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.
If you refer to the amp DC offset, that is extremely low to nil (@ +/-2mV)
I will go to the breadboard and do more troubleshooting.
Thank you!
MBB = Make before break
BBM = Break before make
Even with close to zero DC offset you may get a small click or a loud pop when the switch is BBM. Another term for these is shorting (MBB) vs non-shorting (BBM).
If you have pop even with a shorted source, check the offset at the input pins.
yes, understood, but do you have a part number or a circuit for reference?
Many thanks!
56S36-01-2-04S Grayhill | Mouser India
You can use parametric search to narrow down the options for your use case.
You can use parametric search to narrow down the options for your use case.
thank you!
I am bound to using a push switch given the current front panel design of the amplifier. The mechanical / push switch contacts close the supply circuit to the relay coil. The relay does the actual signal switching.
I am trying to tame the pops associated with the relay signal switching.
At the moment, I removed the entire PCB circuit and have PTP wired the relay directly to the input RCA coax and no source attached on either input. I only have a flyback diode and powering up the relay from a 9v battery to rule out any PSU interference.
Yes, pops still there so I am suspecting my input coax wires playing a number on me. RCA from real panel directly coupled on both inputs now
....or the relay being really bad for this duty...
When using a single relay with a pushbutton, there is no way to ensure a shorting/MBB operation.
All relays needs about 50-100ms to switch between contacts, thus ensuring that even the lightest offset will be grounded through the pull-down resistors after a momentary excursion to a higher value, as the relay actuates. This change in voltage at the inputs is what causes the pop due to base/gate current changing.
To avoid this we use one relay for each signal and the actual switching operation is not broken before the next source is selected.
All relays needs about 50-100ms to switch between contacts, thus ensuring that even the lightest offset will be grounded through the pull-down resistors after a momentary excursion to a higher value, as the relay actuates. This change in voltage at the inputs is what causes the pop due to base/gate current changing.
To avoid this we use one relay for each signal and the actual switching operation is not broken before the next source is selected.
I believe your assessment is very accurate, no mater what I tried so far did not get rid of the pops
won't that create any cross-talk?
do you have a reference circuit with the separate relays?
I am also thinking that a timed sequence where the speaker relays are turned momentarily OFF, until the source select ends, is probably the ultimate solution....along the lines of @RickTH suggestion.
In the mean time I do have the option to turn the speaker relay off manually.
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The reference circuit is a 1P3T (or 4T as occasion demands) switch with each throw connected to one relay and the pole to the relay supply. It's that simple, there's really no electronics involved. The NO contact is hooked to the signal in and the NC contact is left open.
We do however add pull-down resistors on both ends and a reverse diode across the speaker relay for housekeeping. There is a provision for caps on the output side if needed.
Haven't noticed any crosstalk, and if there was any it would still be better than a single relay as the signals are physically further apart.
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 !
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.
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.
LOL
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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