Audio out preference (RCA, 1/4", Mono, Stereo)?

Design Concepts
1

For my MB-SID, I’m trying to figure out the best method wiring up the audio out. Originally I was going to use 1/4" TRS jacks in stereo (just like the MB-6582). It occurred to me that I have plenty of room so perhaps I should use 1/4" Mono jacks or RCA outs instead?

I will be eventually using a sound card that has balanced I/O but offering that for each output might be expensive. So I was going to stick with passive output designs (except for a mix out which will either use a plain op-amp or will offered balanced outs - not sure yet).

Just sort of taking a census of what people are doing and how well it seems to work. I know the stereo 1/4" jacks save space but since I don’t think that is going to be an issue, and since I don’t like playing the adapter dance at RatShack, I’d prefer something that was a bit more straightforward in terms of connections.

Advice welcome!

2

use 1/4 mono, and forget making them balanced, there’s not much advantage with balanced outputs in this application. Just have one 1/4" mono trs for each sid, and then a master L/R pair for the “Mix out” This will give you lots of routing flexibilty to add different effects to each sid.

3.5" sucks, the only time you would use this is for headphones.

RCA is lame too because you’d be using all kinds of adapters to hook it to a mixer.

3

That was my thought as well. I think my mix out might be balanced though just for funzies since I was going to use an op-amp anyway (and the line drivers have op-amps in them).

For the mono, I assume you hook RS to ground (so the tip is the audio)?

4

Yup! If it’s mono, ground the ring and sleeve together. Or use a mono connector with no ring.

5

w00t well I’ll have to redo my audio out board a bit now but sounds like that will work well. I think I’ll keep with my modular idea (trying to figure out if I should have each module be a stereo pair or just make it mono which would make things require more space but lead to more flexibility)…hmm..

6

This imo depends on what the use is.

Personally I prefer balanced outs. And xlr. I use a PCB with 4 opamp buffers. The first two buffers is tied up to a headset amp PCB, and the other two is gonna be tied to a couple of output transformers that goes to the xlr cons.

The buffer also has panning pots, so I have full stereo control on the midibox from the two pan pots.imho active circuit with balancing xformers at the output is the advanced, but the way to go. I made it that way to get headphones out actively seperated from main outs. A passive design, when using several outputs would cause change in volume when u plug in the second output… U know what I mean…

As said before, it all depends on what use. I would in all cases run away from rca and 3.5mm jacks. They work yeah, but it doesn’t stand any real use. Xlr is the most rigid, but 6.25mm trs also works good as long as it is named neutrik hehe. Balanced outs means no di boxes when on gigs, and also fits pro audio standards. Also u electrically isolate the synth from the foh if u use transformers, wich is the only real way to balance the outs…

7

Have any info on balanced outputs with transformers? I was thinking of using an integrated audio line driver, but only for the mix out (which I don’t expect to be all that amazing since, as you mention, it is a passive mixer design). I was thinking about adding volume pots to help adjust the mix out but typically I just live with the audio levels being all over the place until I go to master the track anyway.

I’d like to have balanced outs for everything but that could push up the costs quite a bit, particularly if I rock transformers. Since the synth will be in the same rack as my sound card (probably right above or below it), the cables will be probably 1-2 feet.

I thought about XLR connections but the sound card I plan on upgrading to has 1/4" anyway so might as well keep with it. I don’t plan on moving things around much and if I ever did a gig, I’d just pick the whole rack up and move it.

8

Yeah, this sounds cool. I got some transformers incomming any moment, or that is, some small radioshack toys with transformers in it that sounds very good, says smashtv. So with massive help from him I get some of those shipped to Norway soon.

I have a few circuits soon to be ready, but not tested yet, and also some links and info on output xformer circuits I used for inspiration, since I’m relative new to analog audio electronics. I don’t have them here, since im on the phone right now.

But as I mentioned in ur thread on balancing mb6582, it would be very cool to share some ideas and schematics, since we work on very similar stuff atm.

I’ll try to prepare a proto of the pcbs I designed soon. I still have no etchant, so I have to wait some days for it to arrive before I can make the pcbs. I’m all outta protoboards. But I guess I don’t know how well the design work before I get it on a PCB.

My design is using dual rail 15v psu, so I made a schem for that too. Basically a simple psu with the +/- 15v, a 5v rail and a 9v rail that can be built as a 12v rail. Makes it practical to use with a mbsid with active mixing circuits.

Well thus was just some random chitchat, but as I said, it would be interesting to work together some on this stuff!

9

Yeah I was going to go with +12, -12, +5, +9 PSU. I may use one of those linear ones found in the epic PSU thread a while ago, or I might make my own. Not sure yet. I’ll post info about my stuff in the other thread (and yes, I’m happy to share!)

10

Nice. I have a simple 4ch opamp output board ready tonight, it would be cool to compare PCB design, to see if I’m totally out there or… Hehe.

The board accept line in and the outputs can be used as is for line outs or u can hook up xformers to its output and you have balanced outs.

The big advantage of seperating outs with buffers like this is of course the impedance stuff.

I went with 15v since most opamps will accept it, and the higher voltage, the more headroom. 12 to 15 volt is like 6v difference peak to peak. I don’t see any problems to build a PCB that u can only swith the vregs to support both voltages wo any other changes… Apart from the mains xformer ;-).

I also have a uber psu going for the mb6582, wich features rail voltage sensing and a mute relay on all outputs. This way the psu will not go out of standby unless all caps on the input is charged. Also work as a inrush current sensor, so basically the psu doesn’t supply anything to the load unless all rails are stable. This also means that if one rail goes down for some reason if will immediately go into standby and cut power to the load on all rails.

Most mboxers call that overkill, but I see no disadvantages at all other than a more complex design involving conparators and power resistors. But no big problem. Only thing is I am not sure how much noise the sensor resistors will add to the psu, but again, werent we talking about sids hehe, not hifi audio hehe.

Problems with this kinda design is that if the psu experience instability over and over again, you might sit there with a psu that does hickup restarts, so I might make it smarter with a psu controller ic of some kind to put the psu in standby until a cold start is executed. That way you will be able to have control if something goes wrong. I doubt that it is very healthy for any equipment to experience brown outs like a hicupping psu would do to it.

I’ll share some eagles with ya tonight when I get home from work.

As for me getting the design before publishing, its not important to me at all, but the design should work flawless before u post it though. I am no long term electronics expert, but if I have a look at yours and you look at mine (hehehe trying to not think perv here hehe) both designs could end up as good as can be ;-).

11

Wow that PSU design sounds *nice*. A design like that is one reason I was thinking of just buying an off the shelf linear PSU, though they come with a non-trivial price-tag.

I didn’t think about the 6V swing between 12 and 15, good point. I went with 12 just because that’s a popular choice with the filtering stuff (the AOUT, CV, SSM stuff). Don’t know if 15 would be tolerable for those but if it would I could just as easily use that.

All my outs will be going into my digital capture stuff so loudness isn’t as much of an issue as it is to have clean audio. But “clean” is perhaps a loaded term when it comes to a SID chip :slight_smile:

As far as my designs, not all of them have been done on a protoboard or anything but most of them are simple enough to know it works in theory. There was one design of my audio jack module that had the pins on the wrong side, oops :slight_smile: but other than that those boards can’t really be any more simple.

For the modular boards, I am wanting to use a wire-bus such that there is a single run of copper from start to finish (SID1 to my op-amp board as an example). Each board in the middle would simply tap into the wire. That was suggested from my local hardware group here because it is easier to debug and, while you can have modules fail, the audio bus as a whole will be fairly sturdy.

The problem is that I was going to use the compression crimp DIL connectors but if I go with a mono solution, I would be wasting cables and connectors since I would only need to have 2 wires. So I’m trying to think of something that will do the same thing but perhaps with a bigger gauge wire and which would work with just 2 wires.

12

Yeah, everything I have also is going into the daw. More headroom should mean better s/n ratio within the system, therefore the 15v.

My main issue up till now with the big psu is that I might have set too big standards when it comes to current output of the 5v rail. Was designed to be good for 5 amps, as I was thinking to use it for a psu for more units too, but I will lower the demands down to 2amps on 5v, and it will solve the main issues that kept me from realizing it before.

In the meantime, my WiFi drivers on the eagle computer has done something funky, coz I can’t get online with it. However, I finished the buffer board and post it when I get it online again.

13

To dawg. I have now posted all the designs I got regarding audio design. Including the simple psu. Some pcbs could easily be on the same board, buy free eagle constrain me to keep the PCB size down. Have a look, and comments are appreciated. I will etch the boards as soon as my FeCl arrives, and then some build time, and then we will know for sure how they really works.

14

Where exactly did you post them? :slight_smile:

As far as the S/N, good point, although I may stick with 12V for simplicity sake since the AOUT_NG requires 12V (and I assume the SSM stuff does too) and I already have to juggle 5V, 9V, -/+ 12V. At some point, space may be an issue if I wanted to stuff the synth with everything I want to cram into it :slight_smile:

In terms of FeCl, have you considered Cupric Chloride? It can be started by using Muriatic Acid (HCl) and Hydrogen Peroxide (O3) as a starter solution. It will etch super quick with that but will slow down considerably, and require oxygenation (or continual adding of peroxide, which can dilute the solution) so it takes a bit more work. The benefit is that it will last far longer as it can be used multiple times before you need to add more HCl or dispose of it. Slower etch times but better bang for your buck as far as home etchants go. Plus it makes a pretty green liquid when it’s properly saturated with oxygen (that’s how you know when it’s good for etching).

15

Good tip… I have looked at hydrogen peroxide stuff before, but for a completely other issue. It is really hard to get the right consentration ratio in small quantum here. And u have eBay, but the customs are so much hassle with importing liquids like that. So I think I will stick to the FeCl since I know how to use it, and it is way easier to get here.

The designs are posted as pics in the gallery. I know u already found em, but anyway if others didn’t know… Hehe

16

Muratic Acid is available at the local hardware stores here, or probably pool stores. I don’t think you would need to important and certainly don’t need to be going to eBay - HCl is a fairly common acid and you don’t need it in a concentrated form. With Muratic Acid (which I think is 30%) and peroxide, boards etch extremely quick until you start making the Cupric Chloride.

Here is some further instruction on how the whole process works. It’s rather detailed depending on how interested you are in the chemistry.

I did indeed find the boards in the gallery :slight_smile: I’m going a tad bit more modular with my I/O boards but otherwise we appear to be mostly on the same page. I am still debating on the best option for the PSU though. I already have an MB-6582 compatible PSU which I can likely retrofit into my rackmount SID and use until I need to start doing the bipolar stuff. I’ll need that for any op-amps or line-drivers I have and for the AOUT stuff but it may get me going while I work on the control surface and things.

17

I might have to check the right places for the chemicals… I might look into it when I have used up my 2kg of FeCl hehe.

As for psu, the only reason I have bipolar rails in there is for the opamps at the output board, and of course for the headphones amp. I’m not doing any other bipolar stuff with the stereosid, so that’s why I’m doing it 15v.

For the mb6582 psu I will not include any bipolar rails either. Oof that psu scares me a bit. It’s gonna be some work routing the PCB, especially within the limits of free eagle… I have made several circuit designs, but its some time ago, and I already see I forgot the details of the operation hehe, gotta do some sim work to fresh my mind up a bit, then I will transfer it to eagle for the PCB layout making. I will have to go back on one of my main demands for the design, to be able to pump out 5amps of the 5v rail. I do not longer need that since the mb6582 is the only load, I planned on using it as a central psu for other controllers, but now I have simple PCB for psus, so if u need another one ill just etch it and solder it together, no big deal…

So I am gonna try to see if 1,5 to 2A is enough… I get away with TO-220 cases then. And a slice of my heatsinks too hehe. No bipolar rails also means less concern of isolating the vregs on the heatsink too.

I am scared of beginning on the designing of it, but on the other hand a part of me just can’t wait, so I think I’m just gonna start on it, and we ll see how it goes hehe.

18

An MB6582 PSU should fit well within Eagle I would think. I’m using Eagle Standard now, but the MB6582 PSU I made I used Eagle Lite for and I’m pretty sure I used light for this one as well, though I haven’t built that one yet (and will probably end up modifying it before I do).

19

A standard PSU will sure fit in free eagle :). But I don’t know about the one I have in mind. It contains a lot of ICs (vregs, vrefs, comparators etc) and also relays. So it will be fun to see how it works out.