mod already with an A/B comparison and found that it's a very minor difference in sound, perhaps 5-10% at most, however it's more noticable with all the voices firing in unison. I'm certain that the ECO in the link isn't a deal-breaker change in sound either, but perhaps it's exponential with the Juergen Haible mod.

A clever thing would be to solder the Juergen Haible circuit to your standard glass fiber pcb board, cut the excess of the board off, and as Haible tells us "solder a 4p1t switch across the caps for immediate access to both sounds": Looks more professional and gives one the option to turn the mod on and off.

The switch shorts/unshorts the caps allowing the 120kOhm resistor to work in place of the 1mOhm replacements, which is almost the unmodified's 100kOhm.
Old 2nd April 2018 | Show parent
  #7
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Quote:
Originally Posted by svanis ➡️
I've heard this also alters the sound favourably, you replace 16 resistors:
I thought about doing that one as well. I am curious to what the results. Actually, I never checked those resistors to see what are currently installed. The ECO was for future production, but for all I know, someone may have already made that change as my OB-8 was serviced before I owned it. It had a battery mod done to it. I would have to order some 1% tolerance resistors if I decide to do it.

I feel Juergen's mod really only makes a noticeable difference in the lower frequency range. I agree it is more noticeable when playing chords or unison. Per voice, it isn't noticeable at all once you play up beyond the lowest two octaves. I A/B mine by modifying one board at a time. I was maybe expecting to hear more bottom in the mid range of the keyboard, but that isn't the case. I wasn't even sure the mod did anything at all. Then when I played individual notes in the lowest octave, I could hear more of a "thump" on the modded upper board versus the lower. It is subtle, but there. Once I completed the other board and started playing chords, I had to trim the channel EQ on my mixer as it was too much bass and sounded bad at where it was set pre-mod.

Anyone considering doing the mod shouldn't expect it to make some drastic change to the sound. It still sounds like an OB-8, but with slight more low frequency response.
Old 21st May 2019
  #8
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🎧 5 years
Quote:
Originally Posted by svanis ➡️
A clever thing would be to solder the Juergen Haible circuit to your standard glass fiber pcb board, cut the excess of the board off, and as Haible tells us "solder a 4p1t switch across the caps for immediate access to both sounds": Looks more professional and gives one the option to turn the mod on and off.

The switch shorts/unshorts the caps allowing the 120kOhm resistor to work in place of the 1mOhm replacements, which is almost the unmodified's 100kOhm.
I just did this mod to my OB-8 and I did that. These are the modules I built:

Photo 1

Photo 2

This is how the board is wired (sorry for the poor and un-professional drawing, I don't have the right tools for that).

The jumper is meant to short the capacitor, as suggested above.

I made the modules as small as possible, and actually they fitted pretty well on the voice boards without touching other components (sorry, forgot to take photos of the modules mounted on the voice boards). Just to be sure that the solder points on the module's lower side wouldn't scratch the voice board's protective paint layer, I put a piece of double-sided tape (the soft, "spongey" type) there. This also helps to hold the modules on the voice board when lifting the jumper to switch between normal/bass boost sound.

Well, the difference is pretty obvious, now my OB-8 is definitely on the heavier side. Maybe not heavy as an OB-Xa or an OB-X, but it sounds definitely better.
Old 21st May 2019 | Show parent
  #9
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Quote:
Originally Posted by mima85 ➡️
I just did this mod to my OB-8 and I did that. These are the modules I built:

Photo 1

Photo 2

This is how the board is wired (sorry for the poor and un-professional drawing, I don't have the right tools for that).

The jumper is meant to short the capacitor, as suggested above.

I made the modules as small as possible, and actually they fitted pretty well on the voice boards without touching other components (sorry, forgot to take photos of the modules mounted on the voice boards). Just to be sure that the solder points on the module's lower side wouldn't scratch the voice board's protective paint layer, I put a piece of double-sided tape (the soft, "spongey" type) there. This also helps to hold the modules on the voice board when lifting the jumper to switch between normal/bass boost sound.

Well, the difference is pretty obvious, now my OB-8 is definitely on the heavier side. Maybe not heavy as an OB-Xa or an OB-X, but it sounds definitely better.
Nice! I was thinking of doing something similar, but I will probably just copy your design. Thanks! My current fit is really a half-assed job, but it does work. I certainly would never do that to someone else OB-8 lol.
Old 31st May 2019
  #10
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🎧 5 years
I had to open my OB-8 to redo the volume balance calibration between the lower and upper voice boards. I took the occasion to make a photo of the bass boost modules installed on the voice board. This is how the mod appears.
Old 28th December 2019
  #11
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Bumping since I have now performed this mod on my OB-8. I made modules like mima85, except without the jumper to disengage the bass recovery. Like Jürgen, I hardwired the mod.




Installing was a breeze once I had assembled these modules. Cascading my Marshall head was probably more work. I actually have parts to make four more sets of these, and might be willing to make sets for cheap if anyone wants one.

While I only had one board modified, I recorded a quick and dirty demo with three unison patches (Tom Sawyer sweep, funky bass, sync lead). I have to say the mod gave me that last 5% of the sound and feel I was looking for.



After recording the demo, I calibrated the envelopes, and while it feels better yet to play, I ended up losing much of the smear of the Tom Sawyer sweep.
Old 28th December 2019 | Show parent
  #12
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🎧 5 years
Quote:
Originally Posted by DesolationBlvd ➡️
Bumping since I have now performed this mod on my OB-8. I made modules like mima85, except without the jumper to disengage the bass recovery. Like Jürgen, I hardwired the mod.

Installing was a breeze once I had assembled these modules. Cascading my Marshall head was probably more work. I actually have parts to make four more sets of these, and might be willing to make sets for cheap if anyone wants one.

While I only had one board modified, I recorded a quick and dirty demo with three unison patches (Tom Sawyer sweep, funky bass, sync lead). I have to say the mod gave me that last 5% of the sound and feel I was looking for.



After recording the demo, I calibrated the envelopes, and while it feels better yet to play, I ended up losing much of the smear of the Tom Sawyer sweep.
Nice job! Looks very clever and self contained and I can hear your last 5%


On another topic - does anyone know about the guy who was making replacement keyboard kits? I seem to remember he was doing them for a vintage Sequential or 2 and said the OB-8 was next. I'd really like better keys on mine - anyone know anything?
TIA
Old 9th March 2020 | Show parent
  #13
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Quote:
Originally Posted by AnalogDomain ➡️
On another topic - does anyone know about the guy who was making replacement keyboard kits? I seem to remember he was doing them for a vintage Sequential or 2 and said the OB-8 was next. I'd really like better keys on mine - anyone know anything?
TIA
I had contact with him right before the last Christmas festivities, he told me that was in the process of acquiring an OB-8 to develop his mod. No other contact since them.

Me too I'm waiting for a new keybed for my OB-8.
Old 8th July 2020
  #14
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🎧 15 years
The original haible mods increased coupling caps at two points of the signal path: 1) Between VCO and VCF (this should be CX12 in the schematic) and 2) between VCF and VCA (should be CX21 in the schematic).

The latter mod (applied in this thread) intends to compensate for above number 2) by changing the feedback network of the I/V converter opamp after the VCA.


But what about mod 1)? Does it affect the tuning as well? Has anyone performed both mods? A highpassfilter at 8 Hz still isn't transparent and driving the VCF asymetrically by removing the coupling cap entirely (according to haible like it is done in the P5) might sound good after all...
Old 12th July 2020
  #15
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For reference, I tried a bigger (and technically better) cap between VCO and filter and didn't like the sound as much as the original one. However, the output mod (phase/frequency response correction via the output op amp) by Jürgen Haible was an obvious improvement. My synth is now fully clalbrated and sounds great. ;-)
Attached Files

Ob_Sweeps.mp3 (3.89 MB, 1237 views)

Old 12th July 2020 | Show parent
  #16
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Quote:
Originally Posted by living sounds ➡️
For reference, I tried a bigger (and technically better) cap between VCO and filter and didn't like the sound as much as the original one. However, the output mod (phase/frequency response correction via the output op amp) by Jürgen Haible was an obvious improvement. My synth is now fully clalbrated and sounds great. ;-)
That sound.... you're triggering my post OBXa partum depression....
Old 12th July 2020 | Show parent
  #17
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Quote:
Originally Posted by living sounds ➡️
For reference, I tried a bigger (and technically better) cap between VCO and filter and didn't like the sound as much as the original one. However, the output mod (phase/frequency response correction via the output op amp) by Jürgen Haible was an obvious improvement. My synth is now fully clalbrated and sounds great. ;-)
Sounds beautiful ... will try to program some patches like that on my OBXa tomorrow!
Old 12th July 2020 | Show parent
  #18
Deleted a2500d9
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Quote:
Originally Posted by living sounds ➡️
For reference, I tried a bigger (and technically better) cap between VCO and filter and didn't like the sound as much as the original one. However, the output mod (phase/frequency response correction via the output op amp) by Jürgen Haible was an obvious improvement. My synth is now fully clalbrated and sounds great. ;-)
Sounds really nice
Old 13th July 2020 | Show parent
  #19
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🎧 10 years
Quote:
Originally Posted by living sounds ➡️
For reference, I tried a bigger (and technically better) cap between VCO and filter and didn't like the sound as much as the original one. However, the output mod (phase/frequency response correction via the output op amp) by Jürgen Haible was an obvious improvement. My synth is now fully clalbrated and sounds great. ;-)
Sounds nice, but no longer sounds like an OB-8. :(
Old 13th July 2020 | Show parent
  #20
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🎧 15 years
Quote:
Originally Posted by Don Solaris ➡️
Sounds nice, but no longer sounds like an OB-8. :(
What does it sound like? To my ears this was clearly an improvement, removing frequency and phase response limitations. I can always use the console EQ to dial them back in if I want them.

Here's an example of the synth before the mod (different patch and mono).
Attached Files

ob8test.mp3 (8.70 MB, 1123 views)

Old 13th July 2020 | Show parent
  #21
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Quote:
Originally Posted by living sounds ➡️
What does it sound like?
Definitely no longer sounds like a KLF track. More like Italo disco track. I bet in Italo they used Xa all over the place.
Old 13th July 2020 | Show parent
  #22
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Quote:
Originally Posted by Don Solaris ➡️
Definitely no longer sounds like a KLF track. More like Italo disco track. I bet in Italo they used Xa all over the place.
That's definitely a selling point in my book for this mod.
Old 14th July 2020
  #23
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hmm: would it be possible to make an OBXa sound more like an OB8 by doing the reverse idea, of this?
Old 14th July 2020 | Show parent
  #24
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🎧 15 years
Quote:
Originally Posted by becks bolero ➡️
hmm: would it be possible to make an OBXa sound more like an OB8 by doing the reverse idea, of this?
Yes, but it depends on the OB8 revision. Doesn't make too much sense though, IMO.
Old 14th July 2020
  #25
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I think their filter circuitry is completely different. It would take a lot of modifications to do so.
Old 14th July 2020
  #26
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🎧 10 years
I've never been able to deduce a substantial sonic difference between the OBXa and OB8. That said, I have an early OB8 with Pratt-Read keyboard and CEM3360.

Here is a recent article that chronicles the different OB8 versions...

https://www.rosensound.com/2020/02/0...e-to-ob-x-xa-8
Old 14th July 2020
  #27
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interesting, thanks!

I have never played any of them, just curious

and that was a good link
Old 14th July 2020 | Show parent
  #28
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🎧 15 years
Quote:
Originally Posted by Rangoon ➡️
I've never been able to deduce a substantial sonic difference between the OBXa and OB8. That said, I have an early OB8 with Pratt-Read keyboard and CEM3360.

Here is a recent article that chronicles the different OB8 versions...

https://www.rosensound.com/2020/02/0...e-to-ob-x-xa-8
The latter OB-8 revision is actually technically closer to the OB-XA, so it should sound closer, too.
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