Hi here's the other half of your answer...
RE: Your MB-250 monoblock questions
Yes, the MB-250 is stable during 2 ohm dips, it is a high current output design.
We recommend the MB-250 for 8/4 ohm loudspeakers with dips as low as 2 ohms.
Not many loudspeakers dip below 2.5 ohms (none that I know of). I believe the Thiel's only dip to 2.5 ohms.
The concept of amplifier output power doubling at each halving of the load impedance is sometimes misunderstood and most timed over rated. The 8/4/2 specs you mention sound good but rarely ever hold true.
Mathematically (E=IR) @45V it would be: 45/8*45= 253W @8ohms; 45/4*45= 506W @4ohms; 45/2*45= 1012W @2ohms. It does look good but, in reality power doubling usually doesn't happen at 2 ohms.
A very good amplifier (the more expensive high-end models) will double the output power from 8 to 4 ohms but, generally fall short at 2 ohms.
Most amplifiers today will not even double the output power from 8 to 4 ohms. Many of the manufacturers simply feel it just is not worth the trade offs.
-> The concept usually fails due to these trade offs:
1. cost & size of the required power supply
2. the required number of output transistors
#1 Power supply
For a 250W amplifier to follow the halving rule to 2 ohms the power supply would need to be more than 2X the maximum output power. For 1000W output @ 2ohms the supply would need to be more than 2000W. This would be very expensive and large.
Why 2X? Because output stages are not 100% efficient, a class B is about 50% a class A/B about 40-50% and a class A is usually 40-50%.
Using a class B output stage (very popular) and putting in 2000W in you get 50% or 1000W out.
There are also more losses that I'm simply neglecting for simplification.
Therefore, the power supply would need to be 2000+ Watts per channel!
#2 number of output transistors
Each transistor can output only so many watts, the limit is a thermal one (how hot the transistor gets) and is set by the transistor spec, package size, and heat sink.
Popular transistors can output 60-150W (different types) continuous. So, 1000W output/100W (an average transistor) is about 10 transistors for the positive half cycle and 10 for the negative half of the cycle or, 20 transistors total. This is required to double the power to 2 ohms!
FYI w/100W transistor: 250W=6Trans; 500W=10Trans; 1000W=20Trans
Matching the transistor gains and minimizing crossover distortions, would be a difficult if not impossible job with 20 transistors. This could easily produce a harsh, grainy, brite sounding amplifier.
This is why many professional reviewers (i.e. Stereophile) say the lower power SS amplifiers usually sound better than high power models.
Here is How the MB-250 design looks today:
The MB-250 monoblock uses 10 output transistors. This will produce a cool 500W and require a power supply of 1000+ W.
We use an oversized power supply (as usual) of 1600W. It has a 36 Lb shielded toroidal power transformer and large filter caps (100,000uF [1CH]). This supply and the output transistors used will allow 800Wmax output music peaks at low load impedance dips.
More information and specs will be available on our website as we continue the MB-250s development.
It is a monoblock with an estimated selling price of US$800 each. This price could change at release time. Two MB-250s will be required for stereo applications. The target release date is 4th quarter 2003.
Thank you for your interest and we hope to hear from you when the MB-250s are ready.
