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Old September 29th, 2011, 04:22 PM   #1
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Blocking DC bias voltage.

I just got a Beachtek DXA-SLR to go along with my Nikon D5100 camera. I have immediately run into a problem in that Nikon cameras (like Sony) have a dc-bias voltage to power electret condensors on their mic input. With the Beachtek this means that I get a loud pop every time I go in or out of record as the bias voltage starts and stops. You can also see the bias voltage effect on the recorded waveform (the waveform center is not zero and the top half of the waveform is not as loud as the bottom, losing headroom).

I should be able to make up a short jumper cable with some capacitors inserted, but I don't know the values or how to wire them.

Can anybody point me in the right direction?

Also, does anyone know if the Juicedlink juicedlink dt454 blocks dc bias voltage?
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Old September 29th, 2011, 04:28 PM   #2
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Re: Blocking DC bias voltage.

Sure, it is easy enough to put a DC-blocking capacitor in the line, but not clear whether that will really eliminate the symptoms you are seeing. I use a couple different models of Beachtek boxes with various Sony cameras (all of which have "Plug-In Power"). But I can't say that I have ever seen the symptoms that you are describing. Are you certain that the Beachtek box is the source of the problem? I would contact Beachtek and see what they have to say. Probably the majority of Beachtek boxes are used with cameras that have this DC voltage present.

If you want some guidance on selecting a capacitor...
Camcorder Mic Adapters
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Old September 29th, 2011, 06:54 PM   #3
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Re: Blocking DC bias voltage.

Richard: good find on that info!

I concur with that schematic; note that the caps you're adding are C1 and C2 on the left side of the schematic. (The caps on the right side are already inside the DSLR/recorder.)

The values given are theoretically correct. But you will never find a 3.18 mfd capacitor. Walk into a store and ask for that and you'll hear a lot of laughter. Electrolytics are not accurate to 3 decimal places, and are available on a relatively limited range of sizes. You need to go for the closest value (if it's within 10% or so of what the formula says) or else go to the next highest value. (And note that, for example, if the next highest value is 5mfd (instead of 3.18) then your rolloff frequency will drop from 20 Hz to 12.7 Hz in this example.

Also, there may be a few mixers, recorders, cameras out there that supply more than +5 volts. To be on the safe side, get caps rated at 10 or 12 volts. They will work fine at a lower voltage.

Finally, be absolutely sure you wire the caps with correct polarity, as shown in the schematic.
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Old September 29th, 2011, 09:39 PM   #4
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Re: Blocking DC bias voltage.

You know, I'm not sure that DC bias is the problem. Here is what I'm running into:

I hit record and there is a huge pop through my headphones. This surprises me because I wouldn't have thought that anything happening at the output would affect what I am hearing over headphones. Aside from DC bias, what could this be?
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Old September 30th, 2011, 08:20 AM   #5
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Re: Blocking DC bias voltage.

One way to verify what's happening is if you can disable the "plug in power" in the DSLR.

Otherwise, for a clue, we'd have to look at the schematic for the mixer. If the headphone buffer amp's input is connected directly across the audio output, then anything causing a disturbance on the audio output (e.g. applying DC voltage to the output jack) would certainly cause a disturbance in the headphones.

But actually such a disturbance will still (briefly) get through a coupling capacitor. If you change the voltage level on the side of the cap connected to the camera (say from a steady zero volts to a steady +5 volts), you will still see a spike on the side of the cap connected to the mixer output. After the cap has charged (a fraction of a second) (t=r*c) the voltage at the mixer side should come back to zero. But the initial switching transient has an AC component, and that HF spike will still appear at the mixer output.
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Old September 30th, 2011, 01:09 PM   #6
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Re: Blocking DC bias voltage.

Not sure about DSLRs, but video camcorders don't typically switch the microphone DC "Plug-In-Power" on/off when recording. They leave the DC applied whenever the mic is plugged in specifically to avoid that kind of turn-on artifact, and to allow audio monitoring while not recording.

IMHO, you have some different problem, and your proposed solution is not appropriate.
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Old September 30th, 2011, 03:58 PM   #7
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Re: Blocking DC bias voltage.

Very interesting, Richard. I never would have guessed it, but it may be quite true.

I don't have a DSLR, but I've just tested five mini-recorders, with the following results.

Tascam DR-07: mic power always on.
Tascam DR-03: mic power always on.
Alesis PalmTrack: mic power always on.
Sony ICD-SX700: mic power always on.
Sony ICD-PX820: mic power on only during Record mode. (pocket voice recorder)

Obviously there is not 100% concurrence, but 4/5 of the units I checked do, indeed, leave mic power on even when the unit is not recording.

Well then, the way for Laurence to check this quickly is by using a voltmeter. If the meter shows that DC is always applied, then there is some other problem. If the meter shows that mic power goes on and off depending on the DSLR mode, then that's the source of the "pop."

Last edited by Greg Miller; September 30th, 2011 at 06:53 PM.
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Old September 30th, 2011, 06:35 PM   #8
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Re: Blocking DC bias voltage.

I just got this response from Beachtek:

_____________________________________________________________________________

Hi Laurence,

Thanks for writing!

I understand that you are getting a popping noise when you start or stop recording. Yes, it is caused by the DC voltage becoming active on the camera. However all cameras work the same way and a capacitor will not solve this. I believe the reason it is particularly annoying on your camera is because the input sensitivity of all NIkon DSLR cameras is exceptionally high. We suggest that you use a 25 dB padded output cable between the adapter and camera to match the levels properly. This will also calibrate the level meters on the adapter with your camera. You can purchase these inexpensive cables from Sescom.

I hope this helps.

Regards,
Harry Kaufmann
Product Support
778-478-9872
BeachTek
The Audio Adapter Specialists
Welcome to BeachTek.com!
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Old October 7th, 2011, 01:44 PM   #9
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Re: Blocking DC bias voltage.

I finally got the 25db pad cable and tried it out today. Here is the reply I just sent off to Beachtek's tech support:

_____________________________________________________________________________________

Harry,

Wow was that terrible advice. I got the Sescom 25db padded cable and tried it out. The levels of the camera are a much better match without the pad.

The Nikon D5100 has four level in options: low, medium, high and auto. Without the pad, the Beachtek output is a good match with the D5100 if I put set the audio input level to low sensitivity. With the 25db pad, I have to set the camera mic to it's highest sensitivity setting. This adds a horrendous amount of noise to the input. Yes it lowers (but does not get rid of) the pop in the Beatchtek headphone output, but it is not worth the extra noise (which is about equal to the level of the sound being recorded). I suppose I will just have to live with the pop in my headphones (which fortunately does not get recorded)

I'm just curious, how did you come up with that advice? You obviously never tried it. Please don't give it to anyone else. Just tell them to live with the pop.

Laurence
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Old October 7th, 2011, 01:55 PM   #10
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Re: Blocking DC bias voltage.

Anyway, for anyone looking at this thread in the future, here is my synopsis:

The Beachtek DXA-SLR will match the levels with professional quality mics quite well to the audio input of the Nikon D5100. Just set the Nikon audio input to low sensitivity and the levels will match up with what you see on the flickering green/red LEDs on the Beachtek.

Unfortunately, as the Nikon D5100 goes into record, it applies a small dc-bias voltage to the mic input and this will cause a pop in the headphone output on the Beachtek even though you are not monitoring the audio through the camera. This pop is painfully loud and quite annoying. Fortunately this pop does not get recorded.

Adding dc-bias blocking capacitors to the input of the Nikon will not help with this pop. A 25db pad cable will minimize (but not get rid of) the pop, but if you use it you will need to turn the mic sensitivity way up on the camera and this will add an enormous amount of noise. If Beachtek's tech support suggests this, ignore them.

I have no idea if the same problem exists with the Juicedlink DT454.

Anyway, the good news is that aside from this pop in the headphones, the recorded audio from the Beachtek DXA-SLR is quite good. Just be aware that if you go this route, you will get a loud pop in your ears every time you go into record with the Nikon.
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Old October 8th, 2011, 12:16 AM   #11
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Re: Blocking DC bias voltage.

I'd be curious* to see a schematic for the Beachtek mixer. There's probably a way to change the point where the headphone buffer amp gets its signal, so that a disturbance at the mixer's output jack won't get back into the headphones. I would think such a configuration would be preferable to what you have now.

* But not curious enough to try to locate one on my own. If someone were to send me a link or a file, I'd certainly take a look at it.
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