• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Tube Amp compatible A/B(/C) switch design considerations

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#22
kodabmx said:
I would love to learn how to switch with a microcontroller and pushbuttons...

Care to share more info?
Click to expand...
Thanks for the question, it actually reminds me that I forgot to add the pushbutton which triggers the switching action 🙂

Your question is quite generic so I’m not sure what specifically you want to know, but once I make up my mind, I plan to release all the design items including any SW, schematics, and PCB design I will make.
 
#23
My requirement is I would like to use a microcontroller to control relays like you are doing, but I want to switch ground, not B+ (My friend tells me that's called "Active Low"). I have no idea where to even start but I do have an Arduino UNO
I'll also need to use pass transistors because the relays are 12 volts.
Adding IR support shouldn't be hard after the rest is working as I understand it.
 
#24
Aviv said:
After some thought, I tend to go for the uC controlled relays based design, as it gives much flexibility when it comes to switching sequencing.
Click to expand...
👍
Aviv said:
I plan to release all the design items including any SW, schematics, and PCB design I will make.
Click to expand...
👍 Please do so, I would like to learn how to switch with a microcontroller! My MBB switching method is 1930s technique. 😆
 
#25
kodabmx said:
My requirement is I would like to use a microcontroller to control relays like you are doing, but I want to switch ground, not B+ (My friend tells me that's called "Active Low"). I have no idea where to even start but I do have an Arduino UNO
I'll also need to use pass transistors because the relays are 12 volts.
Adding IR support shouldn't be hard after the rest is working as I understand it.
Click to expand...
Hi kodabmx
It might pay to look up a very old chip still in production and is produced in SMD package if required, a single chip will drive 7 relays.
https://en.wikipedia.org/wiki/ULN2003A
https://www.diodes.com/assets/Datasheets/ULN200xA.pdf
https://microcontrollerslab.com/uln2003-introduction-pinout-example-features/

I would be tempted to use a PIC with enough outputs to drive two ULN2003's enabling up to 14 relays to be operated, the PCB could have a with very low component count. If a simple 4 line by 20 characters per line display is used the entire PCB component cost could be under $20.
 
#27
Tubekwk said:
Hi kodabmx
It might pay to look up a very old chip still in production and is produced in SMD package if required, a single chip will drive 7 relays.
https://en.wikipedia.org/wiki/ULN2003A
https://www.diodes.com/assets/Datasheets/ULN200xA.pdf
https://microcontrollerslab.com/uln2003-introduction-pinout-example-features/
Click to expand...
Great chip, which was used in many industrial devices. If the uC does not have enough i/o, it is possible to use an i/o expander or a shift register with latched outputs. We used these tricks many times with old uCs.
 
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#28
Regarding interesting ic's I have driven 45 x 12 volt relays using some TPIC6C595's, the ic is a shift register with extra components that can drive relays directly, the maximum output current is 100mA per drain, one needs to watch the chip dissipation if many relays on at once at maximum current.
My project drove 45 relays from a single port on a dsPIC. https://sites.google.com/view/kenstubetester
I would not recommend this for such a simple project unless the micro only had 3 pins to spare.
https://www.ti.com/lit/ds/symlink/tpic6c595.pdf
https://forum.arduino.cc/t/what-is-the-best-way-for-controlling-array-of-8-16-relays/123332/4
Ken K
 
#29
Neat project.

Beware that the speaker represents a reactive load. It may not respond in kind to suddenly having its current interrupted. In fact, if the speaker acts inductively it can fry the switch contacts as they open. For this reason I would use MOSFETs for the switching.

Using MOSFETs also has the advantage that the switching can be quiet. This means fewer auditory cues that could sway your senses in an ABX test. A pair of MOSFETs with a photovoltaic gate driver would be a good option - and probably lower cost than a good power relay too. Switching using MOSFETs would also allow you to switch at the zero crossing of the signal so you don't get large clicks and pops when you switch.

I love the uC control. I suggest that you in addition to a manual A/B mode include an ABX mode, where X is chosen randomly for, say, 15-20 trials. Only once the test is over do you get to know what the random sequence was. It still wouldn't be a blind ABX test. After all you do know that something is being tested, but at least it would be ABX. I think that could be revealing.

If you want compatibility with all solid state amps you should switch both OUT+ and OUT-. That'll allow the box to work with bridged amps as well.

Tom
 
#30
+1 for ULN2003 (or the 3.3 V compatible ULN2003LV). Sadly, others think it's a great chip too so it's out of stock these days. You may get lucky if you pick one of the larger packages, though. If you need to drive many relays, Maxim has some interesting relay driver options that allow you to control the relays via SPI or bit pattern. That'll save some I/O lines. Or ... Just get a bigger uC.

Tom
 
#33
16 relay pcb for $11.03
https://www.ebay.com/itm/1433951334...5j239ru4kUBwOPFBUZnDG+wcbw==|tkp:BFBMmuGa1eJg

BTW a FET will only switch DC so for the AC audio signal two FET's will be required for each switch, a floating gate drive circuit will be required for each switch, when "off" considerable gate source capacitance will still be present, when "on" there will be the resistance gate-source Rds(on) by two is series with the speaker.
 
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