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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, 12:42 PM   #91
Mooly is offline Mooly  United Kingdom
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A possible approach to adding a silent start/shutdown to the ACA
An unusual idea

A FET rail switch needn't be lossy. A P channel FET to switch a positive rail will only give a loss dependent on the Rds value and the load current. So absolutely minimal loss and power dissipation, even at several amps of load current.

I'm not sure on the cap idea... the rate at which the ACA output settles to 12 volt and the rate at which the caps virtual ground voltage settle will not be equal, and that will still generate a noise. You have to keep churning ideas around though
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Old Yesterday, 12:54 PM   #92
triode_al is offline triode_al  Netherlands
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Quote:
Originally Posted by Mooly View Post
An unusual idea

A FET rail switch needn't be lossy. A P channel FET to switch a positive rail will only give a loss dependent on the Rds value and the load current. So absolutely minimal loss and power dissipation, even at several amps of load current.

I'm not sure on the cap idea... the rate at which the ACA output settles to 12 volt and the rate at which the caps virtual ground voltage settle will not be equal, and that will still generate a noise. You have to keep churning ideas around though
You are right. I was bound with my thought the Vgate needed to come from the power rail itself.
The two caps solution - often used in power amps to ensure zero DC load & - I am also using this idea in a MC amplifier.
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Old Yesterday, 01:03 PM   #93
Mooly is offline Mooly  United Kingdom
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A possible approach to adding a silent start/shutdown to the ACA
If you are interested in the virtual ground arrangement then have a look at the Quad 606 power amp. That is a 'DC coupled' amp using the junction of two series connected reservoir caps in the power supply as a virtual ground. The Quad achieves the correct DC balance between the caps not with resistors but with two small signal transistors.
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Old Yesterday, 02:02 PM   #94
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 triode_al View Post
... {in a conventional capacitance multiplier circuit} a FET does give at least 4 volts loss though, so maybe a BJT is better suited slooking at voltage drop and internal resistance. The result is that you can suse a 24V LED rail SMPS they cost only several bucks.
Since ACA uses a single ended positive supply, you can build your capacitance multiplier with a "JFETlington" pass transistor, which uses the still-active-production J105 or J107 Nchannel JFET. (deep depletion PJFETs are out of production but ACA has no negative-voltage power supply). And thereby get very low input-to-output "dropout" voltage.

Because there's no base current into the JFET, you can use a high value bias resistor (unlike a BJT cap multiplier) and thus get filtering with a very low corner frequency. Because the JFET operates with its gate voltage significantly less than its source voltage, you can conveniently arrange a simple resistive divider with a multiturn trimmer pot, allowing you to slooooowly dial in the exact voltage drop you desire.

It's all thru-hole, it's all on the shelf and available at Mouser or DigiKey.
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Old Yesterday, 04:19 PM   #95
triode_al is offline triode_al  Netherlands
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Originally Posted by triode_al View Post
The two caps solution - often used in power amps to ensure zero DC load & - I am also using this idea in a MC amplifier.
Forgetaboutut.
In the mean time I ran the simulation (zener is 22v, not in my database; had to use 10+12V); and I used 2SK170. There is a large plop in the output.
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Old Yesterday, 05:34 PM   #96
Michael Bean is offline Michael Bean  United States
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How about something like this:

ACA SPKR DELAY.JPG

Simple, easy and reliable. The one possible negative is the cost of the depletion mode MOSFETs, they are a little on the pricey side.

Mike
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Old Yesterday, 06:51 PM   #97
Mooly is offline Mooly  United Kingdom
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A possible approach to adding a silent start/shutdown to the ACA
That's thinking outside the box

I'd bet you could even dispense with the timer and just use a single FET + R & C to drive the couplers.

(555's can be so cranky sometimes triggering when you don't want as rails rise and fall)
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Old Yesterday, 08:19 PM   #98
Michael Bean is offline Michael Bean  United States
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Quote:
I'd bet you could even dispense with the timer and just use a single FET + R & C to drive the couplers.
Yeah, I originally did try that, but to minimize switching times to keep the MOSFET's power dissipation at the switching transition to a minimum, I decided to go with the 555. I've never had any problems, and in many years of use it's been 100% reliable.

Mike
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