Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Solid State Talk all about solid state amplification.

New Musical Fidelity A-100 nonsense.
New Musical Fidelity A-100 nonsense.
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
Thread Tools Search this Thread
Old 14th April 2018, 05:46 PM   #11
Redbeard is offline Redbeard
diyAudio Member
Join Date: Jun 2010
Hi All,

Yes, there appear to be four zobels in this circuit, one on the output where you'd expect, one between the two phases of feedback and one to ground from each feedback leg. That's a surprising ammount of impedance matching or phase shifting.

  Reply With Quote
Old 15th April 2018, 02:58 PM   #12
chip_mk is offline chip_mk  Macedonia
diyAudio Member
Join Date: Jul 2005
Location: Skopje, Macedonia
Caps I mentioned in my previous post are designated as C11 and C12 in the last schematic shown.

BTW it looks strange, like some transistors are improperly oriented.
  Reply With Quote
Old 15th April 2018, 03:22 PM   #13
Zen Mod is offline Zen Mod  Serbia
Official Court Jester
diyAudio Member
Zen Mod's Avatar
Join Date: Jan 2003
Location: ancient Batsch , behind Iron Curtain
New Musical Fidelity A-100 nonsense.
Originally Posted by Zen Mod View Post
plenty of info here: Hi-Fi Musical Fidelity A1

my Papa is smarter than your Nelson !
clean thread; Cook Book;PSM LS Cook Book;Baby DiyA ;Mighty ZM's Bloggg;Papatreasure;Papa... by Mighty ZM
  Reply With Quote
Old 16th April 2018, 01:11 PM   #14
Redbeard is offline Redbeard
diyAudio Member
Join Date: Jun 2010
Thank you Zen Mod, i found Mark Hennessy's excellent web page before buying the amp. It does indeed contain an awful lot of useful information.

Chip_mk, ok, i started with the wrong ones - C15 and C16. Upping these by a factor of 10 has stopped the HF oscillation. This week (when i have time) i'll gradually reduce them until ringing starts again then up their values as you suggest. Or would i be better starting with C11 and C12 ?

Cheers Folks,

  Reply With Quote
Old 30th April 2018, 08:16 PM   #15
Redbeard is offline Redbeard
diyAudio Member
Join Date: Jun 2010

My humble apologies for not having posted for a while - they've been keeping me too busy at w**k to play with the old A-100.

Attached photograph shows the state of play at the moment.

Ok, the story so far - i changed C15 and C16 for 10 x value (i.e. 220pF) and the oscillation stopped. However, i'd also changes the two "bias" resistors (R6, R11) from the fitted 5.6MΩ to 2.4MΩ to see what that would happen.

The amp sounded great, but thermally shut down after about 20 minutes. Hardly surprising as i was biasing it hot and the attached heatsink (the silver thing scrounged from the scrap pile at w**k) only allows dissipation upwards, isn't of of as great an area as the proper lid, doesn't allow the fan to properly channel through the inverted "U" section as it should &c.

Last change was fitting "Vero" forked pins to the mounting points for R6,11 and C15, 16 to allow for easy component changes from above.

R6 and R11 were swapped back to their original values, C15 and C16 were changed for 100pF.

Last run, the left channel oscillated and sounded rotten but the right channel sounded alright, so i swapped C15 and C16 for 150pF.

Prodding showed that my soldering is dodgy. Back to the soldering board ;o)

On the photograph, the left-channel parts under investigation are circled yellow and green. Chassis modifications i have made are circled pale blue. +VB and -VB fuses, thicker mains lead with earth, earth-lift switch (closed, the chassis is earthed, open the chassis is earthed through 200Ω).

Some thoughts on the A-100.

"The A-100 is biased less into class 'A' than the A-1": Considering the +/- 35Vdc rails against the A-1's +/- 25V (or less ?) rails, maybe so in terms of rail-to-rail operation or maybe not in terms of actual operation.

In terms of servicing, it's the Devil's own job. A hatch underneath exposing the track side of the PCB would have made all this mucking about easier.

Design flaws: The circuit of the A-100 is practically taken straight from the A-1. As a result the +/- 12Vdc rails (for the "tails" of the LTP input stage and also for powering the drive op-amp) are really cooking. In the A-1 (running off +/- 25Vdc or so), R13 and R14 may be right but in the A-100 the resistance values are far too low.
The "tail" of the long-tailed pairs draws very little current, almost all the current through R13 and R14 has to be dumped bu the zeners. In mine i've doubled the 680Ω's to 1200Ω and they still cook. Next step is 2.4kΩ.

Thank you all, i know i'm just talking to myself. Thank you for your patience.

Neil Xx.
Attached Images
File Type: jpg IMG_3165 A-100 insides.jpg (506.2 KB, 78 views)
  Reply With Quote


New Musical Fidelity A-100 nonsense.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

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Musical Fidelity B1 Goetz Solid State 27 25th May 2018 12:48 PM
Musical Fidelity jacob solomans Multi-Way 4 26th April 2015 06:22 PM
Musical Fidelity B1 Mk. II Kris0603 Solid State 0 14th July 2013 04:01 PM
MUSICAL FIDELITY a1 cd pro gio_b Digital Source 0 4th June 2009 04:30 PM
Help Musical Fidelity B1 moatic Solid State 2 6th December 2006 03:52 PM

New To Site? Need Help?

All times are GMT. The time now is 06:11 PM.

Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.00%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio