across my output relay is ............

[Pbassred]

.................a 100R and a 1.U cap. What do they do? Presumably nothing while the relay is closed. Its on a big MOSFET power amp.

[bigwill]

Might be / probably is a zobel network, I don't really know about these things I just happened across this thread in the recent posts list

[PigletsDad]

I tink this is probably a spark suppressor to help damp down oscillations when the relay interrupts a big current to a possibly inductive load.

[Pbassred]

that makes sence. Thanks.

[Bobken]

Hi,

Doubtless you are aware of the fact that (most) power amp output relays are there to prevent DC voltages from potentially harming speaker drivers, should there be an amp fault which outputs PS full-rail voltages, and/or to reduce 'switching thumps'.

Having 'listened' to many relays and the effects they have on the sonic results, myself, if this is a clever manufacturer, he probably realises that these unwanted and degrading sonic effects can be substantially reduced by placing a cap across the relay's contacts.

This cap will still block any DC component in a fault scenario, and yet will be quite effective with passing 'normal' AC signals especially in the upper frequencies, and is a good DIY trick to try with any such relays.

I've not come across a resistor used similarly, but this will also help 'sonically' to some extent, although rather than blocking all DC, it will reduce its amplitude somewhat.

It may also act as a snubber to reduce contact-arcing, but all relays I have dissembled after some use (especially high-current, as here) show signs of contact deterioration, and accordingly the beneficial effects of the bypassing cap will be relatively more significant with the passage of time.

Regards,

[Pbassred]

I get the idea that a cap will block DC while the relay is open. but if the relay is closed, how does it do anything about passing rail voltages to the driver?

[Bobken]

Originally posted by Pbassred
I get the idea that a cap will block DC while the relay is open. but if the relay is closed, how does it do anything about passing rail voltages to the driver?

Hi,

From what you say, the cap and resistor both *permanently* bypass the relay's contacts, unless I have misunderstood your comments here. The effect of this is that the potential loss of resolution due to the relay contacts (which worsens over time) is minimised or reduced, as this is then almost like a conventional output capacitor-coupled power amp from the signal's viewpoint. Although as you suggest these parts appear to do nothing when the relay contacts are closed (as they simply parallel a closed switch), any harmful 'effects' caused to the signal due to this closed switch are effectively bypassed by them.

As the value of the cap is low compared with what would normally be used for capacitor-coupling, the main benefits will be at HF (as I suggested), but this is precisely the frequency region where relays seem to be most destructive of good sonics, in my experience.

I am less certain of the resistor's purpose, but don't think that this would normally be used to prevent contact-arcing as a cap would be more usual here, and as it partly counters the safety-aspect of using a relay in the first place, I am sure that it has some sensible reason for its inclusion. I guess that it is also likely to help with reducing any degradation due to the relay's contacts, and is perhaps more effective at lower frequencies.

Purely from the safety-aspect, i.e. preventing DC from being output to speaker drivers, the addition of a cap will not adversely affect this issue at all, but the resistor would still permit the passage of some DC, albeit that it will be reduced in amplitude and hopefully should be potentially much less harmful in the event of a fault.

Slightly off-topic, but this is similar to bypassing mains fuses in amps with a small capacitor, which some canny manufacturers do, as this helps to reduce the unwanted adverse sonic effects which fuses have on such circuits.

I hope that this helps to understand what I believe these components are most likely there for, but why not ask the makers, who may be willing to tell you?

Regards,

[forr]

An efficient spark surpressor. I saw such an RC circuit in a B&O amp schematics once and duplicated it on every of my loudspeaker relay protection circuits.
Anyway, a good idea to protect the contacts of relays is to turn the volume control full down before switching the amp on or off, so the only present voltage when closed or opened will be the DC offset voltage.

[Bobken]

Originally posted by forr
[B]An efficient spark surpressor. I saw such an RC circuit in a B&O amp schematics once and duplicated it on every of my loudspeaker relay protection circuits.

Hi,

This is certainly the case if the resistor and cap are in series with each other, and this *combination* is across the contacts, but I was reading this as both components being across the contacts, in parallell with each other.

Regards,

[Bonsai]

I think this is probably an 'RC snubber' - designed to absorb electrical energy as the contact s open. This stops arcing across th e contacts, which reduces (stops) contact pitting and prolongs th e life of th e relay.

Arcing also emits RF energy that casues interference, so it is good practice for this reason as well.

BTW, when driving relays, dont forget to use a catch diode (also sometimes called a 'flywheel' diode) across the coil. Again, for EMI resons, locate as close as physically possible to the relay coil.