Beogram 4002 RIAA Preamplifier: First Prototype Installed

I was asked to design an internal RIAA preamplifier for the Beogram 4002 (5521) that I recently restored. An internal preamplifier is an excellent idea since many modern receivers do not feature a dedicated phono input anymore. Also, the RIAA preamps of many classic B&O receivers are pre-opamp designs and therefore have a higher noise floor than what can be achieved with dedicated modern low-noise audio opamps.

After doing a bit of research about modern RIAA preamp designs I settled for the LM833 operational amplifier, which is an optimized low-noise design for audio applications. I based my design on a precision RIAA preamp circuit described in the Texas Instruments application note AN-346. There is really no need to re-invent the wheel when it comes to RIAA preamps, this has been figured out long ago.

I added a rail spitter to create a virtual ground for the opamps since the Beogram only has a positive power rail. It turned out that I needed to run the preamp from the 31V rail, since the regulated 21V rail introduced a significant amount of motor EMF into the amplifier circuit resulting in a loud hum clearly depending on the motor RPM. Using a 24V regulator and appropriate capacitors on the 31V rail solved the problem and now the amp is very quiet in the hum as well as in the signal-to-noise department. I used high precision polyphenylene sulfide (PPS) film capacitors for all RIAA relevant values to limit non-linear distortions due to thermal and microphony effects.

The board was designed with a form factor that allows it to piggyback on the output relay board of the 4002 duplicating the original connectors. In its final incarnation it will replace the original output relay and be soldered into its footprint, which will hold it in place mechanically. The design allows to switch between preamp operation and original non-amplified output by simply plugging the input and output plugs either into the RIAA preamp board or in the original jacks on the relay board. 

Here is a photo of the board:

The 7824 regulator on the left was added after I realized the motor hum issue. On the right end is the new output relay that is integrated into the preamp circuit. In the center is the footprint of the original output relay of the Beogram 4002 together with solder pads for a SMD relay, which I did not solder in yet. In the final incarnation this second relay will be in the original non-amplified signal path, which will allow switching between amplified and non-amplified outputs.

This picture gives an idea how the board plugs into the Beogram:

It seems to work very nicely. I am listening to 'Lonely Woman' by the Modern Jazz Quartet right now through the Tape input of my Beomaster 6000 4-Channel, and it sounds perfect. Previously, I tested the setup with my Analogue Productions Test LP, which has a RIAA test track. My oscilloscope showed a perfectly constant amplitude as the LP went through the frequency ranges. This demonstrated that the circuit indeed has the promised equalization curve as described in the AN-346 application note. A bit more testing and then it is time to think about a final design of the board to get it manufactured...