Mic Preamp -- Is This a Silly Idea?

[chris319]

Is this a silly idea? I'm thinking of an ultra-compact inline mic preamp with about 40 dB of fixed gain and which would plug into a balanced mic input and operate off 48V of phantom power. Something along the lines of this:

http://www.micbooster.com/products/felminimicbooste.html

but with higher gain. The intended use would be to plug into the mic input of a CF recorder having weak mic preamps.

All suggestions welcome.

 

[bcarso]

Why 40dB? That sounds like an awful lot for the application.

 

[JohnRoberts]

I'm not sure what the point of this is... Most preamps that are serious enough to provide phantom voltage are serious enough to have adequate gain. (EDIT- perhaps except for yours)

I have designed phantom powered high impedance FET buffers with modest gain that could be built into a connector body, for DI of bass guitar or such. I never tried it as a product.

I have also scratched up designs to make a crude phantom powered input expander where say 4 dynamic mics could be submixed with individual gain, polarity, pad, mute, etc controls into one normal phantom mic channel. This is compromised somewhat by the lack of EQ on the sub inputs while some crude EQ could be tricked into the gain impedance between the emitters of each individual LTP.

This was "almost" a viable product for small church applications that often run out of mic inputs, but low end mixers are so inexpensive these days most dealers would rather sell them a new mixer. Than carry another low cost, low margin accessory.

JR

 

[chris319]

I'm not sure what the point of this is... Most preamps that are serious enough to provide phantom voltage are serious enough to have adequate gain. (EDIT- perhaps except for yours)

I am evaluating a Fostex FR-2LE CF recorder which is a wonderful device except the mic preamps are weak. By my measurement the mic inputs require -50.5 dBV to come up to 0 dBFS with the mic trims fully CW. With a typical dynamic mic, say an RE50 with its -55 dBV at 1 Pascal, you have to be right on top of it to get even close to 0 dBFS. Granted, one could use an outboard mic pre and go into the line input, but for someone in the field that's one more thing to carry and plug in.

 

[JohnRoberts]

OK, how much can the mic input handle with the trim full CCW?

If you only need 5dB I wouldn't target too much gain, but it may take some work to keep noise and impedances right for low noise.

I would look into hanging a pair of low noise NPN transistors as a differential in/differential output gain stage.

You want to keep the resistor between the two emitters small to keep noise low, but not so low it makes gain too high. You can scrape off some excess gain with a reistor across the two collector outputs that can also drop the source impedance down in the expected 150-200 ohm range.

Have fun,,, I suspect the gear is looking for a hotter mic. but the good thing about doing this outboard is you can always stop using it later.
---------
A friend of mine had an opposite problem where he needed to pad down a mic in a video recorder because his kids band was creaming the input.

JR

 

[clintrubber]

[quote author="JohnRoberts"]I would look into hanging a pair of low noise NPN transistors as a differential in/differential output gain stage.

You want to keep the resistor between the two emitters small to keep noise low, but not so low it makes gain too high. You can scrape off some excess gain with a reistor across the two collector outputs that can also drop the source impedance down in the expected 150-200 ohm range.[/quote]
The booster originally meant for ribbons might be of use:
http://www.groupdiy.com/index.php?topic=5743&postdays=0&postorder=asc&start=97
Not sure if this was the final version (Chris, if possible you might want to check the magazine-article Rossi wrote), but it's at least the thread with alike stuff going on. Let's re-use.

 

[chris319]

OK, how much can the mic input handle with the trim full CCW?

I will test it and get back to you.

I would look into hanging a pair of low noise NPN transistors as a differential in/differential output gain stage.

Got any part numbers in mind?

I suspect the gear is looking for a hotter mic

It works reasonably well with a hot-output condenser mic -- some of the MXLs and Rodes, and the AKG Perception 150.

A friend of mine had an opposite problem where he needed to pad down a mic in a video recorder because his kids band was creaming the input.

I never go anywhere without my Shure inline attenuators. They have saved the day many times.

Thanks.

 

[clintrubber]

Two new messages right next to each other. The thread two messages up has type-suggestions.

Cheers,

Peter

 

[PRR]

> inputs require -50.5 dBV to come up to 0 dBFS with the mic trims fully CW.

3mV

> With a typical dynamic mic, say an RE50 with its -55 dBV at 1 Pascal

2mV @ 94dB SPL

> you have to be right on top of it to get even close to 0 dBFS

Why do you need to be close to 0dBfs?

Can you afford 16-bit recording? (At today's CF meg/$, it should be trivial.)

With the gain given, 97dB SPL gives 0dBfs. At 16-bit resolution, "noise" is 96dB below that, or 1dB SPL. Unless the mike preamp is fabulous, mike+preamp noise will be 15-25dB SPL. In many situations, ambient noise will be 20-50dB SPL.

So it seems to me, for most field speech reporting, you leave the trim full-up, take what you get, be it -10dBfs or -25dBfs, and normalize it in the production room. Putting more signal on "tape" gives a recording that is audible without normalizing, but depends on the field-person to set level, and that person has better things to do than watch levels.

 

[JohnRoberts]

[quote author="clintrubber"][quote author="JohnRoberts"]I would look into hanging a pair of low noise NPN transistors as a differential in/differential output gain stage.

You want to keep the resistor between the two emitters small to keep noise low, but not so low it makes gain too high. You can scrape off some excess gain with a reistor across the two collector outputs that can also drop the source impedance down in the expected 150-200 ohm range.[/quote]
The booster originally meant for ribbons might be of use:
http://www.groupdiy.com/index.php?topic=5743&postdays=0&postorder=asc&start=97
Not sure if this was the final version (Chris, if possible you might want to check the magazine-article Rossi wrote), but it's at least the thread with alike stuff going on. Let's re-use.[/quote]

Yup pretty much what I had in mind.

I'd probably use slightly different values but thats close enough to make something work.

Use 1% resistors for good CMR.

2sd786 comes to mind as a nice low noise NPN, there are others.

JR

 

[chris319]

[quote author="PRR"]With the gain given, 97dB SPL gives 0dBfs.[/quote]

94 dB SPL is the approximate loudness of conversational speech at one inch from the mic. At 12" it's apx. 72 dB SPL. Given a mic with sensitivity of -54 dBV per Pa, the output will be apx. -76 dBV at a distance of 12". That's -21 dBFS. At 24" it will be -27 dBFS.

Funny thing about the mic trims on the FR-2LE. Listening on headphones with the mic trims full CW, there is a certain level of noise. Back the trims off slowly and at a certain point the noise drops abruptly, and I mean abruptly. This leaves the trims at about the 4:30 position. At that setting, the background noise is substantially lower but it now requires 6 mV instead of 3 mV to reach 0 dBFS. So let's add another 6dB of amplification. So maybe I can live with gain in the 30 - 35 dB range.

I have written a short article relating acoustical loudness to mic sensitivity and electrical levels. Peer review is welcome:

http://www.miclisteningroom.org/gain.html

 

[chris319]

OK, how much can the mic input handle with the trim full CCW?

With the trim full CCW it takes 281.1 mV, or -11 dBV, to reach 0dBFS -- a 40 dB range from full CCW to full CW.

 

[StephenGiles]

[quote author="PRR"]> inputs require -50.5 dBV to come up to 0 dBFS with the mic trims fully CW.

3mV

> With a typical dynamic mic, say an RE50 with its -55 dBV at 1 Pascal

2mV @ 94dB SPL

> you have to be right on top of it to get even close to 0 dBFS

Why do you need to be close to 0dBfs?

Can you afford 16-bit recording? (At today's CF meg/$, it should be trivial.)

With the gain given, 97dB SPL gives 0dBfs. At 16-bit resolution, "noise" is 96dB below that, or 1dB SPL. Unless the mike preamp is fabulous, mike+preamp noise will be 15-25dB SPL. In many situations, ambient noise will be 20-50dB SPL.
So it seems to me, for most field speech reporting, you leave the trim full-up, take what you get, be it -10dBfs or -25dBfs, and normalize it in the production room. Putting more signal on "tape" gives a recording that is audible without normalizing, but depends on the field-person to set level, and that person has better things to do than watch levels.[/quote]


Thanks PRR - AMBIENT NOISE - always there in varying proportions!![/b]

 

[Junction]

Check out the Rode D-Power or D-PowerPlug, these give you 20dB gain, powered from your 48V supply, the D Power is neat because it can fit permanently inside most dynamic mics.

http://www.rodemic.com/accessory.php?product=DPower


http://www.rodemic.com/accessory.php?product=DPower_Plug

Michael

 

[chris319]

I thought I was losing my mind! I KNEW I had seen this product before but my feeble brain thought it was made by MXL instead of Rode, so I was looking for the wrong thing in the wrong places. (MXL makes a near-useless mic to USB adapter with only 20 dB of gain for the whole thing.)

I see an 8-pin IC in there. I wonder if there might be a resistor connecting pins 1 and 8 which acts as a gain control as is the case with the THAT 15xx chips. That would enable a user to hotrod it up to 30 or 40 dB. There's one way to find out ...

 

[Junction]

I have one of these at the studio, haven't used it, but I can have a look at it next week and let you know what on the PCB, it is a surface mount IC, thats all I remember at this stage.

Michael

 

[chris319]

[quote author="Junction"]I have one of these at the studio, haven't used it, but I can have a look at it next week and let you know what on the PCB, it is a surface mount IC, thats all I remember at this stage.[/quote]
That would be great, thanks!

 

[Guest]

Something like this:

 

[chris319]

What are the things that look like "8"s in the upper right corner?

 

[Guest]

[quote author="chris319"]What are the things that look like "8"s in the upper right corner?[/quote]

Constant current sources. Actually, couple of JFETs.