Musical Fidelity amp - what voltage transformer? - UK Vintage Radio Repair and Restoration Discussion Forum

Welsh Anorak · 30th Aug 2017, 5:14 pm

Hi there
A friend has given me a stereo power amp to look at - a musical Fidelity Elektra E11. The mains transformer's duff. Does anyone know what voltage secondary I'd need? It's a standard XX-0-XX toroid, around 120VA I'd say, feeding twin bridge rectifiers and 2 x 10000uF 50V smoothers. I was considering a 35-0-35v but would that be sailing a bit close to the wind?
What do you think?
Glyn

mhennessy · 30th Aug 2017, 6:57 pm

It's rated at 60 watts per channel into 8 ohms.

As a rule of thumb, a 25-0-25 transformer should get you around 50 watts into 8 ohms, and 35-0-35 should get you around 100 watts into 8 ohms. This assumes a conventional bipolar output stage with good voltage efficiency rather than a MOS-FET source-follower design.

So in practice, the exact voltage specified originally is likely to fall in-between those values, but there are many other variables to consider. Personally, I'd try a standard 25-0-25, as any loss in output power will be inaudible in practice, but if you wanted to try 30-0-30, then go for it! I agree that 35-0-35 is a bit much for 50V caps, but I've seen worse from Musical Fidelity (in the B200, the 63V smoothing capacitors have over 70V across them!).

Of course, the transformer will have 2 windings rather than a CT (sorry for stating the obvious!)

Welsh Anorak · 7th Sep 2017, 2:58 pm

Hi there
Thanks for the info. I decided on a 30 + 30V (thanks for the correction!) transformer and the amp is now happily running ready for return.
Glyn

30th Aug 2017, 5:14 pm	#1
Welsh Anorak Dekatron Join Date: Apr 2005 Location: North Wales, UK. Posts: 6,918	Musical Fidelity amp - what voltage transformer? Hi there A friend has given me a stereo power amp to look at - a musical Fidelity Elektra E11. The mains transformer's duff. Does anyone know what voltage secondary I'd need? It's a standard XX-0-XX toroid, around 120VA I'd say, feeding twin bridge rectifiers and 2 x 10000uF 50V smoothers. I was considering a 35-0-35v but would that be sailing a bit close to the wind? What do you think? Glyn

30th Aug 2017, 6:57 pm	#2
mhennessy Dekatron Join Date: Jul 2007 Location: Evesham, Worcestershire, UK. Posts: 4,244	Re: Musical Fidelity amp - what voltage transformer? It's rated at 60 watts per channel into 8 ohms. As a rule of thumb, a 25-0-25 transformer should get you around 50 watts into 8 ohms, and 35-0-35 should get you around 100 watts into 8 ohms. This assumes a conventional bipolar output stage with good voltage efficiency rather than a MOS-FET source-follower design. So in practice, the exact voltage specified originally is likely to fall in-between those values, but there are many other variables to consider. Personally, I'd try a standard 25-0-25, as any loss in output power will be inaudible in practice, but if you wanted to try 30-0-30, then go for it! I agree that 35-0-35 is a bit much for 50V caps, but I've seen worse from Musical Fidelity (in the B200, the 63V smoothing capacitors have over 70V across them!). Of course, the transformer will have 2 windings rather than a CT (sorry for stating the obvious!)

7th Sep 2017, 2:58 pm	#3
Welsh Anorak Dekatron Join Date: Apr 2005 Location: North Wales, UK. Posts: 6,918	Re: Musical Fidelity amp - what voltage transformer? Hi there Thanks for the info. I decided on a 30 + 30V (thanks for the correction!) transformer and the amp is now happily running ready for return. Glyn