AMB linestage rebuild

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Ray P
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#31 Seal of Approval

Post by Ray P »

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Sorry, I couldn't resist!
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Thermionic Idler
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#32 Re: AMB linestage rebuild

Post by Thermionic Idler »

I don't think that front panel needs anything more. I've revised it to use a less bulky-looking power / config switch from the same range as the oval ones and that looks a lot better. I tried putting characters next to / around the volume control and it just didn't look right. With this - there's everything you need and nothing more. Still not a perfect match to the Quad, as that was physically smaller, used square buttons on the left and didn't have a vintage RCA-style control knob. But I think it works.

Image

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Deck: Garrard 301 - Audio Origami PU7 - NW Analogue DH3S
Phono: Pete Millett LR, Lundahl SUT, AMB σ22 PSU
Linestage: Bruno Putzeys Balanced Pre + Hans Polak relay mod, LCDuino control
Power amps: 300B PP 'Symmetric Reichert'
Speakers: FF225WK Big Mets
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Thermionic Idler
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#33 Re: AMB linestage rebuild

Post by Thermionic Idler »

With the space saved, I've confirmed in CAD that there is now enough space for separate left and right power supplies to the linestage board. So of course I'll be implementing that, because I'm me.

I think this is going to be very nice once it's done.

I did have a small concern as to how I was going to interface the front panel buttons / LEDs, but I then found that the PCF8574 port expanders that are used for the relays, can be configured for input or output. One chip should be sufficient, and I can just hang it on the same I2S bus that runs the relays. Then it's just a matter of programming. The software just has to interpret a front panel button press the same as the equivalent from the remote, and light up the appropriate LED.
Deck: Garrard 301 - Audio Origami PU7 - NW Analogue DH3S
Phono: Pete Millett LR, Lundahl SUT, AMB σ22 PSU
Linestage: Bruno Putzeys Balanced Pre + Hans Polak relay mod, LCDuino control
Power amps: 300B PP 'Symmetric Reichert'
Speakers: FF225WK Big Mets
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Thermionic Idler
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#34 Re: AMB linestage rebuild

Post by Thermionic Idler »

Just thought I'd post a quick sit-rep. This thread has been rather quiet of late, not because of lack of activity, in fact quite the opposite. I've been completing the required PCB design work in KiCad for 5 other PCB's:

1) Relay driver board, this interfaces to the existing LCDuino PCB and has the port expanders and darlington arrays on it. I've based it heavily on AMB's design but modified to run the relays as configured on the linestage board.

2) Front panel input selection switch / LED PCB - again this connects to the I2C bus to do its stuff

3) Front panel power button PCB - very simple, just a push button switch and connector

4) Power relay for the linestage with proper 'creep' distance between high and low voltages

5) Soft-start power switching board for each 300B amplifier based on the Rod Elliott design.

In addition to this, I've been doing a lot of hacking of the software to 'fit' my new design, taking out a bunch of stuff that isn't needed (sleep timer?? really??) and other general refinements. I actually bought an Arduino Uno and PCF8574 breakout boards so that I could check my coding for the input selection using a breadboard.

The software is done and tested as far as possible, and the order for the PCB's went off to JLCPCB yesterday.

Now I'm working on the BOM for it all and finding out how many of the parts I specced are now unobtanium because of supply chain issues...
Deck: Garrard 301 - Audio Origami PU7 - NW Analogue DH3S
Phono: Pete Millett LR, Lundahl SUT, AMB σ22 PSU
Linestage: Bruno Putzeys Balanced Pre + Hans Polak relay mod, LCDuino control
Power amps: 300B PP 'Symmetric Reichert'
Speakers: FF225WK Big Mets
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Thermionic Idler
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#35 Re: AMB linestage rebuild

Post by Thermionic Idler »

All parts now purchased, present and correct.

I've got some PCB's to populate...

Image

So to recap, this is Bruno Putzey's balanced preamp (but with OPA1612 / OPA191 instead of LM4562 / TL072), with Hans Polak's relay volume control modification, designed to work with latching relays and interface to a modified version of AMB's LCDuino control software. Integrating the preamp circuit, input selection and volume control onto one PCB saved enough space in the chassis to have dual mono power supplies (my favoured AMB sigma 22, bottom left). A lot of the time I've been away from this forum was being spent designing and laying out these PCB's (except for the power supply boards which were purchased from AMB), and reprogramming the software.

Actually the two red boards at the top aren't for the linestage, they are a modified version of Rod Elliot's softstart circuit that I had in the Beasts which are to go in the 300B's. I can still occasionally detect a slight transformer 'thump' when the monoblocks turn on, so I think it would be kinder to the transformers to have some current limiting for the first few cycles.

Note these are actually the same board design, they are just screenprinted on both side and designed in a way that all the components except the op-amp can slot in on either side. This allowed them to be mirror-imaged like the amps are. Also the remote triggering is jumper-selectable so they can be set to allow switch-on independent of the preamp if need be.

They will replace this mess here, which is the part of the 300B build I was least happy with:

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The tracks on the relay board with mains AC run way too close to the low voltage trigger path for my liking. Also the screw terminals in that grey block are not as tight and secure as I'd like them to be. The boards will have better quality Phoenix connector blocks on them with enough connection points for the 4 transformers.
Deck: Garrard 301 - Audio Origami PU7 - NW Analogue DH3S
Phono: Pete Millett LR, Lundahl SUT, AMB σ22 PSU
Linestage: Bruno Putzeys Balanced Pre + Hans Polak relay mod, LCDuino control
Power amps: 300B PP 'Symmetric Reichert'
Speakers: FF225WK Big Mets
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Nick
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#36 Re: AMB linestage rebuild

Post by Nick »

The tracks on the relay board with mains AC run way too close to the low voltage trigger path for my liking.
6mm is required for Class II
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Thermionic Idler
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#37 Re: AMB linestage rebuild

Post by Thermionic Idler »

Nick wrote: Fri May 20, 2022 10:25 am
The tracks on the relay board with mains AC run way too close to the low voltage trigger path for my liking.
6mm is required for Class II
I allowed 8mm, so all good! :)
Deck: Garrard 301 - Audio Origami PU7 - NW Analogue DH3S
Phono: Pete Millett LR, Lundahl SUT, AMB σ22 PSU
Linestage: Bruno Putzeys Balanced Pre + Hans Polak relay mod, LCDuino control
Power amps: 300B PP 'Symmetric Reichert'
Speakers: FF225WK Big Mets
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Thermionic Idler
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#38 Re: AMB linestage rebuild

Post by Thermionic Idler »

There's been something of a hiatus on this project of late, but I decided to use the Xmas break to see if I could get it a bit closer to the finishing line, if not over it.

I already posted CAD pictures of the design, finally here are some real life pictures.

Some progress so far:

Here's the front panel fitted along with the input selection and power switch PCB's:

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I couldn't resist connecting up a 12V supply to the array of LEDs that side-light the etched perspex behind the panel that form labels for each source. This is the 'powered on' look:

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The grey on the panel looks a lot lighter in the pictures than it does in real life - I think the actual colour is RAL9004 which is quite a dark grey.

Here's how the perspex is lit 'behind the scenes':

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PCB from the back:

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The rest of the PCB's bar the one that will sit vertically at the back have now been mounted. Tomorrow's plan is to load up the new software written earlier in the year, and run some initial tests on the control circuits. The bar LEDs you can see are 'telltale' for the pulses being sent to the latching relays.

Image
Deck: Garrard 301 - Audio Origami PU7 - NW Analogue DH3S
Phono: Pete Millett LR, Lundahl SUT, AMB σ22 PSU
Linestage: Bruno Putzeys Balanced Pre + Hans Polak relay mod, LCDuino control
Power amps: 300B PP 'Symmetric Reichert'
Speakers: FF225WK Big Mets
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IslandPink
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#39 Re: AMB linestage rebuild

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We are not worthy !
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Thermionic Idler
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#40 Re: AMB linestage rebuild

Post by Thermionic Idler »

Not a huge amount of progress today due to post New Year hangover. However, I did attach Molex connectors to each of the two 12V transformer secondaries, so that I could connect them to the 5V and 12V regulated supplies (small PCB's to the right of the transformer) and a temporary IEC male in-line connector to the primary for testing. The 5V supply powers the Arduino and port expanders, the 12V supply is for the relay coils.

I also flashed my new software onto the chip, I had originally finalised it back in April but, although bits of the new code had been tested in isolation on a breadboard with an Arduino Uno, it hadn't ever been run in its complete form. Testing it powered from the USB TTS cable with the port expander board disconnected was mostly successful, but I did discover a bug (funny how you think you've thought of everything until the testing stage).

I'd coded it to ignore signals from the IR remote if anyone was pressing the panel buttons. Now, with the port expanders connected and no buttons pressed, the value returned from the PCF8574A will be binary 11111111 or 255. So I'd coded it such that if the value was < 255, ignore the remote.

But... if you've disconnected the I2C connection to the board, the value returned to the software is zero, not 255. You can see where this is going. I was rather perplexed that after initialisation it seemed to 'learn' the remote OK, but then didn't respond to it, until I realised what was going on. So I had to code an additional case for a returned zero value, to treat it the same as 255, so that if something goes wrong with the panel buttons, the remote isn't locked out.

Tomorrow's task - test the 5V and 12V power supplies (should be ok as they were used in my last linestage), then test the LCDuino with the port expander / relay driver and front panel PCB connected.
Deck: Garrard 301 - Audio Origami PU7 - NW Analogue DH3S
Phono: Pete Millett LR, Lundahl SUT, AMB σ22 PSU
Linestage: Bruno Putzeys Balanced Pre + Hans Polak relay mod, LCDuino control
Power amps: 300B PP 'Symmetric Reichert'
Speakers: FF225WK Big Mets
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Thermionic Idler
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#41 Re: AMB linestage rebuild

Post by Thermionic Idler »

OK - so the control circuit for the volume control is working as designed, the one for the input selection is not. The telltale LEDs on the PCB should light up when the set / reset 'pulse' is sent to the relays, this does not happen. Instead the panel illumination LEDs fed by the 12V supply seem to dim slightly instead, indicating that during the 'pulse', some heavy load is being placed on the 12V supply.

I'm a bit stumped at this point, so I'm going to start a separate thread for the problem in the Beginners section as that'll have more visibility, that will have more detail.
Deck: Garrard 301 - Audio Origami PU7 - NW Analogue DH3S
Phono: Pete Millett LR, Lundahl SUT, AMB σ22 PSU
Linestage: Bruno Putzeys Balanced Pre + Hans Polak relay mod, LCDuino control
Power amps: 300B PP 'Symmetric Reichert'
Speakers: FF225WK Big Mets
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Thermionic Idler
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#42 Re: AMB linestage rebuild

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So not-so-happy days - we've proved the issue is the paralleling of the inputs to the darlington array from the port expander - seems that if you do that with these particular chips, the darlingtons don't trigger.

Image

Not an issue as each one can handle 500mA, and the relay coils require a fraction of that, so connecting two to one darlington pair shouldn't be an issue. However, that does mean a redo of the PCB is required.

In addition, I've decided to raise the 12V supply to 15V. Here's why.

This is the equivalent circuit (courtesy of AMB) showing how the relay coils are driven:

Image

The relays are activated by either U2+ or U2- closing a connection to ground, causing current across either R2+ and the relay coil AND R-, or ther other way around. The bit in bold in my last sentence is what I missed when I was resizing R2 for 12V operation, and I selected a resistance with a view to not cause too much voltage drop across the relay coil. Unfortunately, it means heavy pulses of current are drawn from the other side where 4 or 5 relays are flipped at once. (360mA * 5 = 1.8amps).

Had I thought it through properly, I'd have bought 9V relays but I'm committed now - those were a significant outlay and as I have to redo the driver PCB anyway, it's easier to raise the rail voltage to 15V. That way R2 can also be raised significantly and the 'waste' pulse currents should be reduced dramatically.
Deck: Garrard 301 - Audio Origami PU7 - NW Analogue DH3S
Phono: Pete Millett LR, Lundahl SUT, AMB σ22 PSU
Linestage: Bruno Putzeys Balanced Pre + Hans Polak relay mod, LCDuino control
Power amps: 300B PP 'Symmetric Reichert'
Speakers: FF225WK Big Mets
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Thermionic Idler
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#43 Re: AMB linestage rebuild

Post by Thermionic Idler »

Revised PCB design completed and sent off to JLCPCB. This time I made it a four layer board and revised some bits of the layout which made things a little more elegant.

There is something quite lovely about a well-laid out circuit board.

Image

Changes
- 33R current limit resistor arrays replaced with 200R on the input stage (where relays are connected in parallel) and 390R where it's one relay per darlington pair
- 12V rail upped to 15V
- Added in-line voltage drop resistor to connection for power relay which has a 12V coil
- Added another in-line resistor to the power line to the perspex illumination LEDs, both to allow for a 15V rail and bring down the retina-searing brightness a tad (LED strings dropped to 5mA)
- Paralleled 2 relay coils per darlington pair for L/R input selection instead of paralleling the TTS signals
- A few other miscellaneous things I noticed could be improved.

Watch this space...
Deck: Garrard 301 - Audio Origami PU7 - NW Analogue DH3S
Phono: Pete Millett LR, Lundahl SUT, AMB σ22 PSU
Linestage: Bruno Putzeys Balanced Pre + Hans Polak relay mod, LCDuino control
Power amps: 300B PP 'Symmetric Reichert'
Speakers: FF225WK Big Mets
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Nick
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#44 Re: AMB linestage rebuild

Post by Nick »

Did you not try my suggestion of changing the value of the pull up resistors? I still think 47k is wrong even in the case of single input single output. Would/might have avoided a new board.
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Nick
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#45 Re: AMB linestage rebuild

Post by Nick »

Added in-line voltage drop resistor to connection for power relay which has a 12V coil
I would have been tempted to use a 3v3 zener.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
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