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  1. #1

    Direct Injection boxes for synthesizers

    Hello,

    I was wondering if anyone is using a DI box (or any kind of impedance-matching transformer) to balance the unbalanced 1/4'' analog outputs of a synthesizer in order to record +4dBu audio signals.

    If so, have you succeeded in solving this frustrating "unbalanced-to-balanced connections" problem? What kind of DI box? Passive or active?

    -10dBV signals (unbalanced synth outputs) result in low volume/gain recordings, and I was never able to obtain "hot" signals without having to raise the volume both on the synth (all the way up) and in Sonar 2.2 (all the way up also). This results in an increase of the noise floor, which is of course audible in the recordings (and I don't like to process my audio recordings using plug-ins such as DeNoiser etc. It simply destroys the music).

    There are tons of synthesizer brands out there, but only ONE (as far as I know) makes certain models that offer balanced analog outputs: Kurzweil. Expensive stuff though. But lucky are those who own such units.

    All of today's pro audio soundcards provide balanced analog inputs. I don't understand why those synth manufacturers (almost all of them) don't move on and embrace this very old technlogy..

    Thanks for your responses.

  2. #2
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    By using a DI you are making it possible to take your synths output and put it to a balanced mic level. That signal then goes through a preamp in order to obtain the best level. If you are using a mixer, just plug into any mic channel and you will have success.

  3. #3
    Rrybaker,

    My audio interface (Aardvark Aark 24) has 8 analog inputs, so I don't need a mixer (this card itself is a mixer). Plus all I do is synth stuff (no vocals, no acoustic instruments, no mics, etc.). Therefore no need for a preamp.

    Are you saying that I can only use a DI box with synths IF I use a preamp?

    I thought I would just plug the outputs of a synth into the inputs of a DI box then the outputs of the DI box to the inputs of my audio card.

    Audio interface has no XLRs. All are 1/4" inch analog jacks. Can be set to either consumer [-10dBV] or professional [+4dBu] (the latter feature is what made me wonder about using a DI box to balance the synth outputs).

  4. #4
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    Composer,

    You can indeed bypass the DI and go straight into your sound card. A DI is used to match impedence. Coming from your synth you will be at 50K ohms. A DI takes the 50K ohms and outputs it at 600 ohms. The same as a microphone. This is done with a matching matching transformer so yes the signal leaving the DI is balanced, but at at mic level not line level. You can go straight from your synth to your soundcard. True it will be unbalanced, but unless you are getting noise from interference, don't be too concerned about a balanced signal. Remember, if one part of the chain is unbalanced you still are unbalanced. From your synth to your DI is still an unbalanced signal. If you use the DI you will need a preamp to then go into your line level inputs on your sound card in order to obtain something close to a usable signal.

  5. #5
    Rrybaker,

    The synth outputs are 1/4" unbalanced (-10dBV) and the audio cards inputs are 1/4" and can be set to balanced (+4dBu). Yes, I have always set my soundcard to accept unbalanced signals since the synth outputs unbalanced signals, but my question (original post) was all about "how to make the synth outs balanced so that I record +4dBu signals coming from the synth by setting the soundcard to accept +4dBu signals" because (like I mentioned in the original post), unbalanced audio signals are lower in volume compared to balanced signals, which means I have to raise the gain in Sonar (recording program) to get decent levels, but doing so raises the noise floor as well, which results in lesser audio quality.

    I do have a preamp (that I never used), but even if I use it like you're suggesting, its outputs are XLR (mic) and the inputs of the soundcard are 1/4" (line level). In other words, I'll have to use cables that are XLR on 1 end and 1/4" on the other (balanced on both ends) to be inputted into the 1/4" jacks of the soundcard.

    You said, "the signal leaving the DI is balanced, but at at mic level not line level." According to that assumption, a DI box cannot be used to input balanced signals into balanced 1/4" jacks unless the soundcard has XLR inputs. Then what's the purpose of the kind of cable I mentioned above?

    Thanks for your reply.

  6. #6
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    Composer,

    Let's back up and explain some terminology.

    Your Post:
    unbalanced audio signals are lower in volume compared to balanced signals,

    This is entirely untrue. unbalnced vs. balanced signals have nothing to do with volume. It has to do with a keeping your signal clean. Balanced cables use common mode rejection to cancel noise picked up by outside influences such as radio interference.

    Your Post:
    I do have a preamp (that I never used), but even if I use it like you're suggesting, its outputs are XLR (mic) and the inputs of the soundcard are 1/4" (line level).

    The type of connector has no bearing on the type of signal the cable carries. Although it is common to see mics with XLR, they can be 1/4" TRS or some other plug type. It is also very common to see line level use a XLR connector.

    So, let's think of this in terms of signal level and not balanced or unbalnced or connector type. You have a keyboard that outputs line level at -10dB and you have a sound card that has inputs line level inputs -10 dB or +4dB. Set them to -10dB and you are perfectly matched. By using a DI you are taking a line level source and creating a mic level source. Mic level sources then go to a pre amp which make them line level. By using the DI, all you are gaining is a balnced signal from the DI to the soundcard. You have also added noise to the signal by the very fact that you are using more equipment to get it to your soundcard. If you do decide you want to go through a DI and use the pre amp, you can simply get a XLR - 1/4" converter. However, like I said previously, just go from synth to soundcard. If your signal is too low, you are having a different kind of problem. I hope you can get it figured out.

  7. #7
    Thanks for your reply, Rrybaker.

    Yes, I agree that adding more gizmos will introduce more noise to the audio signal. I wish you were here in the studio, you'd understand what I mean by "low volume" because I also use a recording program (Sonar) so it's not just a hardware thing. The software also plays a very important role in setting the gain, etc. What I meant is that a +4dBu signal is louder than a -10dBV signal. I hear the difference (big difference) in my pro monitors when they (and the soundcard's outputs) are both set to +4dBu versus -10dBV.

    Let's forget about the DI box for a moment. The preamp I have inputs unbalanced signals (on TS) and outputs them balanced (on XLR). So it can theoretically be used between the synth and the soundcard by setting the soundcard's TRS inputs to +4dBu of course, and using balanced XLR-to-TRS cables.

    I am sure you know about the noise floor of audio devices such as synths, soundcards, preamps, etc. Since I have some noise in my recordings when recording at -10dBV (synth straight to soundcard), I am thinking I could use the preamp, so instead of having the synth volume at the way up, I could lower it and raise the gain on the preamp, but not too much of course, otherwise I'd raise the noise floor of the preamp itself. I'm saying this because lowering the volume of the synth will also lower its noise floor. What's your take on this?

    I personally never heard of anyone using preamp to boost the signal of a synth, but it doesn't hurt to ask.

    Thanks again.

  8. #8
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    Your right. You dont' hear about it that often. I would try it and see what you get. Being unconventional is one of the best things an engineer can do at times. It raises our understanding of our equipment and often gives us some really neat sounds. Try it out, if you don't like it try something else. The most important thing is how does it sound, not what I did to get that sound. If it sounds good to you, then you did it right. Let me know what you get...

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