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A blast from the past (2005), The scrapped KSA-50 clone
A blast from the past (2005), The scrapped KSA-50 clone
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Old 29th November 2020, 08:52 AM   #1
kmj is offline kmj  Sweden
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Default A blast from the past (2005), The scrapped KSA-50 clone

A friend of mine was cleaning and sorting his garage after moving three times in the last few years and found an old relic.
Instead of recycling it he asked if I wanted it.
When I saw what he had I almost made an Indiana Jones, yelling "it belongs in a museum!"

What he had dug out of the junk pile was a 2005 Pinkmouse Krell KSA-50 clone with a massive 30x2VAC transformer of 1kW and 4x68000uf capacitors TO-3 transistors and a large tunnel heatsink cooled with two 230V fans.

Dusty and dirty as hell and not very well built (if I remember correctly).

Now, this was at least 15 years ago but if I remember correctly around 2x25 VAC and minimum of around 4-500VA was "enough".

If I where to try and put it together and accually use it I need to make some alterations.
I need it smaller and cooler.
So I'm thinking mono blocks.

Cooler means lower rail voltage.

I happen to have two identical transformers but they rate in at 2x18VAC and 200VA each.

So, to get to the point (finally).
Has anyone starved the amp this way? Running it on this much lower voltage and what should I expect?

And what do you think about cooling? It would probably need considerably smaller sinks, right?
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Last edited by kmj; 29th November 2020 at 09:33 AM. Reason: Clarifying title
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Old 29th November 2020, 09:42 AM   #2
duncan2 is offline duncan2  United Kingdom
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Decide what wattage you want out of it and whether to run it in class A or class AB1( push-pull ) .


Its down to headroom before clipping so power supply voltage (continuous ) is a deciding factor .


I have built monoblocks ( true ) with outboard power supplies and designs that output higher wattage (mostly but not all ) can have their rail voltage reduced to a much lower level .


Class A though involves LARGE heat-sinks even at low wattage's .


There is plenty more to say but I am sure some here will add to this.
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Old 29th November 2020, 09:54 AM   #3
kmj is offline kmj  Sweden
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To be honest, it isn't that important.
I have neither space, speakers or even a full system anymore. Only a pair of Bluetooth speakers.

So I am on the level of using what I have lying around or fix at a minimal price or better, through trading.

So if I get 15-20W that is fine by me.
If goes over in class AB that is fine as well.
If I get my speakers up without the small ones poking Thier small fingers at the paper cone I will have some kind of full range design with a sensitivity around 95db.

I'll see if I can trade the large transformer for a smaller but if not I would like to use what I have if possible.

Since I don't know how to design amps I just wonder if I can use the parts from the amplifier as is and just switch the large one for the two smaller and get away with it without replacing other components. Not counting splitting the power supply to true mono that is and of course the heatsinks.

I have a recycling company close by and from time to time transformers, heatsinks and such can be found, so I am not in a hurry to get parts.

As long as I don't have to desolder and replace any components that is.

Sorry if I sound a bit uninterested but I just want it out together with as little quality loss as possible :-)
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Old 29th November 2020, 10:11 AM   #4
duncan2 is offline duncan2  United Kingdom
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I see your pictures show UK standard electrical wiring colours so its either been "built " (terribly ) in the UK of somebody in your area (Sweden ) had access to UK wiring.


Somebody will need to take you through a KSA-50 circuit diagram a bit at a time to give you what you want .


In life you only get out of it what you put in -simple and "easy " answers usually belong to those not of an "engineering mind " philosophy .
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Old 29th November 2020, 10:17 AM   #5
kmj is offline kmj  Sweden
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Haha, I know exactly who has built it (not me). He lives just a few km away. :-D

And I plan dismantle it and do it better.
Perhaps it is just easier to use the transformer and heatsink I have but then I probably won't use it afterwards :-(
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Old 29th November 2020, 10:25 AM   #6
Rallyfinnen is offline Rallyfinnen  Sweden
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I don't know the specifics of the amp, but it sound like you could keep it as is and only adjust the standing current to what you want. (Others please correct me if that's wrong)

If so, you could stand up the heat sink, remove fans, maybe even put a smaller heat sink, and adjust the standing current to whatever temperature you wish for

If you use high efficiency 8ohm speakers, you don't need a lot of standing current to run class A at normal listening levels(0,5A per channel maybe). At temporary peaks it could go to class B.
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Old 29th November 2020, 10:28 AM   #7
kmj is offline kmj  Sweden
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Now that's more like it!
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Old 29th November 2020, 11:34 AM   #8
ferret is offline ferret  South Africa
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The first thing to do is replace the power caps. For one they're probably partially "dried out" if not totally. Also with the trafo being 30VAC on the secondaries, the rated 50V of the caps is too close for comfort to the rail voltage (42 V) after rectification.
My 2 cents, but I prefer to err on the side of caution.
I don't know the Krell electronics and circuit but if you still bias it into Class-A with the 18 V trafos (probably around 23 to 24 V on-load to the boards) it will depend on the speaker sensitivity as to how well the Krell will be able to drive them.
There will be others who know this amp well and will be able to answer your questions way better than me.
All the best with refurbishing this, Kevin
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Old 29th November 2020, 11:45 AM   #9
kmj is offline kmj  Sweden
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Thanks, all cents are welcome.

On the caps, they are probably used for a total of five minutes. If it matters.
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Old 29th November 2020, 11:54 AM   #10
AudioSan is offline AudioSan  Norway
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With your 200VA 2x18Vac transformers and around 1.3A bias you will get around 25W class A @ 8ohm. That will give you a dissipation of around 60W pr ch. If you want it as cheep as possible, do NOT build monoblocks. Stay with a stereo amp, and active cooling.

ferret. How do you figure that 50V caps is to close to comfort? With 30Vac transformer will result in around 39Vdc loaded. +10% margin =43Vdc. Still you are almost 20% below the limit.
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