CV amplifier for Volta output?

Discussion and support for MOTU's Volta software.

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ripe909
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CV amplifier for Volta output?

Post by ripe909 » Tue Aug 25, 2009 11:12 pm

Are there any existing CV amplifiers which would work well with output from a MOTU interface?
I wouldn't need CV control of the amplifier, just a gain knob, and it could preferably be used without a modular power supply/rack case.

All of the amplifiers I could find (except the Cwejman) seemed to have a pretty low maximum output (5V) and/or the gain was just a bias voltage, not a true multiplier.

I would think that something like this would be very useful to get greater range from Volta, I would be using it specifically to control a VCF that is expecting 0-15V for full range control of cutoff.

Thanks
ripe

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doctorvague
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Re: CV amplifier for Volta output?

Post by doctorvague » Wed Aug 26, 2009 7:17 am

That would be fairly trivial to build, other than the redundancy of multiple channels. AFAIK a single non-inverting op amp used as a voltage follower would be all that would be needed. I breadboarded this very thing before Volta ever came out, using a similar (but primitive) strategy to Volta. I posted a long article elsewhere about my exploits.

Image

As an example I got about 2V maximum out of the -10db RCA outs of an older MOTU 2408 interface. If you needed 10V triggers it's simple math of calculating 2 resistor values around the op amp to give 5X gain. Not sure how this fits in with Volta's calibration though, the stuff I was doing was just triggers, not V/Oct, etc. If you're doing v/Oct and not just triggers, you'd want a good quality op amp with low DC offset, etc. I use OPA's for that but perhaps someone else could chime in on that.

NOTE: You do need voltage amplification (gain) for these things, not just an offset (also known as bias, i.e. adding a DC voltage) which will not do what you want.

If I built something like this I'd use trimmers and set the gain appropriately so I'd have known voltages to work with and Volta would have something consistent as far as calibration. The only downside I see is you'd have to be careful feeding voltages to it. If you've got 5X gain and you put 5V in then you've got problems. IOW it would probably need to be dedicated box/module for the task, rather than a general purpose module IMO.

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doctorvague
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Post by doctorvague » Wed Aug 26, 2009 7:27 am

The right side of this schematic is basically a 4 channel version what we're talking about. You could strip away the extra outputs and the LED drivers and use trimmers instead of one of the resistors for each channel.

Image

ripe909
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Post by ripe909 » Wed Aug 26, 2009 8:09 am

Awesome thanks for the info, that may not be too difficult to build.

What would be cool is a dedicated box that has predefined values for each of the various MOTU interfaces, set via a switch or something. That way you could work with a well-defined range of voltages regardless of the interface.

So for example, you would choose you interface type (2408, 24i/o, etc) and the voltage range (1V, 5V, 15V, etc). Although I wouldn't be the person to build such a box!


cheers
ripe

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Metunar
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Re: CV amplifier for Volta output?

Post by Metunar » Wed Aug 26, 2009 9:54 am

ripe909 wrote:Are there any existing CV amplifiers which would work well with output from a MOTU interface?
I wouldn't need CV control of the amplifier, just a gain knob, and it could preferably be used without a modular power supply/rack case.

All of the amplifiers I could find (except the Cwejman) seemed to have a pretty low maximum output (5V) and/or the gain was just a bias voltage, not a true multiplier.
Why don't you take a Doepfer A133 VC-Polarizer.
a. It's possible to multiple the inputsignal by 2.5 (max Output is 10V)
b. CV controllable
c. It's also possible to invert the inputsignal

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Post by Analog>Plugins » Sat Sep 05, 2009 8:08 am

Hi Doctorvague, nice one for uploading those scematics. I only wish I new enough to be able to build it :(

Could you possibly (I know it's a bit cheeky), draw up a simpler version single channel one with easier instructions.

Or maybe I should learnhow to read scematics :doh:

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ndkent
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Post by ndkent » Sat Sep 05, 2009 9:04 am

The Doepfer A-183-3 amplifier is just out.

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doctorvague
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Post by doctorvague » Sat Sep 05, 2009 11:25 am

Analog>Plugins wrote:Hi Doctorvague, nice one for uploading those scematics. I only wish I new enough to be able to build it :(

Could you possibly (I know it's a bit cheeky), draw up a simpler version single channel one with easier instructions.

Or maybe I should learnhow to read scematics :doh:
Try this:
http://en.wikipedia.org/wiki/Operationa ... plications

Scroll down to 'Non-inverting amplifier'
Gain is R2 divided by R1 + 1. In other words, a non-inverting amp always has a gain of at least 1 (unity) or more.
You'll have to figure the resistors based on the voltage gain you need. Use a 100K resistor in series before the input [EDIT - actually I'm not sure about this as I'm used to working with inverting amps - I think I'm wrong about the 100K] and a 1K resistor in series on the output. I'm no expert on this stuff, I just know enough basics to get by. I tend to hook stuff up and try 20 resistors until I get what I want since I suck at math. And I occasionally blow stuff up with that method, BTW. Perhaps someone else can chime in with a better reference for you. It's a PITA to get schematics from my Windows machine to my Mac, etc. or I would do that for you.

I hope I'm giving you correct info here. C'mon all you people smarter than me - you can jump in here anytime!

hope that helps
Phil

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daverj
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Post by daverj » Sat Sep 05, 2009 9:54 pm

I would be using it specifically to control a VCF that is expecting 0-15V for full range control of cutoff.
To get a voltage output that goes all the way to 15 volts requires one of two things. Either a standard op amp powered off of power supplies at least a couple of volts higher than the output range you need, or you need to use newer "rail to rail" op amps.

Standard op amps can only output signals within about 2 volts or so of their power supplies. So a 12 volt power supply can only get an output up to around 10 volts. A 15 volt power supply can get an output up to around 12 or 13 volts.

With a rail-to-rail op amp you can reach the power supply voltage. So to amplify a signal up to a 0-15 volt signal would require standard op amps running off of 18 volts or higher. Or a rail-to-rail op amp running off 15 volts.

The other issue is that if you are amplifying a bipolar DC signal, and maintaining the DC coupling from input to output, your amp needs offset as well as gain.

If you are amplifying an AC signal, like an audio signal, then you can AC couple into the amplifier and preset the DC offset, not having to worry about passing a DC offset.

The non-inverting amplifier that doctorvague showed in his first post is a good amplifier if you don't need both offset and gain. It works well maintaining the DC offset at it's input, or if you are AC coupling to it's input. It's not as simple to calculate gains if you need to mix in a DC offset, because doing so will change it's gain.

If the offset voltage is coming from a constant impedance then the gain can be calculated. But if an offset is involved it is much simpler to use an inverting amp, because an inverting amp has a null summing node which lets you mix several inputs (such as a signal and an offset voltage) without changing the gain of the amp. If a non-inverting path is needed, two inverting amps can be connected in series, with the offset voltage going into either one.

I don't know what range of voltages you are feeding into the amp (I have never used MOTU devices).

If you can define:
- the voltage range going in
- the voltage range you need coming out
- the power supply voltages you have available

I could probably draw you up a simple schematic to do what you need.

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Post by Analog>Plugins » Sun Sep 06, 2009 12:06 pm

Cheers for the reply guys, still a bit over my head though :doh:

I did ask in this forum as well and gfot some other answers.


viewtopic.php?t=7751

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Post by Analog>Plugins » Wed Sep 09, 2009 6:12 am

Would this module do the trick with amplifying my Motu 2408mk3 analog outputs so I can feed my synths a higher voltage for triggering and cv/pitch?

Oakley triple voltage thingy

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Metunar
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Post by Metunar » Thu Sep 10, 2009 6:24 am

Thats a triple VCA. I don't think it makes any amplification.
The gain knob is probably the offset of the VCA.

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krisp14u
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Post by krisp14u » Thu Sep 10, 2009 6:40 am

Analog>Plugins wrote:Would this module do the trick with amplifying my Motu 2408mk3 analog outputs so I can feed my synths a higher voltage for triggering and cv/pitch?

Oakley triple voltage thingy
I think this is what you need

MultiMix

It would need to be built slightly different to the stock version but would X2 your input signal

send me an email with your needs and I'm sure we can work something out for you :mrgreen:
Cheers :guinness:

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ripe909
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Post by ripe909 » Fri Sep 11, 2009 1:28 pm

I decided to go the easy way and get a MOTU 1224 just for high voltage CV.

According to the docs the 1224 has 9.3V on the headphones and 9.2V on the main outs... and they are dirt cheap. I'm hoping the headphones and main outs can have different signals to get 4 usable CVs

cheers
ripe

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1nput0utput
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Post by 1nput0utput » Fri Sep 11, 2009 1:52 pm

If I remember correctly, the phones jack mirrors the main outputs on the 1224.

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Post by Analog>Plugins » Sat Sep 12, 2009 7:08 pm

krisp14u wrote:
Analog>Plugins wrote:Would this module do the trick with amplifying my Motu 2408mk3 analog outputs so I can feed my synths a higher voltage for triggering and cv/pitch?

Oakley triple voltage thingy
I think this is what you need

MultiMix

It would need to be built slightly different to the stock version but would X2 your input signal

send me an email with your needs and I'm sure we can work something out for you :mrgreen:
Just finishing off 6 Oakley modules at the moment. I think I will be putting some business your way for the next modules in a few months as I don't think I want to go through the hassle of ordering the parts to solder again lol.

Will give some thought to the amp.

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