Amplifier design

Thread Starter

milad.norouzi

Joined Dec 23, 2021
35
Hey guys. I want to design an amplifier with the TPA3118 IC. and the source of the audio is the Raspberry pi board. i want to send my music to rpi , then connect the amplifier to the rpi. At datasheet it says:
LINP pin -> Positive audio input for left channel. Biased at 3 V. Connect to GND for PBTL mode
LINN pin -> Negative audio input for left channel. Biased at 3 V. Connect to GND for PBTL mod
I can't find anything to check rpi audio output DC bias level. How should I check that?
 

Jerry-Hat-Trick

Joined Aug 31, 2022
162
Take a look at the first reply to the thread:
https://forums.raspberrypi.com/viewtopic.php?t=200387
" The output impedance is 69 Ohms..... However the output level is rather low at 3.3v p-p, 1.1v rms"
I took a look at the datasheet, it's very informative, including input impedance and recommended AC coupling capacitors and Figure 37 - typical application schematic. I expect the PCB layout is important - see Figures 40 and 41. I'd be tempted to buy the evaluation model it it were not so expensive! Nice component, in itself not expensive for what you get.

Just found https://www.ebay.co.uk/itm/294965519732
Don't know how they do it for the price!
 
Last edited:

Thread Starter

milad.norouzi

Joined Dec 23, 2021
35
Take a look at the first reply to the thread:
https://forums.raspberrypi.com/viewtopic.php?t=200387
" The output impedance is 69 Ohms..... However the output level is rather low at 3.3v p-p, 1.1v rms"
I took a look at the datasheet, it's very informative, including input impedance and recommended AC coupling capacitors and Figure 37 - typical application schematic. I expect the PCB layout is important - see Figures 40 and 41. I'd be tempted to buy the evaluation model it it were not so expensive! Nice component, in itself not expensive for what you get.
Thanks for your reply. in the link thread in firs reply said " The output impedance is 69 Ohms " and in the TPA datasheet input impedance start from 9K ohms.So how can i do the impedance matching for best power transfer?
Just found https://www.ebay.co.uk/itm/294965519732
Don't know how they do it for the price!
Yes, the TPA IC series are inexpensive and easy to use, also reliable. but i'm confused with this 3v bias and and impedance matching.

TPA3118 datasheet (datasheetspdf.com)
 

Audioguru again

Joined Oct 21, 2019
5,166
The datasheet shows that the TDA3118 inputs are already biased at 3.0V and a series input capacitor is needed to prevent the input signal source from affecting the DC biasing.

A chart shows the recommended capacitance vs gain.
 

Attachments

Thread Starter

milad.norouzi

Joined Dec 23, 2021
35
The datasheet shows that the TDA3118 inputs are already biased at 3.0V and a series input capacitor is needed to prevent the input signal source from affecting the DC biasing.

A chart shows the recommended capacitance vs gain.
Yep, but my problem is the audio source (Raspberry pi). The output impedance of RPi is 69 Ohms, but input impedance for 36dB is 9K ohms. i'm confused

1664548056781.png
Shouldnt the output impedance and output impedance match for best power transfer? 69 ohms both?
 

Audioguru again

Joined Oct 21, 2019
5,166
The signal source is almost always a low impedance and the amplifier input is almost always a much higher impedance.
If you match the impedances then the source will be seriously overloaded and the signal level will be reduced to almost nothing.
 

Thread Starter

milad.norouzi

Joined Dec 23, 2021
35
The signal source is almost always a low impedance and the amplifier input is almost always a much higher impedance.
If you match the impedances then the source will be seriously overloaded and the signal level will be reduced to almost nothing.
So, i don't need any impedance match on input?
 

Audioguru again

Joined Oct 21, 2019
5,166
The signal source might be from an opamp that has an output impedance of a couple of ohms but has a minimum allowed load of 2 thousand ohms. It feeds an amplifier input that has an impedance that is much higher.
The output impedance of an audio amplifier is 0.02ohms which is much less than its speaker load of 8 ohms.
 

BobTPH

Joined Jun 5, 2013
5,762
The max power theorem has to be the most misunderstood rule in electronics.

For any given load impedance, the maximum power transferred is when the source impedance is zero. This is obvious if you think about it, adding resistance to the source is certainly not going to increase the power transferred. Try the math if you don’t believe me.

Now if the source impedance is fixed, and you can adjust the load impedance, the max power theorem is correct, but often not useful. In the case of the audio amp with 0.02 Ohms output impedance, you would theoretically transfer the most power to a 0.02 Ohm speaker. But in reality, you would blow up the amplifier because its max current would be exceeded by 2 orders of magnitude. Few audio amps can drive a 2 Ohm speaker, let alone 0.02.
 
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