In need of amp

HuskerTRex

First of all - I am fairly new to the club and enjoy reading posts and have learned quite a bit already. I have been told I need an amp to run my Rti12's. I currently have an HK635 running my entire system. What would be a decent amp to run my RTI's? Also, could someone explain class A & Class B amp's to me? I have heard Adcom & Carver mentioned before? Are they pretty equal?

Thanks in advance!

thehaens@cox.net

Class A operation is where both devices conduct continuously for the entire cycle of signal swing, or the bias current flows in the output devices at all times. The key ingredient of class A operation is that both devices are always on. There is no condition where one or the other is turned off. Because of this, class A amplifiers are single-ended designs with only one type polarityoutput devices. Class A is the most inefficient of all power amplifier designs, averaging only around 20%. Because of this, class A amplifiers are large, heavy and run very hot. All this is due to the amplifier constantly operating at full power.The positive effect of all this is that class A designs are inherently the most linear, with the least amount of distortion.

Class B operation is the opposite of class A. Both output devices are never allowed to be on at the same time, or the bias is set so that current flow in a specific output device is zero when not stimulated with an input signal, i.e., the current in a specific output flows for one half cycle. Thus each output device is on for exactly one half of a complete sinusoidal signal cycle. Due to this operation, class B designs show high efficiency but poor linearity around the crossover region. This is due to the time it takes to turn one device off and the other device on, which translates into extreme crossover distortion. Thus restricting class B designs to power consumption critical applications, e.g., battery operated equipment, such as 2-way radio and other communications audio.

Class C amplifiers conduct less than 50% of the input signal. As such the distortion at the output is gross, but very high efficiencies can be reached—up to 90% or so. Some applications can tolerate the distortion, such as audio bullhorns. A much more common application for Class C amplifiers is in RF transmitters, where the distortion can be vastly reduced by using tuned loads on the amplifier stage. The input signal is used to roughly switch the amplifying device on and off, which causes pulses of current to flow through a tuned circuit. The tuned circuit will only resonate at particular frequencies, and so the unwanted frequencies are dramatically suppressed, and the wanted full signal (sine wave) will be abstracted by the tuned load. Provided the transmitter is not required to operate over a very wide band of frequencies, this arrangement works extremely well. Other residual harmonics can be removed using a filter.


Class AB operation allows both devices to be on at the same time (like in class A), but just barely. The output bias is set so that current flows in a specific output device appreciably more than a half cycle but less than the entire cycle. That is, only a small amount of current is allowed to flow through both devices, unlike the complete load current of class A designs, but enough to keep each device operating so they respond instantly to input voltage demands. Thus the inherent non-linearity of class B designs is eliminated, without the gross inefficiencies of the class A design. It is this combination of good efficiency (around 50%) with excellent linearity that makes class AB the most popular audio amplifier design.
Class AB plus B design involves two pairs of output devices: one pair operates class AB while the other (slave) pair operates class B.

Class D operation is switching, hence the term switching power amplifier. Here the output devices are rapidly switched on and off at least twice for each cycle. Since the output devices are either completely on or completely off they do not theoretically dissipate any power. Consequently class D operation is theoretically 100% efficient, but this requires zero on-impedance switches with infinitely fast switching times -- a product we're still waiting for; meanwhile designs do exist with true efficiencies approaching 90%.

Class G operation involves changing the power supply voltage from a lower level to a higher level when larger output swings are required. There have been several ways to do this. The simplest involves a single class AB output stage that is connected to two power supply rails by a diode, or a transistor switch. The design is such that for most musical program material, the output stage is connected to the lower supply voltage, and automatically switches to the higher rails for large signal peaks. Another approach uses two class AB output stages, each connected to a different power supply voltage, with the magnitude of the input signal determining the signal path. Using two power supplies improves efficiency enough to allow significantly more power for a given size and weight. Class G is becoming common for pro audio designs. Class H operation takes the class G design one step further and actually modulates the higher power supply voltage by the input signal. This allows the power supply to track the audio input and provide just enough voltage for optimum operation of the output devices. The efficiency of class H is comparable to class G designs.


reprinted from screensound.gov

scott

HuskerTRex

What do you think of Carver/Adcom? Any specific models I should look at? Also, forgot to ask in my original post - what type of connections do I need to make to my HK from the amp?
Thanks for your response!

mldennison

husker,
i have an adcom gfa 555 (200W/channel) and like it very much. you can get them on ebay for around $350-400 shipped. you could probably also go for the gfa 545 (150W/channel) and still be fine, those sell for a bit less. i have heard that the newer model adcom's sound somewhat different but still good. anything from adcom is going to be good bang for the buck. unfortunately cant really say anything about carver other than that i have heard others rave about them.

edit - as far as connections, you just need a pair of regular RCA interconnects to run from the pre-outs of your HK to the amplifier and then you speakers plug into the amp.

hope this helps,
mike

HuskerTRex

Thanks Mike!
I was just looking at the Adcom GFA-5500. Is that basically the same model as yours? Is it just a newer model? It is 200wpc, so must be similar if not the same.

mldennison

yeah, i would think that is just the newer model. i think they made a switch from bipolar transistors to mosfets and hence the new model. i have heard that the newer ones sound great! if you can find a decent price for it, i would say you cannot go wrong.

HuskerTRex

Thanks for your help!

dpowell

If you have a Good Guys in your area, they are going out of business over the next 4 weeks. You might be able to pick up a really nice receiver at a decent price there.

cfrizz

He has a receiver, he just needs a separate amplifier to hook up to his receiver to run his RTi12's to their full potential.

dpowell

cfrizz wrote:

He has a receiver, he just needs a separate amplifier...

My bad. I do believe Good Guys does sell separate amps. Maybe not the specific one he's looking for though..

aaharvel

THe GFA-5500 is and the 5xxx and 7xxx models from Adcom in general are great sounding amplifiers. Adcom's newer gear is noticeably more laid back than their older gear, which imo really benefits for music sources/critical listening. Neutral/dry sound.