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NU_NRG 5th May 2002 04:04 PM

Project 3A fusing problem
Just finished my amplifier using Mr.Elliott 3A amplifier design. Had no problem at all with it except the other day when one of my rail fuse ruptured. Has I read somewhere on this forum there is big danger in this kind of failure as the output falls all the way to the opposite rail. It resulted in a burned out crossover and speaker element but hopefully nothing to the amp. I was now wondering if you DIYers had any idea to protect the amp from such a problem? Something that could monitor the circuit and see that the output is cut when one fuse fails.

Thankyou for all your replys

paulb 5th May 2002 06:09 PM

Uh oh, I was hoping that if the fuses went they would at least protect the speakers. I defended this approach some time ago around here somewhere. The idea is to sacrifice the output transistors because they are cheap in order to save the speakers. Obviously this hasn't worked out. I'm a bit concerned now about my version of this amp.
I think somebody suggested adding overcurrent protection to this amp, you might search around to find this idea. Check Rod's original version of the circuit (the Project 3), and perhaps email him and ask, he has been very responsive to emails.
Did you have the correct fuses in place? What conditions caused the fuses to blow?

NU_NRG 5th May 2002 06:23 PM

I should point out that the speaker was not particularely high priced so Its not such a big deal. The fuse died from over current, normal type 5 amp fuse. I've been pushing the amp for some time before it went numb. I read on previous treads that quick blow fuses were thought to be better suited in this design to pretect the rails, wouldn't it be better with slow blow?

I hope we can find an easy solution to this problem.

thank you for your concern paulb


paulb 5th May 2002 10:50 PM

There was disagreement on the question of fast-blow vs. slow-blow. Geoff felt slow-blow were better, as they force the transistors to blow and protect the speakers. In view of your experience, I think he's probably right. Here's the thread:

mrfeedback 6th May 2002 02:32 AM

No Slow Blow
Hello PaulB, standard equipment in commercial gear is " T " type glass fuses.
These often have a tiny blob in the middle of the fusewire link.
These are slower than quick blow, so will handle turn on transient currents, but not so slow as Slo-Blo.
Slo-Blo are not a good idea at all for protection of audio gear.
Hugh Dean mentions output transistor base series resistors to limit fault currents.

Regards, Eric.

trwh 6th May 2002 06:23 PM

I use 5A glass "T" type fuses in my P3A, and have a speaker protection circuit, the Velleman K4700. This disconnects the speakers very quickly (via relays) should a DC voltage greater than about a volt or so be present. It serves to protect my precious Dynaudios should the amp develop a fault, or if one of the supply line fuses blows. Its a good kit, and only cost 13GBP. It also protects the speakers from switch-on transients and allows manual muting via a front panel switch or, as in my case, a thermal switch on the heatsink.

See ya,

chris ma 14th May 2002 07:23 PM

Velleman K4700
Hi trwh,

I am building a pair of mono amps, I wonder would that be better for me to get the K4701 in line DC Protect kit for each amp. I think the K4700 is for the stereo amp and would be difficult fo rmy set up. Is that correct? If so, do you know is that ok to put the kit inside the amp and connect it between the amp output and speaker binding posts?

Thanks in advance,

trwh 14th May 2002 09:34 PM

Chris, a pair of K4701's will work for two mono amps. However, this kit is passive, and does not offer protection from switch-on and switch-off thumps. Also, it draws its operating current from the speaker line and as such may affect sonics (although its effect is probably negligable).

If you are building monobloc amps, you are probably interested in very good performance, so why not use two K4700's? The kit is very cheap, and it can be used in mono or bridged amps. That way you get the switching transient suppression, provision for manual muting (very useful), and I suspect that the active circuit has better (i.e. quicker) performance.

swede 14th May 2002 11:15 PM

I would go for a decent DC protection circuit. You'll find one on Rod's web-pages. It's a cheap insurance when one fuse fail and burn up the transistor on the other rail.


fcel 14th May 2002 11:26 PM

Swede: Vellerman K4700 is just as good for DC protection, right?

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