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Post by mickey242 on May 1, 2021 6:05:34 GMT
Lets say you have a pre amp. It has one pot for gain, thats it nothing else. The amplified signal is then sent to a DAC* that has a pot to control gain. Lets say that said pot has no setting for "unity". Should you set the DAC's* pot fully clock wise then use the pre amps pot for capturing the signal? I hope you know what I'm getting at here. Thoughts/comments?
Thanks.
*Meant to say ADC converter.
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Post by Michael Lawrence on May 1, 2021 13:53:02 GMT
"Unity" indicates "no change in level," or "voltage out = voltage in," and an Analog to Digital convertor (which is what you'd need inline after a preamp, rather than a DAC, since a preamp is an analog device) can't have a "unity" setting because there's no such thing - it's Digital Value out = Voltage In.
The signal is changing forms, so the pot controls the unit's Sensitivity. Generally this is indicated as the input voltage that would bring the ADC's output to full scale 0 dBFS. At a very high sensitivity, it will take a very low voltage signal to bring the convertor to full scale, and that means decreasing the potential signal to noise ratio of the conversion and bringing up the noise contributed by the preamp. Likewise, with a very low sensitivity, the preamp won't be able to toggle all the bits even at max gain and this decreases your SNR as well.
It's best to have the DAC reach full scale with a typical nominal line level signal's peaks (in the pro audio world that means around +20 dBu) and then run the preamp hot enough so its output is a nominal line level RMS signal (+4 dBu or so again in the pro world) which ensures you have room for your crest factor and program peaks without overloading. But you want to check your preamp specs, find out the max output level it can generate, and then trim your convertor so it hits full scale at that level. Both stages should reach clipping at the same time, give or take. In reality you want your convertor to have an extra dB or two above the preamp clipping.
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Post by Hexspa on May 1, 2021 14:01:02 GMT
EDIT: Mr. Lawrence was writing as the same time as me - the 'more qualified' person.
Maybe someone more qualified will chime in. Until then, you have my speculation.
First, a preamp doesn't have unity gain, right? It might if it has part of the knob's range to attenuate the signal but, otherwise, the point of a preamp is to boost a lower-level signal to line level. Faders have unity - 0 - but not anything that's an amplifier.
Second, by DAC you mean a converter? A DAC converts digital to analog but, of what I gather, you're sending an analog signal into this box. From there, you didn't say where the signal goes; to some kind of capture device. If that capture device is digital then what you have is an ADC.
As a whole, this DAC/ADC may have gain controls. A simple ADC doesn't necessarily. Maybe it'd help if you name these devices so we can look into their feature sets.
In the interim, if your 'DAC' has a gain control, then it probably has an internal preamp. If so then you'll pick one preamp over the other unless combining them gives you your desired result in some way. All else being equal (assuming one is not a 'color' box), the more expensive device might have cleaner gain.
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Post by rock on May 1, 2021 14:34:18 GMT
If you did have a setting for "unity" on your ADC, yes, you would start there and bring your level up with your preamp level control. So I think your question should be "how do I determine where unity gain is on my ADC level pot?"
If your ADC is designed to operate with an input as hot as say, +18dBm, measure a test tone oscillator (1KHz) to output +18dBm. Apply this signal to your ADC and adjust your ADC level control to get 0dBFS on your DAW in record mode. Now, if you lower the test tone to 0dBm your DAW meter will read -18dBFS. Now you can adjust your preamp to output the -18dBFS in your DAW. (I'm of course suggesting here that -18dBFS is a good average level to record but that's up to you.)
Of course it's up to you exactly where you want to set your 0dB level (-10, +4, +8?) and how hard you want to hit your DAW but you don't want your preamp all cranked up just to attenuate all that gain with your DAC pot... and end up with more noise from your analog signal. Your ADC manual/spec sheet should help you determine the max level you want to send to your DAW.
On another related detail: If you look at the circuit design of most gain stages, you'll find that the vast majority of "Gain" circuits are actually "FIXED GAIN" and the control pot is just an attenuator either on the input or output (or both). Yes there are some true variable gain designs but it's easier to get the fixed gain design right (without noise whenever you touch the gain control). I'm not an electronics designer/engineer, so I'm as usual, I'm open to correction but the above is from what I've gathered over the years.
Update: OK, I just hit the "Post" button and I see two other members have already posted. Now I'll go back and read what they wrote!
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Post by Hexspa on May 2, 2021 14:36:50 GMT
The funny thing about all three of us writing at the same time is that it's probably statistically likely that it will happen. I can't presently explain why but I heard if you have 100 people, it's likely that at least one pair shares a birthday. Here's the video about the birthday paradox: youtu.be/ofTb57aZHZs?t=1 |
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Post by mickey242 on May 2, 2021 16:56:10 GMT
If your ADC is designed to operate with an input as hot as say, +18dBm, measure a test tone oscillator (1KHz) to output +18dBm. Apply this signal to your ADC and adjust your ADC level control to get 0dBFS on your DAW in record mode. Now, if you lower the test tone to 0dBm your DAW meter will read -18dBFS. Now you can adjust your preamp to output the -18dBFS in your DAW. (I'm of course suggesting here that -18dBFS is a good average level to record but that's up to you.) This is a great idea and I'm totally going to do this! The equipment I'm using is a DAV electroincs pre amp into a Focusrite Scarlett 2i2, so it is one pre amp going into another, then the usb out to a mac. Thank you all for the info. It is really appreciated. |
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Post by mickey242 on May 28, 2021 5:10:49 GMT
"unity" setting because there's no such thing Sorry I'm still coming from the whole mixer to analog world. Where you would have the fader set to unity and the buss send all the way up and then use the gain pot to as the first stage. |
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Post by mickey242 on May 28, 2021 5:22:48 GMT
I"m sorry to beat a dead horse on this topic. Can any of you Recommend a ADC, I almost wrote DAC again, LOL. I looked into RME maybe Burl audio. I just need to channel set up for live MS recording.
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Post by Hexspa on May 29, 2021 22:47:32 GMT
I have an RME interface. Quality gear. Then again, I have a Scarlett I/O also and I can't tell any difference in the conversion.
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