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Old 31st March 2013, 12:25 PM   #2341
AndrewT is offline AndrewT  Scotland
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Is the Allen Wright modification that is being referred to the same as adding the 50kHz filter?
Is this a pole or a zero?
My reasoning is that it is a zero (= going flat beyond 50kHz).
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Old 31st March 2013, 12:31 PM   #2342
vulejov is offline vulejov  Serbia
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Quote:
Originally Posted by Wirehead.be View Post
No schematic, it's a recording/editing program. Please google it.
Why are you convinced that the program is correct?
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Old 31st March 2013, 01:05 PM   #2343
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Well, the same could apply to simulations. Why are you convinced that all simulations hold absolute truth? The thing is, it's more than close enough for me in real life. Just a matter of how you look at things
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Old 31st March 2013, 01:25 PM   #2344
vulejov is offline vulejov  Serbia
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Quote:
Originally Posted by Wirehead.be View Post
Well, the same could apply to simulations. Why are you convinced that all simulations hold absolute truth? The thing is, it's more than close enough for me in real life. Just a matter of how you look at things
I think exactly the same..
I am convinced that the various simulations repeating the same mistakes..
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Old 31st March 2013, 04:40 PM   #2345
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Did some more testing - compared three different opamps in the same, identical test-setup as the previous measurements (whole Rightmark suite). Up for test was the TI NE5532; the BB OPA2134 and the National LM4562NA.

Take a guess as to which one came out best in terms of noise floor/dynamic range.
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Old 31st March 2013, 07:26 PM   #2346
vulejov is offline vulejov  Serbia
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Quote:
Originally Posted by Wirehead.be View Post
Take a guess as to which one came out best in terms of noise floor/dynamic range.
Ask Douglas Self.. NE 5532
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Old 31st March 2013, 08:18 PM   #2347
L-Train is offline L-Train  Canada
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I was intrigued by Wirehead's measurements of his VSPS, so I thought I'd try measuring my VSPS using the same method. Here's my process:
  1. In RMAA, generate 96 KHz/32-bit float test signal files
  2. In Audacity, open the test signal and:
    1. Reduce the amplitude by -27.5 dB via the "Amplify" effect to avoid clipping high frequencies in the next step
    2. Apply the inverse RIAA curve by selecting the "Equalization" effect, choosing the "RIAA" curve, clicking the "Invert" button to invert the curve, and increasing the length of the filter to the maximum 8191 for improved accuracy
    3. Export the test signal
  3. Insert the VSPS in a loopback between an output & input of my E-MU 1616M
  4. Playback the test file in foobar2000 and record in Sound Forge, all using ASIO
E-MU's software/hardware mixer PatchMixDSP was configured to send ASIO outputs directly to the hardware outputs, and hardware inputs to the ASIO inputs to avoid any unwanted bit manipulation by the drivers/OS. The 1616M's input reaches full-scale at 2V RMS so I reduced the output signal from the 1616M in PatchMixDSP by 14 dB to get the VSPS to amplify the signal to that level.

The VSPS was configured as follows:
  • DIY PCB
  • 2x OPA827 on a single-to-dual SO8-to-DIP8 BrownDog adapter
  • Vishay CMF55 1% resistors, unmatched, 105k/732k for R4/R5 respectively
  • Panasonic ECQ-P(Z) 1% polypropylene capacitors, unmatched
  • Nichicon Muse ES 10uF bi-polar capacitor for C3 and 100k for R7
  • 2x 9V batteries in series, with a TLE2426 rail splitter for ground
As you can see the measured performance is very, very good. Even with unmatched 1% components the frequency response reflects RJM's simulations damn near perfectly and with exceedingly good matching between channels. As far as I know the Panasonic ECQ-P(Z) capacitors are now discontinued and unavailable from DigiKey (where I purchased them), but I'm sure there are other companies that manufacturer equivalent caps with the same tolerance.

Attached is the full RMAA test in HTML for you to view, and a .sav file you can load up in RMAA. The "roughness" below ~1 KHz is probably due to all of the digital filtering done for the inverse RIAA and level reduction.

Not sure what to think of the slight rise in frequency response below 50 Hz though, even though it's not enough to be much of a bother.

With all that said, thank you RJM for the VSPS and also to Wirehead for tips on how to measure this thing!
Attached Images
File Type: png VSPS Frequency Response.png (3.1 KB, 94 views)
Attached Files
File Type: zip VSPS-RMAA.zip (247.5 KB, 9 views)
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Old 1st April 2013, 01:59 AM   #2348
rjm is offline rjm  Japan
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Quote:
Originally Posted by AndrewT View Post
Is the Allen Wright modification that is being referred to the same as adding the 50kHz filter?
Is this a pole or a zero?
My reasoning is that it is a zero (= going flat beyond 50kHz).
Yes on both counts.

The zero shows up anyway in a non-inverting opamp circuit, since the gain can be no less than unity. All I did was move the zero up a little to set it at 50 kHz.

@vulejov,

My copy of Horowitz and Hill runs 1125 pages, but you'd need only the first 59, chapter 1 "Fundamentals" explains how impedance and passive filters work and how to calculate the response.

The RIAA is not defined by a circuit, but by three time constants. There are any number of ways it can be realized in practice but the values generated by calculation or software like Audacity are exact by definition.

@L-Train

That's really impressive, I suppose 1% components makes all the difference here, as most people will be only using 5% capacitors. Funny how the bass frequency tips up so very slightly, but that's the result me setting R5 just slightly high. I kinda slipped that in, figuring it will compensate for the rolloff generated by the output coupling cap C3.
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Old 1st April 2013, 04:37 AM   #2349
rjm is offline rjm  Japan
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Quote:
Originally Posted by Wirehead.be View Post
Up for test was the TI NE5532; the BB OPA2134 and the National LM4562NA.

Take a guess as to which one came out best in terms of noise floor/dynamic range.
An educated guess, based off the datasheets:

For an input impedance of 470 ohms, 2134 is 8.5 nV/sqrtHz, 5532 works out to 5.8 nV/sqrtHz, while the 4562 slips in at 3.9 nV/sqrtHz. For completeness, the OPA27 is at 4.2 and the LT1115 at 3.1 nV/sqrtHz.

As expected, at these low impedances the trusty 5532 is more than up for the task. The 2134 is a better choice to sit behind a 100k volume pot for example, its my go to model for headphone amps and preamps.

Douglas Self was basically right though: at the end of the day you can't go far wrong with the 5532-5534. Just a really nice, easy to implement balance of low noise, low distortion, good drive, and stability.
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Last edited by rjm; 1st April 2013 at 04:44 AM.
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Old 1st April 2013, 05:42 AM   #2350
vulejov is offline vulejov  Serbia
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Quote:
Originally Posted by rjm View Post
@vulejov,

My copy of Horowitz and Hill runs 1125 pages, but you'd need only the first 59, chapter 1 "Fundamentals" explains how impedance and passive filters work and how to calculate the response.

The RIAA is not defined by a circuit, but by three time constants. There are any number of ways it can be realized in practice but the values generated by calculation or software like Audacity are exact by definition.
On what basis we are confident that the simulations using the correct equation?

How do we know that the progam is using the correct inverse RIAA?

Except incorrect attempt adding two values ​​so far we have no evidence that the RIAA curve is correct..
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