Switching speakers on the same amp in real time

[CraigGa]

Have fun, make sure you get a 5V relay module not a 12V one, it makes controlling it much easier.

[pjd3]

Hi Craig, I've been wondering about which control voltage to go with for the relays.

There are the little ones to switch out preamp channels in the chassis, but then there would be the ones that would be responsible for switching the speakers. I had to really wonder if the relays switching the speakers would benefit from the 12 volt relays as the current being switched may be much higher and benefit from a more robust contact switching.

The preamp relays are only switching a few milliamps or so and figured 5 volts would be fine in there.

Do you think there is any advantage to ever using a higher voltage controlled relay?

Thanks,

PJD3

[R.G.]

I can really feel my "greenness" on all things microcontroller on little systems like this.

I did have some great results with producing a stealthy MBB with logic gates and relays but it would be satisfying to make that happen with microcontrollers. Maybe its time to grab and Arduino and get down to it.

An Arduino Nano ought to be fine to start with. It's capable enough. You won't get microsecond-level timing, but then you don't need it for this. Tens to hundreds of milliseconds are fine, and even a few hundred milliseconds of overlap on the speakers would probably be unnoticeable to an audience.

The nut to crack is the programming language and the programming tools. When you say "arduino", you're mostly implying programming in a C++ variant. I've been writing code since the 1970s in Fortran(ugh!), Basic, Pascal, APL(!?), and now arduino's C++. If you have had any programming at all, you can work your way in by just writing down pseudocode, then transcoding the pseudocode into the language at hand, then compiling out the syntax errors. This isn't elegant, and pro coders will laugh at you for it as they did at me, but it can get you up and running. If you have had zero programming before, you'll need to work your way through some of the beginner arduino tutorials for a while. But the body of arduino tutorials is large, and some of them are very good.

What I'd like to get under my belt now is how and where to measure a flyback or inductor induced spike on a tube amp. I suppose the EE's at work could help quite a bit with that but, they are often busy and I feel like I'm bothering them. But yeah, I may have to create a thread sometime focusing on that topic and see if we could gather some ideas or approaches on how to look at that say on a scope, and based on what you see, how to go about sizing up components to address the issue.

Good intuition - first you need to be able to see it. The biggest issues are using a 'scope that can see transients fast enough, and getting/using a high voltage probe. If you've used scopes before, you're probably familiar with 1X and 10X probes. For flyback pulses on tube amps you'll probably need a 100x or more scope probe, and it needs to be precision-enough and compensated well enough to get an accurate picture. If you work with EEs you ...might... be able to convince one to show you the secret stash of scope probes and the one dusty, unused kilovolt-capable one in the back of the instruments closet. It depends on the EEs and what they are designing. If they're all logic/controller and I/O device designers, they may have very little experience with high voltages. If they're designing AC mains stuff, they will be more familiar with measuring high voltage transients and have a high voltage probe.

You can make/fake a high voltage probe with high value resistors and some compensating caps, but it can be tricky to get clean scaling and high frequency (transients are spikes, after all) compensation. A scope probe is a resistive divider paralleled by a capacitive divider. The input to the scope is also a resistor divider, with the scaling resistors selected by the range knob or setting.

A 1X probe is a shielded wire paralleled by the capacitance of the wire to the shield braid. 1X is uncompensated, meaning that at high frequencies the frequency response is dominated by the ratio of the parasitic caps more than the resistors. Compensation is the process of tweaking the capacitive divider so that leading and trailing edges of a FAST square pulse look square, not rounded or peaked and ringing. The calibration output on hard 'scopes is there for this, with a custom designed square wave so you can tweak til it looks square on the display 10X probes were designed so that there is a series resistor in front of the 1X wire inside, with a cap parallel to the series resistor and a variable cap on the hidden 1X intput so you can tweak the capacitive part of the division too.

Make/faking a 100x or more probe involves making a capacitor-compensated resistor divider to go ahead of a 10X (mostly) probe to give good-enough frequency compensation. This is the only way to get accurate leading/trailing edges and frequency response on a fast signal.

More if you want it.

[CraigGa]

I was referring to these modules
https://www.ebay.co.uk/itm/255089970590 ... 5071261696

The contact ratings are the same for 5V or 12V but 5V boards are easier to interface with a microcontroller which typically run from 5 or 3.3V

Craig

[pjd3]

Thank you R.G. and Chris.
I work at Zoll Medical that is all about defibrillators and high fast voltages. We have a very good selection of fast scopes (mine at home may be too slow) and high voltage probes. As a matter of fact, I was just on a project dealing with the handful ESD suppression diodes about the units (they were getting stuck in conduction mode). Everyone is pretty relaxed and helpful for the most part. And the Director of EE is just getting into guitar. We talk about that stuff all the time so, he would likely be on board and even interested in this quest. I'll have to chip away at this somehow! I would really be interested to see what happens when speakers are switched with, and without signal going to them. I imagine the approach would be to start with a very low signal and work up while single shot scoping the target places, which I assume would be prim and sec of the OT for one.
Hey Chris, those look really interesting. I took a short look but need to go back for a more in-depth look.

Thank you all for hanging around for this topic! I'm really hoping to make this work safety and repeat-ably one day.

Best,

Phil