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Old 10th December 2012, 05:26 PM   #2491
Struth is offline Struth  Canada
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Hi Guys

Sure I can be serious - but why not have brevity? Life is short.

The Dr.Jordan stuff was the best I could find that presented any description of performance and tests possible. His software allows mining out -140dB distortions and more, just as a lot of software and mathematical manipulation can do, and they used to have some graphs showing the results with the best sound cards available at the time.

Everything else I could find seemed to be designed by guys who thought some arbitrary (usually quite high) level of THD was "good enough", or they simply did not do THD tests, or did not test to 20kHz

Most info I found back when I was looking at such things also suggested that to make the best use of standard sound cards, one has to make best use of the voltage range allowed. To measure a power amp output at anything above milliwatt levels requires attenuating the amp output in a precise way but keeping the level to the card on the high side. You know better than I about the vagaries of the card performance and software compromises to make it "good" but "affordable", Your statement that the "majority" of cards have high THD at high levels suggests that there are some exceptions that work much better? If that is the case, then they would be the units to employ.

Audio Precision set the standard to which all THD measurement systems aspire. They have priced themselves into a very stratified domain which makes them a target as much as an icon. The Dr.Jordan software was the only one I found that claimed similar possible performance. I would hope there are others available now, or other actual measuring systems that are priced more sanely than AP.

Have fun
Kevin O'Connor
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Old 10th December 2012, 07:10 PM   #2492
jcx is offline jcx  United States
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the AP boxes are really good deals for what you get

you aren't going to diy yourself into that level of reliability, usability, have built in the benefits of decades of experience serving diverse customers thousands of units in the field...

you can cobble together stuff to beat maybe one or two parameters in a dedicated test
but by the time you box up the rigs, fixtures, wrote software to do even 10% of what a modern AP does you would have saved time working minimum wage to buy the AP
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Old 10th December 2012, 08:08 PM   #2493
Struth is offline Struth  Canada
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Hi Guys

Yes, AP products are great. I believe they discontinued the portable unit that does just the basic audio measurements most people want, but haven't been to their site for a while. I am fully aware of their history and expertise.

I have zero interest in 90% of the capabilities of the AP systems since I only build analogue audio gear.

Have fun
Kevin O'Connor
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Old 10th December 2012, 08:26 PM   #2494
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Hi Kevin,

>His software allows mining out -140dB distortions and more
Without additional hardware: bandpass and/or bandstop filters?
If so, then we are are on the same track (though probably via a different route).

Cheers E.
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Old 10th December 2012, 11:02 PM   #2495
Struth is offline Struth  Canada
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Hi Guys

Yes, software mining. Virtual picks and shovels.

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Kevin O'Connor
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Old 10th December 2012, 11:33 PM   #2496
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Hi Edmond,

What are you planning to use as the signal source? How are you going to generate a sinewave with distortion low enough to match your analyser?
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Old 11th December 2012, 03:09 AM   #2497
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I am interested in this discussion of software audio test systems but perhaps it should be moved to a new thread? People may feel freer to contribute if they are not worried that they hijack the Cordell discussion.

David
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Old 11th December 2012, 03:35 AM   #2498
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Quote:
Originally Posted by Struth View Post
I think one of the simplest solutions is to use the Wilson CM with fourth BJT added, as this sets both diff collectors at the same potential - or at least the collector in question has a set voltage. A VAS with emitter resistor then has a set base voltage to work against, with R being adjusted for the desired current. So, another compromise, doubling the BJT count for the mirrors.
Bob Cordell's differential shunt resistor is probably the simplest solution to complementary differential inputs with current mirrors. Bob selected a 47K resistor to handle most component variation.

I have been using THAT transistor arrays for the input stage current mirrors and cascode transistors. The tight array transistor tolerances allow a much higher differential shunt resistor value. The higher input stage gain provided by current mirrors does require adding a series R+C differential shunt to control high frequency oscillations. This R+C was not necessary with low gain simple resistor differential loads, and some golden ears favor resistor load complementary differential stages for this reason.
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Old 11th December 2012, 03:49 AM   #2499
wahab is offline wahab  Algeria
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Quote:
Originally Posted by LineSource View Post
Bob Cordell's differential shunt resistor is probably the simplest solution to complementary differential inputs with current mirrors. Bob selected a 47K resistor to handle most component variation.I have been using THAT transistor arrays for the input stage current mirrors and cascode transistors. The tight array transistor tolerances allow a much higher differential shunt resistor value.
Agree on that part.
100K will work as well without degrading perfs and it slightly increase the gain..

Quote:
Originally Posted by LineSource View Post
The higher input stage gain provided by current mirrors does require adding a series R+C differential shunt to control high frequency oscillations.
Increasing the compensation seems more cautious imo.

Quote:
Originally Posted by LineSource View Post
This R+C was not necessary with low gain simple resistor differential loads, and some golden ears favor resistor load complementary differential stages for this reason.
It could be that golden ears like distorsions of the worse kind since only current mirror loading allow radical reduction of IMD , at least in my tries.
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Old 11th December 2012, 03:56 AM   #2500
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Do you mean RC shunt at the CM or across the LTP inputs? And what values approximately? Is this local oscillation or global instability?

Adding RC shunt to an amp is in some cases not unlike adding two-pole compensation, and this could also explain sounding better.
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