Posted by Smarty on 10/07/05 18:51

I went to Google to find the design equations for the two resistor values
used in a standard audio T-pad for different attenuation and found, almost
immediately, a really simple to follow "do it yourself" guide at:

http://www.uneeda-audio.com/pads/

which has a nice table at the bottom showing the resistances for an
unbalanced T bad, the type I would personally recommend.

Looks like the shunt resistor, R2, in either case he uses is 150 ohms. And
the series resistor is 2200 to 7500 ohms for R1 and R3 depending upon how
much attenuation you need.

The nominal input voltage will be maybe a 300 or 400 millivolts or so
peak-to-peak for a typical LineOut, and the attenuated signal needs to be a
few millivolts up to maybe 8 or 10 nominally The "k" value used in the table
for computing the resistors is between 30 and 100 (30-40 dB attenuation
range).

The resistors are not dissipating any power to speak of, so 1/8 watt or
greater are fine. Getting the right resistance values may require buying
precision 1% resistors although 150, 2200 and 6800 ohm resistors are
standard 5% parts.

Smarty




"Smarty" <nobody@nobody.com> wrote in message
news:b4KdnRVlopazJNveRVn-jg@adelphia.com...
> Glad to assist. I need to know 3 things before designing an attenuator
> pad.
>
> 1). Where is the Line Output coming from, a balanced or unbalanced source?
> Balanced sources will have an XLR connector typically whereas unbalanced
> sources typically use an RCA phono jack.
>
> 2).What type of camcorder are you using? It is normally safe to assume a
> consumer camera has -10dBV input but I would prefer to know for sure since
> some require a different signal level,
>
> 3). Although this is a somewhat minor detail, some camcorders provide DC
> power through their MIC-IN jack to support electret or other microphones
> requiring a pre-amp / power. I normally would add a little blocking
> capacitor in the pad to keep this DC from showing up back at the other end
> of the cable at the LineOut device's output stage. If the LineOut device
> uses an output transformer, for example, or does not provide a buffered
> output, this DV voltage can add an offset/bias which you want to prevent.
>
> Let me know and I will calculate the resistances and give you a design.
>
> Smarty
>
>
>
> "Richard Crowley" <richard.7.crowley@intel.com> wrote in message
> news:di69dd$ftn$1@news01.intel.com...
>> "Rick Merrill" wrote ...
>>> Andy & Kim wrote:
>>>> Thanks for the advise guy's. Smarty could you give me some values of
>>>> the resistors I would need for a T-pad attenuator, just as a guide.
>>
>>> I second the request!
>>
>> http://www.uneeda-audio.com/pads/
>>
>
>

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