Musical Fidelity B1 amp - repair and small mod

[tuck1s]

Thought I'd share a recent experience with repair and mod of a Musical Fidelity B1 amplifier belonging to a friend.

The amp needed some minor mechanical repairs to the volume control. On testing and listening I noticed that the amp reached full power very "early" on the dial - on CD or iPod sources, via line level in.

A test on the bench confirmed that it was reaching max power (on a full scale sinewave in from CD) of 23v peak, or approx 33W, at only 10 O'clock on the volume control.

This helps to explain why he had damaged the HF drive unit on one of his speakers - one careless party visitor could have cooked them by driving the amp into clipping. (That was simple to repair, thanks to the nice people at Ruark).

I found the MF B1 schematics here: http://www.audiokarma.org/forums/showthread.php?t=59603

The line-level preamp built around IC2 seems to be the culprit, with a very high gain of 23, set by R27/R28.

I reduced the gain to approx. 8, by just bridging 6k8 resistors on the topside of the board, across R28. This is set to reach max. power from iPod sources (approx. 1v peak) when the volume control is at around 4 O'clock, i.e. nearly full scale. The practical advantages being:

- Reduced chance of accidental speaker damage
- and as a side-effect, should also have better channel balance at usual listening volumes, due to more of the pot range being used.

Sonically the end result seems fine and the amp measures OK after the mod.

More photos of the repair in progress can be seen here: http://www.thetucks.com/Musical_Fidelity_B1_amp/

My friend seems to be very pleased with the repair.

As CD sources can produce typically 3v peak, i.e. more than iPods, I also made a pair of special "attenuating" leads for him to use, with a resistive divider in each lead - in case the amp is 'partied' again.

A final remark on the mechanical construction. The plastic end-cheeks are rather fragile, and are held in place with captive nut plates that seem rather flaky. I accidentally damaged one, so re-inforced the end cheeks using hot-melt glue - ugly on the inside, but invisible on the outside.

In general, comparing the internal construction of the MF B1 to my Cyrus 2, I'm happy to own the Cyrus 2 - much better made.

[Livo]

Hi - I found a MF B1 at our local dump - missing its side and top cover.....cleaned it up and got it going - sounds gorgeous but want to fix that volume control so am very interested in your mod pics and description!
I have an old Sondek but still sounds good with a Leak Stereo 20 attached to Plinius pre and Power amps- am looking at a Cetech mod if I can find more info
Cheers
_I'm new to this forum (and all others too!)