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Old 2nd January 2012, 12:26 PM   #1
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Talking RIAA + IEC

Hi all guys. Im designing my own RIAA equalizer using LF347 op amps. I use as guide, the "On RIAA Equalization Networks" by STANLEY P. LIPSHITZ. In this stuff, the author makes reference to an amendment 4 to place a zero at 7950 microseconds (20Hz aprox.) to attenuate the infrasonic noise. In the text, he makes the zero placing a single capacitor at the signal path. I want to make that zero with a Butterworth hi pass two cells filter (double zero), because I have an unused op amp in the package (Total of 4). What do you think about this? Any ideas are well received.

Very thanks. Best regards from Banfield, Buenos Aires, Argentina.
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Old 2nd January 2012, 02:15 PM   #2
AndrewT is offline AndrewT  Scotland
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I don't like the sound that the extra bass roll off gives.
But I do see it has advantages.
One of those advantages is attenuation of LF information.
Changing the single pole to two pole will increase the attenuation of the LF information.
There is nothing wrong with that.

I would suggest you experiment with the F & Q of the 2 pole till you achieve the LF response that suits you.
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Old 2nd January 2012, 03:18 PM   #3
jez is offline jez  United Kingdom
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I agree with Andrew T.... I don't like the sound of the extra roll off.
I would not use the LM347 either when there are plenty of much better ones around. Try LM4562 or it's many variants
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Old 2nd January 2012, 03:43 PM   #4
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OK, very thanks for your repply. Happens that I have several LF347 in a blister. I look at the ST page that they are low noise, JFET input 4MHZ power bandwidth and gain of 200K.

Thanks.
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Old 2nd January 2012, 05:38 PM   #5
jez is offline jez  United Kingdom
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Quote:
Originally Posted by Osvaldo de Banfield View Post
OK, very thanks for your repply. Happens that I have several LF347 in a blister. I look at the ST page that they are low noise, JFET input 4MHZ power bandwidth and gain of 200K.

Thanks.
There are much worse op-amps than the LF347 out there but by today's standards it is nothing special at all. If you were willing to go to such lengths over the IEC filter then why spoil it by using 2nd rate op-amps?
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Old 2nd January 2012, 06:11 PM   #6
system7 is offline system7  United Kingdom
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Lipshitz is going back a bit, isn't he? I've built a few RIAA MM phono preamps over the years, and always enjoyed them, using TL072 and NE5532. The 5532 can drive 600 ohms, the TL072 and LF347 drive 2k minimum, which means the feedback and load circuit doesn't dip below this ideally.

This is the Rotel design in my RA-931, with usual 50, 500 and 2200 Hz corners:

Click the image to open in full size.

It actually incorporates a 20 Hz filter too (C403/R405) in the cause of DC stability, but loads the amp with (R413) 680 ohm, so not suitable for your opamps without modification.

If you do want to use two opamps per channel, since the LF347 is a quad package, I'd go for a dual circuit like this:
Hi-Fi RIAA Phono Preamp

Should work OK! Interesting article on cartridge loading, FWIW:
Turntable Forum • View topic - Cartridge loading explained
Capacitance at phono input is actually bad, and should be minimised! 47k is not necessarily best either. Depends on inductance of cartridge.
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Old 2nd January 2012, 06:22 PM   #7
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OK, JEZ. Take into account that this preamplifier will work with a stereo 15W/channel into a 8" Karlson speaker in a small house, so I dont need for the best quality. More, this is the first time I will do such a project of my own. And, the ICs will have sockets, so if the "pre" is to much noisier or dont like the sound, I can change the opamp easily.

The stuff from Lipshitz appears to be well mathematically based, and I true him. Like he explains, all noninverting stages had a 0db rolloff I would want to overcome. The circuit Im doing, also has an automatic magnetic/piezo pickup switcher. And works properly!!!

Here is a photo of a turntable like mine, but this isnt my turntable.
Attached Images
File Type: jpg Winco 01.jpg (27.0 KB, 92 views)

Last edited by Osvaldo de Banfield; 2nd January 2012 at 06:28 PM. Reason: make it best
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Old 3rd January 2012, 12:14 AM   #8
system7 is offline system7  United Kingdom
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It's a fine idea to build the moving magnet phono stage into the turntable, Osvaldo. I managed it with a (Thorens TD160) turntable like yours with a simple toroid power supply, 15V voltage regulators and a power switch and LED indicator. I built a balanced input amp using NE5532 opamps on veroboard IIRC. Quite sophisticated really, and I do know a bit about common-mode, but don't fret too much about the actual opamps.

The benefit was I could mount the amp at the base of the tonearm, and get rid of the pesky 0.5m of 100 ohm phono cable with it's 200 pF capacitance due to the mismatch with a 47K load. I recall Stanton and Grado made the best low inductance MM cartridges which gave you the sweet top end.

For all that, moving coil and high mass tonearm is where it's at with vinyl, it simply works better, mainly because everything is matched around 100 ohms. If anyone doubts that MM needs some help, just wire up some 1K resistors on the hot outputs of a MM cartridge, and listen for a dramatic improvement in sweetness in the top end over an admittedly increased noise floor.
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Old 3rd January 2012, 01:14 PM   #9
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>It's a fine idea to build the moving magnet phono stage into the turntable, Osvaldo. I >managed it with a (Thorens TD160) turntable like yours with a simple toroid power >supply, 15V voltage regulators and a power switch and LED indicator. I built a balanced >input amp using NE5532 opamps on veroboard IIRC. Quite sophisticated really, and I do >know a bit about common-mode, but don't fret too much about the actual opamps.

Yes, because you find a solution for two problems: one is that of the cable capacitance, noise pickup, etc., and the other is to make it compatible with audio amplifiers without internal equalization, I out with 1V@47K normalized audio in 2 RCA phono jacks red and white. And the internal automatic switching avoids overload and prevents misequalization between tho types of pickups.


>The benefit was I could mount the amp at the base of the tonearm, and get rid of the >pesky 0.5m of 100 ohm phono cable with it's 200 pF capacitance due to the mismatch >with a 47K load. I recall Stanton and Grado made the best low inductance MM >cartridges which gave you the sweet top end.

>For all that, moving coil and high mass tonearm is where it's at with vinyl, it simply >works better, mainly because everything is matched around 100 ohms. If anyone doubts >that MM needs some help, just wire up some 1K resistors on the hot outputs of a MM >cartridge, and listen for a dramatic improvement in sweetness in the top end over an >admittedly increased noise floor.

OK. Best regards, and happy new year for you and all other forists.
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