Musical Fidelity A1 bias

[tubeing]

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

[tiefbassuebertr]

Originally Posted by tubeing

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

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.

[nigelwright7557]

Originally Posted by tubeing

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

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.

[tubeing]

Originally Posted by tiefbassuebertr

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.

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.

My schematich is different from the one you posted here, but I can use it as a very useful starting point, thanks again!

Stefano

[tubeing]

Originally Posted by nigelwright7557

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.
...

Sure, the amp sounds good as it is (after the beauty farm). I have not measured the output, but I'll do this weekend. I'd like only to know if the amp is correctly biased (as it was originally designed).
Thanks
Stefano

[cchean]

I am trying to restore a MF A1 too. According to Mark Hennesy's pages, the value of C15 and C16 is 33pF but other schematics show them as being 22 pF. Which value is correct?

[Zen Mod]

not of great importance - just leave them in

[jez]

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.....