Scrap-lifier Update.

Okay, so I was having some problems with the 20v transformer; all the expectable ones like no headroom and so on.

So I installed (with some trepidation) the 35v 5A transformer that resided on my test-bench. This had the immediate effect of pushing the speaker protection circuit straight out of it’s comfort zone and into latch-trip-off mode anytime it got pushed past a 6vRMS waveform. So Nurse, board surgery!

First job first, get rid of that awful latch. Option 1 would be to just link-out the output relay, but I kinda like the de-thump delay function, especially with that 6N tube warming-up. So, what controls it? There is a SIL UPC1237 right next door to the relay. The data sheet shows that the latch function is deactivated by an earth on pin three, so link-out C18.

That just left the laughably low level of the trigger. The cct is suppossed to be spec’ed for 68W/ch on a normal heatsink. Mine’s a bit bigger… So, I want to push this thing.

As a start on the trigger-level, with the two 56kRs monitoring the speaker channels summing into a 4u7F electrolytic, I tried a 25% reduction by discharging the 4u7 through a 100kR, but my eyes are getting so bad I got a 180kR instead. So I paralleled that up with another, but realised that it wasn’t transients setting it off, it was sustained bass power. That frequency-dependent characteristic is set by the 4u7.

Therefore, ultimate mod; stick a 10uF across that.

Job done. Now I can get 80w/ch out of it. Seriously, even the smoothing cans get warm. Never seen that before…

AA

About Alacrity Audio

Designed and built in the UK, Alacrity Audio’s loudspeaker systems offer unbelievable sound quality in convenient close-to-wall designs.
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