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OBX voice card (with non-custom CEM3310 equivalent circuits)
MUFF WIGGLER Forum Index -> Music Tech DIY Goto page Previous  1, 2, 3 ... 24, 25, 26 ... 67, 68, 69  Next [all]
Author OBX voice card (with non-custom CEM3310 equivalent circuits)
The Real MC
randel wrote:
1. On the OB-X Control Board, in the S/H circuits, there are unity gain op amps immediately downstream of the Mux chips - A68 for instance. Several of these op amps feed inputs to the voice card directly - FSUS, ASUS, RES and VCFM, for instance.

In each of these cases, the voltage on the input side of the op amp ranges from 0 - 5 volts, and the output ranges from 0 - 5.27 volts, which strikes me as odd for unity gain.


Actually the front panel pots have a rail of (+)5.6v and the DAC can generate above this. It has to because the LM311 comparator (A36) has to be able to detect max pot level.

Quote:
Does the crOwBX host card generate 0 - 5 volts for these signals? If not, do you think the extra 5% voltage contributes to the sound of the instrument? Is there another explanation for the discrepancy?


Yup, the host board pots are on a (+)5V rail. Will half a volt change the sound? Not likely, although the resonance CV is reverse polarity and is maximum at ZERO volts. (+)5V CV gives minimum resonance, will another half volt matter? Highly doubt it given the non-linear behavior of CV to resonance. And the host board includes the correct scaling of the EG CVs to emulate the original.

Quote:
2. The OB-X gate voltage is 10 volts as specified, and measures a little higher on mine. On the OB-X processor board, A40 is responsible for the gate signals, outputs 5 volts, and appears to be connected directly to the voice cards via the mother board. How does this signal get to 10 volts? Does the crOwBX voice card require a 10 volt gate signal?


If you are measuring GATE1 (or the rear panel TRIGGER OUT jack) then it will measure about ten volts because of the buffer circuit for external interfacing on the rear panel jacks. The other GATE signals do not have buffers and will be 5 volts.
The Real MC
oldcrow wrote:
randel wrote:
In each of these cases, the voltage on the input side of the op amp ranges from 0 - 5 volts, and the output ranges from 0 - 5.27 volts, which strikes me as odd for unity gain.

Does the crOwBX host card generate 0 - 5 volts for these signals? If not, do you think the extra 5% voltage contributes to the sound of the instrument? Is there another explanation for the discrepancy?


If you look at the OB-X control board schematic you'll notice the power supply for the 4051s, on both the digitizing/panel pot circuits and the voltage storage array circuits, use a 5.6v positive rail. This extra bit of headroom is to account for (and avoid) "endpoint squashing" as a mux channel approaches 5 volts full-scale off the parameter DAC. If Oberheim had used rail-to-rail analog switches, things like the DG508A. etc. then the extra bit of digitizer supply voltage wouldn't be needed. But as he used cheap 405x series parts, which while extremely useful even today, have rail conditions--see the data sheets.


The 405x switches in the OB-X have a design limitation based on power rails which is why the (+)5.6V rail was supposed to prevent "endpoint squashing". Unfortunately, the OB-X also has a design error that is the reason why so many show up for sale in non-working condition.

These conditions don't exist in the CrOwBX system BTW.
randel
oldcrow wrote:


If you plan to do some kind of CPU-controlled voltage store for CVs then whether you need to goose the demux switches with a bit of extra voltage is up to you. SlayerBadger!



Fascinating.

Thank you for the information, and for posting the schematics. I'm now inclined to use a handful of octal DACs to provide the required voltages, and forget about the 5.6v nonsense!

-Randel
mOBiTh
Hey this smt soldering is a lot of fun - I'm hooked!

It's quite tricky doing it left-handed with chopsticks though, I'm gonna buy some tweezers for the next project.

just waiting for molex connectors (£100??? Argh...) and pots to arrive and then all I'm lacking is a PSU for testing.

so about these psu requirements - why do we need +/-19V exactly?

is +/-18V not enough and +/-20V too much or is there some leeway here? Or are we just aiming for as original spec as possible?

I'm assuming the VRegs are not up to too much voltage and too little causes them headroom (I've forgotten the real term) issues? Any way, would love to know a bit more!

I reckon I'm 75% built It's peanut butter jelly time!

Edit: Ok I re-skimmed the thread, so I can see that not having 19V will affect the bias of the oscillator? I'm still curious to know what would happen if we fed +/-20V or even 24V into it?

Edit more: Gonna check out what I can find lying around - if I can find an LM317/337 based supply I'm guessing it should be pretty easy to modify for 19V assuming it's manly enough. tonights little project...
Tescot
applause
Chok
Thanks Apoisontree 8_) and Thanks Scott for documentation and all things for this big project applause

It is Christmas prematurely and I am going to follow the other cloning with interest, that of the best synthesizer of the world...

mangros
mOBiTh wrote:
just waiting for molex connectors (£100??? Argh...)


woah *gulp*

At least I won't have any trouble hitting the free shipping threshold then...
Chok
I am not going to use connectors Molex but rather to cable boards in wired with potentiometers Alpha separated and new design of face before to make a desktop... meh
oldcrow
mOBiTh wrote:
Just waiting for molex connectors (£100??? Argh...) and pots to arrive and then all I'm lacking is a PSU for testing.

so about these psu requirements - why do we need +/-19V exactly?

is +/-18V not enough and +/-20V too much or is there some leeway here? Or are we just aiming for as original spec as possible?

I'm assuming the VRegs are not up to too much voltage and too little causes them headroom (I've forgotten the real term) issues? Any way, would love to know a bit more!


I imagine 18V is fine, the 4-volt overhead is probably a safety margin to allow for voltage droop on long runs of supply rails as well as dealing with the fan-out of the supply into all those local regulators. If you use "low dropout" regulators that have something like 1.2V overhead, you could probably run it even lower.

One note is the FET buffers on each VCO are tied to the +19V rail, probably to assure the ramp does not flatten as it nears +15V. I have not experimented with lower supply rails yet--I am experimenting with other things at the moment. SlayerBadger!

mOBiTh wrote:
Edit: Ok I re-skimmed the thread, so I can see that not having 19V will affect the bias of the oscillator? I'm still curious to know what would happen if we fed +/-20V or even 24V into it?

Edit more: Gonna check out what I can find lying around - if I can find an LM317/337 based supply I'm guessing it should be pretty easy to modify for 19V assuming it's manly enough. tonights little project...


24V is not a good idea as you would then have the vregs holding back 9 volts and trying to dissipate the extra heat. At that point you would need heatsinks on all of them.

I was going to make a +/-19V supply board using LM3x7 parts at some point as a 4-voice system uses less than 500mA per rail.

--Crow
/**/
oldcrow
mOBiTh wrote:
Hey this smt soldering is a lot of fun - I'm hooked!

just waiting for molex connectors (£100??? Argh...) and pots to arrive and then all I'm lacking is a PSU for testing.


I tried to keep the SMT work as minimal as possible. I just prefer SMT caps as you can put them in close for the ideal bypass spec. Lower ESR, too.

If I had my preference the boards would not have used KK156 series Molex connectors, but as I wanted voice cards to drop into a vintage OB-X I didn't have much choice. They're still better than the connector system a Voyetra-8 uses, however. The only thing that had a flakier connector profile was an old computer kit called the Heathkit H8.

--Crow
/**/
EMwhite
oldcrow wrote:
They're still better than the connector system a Voyetra-8 uses, however. The only thing that had a flakier connector profile was an old computer kit called the Heathkit H8.

--Crow
/**/


No, the worst connectors of all times were used by Gottlieb [Pinball] on the System 1 and Sys80 machines in the late 70s and early 80s.

The fact that there was always batter acid leaking in the general area was a double-bonus but synths suffer from this as well.

mOBiTh
Thanks for the extra info crow!

Ideally I'll find something suitable for testing and then build my own ideal one - I really should buy a nice triple bench supply but I'm not flush enough at the moment.

Those connectors look horrid!!
magman
I found out this evening that an Oakley PSU PCB can be adjusted to 19V quite easily. Coupled with a Yamaha PA30 power supply lump, this should provide plenty of power for a CrOwBX, which is quite good as I had one built already, in the 5U case I was already planning to use. This route also means I don't have to spend time sorting out mains connections inside the case.

I'm also going to connect a CGS66 PSU board onto the other output from the Oakley board to give me a 15V power supply in the same case, for a MIDI interface and one or two other modules. The CGS66 is ideal as it includes a distribution board and it is an easy mod to feed it with a DC supply rather than its normal AC supply.
With a PSU sorted! I can start to get into testing mode. nanners

That can wait till the morning though. Time for bed methinks.

Regards

Magman
thebot
magman wrote:
I found out this evening that an Oakley PSU PCB can be adjusted to 19V quite easily. Coupled with a Yamaha PA30 power supply lump, this should provide plenty of power for a CrOwBX, which is quite good as I had one built already, in the 5U case I was already planning to use. This route also means I don't have to spend time sorting out mains connections inside the case.

I'm also going to connect a CGS66 PSU board onto the other output from the Oakley board to give me a 15V power supply in the same case, for a MIDI interface and one or two other modules. The CGS66 is ideal as it includes a distribution board and it is an easy mod to feed it with a DC supply rather than its normal AC supply.
With a PSU sorted! I can start to get into testing mode. nanners

That can wait till the morning though. Time for bed methinks.

Regards

Magman


Excellent! I thought that might be possible but hadn't got round to reading the user guide again for the Oakley PSU, that's taken a big worry out of this project for me. Guess I'll be ordering some PCBs when they come up then!
mOBiTh
dear mr crow

thank you for including a revA vox board in with my three revB boards just to freak me out at 4am the other morning hihi

this project has definitely kept my brain ticking on my week of nights...

so is there anything I need to be aware of with this revA board other than the errata? I've put in all the same components (ish - some vox boards have more 5% resistors than others, and some different brand ICs) where possible but I believe this board has a few less components overall, is that right? i could well be imagining things, I can hardly see anymore after squinting at those silkscreens all week... Dead Banana

what's the typical lead time on front panels people? still never ordered an fpd one...
mOBiTh
Also I noticed an error on the BOM for the host board - a couple of values were swapped in the pdf compared to the board. I've been a dummy and left my docs at work (not back there for a couple of weeks) but I went with the values on the silkscreen in preference to those in the pdf. Not sure if this was a good tactic or not?!

When I've had some sleep I'd try to figure out which pair are swapped. Anyone else notice it? My brain may be lying to me of course...
magman
I've had a chat with Oldcrow about some of these values already, so here's a copy of this info.

I expect these will make it into revisions of the BOM when he has a bit of time, but I bet he's very busy with other aspects of the documentation (I hope so anyway, as I'm getting very close to testing time nanners ), along with plenty of RL issues.

Quote:
oldcrow wrote:
Heya, sorry for the reply delay, was out of town. See inline comments. --Crow

magman wrote:
Hi Scott,

First, R23 is shown on the BOM as 4K7, but the silkscreen on the PCB shows a value of 33K - what is the correct value for this resistor? If the PCB is correct, then the BOM quantities for these values will need to be adjusted.


The proper value is 4K7. 33K was for single voice kits, but 4-voice needs a lower value.

magman wrote:
Next, R2 and R6 have swapped values from the BOM and what is shown on the PCB silkscreen - can you confirm which value is correct?


R2 is 100K, R6 is 4K87.

magman wrote:
R11 is shown on the BOM as both 1K and 100K. The silkscreen shows the value as 1K, so I have assumed this is the correct value. If this assumption is correct, then the BOM quantity for the 100K value should be reduced to 40.


R11 is 1K. I will adjust the 100K qty.

magman wrote:
Now onto the BOM issues.

C40, C45 and C46 are shown in the list of SMD caps, but they are actually through hole Poly caps, so should've listed separately. C45 should also be in itallics, as it is not needed for a 4 voice.

Next for the resistors, which mainly have quantity issues.

1K - 11 (shown as 7)
10K - 8 (shown as 10)
20K - 1 (shown as 3)
47K - 9 ( shown as 8)


So noted. Will update things here, thanks.

magman wrote:
Lastly, I have a question about some of the presets. I don't think a 3362 type preset will actually fit for R52, as the adjacent components are too close. R83 might fit as a 3362 type, but I suspect it may be tight. I would suggest that 3296Z type pots be used for all presets, at least then adjustments can be made with a front panel installed.


Yeah those should be 3296Z types. I use 3296 on my own host boards.


Regards

Magman
3vcos
HI Michael,

Can you post a link to your BOM or perhaps post an excel file to the the forum? i"m a registered user at Digi and I've spent about an hour on the phone with them. They can seem to find your BOM. :-(



The Real MC wrote:
mOBiTh wrote:
So I want to do a single Vox BOM and then order 4 of them, and tweak up the quantities to hit price breaks - is that even possible?


Sure, that's what it's designed for. I have a BOM in Digikey, it should be public under "analoguediehard"
The Real MC
I put the CSV file here

http://www.retrosynth.com/~analoguediehard/studio/keyboards/oberheim_o bx/crowbx-voice-bom-digikey.csv

It was exported from the digikey BOM manager, it should import right back in. This BOM is only the voicecard, have not ordered for the hostboard yet.

3vcos wrote:
HI Michael,

Can you post a link to your BOM or perhaps post an excel file to the the forum? i"m a registered user at Digi and I've spent about an hour on the phone with them. They can seem to find your BOM. :-(



The Real MC wrote:
mOBiTh wrote:
So I want to do a single Vox BOM and then order 4 of them, and tweak up the quantities to hit price breaks - is that even possible?


Sure, that's what it's designed for. I have a BOM in Digikey, it should be public under "analoguediehard"
oldcrow
Wow, and here I tried to not send out any rev A boards. (There are only 5 of them). It will work fine, they only real differences are the correction shown in the errata and note there are no sustain level trim adjustment trimmers, so you will have to use a fixed resistor around 39K in each case.

--Crow
/**/

mOBiTh wrote:
dear mr crow

thank you for including a revA vox board in with my three revB boards just to freak me out at 4am the other morning hihi
3vcos
Woo Hoo! Thanks MC.

Just wanted to say, it's really nice to be able to put a price on components before you decide to pull the plug on a DIY project. This is a big help.

The Real MC wrote:
I put the CSV file here

http://www.retrosynth.com/~analoguediehard/studio/keyboards/oberheim_o bx/crowbx-voice-bom-digikey.csv

It was exported from the digikey BOM manager, it should import right back in. This BOM is only the voicecard, have not ordered for the hostboard yet.

3vcos wrote:
HI Michael,

Can you post a link to your BOM or perhaps post an excel file to the the forum? i"m a registered user at Digi and I've spent about an hour on the phone with them. They can seem to find your BOM. :-(



The Real MC wrote:
mOBiTh wrote:
So I want to do a single Vox BOM and then order 4 of them, and tweak up the quantities to hit price breaks - is that even possible?


Sure, that's what it's designed for. I have a BOM in Digikey, it should be public under "analoguediehard"
mOBiTh
I had so much 'fun' with the digikey ordering system, I just can't wait to use them again. Even the delivery by UPS was an utter fail...
mOBiTh
oldcrow wrote:
Wow, and here I tried to not send out any rev A boards. (There are only 5 of them). It will work fine, they only real differences are the correction shown in the errata and note there are no sustain level trim adjustment trimmers, so you will have to use a fixed resistor around 39K in each case.

--Crow
/**/

mOBiTh wrote:
dear mr crow

thank you for including a revA vox board in with my three revB boards just to freak me out at 4am the other morning hihi


Haha cool, limited edition hihi

might come back to you later on that when I have the boards in front of me again.


Also thanks Magman smile probably should have reread the thread before commencing as I forgot about the trimmer thing. Will order some replacements today
randel
mOBiTh wrote:
Hey this smt soldering is a lot of fun - I'm hooked!

It's quite tricky doing it left-handed with chopsticks though, I'm gonna buy some tweezers for the next project.



Don't forget to purchase a few extra tempco resistors - one sideways squeeze with the tweezers, and they fly across the room, never to be seen again...
mOBiTh
magman wrote:

Lastly, I have a question about some of the presets. I don't think a 3362 type preset will actually fit for R52, as the adjacent components are too close. R83 might fit as a 3362 type, but I suspect it may be tight. I would suggest that 3296Z type pots be used for all presets, at least then adjustments can be made with a front panel installed.


I can confirm that the 3362 types *will* fit under the following circumstances!

R83: Solder the adjacent 100K 1% after you've fitted the pot - the former will be offset slightly.

R52: If you're making a 4voice with carrier board, most the adjacent components will not be fitted (if I read the BOM correctly!) notably the TL072 that would otherwise be in the way. So this pot type will only need revising if you're making a single voice carrier-free version

M
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