Differential Output

[Apparatus]

Hello,

I am using a Texas Instruments TLV320AIC1106 PCM Codec for some audio
circuitry. This codec is capable of driving an 8 to 32 ohm load
speaker. The suggested connection topology for this speaker is
differential output according to the datasheet, but a schematic is not
included with this recommendation.

I was planning to simply connect one terminal of the speaker to EAROUT+
and the other to EAROUT-, but recently it was suggested to me that in
addition I couple each terminal to ground with a 1kohm resistor. What
are the advantages of this?

Cheers,
Chris

 

[Robert Baer]

Hello,

I am using a Texas Instruments TLV320AIC1106 PCM Codec for some audio
circuitry. This codec is capable of driving an 8 to 32 ohm load
speaker. The suggested connection topology for this speaker is
differential output according to the datasheet, but a schematic is not
included with this recommendation.

I was planning to simply connect one terminal of the speaker to EAROUT+
and the other to EAROUT-, but recently it was suggested to me that in
addition I couple each terminal to ground with a 1kohm resistor. What
are the advantages of this?

Cheers,
Chris

None.

 

[Winfield Hill]

Robert Baer wrote...

None.

Correct. The purpose of a differential speaker output (often called
bridge mode output) is to eliminate the need for electrolytic output
capacitors when using a single-supply power source, and to double the
available speaker voltage, thereby quadrupling the maximum power
obtainable from the power source, which is often a small battery.
A bridge output is fully differential, and has no knowledge of, nor
need to interact with ground. Very small speaker amplifier ICs can
be made using class-D (switchmode) H-bridge output stages, which
dissipate little heat while driving the speaker. But it is necessary
to incorporate four power MOSFETs into the IC for the speaker current.

 

[John Woodgate]

I read in sci.electronics.design that Winfield Hill <hill_a@t_rowland-

Robert Baer wrote...

Correct. The purpose of a differential speaker output (often called
bridge mode output) is to eliminate the need for electrolytic output
capacitors when using a single-supply power source, and to double the
available speaker voltage, thereby quadrupling the maximum power
obtainable from the power source, which is often a small battery.
A bridge output is fully differential, and has no knowledge of, nor
need to interact with ground. Very small speaker amplifier ICs can
be made using class-D (switchmode) H-bridge output stages, which
dissipate little heat while driving the speaker. But it is necessary
to incorporate four power MOSFETs into the IC for the speaker current.

Maybe the resistors are advisable if the loudspeaker can be
disconnected. Not to provide a load on the amplifier but to prevent
possible excessive centre-point offset.

 

[Apparatus]

Thank you all for your informative answers. The speaker is
disconnectable (headphones via phonojack), but the initial center point
offset should only induce a click or pop sound in the headphones. Since
this is not a problem, I'll leave out the resistors.

Cheers,
Chris

 

[Winfield Hill]

Apparatus wrote...

Thank you all for your informative answers. The speaker is
disconnectable (headphones via phonojack), but the initial
center point offset should only induce a click or pop sound
in the headphones. Since this is not a problem, I'll leave
out the resistors.

I don't understand your comment about center-point offset.
In these balanced amplifiers, both outputs are at the same
voltage with no signal, so the difference voltage is zero,
and plugging in a headphone should not make a click. Note,
the outer ring of the headphone jack shouldn't be grounded!

 

[John Woodgate]

I read in sci.electronics.design that Apparatus

Thank you all for your informative answers. The speaker is
disconnectable (headphones via phonojack), but the initial center point
offset should only induce a click or pop sound in the headphones. Since
this is not a problem, I'll leave out the resistors.

Are you sure? There are two aspects:

1. The offset may damage the chip over time;

2. The click may exceed the 'acoustic shock' limits. People tend to put
the headphones on and then plug them in. Since many headphones produce
100 dB SPL for 1 mW or less input, the possibility of getting more than
126 dB SPL (the ITU recommended limit) is significant. I have
experienced a 126 dB click and I assure you it is VERY unpleasant.

 

[Frank Bemelman]

"John Woodgate" <[email protected]> schreef in bericht

[snip]

126 dB SPL (the ITU recommended limit) is significant. I have
experienced a 126 dB click and I assure you it is VERY unpleasant.

Playing Russian Roulette can be worse <G>.

 

[John Woodgate]

I read in sci.electronics.design that Winfield Hill <hill_a@t_rowland-

I don't understand your comment about center-point offset.
In these balanced amplifiers, both outputs are at the same
voltage with no signal, so the difference voltage is zero,
and plugging in a headphone should not make a click. Note,
the outer ring of the headphone jack shouldn't be grounded!

Well, theoretically zero. What actually happens depends on the circuit
configuration and the device characteristics.

 

[Allan Herriman]

I read in sci.electronics.design that Apparatus


Are you sure? There are two aspects:

1. The offset may damage the chip over time;

2. The click may exceed the 'acoustic shock' limits. People tend to put
the headphones on and then plug them in. Since many headphones produce
100 dB SPL for 1 mW or less input, the possibility of getting more than
126 dB SPL (the ITU recommended limit) is significant. I have
experienced a 126 dB click and I assure you it is VERY unpleasant.

John, which ITU recommendation is that?

Thanks.

Allan

 

[John Woodgate]

I read in sci.electronics.design that Allan Herriman <allan.herriman.hat
[email protected]> wrote (in <9g2qr0dl5q244i2c7vtn3l7jggjek4ou
[email protected]>) about 'Differential Output', on Mon, 13 Dec 2004:

John, which ITU recommendation is that?

The 126 dB limit for clicks was applied in UK at the time of de-
regulation, and I am not sure where it came from. The ITU document is
P360, and I don't have a copy of it. The predecessor had 135 dB for very
short clicks and 125 dB for longer disturbances.