But now, its time to tackle a box of parts I have been saving for a while. RJM audios VSPS or Very Simple Phono Stage, for short, at http://phonoclone.com/diy-pho5.html It´s a simple dual op-amp design with a modified Allen Wright RIAA stage and only 27 parts.
You can read a detailed description of the design on RJMs site, above.
http://beavishifi.com/articles/phono-preamps/ has published this simple diagram of the circuit. But note that RJM also suggests adding adjustable gain by switching between three values of R2.
My plan is to build this phono pre-amp with a battery supply and install it in a case that is suitable for display in the living room system. I am using strip board to build on. And I´m planning to power it with batteries.
NE5532 in the top middle, I established the power strips next. The chip is powered by +12V and -12V with reference level, ground, in the middle. I decided to use the bottom strip as ground and the nr 8 pins strip near the top as +12 V. -12V is not yet assigned a strip, I just soldered on a piece of black wire as a reminder, but later changed this to orange to remind me it is not ground.
Next I connected the right and left + INputs from pins 3 and 5 to ground via two 47k resistors to establish input level relative to ground.
Next I decided to solder in three R2 resistors. Considering that the inputs may be more sensitive that the outputs, I soldered these resistors from the -IN strips As suggested by RJM I will use 2.2k for 30dB, 680R for 40dB, and 220R for 50dB. I started with a nice 1% Vishay metal film closest to the middle for 2.2K, but only had ordinary 10% for the 680R and 220R. For the 680R, in the middle, I only had 670s but the nice thing about 10% resistors is that you can find some deviants, and after sorting through 10 of them I had two 675R, which is as close as I will get for now. I also matched the 220Rs and found two that were spot on. All three were soldered to their own strip in the other end where I will later have a 3 way switch to ground to choose gain. Then the strips were Dremmeled away in the middle to create separate channels, like I did, previously, under the op amp.
Next is the RIAA stage between -IN and OUT which is pins 1 and 2 on the left side and pins 6 and 7 on the right side. Due to the amount of hardware going into these strips I need to move one strip down a bit. Intuitively I would move OUT, since -IN might be more sensitive to noise.
I have substituted some values for what I have and this fits RJM´s recommendations for higher precision, which is a plus: 2.2K , 105K and 732K all 1% Xicon Metal Film resistors. For capacitors I am using 1000pF 100V 2.5% WIMA polypropylene and trippeling them for the 3nF value. As shown here:
For the final stage in the build its time to add the OUT components. First the bottom strip of the RIAA stage is connected to OUT on pins 1 and 7 with green wire. Then the output coupling components go into place; a pair of 47R 5% Kamaya Carbon composition resistors followed by a pair of 2uF 250V 10% Sprague polypropylene capacitors. unfortunately I did not have 2.2uF, hope it works OK anyway. I mount all these along the OUT pin strips so I have to cut the strip under the components. Finally a 33k resistor to ground to drain of the capacitor. For this resistor I am using a generic 1/4 watt 10% again, I have a 32k and the value is not critical, according to RJM, so I´ll go with it.
Oh, and the Vcc- with the orange wire - I stuck it an an unused strip so I could power the board from the right edge. Let the the debugging begin!