DIY preamp with multiple inputs
I have designed several preamps in the past with analog single ended and balanced inputs, single ended and balanced outputs, but never with a digital S/PDIF input. Recently I have reviewed a cheap DAC board based on Cirrus CS4398 DAC, so it has been a challenge to use this board in the preamp as a digital S/PDIF input and make the preamp more versatile. I have used one of my older DIY preamp projects (dated 2009 – 2011), with boards, case and panels already built, and I have added the DAC board inside the case.
Design goals
Circuit design
The preamp uses 2 separate analog amplifier boards, one as an input selector and one as an amp with high output current capability.
Input selector board and amplifier board
The input selector board uses high quality relays Axicom D2n type, with 12Vdc coil. Single ended inputs are directed, after input RC filtration (680R + 68pF), directly to amplifier board BUF2. Balanced input goes, after common mode and differential RC filtration (C1 – C5, R1 - R4), to instrumentation amplifiers (IC1, IC2 - INA217) and then to the amp board BUF2. Volume control (P1, blue Alps 2x10k/log) is connected to the input of the composite amplifier consisting of the Analog Devices AD744 opamp and TI BUF634T buffer. This amplifier board has +12dB voltage gain.
S/PDIF input board
I have used a cheap Cirmech WM-013 DAC board based on Cirrus CS4398 DAC. It is able to work with digital input up to 24bit/192kHz. The board was purchased from AliExpress.
Power supply board
As a power supply I have used my ZT15VA with output voltage set to about 2 x 13V. It is a regulated power supply with very low output ripple and it uses Talema toroidal transformer.
Amplifier case
All the boards are placed inside the 19” 1U box. The box was purchased from https://modushop.biz/site/, together with panel drilling and engraving.
Below is the completely assembled box with wiring. One can see the power supply, the DAC board, the input selector board and the amplifier board.
Technical parameters
Measurements
Output noise with volume at minimum, all inputs
Same plot is obtained for SE inputs shorted and volume at maximum
THD vs. frequency at 4V output, comparison of analog vs. digital input
THD+N vs. output voltage at 1kHz, comparison of analog vs. digital input
The DAC board deteriorates the preamp parameters significantly. Its distortion rising with frequency and amplitude is a result of high K ceramic capacitors (highly nonlinear dielectrics) used in the output active low pass filter of the DAC board. I have explained it in the DAC board review. It is a pitty, however, the DAC cost was $18 including VAT and shipment, so I can hardly have objections. The appropriate C0G/NP0 capacitors would be much more expensive.
Attached below are RightMark Audio Analyzer results measured with the balanced analog input and with the S/PDIF input. Attached is also a dxf drawing of the rear panel drilling.
I have designed several preamps in the past with analog single ended and balanced inputs, single ended and balanced outputs, but never with a digital S/PDIF input. Recently I have reviewed a cheap DAC board based on Cirrus CS4398 DAC, so it has been a challenge to use this board in the preamp as a digital S/PDIF input and make the preamp more versatile. I have used one of my older DIY preamp projects (dated 2009 – 2011), with boards, case and panels already built, and I have added the DAC board inside the case.
Design goals
- 2 single ended inputs, 1 balanced input, 1 digital S/PDIF input
- 1 pseudo-balanced output based on a single ended output with close to zero output impedance
- very good immunity to input HF and EMI
- high output current and ability to drive 600 ohm load impedance and headphones
Circuit design
The preamp uses 2 separate analog amplifier boards, one as an input selector and one as an amp with high output current capability.
Input selector board and amplifier board
The input selector board uses high quality relays Axicom D2n type, with 12Vdc coil. Single ended inputs are directed, after input RC filtration (680R + 68pF), directly to amplifier board BUF2. Balanced input goes, after common mode and differential RC filtration (C1 – C5, R1 - R4), to instrumentation amplifiers (IC1, IC2 - INA217) and then to the amp board BUF2. Volume control (P1, blue Alps 2x10k/log) is connected to the input of the composite amplifier consisting of the Analog Devices AD744 opamp and TI BUF634T buffer. This amplifier board has +12dB voltage gain.
S/PDIF input board
I have used a cheap Cirmech WM-013 DAC board based on Cirrus CS4398 DAC. It is able to work with digital input up to 24bit/192kHz. The board was purchased from AliExpress.
Power supply board
As a power supply I have used my ZT15VA with output voltage set to about 2 x 13V. It is a regulated power supply with very low output ripple and it uses Talema toroidal transformer.
Amplifier case
All the boards are placed inside the 19” 1U box. The box was purchased from https://modushop.biz/site/, together with panel drilling and engraving.
Below is the completely assembled box with wiring. One can see the power supply, the DAC board, the input selector board and the amplifier board.
Technical parameters
- 2 analog single ended inputs, RCA, IN1 – IN2
- 1 analog balanced input, XLR, IN4
- 1 coaxial digital S/PDIF input, IN3
- 1 pseudo-balanced XLR output
- Input impedance SE ........................... 10 kohm
- Input impedance balanced .............. 94 kohm
- Output impedance .............................. close to 0 ohm
- Max. output current ............................ 250mA
- Max. input voltage BAL ...................... 8 Vrms
- Max. output voltage ............................ 8 Vrms
- Gain ........................................................... +11.5 dB
- Frequency range analog inputs ....... 2 Hz – 110 kHz / -3dB
- Frequency range digital input .......... 2 Hz – 50 kHz / -3dB / Fs = 192kHz
- S/N SE input ............................................ 110 dB / 4 Vrms / flat 20Hz – 20kHz BW
- S/N S/PDIF input .................................... 106 dB / 4 Vrms / flat 20Hz – 20kHz BW
- THD analog SE input ............................. 0.0003% / 1kHz / 4V output
- THD digital input .................................... please see the plots
- Dimensions ............................................... 450 x 300 x 50 mm
Measurements
Output noise with volume at minimum, all inputs
Same plot is obtained for SE inputs shorted and volume at maximum
THD vs. frequency at 4V output, comparison of analog vs. digital input
THD+N vs. output voltage at 1kHz, comparison of analog vs. digital input
The DAC board deteriorates the preamp parameters significantly. Its distortion rising with frequency and amplitude is a result of high K ceramic capacitors (highly nonlinear dielectrics) used in the output active low pass filter of the DAC board. I have explained it in the DAC board review. It is a pitty, however, the DAC cost was $18 including VAT and shipment, so I can hardly have objections. The appropriate C0G/NP0 capacitors would be much more expensive.
Attached below are RightMark Audio Analyzer results measured with the balanced analog input and with the S/PDIF input. Attached is also a dxf drawing of the rear panel drilling.
