The CNC Phono Stage DIY

Do It Yourself
41

Hi Streets Still Works,

It sounds to me that you are looking for a high quality gain stage. One without the RIAA equalisation, that gives you the desired input impedance and the gain.

The Muffsy MC Head Amp is such a candidate, please contact me on the Muffsy web page for more info on the modifications needed

EDIT: The CNC without the RIAA filter will give you way to much gain. It can be used with modifications though. You need to remove everything between the two op amps, including the 16k resistor, then connect the output on the first op amp directly to the input of the second one. Change the 100 ohms resistors in the first gain stage with 360 ohms, and you’ll get the right amplification.

42

Right, it’s more of a high quality gain stage with impedance matching.

Good catch on the gain on the first stage, thanks very much. I wouldn’t have thought of that.

Also, the output of the CNC stage has that 110k ohm resistor that you don’t show here on the Muffsy circuit. Would that be a help or hindrance to the 30 to 50 kHz rear carrier channels? Or wouldn’t it even matter?

Thanks.

–SSW

43

This circuit (if you disregard the RIAA filter) doesn’t cap the high frequencies in any way.

The resistor was discussed earlier in this thread, it’s just a precaution to make sure the op amps behave right if there’s nothing connected to the output. Feel free to leave it out.

44

Looks like I’ll have to get another CNC board, then, and leave out the RIAA parts and short out the 16 k ohm. I still have two spare op amps from my first CNC build. Is Darren Myers still around?

–SSW

45

Hi SSW,

I’m still here!

I had a question about your set up - whats the gain associated with the RIAA correction in the CD-4?

Is there anything else I can help you with?

If you are interested in another board, you can purchase one here: http://www.psaudio.com/products/diy-cnc-phono-board/

-Darren

46
Darren Myers said I had a question about your set up - whats the gain associated with the RIAA correction in the CD-4?
Not a clue. But remember the "CD" in CD4 stands for "Compatible Discrete" which means that any 2-channel system at the time could play a CD4 record (assuming their needle was able to track the 44 kHz groove) and get the same audio played through their system as the stereo version of the record because the 2 low frequency channels were recorded with LF +LR & RF +RR so they'd get a full playback. It's only when the rear high frequency carrier signals (LF-LR, RF -RR) were added/subtracted properly would one then get individual LF, RF, LR, & RR channels.

But to answer your question I’m guessing that it’s the exact same as any standard RIAA gain because it had to play back on a ‘standard’ stereo system.

–SSW

47
skrodahl said

EDIT: The CNC without the RIAA filter will give you way to much gain. It can be used with modifications though. You need to remove everything between the two op amps, including the 16k resistor, then connect the output on the first op amp directly to the input of the second one. Change the 100 ohms resistors in the first gain stage with 360 ohms, and you’ll get the right amplification.


Skrodahl, should I get some higher resistance values to put there just in case the gain is too high with the 360 ohm? I don’t know a lot about analog circuits, but it looks like that resistor sets the gain of that stage by bleeding off a certain amount of the feedback signal. So the higher the resistor, the more feedback is applied to the subtracting input of the op amp. I was thinking of getting a 400 ohm & 500 ohm resistor to put there in case the gain is too high for the input to the NPC. Of course, there’s the gain setting resistors on the second op amp, but seeing as how the resistors are 10 cents apiece I might as well include them in the order and have them as backup.

Thanks!

–SSW

48

I must apologise for being too quick. The resistor value should be 1k3 (1300) ohms.

Using this value lowers the gain by the same amount as the RIAA filter would. This means that the gain switch settings will be the same as before, at 1 kHz since there’s no equalisation applied. Use a lower resistor value for more gain, or a higher resistor value for less gain.

Here’s the gain calculation for the first gain stage, before the change: http://mustcalculate.com/electronics/noninvertingopamp.php?vin=0.005&r1=100&r2=3300

The attenuation of the RIAA filter (at 1 kHz) is ~20 dB, so you’d need about 20 dB less gain. 1k3 ohms gives you exactly that: http://mustcalculate.com/electronics/noninvertingopamp.php?vin=0.005&r1=1300&r2=3300

My first try was a brainf@rt, I aimed for 20 dB amplification instead of 20 dB lower than before.

Any resistor between 1k and two 2k ohms will get you more or less where you’d want to be, assuming that the cartridge is an MM with 3-5 mV output.

It might be the case that your software is applying the equalisation passively, which would mean that you’d want to keep the original total gain in the circuit. I’d advise against it as it most likely would make your CNC clip the higher frequencies. Let your software apply the equalisation and then amplify (or I guess it would be called normalise) the signal in the software afterwards.

49

Of course this comes an hour after I submitted the order to DigiKey for the 360 ohm parts . . .

Sigh. OK, I’ll figure something out. Do these resistors have to be 1% or is 5% from the local store good enough?

Oh, and even if I do sound like I’m complaining, I’m really not. Many thanks for all your help. I couldn’t have managed without it.

–SSW

50

Sorry man,

Always use 1% tolerance resistors, unless you need even better or if you need power resistors. Metal film is less noisy, and resistors don’t cost much anyway.

Besides, you don’t want the possibility of having 10% difference (if one is +5% and the other is -5%) in resistor values for the gain in each channel.