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Here are some additional measurements....

Posted by John Elison (A) on October 4, 2009 at 20:22:11

In Reply to: Wow, that's great! Thanks for doing this. posted by Ugly on October 4, 2009 at 09:50:04:

One of the things I especially like about my Tektronix digital oscilloscope is its sweep-averaging signal acquisition mode, which removes random noise from the signal being tested. This is really helpful when looking at cartridge output waveforms from test records because it stabilizes the trace and removes all the jitter and noise. I used this acquisition mode to look at power supply ripple in order to get a better idea of its actual amplitude as well as what effect the platter motor introduced. It turns out that the load from the motor is very insignificant compared to the load of the rest of the circuitry. Here is what I am talking about.

The pictures below show ripple from the output of the filtered rectifier on channel 2 (turquoise) and ripple from the output of the regulator on channel 1 (yellow). The power switch is on with the strobe disabled. The platter is not turning in the picture on the left, but it's spinning at 33.33-RPM in the picture on the right. There is only one millivolt difference in peak-to-peak ripple at the output of the regulator with the platter spinning as opposed to not spinning. In other words, the platter motor introduces negligible loading on the power supply.

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Now let's look at ripple from the battery power supply. Oscilloscope sensitivity is set to maximum, 2-mV per division. As before, the platter is stationary on the left and spinning at 33.3-RPM on the right. Additionally, I added a 6-V, 7-Ah SLA in series with my two 12-V batteries to provide 31.6-V to DC input connector.

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Here is what ripple looks like for the DC power supply with the strobe turned on and the platter turned off on the left, the platter spinning at 33.3-RPM in the middle, and the platter spinning at 45-RPM on the right.

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I also made some DC voltage and current measurements. One of your previous concerns was the higher-than-normal internal resistance of my SLA batteries. Well, I now know that their internal resistance is of little consequence as a result of the entry point Kevin selected for his DC power input. Kevin decided to put his DC input ahead of the rectifier bridge so that polarity was of no consequence. In other words, it is impossible to hook up the DC power supply incorrectly because the rectifier will always provide the correct polarity to the regulator. Unfortunately, the rectifier drops just over 1.6-V, which equates to about 7.5-ohms resistance with 0.213-A current I measured from my batteries. Therefore, regardless of how low an impedance the DC power supply has, the regulator will see a minimum of 7.5-ohms source impedance. Consequently, if you want the utmost in power stability, it is better to insert a well regulated DC power supply after the bridge rectifier. On the other hand, it is of little to no consequence with the strobe disabled because the motor and other circuitry do not place dynamic demands on the power supply.

Best regards,
John Elison

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