NorthernLightX

AOUT redesign

56 posts in this topic

No, your proposal is sound in theory. However, you need to check if the DAC itself is designed for uni- or bipolar operation, if there's a reference that needs to be applied etc. I haven't studied the MAX525 in detail, but some checking would be prudent.

Share this post


Link to post
Share on other sites

The MAX525 has an external reference on the board. The Original AOUT module has both unipolar and bipolar modes, bipolar is an extension that is currently placed between the DAC and the opamp.

So, I guess a poll is in order; Bias the DAC output before or after the opamps? :phone:

Share this post


Link to post
Share on other sites

No, your proposal is sound in theory. However, you need to check if the DAC itself is designed for uni- or bipolar operation, if there's a reference that needs to be applied etc. I haven't studied the MAX525 in detail, but some checking would be prudent.

According to the MAX525 datasheet, it can be used for bipolar operations. The internal register works a bit different then. The register is set by the software (if you compile it with the right aout module)

Edit: The settings of the internal register correspond to different output values in bipolar mode.

Edited by Shuriken

Share this post


Link to post
Share on other sites

According to the MAX525 datasheet, it can be used for bipolar operations. The internal register works a bit different then. The register is set by the software (if you compile it with the right aout module)

It is not my intention to change a lot to the AOUT module as it was. For example I don't know much (read: anything) about programming at all, so I don't want to mess up anything that is working fine at the moment. I don't want to change the schematic, just combine existing options on one board.

I tried some changes on the board layout last evening, most of which didn't work out very well. I slept a night on it, and came to the following standpoints:

CV outputs:

A DIL connector for the CV outpunts is not possible without laying out the board from scratch again, and even then I think it would be very hard (unless you want to have the CV outputs not sequentially placed on the pins - an order of 2-1-4-3 could be possible but I don't like it that way). A single 9 or 10 pin SIL connector for the CV outputs with a sequential order is not possible either. Both of the above options also don't match the philosophy of the "split" redesign.

Two 5 pin SIL headers on the other hand are possible by changing the orientation of a few resistors. I like this option and have incorporated it in the last revision (not yet uploaded). :thumbsup:

Power:

I've moved the 4 pin SIL header closer to the molex connector. :flowers:

Bipolar or Unipolar:

This was the hard part. First I thought about abandoning the bipolar option on the board and just make it unipolar - adding a bipolar option after the calibrated control voltages leave the opamp is not much harder than doing it in between the DAC and the opamp. The part cost is about the same as well. Making it like this could also be an option for the existing AOUT, AOUT_NG and AOUT_LC modules. Removing the bipolar parts from the board resulted in a lot of unused space, but moving the parts closer together did result in a mess of traces to get the CVs to the edge of the board. This also resulted in the impossibility of adding the opamp based bipolar option to the board. :sad:

Then again, it's entirely possible to leave the voltage devider/summer based bipolar option on the board, and just not populate it. Soldering 8 bridges in place of the 8 resistors in the CV signal path will make the output standard unipolar. It is a less elegant option than what the AOUT_NG has with the jumpers, but that option has a flaw as well as the proposed resistors are different for bipolar and unipolar operation, so changing the jumpers is not enough; re-calibration is always necessary.

With the soldered bridges I proposed you have no simple otion to change the operation from uni- to bipolar. You'll have to decide which channels will be bipolar and which will be unipolar when building the module. Changing your mind later is possible but you'll have to desolder parts; replace the bridges with resistors, and probably change some other resistor values as well (and of course re-calibrate afterwards). This is still the best option in my opinion.

It's also still possible to add an opamp based bipolar option to existing designs with an extra piece of (vero-)board, the schematics mentioned in this thread should be enough for experienced builders to get started. As soon as I'm going to build the SSM filters I will revisit the idea of the opamp based CV changer to have the modules respond to +/-5V on both channels.

Thus, the only thing I still need to know is how the resistor values for R_x and R_y should be calculated with the voltage devider/summer based bipolar option. Can anyone please give me a an assist? I'll score the point if you let me :D

Share this post


Link to post
Share on other sites

The bipolar option does not require any reprogramming. The circuitry is slightly different. But the values to be set in the register are the same as for unipolar. Just the resulting output is different.

Edited by Shuriken

Share this post


Link to post
Share on other sites

The bipolar option does not require any reprogramming. The circuitry is slightly different. But the values to be set in the register are the same as for unipolar. Just the resulting output is different.

If you feel like elaborating on this: be my guest :flowers:

I'm content with the solution I've come up with so far, but I'm Always interested in additional viewpoints :thumbsup:

Share this post


Link to post
Share on other sites

If you feel like elaborating on this: be my guest :flowers:

I'm content with the solution I've come up with so far, but I'm Always interested in additional viewpoints :thumbsup:

Checkout the datasheet, page 14, figure 11.

Edited by Shuriken

Share this post


Link to post
Share on other sites

Checkout the datasheet, page 14, figure 11.

Doesn't make sense to me at all. Way beyond my skill level. :wacko:

Share this post


Link to post
Share on other sites

Went over the schematic again and made some minor changes to the MAX525 power integrity capacitors. Also renumbered all parts to be sequential now. Just uploaded the most recent revision of all files including pictures.

PM'd TK about the CV plans for MB CV V2 and CV standards and such, and he replied he'll look into this once he has the time. :thumbsup:

Share this post


Link to post
Share on other sites

I got following question from Alex:

I don't want to bother you with all the details, but since I read you have been working on a new Midibox CV, I was wondering if you are considering a CV standard for the Midibox platform. I am of course talking about unipolar or bipolar CV. Of course a "standard" is only a guideline, nothing keeps anyone from taking a different path.

Considering the analog synths that I used in the last years, I would say that the bipolar option (+/- 5V) is the better, because most flexible choice.

Best Regards, Thorsten.

Share this post


Link to post
Share on other sites

Thanks for the answer TK, I will definitely continue the bipolar path then. :thumbsup:

I looked at the LPC core documentation, but could not find if it's possible to connect an AOUT module and if so to which port (I wanted to see if I can add a port with matching pinout). Am I not looking well enough, or is the information not public yet?

Share this post


Link to post
Share on other sites

Here's an interconnection example, which is very similar for the LPC17 core (J19 is used as well, this port is just located at a different location)

http://www.ucapps.de/midibox_seq/mbseq_v4_interconnections.pdf

For a redesigned AOUT module it makes sense to use a 2x5 pin IDC header for a 1:1 connection to the core.

Best Regards, Thorsten.

Share this post


Link to post
Share on other sites

I added the 1:1 DIL header for the 32 bit cores. I made only one such header since the option to split the standard 8 CV AOUT into 2x4 CV is not useful for 32 bit Cores IMHO; the only application I can think of that would benefit from this option uses 8 bit Cores (MBSID and MB6582). If you would want to use the split option with a 32 bit Core that's still possible; you just need to connect to the appropriate pins on JP2.

@TK, can you perhaps shed some light on the R_x and R_y values (with 500R trimpots and the bipolar option)?

Share this post


Link to post
Share on other sites

you know, if you're going towards +/- 5V, you can easily generate that from a positive DC source by using a LT1054 and a 7805/7905 vRegs (like the shruthi). This would get eliminate the need for a complicated power source..

Share this post


Link to post
Share on other sites

I agree, this is a good measure to simplify the PSU

R_x/R_y: I've to search in my records for the best matching resistor values with bipolar option, this can take some time (because I'm currently not at home...)

Best Regards, Thorsten.

Share this post


Link to post
Share on other sites

Regarding the resistors: I'm not in a hurry, so no problem :)

If you on the other hand remember what part of the formula (to calculate the values) needs to be adjusted maybe I can work it out myself.

Regarding the PSU: don't the opamps need the +/-12V for proper operation? Also, good chance that additional modules will be in the same housing as the AOUT that use +/-12V as this is one of the standard supply methods of synth modules. Well performing switching PSU's are nowadays available for as little as €25 (from Mouser and Reichelt for example) so I don't see the need to divert from the current form of power supply.

Share this post


Link to post
Share on other sites

You are right, the PSU voltage should be higher than the operating range, otherwise the amplified voltage will be clipped around the min/max voltage.

I would also recommend to use a rail-to-rail OPA (such as LM324) instead of the TL074 in this case.

A PSU which outputs +/- 6V should be sufficient (if I'm not wrong now, maybe somebody can confirm)

Best Regards, Thorsten.

Share this post


Link to post
Share on other sites

A PSU with +/-6V is not really more simple than one with +/-12V IMHO :whistle:

But if +/-6V has other advantages or preferrence, I'll take over this recommendation.

Share this post


Link to post
Share on other sites

I'm no expert with modular synths, so apologies in advance if this is nonsense!

But, I thought the 1v/oct standard has a range of 0-10v. If the opamps are running off of +/-5v then doesn't that mean you can't get 0-10v from the AOUT?

I already have 1 original AOUT and personally I was planning on doing 4 channels as nonbalanced i.e. 0-10v, for controlling the pitch of VC oscillators, and 4 channels balanced i.e. +/-5v for doing LFO / modulation type tasks.

If the new board can't get above +5v, doesn't it mean the new AOUT board would be limited in this sense?

Share this post


Link to post
Share on other sites

I'm no expert with modular synths, so apologies in advance if this is nonsense!

But, I thought the 1v/oct standard has a range of 0-10v. If the opamps are running off of +/-5v then doesn't that mean you can't get 0-10v from the AOUT?

I already have 1 original AOUT and personally I was planning on doing 4 channels as nonbalanced i.e. 0-10v, for controlling the pitch of VC oscillators, and 4 channels balanced i.e. +/-5v for doing LFO / modulation type tasks.

If the new board can't get above +5v, doesn't it mean the new AOUT board would be limited in this sense?

It has already been discussed in this thread, but I'll give some quick answers based on my current knowledge;

- The original AOUT standard gives a 0-10V response.

- The original AOUT can be expanded (for example with a daughterboard) to give a +/-5V response.

- The redesigned AOUT can give both 0/10V response and +/-5V response without the need of a daughterboard, the response is configured by replacing some resistors with bridges. Attention needs to be paid to certain resistor values (R_x and R_y) with each configuration. The configuration options will be documented as soon as a final design is accepted.

- Both on the original AOUT and the redesigned AOUT the CV response can be configured per channel.

- 1v/oct response depends on the used (target) module, and if it expects 0-10V or +/-5V. In both voltage flavors you have a 10V range.

- It is still possible to use a level shifter to convert +/-5V to 0/10V and vice versa. A level shifter is a simple opamp design. I plan to build a set of shifters as a separate module, and will also build some as a preconfigured expansion on several modules that I have that currently expect 0/10V on the CV input.

Edited by NorthernLightX

Share this post


Link to post
Share on other sites

Hi NorthernLightsX. Maybe I was not clear in my post, sorry!

To put it another way, based on this statement:

A PSU which outputs +/- 6V should be sufficient (if I'm not wrong now, maybe somebody can confirm)

My understanding is that if you run the opamps from +/-6V then you can't generate the 0-10V which we want for controlling VCOs.

Personally I'd stick with running the AOUT from +- 12V to +-15V, which in my experience is what most analogue synthesizers are using.

Hopefully that clarifies what I meant :)

Share this post


Link to post
Share on other sites

Err, true. Level shifting is a fine tactic, but there's no way you can level shift beyond what you can feed your opamps with. One way of doing it would be to have a +-6V PSU, plus use a step-up regulator to get the 10-12V needed for the opamps positive supply.

Or, no inductor dirty tricks, no switchers, just make a proper +-12V or +-15V PSU and feed your final stage opamps that. You can get away with something very close to -5V to +10V PSU if you use rail-to-rail amplifiers. But, that means accepting another constraint versus the same complexity for the bi-polar PSU.

Confused yet? There is a third choice: +12V unipolar DC supply, plus a LT1054 inverter feeding a7905. The positive power is already present, but it can be stabilized using a standard 7810. This can feed a R2R amp -5V to +10V from a normal wall wart. However,the 1054 costs approx 3.50-4.50Eur or about 3.70Usd, depending on where you buy it...

Cheers flowers.png

Share this post


Link to post
Share on other sites

If you ask me a regular +/- 12V PSU and the original method of powering the module, opamps and everything still sounds the easiest, and you gain a PSU for all your other bipolar PSU modules at the same time :)

Share this post


Link to post
Share on other sites

Is it really required to provide 0..10V for controlling VCOs?

This was also my first assumption at the time I started with the MBHP_AOUT module and searched for DIY solutions in the web to get a basic understanding about this topic.

But in the last years I got my hands on analog synths which always provided a TUNE or FINETUNE knob to set the basic offset.

From this offset it was possible to play keys from -5 to +5 octaves.

Some of these synths even didn't allow to play the lowest (audible) frequencies without a negative voltage.

So: which synthesizer (or VCO module) really needs the positive 0..10V range?

Best Regards, Thorsten.

Share this post


Link to post
Share on other sites

When googling the subject of unipolar or bipolar control voltages there seem to be equally many modules (not VCO's per sé) that use unipolar 0-10V as there are modules that use bipolar +/-5V. Both standards give a range of 10V, so with my limited knowledge I guess there isn't much difference in operation other than a +5V offset.

The level shifting I was talking about earlier I would either build as a separate utility module powered by +/-12V, or as an input expansion on the module that expects to see 0/10V on the CV input. Such a module is most probably powered by +/-12V or even +/-15V so power for a small level shifting circuit can be taken from there if you don't want to power the AOUT with +/-12V.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now