Musical Fidelity A1 bias

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tubeing

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#1
Hi to everyone,
I've recently recapped an old Musical Fidelity A1 (MKII I think, PCB David II) to get rid of distortion and other functional problems. Now the amp works flawlessy but I want to be sure about the Ibias value. I measured 0.51A on both channels, but on the web I've found 0.7-0.8A. Could it be because of different PCB version?
Thanks in advance.
Stefano
 
#2
tubeing said:
Hi to everyone,
I've recently recapped an old Musical Fidelity A1 (MKII I think, PCB David II) to get rid of distortion and other functional problems. Now the amp works flawlessy but I want to be sure about the Ibias value. I measured 0.51A on both channels, but on the web I've found 0.7-0.8A. Could it be because of different PCB version?
Thanks in advance.
Stefano
Click to expand...
Yes. This schematic here is from ISS7 (Issue 7 - that means there are at least 7 variations) and a second schematic - only for simulation with DC results (also based on ISS7).
Before you start the troubleshooting replace all electrolytic capacitors through new types.
Use 105 degrees and 63V voltage versions, even if there are much lower presently voltage !!! - this enhances lifetime and soundquality - even by half value of whole capacitance.
After do this cut the wires between preamp and power amp so that you can examine both parts separately. Start with the preamp. After this work fine, then start with troubleshooting at the power amp section.
 

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#3
tubeing said:
Hi to everyone,
I've recently recapped an old Musical Fidelity A1 (MKII I think, PCB David II) to get rid of distortion and other functional problems. Now the amp works flawlessy but I want to be sure about the Ibias value. I measured 0.51A on both channels, but on the web I've found 0.7-0.8A. Could it be because of different PCB version?
Thanks in advance.
Stefano
Click to expand...

If your not getting crossover distortion i would settle with the lower figure.

I am not a fan of high bias currents, they can be wasted power and heat.

I run my amps with a bias current as low as 10mA per output pair and dont get crossover distortion.
 
#4
tiefbassuebertr said:
Yes. This schematic here is from ISS7 (Issue 7 - that means there are at least 7 variations) and a second schematic - only for simulation with DC results (also based on ISS7).
Before you start the troubleshooting replace all electrolytic capacitors through new types.
Use 105 degrees and 63V voltage versions, even if there are much lower presently voltage !!! - this enhances lifetime and soundquality - even by half value of whole capacitance.
Click to expand...
Thanks for your help!
I replaced, before your suggestion, all the elec. caps with Panasonic FC series, but I used 16 and 25V. The amp sounds much better than before (more open, detailed and dynamic) but I've lost some warmth...I noticed that that there were only 4 10000uF - 25V mounted, but there was another footprint for another 10000uF per side (pigreco filter), so I mounted also these smoothing caps.

After do this cut the wires between preamp and power amp so that you can examine both parts separately. Start with the preamp. After this work fine, then start with troubleshooting at the power amp section.
Click to expand...
My schematich is different from the one you posted here, but I can use it as a very useful starting point, thanks again!

Stefano
 
#8
I think the bias was reduced on later versions due to reliability problems.
We certainly used to reduce it somewhat in many samples that were returned as faulty, simply to try and find some reliability and not get the same amp back for repair a month later.....
 
#9
It's amazing how many different variants there exist of this amp; I have a revision 3 there - it does not even have a serial printed on it - it is again a little bit different from this revision 7 and revision 8 from Mark's website.
 
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