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Old 31st May 2009, 09:44 AM   #1
Ardee is offline Ardee  England
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Default Liberty IMP Impedance Measurement

I cannot get any sensible impedance readings from an IMP measurement system. A 6R resistor reads around 30R, and a dead short around 5R. I have tried (and declared in the settings) both a 10R and 20R reference resistor with much the same result. I have also reloaded the 2.05 software to eliminate any rogue settings.

Is there anyone else out there still using this system or who has a good enough memory to recall whether there are any gotchas I might have missed?

Frequency response readings all look OK.
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Old 31st May 2009, 04:35 PM   #2
PLB is offline PLB  United Kingdom
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Hi Ardee,

I used Imp for several years, but now I use Praxis, so my memory on the Imp system is a very vague. However, for the problem you describe, I can think of two things that may be causing your problem;

1) Have you run the calibration procedure recently. To my knowledge, impedance measurements with all these computer based systems employ both left and right channel inputs of a sound card that are effectively connected to either side of the reference resistor. If there are any dicrepancies in the sensitivity between these two inputs, the resulting measurements will be in error.
2) For my setup, I recall that I had to use a small power amp to buffer the output when making impedance measurements in order to provide enough current to drive the low impedance loads.
I assume you are doing the same, so I suggest you check out your power amp to see if that is functioning correctly.

Hope this helps.

Peter
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Old 31st May 2009, 06:17 PM   #3
Ardee is offline Ardee  England
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Thanks, but the IMP uses both a calibration input which is connected to the output from the amplifier and a measurement input connected to the junction of the reference resistor and item under test. Like most systems used to measure impedance it just subtracts one input from the other. In my case, something is upsetting the arithmetic. The cold side of the amplifier output is grounded to the IMP, by the way, as the amplifier has an input transformer.
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Old 1st June 2009, 11:31 AM   #4
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Hi Ardee,

I use the IMP frequently. The only thing I ever had a problem with was the input RCA jacks breaking the solder joints on the IMP PC board. This gave erroneous reading. A quick re-solder fixed the problem. You might want to check.

Just noticed that you said the amp has in input transformer. Have you tried another amp? Or, try using the IMP output directly (no amp in the circuit) with a 1k ohm ref resistor and a 1k test resistor.
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Old 2nd June 2009, 03:49 PM   #5
Ardee is offline Ardee  England
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Hello John K, and thanks for the suggestions.

Input jacks all seem soundly connected, and I did check ground continuity between them.

Using 1k resistors gives a reading of about 3k. Phase looks about right at 0 rising a bit at the HF end. What might be significant is that if I short out the reference resistor and do an electrical test with get-new-reference set, it shows a 0dB flat line. With the reference resistor in circuit it shows a flat line about 2.5dB down. I would have expected this to be either 3 or 6dB, depending on whether it is showing power or voltage. Another odd thing I notice is that, when doing Acquire-Repeat to set the input levels, if I keep turning the level up past overload the displayed level starts dropping. Could a missing bit cause this? I have never really liked the look of that ribbon cable on the Centronics connector.
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Old 2nd June 2009, 11:55 PM   #6
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Have you checked theimpedance of your probes? Have you changed cables? Is the IMP input level set too high?
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Old 3rd June 2009, 08:01 AM   #7
Ardee is offline Ardee  England
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Both probes are 47k, which is declared. I think that any problem with the probes would cause the probe 1 and 2 input levels to show differently when the reference resistor is shorted, but they don't.

I've tried different PC's, so unless there is a setting I have overlooked, I can only think this must be a hardware problem or that a "fast" (Pentium 2) PC is just too much for it. Using SlowDOS makes no difference, but this might not affect the printer port speed.
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Old 3rd June 2009, 10:32 AM   #8
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Try this.

Connect the pulse output of the IMP directly back to the Cal input, (Probe 1). Do not use any resistive probes, just a cable.

Set the probe gain pot to the first mark (almost off).

Select auto_Measure.

Click just_Cal.

You should see a flat line with slight down turn above 10k Hz. The level should be close to 0 dB. If the level is too low, or too high adjust the gain pot appropriately.


On the IMP unit change the cable from Probe1 input to Probe 2 input. DO not change the pot gain setting.

Then go to the auto_Measure menu and click Elec.

You should see a flat response at 0 dB.

If you get this the IMP is working correctly.


One other question. What operating system are you running? I run the IMP on an older W95 machine. As I recall, it won't run on XP. Don't know exactly were the cut off was.
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Old 3rd June 2009, 06:01 PM   #9
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Hi Ardee,

I had a problem with my IMP in that the probe inputs would not work but the microphone input would. The input multiplexer (a 74HC4053) had failed, I think due to me inadvertently leaving the 47K resistors out of the probes. A standard 4053 was to hand and works OK. That is the only failure in 10 years of use. I use an old laptop running Windows ME and the software runs OK in a DOS window
Its great in combination with the DCX2496 you can quickly 'dial in' a crossover.





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Old 4th June 2009, 02:50 AM   #10
bwaslo is offline bwaslo  United States
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Try a different power amplifier. IMP depends on the "cold" [-] side of the amplifier output being always at the same potential as the input ground/shield connection. Bridged amplifiers, or most amplifiers with transformer in their outputs, won't work. The ground from the "pulse out" has to be the same circuit node as what connects to the negative terminal of the speaker (or resistor). That's one of the changes in PRAXIS, the inputs are differential so that input ground doesn't have to be output ground, too. Also, it can work from a current measurement rather than from subtracting voltage measurements (which isn't so tolerant of errors).

Ditto on the 4053 maybe being bad. It will get damaged if it ever gets connected to a power amplifier without the resistors being in the probes. They don't cost much and are easy to replace, so its worth a try.
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