Tom Evans Audio Design

Tom Evans Linear B Mk3 Mono Block Valve amplifiers


The new Mk3 power amps both have 'user variable feedback' a multi position switch sited on the rear of the amps just below the bespoke 'purest' Copper binding posts.
Both amp designs have a low enough output impedance when switched to zero feedback to drive a range of impedances that will cover and drive pretty much all speaker designs. Used in this way they will provide the listener with an accurate reproduction of the recorded event but without the usual dynamic compression and loss of finer detail caused by feedback itself.

Both designs will also match the varying impedance of your loudspeakers across frequency assuring a flat in room frequency response (unlike all transistor power amp output stages!).

Just look for an impedance plot of your loudspeakers, you would be surprised what you find as it wont be flat and may well dip to below half the quoted 'nominal' impedance. This forces any transistor power amp to more than double the power - as an example. you’ve almost certainly read where a transistor amplifiers output is listed as: 60 Watts into 8 Ohms, 120 Watts into 4 Ohms.

Unsurprisingly, this constant up and down moving about of your speakers impedance can really cause it to SHOUT at you! Pasted Graphic
And now we’ve told you that, we bet you’re now thinking - “oh yeah, I’ve noticed that”.

So you can understand now the importance of the amp / speaker interface in being able to match the speakers varying impedance across frequency. This will in turn then provide you with a much flatter “in room” frequency response - on the right track at last!

Matching the amp to the speakers load.

Forget wattage figures, as when it comes to reproducing music they are pretty meaningless figures on their own, far more important is the dynamic range PER Watt and then how many Watts per channel. If the dynamic range figure PER Watt is truly huge, all of the detail will be
present no matter the volume level.

The lower the noise and distortion(s) 'floor' is then the greater the signal dynamic range PER Watt will be and the more natural and lifelike and louder it sounds, it's also exactly where all that finer low level detail is buried.

On both amp designs the HT power supply noise will be around minus 138 db, this is not including the valve circuits 'natural' added rejection figure. The Lithos regulated input and driver stages ( and active valve bias control circuitry) greatly exceed the HT rejection figures!
You need to know the minimum impedance figure your speakers provide as what they don’t tell you is the normally quoted 8 Ohm 'nominal' speaker you own, may well dip to below half that figure, or worse!

It's ok though, as the hifi industry (no names) has been getting away with that one for many years now. Well at least we can provide you with a design solution for resolving that industry wide speaker design issue.
If you are like us, and just want the music to sound as real as can be, you are not going to get the desired end result with the way the current amp / speaker interface has been dealt with industry wide.

Pretty much all power amplifiers on the market are of the voltage feedback type as both of ours are, the lower a speakers nominal impedance figure, the more voltage feedback is needed to drive the load, the more feedback used the worse it will sound!
I have provided both amp designs with enough switchable feedback positions on the rear of the amps for you to entertain yourselves hearing all of the ills of using too much feedback into the speakers you have at home.

Adding any more than 2 db of feedback and it starts to lose the 'alive and real' natural open stereo sound, that “projected into the room” solid stereo 'picture' is gone, and it sounds good, but like 'normal' hifi again, and no longer the very convincing insight into the recorded event.
Using zero feedback, the Mk3 stereo chassis Linear A will drive a minimum impedance of 6 Ohm at 90 db or 8 Ohm at 88 db, adding a couple of feedback switch positions to adjust (only if needed) but any more than 2 db (4 clicks) and you can clearly hear the start of dynamic compression of signal.
The mono chassis differential output Mk3 Linear B's have 4 times the power of the stereo chassis and will drive a minimum of 3 Ohm loads when switched to the zero feedback position.

Choose the model of amp according to the minimum load impedance and sensitivity figure of the speakers you own, you will then be able to run the amps using no feedback at all. This really will bring your recorded music to life!