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Old 15th October 2013, 08:45 AM   #1
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Default Recommend amp project for new member

Hi all!
New member, Henrik from Norway here.
I have some electronics experience building my own tube amps for guitar, but I've never built a solid state hifi amp. I'm planning to build a pair of 4xkt88 monoblocks when time and funds allow (for this project, I was thinking of doing the M-125's from tubes4hifi, but I'm open to other suggestions), but this is beside the point. Before I get to that, I wish to upgrade what's running my speakers these days. I have an old DOXA (Norwegian small-scale manufacturer) power amp, which is very old-school class A. It runs very hot, it hums and it hasn't got a lot of power. What it does have, though, is a massive power supply; namely 2 toroids with twin 250VA 31V secondaries each, which (if I'm not mistaken) makes them 500VA units. I should think that these would be suitable to run an amp with some more power and (hopefully) a lower idle temperature. I was hoping to build a dual mono design with somewhere between 100 and 200W at 8ohms per channel. The existing chassis is spacious and has big heat sinks down each side.
I can see that there are a number of power amp boards available on ebay from Chinese sources and they seem to be great value, but I have no idea if they are any good or which ones are most suited for my project. I'd prefer to not have to design my on PCB's - the boards and parts for the amp modules should be available off the shelf.
Could someone please help me find something sensible to build?
Any tips are greatly appreciated.

Cheers!
Henrik
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Old 15th October 2013, 05:34 PM   #2
Aniket is offline Aniket  India
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DIY AB amp(The Honey Badger). you could get boards from the DIY store. above all you got all the support here.

diyAB Amp The "Honey Badger" build thread


diyAB Amp - The "Honey Badger"


Regards,
Aniket
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Old 16th October 2013, 12:28 AM   #3
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the output power is determined by the voltage swing on the output. and that is determined by the railvoltage. so if you take a transformer from one amp and use it in another amp, you will not get any more power.
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Old 16th October 2013, 03:07 AM   #4
zlast is offline zlast  United States
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Check out the NMOS200, KRELL KSA50, and chip amp projects. You could get more power using your existing power supply with a parallel chip amp setup. If your looking for a hobby, build a chip amp (very simple), then you can maybe try the NMOS200 or Krell.
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Old 16th October 2013, 09:51 AM   #5
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to change a DOXA amp for a chip amp is like selling your ferrari cose it don't performe well enough, and go and buy a volvo it's a hugh step in the wrong direction.
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Old 16th October 2013, 09:53 AM   #6
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you also have the firstwatt F5 turbo.
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Old 16th October 2013, 10:16 AM   #7
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Quote:
Originally Posted by zlast View Post
Check out the NMOS200, KRELL KSA50, and chip amp projects. You could get more power using your existing power supply with a parallel chip amp setup. If your looking for a hobby, build a chip amp (very simple), then you can maybe try the NMOS200 or Krell.
Not with the same speakers you wont. You could go for a bridged amplifier.

The PSU is intended for Class A use, ie lower voltage but higher current. You'd be better looking at one of the myriad of Class A designs on offer but that would still leave you with the heat issue.

Last edited by KatieandDad; 16th October 2013 at 10:19 AM.
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Old 16th October 2013, 11:55 AM   #8
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Originally Posted by KatieandDad View Post
Not with the same speakers you wont. You could go for a bridged amplifier.

The PSU is intended for Class A use, ie lower voltage but higher current. You'd be better looking at one of the myriad of Class A designs on offer but that would still leave you with the heat issue.
Yup, I know. I was hoping that there would be a class AB design that would work well with my voltage, run a bit cooler and still provide enough power. Actually, increasing the power isn't my main concern - I just thought that this design being more than 30 years old, there would be something better and more efficient to run off the same transformers today. My main concern is the heat and hum that his old class A amp generates at idle. It sounds good, but not spectacular. Would the honeybadger work with my voltages?

Thanks for all your input, guys!
Henrik
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Old 16th October 2013, 12:39 PM   #9
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I'd consider repairing it. The hum is most likely being caused by old reservoir caps.
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Old 16th October 2013, 01:18 PM   #10
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by Henrik76 View Post
I have an old DOXA (Norwegian small-scale manufacturer) power amp, which is very old-school class A. It runs very hot, it hums and it hasn't got a lot of power.

What it does have, though, is a massive power supply; namely 2 toroids with twin 250VA 31V secondaries each, which (if I'm not mistaken) makes them 500VA units.
noo, its not classA
maybe it's got 5watt in classA
but never mind

the DOXA I can figure in my head does not have very large classA heatsinks
good heatsinks, but not that big
and so will only be suited for small classA
but maybe I remember wrong

and your trafo voltage is too high for that

but shouldn't be hard to find a nice 100watt classAB design

well, I see now there is an older dual mono Doxa 70 with quite large heatsinks on each side
that one would be fit for something like 4 pair output devices each channel, and drive low impedance
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