Old homebrew linear audio amplifier porn and what shall I do with it? - Page 1
Forum section: electronics / projects

author: MrSlack
author_rank: Frequent Contributor
timestamp: March 03, 2016, 02:43:44 pm
content: Got this free with some other stuff I picked up. It was going with me or to the tip. Weighs about 20kg. Full aluminium and steel chassis dating back to about 1978-1982ish by the looks. Very over-engineered. Massive laminated transformers, huge capacitors, more heatsinks than I've ever seen on anything. Totally rock solid. The underside, not pictured as I can't turn it over at the moment, has a couple of preamp boards and driver boards in it. I have some paperwork that suggests it's a JLH class AB.Also missing bleeder resistors which scared the living excrement out of me when I probed it after a shut eyes behind safety glasses, stick fingers in ears and assume the crash position power up. Unfortunately the entire thing is beyond repair by the looks. I traced some of the circuits and there are a number of scary things in there which I don't have time to resolve including some grounding issues and quite frankly scary dangerous AC wiring.Inside:More insideNice old electrolytic. ESR is still within reasonable limits. My wife hasn't noticed the new window ornament yetSo what to do with it? Caps are good. Transformers are good. The chassis is good and the heatsinks are good. I haven't extracted the power transistors yet but I assume they are good. Seems a shame to sling it.Option 1 - monster bench power supply. If I cut the chassis into three and knock out a simple linear supply with an LM327 something in the middle with current gain and existing power transistors as bypass transistors. Reuse the existing caps, rectifiers and sink resistors as well. I haven't worked out the transformer VA but they push at least 80v RMS by the looks so that's a good bet for a 50v @ 5A supply. 250W of destruction.Option 2 - any ideas?

author: BurningTantalum
author_rank: Frequent Contributor
timestamp: March 04, 2016, 08:07:17 am
content: It looks home-made- or more likely 'under-the-bench' at work (voice of experience... not sure why my first employer didn't go bust)BT

author: MrSlack
author_rank: Frequent Contributor
timestamp: March 04, 2016, 08:53:16 am
content: It's definitely home made. Must have cost a fortune as the entire thing appears to have been built from RS parts on a small scale.I've taken out a single transformer weighing around 3kg - label has degraded so I can't see the VA rating annoyingly. Caps are out, tested and cleaned. Not leaky. Vapourised a few LEDs with one (sorry you just have to do that). Slowly dismantling the chassis to see if I can shorten it.Power amp boards have burns on them and covered in flux and corrosion so I'm not going to bother recovering them.

author: RobertoLG
author_rank: Frequent Contributor
timestamp: March 04, 2016, 09:13:30 am
content: poor old boy, rest in pieces...literaly

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: March 04, 2016, 10:04:25 am
content: Unless you have a lot of time, and don't mind probably inferior sound I would say bin it.While I think of it, I have an old NAD in quite good condition. But with and annoying buzz in the output stage that I never got to the bottom of. I should bin that too.I've got 2 very nice Class A mono amps. Similar style but airtight heavy duty AL project cases with external heatsinking and so no dust. My old boss bought some high end preamp boards for them too.Unfortunately I can't afford to leave these amps on, well except in winter.These days just I want to go class D, but haven't got around to this project yet.

author: MrSlack
author_rank: Frequent Contributor
timestamp: March 04, 2016, 10:36:01 am
content: Indeed. This thing probably burned 60% of the amps going into it in quiescent current on the big ass transistors. I could do with a big linear supply for a couple of things so I'll probably just keep some repurposed remains around for that.I'm building a class D at the moment based on TI TPA3001. Got a working prototype. Far smaller and more efficient, even if it smells a bit funny at the moment. Think the output filter is a bit iffy.This replaces the also broken NAD (C320) which appears to have multiple problems and the shitty HK bluetooth speaker I'm having to live with. Ick - everything is always broken.