Mic Preamp -- Is This a Silly Idea?

[clintrubber]

[quote author="Wavebourn"]What about an average temperature in the clinic, ... sorry, THD curves? How low impedance do they get from 6.8K resistors powering this device to provide low enough distortions?[/quote]
Please elaborate on how those 6.8k would prevent any circuit topology reaching decent THD-performance.

I would never design such a thing, even if it is well marketed commercial project...

That's the benefit of us DIY-ers, we don't need to design stuff that doesn't make sense from the technical side of things.
But if it was your job you could have needed to do so, since despite how silly, if it can make money then there's a commercial reason.


If one was able to start everything from scratch, the inclusion of any gizmo between mic & destination wouldn't be there probably (although in various cases there's obviously a benefit to be had from some local electronics before going on the long cable journey).

But these additions should be understood as being easy-to-add stuff to a certain given not too optimal situation. Like for instance budget-preamps with only just about enough gain. Addition of a good to reasonable in-between will relax the performance-demandment on those budget-preamps.
Like Rossi already said in his mag-article, those preamps can then work al lesser gain and the combination can sound less strained.

But you knew all this already :wink:

Regards,

Peter

 

[Guest]

[quote author="clintrubber"][quote author="Wavebourn"]What about an average temperature in the clinic, ... sorry, THD curves? How low impedance do they get from 6.8K resistors powering this device to provide low enough distortions?[/quote]
Please elaborate on how those 6.8k would prevent any circuit topology reaching decent THD-performance.

[/quote]

I mean convenience.

If one was able to start everything from scratch, the inclusion of any gizmo between mic & destination wouldn't be there probably (although in various cases there's obviously a benefit to be had from some local electronics before going on the long cable journey).

I would review standard levels. It is too high power level to amplify for both mic and power amps so they are bottlenecks. Lot of things may be simplified for more optimal results. A LOT.

 

[chris319]

The Rode D Power units have an Analog Devices AD620AR 8 pin surface mount single op-amp, can find a data sheet at the following;

http://www.analog.com/UploadedFiles/Data_Sheets/37793330023930AD620_e.pdf

Thanks for posting that. Just as I thought, there is a gain-setting resistor between pins 1 and 8. Swap in a 510-ohm resistor and you have 40 dB of gain.

 

[Samuel Groner]

The Rode D Power units have an Analog Devices AD620AR 8 pin surface mount single op-amp.

That's not an opamp, it's an instrumentation amplifier (i.e. about three opamps in one package). But *shudder*, this is not a suitable part for low noise/low distortion audio amplification. About 20 dB more noise than a good mic pre and likely distortion about as good as a HiWatt at medium levels. I'd rather get a decent outboard pre than use this thing.

Samuel

 

[chris319]

I'd rather get a decent outboard pre than use this thing.

In a studio that would be practical. If you're in the field and no AC is available you're not going to split hairs over a few dB of noise (think radio reporter with an RE50 covering a news conference).

Actually if I were a radio reporter I would be looking at one of these:

http://www.hhb.co.uk/flashmic/index.asp?ReferID=int

 

[JohnRoberts]

[quote author="chris319"]

I'd rather get a decent outboard pre than use this thing.

In a studio that would be practical. If you're in the field and no AC is available you're not going to split hairs over a few dB of noise (think radio reporter with an RE50 covering a news conference).
[/quote]

There is no reason why a simple phantom powered pre-pre needs to significantly degrade S/N (as described earlier in this thread).

It looks like they are using the wrong tool for the job.

JR

 

[Rossi]

PRR's ribbon booster actually improves noise performance compared to most normal mic inputs. You have to use special low Rbb transistors, of course. So it is possible, but 40 dB of clean gain is most certainly asking too much. I don't think a phantom powered circuit will do that without noise and/or distortion.

 

[chris319]

40 dB of clean gain is most certainly asking too much. I don't think a phantom powered circuit will do that without noise and/or distortion.

Maybe that's why Rode chose the chip they did.

 

[Junction]

The Rode D-Power devices are pitched at the live sound type market, talking about their uselfullness with long cable runs of 100m or more. I don't think they were intended for studio quality use.

I'll try to do some rough tests on the ones that I have this week.

Michael

 

[chris319]

The Rode also appears to be intended for dynamic mics only, i.e. it doesn't pass 48V back to the mic.

 

[JohnRoberts]

[quote author="chris319"]

40 dB of clean gain is most certainly asking too much. I don't think a phantom powered circuit will do that without noise and/or distortion.

Maybe that's why Rode chose the chip they did.[/quote]

There is little point to put 40 db of gain on a signal being plugged into a mic preamp... If anything you are more likely to overload the preamp and won't be running at optimal gain for that stage's S/N.

JR

PS: and live sound cares about S/N also. The least picky market in my experience was fixed install where they are more interested in saving a few pennies.

 

[chris319]

Here is something I like better than my original crazy idea.

I built a little preamp around the THAT 1512 which has a maximum gain of 60 dB. Yesterday I was poking around on the Web, looking at data sheets for mic preamp chips when I noticed something that had escaped me previously: the INA217 has a maximum gain of 10000 -- that's right, 80 dB. I just happened to have some INA 217s around, opened up my preamp, popped the 1512 and replaced it with an INA217. I tested it, it still worked (that's good), and then removed the 5-ohm maximum gain resistor in series with the gain pot, so at full CW there is now a direct connection between pins 1 and 8 (where the gain resistor is supposed to be).

I got out my RE27 and set it up about 40" from me, and wow! I get 0 dBFS with my normal speaking voice at that distance! This is exactly what I've been after, so I'm going to revisit this little preamp and probably build a second, sturdier prototype.

This preamp is not phantom powered, BTW. It runs off of two 9-volt batteries.

 

[Samuel Groner]

Where the heck to you have that information from that an instrumentation amplifier should have a well defined maximum gain? That's not true, there are second order limits though (e.g. the contact resistance of your pot, which is likely the limiting factor).

In any case you don't want to run any of the mentioned chips much above 60 dB as bandwidth and distortion heavily degrades at these gains. There's nothing to gain above 60 dB anyway--you're just shifting levels (which you can do in your DAW as well, with the additional advantage that you don't ruin your record by clipping), you don't win lower noise.

If you really want 80 dB you need a second gain stage to do it well. A 20 dB noninverting amp is easy and doesn't eat too much power if a proper opamp is selected.

I'd better use the THAT chip (preferably the 1510) for the frontend, it's much better than the INA217.

Samuel

 

[chris319]

If you really want 80 dB you need a second gain stage to do it well. A 20 dB noninverting amp is easy and doesn't eat too much power if a proper opamp is selected.

What would you use? How about an OPA604?

 

[Samuel Groner]

I think a NE5534 might be a good fit. As the gain is high it doesn't need a compensation capacitor and its performance will be very good. There are lower power parts but they ain't as good in amplifying audio, and the frontend IC will anyway eat most current.

Just rechecked the INA217 and THAT1510 datasheets and the THAT part is preferred for its lower quiescent current as well.

Samuel

 

[chris319]

Thanks, Samuel. I will check that out.

 

[chris319]

I'm looking through various circuits using the NE5534 and here is what I get:

Pin 1/Pin 8: Gain resistor? (I'm not finding a gain equation for this)

Pin 2: Output of 1510

Pin 3: Ground?

Pin 4: -V

Pin 5: Ground

Pin 6: Output (output resistor needed?)

Pin 7: +V

What other components are needed to interface this with a 1510 and work into a -10 dB input?

I have been testing the 1510 and INA217 with RMAA and they do get rather squirrely above 50 dB gain.

TIA

 

[Samuel Groner]

You might want to check the basic noninverting opamp configuration for more information. Post a schematic and we will help further with the details.

I have been testing the 1510 and INA217 with RMAA and they do get rather squirrely above 50 dB gain.

Part of that is your DA converter which won't perform well at low levels. You could build an output PAD for the converter so that you can run it at high levels even for high gain circuits.

Samuel

 

[mikep]

to provide an alternate point of view.. on paper these instrumentation amp designs will work at 80dB or whatever, but does it sound good? besides the specs there is always a maximum USEABLE gain for any preamp, above which the sound quality isnt very pleasing. IME this is very noticable with something like a low cost console mic amp, but you need a truly high performance preamp to compare it to, to know what you are missing.

out there in real world recording situations, when ultra high gain is needed for a low output mic on a quiet source, usually a high ratio transformer input preamp is used. a long time favorite for this type of application would be a V76. Im just talking results here, not tech specs but how other successful recordists have made GOOD sounding recordings.

just something to think about. If you are building a pre optimized for high gain, consider a 1:10 or higher ratio input transformer. and get out your wallet!

cascading another opamp might help you get the gain you need, but IMO you need to look at RFI filtering at the input and common mode performance optimization very carefuly if you are going to get good results with a transformerless input operating on such a small input signal.

if you are amplifying speech for internet chat then perhaps I am off-base, but if you are trying to record a sitar with a coles 4038 and have it sound as good as possible, then that is what Im talking about.

mike p

 

[JohnRoberts]

There are practical gain limits on integrated circuits designed to support unity gain but discrete designs or hybrid IC+discrete are not so limited and open loop gain can be increased for any desired closed loop gain.

The only practical limit in transformerless mic preamps is the input stage noise floor. There are solid state devices quite competent for most microphone impedances, only some of these more extreme (low impedance) examples are too low to match up well.

While there are always examples of bad design, we should differentiate between what can be designed around and what can't. (And that line may get moved by future technology advances.)

JR