I haven’t tried yet. In my Navy experience, we just swapped entire boards. The defective boards were shipped to a repair depot or the manufacturer.
I just spent a couple hours testing again and at this point, I am scratching my bald head.🙂 I borrowed a Rigol portable 12 bit scope from a friend and looked at the waveforms coming out of the Quant Asylum analyzer while I ran the distortion and THD tests on each channel.
First I tried a simpler test by just feeding a 1kHz sine wave into each channel with only the scope connected. All 4 channels had a fairly clean sine wave output. With this 12 bit scope, I can see the switching frequency of the Alpine riding on the sine wave which is kind of cool.
Here is the part I can’t understand. When I connect the Quant Asylum analyzer and ran the distortion tests, once again 2 channels looked good and 2 didn’t with very high noise floor. I then looked at channel 4 with no signal but the QA403 still connected. I have attached a screenshot of what I measured. It is an approximately 2.5 Volt RMS sine wave at 154-155 kHz. When I disconnected the QA403, this signal disappeared and the Channel 4 output only showed a very low amplitude sine wave at the Alpine switching frequency.
I can’t see any reason why 2 channels would be showing this ~2.5 volt RMS signal with QA403 connected but no signal coming out of it and 2 channels do not show this signal that is obviously the cause of the high noise floor on the channels I am calling bad. I am using shielded BNC coax cables with RCA adapter on the amplifier end.
One last thing. The 2.5 volt signal I am calling noise is with the gain set at the mid point on the Alpine. The amplitude of this signal varies with the gain setting.
I really like this little Rigol scope. I think I am going to sell or donate my handheld Owon scopemeter and buy this Rigol. It can be powered by a USB C power bank or an AC adapter with USB C output.
First I tried a simpler test by just feeding a 1kHz sine wave into each channel with only the scope connected. All 4 channels had a fairly clean sine wave output. With this 12 bit scope, I can see the switching frequency of the Alpine riding on the sine wave which is kind of cool.
Here is the part I can’t understand. When I connect the Quant Asylum analyzer and ran the distortion tests, once again 2 channels looked good and 2 didn’t with very high noise floor. I then looked at channel 4 with no signal but the QA403 still connected. I have attached a screenshot of what I measured. It is an approximately 2.5 Volt RMS sine wave at 154-155 kHz. When I disconnected the QA403, this signal disappeared and the Channel 4 output only showed a very low amplitude sine wave at the Alpine switching frequency.
I can’t see any reason why 2 channels would be showing this ~2.5 volt RMS signal with QA403 connected but no signal coming out of it and 2 channels do not show this signal that is obviously the cause of the high noise floor on the channels I am calling bad. I am using shielded BNC coax cables with RCA adapter on the amplifier end.
One last thing. The 2.5 volt signal I am calling noise is with the gain set at the mid point on the Alpine. The amplitude of this signal varies with the gain setting.
I really like this little Rigol scope. I think I am going to sell or donate my handheld Owon scopemeter and buy this Rigol. It can be powered by a USB C power bank or an AC adapter with USB C output.
Attachments
It's always good to have an extra scope.
In some alpine amps, the 2v of noise is normal but it's generally on all channels and simply leakage through a cheap output filter. In some instances (most?), the output noise is determined with a A or C-weighting.
If the noise is leakage from the carrier through... whatever, could you possibly add a few caps or RC networks to damp the noise?
Are the noisy channels definitively audibly distorted (could pick them 10 out of 10 times in a blind AB test)?
In some alpine amps, the 2v of noise is normal but it's generally on all channels and simply leakage through a cheap output filter. In some instances (most?), the output noise is determined with a A or C-weighting.
If the noise is leakage from the carrier through... whatever, could you possibly add a few caps or RC networks to damp the noise?
Are the noisy channels definitively audibly distorted (could pick them 10 out of 10 times in a blind AB test)?
The PDX already has what look very much like bypass caps and RC networks that I assume are filters on the input buffers. I have the service manual and can see this on the schematic. This 155 kHz signal is being amplified when the QA403 is connected to the input so I am thinking this carrier is coming from the QA403 or something in the environment.
I am going to try driving a pair of NHT tower speakers with the 2 suspect channels to see what I can hear and if it is audibly distorted.
I am going to try driving a pair of NHT tower speakers with the 2 suspect channels to see what I can hear and if it is audibly distorted.
Could the input circuit of the analyzer be destabilizing the amp?
If you connect the analyzer and a dummy load, do you see the same problem?
If you connect the analyzer and a dummy load, do you see the same problem?