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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

relay as source selector - pop issues
relay as source selector - pop issues
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Old 14th July 2019, 03:14 PM   #31
Fast Eddie D is offline Fast Eddie D  United States
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Quote:
Originally Posted by rottalpha View Post
I have limited options if I want to have a direct input. This is retrofit, not a new design, so I am limited by the existing case design and existing features.
You still need a pull down resistor on the external input. The source of the pop can be your source (CD player etc). If need be, you can use an input capacitor with pull down resistors on both ends.

And don't be lulled by the idea of a DC input. Don't do it. There is much pontificating about the evils of coupling capacitors but in the real world you can make them virtually 100% transparent. It's all in the design.

Direct coupled audio circuits are impractical. You can DC cascade a lot of the circuitry (and you should to minimize the use of coupling capacitors); but at the input, output, and any switch or relay, any DC will cause noise problems.

Quote:
If you refer to the amp DC offset, that is extremely low to nil (@ +/-2mV)
Guaranteed click at line level, especially if it's after the volume control.

Quote:
I am limited by the existing case design and existing features.
If you design your circuits with industry standard specifications (like 47K input impedance and low output impedance) then you will never be limited by existing features, and your builds will be 100% compatible with commercially produced products (power amplifiers, CD players, etc).

Quote:
I will go to the breadboard and do more troubleshooting.
If you're using one of those solderless breadboards then you'll never get your line level circuit to perform to its potential. They are terrible for audio circuits and should only be used for parts testing and prototyping digital circuits. I use small, inexpensive boards like the "Perma-Proto" type, which are designed for op amp circuits. You solder all the connections. Most sizes are under $10 and I try to use them over a few times.

You've received a lot of good advice here from everyone. Heed the advice about not putting a loop around the relay. Use the reverse diode on the relay coil. This is not optional. In fact if you drive the relay with a chip or resistor then the back EMF can destroy the driver.

Final tip - Use a line level relay to short the output of your preamp or line level circuit. Drive this relay with a 555 timer configured as a delay on circuit. Use a time constant of around 2-3 seconds. This will eliminate all your on/off noises.
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Old 14th July 2019, 04:05 PM   #32
rottalpha is offline rottalpha  United States
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Quote:
Originally Posted by Fast Eddie D View Post
You still need a pull down resistor on the external input. The source of the pop can be your source (CD player etc). If need be, you can use an input capacitor with pull down resistors on both ends.

And don't be lulled by the idea of a DC input. Don't do it. There is much pontificating about the evils of coupling capacitors but in the real world you can make them virtually 100% transparent. It's all in the design.

Direct coupled audio circuits are impractical. You can DC cascade a lot of the circuitry (and you should to minimize the use of coupling capacitors); but at the input, output, and any switch or relay, any DC will cause noise problems.



Guaranteed click at line level, especially if it's after the volume control.



If you design your circuits with industry standard specifications (like 47K input impedance and low output impedance) then you will never be limited by existing features, and your builds will be 100% compatible with commercially produced products (power amplifiers, CD players, etc).



If you're using one of those solderless breadboards then you'll never get your line level circuit to perform to its potential. They are terrible for audio circuits and should only be used for parts testing and prototyping digital circuits. I use small, inexpensive boards like the "Perma-Proto" type, which are designed for op amp circuits. You solder all the connections. Most sizes are under $10 and I try to use them over a few times.

You've received a lot of good advice here from everyone. Heed the advice about not putting a loop around the relay. Use the reverse diode on the relay coil. This is not optional. In fact if you drive the relay with a chip or resistor then the back EMF can destroy the driver.

Final tip - Use a line level relay to short the output of your preamp or line level circuit. Drive this relay with a 555 timer configured as a delay on circuit. Use a time constant of around 2-3 seconds. This will eliminate all your on/off noises.
Yes Sir,
I received very good advice and I am very appreciative of that. I want to thank everyone here for the time an patience with my questions !

referring to some of your later observations:

I used a reverse diode in all instances.
I ultimately disconnected the entire circuit board and tested a stand alone relay soldered / wired individually (no bread board), with no external source attached and the input sources RCAs shorted to ground.....and still no cigar.

...but yes, as soon as I use a coupling cap and 47k pot on the lines, that is the only time when I get no pops, when DC, I get pops no matter what.

so....

your last point of regarding the 555 timer...in my circuit that will fit nicely to momentarily turn off the amp speaker relay.

This circuit, if I was not clear, is located inside an existing vintage amp that has a couple of input sources. It is not a new build.

Thank you !
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Old 14th July 2019, 04:59 PM   #33
Fast Eddie D is offline Fast Eddie D  United States
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Quote:
Originally Posted by rottalpha View Post

your last point of regarding the 555 timer...in my circuit that will fit nicely to momentarily turn off the amp speaker relay.
Yes, it works great for that. Relay should be wired normally open and the circuit should energize the relay, closing the contacts.

The same output can also drive a red/green diode, since the 555 sinks as well as sources.
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Old 14th July 2019, 05:27 PM   #34
Fast Eddie D is offline Fast Eddie D  United States
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Join Date: May 2010
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Quote:
Originally Posted by rottalpha View Post

your last point of regarding the 555 timer...in my circuit that will fit nicely to momentarily turn off the amp speaker relay.
Just to be clear, you do not want to short the output of a power amplifier. You can short the input though.

And it's cool modifying and updating old hi fi equipment. I've had a lot of fun doing that.
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Old 14th July 2019, 09:39 PM   #35
rottalpha is offline rottalpha  United States
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Quote:
Originally Posted by Fast Eddie D View Post
Just to be clear, you do not want to short the output of a power amplifier.
LOL

Quote:
Originally Posted by Fast Eddie D View Post
And it's cool modifying and updating old hi fi equipment. I've had a lot of fun doing that.
Yes, I love doing that. Old pots and mechanical switches even after cleaning, they still have a negative impact on sound quality.

Many thanks!
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