Introduction: Turntable(phono) Preamp With RIAA Correction
Hi. This is my first instructable. I see PCB contest accidentally and decided to share this project. This preamplifier was made on march-april 2018. The story start when one friend, who by the way is a producer and DJ, Mihail P, bought a turntable. He quickly realize that cannot use it without phono preamplifier and ask me to made one. To save the time, i started to search for schematic and luckily i find one in one science and technical magazine from my country. The PCB for this project was out of stock so i decided to make my own. And i love it.
The words that you see on the PCB, and on labels on the box, are in Macedonian language with Cyrillic alphabet. But do not worry. I remake all the Eagle Cad files and Labels in English and they are uploaded here.
CAUTION! Risk of electric shock! Part of this project is using high voltage. Be careful.
What is a phono preamp?
A phono preamplifier, is an audio device that amplifies the output audio signal from your turntable. Normally, output from the turntable is very low and if you connect turntable directly to your sound system you will hear very low sound. This is why output signal need to be amplified. Phono preamp need to be placed between your turntable and your sound system.
What is a RIAA correction/equalization?
It is complex, and not necessary to be explained in details here. But in short, when sound is recorded to vinyl, different frequencies are recorded with different amplitude and speed than they really are, and there is a reason for that. This is called RIAA correction/equalization. Because of that, at the other end, when a turntable reproduce the sound, it need to be applied reverse RIAA correction/equalization for sound to be as before recording.
If you want to know more about this, you can find more details on the internet.
The project is separated in two PCB's. First one is for power supply. It is Dual and is based on Transformer with dual secondary windings and positive and negative linear voltage regulators. Second is a preamplifier. It is based on op-amp TL074. As a box for this two PCBs is used old PC PSU. With little work on the box and with two labels, at the end it will look very nice and for sure you will love it.
All Eagle, Labels(Corel DRAW) and PDF files are uploaded here. You can make this project even if you do not have Eagle Cad because PCB's are exported to PDF ready to print.
Sorry if i have made mistacke in the writiing. English is not my native language.
Let see the final product...
Step 1: Overview the Final Product
I believe that is good first to see how final product look, and then to go step by step to make it. Because of that, here are the two pictures of final product.
Step 2: Overview of Schematics
Schematic for the power supply is well know and is easy to find on internet. I am using transformer with output voltages of +15V/-15V and linear voltage regulator 7812(for positive voltage) and 7912(for negative voltage). This means that output voltage of the linear regulators will be +12V/-12V. This is in allowed range for preamp IC(TL074), which is from +9V/-9V to +15V/-15V.
You can use another transformer and linear regulators from series 78XX and 79XX, but be sure that everything is in allowed range. Here is important to know that output voltages of the transformer need to be at least 2Volt higher than output voltages of linear regulators.
Schematic for the preamplifier, as i already mentioned, is from EMITER, science and technical magazine from Macedonia. They give me approval to share this project here. Their web is www.emiter.mk.
Heart of the preamp is IC1, TL074. It have four operational amplifier in his package. Two of them are used for left audio channel and other two for the right. Surrounded with rest of the component it apply necessary RIAA correction/equalization and needed amplification. First stage(IC1A and IC1D) gives amplification of 22 times. Second stage(IC1B and IC1C) gives another 4.5 times. In total it is 100 times amplification or 40dB. This is enough for needs of turntable pick up with MM(moving magnet) or MC(moving coil).
Step 3: The PCB's
I'm making PCS's with a toner transfer, using glossy paper and rolling with inox pipe on hot plate. For etching I'm using 12% hydrogen peroxide and 15-20% HCL. There is many different ways to make PCB. Most of them are here on instructables. This is a reason why I will not show you step by step how to make it, but only the results. If anyway, there is enough people who will like to know how i make PCB's, i will make a new instructable only for this.
When you solder the electronic component, take care for the polarization and orientation of the components.
Part list for Power Supply Board:
TR1 = 2x15V on output. Input will be 110V or 220V. This depend of your country.
D1-D4 = 1N4007
C1, C2 = 2200uF/=>25V
C3,C4 = 100nF/=>25V
C5,C6 = 220uF/=>16V
C7,C8 = 100nF/=>16V
IC1 = 7812
IC2 = 7912
J1 is not necessary. It was used only for testing purpose. This is output of Power Supply and will be connected with wires to the preamp board.
Part list for Preamplifier Board:
R1, R7 = 47k
R2, R8 = 2,2k
R3, R9 = 68k
R4, R10 = 5,1k
R5, R11 = 150k
R6, R12 = 15k
C1, C5, C6, C10 = 100p
C2, C7 = 1n
C3, C8 = 1,5u
C4, C9 = 22n
C11, C12 = 100n
IC1 = TL074
Again, there is no need of J1-J5. They was only for testing purpose.
For better sound use audio grade caps on the preamplifier board.
Your favorite tool for drilling
Step 4: Prepare the Box
As a box for this project will be used box from old PC PSU. It looks like this box is made right for our needs. In this step we will open the cover of the box, remove old electrnoic that is inside and make some drills and cuts. Folow the instructions in the pictures.
After you finished instruction from pictures stick your final labels and mount the following:
1. 16mm round rocket switch. Be sure that ON,(mark as I) is sided up.
2. Two channel, panel mount, RCA audio socket. Use M3 screws and nuts to mount this socket.
Step 5: Put Our Electronic Inside the Box
Take two wooden pieces, wide enough to screw our two PCB's on them. You can glue this two wooden pieces on the box, or you can drill holes on bottom of the box and screw it. It is on you.
Before you screw PCB's on the wooden parts, you will need to solder some wires. First, solder three output wires on the Power Supply board. This wires are -12V, 0V, +12V, and are not so critical. I am using wires from cat5 LAN cable. Here is important to solder every wire to the right place. For this follow the schematic or see in the pictures. For better work i am using wires with different colors. Blue for the -12V, White for 0V and Red for the +12V. Solder other ends of this wires to power input of preamplifier board. In our case this is easy. First to first, second to second and third to third. On this board you will need to solder and audio cables for input and output audio. Now, the quality of the cable is important. Buy audio cable, or use some from old headphones, headsets or speakers. 15cm(~6inch) wires will be enough for every channel, for input and for output audio. See the pictures.
Next, you will need to solder two wires for the HIGH VOLTAGE input on the Power Supply board. I suggest you to use wires that you unsoldered before. Solder one wire from Neutral solder point on Power Supply board to the Neutral terminal on the C14 power socket. Then, Solder one wire from Live solder point on Power Supply board to one of the terminals on the power switch. And solder one wire from other terminal of the power switch to the Live terminal of the C14 power socket. Now you can screw this board on the wooden pieces. At the end, solder one wire on the ground terminal on C14 power socket. This wire need to be 25cm(~10inch) long. Be extremely carefully. This part is HIGH VOLTAGE. Be sure that there is no shorts and that everything is connected correctly. See the pictures and follow the instructions.
After that, solder signal wire to center tap on RCA socket, and ground wire to outer tap of the sockets. Do this for both input channels. See the pictures.
Next, on the upper terminal of the rocket switch, solder the ground wire that we have unsoldered from C14 power connector. Other end of this wire, together with this 25cm(~10inch) wire that we have soldered on C14 ground terminal, need to be tightened to screw on center-top of the cover of the box. For this we will use M5 screw. Place one washer on the screw and put this two ground wires that we just mentioned. Put this screw in the hole of the cover and on the outer side place another washer. Than tight this with a nut.
Place two more washers and at the end tight with wing nut. Between last two washers will be tighten ground wire that will go from turntable to this preamplifier. Wing nut is for easily tight with hand.
Now, solder one wire from pin 1 on 3.5mm stereo audio socket, to terminal of the rocket switch that is sided down and to outer taps of the RCA socket. You can find pin 1 on the pictures.
Next, solder signal wire for right output channel to pin 2 on 3.5mm stereo audio socket, and signal wire for left output channel to pin 5. Twist ground wires for both channels and solder them to pin 1.
Here is one important thing. Be sure that the conductive part on the connector that is for tightening the 3.5mm socket, do not touch the cover. For this reason make this hole a little bigger and place isolating tape in the inner side of the cover and isolating washer before nut on the outer side. See the pictures.
Just to be sure, test conductivity between box and the nut with multimeter. You do not want to have conductivity.
At the end, close the cover.
2 x wooden pieces
4 screw or glue to stick this wooden pieces with box.
8 screws for tightening boards to wooden pieces.
2 x M3 screws and nuts for RCA sockets
1 x M5 screw, 4 x washers, 1 x hexagonal nut and 1 x wing nut for grounding on cover.
1 x Stereo RCA for panel mount (example)
1 x 16mm round rocket switch (black or white)
1 x 3.5 mm stereo audio socket for panel mount(example)
1 x isolating washer
4 x 15cm(~6inch) audio cables
Wires from power supply board to preamp
1 x 25cm(~10inch) wire for connecting ground terminal on C14 power socket to screw on the cover.
Wire for connecting ground on RCA sockets, Rocket Switch and 3.5mm Stereo Socket.
Rotary tool or angle grinder
Flat and philips type screwdriver.
Step 6: Connecting With Turntable
Connect output from your turntable to input of this preamplifier. Then connect output from the preamplifier, to your sound system. It is very, important to use high quality audio cables. At the end, take wires and connect ground from turntable and ground from your sound system(if it have) to ground of preamplifier.
Round rocket switch is used to separate or connect signal and main ground. It can help in avoiding ground loop and humming. Humming can be big problem in audio.
Here are uploade all Eagle Cad, Corel(Labels) and PDF files in one RAR file.
Step 7: Enjoy
Enjoy in listening your music when you made the preamplifier.
Here is a video of preamlifier in action.
Thank you for your time.
Participated in the