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Recommendation for first test equipment
Recommendation for first test equipment
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Old 26th July 2019, 08:50 PM   #21
voltwide is offline voltwide  Ireland
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For identifying noise inside the audio band any wideband oscilloscope is of little help due to restricted sensitivity and intrinsic wideband noise. Instead a soundcard with ARTA delivers far more resolution at small signal levels.
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Old 26th July 2019, 08:53 PM   #22
itsikhefez is online now itsikhefez  United States
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Recommendation for first test equipment
I also wanted to measure the power supply noise+ripple. The Fluke 87-V does not seem to be accurate below 4mV AC (I was trying to measure the ripple+noise of B+ voltage)

Your tip may save me some $.
Which is a good set of tests to run once I have everything set up ?
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Old 27th July 2019, 06:07 AM   #23
trobbins is offline trobbins  Australia
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Unfortunately it looks like the EMU0404 PCIe doesn't have the 1Megohm input that the USB version has (I have the USB version) - which allows the use of a 10:1 or 100:1 scope probe as input, and hence to allow direct connection to much higher input voltage levels (so I don't agree with post #13 as a blanket statement).

It also looks like the PCIe doesn't allow 192kHz ?

I can certainly vouch that the 0404 USB, with 100:1 probe is an excellent audio tool, with bandwidth effectively from 2Hz to 90kHz, especially using REW software and ASIO and a scope probe cal file, and the output level is easily sufficient to drive amp inputs with any kind of signal that REW can conjure up.

For any kind of valve amp testing I'd also recommend a couple of cheap mini multimeters like ANENG AN8009 - as they are dead accurate, and although not a Fluke for ruggedness or meant for mains AC measurement, two meters allows amp circuit probing, and absolute signal gain and output power measurements if you keep the audio signal under 1kHz.

Last edited by trobbins; 27th July 2019 at 06:10 AM.
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Old 27th July 2019, 07:26 AM   #24
ej25awd is offline ej25awd  Canada
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Quote:
Originally Posted by trobbins View Post
Unfortunately it looks like the EMU0404 PCIe doesn't have the 1Megohm input that the USB version has (I have the USB version) - which allows the use of a 10:1 or 100:1 scope probe as input, and hence to allow direct connection to much higher input voltage levels (so I don't agree with post #13 as a blanket statement).
<snipped>
You misread my post. Go back and read it again.

I mentioned the use of add-on attenuator to mitigate PC sound card input damage.

You are using a 10:1 and 100:1 probe, which is an example of an attenuator.
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Old 27th July 2019, 07:44 AM   #25
phofman is offline phofman  Czech Republic
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Quote:
Originally Posted by trobbins View Post
Unfortunately it looks like the EMU0404 PCIe doesn't have the 1Megohm input that the USB version has (I have the USB version) - which allows the use of a 10:1 or 100:1 scope probe as input, and hence to allow direct connection to much higher input voltage levels
I do not understand why a lower input impedance would not allow using an input voltage divider. 30k input + 270k in series - 10:1.

If a higher voltage range is needed for measuring amp outputs, the input impedance is quite unimportant since the measured load is single ohms of the speaker dummy load. Tube amps are a different story, but still you can get hundreds of kiloohms at 10:1 and megaohms at higher ratios.

EDIT:
OK, 1M scope probes could not be used. But any voltage divider would do.
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Old 27th July 2019, 07:56 AM   #26
trobbins is offline trobbins  Australia
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Imho there is a significant advantage for newcomers to just purchase a cheap scope 10:1 or 100:1 probe. There is no need to construct a divider that achieves a flat and substantial bandwidth, given some soundcards are now at 96kHz audio range (ie. 192kHz sampling).

For valve amp work, it is highly recommended to be able to use a scope probe because of the high impedance - not an issue for ss or speaker level probing, but definitely for interstage probing.
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Old 27th July 2019, 03:30 PM   #27
itsikhefez is online now itsikhefez  United States
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Recommendation for first test equipment
Quote:
Originally Posted by trobbins View Post
Unfortunately it looks like the EMU0404 PCIe doesn't have the 1Megohm input that the USB version has (I have the USB version) - which allows the use of a 10:1 or 100:1 scope probe as input, and hence to allow direct connection to much higher input voltage levels (so I don't agree with post #13 as a blanket statement).

It also looks like the PCIe doesn't allow 192kHz ?
...
I had also mentioned I built the Pete Milllett sound card interface to go between the SC and DUT.
If I'm reading the description correctly, it has a 100K input impedance.
Regarding 1MOhm, Pete writes:
One could scale all the input resistors by a factor of 10 and get a 1 megohm input impedance. This might seem desirable as it would allow using 10x or 100x scope probes, which provide very little load to the circuit under test. The problem is that the input noise and offset would also go up (maybe also by a factor of 10), as the input bias current and current noise of the amplifiers will turn into nose and offset voltage. I opted for lower noise and kept the Zin at 100k, like my HP 8903A.

Also, 192kHz looks supported based on the spec sheet
Attached Images
File Type: png E-mu0404PCie-spec.PNG (170.8 KB, 72 views)
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Old 28th July 2019, 01:38 AM   #28
trobbins is offline trobbins  Australia
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If you are aiming to measure ripple and noise on B+, then the typical method used is to insert a blocking capacitor with suitable voltage rating between test node and measurement interface. That cap may alter the absolute voltage, so checking of calibration may be needed if that is a concern. Similarly, the danger of a charged cap is a concern.

An alternative approach is to use a resistive divider off the B+, to bring the measured level down to an acceptable DCV for direct connection/probing. That would need some awareness that absolute levels need calibration, and there may be some bandwidth limitation.

Your interface would likely need a 100:1 type probe to measure levels within any valve amp. I'm not sure what high-frequency frequency response you would get using just resistive dividers, and Pete's webpage doesn't identify the high frequency response range of his interface, only the low end.

I'm not sure about audiotester and ARTA, but like REW they should allow a calibration file to be associated with your interface (and any additional probe) to allow a flat loopback response to be normalised, although an awareness of the natural roll-off is needed so as to allow some caution when interpreting bandwidth edge response where normalisation may be making a substantial compensation (eg. below 10Hz and above say 20-30kHz). And I find I very rarely have a need for a digital or analog scope, as the PC software will likely come with a scope (eg. REW) for when you want to look at a waveshape rather than interpret a spectrum response, especially as the advantage of a soundcard scope/analyser is the huge increase in dynamic range over digital/analog scope products.

From a valve amp perspective, I have had a need to get a good view of response below 10Hz to determine low frequency feedback response, and it is well worth being able to view up to at least 40-50kHz to check there are no parasitic peaks occurring for whatever reason (especially for diy construction).

Last edited by trobbins; 28th July 2019 at 01:44 AM.
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