I have built the design in post #158. Because of size constraints, I have recycled the PCB from a commercial Philips amplifier I had kept. The schematic resembled the design in post #158 well enough for that. I just had to piggy-back R1, R2, R10 & R11. Below is the schematic as I have implemented it, the original Philips design and a few photo's of my project. The issue is that measurements using my inverse RIAA circuit shows that the frequency response starts to rise after 10 kHz (on both channels, L + R). To rule out that this wasn't a measurement error, I've measured an old Pioneer A-110 amplifier that I have lying around. This does not show the rise in amplitude, so my thoughts are it must either have something to do with the design or the PCB layout. Since the issue is on both L & R channels, I am not thinking that it is a wiring issue (for now).
All Inverse RIAA plots have been normalized to 0 dB at 1 kHz.
The direct measurement (input to output) have not. There it can be seen that the amplification is 30 dB @ 1 kHz, which was the design criterion.
Any thoughts on this issue are welcomed.
Schematic as built
Original Philips design
PCB top side
PCB bottom side
Inverse RIAA plot normalized
Bode plot showing gain @ 1 kHz
Pioneer A-110 RIAA Amp circuit
Pioneer A-110 Inverse RIAA plot normalized
All Inverse RIAA plots have been normalized to 0 dB at 1 kHz.
The direct measurement (input to output) have not. There it can be seen that the amplification is 30 dB @ 1 kHz, which was the design criterion.
Any thoughts on this issue are welcomed.
Schematic as built
Original Philips design
PCB top side
PCB bottom side
Inverse RIAA plot normalized
Bode plot showing gain @ 1 kHz
Pioneer A-110 RIAA Amp circuit
Pioneer A-110 Inverse RIAA plot normalized
R4 is too small, 120 ohm instead of 300 ohm...332 ohm.
The problem with single-loop active RIAA correction amplifiers with a low gain is that because the gain drops to 1 instead of 0, you get some extra treble (the ultrasonic zero, in transfer function jargon). You can correct for that with a first-order low-pass after the amplifier. The +3 dB point lies at approximately 2122 Hz times the gain at 1 kHz, about 67 kHz in your case. It's higher in your measurement because with the too low R4, you still partly correct for it.
The problem with single-loop active RIAA correction amplifiers with a low gain is that because the gain drops to 1 instead of 0, you get some extra treble (the ultrasonic zero, in transfer function jargon). You can correct for that with a first-order low-pass after the amplifier. The +3 dB point lies at approximately 2122 Hz times the gain at 1 kHz, about 67 kHz in your case. It's higher in your measurement because with the too low R4, you still partly correct for it.
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Thanks for the response, Marcel! When we discussed R4 in a previous post, I had already soldered in a 120 Ohm resistor, and was too lazy to replace it (also because it would be a small pain to replace it). I'll make the change tomorrow! The HPF works nicely. I hope I can get away with it being single supply, as I read in an Analog Devices application note, that this set-up reduces the PSRR to 6 dB. If I pick up 50 or 100 Hz hum, I will convert both this amplifier and my turntable to a dual supply.
I've written a document about designing this type of phono preamplifier, see the attachment. Section 2 details the method I have used so far, which requires fine-tuning with a pole-zero extraction program (or an ordinary simulator or measurements). A more accurate method that shouldn't require fine-tuning is described in section 3. I'm not yet convinced that the more accurate method is practically usable, but for anyone who should want to try it, see this zip file which contains the document and a spreadsheet that does the most complicated calculations: https://www.diyaudio.com/community/attachments/sriaa-zip.1452225/
I'm not sure MM RIAA with a single 15/18V rail is a good idea. You ju..ust have about 10dBU before O/L so it's difficult to allow for higher output cartridges which might give 10mV at 5 cm/s.
IIRC, allowing for adequate O/L at 1kHz is OK for the rest of the frequency range. Anyone have a link to the SHURE chart for max recorded velocities and their other suggested limits for vinyl?
IIRC, allowing for adequate O/L at 1kHz is OK for the rest of the frequency range. Anyone have a link to the SHURE chart for max recorded velocities and their other suggested limits for vinyl?
See pages 21, 22, and 23.
https://content-files.shure.com/Kno...honograph-cartridge-technical-seminar-faq.pdf
https://content-files.shure.com/Kno...honograph-cartridge-technical-seminar-faq.pdf
The midband gain of the circuit of post #165 is about 30 dB, so assuming that the op-amp output can go up to 3 V from the rails, the overload margin is about the same as that of a 40 dB midband gain amplifier with a +/- 17 V or a single 34 V supply.
Thanks for this rayma. The SHURE chart shows a max peak velocity of ju ... ust below 70 cm/s which is 50 cm/s rms. The most sensitive cartridges are about 2mV per cm/s which would give 100mV. A gain of 30dB gives 3.162V rms which is 4.47Vp. Adding 3V gives 7.47V which is ju.uust under 15V single supply.
Hoevever the peak 70 cm/s is around 7kHz where RIAA EQ is some 10dB below 1kHz.. But da Golden Pinnae designs with passive EQ should beware.
So Marcel is right that a single 15V supply is ample (10dB margin) for '30dB gain'. I am suitably reprimanded. My excuse is designing RIAA preamps with 2 or 3 BJTs in da Jurassic times when THD would be excessive well before the preamp clipped.