Speaker Protection

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Ray P
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#1 Speaker Protection

Post by Ray P »

So let's assume you have a direct coupled amplifier output, (in this case the 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 but leaving the speaker connected?

I ask because the soft start/speaker protection module that I'm purchasing for my Mellow OTL amp has a large relay and it 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 modules yet). That feels as though it's probably the safest but perhaps not optimal for sound quality.

Thoughts/comments?
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Toppsy
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#2 Re: Speaker Protection

Post by Toppsy »

Ray,
Is this ESP Project 198 of any help?

https://sound-au.com/project198.htm
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Ray P
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#3 Re: Speaker Protection

Post by Ray P »

Thanks Colin, I'll check it out.
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Michael L
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#4 Re: Speaker Protection

Post by Michael L »

These seem to be well regarded. I bought a couple myself for my Firstwatt clone

https://www.diyaudio.com/forums/group-b ... ay-gb.html
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Ray P
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#5 Re: Speaker Protection

Post by Ray P »

Michael L wrote: Fri May 01, 2020 8:21 pm These seem to be well regarded. I bought a couple myself for my Firstwatt clone
Yes, I've looked at them before but they're not up to the potential DC voltages in a valve amp, not even the modest 150V in the Mellow OTL.
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Mike H
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#6 Re: Speaker Protection

Post by Mike H »

it's probably the safest but perhaps not optimal for sound quality
Good point. I've had bother in the past (a Pioneer amp) with the contacts going open-circuit due to oxidation, twice I had to get the cover off the relay to clean the contacts.

So the other option of grounding the path may also fail due to "dirty" contacts then voice coils go up in smoke. :shock:

So I'd go with the former, get a decent quality relay and be prepared to inspect / clean the contacts occasionally. Or even replace it.

As an aside, my ancient 1970's Akai recorder uses relays with gold-plated contacts, but even they are not entirely foolproof.
 
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#7 Re: Speaker Protection

Post by Wolfgang »

Yes, I've looked at them before but they're not up to the potential DC voltages in a valve amp, not even the modest 150V in the Mellow OTL.
There is a simple way of how to use them for any psu voltage and without the built-in SSRs. I like them because they put the detection circuit between two opto isolators/coupler. A final test with my InvOTL will show if there is any influence of the SQ but as I plan to use them now just for triggering my relay which can withstand the >200V it's a perfect solution with very little efforts. Disconnecting the speakers is the only effective way in case of high voltage DC at the output and there are no high voltage/high current SSRs (at least for a reasonable price). All SSRs I have tested so far have some minor but clearly noticeable effect on the sound. Grounding the output with a resisor parallel to the speakers works but only for minimizing the "noise" (like in TS OTLs) during the warm up phase.
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#8 Re: Speaker Protection

Post by chris661 »

Question: will an OTL pass enough DC current to worry a speaker?
I'm wondering if there's any need for protection here.

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#9 Re: Speaker Protection

Post by Wolfgang »

Question: will an OTL pass enough DC current to worry a speaker?
I'm wondering if there's any need for protection here.
B+ in case of a faulty tube or a simple discharge if the tube is gassy. This only applies to PP or OTLs without an otuput cap like the InvOTL.
The Pinnacles use a TVS diode against those "dischargings" because of DHTs and a fuse in case of other failures. But the TVS diode influences the sound and should be rather avoided if possible.
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Nick
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#10 Re: Speaker Protection

Post by Nick »

Yep, but Chris said current and you came back with voltage. 1A through a 8 ohm speaker (assuming 8 ohm resistance, in practice they are less) is only 8W, that’s not going to do much damage. Maybe just a 1.6A slow blow in the supply somewhere will be enough?

A discharge is only going to be a problem is the speaker is very delicate, its not DC, so most speaker protection circuits may not see it anyway and it still has to come from the power supply.
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#11 Re: Speaker Protection

Post by Wolfgang »

Ok, then let's rephrase: how much current would flow through a 8ohm speaker if there is a short circuit in a tube or some other unforeseen problem connecting B+ with the output?

I lost a $1200 Lowther DX4 (but they have very good replacement deals) because of a simple short in a tube socket (tiny hardly visible piece of broken solder connecting grid with anode pin). Can happen when you solder 12 sockets and a little broken piece settles in the wrong position right after everything tested ok. How about that? This wouldn't have happened with DC protection.

Anyway, I am not advocating DC protection circuits here. I just tried to answer a question. Now we could ask of course if a fuse with a little thin wire so that it can blow fast enough (500mA in case of Pinnacles) before the voltage at the output can reach dangerous levels will affect the sound more than no fuse and a good DC protection circuit. But I leave this discussion to someone else.
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#12 Re: Speaker Protection

Post by chris661 »

If B+ was connected to the speaker, the current would depend entirely on the output impedance of the power supply.

The 5R4GY rectifier valve, for instance, has an absolute maximum rating of 650mA per plate, so if you did something really nasty (ie, connect it without a series resistor as the datasheet recommends) you might get 1.3A DC out of it for a short while, before something else melts - probably the rectifier itself, since everything else (transformer, chokes, etc) has the thermal mass to suck it up for a while.

1.3A DC for a few seconds wouldn't bother something as fragile as my FE126s - I think Nick's suggestion of a fuse in the supply would be spot on.

If B+ was suddenly connected to a speaker, you'd get an idea of what their impulse response is like. The short-term signal might pose a mechanical threat to the driver, but it would vary by the power supply: you're effectively connecting an 8R (or less, as Nick notes) resistor across the supply, so the voltage will be pulled down very quickly.

Was there a post-mortem on your Lowther driver?

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Nick
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#13 Re: Speaker Protection

Post by Nick »

The main current source would be the caps in the power supply discharging. It would (I think) be a question of if the energy in there was enough to kill the speaker.
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#14 Re: Speaker Protection

Post by chris661 »

So if we take a 10w nominal speaker, that means it'll dissipate 10J/s pretty much indefinitely.

A 300V 100uF cap fully charged holds 4.5J.

If you discharged two of those caps every second, the speaker would hold up thermally. What would happen mechanically would be interesting - the VC inductance would limit the instantaneous current flow, I suppose, but there's still the potential for some fairly large cone excursions since all of that energy is being delivered in a short space of time.

A slow-blow fuse isn't likely to save the speaker, but neither is a traditional protection circuit. I don't even think the typical protection circuits used for PA amplifiers would catch a signal that might go from 0 to 300+V almost instantly.
In fact, the only thing I can think of would be a load of diodes across the output terminals, arranged such that they clamp any output exceeding the amplifier's own peak operating levels. Maybe add a little series R to make sure the PSU doesn't melt.

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#15 Re: Speaker Protection

Post by Nick »

The real problem you have is defining what is DC. Assuming you want to have a 20Hz signal then you have to be ok with full power half cycle for 25ms, so you have to define what a duration counts as DC. You could just clamp current, but again, that becomes a problem as you have to pick a clamping current that wont affect your signal and hope all the extra circuitry doesn't do much harm.

After a bit, output transformers start to look like a good thing :-)
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