So I sampled a few of these interesting BB ICs ages ago with the intention of building one into my CK2III build, but that never happened so I decided to get a little more ambitious. I'm now getting my feet soaked designing a full-on preamp around the chip, complete with digital source selection, display, remote control and pretty much anything else you could dream up. The plan is to make the design as flexible and exposed as possible so it'd be easy to do nutty stuff like tack and I2C temperature sensor inside the power amps and display it on the LCD or even mute/attenuate the output if the amps overheat.

I don't intend this for headphone use, but people around here have far more knowledge than me on the analog side of the electronics, so I'm going to throw this out there. Also a bit of a teaser in case anyone is interested in helping with the prototyping process as I'm not confident I can fab these PCBs myself, and in small quantities they'll be pricey.

The basic signal path has inputs selected through some inexpensive telecoms relays by Panasonic (TQ2 series), buffered by an OPA2132 in unity gain, and then through the PGA2311. I'm still undecided about the output stage - has anyone used this IC that could weigh in? It will be driving a short interconnect in my stereo rack, not a long line or anything. If a buffer is needed, any recommendations for this stage? Also, opinions on dealing with the excessive gain offered by the PGA2311? At +30dB it will drive itself to the rails with a standard 2V line input and only +/-5V supplies. Should this just be limited in software and cut a third of the attenuator's range? I think that's the best solution personally, but maybe a fixed attenuation ahead of the PGA2311 wouldn't be bad? Implementation ideas? Finally the signal will flow through another TQ2 as a hardware mute/soft start that grounds the outputs.

On the digital side is a fairly large Atmel AVR (I've chosen ATmega324 for now) controlling 4 8-bit IO registers via I2C to control the relays, LEDs, receive input from switches and a bunch of GPIO for later use. There is an HD44780-based LCD module for user interface. I plan the main inputs to be a set of pushbuttons (input select & mute) and a nice optical encoder with a switch, though I've placed it on port A so a regular pot could be used via the ADC if necessary. An IR sensor is available for remote control as well. Lots of GPIO as well as I2C will be exposed to make expansion easy. I've also brought out the UART to a serial port for debugging (though maybe there's a use for this in the field? computer control?). Since they're cheap I'll be adding a voltage supervisor ($0.60) to make sure everything starts up nicely, but it's not on the schematic. Also note that there's no coupling/decoupling shown, this will be added later.

I've just pulled the trigger on $150 worth of stuff to play with at DigiKey and should be able to get rolling on the firmware and poking at the design one piece at a time before I commit to ordering boards. There's a fair bit of firmware to write, but I think getting the basic source select and attenuation functions working should be fairly easy. The tough part will be the user interaction and working the timing kinks out to make the best use of muting and attenuation to make sure things are switched smoothly.

Any thoughts on the setup, particularly in the analog section? Would anyone be interested in joining a board order possibly in a month or two?



Edit: Added schematic

Looks nice, might not be exactly what I'd do but it will still perform.

Thought of making it into a somewhat decent headphone amp as well for the output driver? The TPS6120 is a pretty good headphone amp in a chip and will drive cables fine as well. Just gives you the option to plug a pair of cans right into this box too if you want

Another pga2311 preamp:
http://www.twistedpearaudio.com/kook...ookaburra.aspx

Also, this crazy and over the top implementation (very nice): http://www.mhennessy.f9.co.uk/preamp/index.htm

I admit I was a little disappointed when I started researching this project and found it'd already been done to near perfection. Oh well, I'll have fun with it, and avoid the awful PIC architecture

Just sampled some TPA6120, thanks for the heads up DaKi][er. I was thinking of doing a headphone out, but I was going to separate it from the line out with another PGA2311 and configurable gain offset, but just tacking one of these on the output might be a little more sane since I'll probably rarely use the headphone amp output if I implement it.

Want to go insane do you? I had the idea of making a preamp that did cross point switching, 8 inputs, 8 outputs, any input can be simultaneously switched and mixed to any output, volume control and protection circuitry (DC offset) on every output. AD75019 for the cross point analog switch and WM8816 for volume control, and 2 opamps for these for each output. I mostly designed it a while ago but it sort of lost interest a bit (Board, Schematic)

Interesting project. I was going to use something like this for a Pass Aleph P1.7 clone that I am building, but it would need to be balanced, and also support some single-ended inputs.

Also, check out http://electronics.dantimax.dk/Kits/index.html. I had intended on using the RelVol3 and Input2 boards, as these meet most of my requirements.

Pars:

why not use this Aleph project, everything will be nicely integrated in the next board release:

http://r-stens.de/diy/AlephP/diy_aleph_p_en.php

A snippet of what the next rev will have:

"Aleph P V1.7 Rev. 2 new audio board with integrated input, volume, gain and power supply I2C bus plug-in board to control the relays digital communication between control and audio unit via I2C bus"

Another PGA2310 kit w/remote and input selector(w/buffer option.) and display.

Report from user in Chinese, use as OP-Pre.

Hi Error,

How far did you get with your PGA preamp? Having done a similar one to Mark Hennessey's and one of much simpler design, I can recommend you rather go for the simple approach and have a buffer before the PGA and let the output of the PGA drive straight to your amps.

On my design I just limited the maximum gain to +10dB (also, I'm using the 2310, not the 2311 so I have a little more headroom).

If you want my schematics, send me a PM and we can chat.

Cheers

Gert