Pads for preamps and gain-staging

[mjfe2]

Had a few questions re gain staging. Firstly, do many people find use for pads on their preamps? I've recently been experimenting with true omnis on loud bass instruments e.g. bass cab, sousaphone. It seems a nice way to get a full, close-mic'd sound without the proximity effect. Only problem is, neither of my omnis (Neumann KM183 and Sennheiser MKH8020) have pads and even with my pre gain set to 0 the signal still clips my converters occasionally.

I've noticed that my RME OctaMic has a gain range of 6-60dB and no pad so am I right in thinking that I'll always be boosting the mic signal by 6dB even when not adding gain? I then thought I could use my Mackie Onyx pres instead, which go from 0-60dB but these are slightly higher impedance (2.5kOhm rather than 2kOhm) and seem to be slightly louder than the RME's. Does that sound plausible? If I reduced the inputs on my Fireface to 'lo gain' would this solve the problem when taking an output from the Onyx? In other words, how do I know how much headroom is built into these various preamps?

On another note, the only preamp I have with a pad is an old LA Audio MX2, but it's recently starting humming in both channels (R louder than L). Are there any quick fixes I can try here? One of the gain pots was being intermittent but I solved that with some Deoxit. Would more Deoxit inside the unit help?!

[Hugh Robjohns]

Last question first... If a previously quiet preamp has started humming, the chances are the power supply capacitors have dried out and need replacing. It's likely that power supply ripple is getting through to the preamp gain stages...

The headroom of different preamps varies a lot, but in general, the better the preamp the more headroom it will have. Capacitor mics general have much higher sensitivity than dynamic mics and so will tend to eat in to the preamp headroom more.

There are three possible clipping points, each needing a different solution.

If you put a capacitor mic in front of something very loud, there is a danger of overloading the impedance converter inside the mic. Ifthe internal impedance converter headroom is likely o be insufficient some manufacturers include a switchable pad between capsule and impedance converter.

The second clip risk is in the preamp itself. A mic in front of a loud source will generate a big output, and that might potentially overload the front-end of the preamp. If this is a concern the manufacturer usually includes a switchable pad at the preamp input. Again, different designs have differing headroom margins...

The third clip risk is in the converter, and this is down to gain staging. You need to adjust the preamp output and/or converter input to accommodate the maximum signal level. Obviously, in an interface that combines preap and converter the gain structure is probably fixed, in which case you're stuffed.

RME converters often have three headroom margin settings, confusingly called -10, +4, and either low or high gain... Ignore the terms and examine the peak signal levels each mode can accommodate to see what the different headroom margins are. Low/high gain usually has the greatest headroom.

If you're overloading the mic and it doesn't have a pad switch, your only options are to move the mic further way, reduce the volume of the source, or change the ic for something more appropriate.

If you're overloading the preamp, andit doesn't have a pad switch, then use an in-line 20dB XLR pad between mic and preamp.

If you're overloading the converter, then adjust the preamp output or converter input/headroom settings, if possible, or use that in-line XLR pad again to increase the headroom margin through both preamp and converter.

And as for those preamp gain numbers... Take them with a pinch of salt. Some really do give the total gain applied by the whole preamp, but many relate only to the variable gain stage and don't include any additional gain (or loss) provided by other controls or stages.

H

[ef37a]

Said it before.
The only figure for AI pre amps that makes sense is sensitivity, not gain.

That is, "X" mV/dBu/dBV for 0dBFS.

Dave.

[mjfe2]

Thanks Dave and Hugh. I doubt it's the mics that are clipping because they can take such high SPL. It seems the RME is not an option here because the ADC is built in. When it comes to the Onyx though, it's more ambiguous. If I increase headroom by feeding it into the Fireface inputs at 'lo gain' am I right in thinking the only way to identify if the pre is clipping is by ear? For short transients this can be difficult to hear. The metering on any of these preamps is fairly rudimentary and also seems to light up red a few dB before clipping actually takes place.

[Hugh Robjohns]

Line up tones are your friends in situations like this...

[ef37a]

Line up tones are your friends in situations like this...

Heaven forfend Mr R that we should MEASURE anything!

Dave.

[Hugh Robjohns]

Don't be a prat Dave. I'm sure you are perfectly well aware that I have been providing min and max sensitivity measurements in all my preamp reviews for several years.

[ef37a]

Don't be a prat Dave. I'm sure you are perfectly well aware that I have been providing min and max sensitivity measurements in all my preamp reviews for several years.

Yes, but I meant that the PUNTERS should measure anything.
And I don't understand your pique? It is the AI COMPANIES that should give us proper specifications

Dave.

[Hugh Robjohns]

Pique? Good word. I thought you were having another of your pops at me. Didn't realise it was aimed at other punters!

Agree about poor specs from some manufacturers.

H

[ef37a]

Pique? Good word. I thought you were having another of your pops at me. Didn't realise it was aimed at other punters!

Agree about poor specs from some manufacturers.

H

I think you have the direction of any "pops" rather turned around! Still, y'all have been getting some flak lately! FWIIW I am generally pretty happy with the magazine and its content. I especially enjoyed the piece about Roger the Moog synth! Although I had always enjoyed "classical" music (I grew up with "From the Palm Court" on the radio) it was Switched on Bach that got me onto Bach proper.

Dave.

[mjfe2]

http://www.studiocare.com/neutrik-in-line-xlr-attenuator-20d... Is there an attenuator like this that does let through phantom power? Couldn't seem to find anything on the Neutrik website...

[Hugh Robjohns]

http://www.canford.co.uk/Products/21254/20-922_CANFORD-IN-LI...

[mjfe2]

Ah yes, thanks! Actually, is it easy to wire something myself? I have some spare XLR connectors lying around.

[ef37a]

Ah yes, thanks! Actually, is it easy to wire something myself? I have some spare XLR connectors lying around.

Depends how wee the resistors are and how dextrous thee be!
You will find a calculator on Google for the values ( or Hugh might have them OTTOHH?). Try to match the series pair as closely as you can to preserve common mode rejection.

I would like to see inside that phantom power blocker. Hardly looks big enough for any decently sized caps' and surely not a transformer?

Dave.

[Hugh Robjohns]

Yes, simple enough using a 'U' type attenuator circuit. You need very closely matched low-value series resistors for each audio leg, and a third shunt resistor bridging between them. The series resistors will reduce the phantom voltage slightly, but as their value is normally so small compared to the standard phantom 6k8 supply resistors that it makes little practical difference. For modern mics that consume only a few milliamps, the voltage drop will only be a couple of volts.

For a 20dB pad, typical values are 681ohm 1% series resistors and 150 shunt resistor. If you can't get 1% resistors, match two 680s as closely as possible with a multimeter to optimise CMRR.

Wire the 150 directly between pins 2 and 3 of the male XLR, with the 680 ohms between the each of the audio pins (2 and 3) and the corresponding pins of the female XLR.

In use, always put the pad at the preamp input, NOT the mic output.

H