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Old 8th December 2009, 10:22 AM   #11
Elvee is offline Elvee  Belgium
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Originally Posted by Nrik View Post
Interesting stuff.

Elvee: Could you simulate with an output-coil and zobel filter before the switch?
The series RC network being in parallel with the output impedance of the amplifier, it makes little sense to include it.
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Old 8th December 2009, 10:54 AM   #12
Nrik is offline Nrik  Denmark
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Still - its at -80dB at 20khz, and the primary reason for protection relay is to protect the speaker from DC by catastrophic failure, and to prevent turn-on/off peaks.

Chocholic: Did you listen to the amp with this circuit, and does it work as inteded in real life?
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Old 8th December 2009, 01:19 PM   #13
Elvee is offline Elvee  Belgium
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If it is a protection relay only, the isolation is unimportant, so it should be OK.

Combining the battery option with a small circuit could improve the matters slightly for other applications: by reverse-biasing the G-S capacitance, the isolation can be improved by ~6dB (ChocoLv1).
ChocoLv2 shows an example circuit doing that (obviously, not all inputs should be implemented).
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Old 8th December 2009, 06:30 PM   #14
ChocoHolic is offline ChocoHolic  Germany
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Hi Ho !
That's nice, the discussion is started.
Off isolation was also one of my concerns, especially because of the protection varistor. First calculations told approx. 60db at 1kHz.
My measurements at 3.9 Ohms load show -63db at 1kHz and -44db at 10kHz.
As you already noticed for a speaker relay that's fine, for signal selector you would need the traditional arrangement with two switches.
One in series with the load, the second in parallel with the load and both switches driven with inverted control signal. You can of course do this arrangement also with the shown switch and theoretically then you will reach values in the 100db range.
But as you already noticed: For a speaker relay, the OFF-isolation of the simple arrangement is Ok.
This was also the result of the listening test.You can play really loud and then switch off - also perceptionwise it is off.
Also for the undesired pop, the isolation is perfectly fine.
And considering DC-protection, this switch is perfect anyway.

Regarding the artifacts during ON status: Yes, the artifacts are well below my measurement capabilties. The shown artifacts are fully caused by the amp itself, not at all by the switch.

Listening? ...listening to audio equipment?
Of course I do.
This switch is completely neutral to my ears.
I can listen for long time - fine.
I can compare, with and without switch - no difference.
Even when putting a jumper wire across the switch in status ON during listening - it simply does not make any difference, not any change in sound or level comes to perception.

So I decided to use this method in future for my amps.
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Old 11th December 2009, 07:01 AM   #15
Mooly is offline Mooly  United Kingdom
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MosFet Relays
This is a very interesting idea... as a big problem with relays under fault conditions (an output device going short etc) is that the contacts could weld together thus defeating the object of offset protection.
It would be interesting to test the switch at DC measuring the volt drop across it as current flow is increased, in both directions to see how symetrical and linear it is, perhaps up to 10 to 15 amps or so.
I am sure it does appear sonically transparent, unfortunately if it worsened the distortion of say an amp in the 0.00xxx range many would dismiss it.
Perhaps the "distortion" it does introduce is pleasing to the ear...

Interesting idea

Edit... and perhaps incorporate the amp feedback to include the switch in the loop.
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Old 11th December 2009, 08:11 AM   #16
analog_sa is offline analog_sa  Europe
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Originally Posted by ChocoHolic View Post
Hi Ho !
I can compare, with and without switch - no difference.

Do you hear a difference if you use relays?
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Old 11th December 2009, 09:51 AM   #17
BV is offline BV  Slovakia
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Fast switch off at fault conditions with inductive load (speaker) can cause high voltage spikes (and voltage breakdown). Aditional protection clamping diodes at both sides of MOSFET switch are necessary..Internal body diodes are not sufficient.

Last edited by BV; 11th December 2009 at 09:58 AM.
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Old 11th December 2009, 01:28 PM   #18
AndrewT is offline AndrewT  Scotland
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and adding one half of each double FET switch to each supply rail quickly shuts off the amplifier as well.
Would cheap irf540 for the power rails and irf640 for the output do as our switches?
regards Andrew T.
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Old 11th December 2009, 02:14 PM   #19
ChocoHolic is offline ChocoHolic  Germany
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That's a good idea. Especially with respect to the fact that all my meaurement methods so far were less good than the switch.
Unfortunately I cannot measure up to 10A DC, because I am lacking a
strong enough DC source. Also at 10A there will be heat, which is increasing the Rdson, so an unrealistic thermal distorsion would happen.
May be I can do pulsed measurements with the scope. I. e. DC-Pulses of 100ms...200ms duration, simulating realistic load distribution like drums and then seeing a realistic thermal distorsion.

If the relay is high quality and in good condition, then I cannot measure and cannot hear any issue with a relay. Relays are fine, except old/defect/poor quality.
When I took one relay from my shelf I found an ugly high order spectrum with peaks in the range around -80db. Knocking a few times to its plastic cover 'repaired' it...
Similar thing I found with fuses in the speaker path, after some years in the fixture the contacts become poor, found 0.07% THD. Screwing them out and in again - repaired.
And of course I also had the situation that a relay was so obviously bad, that one speaker was fading, somehow distorted.
The above experiences are my motivation to move away from the relays.

Take your time and read the entire thread.
Your proposal with the diodes I mentioned in posting #6.
Furtheron in most cases you will only need them on the side of the speaker , on the side of the amp, these diodes are usually already existing in the amp.
Have a look to the first schematic, there I am using a varistor for protection.
Without that the heavily inductive test with 75A would not have been a good idea.
The simplified schematic is just for starting playing.
Please note: The internal body diodes do not clamp.
But for first playing around the avalanche capability of most MosFets will do the job. For a proper setup of course a varistor or two antiserial TVS-diodes or a bidirectional TVS acrross the switch are necessary. Or at least two diodes towards the rails ... or any other rugged clamping means...

IRF540 and IRF640 will work for first playing around, but due to their slightly high value of Rdson, both types are not the best choice. Of course paralleling will help. If the 100V of the 540 are sufficient in your case, you should prefer the 540 instead 640.
I used IRFB4321. Also a good choice would be IRF52n15d or for higher voltages IRFB4227.
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Old 11th December 2009, 03:05 PM   #20
BV is offline BV  Slovakia
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Sorry, I simply had not noticed text in brackets.
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