AudioApe

modified 6 years ago

LM4562 Phono Test

146

01:38:45

RIAA Phono preamp based on LM4652.
The RIAA curve looks fine. The Bode diagramm shows approx. 12dB at 20Hz, approx. -7dB at 1kHz and approx. -27dB at 20kHz.

The output voltage is at approx. 870mV @1kHz peak-to-peak (standard consumer line level signal).

Based on typical application circuit from here: https://www.ti.com/lit/ds/symlink/lm4562.pdf

published 6 years ago

westelaudio

6 years ago

So it's basically a passive RIAA filter with two gain stages. Why not. I've built an active version I found online which I altered a bit

westelaudio

6 years ago

http://everycircuit.com/circuit/5685754962116608

westelaudio

6 years ago

The original design had a 330k resistor in place of the 390k. Gives you a bit more bass now (+1db), and I didn't have any 330k anyways. I also swapped the 1uF and 3.3 uF (which was initially supposed to be at the input) when I built it, which was a mistake, but it doesn't make a huge difference according to sim. Might aswell use 1uF for both.

westelaudio

6 years ago

Simple curcuit, but sound very good, I used 4558s by the way

westelaudio

6 years ago

You're right, it isn't line level, but remember the bass is also recorded at -20db/20hz on records, and the EQ is there to offset it. If the curve is correct and the output at 1kHz, you're fine.

westelaudio

6 years ago

Ok I see now, your input is only 500uV, if I set it to 3.5mV @1kHz, which would be the output of a typical MM cartridge, the output is definately too high.

westelaudio

6 years ago

I fixed it: http://everycircuit.com/circuit/6106524536274944

AudioApe

6 years ago

Thanks for the info, I'll check out your version, too! I use an MC cartridge, so when I checked only, I read that MC output is at approx. 500uV. Therefore this input. But thanks for your version, I thought about adding a switch to change between mm and mc anyway.

AudioApe

6 years ago

And while the RIAA curve stays the samein your fixed version, it's at almost -12dB @1kHz, is that because of the lower amplification from MM to line or am I misunderstanding something?

westelaudio

6 years ago

-12dBV to give a bit of headroom for more powerful cartridges, -10dBV is max for domestic line level.

westelaudio

6 years ago

MC cardridges are kind of a different beast, because they also need additional equalization to RIAA, not provided by the curcuit.

westelaudio

6 years ago

On my other curcuit, the active one, I used 15mV input so people can see the RIAA curve with 1kHz at 0dbV

westelaudio

6 years ago

But it also gets 3.5mV from my at3600 cartridge of course.

westelaudio

6 years ago

I checked and it seems that I was wrong about the EQ, but when I used MC setting on a preamp for MM, it sounded muffled. That was probably because it was loaded down by the 100 Ohm input impedance, and thus LPFed the signal.

westelaudio

6 years ago

Try this: http://everycircuit.com/circuit/6448414888558592

AudioApe

6 years ago

Updated Description

westelaudio

6 years ago

A switch would have to be multi pole, since you have to switch the resistive dividers at the feedback loop aswell as the input resistor and cap.

westelaudio

6 years ago

This seems alright.

westelaudio

6 years ago

Max line level @ 800uV should give you enough headroom for your 500V cartridge. Adding a resistive divider is fine. You still need to change the input resistor to 100 Ohm and the cap to something higher aswell (3nF is recommended by 2.2 should do, it's a more common value).

westelaudio

6 years ago

*500uV of course

westelaudio

6 years ago

Sometimes it's a bit confusing because EC uses peak values instead of RMS, and I don't know what the manufacturers list. But even if it's RMS, your MC would be 705uV peak, so you're still on the safe side with your new curcuit, not maxed out yet.

AudioApe

6 years ago

Thanks for the great info! I actually changed to 800uV because I checked the manual again and it said 800uV but I didn’t think of headroom, so I might acutally just bump it up to 1mV. I mean it can’t be that bad then, can it? And the Opamps are ideal here so there will be less amplification in real, or not?

AudioApe

6 years ago

Why should I change the input cap and resistor? Sorry, first time into trying to “design” something.

westelaudio

6 years ago

800uV RMS is about 1.13mV peaks. Those values are recommended for MC: https://vinylrestart.com/mm-vs-mc-phono-preamps-explained/

westelaudio

6 years ago

I don't know the exact reason though, but my guess is that the 100 Ohms are supposed to load down the cartrige so that it's internal inductance LPFs the signal a bit, might be too trebly otherwise.

westelaudio

6 years ago

Same with the cap too.

AudioApe

6 years ago

Sorry didn’t see your comments earlier. Great info! Thanks a lot!

add comment in editor