Recommend amp project for new member

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
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
 
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.
 
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:
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
 
Ex-Moderator R.I.P.
Joined 2005
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
 
as i understand it. it is a dual mono with dual 500VA 2*31V transformers. wish also backs up the 40W class A power. and the chassis is huge.
so this can be used for class A amp. or bridged/X'ed class A/B amp:)

Hi guys,
It's a Doxa 70 from (I think) 1979 or thereabouts. It's dual mono, with two toroids and a big chassis with heat-sinks serving as the side members of the enclosure.

Thanks!
Henrik
 
It's amazing what you find if you go looking hard enough.
 

Attachments

  • Doxa-70Signature pwramp[1].pdf
    361 KB · Views: 50
  • DOXA 70.JPG
    DOXA 70.JPG
    282 KB · Views: 204
Last edited:
Ex-Moderator R.I.P.
Joined 2005
Hi guys,
It's a Doxa 70 from (I think) 1979 or thereabouts. It's dual mono, with two toroids and a big chassis with heat-sinks serving as the side members of the enclosure.

Thanks!
Henrik

:smash:

heatsinks appear like real heavy duty
but keep in mind that finning is rather short
lowers effiency more than you might think

but big enough to keep a classAB safe into low 2ohm load
 

Attachments

  • DOXA_70.jpg
    DOXA_70.jpg
    37.9 KB · Views: 201
40W of ClassA into 8ohms requires a peak ClassA output current of 3.2Apk

The quiescent current of the output stage would need to be set to ~1.6A to achieve that 40W, i.e. 320mA through each of those ten 0r56 emitter resistors. That is 179mVre

Could the bias be set that high?
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.