Topic: Brickwalling with Zefiro Inbox/Denecke AD-20

[hcouch]

Hi guys,

I have been running a simple Rode NT4 > Sony MZ-RH910 (HiMD) rig for the past few months.  I've made some decent tapes, but I was looking to improve them a little bit.  I shopped around and thought that an Inbox/AD-20 would be a good addition.  I'd have a better pre than the internal one on the Sony, plus adding the AD would let me run digi into the MD and hopefully avoid any noise from the mechanics of the spinning MD.

Well, I ran it for the first time last night:
Rode NT 4 > XLR > Zefiro Inbox / Denecke AD-20 > Toslink digi > HiMD

I was taping Derek Trucks in a loud music hall.  I set the knobs on the Inbox at 12 o'clock.  Once the show started, the levels on the MD were peaking around -8 dB.  I figured I was in good shape.  I did notice that the meters weren't bouncing a lot, but I figured it was no big deal.

Well, I get home and upload the WAVs and lo and behold, I am brickwalled all to hell. 

I am guessing the Rode was driving the Inbox too hot?  Am I going to need to add in-line attenuators?  Am I missing something?  Could something else be causing this?


Thanks,
Harvey

[DSatz]

The AD-20 doesn't have a problem with input overload when properly powered; no matter where you set its gain controls, its preamps can handle whatever signal levels it takes to drive the converter outputs to 0 dBFS. So unless your AD-20 is defective in both channels in exactly the same way or its battery was weak, you can rule that out.

When the gain controls on my AD-20 are set to 12:00 it takes around 100 mV at the mike inputs to get 0 dBFS from the converters. Given the Røde NT4's rated sensitivity of 12 mV/Pa, 100 mV would occur at about 112 dB SPL. But if your MD recorder's meters were telling you the truth, then only about 104 dB SPL would be required because the meters only got to -8. 104 dB is a plausible sound pressure level in some venues, but I wasn't there, so I don't know how loud it was at your microphones.

In your waveforms the peak levels aren't exactly uniform throughout--there are little whiskers of slightly higher momentary levels. It looks to me as if the signal was originally some 6 - 10 dB higher than the level that it was forced to fit into. Are the peak levels in the WAV file nearly 0 dB? There's no vertical scale, so it's hard to tell. If so, does your transfer software from MD to WAV have an option setting for "normalizing" a recording, and if that's the case, could you switch that option off and try the transfer again? That would eliminate one uncontrolled variable at least.

--best regards

[jlykos]

I ran a similar setup for a while and only had problems brickwalling with the AD-20 a couple times, each of which was in a very loud environment.  It is possible to brickwall the AD-20, but it takes a hell of a lot of volume to do so.  I am not familiar with the Rode microphones, but the issue may be that you were not adequately powering them.  Are the Rodes self-powered (meaning if they have an internal battery)?  If they are not self-powered, you will need a battery box like the Denecke PS-2 or something similar to power the microphones to provide phantom power to them.  You would run Rode > PS-2 > AD-20 > MD in that case.  I suspect that by running them direct to the preamp on the MD, that was providing some kind of power to the microphones in your setup before you purchased the AD-20.  The PS-2 > AD-20 combination will sound much better than the internal preamp on your MD, but you need to ensure that your microphones are adequately powered.

If the mics are adequately powered with this setup, you will find that you will not need to run the levels on the AD-20 much lower than the 12:00 position; more like 9:00 or something like that in a loud concert environment.

[hcouch]

Thanks a bunch for the replies.  I'll see if I can't address any issues raised...

The AD-20 doesn't have a problem with input overload when properly powered; no matter where you set its gain controls, its preamps can handle whatever signal levels it takes to drive the converter outputs to 0 dBFS. So unless your AD-20 is defective in both channels in exactly the same way or its battery was weak, you can rule that out.

Battery was fresh


When the gain controls on my AD-20 are set to 12:00 it takes around 100 mV at the mike inputs to get 0 dBFS from the converters. Given the Røde NT4's rated sensitivity of 12 mV/Pa, 100 mV would occur at about 112 dB SPL. But if your MD recorder's meters were telling you the truth, then only about 104 dB SPL would be required because the meters only got to -8. 104 dB is a plausible sound pressure level in some venues, but I wasn't there, so I don't know how loud it was at your microphones.

It was pretty loud...  It was a rock show, but it's not like my ears were bleeding. 


In your waveforms the peak levels aren't exactly uniform throughout--there are little whiskers of slightly higher momentary levels. It looks to me as if the signal was originally some 6 - 10 dB higher than the level that it was forced to fit into. Are the peak levels in the WAV file nearly 0 dB? There's no vertical scale, so it's hard to tell. If so, does your transfer software from MD to WAV have an option setting for "normalizing" a recording, and if that's the case, could you switch that option off and try the transfer again? That would eliminate one uncontrolled variable at least.

The vertical scale is on the right side of the attached image.  I zoomed in vertically, so you could see the waveform better.  It's peaking at about .45 out of 1.0.  There is no normalization feature that I have seen in the software and I doubt that is the problem, because I could hear the brickwalling when I played it back after the show straight out of the MD using headphones.

I ran a similar setup for a while and only had problems brickwalling with the AD-20 a couple times, each of which was in a very loud environment.  It is possible to brickwall the AD-20, but it takes a hell of a lot of volume to do so.  I am not familiar with the Rode microphones, but the issue may be that you were not adequately powering them.  Are the Rodes self-powered (meaning if they have an internal battery)?  If they are not self-powered, you will need a battery box like the Denecke PS-2 or something similar to power the microphones to provide phantom power to them.  You would run Rode > PS-2 > AD-20 > MD in that case.  I suspect that by running them direct to the preamp on the MD, that was providing some kind of power to the microphones in your setup before you purchased the AD-20.  The PS-2 > AD-20 combination will sound much better than the internal preamp on your MD, but you need to ensure that your microphones are adequately powered.

The Rode is self-powered with a 9v.  It was a fresh battery, too.  It was running off of a battery when I was running straight into the MD, so I don't think that is the problem.


If the mics are adequately powered with this setup, you will find that you will not need to run the levels on the AD-20 much lower than the 12:00 position; more like 9:00 or something like that in a loud concert environment.

Any other thoughts?

[Brian Skalinder]

Battery was fresh

Fresh != known-good.  I've had more than my share of "fresh", i.e. newly bought, 9v batteries that simply weren't up to snuff.  So I wouldn't rule out the battery as a root cause simply because they were new.

[hcouch]

Battery was fresh

Fresh != known-good.  I've had more than my share of "fresh", i.e. newly bought, 9v batteries that simply weren't up to snuff.  So I wouldn't rule out the battery as a root cause simply because they were new.

I will check it on a battery meter tomorrow at work.  It is a Duracell.  Rode rates battery life in the NT4 at 400 hours per 9v.  This one had been used about 2 hours.  I had run it the previous week and had no problem with it when I running straight into the MD. 

Is it possible that a battery would be fine one week and then "not up to snuff" the next?  And yes, I am sure that the mic was powered off.  In fact, I removed the 9v after the show the previous week.

[MattH]

Can you tell from listening to the wave if it's just the low frequencies causing the (apparent) brickwall? Rolling off those frequencies can often yield a better looking wave and a much more listenable recording.

[DSatz]

OK, if the peak amplitudes are at about 45%, that matches pretty closely what the LEDs on your MiniDisc recorder were telling you (45% modulation = 6.9 dB below full scale).

Again: If your AD-20 is similar to mine, was working properly, and was properly powered, you couldn't possibly have overloaded its preamps given that its digital output was below 0 dB (full scale) at all times. With apologies to jlykos, volume alone doesn't overload a mike preamp; voltage does. The voltage at the input of a preamp is a function of volume, of course, but it depends equally on the sensitivity of your microphones.

The AD-20 has enough input headroom across its entire range of gain settings so that any signal that could possibly overload its preamps would also drive its A/D converters to full scale (and beyond if that were possible, which of course it's not). But yours was driven only to -7. So we can safely conclude that the preamps weren't being overloaded--provided that the 9V battery was really OK at the time.

Can you recreate the hookup that you had, and play something equally loud at the mikes? If so, you could try putting resistive pads (attenuators) at the inputs of the AD-20 to see whether the distortion goes away or whether it merely occurs at a level that's correspondingly lower (by the amount of the pad's attenuation). Based on what you've said so far, I would expect the latter situation to occur (e.g. if you use 20 dB pads, the same clipping would occur 20 dB lower in the recording). If on the other hand the distortion goes away or is greatly reduced, then the AD-20 would be highly suspect because it absolutely should not overload that way, as I've said "many times, many ways" by now.

--best regards

[hcouch]

Can you tell from listening to the wave if it's just the low frequencies causing the (apparent) brickwall? Rolling off those frequencies can often yield a better looking wave and a much more listenable recording.

It doesn't appear to be the low freq's causing the brickwall.  Are you talking about rolling off those frequencies during recording or in post?

OK, if the peak amplitudes are at about 45%, that matches pretty closely what the LEDs on your MiniDisc recorder were telling you (45% modulation = 6.9 dB below full scale).

Again: If your AD-20 is similar to mine, was working properly, and was properly powered, you couldn't possibly have overloaded its preamps given that its digital output was below 0 dB (full scale) at all times. With apologies to jlykos, volume alone doesn't overload a mike preamp; voltage does. The voltage at the input of a preamp is a function of volume, of course, but it depends equally on the sensitivity of your microphones.

The AD-20 has enough input headroom across its entire range of gain settings so that any signal that could possibly overload its preamps would also drive its A/D converters to full scale (and beyond if that were possible, which of course it's not). But yours was driven only to -7. So we can safely conclude that the preamps weren't being overloaded--provided that the 9V battery was really OK at the time.

Can you recreate the hookup that you had, and play something equally loud at the mikes? If so, you could try putting resistive pads (attenuators) at the inputs of the AD-20 to see whether the distortion goes away or whether it merely occurs at a level that's correspondingly lower (by the amount of the pad's attenuation). Based on what you've said so far, I would expect the latter situation to occur (e.g. if you use 20 dB pads, the same clipping would occur 20 dB lower in the recording). If on the other hand the distortion goes away or is greatly reduced, then the AD-20 would be highly suspect because it absolutely should not overload that way, as I've said "many times, many ways" by now.

--best regards

Thanks for your continued assistance...  I can attempt to recreate the volume level, but I don't have any pads to use.

If the result is the latter (as you suspect), would that not make the AD-20 suspect as well?  Meaning that it needs to have attenuation in line order to handle the input from these mics?

[DSatz]

> If the result is the latter (as you suspect), would that not make the AD-20 suspect as well?  Meaning that it needs to have attenuation in line order to handle the input from these mics?

The AD-20 should be suspect only if it is underpowered or defective. Maybe I haven't been clear enough with the logic behind that statement. If you turn the gain all the way down on the AD-20, it takes a stronger signal to drive its converters all the way to 0 dB, but at (say) -0.25 dB the signal is still clean and undistorted; the preamps aren't overloading. If you turn the gain all the way up, it takes less of a signal to drive the converters to 0 dB but again, at -0.25 dB there's no audible distortion. Nor is there at any other setting in between.

Therefore you can conclude absolutely that the signals reaching the AD-20 were NOT too strong for it, since its converters didn't reach (or even approach) 0 dB. Do you see why this is so? It wouldn't necessarily be true for other preamp/converter combination units; you'd have to test them individually to see.

Also, if the gain range of the AD-20 were wider than it is, there might be a problem at one end of the scale or the other, but with the particular gain range that it has there is no problem.

I suggested the attenuator test because it is quick, (relatively) easy and definitive. Regardless of the outcome, one way or the other you'll have narrowed the problem way down.

--best regards

[DSatz]

Since some other people are reading this thread for information, I'd like to reply to your private message "out here" if I may.

Background: You partly recreated the problem situation as an experiment, except that you probably didn't reach the original sound levels, and your batteries may have been fresh--you didn't say. There was no clipping on the recording, and the levels came close to 0 dB full scale when you increased the gain on the AD-20.

> Please forgive my confusion, but, if my test has excuplated the AD-20, can I therefore deduce what it is, exactly, that caused my recording to brickwall?

Your test exculpated the AD-20 in that it showed the unit isn't broken in some way that would limit its usual ability to handle high signal levels. But it didn't rule out the possibility that (contrary to your impression at the time), its battery was weak during the first recording.

The similarity of the clipping between the left and right channels is striking, and hints at a power supply problem in either the microphone or the AD-20 (since there is no limiter circuit in the AD-20, and its input overload margin is definitely sufficient with normal powering)--or possibly that the microphone itself was being overloaded internally.

Capsules in condenser microphones can generally handle far higher sound pressure levels than the output circuits of the same microphones can convey. When a manufacturer gives the maximum SPL for a condenser microphone, that is nearly always the limit of the microphone's circuitry. That limit, in turn, depends both on its design and on its having proper powering.

--best regards

[hcouch]

Alright, I was able to do a test with some attenuators at a band practice where the volume got pretty darn close to live concert levels...

Here are my results:


This is the mics into the AD-20 (levels set at 12:00).  You can see levels are peaking at 0.5  Brickwalling galore.  No attenuators used.



This is the mics into the AD-20 with a 15dB pad.  Levels on the AD-20 are at 2:00  You can see the levels only reach 0.08



This is the mics straight into the Hi-MD, with no AD-20 involved.



This is the PA monitor feed into the left channel (with a 20 dB pad) and the mics into the right channel.  Both channels running through the AD-20.  Left channel at 1:00, right channel at 3:00.  You can see some brickwalling on the right channel...


Thoughts, opinions, concerns

[Javier Cinakowski]

That is interesting.  I can't add much other than what has been said.  That Hi-MD wavform looks fine.  I was very impressed with the Hi-MD preamp, I'd be supprised if you have much improvement with the Denek, esspecially in light of your problems...

I don't think the NT4 has anything to do with your current problems, but it does perform a little better on 48v phantom rather than battery....

I do like the Deneke's optical connection to the Hi-MD as well.  Not only does the optical cable eliminate the disc spinning noise, but it decouples any noise downstream from the recorder.

good luck!

[illconditioned]

That looks like brick wall to me!

IIRC the AD20 has a gain of 17dB minimum.  I had one for *one day* and never got brickwall (moderate mics).  But I didn't like the sound *at all*.  IMO the preamp on this thing is not great.  It is *quiet* but I don't think the sound quality is good.  That was confirmed by a quick look at the circuit.  The circuit is a discrete transistor input followed by an OK opamp.  The problem is the discrete circuit is too simple.  I *think* they designed this thing for film use, where low noise on speach or foley mics is more important than perfect hifi sound quality or transparency.  Just a theory, but I don't like it.

I would stick with the minidisc if I were you!

  Richard

[fmaderjr]

I would stick with the minidisc if I were you!

The Sony Hi-MD's have great preamps. For recording anything at least moderately loud, adding even a great sounding pre to the chain isn't going to make much of an audible improvement. Keep things simple and ditch the AD20, especially since it is causing problems for you.

However, as has already been mentioned, the NT4 does sound somewhat better when run on phantom power instead of a 9 volt battery. At some point, maybe when you upgrade the recorder, you might want to try a phantom power source that works better for you than the AD20. If you don't insist on an on board ADC, the Naiant Littlebox is a very reasonable priced alternative.