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Musical Fidelity A370 refubr
Musical Fidelity A370 refubr
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Old Today, 05:20 AM   #11
Ian Finch is offline Ian Finch  Australia
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Location: Coffs Harbour, NSW
I stand corrected if that is in fact a bias pot. but how would offset be adjusted, if not by swapping components in the production line? Then it probably also requires the same parts swapping process whenever semi replacements are necessary.

A class AB amplifier certainly needs bias adjust but I would have thought it unnecessary for class A, where fixed resistors are generally fine, cheaper and superior for reliabilty. If it's only a partial class A or high bias class AB design, it becomes arguable whether bias adjustment is a wise inclusion or not because there is a likelihood of overheating and serious damage in the hands of unwise tweakers.
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Old Today, 07:15 AM   #12
duncan2 is online now duncan2  United Kingdom
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While I cannot fault your engineering logic in your second paragraph it is a fact of life that there are many class "A" power amplifier designs that include adjustment with or without the use of a BJT .


Heat has to come into consideration as does the size of the analogue power supply.
As JLH has pointed out many people make use of a portion of full class A say- 5W -10W then sliding (smoothly in a good design) into AB.
If the customer lives in a suburban setting with neighbours even 10W pure class A is bound to attract the attention of the police.


Cost certainly enters into it for the general public ,class A is always dearer but the attraction of an amplifier to the public which is cheap but advertises "class A " but is really limited does sell equipment .


On the other hand--
you live in the "Outback " in isolation and are well off then a very large ,heat inducing piece of equipment is just the job, that is if the "other half " allows it in the living room.


I repeat I am NOT questioning your engineering logic just saying "horses for courses ".
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Old Today, 12:05 PM   #13
Ian Finch is offline Ian Finch  Australia
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Join Date: Apr 2010
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Quite so and no problem with questioning. For starters, small class A designs like JLH's 1969 design and some of Sugden's commercial offerings can certainly be adjusted from modest levels to full class A bias.

However, if I were concerned at all about the effect of DC at the output of a balanced rail supply amplifier, I'd hope for either a blocking cap, DC servo (nulling circuit) or adjustment pot. to reduce it with certainty to <50 mV. I wouldn't trust the 5% tolerance carbon resistors in Sugden's early designs, for example, to achieve that without trimming.

For interest, MF products arrived in Oz back in the 1980s, with a splash of retail promotion and loads of models to choose from, busy shops and brisk sales, even here in my current location, hundreds of km from capital cities. They left a trail of failures behind them though - just not designed with enough tolerance to higher ambient temperatures, nor with appropriate component ratings for safety and reliabiity.

They returned in more recent times with the high end A series and I wound up with a shop demo A5 model CD player - promising much at $4k but failing to read any disc at all after some 40 plays. They were credited with using trivista valves in the line amplifiers but even they couldn't save the day for this fragile and by comparison with contemporary Japanese brands, poorly engineered piece of work. It did sound OK if rather dry, but other models with 24 bit DACs at far lower prices sounded quite OK too.
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Old Today, 01:49 PM   #14
duncan2 is online now duncan2  United Kingdom
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Interesting Ian this is just the type of conversation I enjoy --interaction on a technical level.
Never owned MF equipment but I see its now owned by an Austrian company -Audio Tuning-2018 .
I remember the A1 amplifier got a good write up by UK "golden ears " at the time.



Antony wasn't a enthusiast of exotic cables and neither is D.Self who used a ring main electric cable in EW to prove his point.
Certainly caused some arguments there ,a lot of the fire has gone out of EW now I enjoyed the arguments in the Letters section.
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