Article

DC Protection for Speakers

While building a single-ended class A MOSFET amplifier consisting of two mono blocks, this designer did not want to use a coupling capacitor between the amplifier and the speaker. He needed a circuit that would protect the speaker against DC voltages. Check out his solution.

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Discussion (0 comments)

F1Andy 2 years ago

Thank you for the circuit.

I was wondering if there is there a reason to use op-amps instead of comparators (e.g. LM339)?

Many thanks,

Andy

0 Comment(s)

audeq2 2 years ago

There are a few reasons not to use op amps as comparators. Download Analog Devices application note AN-849 for a detailed discussion. The four main reasons are

• Speed
• Inconvenient input structures
• Inconvenient logic structures
• Stability/hysteresis

Op amps are meant to be operated in a closed feedback loop. Some op amps have back-to-back diodes in parallel with the inputs. Some op amps inputs could be damaged or the input parameters degraded by using them as comparators.

0 Comment(s)

EN0167479ID 2 years ago

why not use an output transformer? there are numerous SE FET amps around that use a lump of iron with plenty of copper windings between the FET and the tweeter…

Nelson Pass built SE FET amps inspired by Jean Hiraga's late 1970s Nemesis amp. a very simple design that can sound excellent, IMO not despite, but because of having an output transformer. (the best protection against DC… – much more reliable than any electronic circtuit, and w/o eight-legged fuses and noisy 555s. and also w/o relais contacts that can and eventually will weld together and thus not open when they should.)

btw check out Tom Christiansen's (Neurochrome) Guardian circuit. it does exactely what you were looking for. they work very well.

0 Comment(s)

Dego 2 years ago

Agree, comparators would have been better, and they are often open collector, making the or-ing diodes obsolete. However, LM358 can withstand even greater than 5V differential voltage at the inputs without any degradation, so that is not a problem here.

The trimmer in series with the transistor base - to compensate for different relay behaviors (!) - is certainly not recommended. It will run the transistor into its very much temperature-dependent linear region and also hot, as the author pointed out.

The author might have added the trimmer to solve a problem due to the diodes in series with the output of the opamps - probably not being able to turn off the transistor properly (without tweaking the trimmer...). LM358 does not swing close to the ground with a 1mA sinking current.

Many of the bypass capacitors could be left out in this circuit without problems. Using a QUAD comparator you could easily remove the 555 timer from the circuit...

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