Remote-controlled preamplifier

Posted on Feb 6, 2014

This is a distillation of my ` design notes `, with more practical details as I move along with the construction, and (in due course) some pictures. Right at the moment I`m using it as a kind of progress report for myself - it`s a mixture of immediate plans and actual construction, so isn`t too clearly structured! The signal control circuit is on

Remote-controlled preamplifier
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0. 1" matrix board for the time being - after hours of trying to work out exactly what was going to go where on a PCB, I decided to start soldering everything together anyway to make a prototype. Right now the board is getting pretty crowded, and there isn`t room for all the traces. For the moment I want to keep one or two options open, for instance the volume control and the layout of the balanced connections. At some point in the not-too distant future, though, I hope I`ll be more or less satisfied with what I`ve got, and then I`ll build a proper set of PCBs for the project, which will be a chance to tidy it all up, and of course add such niceties as a ground plane if I can. I use sealed relays with gold-plated silver contacts, type 53W/1 from Fujitsu, for all switching, including output polarity and mono/stereo selection. The 5V power supply for the relays is separately regulated from the ones for the TTL logic and the indicator LEDs to avoid glitches. All op-amps are Burr-Brown OPA2604, with one pair of LM317/337 regulators for each stereo channel. The gain of the output stage is +10dB, while the input buffers are unity gain except for the CD input (see below). If an unbalanced input is selected, the balanced signal is generated by inverting the input before the volume control, so that antiphase signals are always present in the volume control and output stages - this ought to decrease hum and RF pickup and improve the...

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