Digital Amplifier Questions

If you look at second hand OHMS, be sure to identify what version/generation drivers on them. They all tend to look similar on teh outside but each version will sound differnet with the most current ones being the most refined. Mine are series 3 which is one generation old now (current sereis are designated X000) and superceded original X series (including Walsh 4's) and mkII series. Also custom mods to all of those have been done over the years. So unfortunately it can be very difficult to know for sure sometimes unless the source is trusted or maybe John Strohbeen at OHM can help confirm.

Also with Bel Canto, newer ref1000m and ref500m include teh BC custom power board internally as well as custom input board with higher input impedance to work better with tube preamps. These make a difference and go for a premium accordingly. PRior ref1000 and ref500 models were basically stock Icepower modules in a nice box and can be had for much less accordingly. Also I believe 500m uses a newer version of Icepower than 1000m last time I checked about a year ago FWIW.

My experience has taught me that there is a vast difference in the approach that some manufacturers have taken with Class D technology versus others. An example would be Rotel vs Jeff Roland. Albeit, that we are talking about two companies targeting different market segments, but spending time with each has caused me to love one and despise the other. I'll let you guess which is which!

And yes, it loves to be kept on all the time.

RF leak thru power cord depends on the type of power supply. Rowland's supplies have zero current switching and power factor correction presenting load similar to resistive. No narrow current pulses. Ref1000m, a second generation uses also power factor correction. In contrast linear power supplies are, in reality, a very primitive switchers operating at 120Hz. They produce a lot of electrical noise since switching is done at max voltage (producing narrow current spikes of high amplitude). Higher switching frequencies of SMPS are easier to filter. Linear supplies in power amps require a lot of capacitors not only to filter out 120Hz but also to reduce voltage drops since they aren't regulated (line or load). For that reason Jeff Rowland uses zero current switching (+PFC) supplies operating at 1MHz, even in class AB amplifiers (Models 625, 725).

One comment on Damping Factor. Speakers are mostly resistive meaning that 4ohm rated speaker will measure likely 3ohm with DC ohmeter. This resistance is in the circuit limiting effective DF to less than 3. Adding 1/10 (0.3ohm) equivalent to DF=27 should not affect the sound. Above that it is only spectmanship. In class D amps very high DF, especially for low frequencies, is caused by the fact that one of the output switchers (Mosfets) is always connected to low impedance point (GND or Vs), making low output impedance to start with. Feedback reduces it even further.

Also, transition from A to B in class AB is audible - it is only matter of extend. It is because trans-conductance (Iout/Vin) is different for one transistor (class B) operating at given time vs two transistors (class A). It is called "gm doubling" and it adds to distortions in class AB amplifiers. It is also the reason why extending area of class A operation (increasing bias) increases distortions.

B_limo,

I had a Peachtree 220, 220se, and Grand Integrated. The 220se has the best overall sound. With the exception of the Grand, initially I enjoyed the sound of both the 220 and 220se... I never liked the Grand. After a couple of months, to my ears the Peachtree had a thin sound and something just never sounded right.
Recently I've listened to a friends Jeff Rowland 125 and with the exception of bass impact, it was one of the best amplifiers I've ever heard. I confess that's the only amplifier that I've had in my system that presented a three dimensional image.