So let's assume you have a direct coupled amplifier output, (in this case a Tim Mellow OTL) and you are looking to protect your speakers from DC on the output using a relay. Would you use the relay to (a) disconnect the output to the speaker or (b) use the relay to shunt the output to ground (leaving the speaker connected)?
I ask because I'm purchasing a set of PCBs from Ares for my Mellow OTL amp and the the soft start/speaker protection module has a large relay that appears to be in line, so disconnects the loudspeaker if DC appears on the output (I assume above a given small theshold - haven't actually got my hands on the PCBs yet so working from diagrams/pictures). That feels as though it's probably the safest but perhaps not optimal for sound quality and my inclination would be to shunt the output to ground in a fault condition.
Thoughts/comments?
I ask because I'm purchasing a set of PCBs from Ares for my Mellow OTL amp and the the soft start/speaker protection module has a large relay that appears to be in line, so disconnects the loudspeaker if DC appears on the output (I assume above a given small theshold - haven't actually got my hands on the PCBs yet so working from diagrams/pictures). That feels as though it's probably the safest but perhaps not optimal for sound quality and my inclination would be to shunt the output to ground in a fault condition.
Thoughts/comments?
Wouldn't it be better to switch the load from the loudspeaker to a power resistor instead? You will definitely destroy more if shorting to ground.
Actually… OTL doesn't give a warlock's bâhlls whether it has an open or a rated load, as far as I know. There is no 'buck EMF' coming to the output valves when it is open. Indeed, if one's modestly concerned, just solder a 100 Ω resistor in parallel to the output channel (on the amp proximal side!), so that if the relay trips, it “opens to 100 Ω” instead of infinity.
Seriously.
OTL.
However, also note that OTL can also be damaged fairly easily if it sees ZERO Ω. The (typically, but not exclusively) cathode-followers-in-parallel output tries to 'follow' the incoming signal, and ends up conducting current asymptotically equal to the transconductance of the valves, together. Whether this fissions the valves, is a matter of valve, and size of input, I'd venture. But the power supply may not be amused, no matter what.
⋅-⋅-⋅ Just saying, ⋅-⋅-⋅
⋅-=≡ GoatGuy ✓ ≡=-⋅
Seriously.
OTL.
However, also note that OTL can also be damaged fairly easily if it sees ZERO Ω. The (typically, but not exclusively) cathode-followers-in-parallel output tries to 'follow' the incoming signal, and ends up conducting current asymptotically equal to the transconductance of the valves, together. Whether this fissions the valves, is a matter of valve, and size of input, I'd venture. But the power supply may not be amused, no matter what.
⋅-⋅-⋅ Just saying, ⋅-⋅-⋅
⋅-=≡ GoatGuy ✓ ≡=-⋅
I would prefer shorting the amps output to ground.
Why ? Breaking DC with a relay, especially with some current involved
will take time as it will cause an arc that continues to flow current.
Shorting on the other hand will be instantant.
I do not care if something MORE get broken in the amp as the amp already
is broken ( that's why the relay closes), in fact the amp's fuses might blow
earlier as the load will be a short instead of a burning loudspeaker.