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Old 11th March 2010, 05:11 PM   #1
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Default Install a switch to prevent turn on hum

Can I install a DPDT to prevent turnon thump when i turn on my LM1875T amp - that way, i can turn off the speakers when i first turn on the amp and flip the switch to engage the speakers after the amp is on to prevent the thump noise. If so can someone please show me how to connect the thing. Sorry for the dump question as i am not much of a tech. Thanks.
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Old 11th March 2010, 06:26 PM   #2
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Yes, you can. The COM pins should go to the amplifier's outputs and the NO pins to the speakers.

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Old 11th March 2010, 07:15 PM   #3
tomchr is offline tomchr  United States
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You could use an output relay and have it turn on a short time after the supplies come up. See attached as an example.

I don't know what rail voltages you're using. I assume something in the +/-20 ~ +/-30 V range. If that's the case, skip R1 and use a relay with 24 V coil. You might have to tweak the resistors a bit to get the delay you want. The idea is that the BC327 transistor turns on when the supply voltage exceeds 15.7 V (zener voltage + Vbe), thereby, turning on the relay. The relay will turn off quickly after you turn off the amp as it's on its own separate supply created with the 1N4004 diode and 1000 uF cap.
If you get a thump at turn-off, you can make the relay drop out faster by connecting a diode across the 2.2 kOhm resistor (diode anode to the junction between the zener, 470 uF cap, and resistor; diode cathode to the (+) pin of the 1000 uF cap).

The two 10000 uF caps I've drawn are your main supply caps. I don't know what capacitance you're using so I just put 10000 uF...

~Tom
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Old 11th March 2010, 07:28 PM   #4
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Quote:
Originally Posted by pacificblue View Post
Yes, you can. The COM pins should go to the amplifier's outputs and the NO pins to the speakers.
The switch has 3 rows: lef, right and middle. Which terminal is 1,2,3,4,5,6 - is 3 and 4 stands for the middle row. Thanks.


tomchr, i think the switch would be much easier for you to do. Thanks for the suggestion though.
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Old 11th March 2010, 08:34 PM   #5
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I assume you are talking about a toggle switch. If the switch is mounted in a front panel so it toggles up/down, the center row of pins usually connect to the common point of the switch, i.e. pins 2, 5 in the schematic above. So wire the amp output to these pins. Left channel output to the left side of the switch, right to the right. Then if you want the speakers on when you flip the switch up, connect the speaker to the bottom pins of the toggle switch. Left to the left side, right to right. If you want the speakers on when you flip the switch down, you, of course, wire the speaker to the top pins on the switch. Simple, eh?

What you designate as the left and right side of the switch is arbitrary and really up to you. I generally make the "right" side the one that's on the right when I look at the amp from the front. That's my system. It just helps me so I don't get the channels crossed.

If you're in doubt about the connections on that switch, I suggest getting one of these el-cheapo multimeters. Seriously! A low-end digital multimeter can be had for less than $10 and can save you a lot of trouble.
...and if $10 is too rich for your blood, a bulb from a flashlight and a couple of batteries can be rigged as a continuity tester. Easy. Figure out how the switch works, then wire it up.

~Tom

Last edited by tomchr; 11th March 2010 at 08:37 PM.
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Old 11th March 2010, 10:54 PM   #6
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Quote:
Originally Posted by tomchr View Post
You could use an output relay and have it turn on a short time after the supplies come up. See attached as an example.

The two 10000 uF caps I've drawn are your main supply caps. I don't know what capacitance you're using so I just put 10000 uF...

~Tom
If you want to be adventurous you could use a PIC to delay the turn on by a couple of seconds.

Then given you use a PIC with A2D you could then monitor for fault DC conditions and drop the relay out on a fault. I must admit I cracked doing it with just using i/o pins as comparators and didnt use a A2D PIC.

I use this in my disco amps and have saved a fortune in speakers !
I got caught out with faulty leads shorting a couple of times blowing up the amp but the PIC saved the speakers.

Leads on a disco tend to get pulled and tripped over so take a lot of abuse.
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Old 11th March 2010, 11:58 PM   #7
tomchr is offline tomchr  United States
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Off topic and largely irrelevant, but I've ditched the PIC in favor of the Atmel AVR family of micro controllers. I got tired of the ad-hoc "architecture" of the PICs. I haven't looked back...

I considered using a micro controller for controlling the relays in my power amp, but decided to go with a dual D flip-flop and a "diode OR" gate instead. No software to debug... The D flip-flop turns a low-voltage push-button switch into a momentary switch driving the power relay. A circuit with an NPN and a zener diode monitors the supply voltage and starts an RC delay for the output relay after the supplies have reached about 2/3 of their nominal voltages. If the AC mains disappear, DC develops across the amp output, or the heat sinks reach 95 deg C, the power is cut and the output relay turns off. A fault condition also turns the ON indicator from blue to red (two LEDs with an acrylic light pipe).

~Tom
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Old 12th March 2010, 07:37 AM   #8
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Quote:
Originally Posted by tomchr View Post
I assume you are talking about a toggle switch. If the switch is mounted in a front panel so it toggles up/down, the center row of pins usually connect to the common point of the switch, i.e. pins 2, 5 in the schematic above. So wire the amp output to these pins. Left channel output to the left side of the switch, right to the right. Then if you want the speakers on when you flip the switch up, connect the speaker to the bottom pins of the toggle switch. Left to the left side, right to right. If you want the speakers on when you flip the switch down, you, of course, wire the speaker to the top pins on the switch. Simple, eh?

What you designate as the left and right side of the switch is arbitrary and really up to you. I generally make the "right" side the one that's on the right when I look at the amp from the front. That's my system. It just helps me so I don't get the channels crossed.

If you're in doubt about the connections on that switch, I suggest getting one of these el-cheapo multimeters. Seriously! A low-end digital multimeter can be had for less than $10 and can save you a lot of trouble.
...and if $10 is too rich for your blood, a bulb from a flashlight and a couple of batteries can be rigged as a continuity tester. Easy. Figure out how the switch works, then wire it up.

~Tom
Perfect. That's what i need. thanks Tom for your help.
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