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jojjelito

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Posts posted by jojjelito

  8. AOUT redesign

    in Design Concepts

    Posted

    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.

  10. AOUT redesign

    in Design Concepts

    Posted

    Basically the op-amp offset is the error voltage when the input voltage difference is zero you expect the output to be zero. That is not the case as the transistors inside the op-amp aren't perfectly balanced. Here's an explanation. You can either use external trimming, or the op-amp is trimmed at the factory. There are some low-offset op-amps, for instance it might be a good idea to use TL082/084 for CVs instead of 072/074 (which are generally better for audio) since the 08x has lower offset. Anyway, the error is usally very small but one should be aware that it exists and that there are ways to mitigate it if necessary.

    The gain-bandwidth product is basically telling you something like x Mhz from the datasheet. For the TL08x it's min 2.5MHz, typ 4.0 Mhz. It's typically 3.0MHz for TL074. The 074 is less noisy though so it's usally preferred for audio. OK, what does this mean?

    Let's say you need to make sure that an AC signal of 100kHz can be amplified without too much distortion. In this case you can amplify up to 25-40x before you lose signal integrity. More for a signal of lower bandwidth.

    In the example with the CV mixer we have a gain of 1. A signal of 2.4 to 4.0MHz will pass trough. Good enough for audio, less so for analog SD video.

    All in all, I would take a look at the offset (which is likely to be small), I don't need to worry about the gain-bandwidth in this application since we don't amplify and don't have a high-frequency signal to worry about.

  11. AOUT redesign

    in Design Concepts

    Posted · Edited by jojjelito

    Don't get me wrong; I'm very grateful that you provide input, but according to the stuff I read yesterday the voltage clamp is not complicated at all, and it uses less parts than your suggestion :wink:

    Look at this picture: http://en.wikipedia....lamp.svg&page=1

    I see one opamp, two resistors, a diode and a capacitor. Or am I missing something?

    No, the less parts (marginal, it's only a couple of resistors) is fine. Less parts is always nice. flowers.png

    But, please run a Spice or similar simulation of this (iCircuit or...) and you'll see the effect of the RC constant. Also, I don't think the clamp is OK with slowly changing or more or less constant DC voltages due to the influence of that RC element. It introduces a frequency dependency that might affect any FM or lag or constant voltage CV. The mixer won't exhibit those effects. You might even want to breadboard those circuits for checking.

    For the mixer you only need to worry about op-amp offset (a small problem) plus the bandwidth product of the op-amp. thumbsup.png

    Have fun!

  13. AOUT redesign

    in Design Concepts

    Posted

    Yes, I found that a "Positive voltage clamp circuit using an opamp" is what we want; this will act as a voltage doubler and force the signal at or above 0v. The opamp part is because is introduces percision over the standard diode clamp, which suffers from the voltage drop of the diode.

    Again a bit of theoretical knowledge without knowing the how to apply it in practice. Wikipedia has an article about it but I still don't know which value of components to choose :pinch:

    Link to the article: http://en.wikipedia....er_(electronics)

    Errm, I think that's over-complicating things. Why not just use an non-inverting mixer where you add or subtract 5V?

    You can use a stable precision +-5V reference if you're really picky, but that's overkill IMHO.

    Something like:

    CV In->100k (R3)

    5V offset->100k (R4)

    R2=100k

    100k (R1) plus 22pf cap across (in parallel) to prevent oscillation in the feedback connection

    opamp-sum2.gif

    vo =v1 + v2 (for all resistors equal)

    vo = (R1+R2)/R2 (v1 R4 + v2R3)/ (R3+R4)

    This makes -5V become 0V, +5V becomes +10.

    By adding a -5V you can go the other way.

    Or, you can use an inverting mixer plus a unity gain inverting op-amp to make this less impedance sensitive at the cost of more op-amps used.

    This makes the circuit look like this:

    opamp-inv-sum.gif

    followed by a standard inverting op amp.

  14. AOUT redesign

    in Design Concepts

    Posted

    The rabbit hole goes a bit deeper: The Yamaha CS series also use Hz/V as well as the old monophonic preset boxes Yamaha SY-1 (needs added sockets) and SY-2. Then you have the exotic stuff like Buchla that uses 1.2V/Oct. All of that won't have much bearing to you maybe, but there's weird gear out there for sure... :sorcerer:

    However, uni- to bi-polar: Can't you just add or subtract an offset depending on which way you convert? The slope at 1V/Oct should be the same no matter what.

  23. GM5: Least-cost USB-MIDI Interface Chip for 4.50 EUR

    in Bulk Orders

    Posted

    I definately think there is demand. I advertised the goodness that springs from the GM5 IC around at Mutable Instruments, and this is what filled the list with takers. Others, like Altitude testified to the fact that the interface is really good, stable and fast. I have some extra GM5x5x5 PCBs, but that's why I ordered more ICs than boards this time around. It shouldn't be a problem to sell both the mini-PCBs for Shruthis, Ambikas, and other DIY synths and such as well as shifting the 30 odd existing 5x5x5 PCBs and more flowers.png

    Cheers,

    Reckless youth in Stockholm

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