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Author Topic: Are preamps for pu$$ies?  (Read 15738 times)

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Offline chris319

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Re: Are preamps for pu$$ies?
« Reply #75 on: September 20, 2008, 08:12:26 AM »
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match it with a resistor from pin 3 to ground.

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Pin 3 is the non-inverting input of the final opamp.

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The impedance balancing resistor does not connect to either input of the opamp.

Do you mean pin 3 of an XLR connector or pin 3 of the opamp chip?

Offline DSatz

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Re: Are preamps for pu$$ies?
« Reply #76 on: September 20, 2008, 02:09:22 PM »
Arriving late in this thread ... seems to me that there are a few things some people might want to think about.

One is that you need to make a fair comparison. Yes, theoretically every piece of equipment contributes some noise and distortion. But when you're asking whether or not to use a preamp, the preamp isn't the only source of noise and distortion to be considered! The recorder's input circuit--which you've been using in the absence of a preamp--also has noise and distortion. If you leave that out of the comparison, you'll be off on a completely wrong tangent.

Practically speaking, the mike inputs on most mass-produced portable recorders are mediocre because there is no economic reason for them to be better; people buy them anyway. A 30-cent increase in parts cost either removes a dollar from the profit picture of the manufacturer, distributor and/or retailer, or forces them to raise the selling price a dollar, making them vulnerable to price competition from other manufacturers' products. And the present-day audio equipment market is driven much more by price than ever before.

Equipment built to professional standards of quality, reliability and adaptability is usually quite a bit more expensive. It seems as if we all hope for a miracle each time we buy a (relatively) bargain-priced piece of mass-produced equipment. There may be something like a gambler's psychology at work. The moment a low-cost alternative can really replace a high-cost item of equipment, you see the professionals adopting the low-cost gear.

I'm not saying "more expensive is better;" I'm only saying that in some things, "better is more expensive." And since most people who buy recording equipment don't do any live recording at all, or do so only in their own homes and/or with consumer-type microphones, the biggest place where the mass-produced recorders almost never meet professional standards is their mike inputs. We can get more specific, but I just wanted to lay down a carpet of what I consider the reality to be.

--best regards
« Last Edit: November 03, 2009, 10:20:51 PM by DSatz »
music > microphones > a recorder of some sort

Offline jerryfreak

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Re: Are preamps for pu$$ies?
« Reply #77 on: September 20, 2008, 04:14:15 PM »
dsatz - on point as always

one flip side to your discussion of 'pressing the limit of input noise' is the concept of my original post

schoeps>ad2k (pro mic>pro converter) reaches its limits in a quiet studio, as the input noise of the converter is *slightly* greater than that of the mic. Is very but This setup however is perfectly applicable to our environment as the noise floor of both the mics and converter are far below even the quietest background noise of a live venue. so in this case, it works!

oddly enough, i stubmled across this setup by accident, while doing a test of a bunch of schoeps capsule/preamp setups micing monitors in my home, i found that the cmr>ad2k was pretty much as good as it gets, so it became my field rig. (for now)

I've got some new things up my sleeve that are along the same lines as this setup, technically addressing the fundamental 'limitations' if you call them that, the goals are two fold:

-ideally reduce the input level of the converter to 6 dB or more below the mic noise
-increase the input sensitivity of the converter to get a signal thats in the 0 to -20 dB range as opposed to -24 to -36 range, to reduce or eliminate normalization. but it all goes back to #1, converter noise vs mic noise. if you can get the converter noise 6dB below the mic noise, it is really irrelevant what level you record at, as you have essentially allowed for the whole range of mic input levels to be accurately recorded.

a trick is to have a device that can allow for low level inputs but maintain enough headroom to take a signal off a pre when you want to use it.

theres a couple other features my 'ideal box' would have, its really in its infancy, it is a modded piece of gear, but not an ad2k per se.

Offline jerryfreak

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Re: Are preamps for pu$$ies?
« Reply #78 on: September 20, 2008, 08:37:17 PM »
how not cheap are we talking? also what would be the rough size and power requirements of a simple assembled fixed or trimpot-variable gain? are wer talking larger battery box or ad20/mic2496 size?

 
I've got some new things up my sleeve that are along the same lines as this setup, technically addressing the fundamental 'limitations' if you call them that, the goals are two fold:

-ideally reduce the input level of the converter to 6 dB or more below the mic noise
-increase the input sensitivity of the converter to get a signal thats in the 0 to -20 dB range as opposed to -24 to -36 range, to reduce or eliminate normalization. but it all goes back to #1, converter noise vs mic noise. if you can get the converter noise 6dB below the mic noise, it is really irrelevant what level you record at, as you have essentially allowed for the whole range of mic input levels to be accurately recorded.

An easy solution is a good transformer.  A converter usually has a 10K input; the CMR is 15 ohm output, right?  So something like the Jensen JT-13K7A does +14dB of almost noiseless gain.  It's not cheap though!

Offline SparkE!

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Re: Are preamps for pu$$ies?
« Reply #79 on: September 20, 2008, 10:16:23 PM »
An easy solution is a good transformer.  A converter usually has a 10K input; the CMR is 15 ohm output, right?  So something like the Jensen JT-13K7A does +14dB of almost noiseless gain.  It's not cheap though!

Transformers are often a useful addition to the front end of an amplifier, but you have to be careful about getting too much gain out of your transformer.  It really depends on where the noise in your signal path is coming from.  Your mic can be modeled as a signal voltage source in series with a noise voltage source and a resistor whose value is the output resistance of your mic.  The noise voltage source is to model the acoustic noise due to random motion of air molecules that impinge on the face of the mic's diaphragm plus the thermal noise that is produced by the output resistance of the mic (equal to the sqrt of 4kTBR, where k is Boltzmann's constant, T is the absolute temperature in degrees Kelvin, B is the Bandwidth of the signal path and R is the output resistance of the mic).  We'll call this the microphone noise.

You also have a couple of noise sources right at the input to your amplifier.  One can be modeled as a series noise voltage source and the other as a shunt current noise source.  The resulting noise at the output of the amplifier is given by:

Vnampout = (Inamp * Rmic * N^2 + Vnmic * N + Vnamp) * A

where:

Vnampout is the noise at the output of the amp
Inamp is the current noise of the noise current source at the input of the amp
Vnmic is the noise voltage of the mic
Rmic is the output resistance of the mic
N is the turns ratio of the transformer
Vnamp is the noise voltage of the voltage noise source at the input to the amp
A is the gain of the amp

The signal voltage at the output of the amp will be

Vampout = Vmic * N * A

So, the S/N is given by:

S/N = Vampout/Vnampout = Vamp/(Inamp*Rmic*N + Vnmic + Vnamp/N)

Typically, the largest contribution to the noise at the output of the amp is Vnmic, but notice how Inamp*Rmic is multiplied by N.  You have to be careful not to use too large of a turns ratio, N, or your input current noise of the first amplifier stage after the transformer will begin to dominate the noise budget.

So, if you have mics with lots of self noise, then you can use lots of "free" gain from a transformer with a high turns ratio, but if your mics are pretty low noise, then you have to be careful not to use too much transformer gain or the input noise current of your amp will start to become the dominant noise source.  The goal of any preamp should be to get the signal level boosted to a useful voltage level without significantly degrading the S/N ratio.

How'm I supposed to read your lips when you're talkin' out your ass? - Lern Tilton

Ignorance in audio is exceeded only by our collective willingness to embrace and foster it. -  Srajan Ebaen

Offline DSatz

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Re: Are preamps for pu$$ies?
« Reply #80 on: September 21, 2008, 02:13:54 PM »
> [ ... ] I thought the idea was to operate at a fixed level where the mic noise slightly exceeds the ADC input noise?

OK, I think it's time to call "shenanigans." Why bother to point out again and again that noise can't be adequately described by a single numeric value? People nod their heads sagely but then they say, "Well, I have to use SOME number,"--so even though the basis of the numbers may 10 to 12 dB apart, they just turn right around and act as if it's OK to compare them and to make engineering decisions on that basis. Sorry, it doesn't fly.

The noise spectrum of a typical condenser microphone is quite far from flat. At low frequencies it's dominated by 1/f noise from the capsule--the source impedance driving the FET or tube input stage of the amplifier. The spectrum of this noise has an inherent downward slope. At some higher frequency the noise of the amplifier's input stage gradually takes over--generally with a flatter noise spectrum, though this varies.

See the attached graph, for example, in which a good professional-quality preamp was driven by a very high-quality transformerless, low-impedance, balanced condenser microphone. A measurement test head was used in place of the capsule; it offers the same source impedance as a capsule of the microphone's own type would have (ca. 35 pF), but doesn't pick up sound from the room. The gain of the preamp was set at a typical level for the kind of recording I mostly do. Phantom powering was turned on, of course.

The rise at low frequencies is a realistic representation of what's generally going on beneath most of my recordings. The noise at the lowest frequencies is ~25 dB (!) above the noise at high frequencies where the ear is most sensitive.

To quantify that noise, you have to take into account the ear's drastically different sensitivity to different frequencies at different sound pressure levels--and that throws the whole thing into a cocked hat, because that relationship depends greatly on the playback levels. There's just no way to condense all that into a single, meaningful numeric value--and we haven't even touched on whether the noise maintains a steady amplitude over time or whether it's full of moment-to-moment variations and impulses ("shot noise").

So yeah, we'd all like the equivalent input noise of our preamps to be well below that of the microphone. But at what level does that occur if the input noise of the preamp/converter has a flat spectrum while the microphone's noise spectrum is tilting downward? Frankly, it's not very hard to build a preamp that will be quieter than even the best condenser microphones at low or mid frequencies. Measure around 4 kHz, though (where the ear is most sensitive at low sound pressure levels), and it's a rather different story.

Let's make the discussion as simple as possible, but please--no simpler ...

--best regards
« Last Edit: November 03, 2009, 10:17:08 PM by DSatz »
music > microphones > a recorder of some sort

Offline DSatz

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Re: Are preamps for pu$$ies?
« Reply #81 on: September 21, 2008, 04:28:46 PM »
mshilarious, apparently you feel personally affronted by what I wrote. I had no such intention at all; I was actually trying to be adorable by quoting something from "South Park," which no one expects somebody my age to know. But since I quoted part of your message, I can perfectly well see why you got that impression, and I apologize for that.

All I'm trying to say is that a few more specifics need to be taken into account before you can know whether or not a given input arrangement will have lower input noise than your microphones (a) at one particular frequency or (b) across the audible band. In practice it is very hard to predict this sort of thing from specifications and theory; you usually need a real, working circuit as a starting point.

--best regards
« Last Edit: September 23, 2008, 08:35:17 PM by DSatz »
music > microphones > a recorder of some sort

Offline Church-Audio

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Re: Are preamps for pu$$ies?
« Reply #82 on: September 22, 2008, 02:01:20 PM »
> [ ... ] I thought the idea was to operate at a fixed level where the mic noise slightly exceeds the ADC input noise?

OK, I think it's time to call "shenanigans." Why bother to point out again and again that noise can't be adequately described by a single numeric value?

That's a very unfair statement to make when you've arrived on a thread two months after I began trying to explain this as best I am able.

For example, on July 16 I wrote:

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S/N ratios are either stated as unweighted or A-weighted.  A-weighting is quite a bit broader than the 1kHz tone; more like 500Hz to 10kHz, with the peak at 2.5kHz.

Microphone self-noise is a combination of capsule noise (1/f) and white noise, usually with capsule noise dominating the A-weighted range.   Once you get into preamps and converters, the noise spectrum is much more uniformly white in the A-weighted range.

And just a few days ago:

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If you are constructing an amp for a single application, you could safely select a fixed gain.  Otherwise, I would add at least a couple of switching options.


jerryfreak is looking at interfacing two very specific pieces of gear, and with that in mind, yes, a gain figure can be derived.  And that was also described by the designer of one of the bits back on July 16.



Jon dont take it personal Dsatz really does know his stuff. I can personally attest to that :) He has corrected me on more then on occasion. I for one welcome his expertise to the community!

Chris
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Offline chris319

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Re: Are preamps for pu$$ies?
« Reply #83 on: September 23, 2008, 01:07:57 AM »
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What you probably want to do is build the schemo as shown in the datasheet, but feed its output to pin 2 out AND the negative input of an opamp in inverting configuration, with that opamp's output to pin 3.

mshilarious -

I built the circuit as you suggested above and it works. Thank you for all your help. One last question: do I need anything between the output of the third opamp (on the right) and the new, fourth inverting output opamp, such as a resistor or capacitor?

Here is a link to the data sheet for the four-banger version of this chip:

http://www.national.com/ds/LM/LME49740.pdf

Thanks in advance.

Offline chris319

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Re: Are preamps for pu$$ies?
« Reply #84 on: September 23, 2008, 12:03:54 PM »
Actually I added three resistors: one between the output of the third opamp and the inverting input of the fourth opamp, a feedback resistor between the output of the fourth opamp and its inverting input, and one from the non-inverting input of the fourth opamp to ground, all 10K.
« Last Edit: September 23, 2008, 12:34:13 PM by chris319 »

Offline jerryfreak

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Re: Are preamps for pu$$ies?
« Reply #85 on: August 10, 2009, 02:36:54 PM »
just wanted to unearth this topic now that i have more expereince with this rig under my belt.

and yes im quite happy with the results

check out red rocks and gorge if youre curious

http://bt.etree.org/?searches=jerryfreak&cat=0

 

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