Fast Work - almost done!

[Oren]

As your process unfolds, I feel blonder and blonder...
Lotsa fun witnessing the build, however, and some fine day I may actually get a grip on what you've accomplished.

[folderol]

Thanks for sticking with me trooper

[folderol]

Things had been going well, perhaps too well.

The preamp board is built, but as there is a lot of gain around the opamps I've been populating it stage at a time looking carefully for any hint of instability. I'd just completed the tone controls and had needed, as I expected, to add a couple of extra caps to kill a bit of ringing when set for maximum treble. Controls were behaving really nicely with just the right amount of cut and lift.

By chance I shorted a makeshift attenuator I had hanging on the inputs, resulting in a massive overload that pushed the amp so far into limiting it went asymmetric. The good news is that the logic (now also fully connected) picked up the effective DC offset and disconnected the output relays as it should. Once I'd pulled the input, the relays snapped back again quite correctly.

The bad news is that there was now about 1/2 volt of really weird HF crap on the outputs. A quick check all round and voltages all seemed normal, so was the current on the output stages. Switched off, did some more checks and had a good look around, then switched on again and everything was back to normal. Deliberately overloaded the input and the same thing happened. What's more I found I could set this off with just a high amplitude pulse on the input. Now I was worried!

Disconnecting the preamp and going straight into the main amp inputs cured the problem, so I started to think it was a preamp stability problem, even though that seemed really unlikely. This actually tuned out to be a red herring. Eventually I thought about putting the scope on the supply rails. Everything was fine ... till I looked at the -12V rail which was going absolutely nuts!

If you look at the PSU drawing, you'll see the supplies are symmetrical, so why was just the negative regulator misbehaving? I carefully checked the wiring for silly mistakes and examined the decoupling capacitors but everything seemed OK. Then I just tapped an additional cap across the -12V line to ground. The oscillations instantly stopped. Thinking I had maybe a dud cap I replaced it, but the fault was still there. However, additional caps on just the negative line cured the fault.

Confession time!

I'd looked up the LM7812 data sheet to confirm the decoupling I needed, which was >= 330nF on the input and >= 100nF on the output, then just repeated that for the LM7912. Just a mirror image, right? Well, wrong! When I thought to look up that one's data sheet it had >= 2u2 on the input and >= 1u on the output.

I've left the 100nF caps in place and now added 10u caps on both + and - outputs. The thing is now perfectly stable no matter what I hit the input with. Only a couple of days wasted. Phew!

[Oren]

...live and learn - ideally...

[folderol]

Well first off I found another 'gotcha'. Wiring the sub signal and power channels together was a mistake. If the sub channel develops an overdrive fault it will switch off of course. This presents two problems. The first is that the normal speakers will now be handling the bass (and in some future setup that might not be advisable). The second is that with the routing changed, there will no longer be the signal condition that caused the fault, so after a second or two it will be reconnected. Rinse and repeat!

The advantage of having spare I/O comes to the rescue. I already had separate drivers for the relays, so all I needed to do was take another line back to the controller and make a minor software change. Now, when selected, the sub signal routing remains in place if there is an output fault registered, and just the output is disconnected with an appropriate warning on the display.

The following drawings have been updated to reflect this:
http://www.musically.me.uk/drawings/Logic.pdf
http://www.musically.me.uk/drawings/Control.pdf


A minor change has also been made to the preamp as I found there was an overall 3dB 'hump' at the crossover point when the sub channel was enabled. At the moment I've just lowered the value of two resistors to reduce the 'Q' of the filters, but I'm considering making this user adjustable too so that there is a better match to whatever speakers are employed.

New drawing is:
http://www.musically.me.uk/drawings/PreAmp.pdf


The board itself (without the 56pF caps I added later) is:
http://www.musically.me.uk/images/PreAmp_Bare.JPG


The idea of switched resistors for the headphone gain matching has been tested and seems to work reasonably well, but still isn't installed in the amp yet. I don't exactly have a huge range of phones, so there's a fair bit of guesswork involved, and (no doubt) later adjustment.

Finally here is a picture of the current state of the chassis:
http://www.musically.me.uk/images/Chassis_2.JPG


You can clearly see the new voltage regulators above the main reservoir caps. Its mounting plate is also the heatsink for the 4 voltage regulators, and is easily removable (hence the two cut-outs in the PCB. The logic is also pretty thoroughly shielded from the rest of the amp, with just very short leads to the display. Also, all the signal connections between the preamp board, controls & main amp are fully screened, and I did a quick crosstalk check which seemed to indicate better than -50dB @ 10kHz, and buried in the noise @ 1kHz. The screened wires to the input sockets tuck nicely into the groves in the side frame!

[Oren]

Looking like the real deal, Will!
As we are far out of my technical depth, may I compliment you on your soldering?...

[folderol]

Well, thank you my friend

[folderol]

Well, we've had a major shunt around at work, (where all this fun stuff gets done in my free time) and with the holiday period as well, things have rather come to a standstill

Our office is a very old converted granary. Since the shuffle I'm now one floor up in the building. which means more light and some nice countryside views.

Anyway, all my bits and pieces - gathered over many years - are in place, and I've actually got a little more floorspace than I had before. The bench itself is my pride and joy and has followed me around for about 12 years, even though the bosses keep saying I should have something more 'modern' (i.e. smaller).

 

[Oren]

Stand firm, and keep the bench, pardner. It's a dandy!

The room is premium, too... and some nice gear strewn about.
  Which reminds me - I've been meaning to buy a good digital multimeter soon. Any recommendations?

[folderol]

I intend to hold onto it until I eventually retire! Incidentally, it all unbolts and goes flat-pack
You know, I reckon if I were to suggest taking it with me in a few years time, I bet they suddenly find they can't do without it

Meters are pretty universally good for their price these days. I quite like the Tenma range which cover the price range from about 50ukp to over 100ukp. Mine is this one, but a cheaper 3.5 digit one will be more than adequate for most purposes.

[Oren]

Thanks!

I don't normally apply pet names to inanimate objects, but when the meter arrives at Casa del Fisher, I'm going to name it "folderol"...

[folderol]

Thanks!

I don't normally apply pet names to inanimate objects, but when the meter arrives at Casa del Fisher, I'm going to name it "folderol"...

Hmmm. I missed that

Anyway...

Well, so much for having more free time these days. I seem to find it harder to get to work on this project.

Sooo, we're approaching completion

First off, we have the completed phones resistor network. This gives approximately 6dB steps (depending on the actual phones) and for my AKG240s half way up the range gives a pretty close match to my speakers. The choice of values was a multiple compromise between power dissipation, S/C protection, impedance and level. In the drawing there's a greyed out inductor. If I can work out the correct value I'm debating being able to switch this in to approximate the behaviour of speakers.
http://www.musically.me.uk/drawings/Phones.pdf

And now and underside view. The resistor network is pretty obvious in the top LH corner. and the switching relay is in the tiny board to the right and below it. Further across is the Relay drivers board. The decisions about these were all rather piecemeal, and I should probably tidy it all up with a new board for everything. It works, so I probably won't! The three relays at the bottom are for the speakers. I changed from my original choice as these are half the price for the same rating.
http://www.musically.me.uk/images/Underside.JPG

I also decided to add in the ability to change the Q factor of the crossover point and this is shown on an updated pre-amp drawing. You'll see I've started putting in function names and component numbers - there was no point while everything was still in development. This is the sort of work I always hate doing.
http://www.musically.me.uk/drawings/PreAmp.pdf

The front panel layout was settled a long time ago, and I chose control knob positions and sizes to reflect their importance. This is likely to be the biggest headache now, actually producing a front panel overlay. I not only need to get the legends in place, but also to mask over the edges of the display and cover up an 'oops' (a hole in totally the wrong place!)
http://www.musically.me.uk/drawings/Layout.pdf

Some minor alterations to the software. In the picture below, the amp was switched on while there was a strong signal being applied to the RH channel. You can see it on the bargraph, but there is also an 'X' at the end giving visual indication that the output has been disabled. As soon as the level is dropped, the relay will click in and everything behaves normally.
http://www.musically.me.uk/images/Start_Hold.JPG

In a similar way it has been blocked due to a DC offset (I faked it with a resistor on the logic board). Once again if the fault clears everything returns to normal.
http://www.musically.me.uk/images/DC_Hold.JPG

So there you go, only six years to not quite finish

[Oren]

Damn, I'm so jealous!

Soon you'll have your own custom designed and constructed integrated stereo amp. That rates very high on the cool-meter...