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Interstage Distortion Measurements

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rdf

rdf

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Joined 2004
#1
Hi all. It's one of those topics that strings common words together, resulting in searches which return nothing or everything. Is anyone doing interstage noise and distortion measurements with a sound card? I'ld like to get a better picture between the input and output stages of circuits I'm playing with but the 10 kohm input impedance and 1 VRMS input limit of my M-Audio makes it a dicey thing. One possible option is 'tapping' off the grid resistor with a cap, however it only appears useful where voltage swing is sufficient to allow a low enough tap to present a low impedance drive to the card.

Any recommendations? Thx in advance.
 
#2
Hi rdf,
Low distortion buffer time. Many points before the output are high impedance points and you don't want to load them.

This could be as simple as a FET input op amp after your high impedance attenuator.

-Chris
 
#3
I use a high speed buffer amp (BUF-03) with a 1M input impedance and a 10x scope probe. Works a charm. A second BUF-03 is used between the soundcard and any nasty load (like the low input Z of a step-up interstage); source Z can then be easily adjusted with a series resistor.
 
#4
anatech said:
Hi rdf,
Low distortion buffer time. Many points before the output are high impedance points and you don't want to load them.

This could be as simple as a FET input op amp after your high impedance attenuator.

-Chris
Click to expand...


Ah...interesting....As SY mentions low point loading.....I use 10Meg min. It may sound old fashioned but for push-pull amp work I always use matched probes and straight into a god quality XY scope (Lissajous). Any capacitive loading or effects of crummy layout becomes phase sensitive and immediately apparent as 0°/80°/360° line on scope bows out of line assuming the oscilloscope amps are matched within a few %. This is only one of a raft of sim measurements one can do.
SUprisingly I couldn't find a description of the Lissajous method in the Radiotron designers handbook 4th ed......I presume the quality of dual channel scopes in early days wasn't good. I seem to remember that the Tektronix 545 series had this facility.

richj
 
#5
Thanks everyone! I think I like SY's approach. The only downside is both the BUF-03 and -04 are now obsolete product. AD recommends the AD811 video buffer as a replacement and it appears to have suitable distortion performance. Cheap too. In your experience is there any reason one of these video buffers can't maintain performance into a 1 kohm ten-turn? The M-Audio drivers disable the Windows mixer leaving no way to adjust input level in software.
 
#9
Yeah, the trick is to match the scope's input resistance and capacitance; that way, you know you can get the probe compensation adjusted properly. With a 10x attenuation network (100k + 900k) as the input to the buffer, you can get a 100x reduction, very useful for those of us who often build high voltage driver stages.
 
#11
M-audio cards that disable the windows mixer generally have hardware mixers and they provide the driver software for that function.
I would go to their site and make sure you have the latest and greatest.

I use several m-audio cards for my media server and laptop measurement system which uses software dsp based audio measurement software. (audiotester)

The cards I use are the usb transit which is compatible with the windows mixer, and the 2496 which is not. (Has excellent hardware mixer instead.)

You will still need lots of attenuation ahead of the audio card though as it can't handle input voltages in excess of a volt or so. I use an output amplifier with gain as well so that I can get as much as 5Vrms drive to the device I am testing.
 
#13
kevinkr said:
M-audio cards that disable the windows mixer generally have hardware mixers and they provide the driver software for that function.
Click to expand...

That's partially correct in the case of the Audiophile USB. Hard to pass up at $200 CDN. This stand-alone has a volume control for output only, the input is fixed at about 1 Vrms best I can tell. The drivers are also very touchy. Many times my machine has done a 'rapid shutdown' on exiting Windows, fast like pulling the plug.
The max output level of 1 Vrms suits my need for now. It's enough to clip anything I've built, or in the case of a headphone amp deafen. When more is required the headphone output works in a pinch and is pretty clean. The Audiophile USB has stellar measured performance with a few provisos. There's something odd about the gain structure, there isn't enough output to reach 0 dB loopback. I don't know if its software or hardware related. It's maximum duplex R/P sample rate at 24 or 32 bits is 48 kHz no matter what the specifications imply. More disconcerting, though max and min level performance is outstanding, with the output pot at about 11 o'clock a severe series of low level harmonics appears in loopback. I haven't determined if it's on the input or output and it may be related to the removal of a pair of 10 uF coupling caps ahead of the 10 kohm volume pot. No DC was measurable at any setting but the possibility exists the changes resulted in some instability.
All things considered it's still well worth it to lose the 'Creative' mixer.
 
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