Chaining Mic preamp channels?

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tmix

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With the proliferation of new ribbon mics out there requiring hi gain pres, I was wondering if it would work ( on say... a DMP-3)to run the mic t o channel one, the output of channel 1 to the input of channel 2 and run them at a more moderate and quiet level to have enough gain for a ribbon?

Is this just going to horribly overload the pre?
I thought I'd see if anyone else has blown up their pre before I offered to!
 
Hey Tom,
It's unlikely to give the results you want, but heck, it may be worth a try. There's two approaches that come to mind; either use a very small amount of gain in the first stage and come into the second stage as a mic input, or use the first stage to do the heavy lifting, and go into the second stage as a somewhat less than line input for some additional gain to bring it up to snuff. As long as you start with everything turned down and proceed carefully your not likely to break anything. Let us know how it goes if you go for it.
 
Hey RD,
Thanks for the answer.
I thought I'd lift it mostly with the first channel then run it into the line in and boost it as needed from there. I'll let you know how it goes.

I went to your myspace to listen, but the songs would not stream. Probably just my work connection. I'll try again tonight.

I do the progressive Celtic / Folk Fusion myself.

Later
Tom
 
Read that entire thread - however, it seems you have the jist of it - get as much clean gain out of the first preamp as possible, and then only use the second to make up the very last of it you need.
 
I forgot about that thread. My answer at the time was;

"I think there's a a couple of things here that are being taken as absolutes that aren't really. Saying that a preamp puts out line level is one. A preamp is a variable gain stage that we use to drive our signals approximately to +4 dBu (1.228V (RMS)) or -10 dBV (0.3162V (RMS)), but that's totally a function of how much of it's available gain we use and the signal level we start with. Secondly, input voltages don't always fall into "line level" or "instrument level" as absolutes.
The DMP3 can take a maximum input of +14.6dBU, which calculates to 12.38dBV, or 4.15V (RMS). So, it will have no trouble at all accepting the line level output of a previous preamp stage, which isn't even at line level or we wouldn't be discussing this.
The other consideration is input impedance. We usually talk about just high impedance and low impedance, but mid impedance should probably be in our lexicon. High impedance in this discussion would be around 1M - 3M ohm, typical for instrument inputs. Mid impedance might be around 10k - 20K ohm, typical for line inputs. Low impedance is usually in the 600 ohm range. I believe there is more to be gained by having a high impedance input for pickup based sources than there is to be lost by presenting a line level signal to a high impedance (instrument) input. Someone can correct me if I'm wrong, but I don't think this presents a problem."

Also, I'd like to address an error someone made in that thread.........

"I think the most confusion surrounding this issue is the misconception that the gain pot on a preamp boosts the output of the amplifier. The gain of an amplifier chip is static... The gain pot is nothing but a variable attenuator... By turning it up you're actually decreasing the attenuation at the input to the preamp. The net result is that the output of the amp increases, but the amplifier doesn't provide additional amplification, it's just passing along what it sees on the input".

That simply is not true for most mic preamp designs. The gain control usually varies the feedback, changing the gain of the preamp. This is true for the classic Jenson twin servo design, and most other feedback amplifier topologies. That's why things get hissy when you turn them all the way up, even with the input grounded.
 
Robert D said:
The other consideration is input impedance. We usually talk about just high impedance and low impedance, but mid impedance should probably be in our lexicon. High impedance in this discussion would be around 1M - 3M ohm, typical for instrument inputs. Mid impedance might be around 10k - 20K ohm, typical for line inputs. Low impedance is usually in the 600 ohm range. I believe there is more to be gained by having a high impedance input for pickup based sources than there is to be lost by presenting a line level signal to a high impedance (instrument) input. Someone can correct me if I'm wrong, but I don't think this presents a problem."
Click to expand...
What one usually wants to try to avoid on mic pre inputs is actually matching the impedance, as that can actually cause an inherant signal drop of up to 6dB.

The general rule of thumb (actually there are two, but I'll combine them here) in matching a mic to a pre is that the input impedance on the pre should be somewhere between 5-10x the output inpedance of the microphone for optimum gain and sound. What I'm unsure of is exactly what the deal is when one mismatches the other way; i.e. if line out is 10k-20k ohms, and you have that going into (for typical example) a preamp in rated at 1.6k ohms, then we have an impedance mismatch of about 10x in the other direction.

I don't believe this is all that dangerous, as plenty of folks have gone against the conventional wisdom and chained preamps, but that mismatch could have a damping effect on the gain potential of the second preamp (or am I wrong about that?). This would help explain why you'd want most of your gain boost at stage one (before adding noise and distortion, of course.)
Robert D said:
Also, I'd like to address an error someone made in that thread.........

"I think the most confusion surrounding this issue is the misconception that the gain pot on a preamp boosts the output of the amplifier. The gain of an amplifier chip is static... The gain pot is nothing but a variable attenuator... By turning it up you're actually decreasing the attenuation at the input to the preamp. The net result is that the output of the amp increases, but the amplifier doesn't provide additional amplification, it's just passing along what it sees on the input".

That simply is not true for most mic preamp designs. The gain control usually varies the feedback, changing the gain of the preamp. This is true for the classic Jenson twin servo design, and most other feedback amplifier topologies. That's why things get hissy when you turn them all the way up, even with the input grounded.
Click to expand...
To give credit where credit is due, what that guy said is true for many loudspeaker driver amplifiers and their associated preamps. Most home stereo and entertainment center integrated amps work that way...or at least they did as of a few years ago when I last looked.

My guess was he was trying to blanket that statement over mic preamps as well, and apparently got that wrong.

G.
 
SouthSIDE Glen said:
The general rule of thumb (actually there are two, but I'll combine them here) in matching a mic to a pre is that the input impedance on the pre should be somewhere between 5-10x the output inpedance of the microphone for optimum gain and sound. What I'm unsure of is exactly what the deal is when one mismatches the other way; i.e. if line out is 10k-20k ohms, and you have that going into (for typical example) a preamp in rated at 1.6k ohms, then we have an impedance mismatch of about 10x in the other direction. .
Click to expand...

Right. So, in this case (DMP3), I think everyone agrees the right approach is letting the first stage do the heavy lifting, so we're talking about going into a truely high impedance input, an instrument input with a Z of over a Meg. So the rule of driving a high impedance input with a lower impedance output is more than satisfied.

SouthSIDE Glen said:
To give credit where credit is due, what that guy said is true for many loudspeaker driver amplifiers and their associated preamps. Most home stereo and entertainment center integrated amps work that way...or at least they did as of a few years ago when I last looked.

My guess was he was trying to blanket that statement over mic preamps as well, and apparently got that wrong.

G.
Click to expand...

Yuppers. :)
 
Thanks guys!
I appreciate all the links and info.
At least I feel safe in experimenting.
It may be a total non issue .
If I get a ribbon mic (which started this whole mess) and I am able to get enough clan gain from the DMP3 or RNP or what ever I have, That is where I'll probably stop!

Notice I DID say Probably.....Now I WANT to know how it will sound!

Tom
 
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