Preamp (Updated april 2018)

The decision to use a touchscreen display on the Pre amplifier as control interface had an immediate impact on the way the Pre amplifier design had to be implemented. All controls had to be controlable in a digital way.
For volume control I could have chosen for motor driven potentiometers but in stead I chose the PGA2320 from Texas Instruments.
The PGA2320 is a stereo audio volume control device that sets its volume in 255 steps from -95.5dB to +31.5dB in 0.5dB steps with a TDH of 0.0003% @ 1KHz and has a nominal output noise of 10uVeff.
The PGA2320 has a serial control interface that can be daisy chained and easily interfaced to the SPI of a microcontroller.
The input selectors and relays of the Phono amplifier can be controlled by SN74AHC594 shift registers with parallel buffered outputs that are in the daisy chain with the PGA2320.
The serial daisy chain contains three signals;clock, data and enable and run over the whole board to control all settings. These signals are only active during changes in settings.

For the input selector there are a number of choices available, reed relays, analog switches, discrete FETs and maybe others that I did not consider.
Reed relays have the advantage of having no noise or distortion contribution but are bulky, I would need 10 of them. 
Discrete FET's can be used but require quite some additional circuitry to make them operate properly.
I decided to use analog switches but there are some properties to take into account.

Analog switches like the ADG5207 from Analog Devices or the MUX36S08 from Texas Instruments have an inherent non linear behaviour. The R-on of the switch varies with the voltage applied to it.
This non linearity is also not symmetrical around zero (ground), (see Fig.1), the R-on variation is different for positive voltages and negative voltages.
One way to reduce the impact of this behaviour is to add a ~2V DC offset and/or minimize the voltage level on the in- and output of the switch. I decided to leave the DC offset. Minimizing the voltage level can be done by placing the switch at the summing point of an inverting amplifier, where the voltage swing is zero,  at least in theory.
Since the R-on of the switch isn't zero, there will be a small voltage swing at the input side of the switch that will cause distortion.
For a gain of -1x in a summing amplifier topology, the feedback resistor must have the same value as the resistor in series with the signal plus the R-on of the switch, connected to the - input of  the summing amplifier.
The higher that total resistor value, the higher the ratio between the series resistor and the R-on resistance and the lower the voltage swing on the input of the switch will be.
However we want a low noise design and use therefore low resistor values in the signal chain, preferably 1KOhm or lower.
Fig.2 shows the schematic for connecting the MUX36S08 in the inverting summing point topology with a -1x gain. 
The R-on of affordable high voltage analog switches is usually in the 100Ohm - 200Ohm range. I found the average R-on of the MUX36S08 to be at 180Ohms.
With 1KOhm resistor values and a gain of -1x of the summing amplifier there will be around 18% remaining voltage drop at the input of the switch when closed, the output of the switch is connected to the - input of the opamp and therefore virtually zero.
The distortion caused by the analog switch in this topology is with an 1Veff. signal at -85dB, an acceptable level without compromising the SNR. Fig.3 shows the FFT plot plus THD taken over the analog inputs at 1Veff. The rise at low frequencies comes from the analog I/O of the audio card. SNR measured on an analog input referenced at 1Veff. is around 110dB

This plot was taken using the analog I/O's on my ESI Juli@XTe card and Arta audio analysis software on my PC.

Using analog switches to mute the outputs of the Pre amplifier leaves the full voltage swing over the input and output of such switches. This would have a more severe impact on distortion.
I decided to use reed relays for that. Fig.4 shows the plot over frequency of a reed relay in the output of an amplifier. The blue line is when the reed relay is active, the red and green lines show the reed relay switched off at -90dB, increasing to -80dB at 20KHz.

The analog inputs are straight forward non inverting Opamp topologies. Left and right channel are implemented with a dual OPA1642.  TPD2E007 ESD protectors are used at the inputs.
I used high quality Polypropylene capacitors in the signal chain and for the servo amplifiers where nessesary.

The digital inputs are all switched using logic gates before the selected digital input is sent to the SPDIF receiver DIR9001. This SPDIF receiver can only receive signals up to 96KHz. samplerate so not supporting 192KHz.
I haven't seen equipment like TV's, settop boxes , CD or DVD players transmit SPDIF signals at higher frequencies than 48KHz.
The error outputs of both the DIR9001 and the SRC4192 are combined and sent to the microcontroller to indicate if the selected digital audio input presents a valid audio signal to the Pre amplifier. This SPDIF error is shown on the display.

The converters in the Pre amplifier all work at 48KHz. driven from a low noise master Xtal from TentLabs. To accomodate other SPDIF input sample frequencies the output of the DIR9001 is passed through a samplerate converter  SRC4192 that provides 48KHz. I2S digital audio data to a PCM1794A stereo DAC.
Differential I-V amplifiers transform the current outputs of the PCM1794A into a single ended analog voltage, that is fed into the MUX36S08 analog input selectors.

The analog output of the stereo volume control device PGA2320 is passed to both an output buffer (OPA1611 with OPA192 DC-servo amplifier) and a stereo ADC (PCM4202) followed by a digital transmitter (DIT4192) that drives an optical transmitter (TOTX1952F).

The daisy chain is driven from the on board microcontroller, a MSP430FR5728, that has non volatile FRAM memory where all parameters are stored to startup the Pre amplifier in case of a power down.
Communication to the microcontroller on the Display unit runs over a RS232 link at 115.2KBaud. The SPI link runs at a clock speed of 100KHz.
The micro is normally in sleep mode and powered from the 3V3 "Always ON" supply. Commands from the Display unit are single bytes for selecting an input, increase or decrease volume, balance or volume presets.
Balance and presets are simply offsets compared to the master volume with a maximum of +/-20dB, leaving 11.5dB of headroom (3.75Veff. max output voltage).
The Pre amplifier micro controller responds to the Display controller with acknowledge commands or sends packets of volume, balance and preset values upon request packed into a so called small data protocol, the same protocol that the intelligent display is using.
When the Pre amplifier is activated by either an IR "power-on" command or a manual ON/OFF pushbutton command all settings are verified with the Display controller to ensure both are in sync.

Besides the SPDIF error signal I also feed the combined left and right ADC clip outputs of the PCM4202 to the Display unit to flag a clipped Digital output signal.


On the left you can download and open the PDF schematics of the Preamp board.


The PCB design of the Pre amplifier is the largest board I ever designed. It is a 4-layer board of 28cm x 20cm.
The top layer is the primary signal layer, the second layer is a continuous ground layer, the third and fourth layers are power planes and carry some non critical signal paths that could not be routed on the top layer.
The placement of the different sections was carefully chosen. Phono amplifiers are located on the right-bottom side and the analog input buffers on the right top side. Analog outputs in the middle top side, right to the digital output.
Digital inputs on the left top side so that all I/O was placed at the top side of the PCB to be mounted at the backside of the cabinet.
The D-A section was placed in the middle-left of the board and the A-D section plus input selectors and volume controls in the middle right section.
This allowed me to use only two power planes for both the +/- 15V and for the analog 5V and 3V3 planes.
The Microcontroller was placed at the bottom left corner, as far away from the analog circuits as possible.

The ground plane was placed at the second layer for a good reason.
To route all analog signals over the board they had to travel tens of centimeters in distance, prone to pick up of noise and mains frequencies.
By separating them with ground traces and an underlying ground plane they were all becoming virtually shielded signal traces minimizing crosstalk and pickup of noise.
Fig. 5 shows a detail of the Pre amplifier PCB with these shielded traces.