Developing the MA100 amplifier

[petrat]

Will sure does know his stuff .... and he's a perfectionist, so these will be something special, I'm sure.

A year on, and I'm still awestruck by the STA-100 he rebuilt and kindly sold to me. I thought it was my last amplifier, but looks like the new MA-100 will be very, very tempting indeed.

[nickbaba]

Any thoughts about the form, Will? I imagine form will follow function to a degree, but I know the old TT100 looked very different to the Radford STA series - it's basically a big black metal box!

Will your MA100 look similar to the Radford STA series, or the old TT series, or be something else again?

[Radford Revival]

Will sure does know his stuff .... and he's a perfectionist, so these will be something special, I'm sure.

A year on, and I'm still awestruck by the STA-100 he rebuilt and kindly sold to me. I thought it was my last amplifier, but looks like the new MA-100 will be very, very tempting indeed.

Great to hear you're still enjoying your amp Let's see how much I can temp you with the shiny new ones once they're ready

Any thoughts about the form, Will? I imagine form will follow function to a degree, but I know the old TT100 looked very different to the Radford STA series - it's basically a big black metal box!

Will your MA100 look similar to the Radford STA series, or the old TT series, or be something else again?

The new amp will be visually very much in keeping with the classic older Radfords with handles etc - though slightly rejigged so that it is safe to run without the perforated cage (but it will come with one). They will quite closely resemble the STA100 but be a bit smaller. As monoblocks they won't be so heavy either so far easier to deal with, the STA100 and TT100 are very awkward as a result of their weight - this was one of the main reasons I went with monos.


One feature I have developed worth mentioning is an auto-standby system which will reduce the bias current to around half after a number of minutes with no signal. On the detection of a signal the bias will recover to the full value within a couple of seconds (the transition will be inaudible and seamless)

As many amps spend a signification proportion of their time idle this will have an obvious benefit in terms of valve life and power consumption - a valve amp sitting there reproducing silence clearly doesn't need full idle dissipation. It will be possible to disable this feature with a simple jumper or switch if this does not appeal.

All the advanced features such as the automatic biasing and standby have been kept as simple as possible to avoid overcomplexity - they are all simple analog circuits using very ordinary parts working with no stress or dissipation, they have been carefully specified so that their inclusion is worth the few extra components while not adding needless complication.

There will also be robust protection circuitry to ensure no damage occurs to the amp in the event of a valve failure. No one likes sending things back for repair, or smoked up boards. The aim is for it to be pretty much impossible to blow up even with extremely faulty valves.

[CliveG]

Like Petrat I would not have thought I could be tempted away from my STA100, but this sounds an attractive proposition. Look forward to hearing more and seeing some pictures!

[nickbaba]

Sounds nice, Will. Incidentally that would be my preferred look - love the style of the STA series.

[Radford Revival]

Development has been continuing in the background on this - today I managed to massively increase performance as I had been unintentionally hampering the thing!

[thingfish]

Will....."You build it, they will come"

Anyway Wot about this new pre-amp ???

[Radford Revival]

Will....."You build it, they will come"

Anyway Wot about this new pre-amp ???

That's in the pipeline after these monos

It has become apparent that they (the amps) will be something rather special.

Essentially this will be the valve amplifier for those who love the idea of one, but also want low distortion and IMD, impeccable overload recovery, tolerance of difficult to unreasonable loads and a wide bandwidth. As far as I can tell this design has indeed pushed the envelope.

[southall-1998_mk2]

Good stuff, Will. Still lusting after the STA15 Series 5. Maybe someday!

S.

[Radford Revival]

Good stuff, Will. Still lusting after the STA15 Series 5. Maybe someday!

S.

Tentative rough specifications of this new design, subject to changes as things are tweaked / refined:

Bandwidth 10Hz - 50Khz (100Khz -6dB approx)

THD into 8R / 1Khz:

Just over 1W: 0.004% (Almost entirely just 2nd and 3rd harmonic)

Approx 10W: 0.008% (Almost entirely just 2nd and 3rd harmonic)

Approx 25W: 0.012%

Full power: 0.025% (amount of power slightly curtailed in prototype due to unregulated screen supply dropping significantly)

Yes, this is with a valve output stage, and this is actually significantly better than many solid state amplifiers, even currently sold ones, especially in terms of harmonic spectrum at low levels.

Output impedance approximately 0.3 ohms, substantially resistive even until top of audio band (not directly measured as of yet)

Minimal peaking / ringing into capacitive loads, unconditional stability