The No.1 Website for Pro Audio
 Search This Thread  Search This Forum  Search Reviews  Search Gear Database  Search Synths for sale  Search Gearslutz Go Advanced
OB-8 "OB-Xa sound" modification Keyboard Synthesizers
Old 31st March 2018
  #1
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
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
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, 404 views)

SD01.wav (30.8 KB, 368 views)

CH01.wav (28.5 KB, 349 views)

OH01.wav (82.7 KB, 414 views)

CY01.wav (116.6 KB, 413 views)

Old 31st March 2018
  #4
Lives for gear
 

Quote:
Originally Posted by BM0 View Post
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
  #5
Quote:
Originally Posted by Sir Ruff View Post
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
Here for the gear
 

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
  #7
Quote:
Originally Posted by svanis View Post
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.
Post Reply

Welcome to the Gearslutz Pro Audio Community!

Registration benefits include:
  • The ability to reply to and create new discussions
  • Access to members-only giveaways & competitions
  • Interact with VIP industry experts in our guest Q&As
  • Access to members-only sub forum discussions
  • Access to members-only Chat Room
  • Get INSTANT ACCESS to the world's best private pro audio Classifieds for only USD $20/year
  • Promote your eBay auctions and Reverb.com listings for free
  • Remove this message!
You need an account to post a reply. Create a username and password below and an account will be created and your post entered.


 
 
Slide to join now Processing…
Thread Tools
Search this Thread
Search this Thread:

Advanced Search
Forum Jump
Forum Jump