The CNC Phono Stage DIY


Yup, that’s the correct orientation. I didn’t design it upside down :wink:


I have no doubt about your design abilities! :slight_smile:

I just wanted to be sure I got the switches in right because desoldering to reverse them is a PITA.



I just said it jokingly, looking forward to seeing the end result.


Components in board today:

Are the resistors in the right spot? :slight_smile:



Looking good, are you getting ready to test it soon?


Not yet, I still have to make the case to mount the two 12 volt SLAs plus make a mechanical switch guard that prevents the circuitry from being turned on when the batteries are being charged.


Did anyone finish one of these yet? How does it sound?


Don’t know just yet how it sounds, but I’m almost done with the mechanical design. I just have to drill a hole for the LED and figure out how to put on a handle that connects to the bar over the batteries. But at least I’ve figured out how the mechanical interlock to keep power off the phono stage when the SLA batteries are being charged (the aluminum bar can’t slide to uncover the power connectors when the power switch is in the “on” position and likewise when the charger is plugged in to either port the phono stage can’t be powered on.)

Now all I need to do is wire it up. And that’s the easy part.


(above) Phono stage switch is ‘on’, interlock can’t move to the left to expose charging connector.


Phono stage power is ‘off,’ interlock can’t move to the right when battery charging connector is plugged in.


Oh that looks excellent! Thanks so much for the hi-rez photos so that others can look at a finished board.

Those PRP’s look so good! Let us know how your first listening session goes! happy-132_gif


I just ordered the PCB and was about to order the parts from Digikey, but I noticed an anomaly in the BOM. The board shows four caps of .1u, but the BOM shows four caps of 10000PF (which is .01u). I assume the board is correct and I should change the order before it goes in. Could someone please confirm.

Thank you for your help, I am looking forward to putting this together!



Hi Frederick,

Yes, you’re correct. The link in copper hasn’t been updated but the link that’s on the page where you purchased the board has been.

The link is



Thank you, Darren. One other question, what are the 110k resistors that are by the outputs on the board for? I do not see them on the schematic.

Thanks again.


Hi Frederick,

The original schematic that Jim posted is not the exact schematic of this PCB board. From my understanding, the schematic that was posted in copper was pulled from the LM4562 datasheet that the CNC is based on.

Since I didn’t design this preamp, I’ll have to guess what the designers intent here was since there are a couple of reasons why one might use a resistor in this location.

The two main explanations that are jumping out to me have to deal with cutoff frequency and leakage.

If we assume that the line preamp after the phono stage has a 100K input impedance, placing ~100K here increases the cutoff frequency to around 3Hz . This can help reduce rumble - very low frequencies that sometimes a tonearm that is “bouncing” around can produce (perhaps because of a warped record). When playing certain records, you may have seen this before when your woofers move in and out very slowly. This uses unwanted power from your amplifier and can compromise SQ.

It also can help with leakage from the capacitor. Capacitors are far from perfect. They have a leakage resistance that creates a voltage divider with the input resistance. Depending on this divider and the DC voltage before the cap, there can actually be DC after the cap. Lowering the input resistance decreases the DC that’s leaking. I’d expect the DC offset to be fairly low even before the cap, so leakage is not a large concern unless an op amp rails out.

Hope this helps4_gif


Thank you, that helps a lot. FYI, I noticed that the minimum quantity on the .1u cap added to the BOM is 2,000. I found a similar cap 445-4756-ND with no minimum quantity. It is the FK series rather than the FG series, but it appears that it should work for what is needed.

Thanks again.


Frederick, Darren,

The 110k resistor is mainly to make sure that the op amp outputs aren’t floating when nothing’s connected. They will create a high pass filter, that’s why the value is relatively high.

It does not mess with the output impedance, as it is in parallel with the output (and two resistors in parallel will always have a resistance below the lowest value resistor).

You will see more changes from the original CNC circuit, as this is my take on this textbook phono stage. The input impedance values are (imho) more useful, giving you 17k to 150k in fifteen steps. The variable gain is done in the second gain stage, after the RIAA equalization, to avoid clipping the higher frequencies.

This circuit also gives you selectable gain of 36-46 dB in four steps, and the RIAA equalizer circuit has been improved. It has lower resistance which means lower noise, it’s not as sensitive to changes in component values (even outside of normal component tolerances), and all components are standard E24 values so you won’t have to search for components that are hard to find.

Finally, even though this board is larger than the ones in my kits, it’s smaller than the CNC boards. It’s even got ground fill, and it will slide into a B0905 enclosure without having to do any drilling.

The design files for my power supply are available here:

It’s fairly easy to etch yourself, as there’s very little copper that needs removing. Alternatively, order it from the fab house of your choice.


Thank you very much, skrodahl. I had been thinking of using batteries, but you’ve piqued my interest with your ps. I will give it a try on a stripboard or protoboard (this one looks interesting - )


Cool tip about the protoboard :slight_smile:


After months of vacations, family matters, and other get-in-the-way-of-life annoyances, I finally wired the board to my chassis and gave it a whirl on my 30 year-old turntable and cartridge resurrected from the closet.

And it worked! Just fine. My albums are buried in another closet, so I was stuck testing with the one 45 I had laying around (Olivia Newton-John “Physical”) It sounded fine.

I still have to make the handle for the chassis, as the batteries are heavy, And the #1 pin on the Burr-Brown OPA2134 wasn’t in the center, so I had to guess on how to put that in, but seeing as how I had a 50-50 chance of getting it right, I guessed right the first time.

Here are pics. Turntable is a Denon DP-52F that hadn’t been used in 30 years, coupled with a Grace F-8 cartridge of same vintage. Worked perfectly (who’d a thunk?)

Turning the unit on and off did overload the BHK 250 and got it into rapid-blink mode from the transient spike, but all is well after a reset. Just have to remember to turn volume down when powering the preamp on and off.

Now about those old records . . . .


Edit: Oh, and I did change the output RCA jacks from the grounded versions to that Darren had recommended to the insulated ones as my chassis would have grounded everything together in multiple points (you don’t want that to as to prevent ground loops) so I got some insulated jacks from Mouser. And that took a few weeks, too.


A little off-topic, but I have stumbled here and DIY Audio is my budget.

Along with my PS IV serial # 9941 waiting to be recapped, this LM194 RIAA Ultra Low Noise Preamp has been a long-delayed project of mine that I wonder if those with better hands on experience would comment on pertaining to it’s possible performance and sonic capabilities for it’s age, alongside the CNC, it’s derivatives, or the PS IV.

Many thanks in advance




Question for skrodahl: I want to digitize my Dad’s old CD-4 record collection so that I can play the CD4 records without any further wear on the discs. I’d use the PSAudio NuWave Phono Converter (NPC) to digitize the CD-4 sum channel (front audio level) + the 18-45 kHz carrier (rear) into 2 digital channels at 176 kHz, then for playback send that into a DAC who’s analog output goes into the ‘phono’ input of the CD-4 decoder.

If I take the output of the cartridge and connect it directly to the analog input of the NPC, then there’d be an impedance mismatch between the cartridge and the NPC. So I need a phono stage with input impedance matching.

Here’s the rub: When playing through a normal phono stage like the CNC before the carrier + sum is sent to the NPC, the RIAA circuit in the CNC will convert the sum channel bass to higher levels. I don’t want it to do that because during playback through the CD-4 decoder it will have another RIAA correction. So when I record the record into digital I need a phono stage that has no RIAA circuit.

If I wanted to use your CNC circuit, would I take out the 47 nf, 68 nf, 130 ohm and 2.2 k ohm resistors to eliminate the RIAA? Should the 16 k ohm resistor on the output of the first op amp be changed? If so, what would be a good value?