AP1 speaker protection circuits

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Robin Daly

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Dec 17, 2019, 9:28:56 AM12/17/19
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Hi All

I'm in need of some help! My trusty AP1 developed a fault a while ago whereby it needed a 'warm up' period before it would come on. I guessed it was something to do with the relay system that turns the speakers on, but not being a technician, I went in search of someone who might be able to fix it for me.

Well he had a go but eventually has given up, saying:

" The schematics I found online are quite helpful and include circuits of the amplifier board and preamp, but not the two extra cards which control the speaker protection. 
I fears these may prove very difficult to find."

Well if they can be found anywhere, this is the place! Can anyone help me out with the diagrams, and it may be I need another technician as well!! In the past Spiny from this forum has helped me, but so far I've not been able to contact him.

Huge thanks for any assistance.

Pete K

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Dec 18, 2019, 10:04:56 AM12/18/19
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Are the protection boards an after market addition to the AP1? As your expert says, the schematics on line don't show such a board and the AP2 I have which is supposed to be identical to the AP1 in layout doesn't have such a board. 

One idea is that your board is picking up a developing problem with the AP1 amplifier board and is therefore protecting your speakers from such as DC offset. 

Perhaps a long term AP1 owner could comment. 

I have thought of adding speaker protection to my AP2 but one problem would be a future owner would be in the same position as you if the protection board malfunctioned. My design would be based on a microcontroller and maintaining the software long term on a one-off device is next to impossible.

On my AP2 the output relay only mutes the speakers when triggered by a reed switch associated with the headphones slider. The 'phones' channel lines are always connected via an attenuator, so DC level could be monitored via that route. If there's a large DC offset upon power-on which decays to a certain level before the protection board connects the speakers, then that's one clue to the problem. 
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