eptheca

MBFM 5V option?

17 posts in this topic

I wan't to build a MBFM, and have got hold of the chips.

I wonder if it is possible to modify the OPL3 module PCB from SmashTV to use only +5V op-amps like the sammichFM.

I would like to avoid the whole 5V & +/- 12V power supply.

 

Sauraen, what did you end up doing?

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The only OPA that we know (proposed by Nils) is SMD based, and not pin-compatible to TL074; therefore it won't be so easy to modify the OPL3 module accordingly. :-/

 

Best Regards, Thorsten.

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I wan't to build a MBFM, and have got hold of the chips.

I wonder if it is possible to modify the OPL3 module PCB from SmashTV to use only +5V op-amps like the sammichFM.

I would like to avoid the whole 5V & +/- 12V power supply.

 

Sauraen, what did you end up doing?

It should absolutely be possible. My concern was not going to a single +5V rail, but using +/- 5V instead of +/- 12V, so I used a more complex setup than below.

 

The op-amp MCP6004 is rail-to-rail at 5V, is suitable for audio applications, and is pin-compatible to TL074 (I used MCP6004 for IC3 and IC5 but TL074 for IC6, and inserted biasing circuits between the two amplifier stages, because I wanted to convert between +5V and +/- 5V). So I would recommend you replace all the TL074s with MCP6004s. But then the one problem I can see is that the circuit around IC6 will no longer work--both because it's a non-inverting amplifier configuration referenced to ground (while your audio signal is biased to +2.5V), and it has a gain x2 which (if the input signal is rail-to-rail) will make it clip horribly. So you have a couple options: make it a unity-gain non-inverting amplifier by replacing R5 and R6 with jumpers and leaving out R4 and C27 (and the corresponding components on the other three channels, look at the schematic); or you could leave out the second stage of buffering altogether, leave out IC6, replace R5 with a jumper, leave out R4, R6, C27, and short IC6 pins 1 and 3 (and the corresponding spots on the other three channels). As far as power, connect +12 to +5, GND to GND, and -12 to GND; and replace all the capacitors between the -12V rail and GND with jumpers.

 

I will guarantee that the MCP6004 will work, in so far as I used it in my build and it works fine; but I will not guarantee that the modifications to the IC6 circuit will work. Buy a couple extra MCP6004s in case. :) (Though none of this should fry the OPL3 or YAC512s, since they're powered from the separate +5V supply, as long as you have that hooked up with the polarity correct, heh heh!)

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Thank you Sauraen,

That is a is a great help.

I'll replace IC3 & 5 with MCP6004's and go for the last option, and leave out the second buffering.

So can I use the same values for the R2&3 and C7-22?

 

On the sammichFM the analog audio circuit, including the YAC512's, is powered by a separate 5V source, what is the benefits of that?

Edited by eptheca

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Thank you Sauraen,

That is a is a great help.

I'll replace IC3 & 5 with MCP6004's and go for the last option, and leave out the second buffering.

So can I use the same values for the R2&3 and C7-22?

 

On the sammichFM the analog audio circuit, including the YAC512's, is powered by a separate 5V source, what is the benefits of that?

R2-3 and C7-14 are for the YAC512s, I would leave them alone. C15-22 are power filtering capacitors, as I said I would replace those ones that were originally to the -12V rail (i.e. C17, C18, C21, C22, C25, C26) with jumpers (because they would be between two different wires that are both supposed to be ground), but leave the others alone.

 

That might actually be a good idea, it should reduce the noise at least a bit. But that will only work if you're powering the synth originally from something higher than +5V; what were you going to use? (Hopefully not your computer's USB port--that'll be noisy enough to ruin your audio from the start! Oh wait, you're using the old 8-bit core, you don't have USB...)

 

I wonder how noisy these DC buck/boost dual +/- converters are? Because it would sure be handy to get +/-12V from a simple 5v supply.

Probably very noisy, but why don't you buy one and test it out? :P

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Could try a charge pump like on the Mutable stuff, LT1026 does up to +/- 18V

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I've seen a charge pump done by simply putting a few electrolytical capacitors and rectifier diodes in the right configuration, that was on a PSU for microphone preamplifiers (those often need +48v for phantom power).

 

I've been working on a headphone amplifier for my father the last 2 weeks and tried 3 different wall warts; all were noisy as hell. Good linear wall warts are hard to get these days. I ended up just using a mains transformer, a few capacitors and an 7812.

 

If you end up building a PSU anyway (because a good off-the shelf 5v PSU cannot be found) it's probably easier to just build a +/- 12v PSU and regulate +5V from the unregulated +12V line; AFAIK the MBFM does not stress the +5v line as much as for example the MB6582.

Edited by NorthernLightX

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linear supplies based on 78xx vRegs are always going to be very sensitive to the DC source noise since they have a very poor noise rejection.  I dont even use unregulated supplies for high current applications, switching supplies are usually cheaper, a fraction of the size, and perform much better than a huge unregulated linear one.  I am just finishing a MB6582 with a 1.6A 15V switching wall wart with recom 5V and 9V switching vRegs and then a 7812 for the 6581s and it's as quiet as any big linear supply setup but no heat and a fraction of the weight

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I am not aware of any Mutable Instr. projects using the LT1026 (he does use the LT1054 a lot). Looking at the datasheet of the LT1026 with 5 Vin you will get to max 6,5-7V out. So that would not be feasible.

 

The simplest (and yet most expensive options) are the Traco TEN3 and the Recom RS3 units. They can convert 5V to +/-12V and still deliver 125mA.

 

@NorthernLightX, For audiophile powersupply options, the LM317 is a better option then the LM78xx series. But there are better options available.

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all use the 1054 other than ambika, I wasn't implying that the 1026 should be driven with 5V though, drive with what is needed to get +/- 12 and then step down off the DC to +5 with a recom.  How much current do the opamps need?

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@NorthernLightX, For audiophile powersupply options, the LM317 is a better option then the LM78xx series. But there are better options available.

 

My point was that a complete ready made PSU that delivers +12/-12/+5/GND is as cheap as €25. If you want to DIY something it must be either because you want to do it cheaper or even quieter than a good off-the-shelf switcher. I'm perfectly happy with my MeanWell switchers myself.

 

The LM317 is a good IC, but the 78xx requires less parts. The headphone amp was for my father because he's getting a bit deaf, not for audiophile purposes. He didn't even hear the switching noise from the cheap PSU's  :rofl:, but I wanted to deliver something at least decent. I only hear a little white noise at full volume (with no audio source), more than good enough for this application.

 

Anyway, I'll stop hijacking this thread and let the experts do the talking :wink:

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Got the OPL3 module from SmashTV today, and finished my MIDIbox FM :)

 

med_gallery_7274_109_47204.jpg

 

 

OPL3 module modified with rail-to-rail 5V op-amps, MCP6004, + separate digital and analogue power

Like the sammichFM, the DACs and the op-amps are powered with a dedicated 5V supply
Second buffer stage left out.

Hardly any noise, and enough gain

 

Thank's for the help, it works and sounds perfect.

 

med_gallery_7274_109_57031.jpg

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Well done!!! Great case, great buttons! :)

 

Could you please create a new Wiki page which explains the MCP6004 approach?

I would like to link it to the MBHP_OPL3 page to inform other builders about this option.

 

Best Regards, Thorsten.

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Geez, you're fast! It took me a few hours to assemble the OPL3 boards for my build, and it's been hundreds of hours over the last two months for everything else. Then again, both of my builds have been big, flashy, and required a ton of custom code (which is still not done for either of them, heh heh!).

Edited by Sauraen

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Looks good, nice work! And glad to be able to help. :)

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