DUOr interference from headphone outputs #eladduo #fdm-duor #duor


Ad Dieleman
 

This afternoon I was doing some measurements on audio signals on the DUOr's headphone outputs. Suddenly I saw a 2.4 MHz signal on my oscilloscope with an amplitude between 0.5 and 1 Vpp. The occurrence of that signal depends on the length of wire connected to the headphone output and crucially occurs most often when no load is present apart from the high-impedance oscilloscope input. Once the output is loaded with 10 ohms or so the 2.4 MHz disappears. I already use a ferrite core through which the headphone cord is wired but that doesn't suppress it. Ultimately I made a Boucherot filter-like setup, connecting a resistor of 4,7 ohm and a capacitor of 612 nF in series across the output. See the picture. No more interfering signals this way. I'd suspect the audio amplifier starts to oscillate under certain load conditions.



The filter box. Decided to resist the temptation to open the radio, so built a separate box to be inserted between loudspeaker/headphone and the radio output.


Andy G4JNT
 

I saw something like that.  I guess the Duo might use a Class-D switch-mode audio amplifier chip for speaker and headphones
There's no real reason NOT to use one in that position, they're very efficient.



On Thu, 24 Sep 2020 at 19:35, Ad Dieleman <addieleman@...> wrote:
This afternoon I was doing some measurements on audio signals on the DUOr's headphone outputs. Suddenly I saw a 2.4 MHz signal on my oscilloscope with an amplitude between 0.5 and 1 Vpp. The occurrence of that signal depends on the length of wire connected to the headphone output and crucially occurs most often when no load is present apart from the high-impedance oscilloscope input. Once the output is loaded with 10 ohms or so the 2.4 MHz disappears. I already use a ferrite core through which the headphone cord is wired but that doesn't suppress it. Ultimately I made a Boucherot filter-like setup, connecting a resistor of 4,7 ohm and a capacitor of 612 nF in series across the output. See the picture. No more interfering signals this way. I'd suspect the audio amplifier starts to oscillate under certain load conditions.



The filter box. Decided to resist the temptation to open the radio, so built a separate box to be inserted between loudspeaker/headphone and the radio output.


Klaus Brosche, DK3QN
 

Possibly one of these low power class-D PAM-xxxx chips?

Klaus, DK3QN

Am 24.09.2020 um 20:52 schrieb Andy G4JNT:

I saw something like that.  I guess the Duo might use a Class-D switch-mode audio amplifier chip for speaker and headphones
There's no real reason NOT to use one in that position, they're very efficient.



On Thu, 24 Sep 2020 at 19:35, Ad Dieleman <addieleman@...> wrote:
This afternoon I was doing some measurements on audio signals on the DUOr's headphone outputs. Suddenly I saw a 2.4 MHz signal on my oscilloscope with an amplitude between 0.5 and 1 Vpp. The occurrence of that signal depends on the length of wire connected to the headphone output and crucially occurs most often when no load is present apart from the high-impedance oscilloscope input. Once the output is loaded with 10 ohms or so the 2.4 MHz disappears. I already use a ferrite core through which the headphone cord is wired but that doesn't suppress it. Ultimately I made a Boucherot filter-like setup, connecting a resistor of 4,7 ohm and a capacitor of 612 nF in series across the output. See the picture. No more interfering signals this way. I'd suspect the audio amplifier starts to oscillate under certain load conditions.



The filter box. Decided to resist the temptation to open the radio, so built a separate box to be inserted between loudspeaker/headphone and the radio output.



Andy G4JNT
 

This sort of device



On Thu, 24 Sep 2020 at 20:58, Klaus Brosche <klaus.brosche@...> wrote:
Possibly one of these low power class-D PAM-xxxx chips?

Klaus, DK3QN

Am 24.09.2020 um 20:52 schrieb Andy G4JNT:
I saw something like that.  I guess the Duo might use a Class-D switch-mode audio amplifier chip for speaker and headphones
There's no real reason NOT to use one in that position, they're very efficient.



On Thu, 24 Sep 2020 at 19:35, Ad Dieleman <addieleman@...> wrote:
This afternoon I was doing some measurements on audio signals on the DUOr's headphone outputs. Suddenly I saw a 2.4 MHz signal on my oscilloscope with an amplitude between 0.5 and 1 Vpp. The occurrence of that signal depends on the length of wire connected to the headphone output and crucially occurs most often when no load is present apart from the high-impedance oscilloscope input. Once the output is loaded with 10 ohms or so the 2.4 MHz disappears. I already use a ferrite core through which the headphone cord is wired but that doesn't suppress it. Ultimately I made a Boucherot filter-like setup, connecting a resistor of 4,7 ohm and a capacitor of 612 nF in series across the output. See the picture. No more interfering signals this way. I'd suspect the audio amplifier starts to oscillate under certain load conditions.



The filter box. Decided to resist the temptation to open the radio, so built a separate box to be inserted between loudspeaker/headphone and the radio output.



Klaus Brosche, DK3QN
 

Hi Andy,

That's exactly was I was wondering about :-))

The average designs around this class-D chip family go w/o any LPF at the output.
I do not know how this is implemented in the DUO(r).

In case the outputs (stereo) are not LPF-limited, the PAM-xxxx devices turn
into 'transmitters', as is true for any class-D amp, especially if speaker/headphone
cables are longer than a few inches/centimeters!

Thus, a proper designed LPF at the amp output is a real 'must' with such kind
of amps.

The 'PAMs' are typically used in low power one-box FM receivers with internal
speakers and a very short cable connection between PAM-xxxx out and the
(internal) speaker chassis.

Klaus, DK3QN


Am 24.09.2020 um 22:01 schrieb Andy G4JNT:

This sort of device



On Thu, 24 Sep 2020 at 20:58, Klaus Brosche <klaus.brosche@...> wrote:
Possibly one of these low power class-D PAM-xxxx chips?

Klaus, DK3QN

Am 24.09.2020 um 20:52 schrieb Andy G4JNT:
I saw something like that.  I guess the Duo might use a Class-D switch-mode audio amplifier chip for speaker and headphones
There's no real reason NOT to use one in that position, they're very efficient.



On Thu, 24 Sep 2020 at 19:35, Ad Dieleman <addieleman@...> wrote:
This afternoon I was doing some measurements on audio signals on the DUOr's headphone outputs. Suddenly I saw a 2.4 MHz signal on my oscilloscope with an amplitude between 0.5 and 1 Vpp. The occurrence of that signal depends on the length of wire connected to the headphone output and crucially occurs most often when no load is present apart from the high-impedance oscilloscope input. Once the output is loaded with 10 ohms or so the 2.4 MHz disappears. I already use a ferrite core through which the headphone cord is wired but that doesn't suppress it. Ultimately I made a Boucherot filter-like setup, connecting a resistor of 4,7 ohm and a capacitor of 612 nF in series across the output. See the picture. No more interfering signals this way. I'd suspect the audio amplifier starts to oscillate under certain load conditions.



The filter box. Decided to resist the temptation to open the radio, so built a separate box to be inserted between loudspeaker/headphone and the radio output.




Ad Dieleman
 
Edited

I don't think that the interference comes from spurious switching signals from a class D amplifier: the 2.4 MHz signal disappears completely when loading the output with a low enough resistance, i.e. no high-frequency filtering at all. The filter I applied also doesn't really filter out HF signals, it just loads the output with 4.7 ohm for frequencies over roughly 50 kHz, this being the 3 dB cutoff point. So I still think that the output stage is oscillating under certain circumstances, be it a class D amplifier or something else.


Klaus Brosche, DK3QN
 

As you wrote, you are loading the output with 4.7 ohms in series with cap. Which results, as your
indications say,  in an LPF at minus 3dB @ 50KHz.

I have currently no way of verifying that. I don't have my Ham Radio Laptop at hand right now
with the simulation software. It is just my guess.
So, maybe I'm wrong. In such case: forgive me ;-)

Klaus, DK3QN


Am 24.09.2020 um 22:36 schrieb Ad Dieleman:

I don't think that the interference comes from spurious switching signals from a class D amplifier: the 2.4 MHz signals disappears completely when loading the output with a low enough resistance, i.e. no high-frequency filtering at all. The filter I applied also doesn't really filter out HF signals, it just loads the output with 4.7 ohm for frequencies over roughly 50 kHz, this being the 3 dB cutoff point. So I still think that the output stage is oscillating under certain circumstances, be it a class D amplifier or something else.



Klaus Brosche, DK3QN
 

The 'Boucherot' filter is also known as the 'Zobel' filter in HiFi amp output stages.

Klaus, DK3QN


Am 24.09.2020 um 22:49 schrieb Klaus Brosche:

As you wrote, you are loading the output with 4.7 ohms in series with cap. Which results, as your
indications say,  in an LPF at minus 3dB @ 50KHz.

I have currently no way of verifying that. I don't have my Ham Radio Laptop at hand right now
with the simulation software. It is just my guess.
So, maybe I'm wrong. In such case: forgive me ;-)

Klaus, DK3QN


Am 24.09.2020 um 22:36 schrieb Ad Dieleman:
I don't think that the interference comes from spurious switching signals from a class D amplifier: the 2.4 MHz signals disappears completely when loading the output with a low enough resistance, i.e. no high-frequency filtering at all. The filter I applied also doesn't really filter out HF signals, it just loads the output with 4.7 ohm for frequencies over roughly 50 kHz, this being the 3 dB cutoff point. So I still think that the output stage is oscillating under certain circumstances, be it a class D amplifier or something else.