The relationship between working bandwidth and square wave response and intermodulation distortion
Intermodulation distortion has been proven to be the main cause of sound deterioration
For example, some external narrow pulse signals radiate to the input terminal of the transistor
Caused transient overload of the transistor
As a result, severe distortion occurs. This is intermodulation distortion.
This is one of the reasons why low-frequency audio amplifiers also need good shielding.
At the same time, a well-shielded wire is also very helpful in reducing the probability of intermodulation distortion.
This is an external solution.
However, the interference signal may also be generated by the internal signal channel, which will also cause intermodulation distortion.
For example, the signal source adopts a switching power supply, and the switching power supply has a carrier signal that works at about 100k.
This signal is serially connected to the input of the signal amplifier. Intermodulation distortion will occur when the amplifier bandwidth is not high.
Also, the residual carrier signal of the CD player dac, 44.1kHz 96khz 192khz, these are bad signals
Also, the interference introduced by the external mains
Such as mobile phone chargers, air conditioners, etc. Now the popularization of switching power supplies is an audio nightmare.
You can't use shielding to solve internal interference
In addition to the selection of the grounding point and the filtering of the power supply
More important is the bandwidth design.
If we have an amplifier with a very high bandwidth, linearity can be maintained within this bandwidth range
If we design a low-pass filter in front of the amplifier to limit the high-frequency interference signal so that it cannot enter the amplifier
Then, our intermodulation distortion can be reduced to the limit level.
This is how we designed it.
Inside the amplifier is a conversion rate of 200v/us
The whole machine is limited to 100v/us after adding the input filter
Therefore, the generation of intermodulation distortion can be extremely effectively suppressed.
Even so, we still maintained a 1.2mhz bandwidth,
Keep the audio range perfectly responsive
Moreover, different bandwidths can be selected through the remote control 22k 33k 60k 1.2mhz
You can slowly test the difference between the different filters.
This picture shows the intermodulation distortion IMD of mhzpreampII (Ref) tested by ap 2322, using SMPTE4:1 standard.
The light blue is the self-test of AP2322's intermodulation distortion.
In other words, AP2322 cannot measure the intermodulation distortion of our previous stage.
So it can only be defined as IMD=0.0007%, which is a bit of a disadvantage.