Building the AudioKarma CNC Phono Preamp





Introduction: Building the AudioKarma CNC Phono Preamp

Visit the Muffsy Home Page for even more resources.

Welcome to what appears to be the first complete phono preamplifier at Instructables.

At the end of these instructions, you will have built a fully functional and highly exceptional phono stage.

This project is very suitable for inexperienced builders, as it uses only through-hole components for easy soldering.

True Hifi

With its no-nonsense, straight forward design that focuses on both performance and cost, this preamp caters to audiophiles and music lovers alike.

For once, there is no magic involved in a true audiophile piece of equipment. You get very precise RIAA compliance (within +/- 0.1 dB), ultra-low noise, a wonderful great sound stage and superior detail. All done with with a 90x51 mm PCB and standard, off the shelf, components.

This phono stage is suitable for:

  • Moving Magnet (MM)
  • High Output Moving Coil (HOMC)
  • ...and even Low Output Moving Coil (LOMC) with a step-up transformer

PCB Availability

This phono stage is available in two versions:

  • PCB - Covered in these instructions
  • Kit - The new version of the phono stage, Muffsy PP-3

Instructions and information for the new kit is available here.

A PDF with the single-sided version of this PCB is available for download at the end of this step, so you can etch your own.

You can even get a very high quality regulated power supply for this preamp. It is safely powered by a wallwart, and has only 0.1 mV ripple.

Where it all Started

The Muffsy Phono Preamp is an improved version the great AudioKarma CNC Phono Stage, which has gained well deserved fame for both its simplicity and great audio quality.

There is a humongous thread over at where you can read all about it: AudioKarma CNC Phono Stage.

Hackaday Project Page

For more information on this project, have a look at my project page.

Step 1: Bill of Materials

Yes, all those components are going to be mounted on that small board. Here's the full bill of materials:

  • 26 resistors, 0.25W metal film 1%, all from the E24 series:
    • 2x 100 ohm
    • 2x 130 ohm
    • 2x 150 ohm
    • 2x 300 ohm
    • 4x 2,200 ohm (2k2)
    • 4x 3,300 ohm (3k3)
    • 2x 16,000 ohm (16k)
    • 2x 33,000 ohm (33k)
    • 2x 82,000 ohm (82k)
    • 2x 110,000 ohm (110k)
    • 2x 150,000 ohm (150k)
  • 8 polyester film capacitors, 5mm leg spacing, 63V or higher:
    • 2x 47nF
    • 4x 68nF
    • 2x 1uF (5 or 10mm leg spacing, alternatively 2x 2.2uF)
  • 4 ceramic disc capacitors, 5mm leg spacing, 50V or higher:
    • 4x 100nF (0.1uF)
  • 4 electrolytic capacitors, 25V or higher:
    • 4x 10uF (alternatively 4x 22uF)
  • 2 operational amplifiers, almost any dual opamp will do, these are the recommended ones:
    • 2x OPA2134 (alternatively 2x LME79720, or 2x NE5532)
  • 2 DIL-sockets:
    • 2x 8-pin DIL-socket
  • 4 DIP-switches:
    • 2x 2-way DIP-switch
    • 2x 4-way DIP-switch
  • Other/Optional components (this will become clear as we go along):
    • Dual power supply +/-9V to +/-15V
      • The preamp can also run on batteries
    • Enclosure
    • Terminal blocks, 5mm leg spacing
    • Cabling, two-lead shielded is preferred
    • Connectors, RCA and others
    • LED
    • Power button

Selection of Components:

It's possible to spend hilarious amounts of hard-earned cash on components, and there may be many reasons for why you'd like to do that. It's important to note that you don't have to take up a second mortgage build this project. A reasonable amount to spend, using ordinary standard components, is somewhere between US$ 20-30. In doing so, you will have built a very sweet sounding preamp indeed.

I suggest that you avoid the absolutely cheapest online sources, as you may well end up with counterfeits. They will do you no good at all, and you will have a very poorly performing phono stage at best.

Step 2: Solder the Components

Let's build this thing!

To help you along, there are pictures above to show you the placement of each family of components.

Just one small thing before we move on...

How to Solder

If you're just starting out soldering, or if you need an update on how to solder, SparkFun has a great tutorial for soldering through-hole components.

No more hesitation, it's time to do some actual construction. Follow the steps below, and your phono preamp is bound to turn out perfect!

Soldering the Resistors

When starting on a project, it is customary to solder the components closest to the board first. If the PCB is filled with bulky components, it can be quite fiddly to get the smaller ones in later on. In this case, it's all those resistors.

All twenty-six of them are shown in the picture.

  • Measure the resistor to make sure it's the correct value, replacing it later is quite a hassle.
  • Bend the legs (preferably with needle-nose pliers) so the resistors will fit in the holes.
  • Stick a few of them in place, turn the PCB around and bend the legs slightly so they hold in place.
  • Solder.
  • Cut the legs with flush cut pliers.
  • Repeat until all resistors are soldered in place.

Do a visual inspection to confirm that all solders are good, and that there are no excess solder outside of the soldering pads.

Soldering the Switches and Sockets

Next up are the four switches and the two sockets. It is advised to do them one at a time, use some tape to hold them temporarily in place. You want them to be perfectly flush with the board.

Note: These components have a specific orientation, make sure you mount them the right way.

There shouldn't be a need to cut the legs of these components after they have been soldered in place.

Again, make sure that there is no solder where there should be none.

Step 3: Soldering - Continued

Soldering the Ceramic and Polyester Capacitors

The picture shows the placement of all twelve smaller capacitors.

Start out with the four ceramic capacitors, followed by the 47nF and 68nF capacitors in the RIAA filter. Finally solder the 1uF capacitors in place. These capacitors can be oriented whichever way you desire.

Note: The 1uF output capacitors can have 10mm and 5mm leg spacing. One of the pins MUST be in the hole closest to the center of the board. The pins have to be across the "-||-" symbol marked with "1uF".

When done, make sure all solders are good.

Soldering the Electrolytic Capacitors

The four 10uF electrolytic capacitors are the last components to go on the PCB. They stick out the furthest, so it's only logical to leave them for last.

Note: The orientation of electrolytic capacitors does matter. Mount them the wrong way, and you'll let all the magic smoke out of them. Electronic components are no good without magic smoke.

You'll find that the electrolytic capacitors has a white band on one side. This white band has a few minus signs on them, like so: "- - - -". This is the negative side. The other side, without the band, is the positive, or +, side. The positive side usually has a longer leg than the negative side.

Stick the legs through the board, flip the board over and bend the legs to hold the capacitors in place, and solder. Snip of the legs and you're done soldering components!

Step 4: Assembly: Enclosure and Connectors

This is where you can get creative. I have found an enclosure that suits my needs, but you may want to do it differently. Because there are so many possibilities, this won't be a part of the Instructable.

To see how I put together my enclosure and back panel, check out my project pages:

Step 5: Assembly: Powering the Phono Stage

From here, you can choose if you want to power the preamp with a power supply, or with batteries.

Alternative 1: Use a Power Supply

To power the phono stage with my power supply, you'll need:

  • The power supply itself (of course)
  • A wall wart that gives from 15 to 18 volts AC (preferably one with a transformer)
  • A power switch
  • A power connector, 2.1 mm, female

The first picture shows how to connect them all together.

Power comes from the AC wall wart into the power connector. One cable goes directly to the PSU, the other through the power switch and into the PSU. Which one doesn't matter, they are both the same.

There are three cables going from the PSU, marked +VE, GND and -VE. They go into the corresponding connections on the phone stage. This is the +/-15VDC that powers the phono stage.

You should twist the two cables coming from the power connector and braid the three cables between the PSU and the phono stage. This is done to avoid interference.

Make sure that the power switch and the power connector is completely isolated from the chassis/enclosure, as both cables are live.

LED - Power On Light

If you want to add an LED, connect it between -VE and +VE. Connect a 1k5-4k7 ohm resistor, rated for 1/2W in series with the LED. Lower resistor value gives brighter light. (If the resistor is 3k9 ohm or higher, 1/4W can be used.)

Suitable AC to AC Wall Adapters

120V Input:

230V Input:

Alternative 2: Battery Power

You can even power the phono stage with two 9 volt batteries, and it will run on them for weeks. Here's how, referring to the second and third picture above:

  • GND: Connect the positive side of one battery to the negative side on the other battery.
  • +VE: The remaining positive side is your +VE.
  • -VE:The remaining negative side is your -VE.

A suitable power switch would be a DPST, since you need to break both +VE and -VE. The pictures show how to connect your batteries with or without a power switch.

LED for batteries will also have to be connected between +VE and -VE. The resistor in series with the LED can be from 820 to 2k2 ohm rated at 1/2W. Lower value gives more brightness. Just keep in mind that an LED at max brightness uses the same amount of power as the whole preamp. I would recommend using 2k2 ohm and sacrifice some brightness.

Step 6: Assembly: Audio Wiring

It's time to wrap things up, this is the last step before the preamp is done. Now we need to input the sound from the turntable and output the amplified and equalized signal.

These are the parts involved:

  • The phono stage
  • Shielded audio cable
  • Back panel with:
    • Two female RCA panel connectors for input
    • Ground screw for input
    • Two female RCA panel connectors for output

A Word on the Ground Screw

Most turntables come with a ground screw. This is to ensure that the ground levels of the turntable and the preamp are equal. This removes any problems with hum. Some audio interconnect cables come with an attached ground cable. If your's don't, you need to run a separate ground cable alongside it.

Back Panel - Grounding

The first picture shows how to connect all grounds together. This is done to make sure the input, output and ground screw are at the same ground potential. Not doing this will invariably introduce a lot of hum.

Back Panel - Audio In/Out

The second picture shows how to connect the audio cable (only the input is shown here). Connect the red cable to the right input RCA connector and the black (which can be white in some cables) to the left input RCA connector.

Connect the shield of your audio cable to the ground of one of the RCA connectors. We just connected them all to each other in the previous step.

Now do the same for the output RCA connectors, and the back panel cabling is completed.

Hooking up the Phono Stage

Now that the back panel is sorted, all we have to do is connect the audio cables from the back panel to the phono stage. How they connect together is shown in the third picture.

You'll want the input side facing the back panel. Having shorter input cables is good, because it lowers the cable capacitance and the chance of picking up interference.

All Done!

This is the moment we've been waiting for. The phono stage is complete, and you get to enjoy your records. Congrats!

Don't forget, there is much more information available on my project pages.

Continue to the next step to learn how the phono stage works. This is also where you'll find instructions on how to set the DIP-switches to suit your needs.

Step 7: The CNC Phono Stage Explained

The CNC Phono Stage is an exemplary phono preamp that's ultra-low noise and has a wide sound stage. If you measure and match the components, the maximum deviation from the RIAA curve will be in the 0.05 - 0.1 dB range. By using a passive RIAA filter, there are very few components in the audio path.

You're probably wondering what goes on inside a phono stage.The different parts of the circuit are color coded in the picture above, here's a short discussion of each part.

Printed Circuit Board

The 90x51 mm credit card sized, shiny red PCB is designed to make the circuit operate in a stable manner and to keep noise and interference to an absolute minimum. This is the case for the PCB that you can etch yourself as well, the only difference is that it has a ground fill instead of a full ground plane.

Note: This PCB is plated through hole. Any attempts to drill through the holes to make them bigger will destroy the PCB

If you're adventurous, there is nothing stopping you from building this preamp on a veroboard/breadboard. While there's every chance of succeeding, you will always get a better result using the PCB.


  • 1x Printed Circuit Board

Input Impedance

These resistors, and the switches to select between them, are here to match the input impedance with your turntable pickup/cartridge. Experiment with the input impedance to find a perfect match for your turntable. The default value is 47 kOhm, which is achieved by setting switches 2 and 3 to on.

A full list of all available input impedances is included in the picture above.


  • 2x 4-way DIP-switches
  • 2x 33k ohm resistors
  • 2x 82k ohm resistors
  • 2x 110k ohm resistors
  • 2x 150k ohm resistors

Component choice and matching:

Use 0.25W, 1% metal film resistors. There's no need to match these components.

Input Protection

These two resistors, one per channel, are here to dampen any sudden signals. This circuit has a very high amplification, and these resistors are here to reduce sensitivity. Without them, a small grain of dust, or just touching the tone arm on your turntable could create a very loud thump. Besides scaring your pet, you don't want this to happen to your loudspeakers.


  • 2x 2k2 ohm resistors

Component choice and matching:

Use 0.25W, 1% metal film resistors. These components are in the audio path. Matching them is preferred, but not critical.

Opamps and Gain

The operational amplifiers (opamps) are doing the amplification. The resistors in the opamp circuits decide how much the signal will be amplified. The first opamp (left to right) has a fixed amount of gain, while the second has adjustable gain. Think of it as a volume control with three fixed presets if you like.

For a more interesting read about this part of the circuit, have a look at my project log.

The opamps have high input impedance and low output impedance, which is perfect for securing the best operation of the RIAA equalization filter.

The list of gain levels is included in the picture above.


  • 2x 8-pin DIL-sockets
  • 2x OPA2134 dual opamps
  • 2x 100 ohm resistors
  • 2x 150 ohm resistors
  • 2x 300 ohm resistors
  • 4x 3k3 ohm resistors
  • 2x 2-way DIP-switches

Component choice and matching:

Use 0.25W, 1% metal film resistors. The resistors will decide the gain of the preamp. You should match them to make sure the sound volume will be equal on both channels.

RIAA Equalization

After the initial amplification of the signal, it goes through the RIAA equalization filter. In this circuit, the RIAA filter is passive. Having a passive filter means that none of the components (except for the one 16 kOhm resistor) are in the audio path.

A passive RIAA filter will attenuate, or dampen, the signal in varying degrees depending on frequency. Higher frequencies are reduced the most, while lower frequencies are reduced the least. This has another nice property that we value. It will even filter out higher frequency noise, giving you a much cleaner signal in the process.

The signal is now something that you can actually listen to, although it's too low. The second opamp stage is where it will be amplified to its final level.

This resistance in this filter is more than 40% lower than in the original CNC Phono Stage, which will give even less noise (given that the original CNC is pretty damned quiet).

The redesigned RIAA equalization filter uses only standard component values, and is very, very close to the calculated optimal values. If you have equipment for measuring capacitors, you can use the information below to get an absolute match. If you can manage both, it will of course be for the best.

If you take a look at the image of the schematic of the RIAA filter, you'll see that there are some odd values in there. We've got R2 at 2k33 ohm and C2 at 136nF. These are made up by combining two and two components. For R2 it's 2k2 ohm and 130 ohm, and for C2 it's two 68nF capacitors. This way, we get nice standard component values that are available everywhere.

Here's how the component values of the RIAA filter stack up, referring to the schematics above:

  • R1 Optimal Value: 16,000
  • R1 Actual Value: 16,000 (0% off)
  • R2 Optimal Value: 2,326.515
  • R2 Actual Value: 2,330 (0.15% off)
  • C1 Optimal Value: 46.876
  • C1 Actual Value: 47 (0.26% off)
  • C2 Optimal Value: 136.681
  • C2 Actual Value: 136 (0.5% off)


  • 2x 130 ohm resistors
  • 2x 2k2 ohm resistors
  • 2x 16k ohm resistors
  • 2x 47nF polyester film capacitors
  • 4x 68nF polyester film capacitors

Component choice and matching:

Use 0.25W, 1% metal film resistors and maximum 5% capacitors. These are the most important components to match. It is more critical to have equal values on both channels, than to have the values match the listed ones.

If you don't have equipment for measuring capacitors, try to get some with 2.5% tolerance or lower.

Output Capacitor

The output capacitor is there to make sure no DC (direct current) is passed over to your amplifier. If your amplifier receives a DC signal, the sound could be distorted.

This is technically a high pass filter, designed to let through the audio signal down to very low frequencies. With a 1uF capacitor it will be between 3-13 Hz and up, depending on the input impedance of the next stage.

Most amplifiers have input capacitors to make sure this won't happen though, but don't remove the output capacitors unless you are absolutely sure about what you're doing.


  • 2x 1uF polyester film capacitors

Component choice and matching:

Using max 5% tolerance capacitors ensures that you don't have to match them further.

Opamp Bypass

These capacitors have two functions. The first is to further filter the power from your power supply, and the second is to keep the opamps stable. Without these, some opamps could oscillate. An oscillating opamp has a particularly annoying noise, and it will invariably disintegrate after a short period of operation.


  • 4x 100nF (0.1uF) ceramic disk capacitors
  • 4x 10uF electrolytic capacitors

Component choice and matching:

Use bog standard components, rated for 50V or more. There is no need to match these capacitors, but note that the 100nF capacitors must be ceramic.



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    9 Discussions

    Hi skrodahl, great project! :-)

    I'd like to ask you one thing: how is important the capacitance tolerance of the capacitors used in this preamp? I refer especially to the polyester capacitors - which are responsible for the equalization if I'm not wrong. Should I choose M (20%), K (10%) or J (5%)? Thanks in advance!

    2 replies

    The 47 and 68 nF capacitors form the RIAA filter. I'd choose 5% or better. The other capacitors can be 20%, with no ill effects.

    Going to try this as sson as possible. One question though - is it possible to set an adjustable gain through a potentiometer knob?

    2 replies

    Thank you for your reply... Indeed that was what I suspected, however I saw that you used switches to vary the gain and it got me wondering. Maybe if I find a way to limit the pot's range to the same values you set with the switches (just wondering, probably going to try the PCB as is first)

    Snyggt jobbat Håvard :) Göran låvorda den, men själv har jag lämnat vinylspelare bakom mig.

    1 reply