Daisy Chaining Mic Preamps....

[kidvybes]

...I've been using dynamic mics for vocals lately (SM7B, Heil PR40) but a few of my mic pres come up a little short in the gain output needed to get an ample signal from the SM7B in particular...I've noticed references in different threads to "daisy-chaining" pres to get more gain output...I've been hesitant to do so, but wondered if anyone on this site has any experience with this option...I'd like to hear my GT Brick (only 55dB) drive the SM7B, so I'm considering daisy-chaining it through my Rane MS1B (clean gain)...any thoughts before I blow up the neighborhood?...

 

[MOFO Pro]

I'd put the Rane in front, turn the output all the way down, set up the brick at expected levels and then work the gain up on the Rane... you should get interesting results... without blowing anything up.

 

[mshilarious]

I'd put the Rane in front, turn the output all the way down, set up the brick at expected levels and then work the gain up on the Rane... you should get interesting results... without blowing anything up.

I would also use the Rane first, because of its lower input noise. However, you want to take enough gain at the Rane to get the Rane's output noise a comfortable margin above the Brick's input noise. So set the Brick at minimum gain, and raise the gain level on the Rane until you get the necessary level.

 

[kidvybes]

...thanks guys...I'll give it a try...so the rule of thumb is to keep the "noisier" pre at the end of the chain, quietest at the front?...makes sense...

 

[MOFO Pro]

I would also use the Rane first, because of its lower input noise. However, you want to take enough gain at the Rane to get the Rane's output noise a comfortable margin above the Brick's input noise. So set the Brick at minimum gain, and raise the gain level on the Rane until you get the necessary level.

Good point... but if you're trying to impart the sonic character of the brick, you'd want to push the brick harder... It's a balancing act... that I guess could actually be better acheived by moving the Brick in front of the Rane. The preamp that you most want to effect the signal should be the preamp with the most gain

 

[mshilarious]

Good point... but if you're trying to impart the sonic character of the brick, you'd want to push the brick harder... It's a balancing act... that I guess could actually be better acheived by moving the Brick in front of the Rane. The preamp that you most want to effect the signal should be the preamp with the most gain

The question is, are you pushing the Brick more with a hot input level or a high gain figure? I don't know the Brick's circuit, but if we take a simple tube circuit like say a Fender Champ, where there are two tube stages with a pot in between, a goal could be to crush the second stage with the gain from the first. Such that if the incoming signal level was hot enough, you could get the same effect by crushing the first stage.

The Brick won't be like that though, I imagine. Also it has an input transformer, does it not? There will be a difference between running a mic into a transformer input and transformerless pre.

But again, the issue here is insufficient gain in certain circumstances with the Brick. That's an indication of a low input signal. The Brick has relatively high input noise. So that combination will result in low signal to noise ratio, which cannot be improved with a subsequent gain stage. In fact, if this setup is feeding a converter, there is really no point in chaining a second pre, because the Brick's output noise will well exceed the converter's input noise. Such that a digital gain change will yield the same signal to noise ratio as another analog gain stage.

So let's back up a step: I think it is likely the Brick will show more character with a hotter input signal, as every component in the thing will therefore see a hot level. I do not know if that is good or bad, but it's character!

Still, the principle of chaining gain stages is you always want to take enough gain in the first stage such that the output noise exceeds the input noise of the following device. You would normally also want the quietest gain stage first. That is how you maximize signal to noise.

 

[MOFO Pro]

You are correct sir... I can't argue any of your points... and all kidding aside, you are infinetly more versed in this topic than I, and I respect your opinion greatly...

but my response was based on this:

I'd like to hear my GT Brick (only 55dB) drive the SM7B

He want's the bricks sonic signature on the SM7 which he can't get because of the limited gain on the brick...

I guess my only question would be on the actual circuitry in the brick... Is the gain knob really just a variable attenuator on the input of a fixed gain amplifier stage? That was my thought...that by turning the gain knob down on the brick you were really just reducing the input signal... so reducing the input signal would significantly reduce the brick's impact on the character of the output.

 

[mshilarious]

You are correct sir... I can't argue any of your points... and all kidding aside, you are infinetly more versed in this topic than I, and I respect your opinion greatly...

but my response was based on this:


He want's the bricks sonic signature on the SM7 which he can't get because of the limited gain on the brick...

I guess my only question would be on the actual circuitry in the brick... Is the gain knob really just a variable attenuator on the input of a fixed gain amplifier stage? That was my thought...that by turning the gain knob down on the brick you were really just reducing the input signal... so reducing the input signal would significantly reduce the brick's impact on the character of the output.

Gain controls on tube amps often are variable attenuators, but normally after a fixed gain stage. Otherwise the noise performance of the amp would suffer greatly. Or it can be done by changing the gain with feedback, similar to how it would be done with an opamp. Have a look at the note for SW2 in this schemo of the Gyraf G9, it does both:

http://www.gyraf.dk/gy_pd/g9/g9_sch.gif

Again I dunno what the Brick does. But I'm still guessing that the better noise performance of the Rane will help with an SM7 on a quiet source.

 

[Madguitrst]

All technicalities aside, the answer is simple:
Try both ways and see what you prefer.

In my experience doing this, I usually like both pre somewhere clloser to the middle. Sounds to me like this results in less noise as pushing one of the preamps to near it's max usually results in more noise.

 

[DigitMus]

There is no reason to run one mic preamp into another. If you like the sound of a particular pre, and it doesn't have enough gain (a very rare circumstance, unless you like running everything "in the red"- or are using the wrong type of mic for the source), make the gain up further down the chain (i.e. compressor, E.Q., ADC, console line input, etc.).
But then, what do I know, I've only been at this for 40 years...

Scott

 

[mshilarious]

There is no reason to run one mic preamp into another. If you like the sound of a particular pre, and it doesn't have enough gain (a very rare circumstance, unless you like running everything "in the red"- or are using the wrong type of mic for the source), make the gain up further down the chain (i.e. compressor, E.Q., ADC, console line input, etc.).
But then, what do I know, I've only been at this for 40 years...

Scott

Compressors, consoles, and EQs all have amplifiers, so I fail to see the difference . . . and I already pointed out that if the output noise of a single preamp exceeds the input noise of your converter by enough (at least 6dB), you don't need to bother with an interim analog stage.

I reiterate that the Brick has high input noise (-90dBA, which if accurate is really extremely high), so following the Brick with a gain stage of any sort cannot improve its SNR (in fact, if you are using a compressor to compress, rather than just a gain stage, you will degrade SNR), whereas a quieter gain stage in front of it can help.

 

[kidvybes]

...I guess I'll try both ways, just to see what the outcome will be...the odd thing is that I have a "reconstituted" 1290 Neve pre (actually an original 2 stage 1272 preamp that has been modified with a 3rd stage as per the original 1290/1073 designs)...when I drive the SM7 with this pre, the gain is quite clean through the first two stages (up to 55dB) but as soon as I boost the gain upwards of the 65-70dB that I need to get a good signal (kicking in the preamp's 3rd stage), the "noise" factor becomes somewhat objectionable...I'm not sure how a three-stage design like the 1073/1290 relates to daisy-chaining separate preamps, but it's clear that even in a "holy-grail" pre like the Neve, the gain structure has it's drawbacks...

...I have since sent the Neve back to the builder to examine the 3rd stage components to see why the noise/distortion becomes so noticeable (far beyond what is normally referred to as signature Neve "colour")...in the meantime, I plan on experimenting with the daisy-chain concept...

...thanks to all for the input...interesting discussion no doubt...

 

[DigitMus]

Compressors, consoles, and EQs all have amplifiers, so I fail to see the difference . . .

Really, you don't see the difference between a MIC preamp and a line amplifier?
I stated there was no reason to plug a MIC preamp into another MIC preamp.
Pick your fights better.

 

[mshilarious]

Really, you don't see the difference between a MIC preamp and a line amplifier?
I stated there was no reason to plug a MIC preamp into another MIC preamp.
Pick your fights better.

OK, go ahead, fill me in. What is the difference between a mic amp and a line amp? Could, for example, the INA163 used by the Rane also be used as a line receiver? Or would you prefer something like a 5532? Can a mic amp use a 5532? What about the input impedance of the mic amp? Will its 1K ohm impedance excessively load another mic amp with output impedance of 100 ohms? Or if the input impedance drops below 10K ohms, does the world end

Please, inform us

 

[DigitMus]

I never stated, implied or suggested that you CAN'T put a line level signal into a mic preamp, nor did I say that anything horrible would happen if you did. I did try to point out the "correct" way to deal with gain staging - there are many good reasons to adhere to proper gain staging (S/N ratios and headroom not least among them). If you properly buffer the input stage, a mic amp can certainly be used as a line amp, but one cannot assume that to be the case (especially with 'low end' gear common in most home studios).

I don't know why you feel the need to argue - there are many good reasons not to daisy-chain mic preamps, and few to the contrary.

mshilarious - while you may have more knowledge and experience than the majority around here, it might do the community a disservice to imply that there's no difference between a mic preamp and a line amp. While YOU may know enough to get away with "bending the rules", a lot of people here need to learn the rules before they experiment with alternatives.

No hard feelings, I just was a bit nonplussed that a knowledgeable poster would choose to quibble with accepted "best practices" used in studios around the world.

Scott

 

[mshilarious]

I never stated, implied or suggested that you CAN'T put a line level signal into a mic preamp, nor did I say that anything horrible would happen if you did. I did try to point out the "correct" way to deal with gain staging - there are many good reasons to adhere to proper gain staging (S/N ratios and headroom not least among them). If you properly buffer the input stage, a mic amp can certainly be used as a line amp, but one cannot assume that to be the case (especially with 'low end' gear common in most home studios).

I don't know why you feel the need to argue - there are many good reasons not to daisy-chain mic preamps, and few to the contrary.

mshilarious - while you may have more knowledge and experience than the majority around here, it might do the community a disservice to imply that there's no difference between a mic preamp and a line amp. While YOU may know enough to get away with "bending the rules", a lot of people here need to learn the rules before they experiment with alternatives.

No hard feelings, I just was a bit nonplussed that a knowledgeable poster would choose to quibble with accepted "best practices" used in studios around the world.

Scott

Dude, reread your first post and you'll see why you got the blowback.

But then, what do I know, I've only been at this for 40 years...

Which STRONGLY implies that nobody else knows what they are doing. That ain't gonna make the other people before you on the thread real happy, is it?

If your first post had read like your last one, this could have been quite a bit more civil.

Anyway, you make a good point in your first post about mics and applications. I'd extend that to pres and applications. It isn't good engineering practice to use a low sensitivity mic on a quiet source into a noisy preamp. But you know what? I can't stop people from doing it, so I just try to help make it work better.

 

[DigitMus]

Hmm... I guess my attempt at irony didn't come across too well - I thought I was kind of dissin' myself and highlighting the increasing lack of relevance of "Old Schoolers" like me. I guess I'd better figure out the proper emoticon for that. (things never read the way we hear them in our heads)

...nothing to see here...move along, move along...

 

[dgatwood]

I don't know why you feel the need to argue - there are many good reasons not to daisy-chain mic preamps, and few to the contrary.

In this particular case, though, the limited gain and high noise level of the Brick somewhat contraindicates what you suggest. If you boost the gain later on in any form, you're boosting all the noise that the Brick adds, which can be substantial at high gain settings.

If you had a clean FET or op-amp preamp feeding into it at a low gain setting, even if you were throwing away a lot of that gain by running it through a pad to bring the level down closer to mic level, you'd still get much cleaner gain than if you brought up the gain later in the chain. In effect, you'd be using the non-tube preamp to bring the level of the SM7 up to a condenser mic level or perhaps a little beyond that.

What I'd love to see (MsHilarious, have you ever considered building one of these?) is about a 10-20dB variable-gain inline phantom-powered mic booster. In addition to being nice for bringing dynamic and ribbon mics up to a hotter level when working with underpowered pres, it would also effectively serve as a complete phantom block for ribbons (for the paranoid). Just a thought.

I wonder if these are usable:
http://www.samedaymusic.com/product--RODDPOWERPLUG

 

[mshilarious]

What I'd love to see (MsHilarious, have you ever considered building one of these?) is about a 10-20dB variable-gain inline phantom-powered mic booster. In addition to being nice for bringing dynamic and ribbon mics up to a hotter level when working with underpowered pres, it would also effectively serve as a complete phantom block for ribbons (for the paranoid). Just a thought.

I wonder if these are usable:
http://www.samedaymusic.com/product--RODDPOWERPLUG

Yeah, I've thought about it. In fact I even have parts and a PCB, I could do one today!

The problem again is noise. You need a low-current opamp to run off of phantom power, and that generally means higher noise. For a 3mA opamp, you are looking at chips that are around 8nV/Hz^.5, which translates to about -118dBV unweighted. Rode doesn't do quite that well; they rate their plug at -116dBA, which would be around -113dBV unweighted. There's no reason to plug a ribbon mic into anything that noisy, unless your only alternative is a Brick

There are a couple of ways to fix that problem: you can use a sexier chip like OPA227, which is only 3nV/Hz^.5 (-127dBV). You can use a low noise discrete front-end for the first 6dB of gain into a noisier opamp. You can use a step-up transformer.

The PCB I designed wasn't strictly for this purpose, or I would have done those things. I can't use OPA227 because my PCB is for a dual opamp (I could use OPA2227, but then current would be 8mA, which would lower headroom (not a big deal) and also not play nice with nonstandard phantom power). I didn't incorporate a discrete front-end, because for my other applications I didn't need the low noise figure. I can't fit a transformer inside an inline plug Well, I could, but the thing would be like 6 inches long

Variable gain is a pain because that requires careful machining of the inline connector. I'm not into careful machining

Hey, looking at the single-opamp pinout, I think I could probably hack the trim pins off and bridge the V+ and output pins to where they need to go. Hmmm.

 

[kidvybes]

...another thought...while starting to "play" with some of the Daisy-Chain possibilities, I realized that my interface (M-Audio Omni Studio) has 2 built-in pres (DMP-2) that are already in the chain...so what about "Brick into DMP-2" as a possibilty?...and what would be the difference of going into the "line" 1/4" input of the pre as opposed to the XLR input?...sorry if this is a sophmoric question, but my skills are more focused on the creative process than the hardware configurations...