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...
Inputs?
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?

The Korg Nutube 6P1...

It is with much excitement that I opened a box from RS Components....

It's here!

Possibly the only new thermionic device made this century! Oh the excitement!!

It's a tiny double triode which promises "Real vacuum tube sound. The real triode structure produces a warm, unique vacuum tube sound, delivering excellent linearity."

It's built using vacuum florescent display technology, and is tiny. 

Marketing guff can be found here ... http://korgnutube.com/en/

and real data can be found here:

https://docs-emea.rs-online.com/webdocs/15db/0900766b815db1b5.pdf

It already is appearing in guitar pedals all over the internet, and it's going to feature in a project near here very soon.

Tip for drilling homemade PCBs.

Here's a quick tip .... if you're as myopic as I am, try this out...

Here's my PCB drill in it's stand. It's a foul thing. It was dirt cheap when I bought it 15 years ago ... The rubber coating on it has gone all sticky ... yuck...

Now look at the bottom...

I've mounted one of those small 3w LED's there, left over from another project...










Makes alignment of the drill bit with the pad on your PCB a breeze.....


You'll notice it's slightly off-centre, this is avoid drilling through it ....

.... yeah, you know how I found that out!

Modifying the Fellows A75 A4 laminator for making PCBs. Toner transfer the posh way!

Now, if I'm making something up, it's often easy to make a simple single-sided PCB up quickly, and professionally by using the toner-transfer method of PCB manufacturing. I've been doing it for years, but the results are always a bit hit-and-miss. Use google if you haven't heard of it before, there are many guides on the web (although no two the same!) , and I don't intend to duplicate those here.

I have tried many different types of transfer paper over the years, from the proper stuff, to plain old A4 paper, the best I found for years was a copy of the New Scientist magazine! Sadly they changed to thicker, shiney paper a few years ago, which doesn't work well at all. The best at the time of writing seems to be "The Economist", which is a shame, as it's nowhere near as good a read as "New Scientist" before it get's recycled into PCB making materials!!

... anyway ...

The last board I did, I tried an experiment, by borrowing Mrs Doz' laminator from her office. I idea was to see if it would transfer the toner from the paper to the board, whilst keeping a constant and steady pressure on it. Well, results were mixed... it really pushed the board through too fast for proper transfer to take place. I ended up transferring the toner with the conventional clothes iron, and then passing the board through the laminator, which resulted in an improved board after a couple of goes. It did, however give me an idea ... make the motor run slower....

So, rather than incur the wife's wrath and modify her laminator, I picked up a Fellows A75 A4 laminator from Argos for £16....

I initially tested it would take the thickness of the board by passing a blank piece of PCB stock through it... yep, works a treat.

So, to disassembly... There are 4 pozi screws underneath (great, not even some torx or bizarre security screws to bother me!)

So (from left to right) we have an essential mug of tea, the motor, drive rollers and heaters, and finally a PCB on the right....

A couple of things to notice about the PCB... 

There's some sort of timer in there to shut the unit down after it's been on for 30 mins (just in case you leave it running). It's unlikely to bother us much....

The low voltage electronics (the timer, and LED's etc) are driven from a power supply formed by a wattless capacitor dropper... so a word of warning... all of these voltages on this board will appear to be referenced to the mains. This would present us with a shock hazard and a problem if we want to interface to them...

On to the motor....

It's something I used to refer to as a "Le Cruzet" motor. A synchronous motor with built in gear box. As we can see from it's label, it rotates at 4 RPM (4.8 RPM if you live on that funny bit of the planet that uses 60Hz mains). That's a bit of a pain. I was hoping for a small DC motor I could drive with some PWM to get the speed nice and regulated (although if I was building a cheap laminator, I'd be using exactly this motor too!) 

So, how can we control it's speed?

Well, it's synchronous, so we'll need to lower the AC frequency driving it .... Ugh ... don't fancy this idea much.. generating a few tens of hertz at 240 V RMS is not going to be a cheap fix....

What about phase modulation? Daft idea. It may vary the speed, but will destroy the motor's torque, and won't vary the frequency. May cause overheating and failure in the motor's windings because of the resultant unpleasant waveform.

What about PWM? Nope, DC only ..... or is it????

How about really, really slow PWM? Really really slow.... 

I'm talking about switching the AC to the motor on for ~.5 second, off for a second, and so on ......

It'll inch the board forward, cook it for a bit, and inch it forward again ....

It's just stupid enough to work....

So what do I need? 

A 555 timer, a slack handful of passives, a transistor and a relay...

or an arduino (or similar, running a modifed blink sketch!),a transistor and a relay...?

I'll go with the latter, as it'll be easy to adjust by fudging the software, and most of the hardware is built for us.

Enter the relay module, and the weird and wonderful Digistump Tiny85 module... just the ticket, and small enough I should be able to squeeze them into the case.

I've used the normally closed relay contacts, so if I do want to use the laminator without the speed control, it will work fine with the speed control switched off. The relay contacts are connected in series with the motor, and the relay control pin to P0 of the digistump.

A design is laser printed onto a piece of paper from "The Economist", and a test piece of board stock is cleaned up (I know they're a different size, but it's only a test)

The paper is tightly folded round the board so it doesn't move, and stuffed into the mouth of the hungry laminator! The "printed" side needs to be facing the bottom roller, as that appears to be the roller that's heated.

I left the speed controller off (so the unit was running at normal speed) whilst it reached temperature.

I ran a couple of tests, trying different speeds.




and into the sink!













I finally settled on 4 seconds off time, and 0.5s on time.... which produced nice, repeatable results ....

and left us with the digistump code looking like this....

void setup() {
  pinMode(0, OUTPUT);
}

void loop() {
  digitalWrite(0, HIGH); //Turn the motor off
  delay(4000);
  digitalWrite(0, LOW); //Turn the motor on
  delay(500);
}

So the electronics is all boxed up, and driven from a redundant mobile phone charger...

A few words of warning....

We're putting paper in here, which is being exposed to high temperatures for more time than it would be in normal use.... If it should jam, there's a risk it could smoulder and catch fire, and ruin your day... DON'T USE IT UNATTENDED!

It clearly states "Up to 80 Micron" on the box. We're really taking the P*$$ here, as my boards are 1.6mm thick, plus the paper (folded) so don't expect it to last ... but while it does .... get making!









... oh, and you can use the same idea to print a "slik screen"...