Weird preamp behavior

niclaus

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Dec 28, 2007
Location
Burbank, CA, USA
hello guys,

I have kind of a weird situation here. Let me explain.
We are working with that guy who developed a great machine that uses lasers and such to read sound out of different supports.
The technology is great and everything but there is one thing I don’t understand.
The guy came and plugged the audio outputs of his thing into a RME quadmic preamp. So I went “oh no, I have better preamps you can plug that into” and he says “actually no, this only works with that rme pre”. Since I couldn’t understand why, I still plugged the audio output of his rack into our new grace preamp. All I got was a really distorted signal, not usable. Went to the RME. Perfectly fine. Tried with a presonus preamp, weird distortion again...
So he’s right. The audio outputs of his machine only work with that RME quadmic pre. But I do not understand why! I am thinking some kind of impedance thing but still, I find it really weird.

Could it be that his audio outputs are not really proper line audio outputs and that for some reason, only the RME is, by luck, designed to be ok with his signals!
Anyway, if some tech guy has any input here, i would be glad to hear it.

Thank you and good day!

N.
 

bouldersound

Real guitars are for old people.
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Jan 23, 2010
Location
Boulder, Colorado
That sounds a bit like what can happen when you connect a high impedance output to a circuit that isn't substantially higher, like running a piezoelectric pickup into a typical mic pre.
 

niclaus

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Dec 28, 2007
Location
Burbank, CA, USA
Yeah that is what I thought.
But I forgot to mention something.
So let’s say that we are using the RME, when gain is turned all the way down, the levels are ok, not too high but ok. When you start adding some gain everything is fine for a few dbs, but then you cross a point when the pre starts indicating some clipping and starts distorting like hell, even though the output levels are not that high. And it is on and off. One minute you are good and then you add just a little gain, and boom, clip led and high distortion!
I am puzzled.
Does that also sound like what you are mentioning?

N.

PS : thanks for your answer by the way.
 

niclaus

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Dec 28, 2007
Location
Burbank, CA, USA
I guess you mean line output.

I am sorry but i have to disagree with that.
A lot of preamps can totally handle line level. And the ones I have do. That is not the issue here. Nobody’s clipping or anything.
 

Kurt Foster

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Location
77 Sunset Lane.
I guess you mean line output.

I am sorry but i have to disagree with that.
A lot of preamps can totally handle line level. And the ones I have do. That is not the issue here. Nobody’s clipping or anything.

i've been recording and doing audio for over 40 years. i think i know what i'm talking about.


A line-level signal is approximately one volt, or about 1,000 times greater than a mic-level signal. ... Connecting a line-level source (such as mixer output) to a mic-level input will cause the sound to be loud and distorted because the line signal is much stronger than what the mic input will accept.

Line level is in the region of 0 dBV (1.000 volt). A line-level signal is approximately one volt, or about 1,000 times greater than a mic-level signal. Connecting a microphone to a line-level input will result in almost no sound at all because the mic signal is so faint that the line input cannot hear it."


https://www.thebroadcastbridge.com/...e-difference-between-line-and-mic-level-audio
 

niclaus

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Joined
Dec 28, 2007
Location
Burbank, CA, USA
Yeah and when you add 30db you multiply your signal by 1000.
Most preamp have more 30db of gain.
Again, most preamps can totally handle line level inputs, but I am not here to argue about that.
That is not the problem here.
 

Kurt Foster

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Jul 2, 2002
Location
77 Sunset Lane.
i think you may be confusing things a bit. many "mic pres" have both mic and line level inputs. if you are putting a line level signal into a mic input you will get distortion, even if you aren't "clipping" the signal in your DAW. it's the same thing as a distortion box in front of a guitar amp. you are overdriving the signal.
 

niclaus

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Joined
Dec 28, 2007
Location
Burbank, CA, USA
Ok then, from the grace manual for example :


AUDIO CONNECTIONS
Microphone Inputs: Female XLR, pin 2 positive, pin 3 negative and pin 1 ground. 48V phantom power is supplied on pins 2 and 3. With a gain range from -6dB to 69dB these inputs can be used for any type of microphone or as line level inputs.
 

niclaus

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Dec 28, 2007
Location
Burbank, CA, USA
You know what, i got carried away here, that is not even the problem.
When I take this signal that is supposed to be line and feed it to the RME (which is supposed to be mic level), it works. When I feed it to a line level input of some sort I have distortion.
If I follow what you are saying (and which I understand perfectly) that would be the other way around, right?
I mean, if I had distortion in a mic input, I would understand, the signal is too hot, impedance is wrong, I should get distortion, but I don’t!
I get distortion in a line level input!
Which is what I don’t understand.
 

Boswell

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Apr 19, 2006
Location
UK
My guess is that the output from his gear has a d.c. problem. This could take the form of different d.c. levels (unbalance) on the positive and negative outputs, or else an output overload or instability when connected to a low-resistance d.c. path such as a transformer input of a high-end pre-amp.

I would make the following test: use an XLR splitter cable (not an insert cable) to connect the two pieces of gear, and then take measurements with an oscilloscope or a multimeter at the open split end. What you need to read is the d.c. voltage between pins 2 and 3 (multimeter only), or else the two readings between pins 2 and 1 and between pins 3 and 1. Try this when the RME is the receiving device (it has a true differential input) and then with the Grace or another pre-amp that causes the distortion problem. Transformer-coupled inputs require true balanced input signals to avoid core saturation due to excess d.c. flowing in the windings.

This is an illustration of just one of the differences between audio and instrumentation setups.
 

niclaus

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Dec 28, 2007
Location
Burbank, CA, USA
Thank you so much Boswell. That makes total sense.

I will try to make those measurements as soon as I can.
So if that is a DC problem, is there a remedy that you can think of? You would probably have to see the unit itself I guess, but anyways, if you ever think of something on the top of your head, let me know.

Thank you so much.

N.
 

Boswell

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Apr 19, 2006
Location
UK
I would avoid transformer-input pre-amps for this job. If you need the variable gain that a pre-amp gives you, stick with the RME Quadmic. Otherwise, if the setup can cope with a fixed 0dBFS level of (say) +18dBu, then try something like the Audient ASP880, which has balanced insert returns direct to its ADCs.

I would class the use of a suitable pre-amp as a "remedy". If you are obliged for other reasons to use one of the problem pre-amps, try a large-ish capacitor in series with one pole of the input. A 100uF 10V electrolytic should be sufficient, but be sure to connect it with the correct polarity to match any unbalanced voltage. What's the frequency range of interest in the signals coming out of this rig?

BTW, a voltage gain of x1000 is 60dB.
 

niclaus

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Dec 28, 2007
Location
Burbank, CA, USA
Really interesting. Yeah, I need the variable gain as sometime the output level of that thing is quite low, depending on what it is reading (old cylinders for example).
But even with the quadmic, we do have some weird things happening. Like I said earlier, sometime you just want to add a little gain, but for some reason the preamp starts acting up. When you pass a certain point, a lot of noise starts appearing. And it is not the background noise beeping brought up, that is something else. You add one dB, and boom you have 10db more noise. As if it was reacting to something when you were turning the pot.

The frequency range of interest? Well, it is supposed to be audio so i’d Say up to 20000hz, but the reality of things is that the bandwidth of the material we are reading is not that wide.
I am not obliged to use another preamp, I can stick with the quadmic if that is the best option here.

Anyways, I am going to make the measurement today and let you know what I read.

Thanks a lot for your help.

N.

Oh, and by the way :

BTW, a voltage gain of x1000 is 60dB.

Yes, you are correct (as always), sorry, I got mixed up with power (Saturday night!). But that is still making my point.
 

niclaus

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Joined
Dec 28, 2007
Location
Burbank, CA, USA
So when plugged into the rme the d.c. measurement between pins 2 and 3 shows 0.15v.
With the grace 0.40v.

What do you make of that? there is less D.C. current with the RME but there is still something? Which would justify why it behaves better but still erratically?
 

Boswell

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Apr 19, 2006
Location
UK
But even with the quadmic, we do have some weird things happening. Like I said earlier, sometime you just want to add a little gain, but for some reason the preamp starts acting up. When you pass a certain point, a lot of noise starts appearing. And it is not the background noise beeping brought up, that is something else. You add one dB, and boom you have 10db more noise. As if it was reacting to something when you were turning the pot.
I've met this type of effect before, and at that time it turned out to be due to h.f. oscillation being rectified and turning into audio noise. It may be nothing of the sort in this case, but it's worth putting an oscilloscope on the equipment output lines and checking for oscillation.
So when plugged into the rme the d.c. measurement between pins 2 and 3 shows 0.15v.
With the grace 0.40v. What do you make of that? there is less D.C. current with the RME but there is still something? Which would justify why it behaves better but still erratically?
Yes, interesting, but both those values are uncomfortably high for a transformer-input pre-amp. The d.c. input resistance of the Grace would be only a few tens of Ohms. Did you happen to note the voltage at the equipment output when it was open-circuit?
 

niclaus

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Joined
Dec 28, 2007
Location
Burbank, CA, USA
Hey Boswell,

So yesterday I copied pasted your first message to this inventor guy. Here is his reply :

“Well - if only he knew how right he is :)
only, its not a problem, it is the very concept.

By nature the output signal is asymmetric. It is a wonder that it works so well, but it does.

Greetings to your friend.”

So I guess I just have to accept that for some reason this crazy machine works as it does with the RME...

But thank you so much for your help Boswell. You were spot on!
 

niclaus

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Joined
Dec 28, 2007
Location
Burbank, CA, USA
Or maybe I should try and understand what is special about the RME and find something with which it would work even better. But my guess is he already tried that.
Crazy inventor guy!!
 
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