The Pre-Amp Chain

As you may or may not know I’m building an audio/video pre-amplifier/pre-processor. Since this a quite a bit DIY project I decided to make it very modular.

It will accept either SPDI/F coax and optical as input along with the more basic analog inputs. Every input module handle one input and will output an I2S signal over a 5 pin header (4 for I2S and the last bit will be for if audio is present). Every input will then go to a mux board which will simply be relays controlled by the main logic board (possibly by some simple PIC) this board will have only one output and will be I2S. Exception to this will be the SPDI/F inputs; the CS8416 in software mode will be responsible for the mux and the output will then go to the mux board.

The fun stuff will be on the main board; the I2S input will go into a SRC (Sample Rate Conversion) the output will always be at 24-bit 192kHz and will go into a DSP. The SRC will be pulled into bypass if the “audio present” bit is not present – this will allow me to pass raw SPDI/F data and possibly implement AC-3. The main control logic will be a Freescale Coldfire MCF5282 at 80Mhz and will possibly interface with a Freescale DSP56367. The Coldfire will handle the infrared, the VFD display the power (ON/OFF), programming the DSP and monitoring the pre-amp for over-temp and such.

The video will follow the same pattern; Inputs will be converted to digital, will be muxed and then back to analog for the output – however I will not put a DSP in the single path. This project is already complex as it is.

For the power I decided to keep it simple and will use a ATX12v power supply.

Right now, the output PCB is done and I’m designing the main board. Fun stuff !

Differential Output Stage

It’s one thing to build and amplifier, it’s another to build a pre-amplifier. I selected a CS4398 [PDF] as DAC; this chip has a differential output for both channels, an I2S input and up to 120dB dynamic range.

What’s nice is that Cirrus Logic made a very well designed evaluation board for this chip, the CS4398. I’ve used the XLR output as a starting point for my own output stage.

The DAC will run at 96kHz with an oversample of 512x. The master clock is just over 49MHz (49.1520MHz). The input (I2S) will be feed from either a DSP or my “Core Module”; I have not yet tested if the ColdFire v2 @ 66MHz will be able to keep up with video & audio. As for the PCB, all the components will be surface mount to minimize space and keep the trace length to a minimum.

Differentail Output Stage


Well I just finished my PCB layout for one of my class A power amplifier;
PCB for my JLH Amp

I will now attack the blanced input stage. I was thinking about a simple op-amp with a buffer right before the “input”.