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blocking the DC power from my camera

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zuma

New Member
Hello all. This is my first post and I am a total newbie. I have done quite a bit of research over the past week, however.

This is my situation in a nutshell - I need connect a line level signal to a mic level and, more importantly, block the DC power coming from my Sony minidv camera's mic input.

Here are some diagrams I found:
Camcorder Mic Adapters
and WITH attenuation:
Line signal to microphone input

These are the specs on my camera's input:
Minijack, 0.388 mV low impedance with DC 2.5 to 3.0 V, output impedance 6.8kΩ (kilohms) ( Φ 3.5mm), Stereo type

According to this website a 1uf cap would do nicely (but I don't think using anything up to 10uf would be that wrong either):
Opamp Labs Inc <> RFC Calculator <> www.opamplabs.com

Any way, let's forget about attenuation for the moment. I just want to block the DC power first. Here is what I did. I bought a variety pack of ceramic resistors (no polarity of course). I spliced a 3.5mm cable in half and connected 1uf capacitors in line with the red and black wires. Guess what? There is NO SOUND at all when I do that. What am I doing wrong? As soon as I remove the capacitors there is sound again.

It was suggested that I go to electrolytics and give it another shot. I will try this tomorrow but the puzzling thing is that this page:
Line signal to microphone input
specifically states "NOTE: The polarity of C1 is marked to the circuit in case you use an electrolytic capacitor. A "dry" plastic or ceramic capacitor is preferred in this circuit.

What the heck?

So, O Wise Electronic Masters, please advise this troubled soul on how to block a measly 3v.
 

MikeMl

Well-Known Member
Most Helpful Member
Sounds to me that you need a whole lot of attenuation as well as dc blocking. Your input level is only 0.38mV, while things that produce "line level" usually produce about 1V, so you need an attenuator which drops the level by 0.00038/1 or -68db.

As for blocking the DC, the cap follows the attenuator; Here is my guess at what you need. Note the attenuation as a function of R1, the low freq roll-off as a function of C1, and the optional high-freq roll-off.
 

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bailey45

New Member
Impedance Matching

Most 'line level' signals are meant to drive an input with an approximate impedance of 50K. I would suggest increasing the resistors on the input of the circuit to something closer to 50K to increase sound quality. A variable resistor in the last attenuation stage would also help with volume adjustment.
 

MikeMl

Well-Known Member
Most Helpful Member
Good point. Here is a revised circuit that has an input impedance of 50K, has a trim pot for adjusting level over a 30db range, and keeps the low-freq corner more or less in the same place as the pot is adjusted.
 

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zuma

New Member
Thank you thank you!

Now, before I even try to understand the awesome diagrams...are you guys saying that without the attenuation the DC blocking won't work?

What I am doing now is just reducing the level on my mixing board by running the faders really low. I know it's not the same thing but I can actually get audio in the camera without clipping.

Any how, I am still wondering why the capacitors (in general) are blocking any sound from my cable.
 

MikeMl

Well-Known Member
Most Helpful Member
...
Any how, I am still wondering why the capacitors (in general) are blocking any sound from my cable.
It could be that your camera is detecting the bias current that flows when a mic is plugged in and it uses that to turn something on. In that case, leave out the coupling capacitor and build just the attenuator...
 

zuma

New Member
Mystery solved. The ceramic capacitors were the problem. I don't know enough about this stuff to know why but when I plugged in the electrolytics my audio was back. I did take care to point the positive side towards the camera side.

I basically just cut open a 3.5mm stereo cable and put and electrolytic in line with each black and red wire. It works with my test setup but tomorrow will be the real test when I actually use my mixer.

Here is something really interesting. The cable sounds fine (not perfect, a bit less crispy in the upper frequencies) when I hook up to the camera, however, if I plug in some mp3 player headphones in place of the camera, the impedance difference (I think) caused a severe roll off of low frequencies. So, this cable I made is "custom" built for this exact setup now, isn't it?

After my test tomorrow I will probably cut the cable again and put in the resistors. THEN, I will think about making a proper box of it all and do it right. Seems like a good first project for me.
 

MikeMl

Well-Known Member
Most Helpful Member
Are you sure the ceramic was 1 to 10uF, or where you using one which was in nF?
You need ~10uF, which might be marked "106".
 

zuma

New Member
Hmmm...it's simply marked 1. Radio Shack part # 272-0801

It's a 100 variety pack. I also tried 10. There are no other markings beside what looks like a dot of black tar on top of each one.
 

MikeMl

Well-Known Member
Most Helpful Member
I'd be surprised if the largest capacitor in that variety pack is 0.1uF (100nF or 100000pf or 104), a factor of 100 too small for your application. At that, it should have passed some audio, albeit very tinny sounding because it would tend to pass high frequencies much more than low freq.
 
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zuma

New Member
When I actually build this thing right I think I will try the other values and see if anything passes audio at all. For now I'll be inserting the resistors.
 

zuma

New Member
I'd be surprised if the largest capacitor in that variety pack is 0.1uF (100nF or 100000pf or 104), a factor of 100 too small for your application. At that, it should have passed some audio, albeit very tinny sounding because it would tend to pass high frequencies much more than low freq.

You were spot on with this one. I found this after googling the markings on some of the caps in the pack (2nd and 4th posts):
My trip to Radio shack - All About Circuits Forum
 

zuma

New Member
Success!

PERFECT!

It worked like a charm! I ended up going with 30K and 100. I only tested with my mp3 player so far but it sounded great. I turned it all the way up (a Sansa clip) and it didn't distort. It was actually .2db under 0 on a pretty loud song. I'm thinking with my mixer I might even need to ride my faders a touch low.

Don't laugh if I did something foolish. It's my first time building something like this and I was just making it up as I went. Hey, it works!





Thanks to everybody!
 
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