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Author Topic: Attenuator Help  (Read 2569 times)

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Offline techgui

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Attenuator Help
« on: April 06, 2011, 08:40:27 AM »
I want to run the "line out" from my Tascam DR-2d recorder to the external "mic input" of my Canon Vixia HfR10 camcorder.  Specs are as follows;

Tascam Dr-2d Output
Output impedance   12 ohm
Nom output leve   -14 dBV (10 K ohm)
max output level   +2 dBv (10 k ohms)

Canon Mic Input level
Per Canon, is -57 dBV (with 600 ohm microphone), Impedance  5 kohms or more.

Also, I would like to build this attenuator myself, so help with resistor values would be appreciated.

Thanks,
John

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Re: Attenuator Help
« Reply #1 on: April 06, 2011, 04:43:23 PM »
+2dbV to -57dbV is a difference of 59db.  Call it 60db.  But if it were me, I'd start off with the lazy man's approach and see if that works.

Option A) skip the attenuator, turn down the output of the Tascam recorder.  Use the home brew "calibration" procedure below to figure out "if I turn the headphone volume of the TASCAM down to XXX it works fine."  If it works, it's easy.

Option B) The output jack on the recorder is 1/8 TRS, and the input jack on the camera is the same.  I'd get something like this headphone jack volume control, available at a bunch of different places, along with a 1/8" to 1/8" male cable.
http://www.radioshack.com/product/index.jsp?productId=2102975

To "calibrate" it to 60db I would try this procedure (on a quiet night at home):
- find/edit/create a wave file which frequently has peaks going to zero,
- copy that to the TASCAM,
- play the song on the TASCAM, running into the camera, and listen carefully with headphones to the sound on the camera.
- turn the volume control way down, then turn it up until it's starts to crackle, back it off a little, and tape down the adjustment so it doesn't change.
- if the camera has an audio VU meter inside, obviously use that, I'm assuming it doesn't, so you have to go by ear.
If this doesn't work out so great... well, you are only out about $10, but I think it's easier than trying to solder tiny resistors in a 1/8" mini plug.

Option C) to answer your original question... +2dbV to -57dbV is a difference of 59db.  Call it 60db.
Go to http://www.uneeda-audio.com/pads/  and about half way down the page he has a link to an excel spreadsheet.
You can get away with a simple L-circuit, and the ratio for 60db is 1000:1.  So R1 needs to be about 1000 times as big as R2.
The output of the Tascam sees R1 + R2 added together.  The input of the camera sees just R2.



Go to Radio Shack and see what you can find for pairs of resistors in the 1000 : 1 ratio.  I'd look for something in range of 50K ohms and 50 ohms, or 33k and 33ohm.  I'd try to keep it below 100K/100ohm, but above 10K/10ohm.

Remember you need 2 sets, one for the left channel, and one for the right channel.
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Offline techgui

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Re: Attenuator Help
« Reply #2 on: April 06, 2011, 10:15:14 PM »
Thank you both.  That's exactly what I needed. 

John

Offline techgui

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Re: Attenuator Help
« Reply #3 on: April 10, 2011, 10:45:04 AM »
My camcorder has a mic attenuator switch.  By creating some reference test tones (0bd, -12db and -30db), I was able to determine that it applies -12db of attenuation.  And for reference the Tascan DR-2d has 12 bars on the output indicator.  Each bar takes 3 clicks of the +/- control.  So here is what I determined.

1.  With the camcorders mic attenuator switch off, I have to turn the DR-2d line out all the way down to the first click of the first bar for the camcorder to show the same signal level as the DR-2d.

2.  With the attenuator switch on, I have to turn the DR-2d line out up to the first click on the third bar to show the same signal level as the DR-2d.   

Basically I've come to the conclusion that an external attenuator would be best, since I don't want the camcorders attenuator engaged.  Other family members will be using both the camcorder and DR-2d, so I want the technology to be invisible to them.  I can see them using just the camcorder and forgetting to turn the attenuator off. 

Questions
1  - What would be the best range to set the DR-2d's output for the best possible signal?  Middle or the range? 
2 - Headphone volume control.  Would this present any impedance match problems?




 

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