# attenuator circuit to connect headphone to 50x2 Amplifier

#### xiles

Joined Oct 16, 2015
17
Hi all
please consider the following
1. i have Philips MCD 710 micro theater system which lacks headphone output . RMS wattage is 50W x2 output ic is TDA8920BTH
2. to connect a 32 Ohms headphones (200mw max input) to it is speaker output could someone please provide me an attenuator circuit ?

Thank you in advance

#### Ian0

Joined Aug 7, 2020
8,370
If that's 50W into 8Ω then that's 20V rms output.
200mW into 32Ω is 2.5V rms.
The simplest way would just be a 220Ω 2W resistor in series. It assumes that the impedance is fairly flat with frequency, which is often the case for headphones. If the headphones have a lot of inductance it will sound a bit trebly.
If that's a problem, measure the inductance, and put a Zobel network of 32Ω and C=L/(R^2) across the headphones.

#### BobTPH

Joined Jun 5, 2013
7,509
If you just keep the volume down, you might not need any attenuation. If the output is 20V at full volume, then it will be something like 2V at half volume due to the logarithmic nature of volume controls. This voltage should be okay for your headphones.

You will need attenuation if there is too much noise at the lower volume level, but I doubt that would be a problem.

Bob

#### Ian0

Joined Aug 7, 2020
8,370
If you just keep the volume down, you might not need any attenuation. If the output is 20V at full volume, then it will be something like 2V at half volume due to the logarithmic nature of volume controls. This voltage should be okay for your headphones.

You will need attenuation if there is too much noise at the lower volume level, but I doubt that would be a problem.

Bob
True, but if you accidentally turn it up, you'll kill your headphones - because of the logarithmic volume control the voltage goes up very rapidly above halfway.
And if your headphones are quite sensitive (102dB @1mW for a pair of Shure SRH840) then 123dB @ 200mW, threshold of pain is reached at 4.4V.

#### Audioguru again

Joined Oct 21, 2019
6,151
The output power depends on the supply voltage and the speaker impedances, both you did not say.
The class-D filters are designed only for the impedance of the speakers. Then if the speakers are disconnected when you use headphones, the amplifier must be terminated with power resistors having almost the speaker impedances.

#### Ian0

Joined Aug 7, 2020
8,370
Class D amplifiers are designed to be connected to 8Ω loudspeakers, not 8Ω resistors. An 8Ω loudspeaker is probably 6Ω+1mH, which is about 140Ω at the frequency of the output filter, or the loudspeaker may have a treble unit with a high pass filter which may give an impedance around 8Ω at 22kHz, so the amplifier will be designed with a load impedance stabilising network or Zobel network, so that it will cope with a range of load impedances.

#### xiles

Joined Oct 16, 2015
17
If that's 50W into 8Ω then that's 20V rms output.
200mW into 32Ω is 2.5V rms.
The simplest way would just be a 220Ω 2W resistor in series. It assumes that the impedance is fairly flat with frequency, which is often the case for headphones. If the headphones have a lot of inductance it will sound a bit trebly.
If that's a problem, measure the inductance, and put a Zobel network of 32Ω and C=L/(R^2) across the headphones.
thank you so much for your input
hi it is 50w for 4Ω speaker

#### Ian0

Joined Aug 7, 2020
8,370
thank you so much for your input
hi it is 50w for 4Ω speaker
So it's 14V not 20V.
150Ω 2W will do the job.

#### xiles

Joined Oct 16, 2015
17
Class D amplifiers are designed to be connected to 8Ω loudspeakers, not 8Ω resistors. An 8Ω loudspeaker is probably 6Ω+1mH, which is about 140Ω at the frequency of the output filter, or the loudspeaker may have a treble unit with a high pass filter which may give an impedance around 8Ω at 22kHz, so the amplifier will be designed with a load impedance stabilising network or Zobel network, so that it will cope with a range of load impedances.
according to the manual this is for 4Ω

#### xiles

Joined Oct 16, 2015
17

#### Ian0

Joined Aug 7, 2020
8,370
according to the manual this is for 4Ω
At a guess, it specifies 4Ω minimum but doesn't specify a maximium.

#### xiles

Joined Oct 16, 2015
17
If you just keep the volume down, you might not need any attenuation. If the output is 20V at full volume, then it will be something like 2V at half volume due to the logarithmic nature of volume controls. This voltage should be okay for your headphones.

You will need attenuation if there is too much noise at the lower volume level, but I doubt that would be a problem.

Bob
Thank you for your reply. but i am partially deaf. last time this method fried my AKG K44 headphone

#### xiles

Joined Oct 16, 2015
17
At a guess, it specifies 4Ω minimum but doesn't specify a maximium.
i dont have a very good knowledge about this it just say 50wx2 at 4Ω

#### Audioguru again

Joined Oct 21, 2019
6,151
Class D amplifiers are designed to be connected to 8Ω loudspeakers, not 8Ω resistors.
No. The datasheet for the amplifier says its load can be 3 , 4, 6 or 8 ohms and shows a chart for matching filter values.
Then the output load impedance is important and must be matched if the speakers are disconnected and attenuated headphones are used. The class-D oscillator frequency is also not known. The filter is designed for that frequency.

Is the output power 50 Whats of high distortion? Or is it 50 Watts at low distortion. The maximum output level is determined by which type of number.

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#### Ian0

Joined Aug 7, 2020
8,370
The speaker impedance is a nominal value. An "8Ω" speaker is probably 6Ω of DC resistance in series with an inductance that can vary over two orders of magnitude, depending on make and model.
At the PWM frequency (317kHz nominal, according to the datasheet) even a tweeter with an inductance of 100uH will have an impedance of 200Ω, so the impedance seen by the filter will be dominated by R9 (the Zobel resistor).
The filter has a cutoff frequency of between 40kHz and 50kHz. The cutoff frequency is determined by L3/C22 varies very little with load impedance. The Q of the filter will vary with load impedance, but there should be very little energy output in that region that could excite the resonance. The highest Q will occur with no load connected, and the datasheet quote all sorts of parameters as being measured with no load, so one can assume that it is stable with any output impedance.

#### xiles

Joined Oct 16, 2015
17
@Ian0 @Audioguru again
if you guys are interested here is the service manual. refer page no 19

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#### Audioguru again

Joined Oct 21, 2019
6,151
Most of the pages in the service manual have no page numbers except the last few (pages 41 to 46) of the parts list.
The schematic of the power supply does not show the supply voltages but the layout shows +27V and -27V as in the TDA8920 datasheet.
Then the max low distortion output power is 69 Watts per channel onto 4 ohms.

#### Audioguru again

Joined Oct 21, 2019
6,151
The manual shows the part number and location of the "Headphones Circuit Board".
It also shows two different part numbers for the amplifier IC, which is why it shows two supply voltages for them.