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Old 7th June 2012, 02:18 AM   #1
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Default Power Supply Problem Help!!!

Hi there - i have a pair of vintage pioneer active CS speakers, of which i know nothing about and cannot get the specs for. They each have an 8" bass and 3" tweeter alnico drivers, they have active amplifier boards inside each spkr. I think they date from the mid-70s. They have TO3 output transistors - two for each driver, producing i would say, about 25 watts per speaker box.

The problem i have is that i am entirely guessing at the power supply requirements. I have a centre-tap torroidal transformer (rated to deliver about 5A) which i currently have in parallel, so both set of windings on the tranny are presently giving about 25VAC, rectified to 36VDC. This is going thru a standard bridge and Panasonic power caps. The amount of power is too high. The output transistors are getting too hot to touch, and the heatsink heat sensors cutoff the amplifier at times. I cant leave them running for too long at the mo becuase of this. However the sound hasnt been getting distorted. I have tried:

(a) running only one set of windings from the tranny - giving about 12VAC and 18VAC once rectified. This wasnt nearly enuf power. the output transistors werent even getting warm, and the sound was powerless and bass was distorting at about 5watt output power. This made me realise i needed more power.

(b) putting power chip rectifiers on the +VDC side of each amplifier module. These couldnt handle the input voltage and starting "motorboating" and getting very hot. I was running these rectifiers close to their max input voltage tolerance. So I gave up this idea. Should i try this again and have one rectifier for each amplifier module voltage input, one of the + and one for the negative?

(c) putting four 5W resistors on the +VDC rail to try to bring the power down. These were the equivalent of 8ohms, and reduced the voltage down from 36VDC at the amplifier module power input, to about 24VDC. However they were getting very hot, and still the output transistors were getting very very hot.

Does anyone have a simple (and hopefully cheap) way to bring the power down safely and effectively? From reading the above, i think i need to have a fully regulated PS, delivering approx 20VDC but at a high current, say about 2 amps. Should i try again and just get another transformer, maybe a single-tap delivering about 15VAC at about 2A?

Any ideas appreciated! I am gettign desperate to get these classic beauties going properly! Thanks.
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Old 7th June 2012, 02:24 AM   #2
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Make a simple (and cheap) regulated power supply using LM317 & lm337 paralelled with 2955 and 3055 transistors, I'm using this on an amp that I'm prototyping at the moment and it works very well. I'm at work at the moment but I can send you a circuit diagram when I get home later this evening (unless somebody else beats me to it).
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Old 7th June 2012, 03:56 AM   #3
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Hi Oliphant - that would be really great! If you wouldnt mind coud u plse email the diagram to slimecity@yahoo.com? These components will tolerate the voltages the transformer is outputting OK? Thanks!
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Old 7th June 2012, 04:14 AM   #4
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Yes they will tolerate them just fine - I'm using them to regulate 43V down to 30V. I'll email you the circuit diagram and post it here as well.
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Old 7th June 2012, 07:07 AM   #5
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As promised here is the regulated supply schematic. To adjust the output volage use the following: VO= -1.25V (1+R2/R1).

You must heatsink the transistors and regulators and ensure that they are well insulated from each other.

This solution is to suit a centre tapped transformer. If you have a dual secondary transformer you could just use two lm338's instead.
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Old 7th June 2012, 08:40 AM   #6
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R3,4,5 & 8 need to be 5W resistors. All others can be standard 1/2W type.
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