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New Musical Fidelity A-100 nonsense.
New Musical Fidelity A-100 nonsense.
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Old 14th April 2018, 04:46 PM   #11
Redbeard is offline Redbeard
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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.

Xx.
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Old 15th April 2018, 01:58 PM   #12
chip_mk is offline chip_mk  Macedonia
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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.
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Old 15th April 2018, 02:22 PM   #13
Zen Mod is offline Zen Mod  Serbia
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New Musical Fidelity A-100 nonsense.
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Originally Posted by Zen Mod View Post
plenty of info here: Hi-Fi Musical Fidelity A1

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Old 16th April 2018, 12:11 PM   #14
Redbeard is offline Redbeard
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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,

xx.
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Old 30th April 2018, 07:16 PM   #15
Redbeard is offline Redbeard
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'Phew,

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.
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File Type: jpg IMG_3165 A-100 insides.jpg (506.2 KB, 72 views)
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