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 Post subject: Speaker Protector
PostPosted: 22 Aug 2016, 03:34 
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Joined: 08 Aug 2009, 03:11
Posts: 2229
Location: Chilliwack, BC
Here's a little speaker protector unit that I've been using to add to vintage amplifiers that don't come with them (Sansui, Marantz, Technics, Luxman, Pioneer, etc.)

It combines a reliable, repeatable 555 for a 5-second turn-on delay to let the DC bias settle and avoid the typical turn-on "thump" that's so common in the lower priced units.

The protector is based on a Rod Elliot design with a couple of additions - the diode on the KSC945 collector to keep a negative-rail-fault from dumping a large enough V- on the 2SC3940 base to cause future unreliabilty and the 1Meg resistor on the B-E junction to bleed off charge that can cause false triggering with stray RF fields.

The transistors were chosen to be active production high-beta types. The circuit will not work properly with typical 100-150 hfe 2N-type devices.

The 15K-100K input resistors can be chosen by rule of thumb - from the service manual (or measuring) the PA supply rail, choose the value so that no more than ~2mA will flow through the base of the KSC945 if all heck breaks loose and an output transistor shorts.

The supply droppers will depend on what free AC voltage you can find off the power transformer. Use something if you can, to give over 12V to the circuit for best operation. With simple power supplies, you will likely have to power it from the AC winding for the PA.

Don't be tempted to use the amplifier's DC rail and dispense with the rectifiers all together - the amp's caps will take longer to bleed and will not cut off the speakers fast enough to stop the turn-off "thump". The low value cap in this is designed to drop the relay out almost immediately.

Many speaker protectors use filter caps as low as 4.7uF for this to happen - they also don't use a 555. Ripple can play havoc with the chip and the G2RL relays are sensitive enough to "chatter" with more than 20% ripple.

I specified a Nichicon UKL because it's a low leakage and fairly tight for an electrolytic. Use what you have, but note the turn-on delay can vary significantly if the unit is hot or cold.

If possible, use real bi-polar caps on the sense line. I use Nichicon UES simply because I stock them. They're designed for AC signals floating about 0DCV on both sides. Polarized caps will eventually short and putting two polar ones back-to-back again are very unreliable (80% of the 20+ year old commercial protectors that do this, fail testing. Those that used a bipolar (typically Sansui), had only a 2% failure rate under test).


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-= Gregg =-
* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
Work: Classic Valve Design

 Post subject: Re: Speaker Protector
PostPosted: 24 Nov 2016, 00:31 

Joined: 23 Nov 2016, 06:09
Posts: 5
I'm actually facing a similar problem of DC in the output terminals in my tda2050 amp. Is there a much simpler way to eliminate the DC in output,since I'm afraid that ive damaged one of my speakers while testing. I was able to hear a banging sound while I switched off the power.

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