Calibrating the AP System One

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
I have been using the System one for several weeks now and its been working great but has been giving me about 3 - 5 dB difference in THD measurements between channels. I figured a cal was probably in order. It took me several hours to figure out how to get the S1 cals operating in a DOS window. I needed to add that /i1 to the command line, or s1.exe /i1. I'm using Duke's APIB-LPT translator with a Win98SE laptop in case you were wondering. The cal's are fairly straight forward, easier than many pieces of equipment I've calibrated in the past. About the only anomaly I saw was that CMRR for the A input is about -92dB and the B input was able to get to about -104dB. Both are well below spec, but it would be nice to have both of them closer.

The performance that this old analyzer can achieve exceeded my expectations for sure. I had expected it to go to -102 to -103db based on what I had read online but I wasn't how much more was possible. I attached a chart of THD vs Freq of my unit in GenMon mode (loopback essentially) and a chart of THD vs Volts. Both are with the 80kHz LP active. These two tests had about 3 - 5 dB of difference at various points in the curves between the two channels before the calibration. They're reasonably consistent with each other after the cal. Needless to say I'm happy with the results. This unit is an early 90's model and appears to be 100% original. I'll probably recap the power supply eventually and I need to do something with that noisy fan (does anyone have any suggestions for a quiet replacement fan?), but other than that I'm not going to mess with it. :D
 

Attachments

  • THDvsFreq2.jpg
    THDvsFreq2.jpg
    152.3 KB · Views: 276
  • THDvsVolts.jpg
    THDvsVolts.jpg
    159 KB · Views: 273
  • Like
Reactions: 1 user
Hi Kevin
The CMRR is the match of the 1uf input caps and the light bulbs @ 20Hz. This is GOOD as it meets spec.

The THD vs Freq indicates that there may be a slight problem in the GEN or ANALYZER in the #4 band (22kHz to 204kHz) as the distortion is >0.001%

My tests of THD vs LEVEL are more detailed. Start @26 volts and sweep to 10mv and use 500 points.

The PS CAPS are not high on my list of problems. I do FFT of the PS to check. The main problems are DRY Electrolytic on the local modules.

Any good replacement fan and filter (see late notes on LinkedIn, Classic AP Audio Systems Group LA) are easy to find.
Duke
 
Hi Duke,

Thx for the response! Its good to have an AP expert on the forum.

I had thought the the sudden distortion rise at 20kHz was abnormal, but the system check routine shows the high freq distortion to be well below the pass/fail red line so I thought it must be normal. I ran another pair of tests using 500 sample points this time. For this THD vs Freq test I disabled the bandwidth filtering, prior it was 22Hz - 80kHz, so these are 10Hz - 500kHz now and up to 100kHz signal. The THD vs Volts Out test is with a 1kHz signal.
 

Attachments

  • THDvsFreq_100kHz_No_Filter.jpg
    THDvsFreq_100kHz_No_Filter.jpg
    156.5 KB · Views: 247
  • THDvsVolts_26V.jpg
    THDvsVolts_26V.jpg
    160.2 KB · Views: 232
Kevin

Run the FREQ vs THD @ 2 volts. Is it the same as 9.5 volts?
Duke


Hi Duke,

I attached the revised chart at 2V out. Its similar, distortion #'s are slightly higher and high freq dist at >80kHz is a bit closer between channels.

I performed a couple of the tests we spoke of over the phone. My 2nd analyzer reading the distortion of the AP generator and the AP analyzer measuring the distortion of the generator output from my 2nd analyzer's generator.

Observations,
The AP shows little difference when measuring the signal output of my 2nd generator. I varied the output of the 2nd generator sweeping from 19kHz to 23kHz back and forth and the distortion is fairly constant at -96.5dB +/-0.5dB when measured by the AP analyzer.

My 2nd analyzer shows a step in noise output from the AP generator when transitioning from 20k to 21kHz. That step is about 4 - 5dB with the 80kHz BW filter active and about 2 - 3dB with the system set for full bandwidth measurement.

So it appears that the large change in step noise at that frequency is coming from the AP generator.
 

Attachments

  • THDvsFreq_2V_100kHz_No_Filter.jpg
    THDvsFreq_2V_100kHz_No_Filter.jpg
    157.3 KB · Views: 229
Hi Kevin & Hardi
I need more information to help resolve the specific problem. I would first run a NOISE CHECK that is similar to the basic noise check; mine has 100 points to do a fine look @ the noise. I them may do a FFT of the 5 volts on the GENERATOR BOARD across the 1000uf 10 axial cap near the rear of the board.

Look @ LinkedIn, Classic AP Audio Systems Group LA. (Noise problem post) This show a real problem form a system a few years ago.
Duke
 
For comparison THD&N vs. Frequency from S322 and S022 (my analog only 022 is sligthly better). 2Volts , >500KHz bandwith. The scaling is different from yours, and i stopped at 40 KHz, but in my opinion this is typical for S1.

https://abload.de/img/022gifk3ci7.gif

https://abload.de/img/332gif2pi05.gif


Hardi,
Yes your 022 is somewhat identical to my 22A, HF distortion seems to be rising more in mine than yours.


Hey Duke,
By any chance do you have a freq scan on file from a S1 that you refurbished? I looked through all of the postings on the Linkedin.com forum and I do not see any prior info like that.
 
As is typically the case. I was looking at something totally unrelated and stumbled across a baseline of an AP System One. The attached image is taken from the OP AMP distortion article here which was written by Samuel Groner. He baselined his S1 for the testing and chronicled the results. This is a 500kHz BW measurement at 20dBu output from his S1.

This is not to imply this test is good ..... or bad for that matter. I'm just adding to the data.
 

Attachments

  • S1_THDvsFreq.jpg
    S1_THDvsFreq.jpg
    229.7 KB · Views: 94
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.