This project was designed so that I could monitor and control all types of recording through my PC sound card . It allows for direct recording to the sound card by using a by-pass function and also controls the volume input as well as enable to monitor the PC output using headphones . By integrating the Phono pre-amp described on page twelve as discussed later , we can have complete flexibility in monitoring with pre-amp control in one small enclosure .

To start with I designed a simple pre-amp with low gain with the simple task of input and output impedance matching between the PC sound card and any inputs that may be available to you such as Headphone output , Auxiliaries, FM radio headphones , tape output and with the addition of the magnetic pre-amp on page 12 , direct recording from a turntable .

The preamp shown below needs only a few miliamps of current and a LED/On indicator has been added when power is required only for the pre-amp function . Two are required for stereo and only one LED is required .

Layout for Stereo . PCB size shown 1 to 1

Shown below is the basic switching and connection arrangement for the monitor box . All input and output cables must be the shielded type and all shields must be connected to the circuit ground at one main point to reject any interference and Hum pick up .
For all input and output connection on the enclosure (P1 and P2 ) I used 1/8" stereo phone adaptors and sitches are of the miniature types .

RV1a and RV1b are a dual 10K pot labelled "Volume" to control the stereo input signal wich can be fed throught the dual pre-amps which has a fixed gain of ten . When amplification is not required the power can be turned off with S1 to save the battery and the signal can by-passed with S2 directly from the volume control to the output jack P2 marked " OUT PC ". Label S2 as "AMP & By-Pass " .

If you wish to have headphone monitoring facility you can include the following circuit into your enclosure .You will need to addl three 1/8" phone plugs ( P3,P4,P5 ) and one addtional miniature DPDT switch (S3 )as shown below .

This set-up is inserted between the PC sound card output and the PC sound system . As shown S3 is used to select the sound ouput from the sound card to either the PC sound system (amplifier input ) or a stereo headphone . Note that the PC volume control and NOT the input volume control should be used to control the headphones .
Label S3 as " Headphones " and " Speaker" , P3 as "From PC " , P4 " To Speakers " and P5 " Headphones " .

Turntable recording option

Below is the set-up used to integrate the Magnetic pre-amplifier described on page twelve. To include the mag. pre-amp an additional miniature DPDT switch ( S4 ) is required to switch the inputs to the dual volume control from direct inputs to the pre-amps outputs . With the mag.pre-amps in the circuit the left and right amplifiers may be switched in series if more volume is required or by-passed for recording .
NOTE : For this purpose the voltage amplifier described on page twelve is not required .