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A possible approach to adding a silent start/shutdown to the ACA
A possible approach to adding a silent start/shutdown to the ACA
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Old Yesterday, 02:18 PM   #61
Mooly is offline Mooly  United Kingdom
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A possible approach to adding a silent start/shutdown to the ACA
Digital logic with its normally hard and fast rules really is a different ball game compared to analogue and all its intermediate states.

I'm not rally sure what to make of it tbh (hint... if you want to simulate using a sine wave output then selecting a much lower frequency of say 100Hz will speed the simulation time up no end)

The goal in the simulation is to get a flat line output when no signal is applied, meaning that the speaker sees absolutely no voltage change. That doesn't happen I'm afraid.

You also seem to have the same issue that I keep getting in that the simulated relays/switches etc allow the turn off pulse through. Some hazy thought has just appeared to me... is it because the rail has disappeared and the switch needs the supply present in order to work in the simulation i.e does the switch have zero resistance with no supply present. I wonder.
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Old Yesterday, 02:20 PM   #62
Mooly is offline Mooly  United Kingdom
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A possible approach to adding a silent start/shutdown to the ACA
Quote:
Originally Posted by wayne View Post
Just use it balanced, more than 2X the power, lower distortion and the thump is cancelled at the speaker by appearing equally on both sides.
That's a fiddle though. Any amplifier should (in my book) be silent at power on and power off. Its all part of the user experience that makes a product a delight to use.
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Old Yesterday, 03:58 PM   #63
Mark Johnson is offline Mark Johnson  United States
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A possible approach to adding a silent start/shutdown to the ACA
Quote:
Originally Posted by Mooly View Post
... is it because the rail has disappeared and the switch needs the supply present in order to work in the simulation i.e does the switch have zero resistance with no supply present. I wonder.
If you use the NC (Normally Closed) contacts of the relay to connect an X ohms resistor across the speaker, then the job of the control electronics is to de-energize the relay coil at power-off. Which is easy to do even with a collapsing power supply.

To simulate this using the LTSPICE voltage controlled switch as a handy substitute for a relay: connect the switch control circuitry to the always-present +24V output of the SMPS, upstream of the front panel ON/OFF switch.

If that disgusts you, build a little subcircuit in LTSPICE that implements a current controlled switch, using the LTSPICE voltage controlled switch and a small number of additional components. It might be as simple as a single resistor and an LTSPICE voltage controlled switch, with cleverly chosen switching threshold (model parameter Vt). This current controlled switch does not need any supply rail to operate correctly; when the input current is sufficient, the relay closes. By the way, that's how real-world relays work too! When coil input current is sufficient, the electromagnet's field is now strong enough to pull the armature.

One way to simulate NC relay contacts is to swap the + and - control voltage input pins on the LTSPICE voltage controlled switch, and change the Vt parameter appropriately.

Another way to simulate NC relay contacts is to define "Ron" to be 66 megohms and "Roff" to be one milliohm. Aha, now open means closed and vice versa.

Last edited by Mark Johnson; Yesterday at 04:01 PM.
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Old Yesterday, 05:59 PM   #64
wayne is offline wayne  United States
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A possible approach to adding a silent start/shutdown to the ACA
Quote:
Originally Posted by Mooly View Post
That's a fiddle though. Any amplifier should (in my book) be silent at power on and power off. Its all part of the user experience that makes a product a delight to use.
Very true not acceptable in commercial product. Some people put up with it but I wouldn’t, especially at some of today’s prices. I think the mono ACA is a cute package though.
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Old Yesterday, 06:09 PM   #65
Mooly is offline Mooly  United Kingdom
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A possible approach to adding a silent start/shutdown to the ACA
Quote:
Originally Posted by Mark Johnson View Post
Another way to simulate NC relay contacts is to define "Ron" to be 66 megohms and "Roff" to be one milliohm. Aha, now open means closed and vice versa.
Thanks Mark That's actually the way the sim is set up. I have found some weirdness in the simulation that I need to try and figure out. The relay works as expected with a static control voltage, however if I source the control voltage from a 'pulse generator' set to unmute at say 12 seconds then I find the relay doesn't work even though the control voltage has flipped.

Crazy thought... its behaving like a latching relay. Hmmm... is that possible ?

I've actually posed this relay question over in the LT thread so I'll see what develops.

Quote:
Originally Posted by wayne View Post
Very true not acceptable in commercial product. Some people put up with it but I wouldn’t, especially at some of today’s prices. I think the mono ACA is a cute package though.
Very cute, yes. It would be great if this noise aspect of its operation could be sorted easily though.
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Old Yesterday, 06:27 PM   #66
tomchr is online now tomchr  Canada
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Quote:
Originally Posted by Mooly View Post
One quirk of the hugely popular ACA that seems to regularly get a mention relates to the strange noise it can make at power on and the loud thump that occurs at switch off. This kind of behaviour is actually 'normal' for pretty much any AC coupled amplifier.
Yep. The output cap has to fill with charge somehow. Unfortunately, the charging current flows through the speaker.

Quote:
Originally Posted by Preamp View Post
Just a quick thought while skimming through your thorough work: What about putting that solid state relay in parallel with the speaker and shunt that thump to ground?
That might work in a tube preamp where the output current is limited. In a solid state power amp like the ACA, you'll end up with full signal across the cap if someone turned on the source while the relay was closed. I doubt either the cap or the amp would appreciate that.
I suppose you could add a mute relay on the amp input, but it may be worthwhile to take a step back and look at which problem you're trying to solve exactly.

Quote:
Originally Posted by Preamp View Post
I for one would not like to wait half a minute for my amps to come on... (well, I'm already annoyed by two seconds turn-on delay )
In that case, an AC coupled amp may just not be for you. Or you'll have to accept the turn-on/off thump.

It's perfectly possible to build a speaker protection / turn-on thump eliminator circuit with MOSFET switches and floating photovoltaic drivers. That's what I did in my Guardian-86. I wasn't able to measure any meaningful difference in THD vs output power between the output of the amp and the output of the speaker protection circuit.

I would shy away from mechanical relays. As this guy demonstrates, they're not great for speaker protection due to their tendency to arc over, which causes contact wear (and resulting distortion) and may not actually protect the speaker. I'd certainly perform thorough testing on a batch of relays before committing to use one in production.

Tom
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Last edited by tomchr; Yesterday at 06:30 PM.
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Old Yesterday, 06:42 PM   #67
Mooly is offline Mooly  United Kingdom
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A possible approach to adding a silent start/shutdown to the ACA
With many thanks to Hans Polak over in the LT thread and a little tweak of the switch simulation parameters and we have a great result. This is how the real relay would behave.

(I still don't understand why the simulated relay isn't behaving as I expect but that's one for tomorrow probably)

Anyhow, this simple version should give a silent start up and shutdown and is non invasive to the ACA.
Attached Images
File Type: jpg Capture7.JPG (280.3 KB, 25 views)
Attached Files
File Type: asc ACA1 plus Shorting Speaker Relay Simple Version.asc (5.2 KB, 3 views)
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Old Yesterday, 11:48 PM   #68
Old'n'Cranky is offline Old'n'Cranky  Australia
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No idea if this is a valid thought or not...

What if the 24vdc power supply was slowly ramped up and down over say a few seconds ?
I imagine you'd still have the oscillation etc, but wouldn't they be much reduced due to the power supply being lower ?
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