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#31 Re: Millett LR + TP Opus + AMB α24 reboot

Posted: Mon Sep 20, 2021 5:40 pm
by jack
Thermionic Idler wrote: Mon Sep 20, 2021 12:50 am
jack wrote: Sun Sep 19, 2021 10:50 pm I may have some genuine 797s in stock. I use them in low noise head amps for metal detectors.
If they're the DIP-8 ones Jack, you could be a lifesaver! :)
Take your pick. DM for details. Two options - just send me replacements when the become available again (I don't need the DIP ones any time soon) or fork out for the current Digikey UK price (about £14.20 each). Mouser is much the same.
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#32 Re: Millett LR + TP Opus + AMB α24 reboot

Posted: Mon Sep 20, 2021 7:34 pm
by Thermionic Idler
Cheers - I'll send you a PM.

#33 Re: Millett LR + TP Opus + AMB α24 reboot

Posted: Mon Sep 20, 2021 7:59 pm
by IslandPink
You could send him a DM, as he requested !

#34 Re: Millett LR + TP Opus + AMB α24 reboot

Posted: Tue Sep 21, 2021 9:50 am
by jack
IslandPink wrote: Mon Sep 20, 2021 7:59 pm You could send him a DM, as he requested !
I'd prefer a pair please. Size 13, G or H fit...

#35 Re: Millett LR + TP Opus + AMB α24 reboot

Posted: Tue Sep 21, 2021 11:20 am
by ed
overrated!
my last pair are now 31 years old and the inner sole has started to bulge, right under the arch of my toes....Pah!
manufacturer said buy a new pair but Mr Cheapskate would rather put up with the discomfort,
but I still wear them regularly to go slope soaring on the north York moors

#36 Re: Millett LR + TP Opus + AMB α24 reboot

Posted: Tue Sep 21, 2021 11:47 am
by jack
AD797s are actually quite old technology - from 1992 I believe... still a stupidly good audio op-amp though...

Interesting article (extract from Doug Self, Small Signal Audio Design - IMHO an excellent read):

https://www.eetimes.com/op-amps-in-smal ... roperties/#

#37 Re: Millett LR + TP Opus + AMB α24 reboot

Posted: Sat Sep 25, 2021 12:27 pm
by Thermionic Idler
Progress this morning. Got up bright and early, and pulled the AMB onto the bench to fit an additional Molex header connector to J8 and load the new software that I hacked about a couple of weeks ago. Bit nervous as I wouldn't be very popular if I broke it and left us with no living room audio.

Thankfully everything flashed onto the chip smoothly and the functionality I programmed worked perfectly. Input 1 = Analogue Input 1, Port 2 = Analogue Input 2 + 0V on J8, Port 3 = Analogue Input 2 + 5V on J8, Port 4 = Analogue Input 4 and so on.

There are a couple of very minor outstanding things I'd like to fix. When you do the initial setup, you have to tell it how many I/O and volume boards you have, and also which "ports" are configured as inputs and outputs. The default was 1x I/O and 1x volume board with 4 inputs / 4 outputs. I have double the number of boards because it's a balanced system, and they are wired with 6 inputs / 2 outputs. So I also tried to match the defaults in the code to the installed hardware - managed to do it successfully for the input / output config but I must have missed something on the number of boards, because it defaulted to 2x I/O and 1x volume board. Not a big deal as it can be corrected on setup, but it would be useful to fix that in case one of us accidentally re-initialises the EEPROM after too many G&T's (it has happened). The other thing I want to get rid of is the sleep function because it's completely pointless.

Also, a package arrived from my friend in the States containing: 3 Silentswitchers, the OTTO DAC source switcher module and the 4 Cinemag inductors for the LR phono stage.

#38 Re: Millett LR + TP Opus + AMB α24 reboot

Posted: Sat Sep 25, 2021 6:47 pm
by Thermionic Idler
I've just been examining the SilentSwitcher modules. The output voltage on the dual rail supply can be adjusted down from ±15V and I had originally thought about doing that, in order to run things a little more efficiently.

However, it seems you have to solder on 4 SMD resistors, in locations that are rather worryingly close to other somewhat delicate looking parts. The only SMD components I've ever soldered were the SOIC-8 opamps on the AMB linestage with no other components close by (they were the first things on the board), and that was enough for me.

I know Jan says they are huge for SMD components but I'm just not going there. These things cost a lot of money and were a hassle to get hold of. Everything I'm feeding with that dual rail supply has a standard operating range (not absolute max) up to at least 16V, so I"m going to leave well alone. I don't think it's going to impact battery life too much but I will need to do a recalculation.

#39 Re: Millett LR + TP Opus + AMB α24 reboot

Posted: Sat Sep 25, 2021 6:48 pm
by jack
I'm sure one of us who is happy with SMD stuff would do those for you...

But, honestly, it's not that tricky if you have a decent fine tipped iron and a steady hand...

#40 Re: Millett LR + TP Opus + AMB α24 reboot

Posted: Mon Sep 27, 2021 12:50 pm
by Thermionic Idler
I think, for the phono and DAC boxes, I'm going to go one better than some boring 'on' LED.

All I need for this are two small pieces of 2mm perspex, etched on the back with something like the following:

Image

These are then attached behind the front panel (in the same way that I have a piece of perspex over the 16x2 display on the preamp), and sidelit with orange LED's.

Just got to find a craft supplier that can supply such a thing...

#41 Re: Millett LR + TP Opus + AMB α24 reboot

Posted: Tue Sep 28, 2021 6:56 am
by Ray P
It wouldn't be cheap Dave but Schaeffer can do that sort of work.

#42 Re: Millett LR + TP Opus + AMB α24 reboot

Posted: Thu Sep 30, 2021 3:13 pm
by Thermionic Idler
I'm currently seeing if one of those wedding suppliers on Etsy can oblige - they do little 2mm thick pieces of perspex with nametags on and the like for dinner guest seating.

#43 Re: Millett LR + TP Opus + AMB α24 reboot

Posted: Thu Sep 30, 2021 3:33 pm
by Thermionic Idler
Completed the design for the power management system that'll sit inside the preamp along with the batteries. It'll also route the DAC source switch trigger signal.

Here's the schematic:

Image

the KiCad PCB layout (my first time generating a PCB from a schematic, managed to do it on two sides with no vias):

Image

and the 3D view from the top, it's easier to see the silkscreening from this and ensure everything is properly labelled. Not every part has a 3D model defined so there are a few gaps, but it gives you an idea:

Image

#44 Re: Millett LR + TP Opus + AMB α24 reboot

Posted: Sat Oct 02, 2021 10:01 am
by Thermionic Idler
Order placed to JLCPCB this morning - watch this space.

Now to roll up my sleeves and sort out those 300B operating points, which I meant to do back in July...

#45 Re: Millett LR + TP Opus + AMB α24 reboot

Posted: Tue Oct 12, 2021 8:39 pm
by Thermionic Idler
All the outstanding parts have now arrived from Digikey, Farnell and RS. PCB is still in transit, should be here in the next day or two. Tonight I breadboarded the battery monitor circuit with some of the newly arrived parts, to verify that reality matched the spice model - it did, precisely! :D

The picture shows the variable voltage being fed in from the Analog Discovery 2. The multimeter on the left shows the mA consumption of the circuit (which also matched expectations) and the one on the right, the voltage. The optimum cutoff of 7.43 - 7.44 volts was easy to dial in via the 12-turn trim pot and was well within the range of adjustment. The orange LED shows the output which will go to the SSR to activate the latch circuit that flips the relays over to the supply from the wall wart in the event of excessive battery depletion.

It is rather satisfying wrangling electronics, I have to say.

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