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Upgrade of RIAA stage in a commercial amp
Upgrade of RIAA stage in a commercial amp
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Old 5th June 2019, 11:59 PM   #11
Conrad Hoffman is offline Conrad Hoffman  United States
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Location: Canandaigua, NY USA
First thing I'd do is fill in the cap values and sim the circuit to see how good the RIAA curve is. Use an inverse circuit to feed it. If the curve is off, fixing that will probably accomplish more than opamp changes. What pickup are you using?
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Old 6th June 2019, 12:00 AM   #12
LJT is offline LJT  Norway
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Cross-posted with stocktrader-200. His post address some of my questions.

As for comparison with digital source: Vinyl is giving a much more lifelike presentation in terms of "space". This compares not only to "CD" and "Master" quality sources from Tidal but also to CD from my Raysonic CD128.

The pre-amps DAC is better than that of my Bluesound Node-2 streamer, but still not as good as Vinyl.

The Phone input has an anomaly in that it gives a muted "thump" about 2 seconds after needle has been lifted of the record.
Other than this I have no grevings with the sound quality of the Phono Input as it is, other than that I Would like to make it better if I can.

I have compared by using an old 80's Denon pre as RIAA only (Connected tape out of the Denon to line in on my pre). That is an map using 8 2SK389 in an LTP on the input. The Denon RIAA gave a much better reproduction, but unfortunately it did not have the same resistance to scratches, making blemishes that went un-noticed on other amps clearly audio-able and anoying.

Adjusting the resistive network around the input stage may be a good place to start.
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Old 6th June 2019, 12:11 AM   #13
LJT is offline LJT  Norway
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Quote:
Originally Posted by Conrad Hoffman View Post
First thing I'd do is fill in the cap values and sim the circuit to see how good the RIAA curve is. Use an inverse circuit to feed it. If the curve is off, fixing that will probably accomplish more than opamp changes. What pickup are you using?
I have only reverse engineered the circuit layout, not taken out and measured the SMD capasitors. However, resistor values in the feedback circuit are the same as in the LME49710 datasheet (attached here). inverting input to ground resistor is 1200 instead of 390, so slightly higher gain even with the pre-amplification of stage-1. I assume this is to level mach the phono input with the internal DAC of my unit.
Attached Images
File Type: png LME497x0 RIAA.PNG (24.9 KB, 92 views)

Last edited by LJT; 6th June 2019 at 12:23 AM.
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Old 6th June 2019, 12:38 AM   #14
stocktrader200 is offline stocktrader200  Canada
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LJT, it sounds like your current preamp is performing very well seeing as you prefer the sound to your Tidal and CD sources played from quality equipment. If you want to lower the noise a bit you can replace the 1st op amp feedback network parts. I would use 2.2k for 10k, 1k for 4.7k , 68 for 390 and Cfb 5x larger. The input resistor 47k and 100p capacitor remain unchanged. Metal film 1% resistors should be used.
This does change the differential loading of the op amp which could be noticeable.

Last edited by stocktrader200; 6th June 2019 at 12:46 AM.
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Old 6th June 2019, 02:52 AM   #15
Conrad Hoffman is offline Conrad Hoffman  United States
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I don't think one can change that 390 ohm resistor without changing other things. Hopefully they did that, but a bench test with an inverse network would be a good place to start. It's hard to know what's being "fixed" without measurement.
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Old 6th June 2019, 03:59 AM   #16
MarcelvdG is offline MarcelvdG  Netherlands
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Originally Posted by LJT View Post
I do not fully grasp the calculations as shown by MarcelvdG above, but it seems I should assume about 12K source impedance for the MM calculations?
Yes.

In 2003, I wrote an Electronics World article that shows that minimizing the RIAA- and A-weighted noise integrated over the band is roughly equivalent to minimizing the spot noise at 3852 Hz. A typical moving-magnet cartridge with an inductance of about 0.5 H has an impedance of about 12 kohm at 3852 Hz.

I later extended it with ITU-R 468 weighting, see Linear Audio volume 8. The magic frequency then becomes 5179 Hz, if I remember well, so the impedance gets even higher. Then again, many cartridges are a bit lower in inductance than 0.5 H and noise optima are rather broad anyway, so I always kept 12 kohm as a rule of thumb for moving magnet cartridge impedance.
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Old 6th June 2019, 04:12 AM   #17
dotneck335 is offline dotneck335  United States
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It would seem to me that signal-to-noise ratio for any modern op amp-driven RIAA amplification circuit VERY much exceeds the ~ 70 db S/N of the vinyl record source; therefore OTHER sound characteristics (THD, slew rate, IMD, etc.) would make much more difference.

Last edited by dotneck335; 6th June 2019 at 04:15 AM.
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Old 6th June 2019, 04:32 AM   #18
MarcelvdG is offline MarcelvdG  Netherlands
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Not necessarily, by neglecting the current noise term altogether, the people at Elektor have managed to make a moving-magnet amplifier that is so far from the noise optimum that its noise is of the same order as record surface noise, so you lose several dB of signal-to-noise ratio. That is, they have used lots of expensive ultra low noise op-amps to make the highest noise moving-magnet amplifier in history:

Supra 2.0 - High End Preamp for Record Player [150616-I] - Elektor LABS | Elektor Magazine

Regarding THD and IMD, you really have to do your best to make those worse than the distortion of a record, which to the best of my knowledge is usually of the order of a percent. Maybe that will be Elektor's next project, to use ultra low distortion op-amps and still somehow end up with a higher distortion than a record or a cartridge.

Last edited by MarcelvdG; 6th June 2019 at 04:34 AM.
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Old 6th June 2019, 05:27 AM   #19
dotneck335 is offline dotneck335  United States
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Quote:
Originally Posted by MarcelvdG View Post
Regarding THD and IMD, you really have to do your best to make those worse than the distortion of a record, which to the best of my knowledge is usually of the order of a percent. Maybe that will be Elektor's next project, to use ultra low distortion op-amps and still somehow end up with a higher distortion than a record or a cartridge.
Methinks that all this points to the fact that we STILL have not achieved measurements that clearly reflect the true SOUND of audio gear. Why does a Fender tube guitar amplifier sound so GOOD, with all its high THD? Why do some amps sound so different when they are both equally rated at much less than 0.1% THD(the supposed threshold of audibility)? Why do vinyl records and magnetic tape sound better than CDs?
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