OK to use .wav files with RIAA EQ reversed to test phonostage? - diyAudio
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Old 12th November 2006, 12:42 AM   #1
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Default OK to use .wav files with RIAA EQ reversed to test phonostage?

I have a phonostage that I built a while ago that has electrical problems. I'd like to work on the thing but my TT is currently out of service (waiting on arm wires).

For diagnostic purposes:

Using Sound Forge, I'd like to create a CD whose audio clips have had an inverse RIAA EQ applied, and the gain dropped quite a bit (basically as it would be coming off a record before it hits the phonostage). I'd then run the CD audio through the phonostage.

Again, this is only for basic electrical diagnostic purposes (trying to pinpoint a source of oscillation in the phonostage).

Am I missing anything that would make this a bad idea?
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Old 12th November 2006, 09:24 AM   #2
Calvin is offline Calvin  Germany
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Hi,

maybe the source impedance of the cartridge =output impedance of the soundcard?

jauuu
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Old 12th November 2006, 12:30 PM   #3
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Yes, it can be done, in fact there is a commercially-produced CD for use with stancard 2V-output CD players for 'breaking in' phono stages.

I made my own 'inverse-RIAA' CD to break in a new Gram Amp 2 phono a few months ago.

I then used an AIWA CD-walkman which outputs 500mV (0.5v) on it's headphone output at maximum volume (according to it's specs), so I was able to attenuate the signal using the volume control, in addition to having applied 20-odd dB attenuation to the tracks on the CD itself. It works very well.

Advantage with this (using a personal player) is that you use more of CD's 16-bit resolution - that commercial disc has to be using no more than 2 or 3 bits to produce a 5-odd mV signal from a 2V CD player, I doubt the resulting sound quality would be very useful in judging if a phono is working properly or not.
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Old 12th November 2006, 01:30 PM   #4
ebrjvd is offline ebrjvd  Belgium
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Default RIAA EQ reversed to test phonostage

You can use Diamond Cut DC6 which has built-in RIAA and reverse RIAA equalisation.

Also with CoolEdit or Audition you can make an inverse RIAA function:
<http://www.a-reny.com/restauration/avance.html#Riaa>

Jos
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Old 12th November 2006, 01:30 PM   #5
ebrjvd is offline ebrjvd  Belgium
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Default RIAA EQ reversed to test phonostage

Another solution is to build a simple anti-RIAA filter:
http://www.frihu.com/grundlagen/filter/5b.html
You put it in front of the RIAA preamp instead of the cartridge.

Jos
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Old 14th November 2006, 04:10 AM   #6
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Here's another filter you can use (schematic in manual):

www.hagtech.com/pdf/iriaa.pdf

And the paper that talks about it:

www.hagtech.com/pdf/riaa.pdf

jh
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Old 18th November 2006, 11:24 PM   #7
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The CD based reverse RIAA is superior to the analog reverse RIAA.

The way I do it is to record the CD with the max level of the reverse RIAA within 1dB of "O" and then use an attenuator to feed the proper level into the phono stage. This way one gets the max number of bits and max S/N to work with. You can even split the file at 1kHz. if you like and reset that segment back up to "0" so you can check the upper and lower halves of the curve separately - if you like.

Btw, that site on the Cool Edit tricks is really nice, thanks!

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Old 19th November 2006, 01:29 AM   #8
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Quote:
The CD based reverse RIAA is superior to the analog reverse RIAA
Say what? To implement the inverse curve you have to cut bass by 20dB and boost treble by 20dB. Aproximately. That's a 40dB tilt from one end to the other. Or 100:1. Or about 6.6 bits.

So when you start with 16 bit resolution (CD quality), you have to drop the bass by 40dB, midrange by 20dB, and leave treble as is. That means you've scaled the bass down by 6.6 bits, leaving you with the equivalent of 9.4 bits of resolution in the bass.

Now tell me how that is supposed to be better than analog?

jh
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Old 20th November 2006, 05:13 PM   #9
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Oh dear...

It's absolutely more accurate in the terms of amplitude.
You don't need any more "bits" since ur producing a simple sinewave.
DACs do a very excellent job of producing a sinewave.

The "inverse RIAA" filter is no better than your implementation, which means that it can have errors as great or greater than the RIAA you are supposed to be testing. One has to watch the input impedance and the output impedance - which in practice means that it really ought to be buffered on the input and output side and then tested to make sure it "tracks" the curve properly... which I suppose is an ok way to do things, but one heck of a lot more work than simply burning a CDR and driving the input end of the RIAA preamp through an attenuator or similar means.

I also mentioned that you can always split the two halves of the RIAA curve at the 1kHz inflection point and then ur "bitloss" is minimized, if one is concerned about this... Yes?


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