Idea for a 'Re-Preamp' box

[brazilianwonderboy]

I was lying in bed last night thinking about a preamp that I am building and I started thinking how cool it would be to have a way of recording the information that comes directly from a microphone without putting it through a preamp or other signal altering gear. This would be for the pourpose of being able to compare the effects that different preamps have on exactly the same piece of audio. So for example if I wanted to hear how my newly built pre sounded compared to another, and I wanted to use exactly the same audio source, I could simply put the un-preamplified audio though both of the pre's and record them into my computer and then have a side by side comparison of the differences. Any ideas how this could be done?

 

[lofi]

http://www.twin-x.com/groupdiy/albums/userpics/ampinterface.jpg

starting point maybe?

 

[brazilianwonderboy]

That was my initial thought. The problem with an impedance matching device like that is that it presumes somthing that has been recorded at line level and needs changing back to mic (or in the case of that schem, guitar) level. The result is that you still get all the colouration from transformers or other attenuating circuitry that you are trying to avoid.

What I have in mind is an ADC/DAC that records the mic output at mic level and then can play it back at mic level without any impedance changes or amplification/attenuation taking place. That way, I can just plug the output of this device into a preamp, and as far as the preamp is concerned, the signal it recieves is coming from a microphone. Maybe 're-preamp' is the wrong name to give it, 'mic-level ADC' is more accurate,

 

[clintrubber]

Hi,

Not saying it couldn't be done to a degree, but there's the issue of the interaction between
the mic & the preamp, how to capture that ?

You could use some kind of dummy-loading, but then you're already selecting a certain preamp.

An alike issue pops up at the gtr-amp interface and at the poweramp-speaker interface.

Hah, in fact everywhere, your words could be recorded & played back to me & I could respond.
It'll never be the same compared to us directly speaking to each other.

Again, things could be done to a certain degree, but there'll always be a part missing.

Bye,

  Peter

 

[abbey road d enfer]

I was lying in bed last night thinking about a preamp that I am building and I started thinking how cool it would be to have a way of recording the information that comes directly from a microphone without putting it through a preamp or other signal altering gear. This would be for the pourpose of being able to compare the effects that different preamps have on exactly the same piece of audio. So for example if I wanted to hear how my newly built pre sounded compared to another, and I wanted to use exactly the same audio source, I could simply put the un-preamplified audio though both of the pre's and record them into my computer and then have a side by side comparison of the differences. Any ideas how this could be done?

Whatever way you're looking at this, you have to have some kind of preamp to drive the ADC. Connecting the mic directly to the ADC chip is NOT an option (not enough level, wrong impedance, bad NF). You have to compare your preamp to reference preamp, one that is designed and accepted as neutral as can be. An instrumentation input stage, which can be considered as neutral as can be in terms of coloration, is not a proper candidate, because it will exhibit a poor NF.
Grace, Lavry, Earthworks mic pres are considered to be amongst the most neutral you can find...

 

[keefaz]

What about you play a recorded voice through your monitors and re-record it with different preamps using a mic placed at a defined position in front of the speaker, then you A/B the recordings ?

 

[brazilianwonderboy]

Hi,

Not saying it couldn't be done to a degree, but there's the issue of the interaction between
the mic & the preamp, how to capture that ?

You could use some kind of dummy-loading, but then you're already selecting a certain preamp.

  Peter

Yes, it is true I could never re-create the physical charactoristics of the mic that cause certain interactions (and to be honest I wouldn't know where to begin, or what sort of interactions even go on) but I think it would be possible to recreate some characteristics such as the particular impedance of the mic used. I'm thinking of the way that some mic pres have a switchable load resistance to match your mic. This idea could be used in reverse to present a source to a preamp that is similar to the impedance of the original mic that was used.

Whatever way you're looking at this, you have to have some kind of preamp to drive the ADC. Connecting the mic directly to the ADC chip is NOT an option (not enough level, wrong impedance, bad NF). You have to compare your preamp to reference preamp, one that is designed and accepted as neutral as can be. An instrumentation input stage, which can be considered as neutral as can be in terms of coloration, is not a proper candidate, because it will exhibit a poor NF.
Grace, Lavry, Earthworks mic pres are considered to be amongst the most neutral you can find...

But isn't an ADC essentially a comparator or group of comparators that compare a voltage signal over time to a particular reference voltage, and then puts out a binary stream to represent the changes? In theory couldn't this voltage be as low or as high as it wanted so long as Vref is in the same ballpark? Don't get me wrong, I'm no expert on the subject, I do not know if a particular ADC chip exists out there that is designed to reference against very low signals but it would surprise me if there wasn't somthing (even if it was not designed with audio necesarily in mind).

What about you play a recorded voice through your monitors and re-record it with different preamps using a mic placed at a defined position in front of the speaker, then you A/B the recordings ?

It would work as a rough way of making a comparison but not as interesting a project though. I feel like gettin' ma geek on.

 

[keefaz]

It would work as a rough way of making a comparison but not as interesting a project though. I feel like gettin' ma geek on.

Hey sorry  I didn't get your goal
What about you put some mic circuit in a box, but instead of using a mic capsule, you use a signal generator with very low level,
then you connect output to any mic preamps.
For sure you won't get listenable comparaison this way, maybe just one for measurement purpose
(I guess I still don't get what is your goal  )
Edit
I also guess the signal generator used would have to simulate the mic capsule characteristic in some way

 

[Samuel Groner]

But isn't an ADC essentially a comparator or group of comparators that compare a voltage signal over time to a particular reference voltage, and then puts out a binary stream to represent the changes? In theory couldn't this voltage be as low or as high as it wanted so long as Vref is in the same ballpark?

You want to design a decent ADC with 50 mV reference voltage? Good luck with that one . Personally I'd rather prefer to add a 40 dB gain stage before a decent ADC with 5 V reference voltage. Saves a couple of years research, and leads theoretically to the same result if you think about it.

The preamp comparision probleme is unlikely to be ever solved completely; one of the closest things I've found is a repeatable acoustic source such as a self-playing piano. But even this ain't perfect as the instrument will get out of tune sooner or later. A speaker probably sounds too ugly in itself for meaningful comparison.

Fortunately I've convinced myself after several years in the recording business that my recordings don't sound good because of my gear but because I'm working with excellent musicians, good instruments, suitable recording spaces, decent mic placement, my full musical background and careful editing.

Samuel

 

[MagnetoSound]

This would be for the purpose of being able to compare the effects that different preamps have on exactly the same piece of audio.

You could do that at line level.

Not saying it couldn't be done to a degree, but there's the issue of the interaction between
the mic & the preamp, how to capture that ?

This is a very important point. Even if you use (or simulate) a mic at the input, you will only know the effect of the interaction with that one specific source impedance, which might kind of defeat the object, don't you think?

If you just want to know the audible effect of the circuit in question on an audio signal then, as I say, you could just pad the line level output of a recorded source and be done with it, no?

 

[mikep]

I think it is much easier to get the desired result than you might think.  All you need is a 40 db pad with 200 ohm output Z.  Three resistors in a male-female xlr barrel. I have used one of these to test mic preamps (repairing), and to evaluate new designs. You could use to to impart a preamp's tone to previously recorded audio.  Attenuators like this do not have a "sound".

 

[Svart]

just use a quality transformer.  without serious levels going through it you won't even notice any "transformer sound".

 

[lagoausente]

  Our mind have two ways of work. Intuition, and reason.  If you think it in depth, even you want to make a "clinical" analisis of a device, what you are looking at least is an audible result. I mean, whatever you want colored or neutral device, I find more insteresting listening training, then waveform analisis. This last one may show you things that you really don´t hear, and you may hear things that you cannot find a direct explanation.
  So why not enjoy recording, hearing and comparing? At least, what you hear is what you need. Or do you plain to convert your recordings in waveform pictures?