Friday, 9 February 2018

The big hifi preamplifier project - input selection.

Amongst the ever changing line up of hifi kit at home, I feel the need for more homemade bits of it...

I mean there are some...

This lash up, for example. Built many, many moons ago, and largely undocumented...

It's a Systemdek with a unipivot arm, and a nasty otofon Red cartridge, home made all valve pre-amp with posh stepped attenuator, RIAA and EM84 indicator tubes, and an amp, made from junque parts at the time, featuring a GZ34 rectifier, an ECC82, and two something-or-other pentodes in SE output. A staggering 3 watts on a good day...

The current line up is a bit "not home made enough" ...  The BSR (here) is turntable of the minute, although there are more than a few turntables here! The amp is good, a Rotel 6 channel thing, with two channels bridged (and two unused), and the pre-amp is a Rotel. There's an Arcam CD which I ought to get round to putting a new optical ass'y in, as it's as weak as dishwater...

The Rotel pre-amp is OK, but it's MC phono input is as noisy as a hen with one chicken.

So, after watching a  Techmoan video on youtube (Here) I was somewhat inspired. Yes, I would like more inputs, especially two turntable inputs. It needs to have a phono stage for MM (Moving magnet) as well as MC (Moving coil). I'd like some subtle tone controls. I'd like remote control. I'd like some VU meters or similar. I'd like a nice colour display with touch screen. I like an option to use the new Korg Nutube . I'd like it all to fit in a 2U case.... not much to ask is it?

So I set about sketching some ideas down. Power supply board, main pre-amp board, input and output switching board, microcontroller board, and finally, but my no means least, a decent phono stage.

I'm going to tackle this in a few posts, as it's a big project.

First off... Input selection.

A couple of options come to mind to deal with input selection.
First though was some nice analogue switches. A slack handful of MAX4066 switches would work well, in a similar fashion to those I fitted to the Quad 44 here.  Reasonably easy to implement, but they do add some distortion and noise.
What about a bunch of small signal relays? Essentially noise-free, switching not quite as easy, and expensive ... but I don't want to be doing this twice...
Two phono. 1 CD (1dBV adjustable) , and then 4 line level (-10dBV)
Outputs? Recording output (although I don't really have any recording devices, the option to connect a laptop or something would be nice), and the main output to the power amplifier.

Right .....

From the top... 5V and the control signals come into JP1. K1 (the phono switching relay) is switched by Q1. All the other relays are driven by a ULN2003 darlington array. K2 is a "mute" relay and grounds the output. K1 switches between the turntable inputs. This is output to JP4, which will feed the phono stage. The phone stage will then feed the signal back to JP7, and via relay K3 to the output JP5 which will feed the main pre-amp board. The main pre-amp board then feeds the signal back to JP2 and JP3, and is routed through to the output sockets (via the mute relay in the case of the main output). K3 is the CD player input. Notice it has two attenuators, R2 & R3 to reduce the level a bit to match the other sources. K4 through K8 are the other normal line level inputs. Notice the different grounds, GND is the +5V ground, and used by the relays and associated switching. AGND is the audio ground. GNDA is the phono ground. I've included a link at JP6 should we wish to ground them here. I think it's going to be best to make a start point of grounds at the power supply, but I've included this here for a bit of flexibility.

Want more inputs? Add more relays another ULN2003 and some more sockets! (Don't worry about the number of selector inputs, we can use a shift register if you get seriously carried away)

Got that? Right ....

It should all fit nicely on a single sided board.

The red trace is a wire link.

It's duly created using toner-transfer and etched...

... silk screened ...

... and finally assembled.

Relays are Omron G6A-274P (available everywhere, but shop around, prices vary. I managed to get these for £1.79 each from Farnell) and the double phono sockets are PSG01550.

Coming next, the main pre-amp PCB ...

... and just what does CoaSt stand for?


  1. Analogue switching with a CD4066? No thanks. Use a DG508 (8x1) or DG509 (4x2) IC instead (other switches are available). I have used these in the past in instrumentation, although they might now be a little long in the tooth.

    1. The '44 was designed by Quad many moons ago. I wouldn't want to switch anything with a CD4066, but the MAX4066 is a far superior component, but I'm not using any analogue switch.