krish888
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3.5 mm RCA Jack output characteristics

Fri Oct 19, 2018 2:18 pm

Hello,
I am a newbie to this forum. I am wondering:

1) what is the maximum audio power output from the 3.5 mm RCA jack?
2) what is the RMS voltage at the 3.5 mm RCA jack?
3) what is the output impedance at the 3.5 mm RCA jack?

Thanks in advance!

Krish

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mahjongg
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Re: 3.5 mm RCA Jack output characteristics

Fri Oct 19, 2018 2:55 pm

Its a "line level" signal suitable to drive an audio amplifier.

The signal is generated from a 3V3 specially optimized PWM signal (optimized for audio quality), and the maximum amplitude of it is about 1V peak to peak out of a 1000Ohm impedance. as such you should use an input impedance that is (much) higher, or you will generate signal loss. For example if you load it with 1K, then you will use the maximum power out of the port, but the output level will be halved, so don't think in terms of "output power".

Also, If you don't output the maximum signal, the dynamic range of the signal will suffer, so don't use any software controlled volume regulation, but always use the maximum volume setting for best quality, and control the actual audio volume with the audio amplifier.

The point is, the output is NOT suitable to drive any speakers or headsets, or other low impedance devices, as quality and volume will suffer when you try. Only use an amplifier with 10K input impedance, or more.

krish888
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Re: 3.5 mm RCA Jack output characteristics

Fri Oct 19, 2018 5:01 pm

Thanks for your reply! I forgot to mention that I have a Raspberry Pi 3B. Would the numbers you mentioned change for the 3B?

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mahjongg
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Re: 3.5 mm RCA Jack output characteristics

Fri Oct 19, 2018 5:21 pm

No.

Changes that have been implemented in the past were to buffer the PWM signal with a buffer IC powered with a clean 2.5V supply, (to make the audio less noisy) and the audio which was earlier on a stereo TRS 3.5mm jack has been integrated into the TRRS (two rings Tip Ring Ring Sleeve) A/V connector . The various PI Zero's don't have analog output, but you can redirect the PWM signal to to GPIO's, and add your own "filtering circuitry".

In the last couple of years the PWM driver has been dramatically improved, no longer generating clicks when turning on and off, and with special PWM algorithms to create better sounding output.

Besides analog audio, there is also HDMI audio, and a codec I2S output (which can also do audio input).

krish888
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Re: 3.5 mm RCA Jack output characteristics

Fri Oct 19, 2018 7:01 pm

Thanks for the prompt reply! I'm sorry, did you mean to say 100 ohms or 1000 ohms in your earlier reply? I ask cuz I read elsewhere that the impedance is 69 ohms for a 3B and 97 ohms for a 2B.

Thanks in advance!

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mahjongg
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Re: 3.5 mm RCA Jack output characteristics

Sat Oct 20, 2018 10:44 pm

the 1K was from memory, but after checking my older schematics I realised the original filter circuits had much higher output resistors but not 1K, but rather 270 Ohms, because without the 2.5V powered driver the GPIO's the drive capacity was less, and they used divider resistors that resulted in an about 250 ohm output resistance.

It seems that later, (when they started using the 2.5V powered buffer) they decided the driver could drive a lower impedance, so now they use a resistor divider with lower resistances so yes for newer PI's the output impedance is nearer to 100 Ohms than to 250 Ohms.


that doesn't change much to my conclusion, except it explains why, with some loss, the output can just about drive a 32 Ohm headset speaker.

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Burngate
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Re: 3.5 mm RCA Jack output characteristics

Sun Oct 21, 2018 8:14 am

Funnily enough, almost exactly a month ago there was a similar thread.
viewtopic.php?f=91&t=223061
Burngate wrote:
Fri Sep 21, 2018 9:17 am
The schematics of the latest Pis are available at https://www.raspberrypi.org/documentati ... /README.md
From there, it's relatively easy to calculate both output impedance and p-p drive capability

For the 3B & 3B+ it's theoretically 1v p-p from 68Ω
For the 2B it's 1v25 from 50Ω
The B+ schematic doesn't show that bit of the circuitry
Original Pis were 1v18 from 95Ω

drgeoff
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Re: 3.5 mm RCA Jack output characteristics

Sun Oct 21, 2018 9:27 pm

BTW there is no such thing as a "3.5 mm RCA" jack.

There are RCA jacks/sockets. Also known as phono sockets. This type was used on the original RPi model A and B for the video output. Two contacts.

There are 3.5 mm sockets which come in 2, 3 and 4 contact varieties. The 3 contact version was used on the original model A and B for stereo analogue audio. The 4 contact version is used on the current models of RPi which provide composite video and analogue stereo audio on one socket.

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