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Thread: Developing the MA100 amplifier

  1. #1
    Join Date: Apr 2012

    Location: South West of England

    Posts: 268
    I'm Will.

    Lightbulb Developing the MA100 amplifier

    Among seemingly a million other projects, I have been developing a new flagship amplifier, using a newly developed topology (for lack of a better word, it pains me slightly to use it as it is so thrown about these days - but I think this just about qualifies)

    The first amplifiers to use this "topology" will be 100W KT88 or compatible monoblocs. Somewhat like Radford's own ISA series of amplifiers that were conceptualised but sadly never materialised, the aim is to achieve a genuine step up in performance in valve based amplifiers which I believe I have achieved with this design, and a working proof of concept exists.

    The design is inspired by Radford's own TT100 amplifier, in that it is a hybrid design. The output stage is of course valve, but driven via a highly linear differential Class-A transistor stage. The actual amplifier circuit is extremely simple, but afforded very high performance thanks to other innovations, and perhaps to the simplicity itself.

    The basic thinking behind this new design is to eliminate or significantly reduce some of the misbehaviours the vast majority of valve amplifiers exhibit - ones that cannot possibly be argued to contribute a pleasant coloration. In doing so, both the subjective and measured performance significantly improves, and the benefits can clearly be demonstrated.

    Chasing low distortion was not the goal, but measurements taken from the prototype amplifier, show THD on the onset of clipping at 1Khz, as low as 0.03%. Distortion reduces further with level and at typical listening levels, practically nonexistent. No extreme measures were taken to lower the THD, but this improved linearity is a natural result of removing sources of "misbehaviour" elsewhere in the amplifier.

    THD at full power at 10Khz is still as low as 0.2%, which for a Class-AB amplifier using a single pair of output valves, is remarkably low.

    This new amplifier will feature a few innovations such as fully automatic biasing (not a new concept in itself), using simple analogue circuitry to set the true quiescent point of the amplifier at all times. This affords a useful advantage in that valve matching becomes far less important for good performance, and eliminates all biasing worries - the bias will be set to a higher degree of precision than is strictly needed, at all times.

    There are more innovations to list, so more details, including prototype pictures, to come soon!!
    Radford Revival
    www.radfordrevival.co.uk

  2. #2
    Join Date: Sep 2014

    Location: Northern Ireland

    Posts: 1,060
    I'm John.

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    Sounds great Will.

    Would be nice if the filament supplies were heavy enough to allow KT120 and KT150 to be used too. Would this require the auto bias set point to be adjusted?

    What size of chassis is required roughly?

  3. #3
    Join Date: Oct 2012

    Location: The Black Country

    Posts: 5,008
    I'm Alan.

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    Quote Originally Posted by Radford Revival View Post
    The actual amplifier circuit is extremely simple, but afforded very high performance thanks to other innovations, and perhaps to the simplicity itself.
    Loving the sound of this Will, just the approach I favour, to good result as well. However my amp is valve driver and MosFet output.
    Vive la difference.

  4. #4
    Join Date: Apr 2012

    Location: South West of England

    Posts: 268
    I'm Will.

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    Quote Originally Posted by JohnJo View Post
    Sounds great Will.

    Would be nice if the filament supplies were heavy enough to allow KT120 and KT150 to be used too. Would this require the auto bias set point to be adjusted?

    What size of chassis is required roughly?
    KT120 and 150s will indeed be supported and heater supply specified to cope, and I have tested KT150s. The trade-off with using KT150s appears to be higher peak current delivery at the cost of slightly higher overall distortion. These would work well with particularly difficult loads. The power supply I intend to use will easily cope.

    Zero adjustment will be necessary to change between valves, just plug and play. All variances are adjusted for by the bias servo. Zero worries about biasing.


    Quote Originally Posted by Firebottle View Post
    Loving the sound of this Will, just the approach I favour, to good result as well. However my amp is valve driver and MosFet output.
    Vive la difference.
    Another way to do things! Typically this is what people think of in terms of a hybrid, so I guess these MA100s will be "backwards" hybrids
    Radford Revival
    www.radfordrevival.co.uk

  5. #5
    Join Date: Sep 2012

    Location: London

    Posts: 418
    I'm Nick.

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    Very nice, Will. I have an original Radford TT100 and a modernized, upgraded version of that concept will be awesome. Looking fwd!

  6. #6
    Join Date: Apr 2012

    Location: South West of England

    Posts: 268
    I'm Will.

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    I will try to detail some of the philosophy behind this new design in this post.

    Valve amplifiers often have fairly poor behaviour near or at full power, and especially for a period after time after even brief clipping. This is responsible for the well known phenomena of valve amplifiers "falling apart" or "turning to mush" into a difficult load or being used near their power limit.

    The occasional clipping isn't itself the problem - in fact this isn't even normally noticed. The issue is the long recovery time most valve amplifiers exhibit after the fact. There are actually a few mechanisms responsible, and unfortunately they all interact to make the whole problem even worse. If a valve amplifier is being used near its maximum output power, it is likely these problems will show up.

    This new design eliminates these issues allowing much more apparent dynamic range and no "falling apart" even when tasked with driving a horrendous load. The resulting amplifier is solid-state like (to the point where it's uncanny) in its bass grip and damping, yet retains the pleasing low-order harmonic profile of a valve output stage.
    Radford Revival
    www.radfordrevival.co.uk

  7. #7
    Join Date: Feb 2013

    Location: W Lothian

    Posts: 57,408
    I'm Grant.

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    Sounds intriguing
    Regards,
    Grant .... ؠ

    I've said it before and I'll say it again: democracy simply-doesn't-work
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  8. #8
    Join Date: Sep 2014

    Location: Northern Ireland

    Posts: 1,060
    I'm John.

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    Does it use two output valves per channel?

  9. #9
    Join Date: Apr 2012

    Location: South West of England

    Posts: 268
    I'm Will.

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    Quote Originally Posted by JohnJo View Post
    Does it use two output valves per channel?
    Yep - a pair of KT88 or compatible for a target of 100W. The output valves will be pure tetrode connected and not ultra linear (genuine technical reasons behind this) so this power figure shouldn't be too out the way, nor hard on the valves, especially with the features I've not even disclosed yet
    Radford Revival
    www.radfordrevival.co.uk

  10. #10
    Join Date: Apr 2012

    Location: South West of England

    Posts: 268
    I'm Will.

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    Developments still being made - I also managed to get as low as 0.025% distortion out the thing at full power / 1Khz!
    Radford Revival
    www.radfordrevival.co.uk

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