budget (usb) mic for classical guitar recording needed

[steve]

Meanwhile I'll try to find or build a simple 48V power supply for the phantom power

There's plenty of DIY circuits for that on the Internet. Really good regulation/smoothing is the key. At one time I was considering building a box for 5 pp3 batteries and a cap across the output - 45v should be close enough, but the cost of batteries put me off. For solid state microphones the current draw is really tiny.

If you get round to making one, good quality capacitors are essential as many off-the-shelf electrolytics will produce an unacceptable amount of leakage noise.
Black Gate caps are probably OTT and are hard to get hold of. Elna Cerafine and Nichicon FG are good and more reasonably priced, but again are difficult to get hold of in Europe, but Panasonic FC are good and readily available (CPC, Farnell, RS, ...)

So 7 pages of posts later I went to read back the first posts...
....
Thinking out loud: maybe I should simply get both and then return the one I don't want to keep (which could get me in trouble if I don't want to return any )

LOL - I spotted that yesterday.

Finally got to hear and play with your latest audio sample. This is a pleasure to work with
I've had a go at tackling the noise problem.

In this sample I first ran a 6dB/octave high pass filter at 20Hz to remove the DC off-set and a bit of sub-sonic rumble (was that a car going past near the end of the track?) The HP filter seems to work a bit better than the "Normalize" effect for removing DC off-set on this sample as the DC off-set amount drifts a bit (sub-sonic). Then Amplified close to 0dB.

Then I used a subtle amount of noise reduction using Gnome WaveCleaner. The amount of noise reduction was set very low (0.25 on a scale of 0 to 1.0) and the "gamma" setting dropped down to 0.5 (this helps to preserve transients).
Finally I used the noise gate with a fairly slow attack/decay.

Definitely useful doing this as it appears that there is an undocumented change in Nyquist which means that the Noise Gate effect is not compatible with Audacity 1.3.11! (I'll have to investigate this further).

The result is very close to the sample from Koz (23-sample-with-silences-Keypex). You can hear a slight difference if you listen carefully to the decay on the final note. In the Keypex version you can hear the noise level creeping in as the note decays (before the gate kicks in). Also, in the original track there is a slight "bump" sound just after the note stops (your hand on the guitar?) which has disappeared in the Keypex version. In this version the noise reduction has removed the hiss from the last note and the "bump" is still audible (though on a production job I'd probably remove it).
http://audacity.easyspacepro.com/exampl ... es-NR.flac

[steve]

Connecting the mic to a preamp and then the preamp to the line-in on the soundcard (selecting the input as line-in and not mic) is another setup that I'd like to try...

That will probably work pretty well. A lot of the poor reputation of AC'97 comes from the very poor quality analogue components that are typically used in the analogue end of on-board sound cards. Also the high gain required for microphone pre-amplification is much more demanding than dealing with line-level.

My quest here would be mostly to find out which ADC would be better... the ADC in a mic preamp + usb output VS. soundcard ADC + a preamp with 1/4" line out

With a reasonable mic pre-amp I'd guess that the difference will be subtle. Getting the 50dB or so of gain to boost the mic level signal up to line level without introducing noise and distortion is the hardest part.

If you want another project to hang around for several years, you may be interested in this snippet that I stumbled upon recently: http://forum.audacityteam.org/viewtopic ... 27&t=27426

[whomper]

or an outboard interface w/o a mixer

Please read the full topic before posting. bgravato has already made his decision and bought his new hardware.

=======================================
apparently he has changed his mind as of page 7

and others reading the thread may not want his solution and would welcome some alternatives to consider

[bgravato]

a bit of sub-sonic rumble (was that a car going past near the end of the track?)

Most probably... I was recording in the living room (which is where I temporarily have my PC), where the cars passing in the front road can be heard more than I wished for...

The 10m cable I got is a couple of meters short to allow me to take the mic into the free room on the other side of the appartment... (option here is to use the laptop instead). Other place I tried to record the acoustics where terrible and the sound was too boomy...

You can hear a slight difference if you listen carefully to the decay on the final note. In the Keypex version you can hear the noise level creeping in as the note decays (before the gate kicks in). Also, in the original track there is a slight "bump" sound just after the note stops (your hand on the guitar?) which has disappeared in the Keypex version. In this version the noise reduction has removed the hiss from the last note and the "bump" is still audible (though on a production job I'd probably remove it).

The "bump" is probably my fingers going back to the strings after the final chord... You can also hear some breathing during the play

If you want another project to hang around for several years, you may be interested in this snippet that I stumbled upon recently: viewtopic.php?f=27&t=27426

That looks interesting... and it's available for less than 5 euros at farnell... I may as well add it to the order I'm been thinking about placing at farnell for the last year or so... (still trying to finish that never-finished list of components...)
Although the problem here, as in any "sensitive" electronic gear, is to get/make a good power supply...

and others reading the thread may not want his solution and would welcome some alternatives to consider

I plan on posting an update on the initial post of this thread once I got everything sorted out and settled... So that the newcomers may just quickly read the short version of a very long thread...

[kozikowski]

<<<Definitely useful doing this as it appears that there is an undocumented change in Nyquist which means that the Noise Gate effect is not compatible with Audacity 1.3.11!>>>

That would go right around me. I'm doing this in 1.3.7.

And yes, that's why I wanted at least one long slow note decay at the end. If you're going to have troubles with processing tools, that's the place damage is going to occur.

Koz

[kozikowski]

Phantom power isn't all that crazy, but you do need to pay attention to a couple of items...

http://www.jensen-transformers.com/as/as016.pdf

This is a very nice way to design a microphone input system and it's next circuit is one of their transformers.

http://www.jensen-transformers.com/as/as017.pdf

If your circuit has no coils at the input, then you get to fool around with not only balancing the 6k8 DC feed resistors very carefully, but balancing the coupling capacitors as well. Remember, they have to be large enough to pass the lowest frequency of interest plus extra to avoid low frequency current noise, and well behaved enough to work with no DC bias on them in the event you never turn the +48 volts on.

The worst curse of phantom power is not what happens when it's running, although that can be fun. When you turn it off, it dumps a sudden 48 volt charge into the amplifier system. This would be the delicate, noise tuned, super low distortion amplifier system. I can't find it immediately, but someone made a phantom system that faded the battery supply on and off to avoid that sort of damage.

Koz

[bgravato]

The worst curse of phantom power is not what happens when it's running, although that can be fun. When you turn it off, it dumps a sudden 48 volt charge into the amplifier system. This would be the delicate, noise tuned, super low distortion amplifier system. I can't find it immediately, but someone made a phantom system that faded the battery supply on and off to avoid that sort of damage.

I've seen a couple of schematics where they use some zener diodes for that matter I believe...

[kozikowski]

<<<Also, in the original track there is a slight "bump" sound just after the note stops (your hand on the guitar?) which has disappeared in the Keypex version.>>>

But not all of the versions. The one before this one had a different threshold of noise sensing and the bump came through. I changed the threshold to get rid of it because it came through not as a polished-wood/string/musician activity, but a "pish" of noise.

Delicate events near the noise threshold are very important when messing with this process.

Koz

[steve]

<<<Definitely useful doing this as it appears that there is an undocumented change in Nyquist which means that the Noise Gate effect is not compatible with Audacity 1.3.11!>>>
That would go right around me. I'm doing this in 1.3.7.

I guessed that, and it helped to identify the problem. Since Nyquist has been updated it is now (correctly) more fussy about whether values are integers or floats. Just adding a dot has fixed that problem. However, I've clearly learned a thing or two since I wrote that (almost a year ago) and I've identified a couple of other bits that should really be improved. Apart from the coding being rather inelegant, there is virtually no error checking in it and the attack/release times are totally inaccurate. I've added some error checking to prevent silly values from breaking it so I just need to sort out the timing issue and tidy it up. Hopefully get that done this week.

@Koz, this would be a good time to say if you want a feature for measuring the noise level, or are you now happy with using the Amplify effect to do that?

I changed the threshold to get rid of it because it came through not as a polished-wood/string/musician activity, but a "pish" of noise.

As a matter of taste, I'd do that as well. Creative use of noise gating.

I've seen a couple of schematics where they use some zener diodes for that matter I believe..

Here's one such circuit, though I'm not convinced that the voltage regulation is really adequate for high quality recording, though that will depend on how forgiving the microphone is:

http://www.tangible-technology.com/powe ... rying.html

There's a more sophisticated one on this page that uses a LM317 regulator. http://www.uneeda-audio.com/phantom/

[kozikowski]

<<<are you now happy with using the Amplify effect to do that?>>>

Amplify isn't enough. It will not go over 50dB -- at least the one I'm using won't.

Koz