audio attenuator

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I have a device that processes audio, and has balanced audio input and
output. It supposed to accept +4dBu Line level, but it is right now
calibrated to lower level. I need to add up to 15-25dB attenuator to
its input. The input is balanced XLR, and somewhere on the PCB it is
converted to unbalanced signal.

The easiest thing would be to add is stereo audio potentiometer before
the input balanced signal (to the XLR input).

1. Is adding audio potentiometer as attenuator the best solution?

2. If so, is adding stereo audio pot to the unbalance signal better
than modifying the PCB having the unbalanced signal? (in terms of
not introducing distortion or noise)

3. which (professional) audio potentiometer would be recommended:
3.1 brand? (professional level, balanced stereo, low noise,
low distortion, durable, small about 0.5" diameter, etc.)

3.2 model number or technology / material description of the
component?

3.3 What value should I use? (I am guessing large impedance of 1M or
more is needed. Should comply with Line signal specs.)

3.4 Log or linear pot?

3.5 reference to where to purchase such.

.... and Shine!

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The Rane website has instructions for how to build attenuators and
terminating resistances.

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bcrowell@excite.com wrote:
> The Rane website has instructions for how to build attenuators and
> terminating resistances.
>
I see there the transformer used to convert between unbalanced line to
a balanced one. But they don't specify the transformer having +12dB.
Can anyone help in that?

Another issue, on the balanced side the XLR's GND pin is connected to
the chassis, but I don't see any connection to the transformer's winding
middle wire. Is there any missing additional connection to the chassis
(between the chassis and the transformer)? So what makes the chassis the
"zero" reference with respect to the two other pins of the XLR /
transformer?

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OK, thanks,
So now the issue of the right transformer to be used. I want to convert
between -10 dBV unbalanced line level to +4dBu balanced line level.
Most transformers I find at Mouser & Digikey are for 1:1 conversion,
For example:
Triad Magnetics TY-146P 553-TY146P
(it has access to the middle of the coils, is that useful for anything?)

I found such at Rane, does anyone know what such transformer is used for
example at the following Rane?
http://www.rane.com/bb22.html
http://www.rane.com/pdf/bb22dat.pdf




Stephen Cowell wrote:
> "SunShine" <Sunshine@NoSpam.com> wrote in message
> news:423DBD11.6050704@NoSpam.com...
>
>>bcrowell@excite.com wrote:
>>
>>>The Rane website has instructions for how to build attenuators and
>>>terminating resistances.
>>>
>>
>> I see there the transformer used to convert between unbalanced line to
>>a balanced one. But they don't specify the transformer having +12dB.
>>Can anyone help in that?
>>
>>Another issue, on the balanced side the XLR's GND pin is connected to
>>the chassis, but I don't see any connection to the transformer's winding
>>middle wire. Is there any missing additional connection to the chassis
>>(between the chassis and the transformer)? So what makes the chassis the
>>"zero" reference with respect to the two other pins of the XLR /
>>transformer?
>
>
> With balanced lines, you don't want or need a ground reference...
> that's the beauty of them, and why they are used for very long
> (*very* long) runs. Let the ground float.
> __
> Steve
> .
>
>

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"SunShine" <Sunshine@NoSpam.com> wrote in message
news:423E3722.3070103@NoSpam.com
> OK, thanks,
> So now the issue of the right transformer to be used. I want to
> convert between -10 dBV unbalanced line level to +4dBu balanced line
> level. Most transformers I find at Mouser & Digikey are for 1:1
> conversion, For example:
> Triad Magnetics TY-146P 553-TY146P
> (it has access to the middle of the coils, is that useful for
> anything?)
>
> I found such at Rane, does anyone know what such transformer is used
> for example at the following Rane?
> http://www.rane.com/bb22.html
> http://www.rane.com/pdf/bb22dat.pdf

Check out the Jensen transformer's site. You want a transformer that
provides about 11 dB voltage gain, which would come from an
approximate 1:3 turns ratio. You want a line level transformer so that
it won't distort at higher levels and low frequencies.

http://www.jensentransformers.com/ [...] 3alcf3.pdf

This is a very high quality part, and priced accordingly. You can pay
less and you will probably get less. Audio transformers need to be of
very high quality if minimal degradation of sound quality is desired.

If you want something that isn't a raw part, this product seems to be
very appropriate:

http://www.jensentransformers.com/datashts/dinpc.pdf

Jensen is not a huge company and IME their staff is very
customer-oriented. You might be able, with minimal effort, to coax
them into building you one of these with a different transformer:

http://www.jensentransformers.com/datashts/ci2rr.pdf

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"Steve Urbach" <dragonsclaw@NOTmindspring.com> wrote in message
news:lqqt311at6fo1a0buobuir4grjbs2qkru0@4ax.com
> On Mon, 21 Mar 2005 03:27:06 GMT, "Stephen Cowell"
> <scowell@sbcglobal.net> wrote:
>
>> It's actually a physical impossibility to convert from a low-level
>> high-impedance circuit to a high-level low-impedance circuit
>> without doing some amplification. Passive devices won't do it.
>> __
>> Steve
>> .
> 11 db gain? where is the extra energy coming from if you do not use
> amplification.


Obviously with a transformer, the energy comes from nowhere. Instead,
a transformer works when the source impedance is considerably lower
than the load impedance. This is usually the case in audio.

Archived from groups: alt.guitar.amps,rec.music.makers.guitar,rec.audio.tech,rec.audio.misc,rec.audio.opinion (More info?)

SunShine <Sunshine@NoSpam.com> wrote:

>I have a device that processes audio, and has balanced audio input and
>output. It supposed to accept +4dBu Line level, but it is right now
>calibrated to lower level. I need to add up to 15-25dB attenuator to
>its input. The input is balanced XLR, and somewhere on the PCB it is
>converted to unbalanced signal.

Buy two Shure A15AS attenuators. They're switchable between -15, -20 and
-25 dB. Reliable. Inexpensive.

--
Len Moskowitz PDAudio, Binaural Mics, Cables, DPA, M-Audio
Core Sound http://www.stealthmicrophones.com
Teaneck, New Jersey USA http://www.core-sound.com
moskowit@core-sound.com Tel: 201-801-0812, FAX: 201-801-0912