Good Evening from Co. Durham

[Naughty Nigel]

Hi Nigel and welcome from lovely Gateshead!

Great intro mate, sounds like some lovely systems you have there. I used an a400 for many years, it was my main amp for at least 10 years then i used it in my study/second bedroom system until last year when the transformer finally gave up and I went down the mini-t amp route with a pair of vintage Ditton 15s. This system still gets a lot of use and gives me joy whenever I use it.

So welcome aboard and as Mick mentioned perhaps we'll get to meet you at a future meet up.

All the best

Rich

Do you still have the A-400 Richard? If you don't want it I would be very happy to have a go at fixing it!

Nigel

[RichB]

Do you still have the A-400 Richard? If you don't want it I would be very happy to have a go at fixing it!

Nigel

Sorry mate, it went in a clear out some time ago. You know that click it makes a few seconds after its switched on, well this was taking ever longer to make the click and I wasn't prepared to risk losing a pair of speakers or worse waiting for it. As it was it seemed uneconomical to fix and I was curious about mini-t amps which have worked well for me since. Cheers for the offer though.

[Naughty Nigel]

Sorry mate, it went in a clear out some time ago. You know that click it makes a few seconds after its switched on, well this was taking ever longer to make the click and I wasn't prepared to risk losing a pair of speakers or worse waiting for it. As it was it seemed uneconomical to fix and I was curious about mini-t amps which have worked well for me since. Cheers for the offer though.

Hi Richard, the 'click' that you hear is a protection relay, which disconnects the loudspeakers for a few seconds if the amplifier is overloaded. (It is basically an overcurrent protection system. Some amplifiers also incorporate output transistor overheat protection into the same circuit.) If you were to drive (say) two pairs of 4Ω loudspeakers the relay would disconnect them as soon as the output current reached a certain point.

The delay in the relay switching on also prevents the switch-on 'thump' that you get with some transistor amplifiers - by allowing the capacitors time to become fully charged before connecting the loudspeakers. So I very much doubt that there was ever any risk to your speakers.

Oddly enough, the greatest risk to loudspeakers comes from driving them with underpowered transistor amplifiers. If the amplifier is driven to clipping (distortion) the high frequency harmonics produced can easily damage tweeters. Also, if the amplifier is driven to severe distortion, the output waveform will effectively be a square wave, which your loudspeakers may see as a DC voltage. The 'impedance' of loudspeakers is quoted at 1 KHz, but their DC resistance will be much lower, so the high current can overheat woofer voice coils. This too is prevented by overcurrent relays, although purists might not like the idea of this 'unnecessary' switchgear in the signal path.

Nigel.