LCR Phonostage With Variable EQ Curves, Part 2: Implementation

Hi!

In the first article about the variable EQ phonostage I wrote about some initial thoughts. In the meantime things progressed considerably. In fact the phonostage is almost completed. So it is time to continue this series with the second part. As I wrote in the first part, the same basic circuit topology as in my EC8020 LCR RIAA phonostage was chosen. A two stage circuit: First gain stage with low output impedance driving a LCR EQ network, followed by a second gain and output stage. Only this time the EQ should be selectable. This is the block diagram of the circuit:

Two switches for independent selection of low frequency turn over and high frequency roll off. Since the EQ should be done with LCR networks, a completely separate LCR circuit needs to be built for each turnover and roll off. LCR networks are quite large, so a good compromise had to be selected to cover as many EQ curves as possible but limit the amount of networks. The given chassis size would allow for 8 separate networks. In addition a flat setting can be easily implemented for the roll off, by just selecting a feed through.

This is the list of turn over and high frequency roll off points which was finally selected:

Turnover frequency / low frequency cut:

400N : 400Hz / -
500N : 500Hz / -
500R : 500Hz / 50Hz (RIAA)
500C : 500Hz / 100Hz

It is actually not possible to make a LCR network without low frequency cut. This would require infinitely large inductances. Instead the cut was moved to a very low frequency, around 20Hz.

For the treble EQ, 4 settings were chosen, plus flat. Listed as the attenuation at 10kHz:

0
6dB
10.5dB
13.7dB (RIAA)
16dB

The two schematics on the left show the basic principle of LCR networks. The one on the top is a network with a single corner frequency below which it is flat (low pass). At the corner frequency the attenuation is 3dB and the signal rolls off with 6dB per octave above. For the RIAA high frequency roll off (2122Hz), values for a 600 Ohm network are: 45mH for the coil and 125nF for the cap. All resistors 600 Ohms. With just an additional resistor such a network can have two corner frequencies. Between those the signal has a slope of 6dB per octave and is flat below the lower frequency and above the upper corner. Again the values for the RIAA network (50/500Hz): 1,8Hy for the coil and 5uF for the cap. The two resistors parallel to the inductance are 495 Ohms each. The resistor in series with the cap is 116 Ohms. The terminating resistor is 600 Ohms. Sometimes another resistor is added in parallel to the cap. The purpose of that is a compensation of the DC resistance of the coil. Of course the networks can be adapted for other resistances than 600. L and C need to be changed then. And of course such networks can be calculated for just about any set of corner frequencies.

Now some photos from the construction of the phonostage:

Since it is mono, only two tubes are needed. The two chokes will later be hidden under transformer covers. As much as possible was moved to the top side to keep space inside the chassis free for all the EQ networks. Here a photo of the inside with some initial wiring:

The power supply:

Before the EQ networks got installed, I tested the preamp without any EQ, just as a linear gain preamp. To make sure everything works as intended here a photo of the phonostage ready for initial testing, just lacking the EQ networks:



Everything is working as it should, ready to finish it up with the EQs and for final testing. This will be covered in part 3. Stay tuned.

Best regards

Thomas