Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 27th March 2009, 02:56 PM   #1
tuck1s is offline tuck1s  United Kingdom
diyAudio Member
 
Join Date: Jul 2008
Default Musical Fidelity B1 amp - repair and small mod

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.
Attached Images
File Type: jpg img_0072 lores.jpg (55.8 KB, 243 views)
  Reply With Quote
Old 8th January 2013, 08:57 AM   #2
diyAudio Member
 
Join Date: Jan 2013
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!)
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Musical Fidelity A3.24 repair ronenash Digital Source 54 10th September 2009 09:02 AM
Help Musical Fidelity B1 moatic Solid State 2 6th December 2006 02:52 PM
Musical Fidelity CD Pre 24 devron12 Solid State 5 7th January 2006 06:01 PM
musical fidelity repair mistamovie Solid State 14 3rd March 2005 11:19 AM
JLH Or Musical Fidelity A1? Jay Solid State 1 16th September 2003 12:40 PM


New To Site? Need Help?

All times are GMT. The time now is 12:24 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2