Preamplifiers

Classe CP-800 Stereo Preamplifier

ARTICLE INDEX

The Design of the Classé CP-800

The front of the CP-800 has an on/off button (standby), with the main power on/off toggle on the rear. There is an LCD panel that shows the volume level, but also has a menu for you to select DSP options, such as the frequencies where you would like to decrease the level (such as a room mode). The volume control dial is huge, which I like, because I have big hands. There is also a USB port, which is configured for connecting to your iPod (the oldest versions of iPods will not work with this port, however).

The rear panel is shown below.

You can see the on/off toggle, a grounded AC socket, digital input connections consisting of a USB jack, XLR, three coax, and four Toslink optical jacks. There are also trigger jacks, and RS-232 jack, and an Ethernet port (for future use). Analog inputs include three sets of coax and two sets of XLR. Analog outputs are two sets each of XLR and coax. The USB jack on the rear is asynchronous (as is the one on the front), and is for use with a computer's digital music output (so, consider this input a "USB DAC").

 

 

When I said the CP-800 is unique, I mean unique. Innovative is another word that comes to mind. The ability to take all digital circuitry out of the signal path so that the preamplifier is totally analog is a brilliant stroke of design engineering.

The switch mode power supply takes the incoming AC power, coverts it to DC, switches it at 140 kHz, ± 10 kHz depending on the music signal, and then smooths it with capacitors and inductors much the same as a standard power supply.

Below is shown a generic schematic that I came up with (by looking at many schematics on the Internet) that shows the path of power, so to speak, of a switch mode power supply.

 

 

So, the incoming 120 volt AC goes through an EMI filter to remove radio frequency noise, then is rectified (converted to DC), and in the case of the CP-800, passes through a Power Factor Correction circuit (described below), then to the switcher, a voltage stepdown transformer, a second rectifier, and finally, the filters that smooth the DC so that it does not have any ripple. The fact that the voltage going through the stepdown transformer is at a high frequency, means that the transformer can be much smaller than if it were used at 60 Hz from the wall. The capacitors can be smaller too. The power supply is the most expensive part of hifi equipment, and being able to use smaller components due to the high freuquency reduces the cost to the consumer considerably. As such, this product, at $5,000, would cost somewhere around $8,000 if it were to use a conventional power supply of the same quality.

Now, about the Power Factor Correction: When AC comes into your hifi component and passes through a transformer (used to create the rail voltage), the transformer, being an inductor, stores power for a very short period of time, and then releases it back into the power cable, and therefore, back into the wall AC. This released power, being slightly out of phase (because it takes time for storage and release) with the incoming AC, distorts the flow of current in relation to the height of the voltage at any one instant in time. Seen below is a spectrum of AC power at the wall socket, but with an amplifier plugged into the wall socket, and with the amplifier turned on. The red spectrum is the 60 Hz AC, and the blue spectrum is the flow of current. Notice how the current flow is a very narrow peak, slightly off center with respect to the 60 Hz peaks. This causes multiple harmonics to be released into your house wiring which can contaminate the power supply of other components (which are also contributing to the problem themselves).

classe-cp-800-preamp-demo-no-pfc

Through the use of a Power Factor Correction circuit, the wall AC can be made to look like this:

classe-cp-800-preamp-demo-with-pfc

See how the current flow is in synch with the rise and decline of the 60 Hz voltage sine wave. This eliminates all the harmonics that a non-power factor corrected circuit produces. The area under the current curves is the same for both graphs, but power factor correction's purpose is to eliminate harmonics being released back into the AC wall supply.

I should point out that many switch mode power supplies do not contain power factor correction. It is not necessary to have power factor correction for the switch mode power supply to function. However, the Classé CP-800 has power factor correction because it is a high performance product, and the design engineers want to eliminate as much harmonics in the circuit as possible.

Here is a photo of the inside of the CP-800:

A large part of what you see is the DSP portion of the preamplifier circuit. The output stage is, in fact, a very high performance op-amp, which is small in physical size. Op-amps are another item that purists don't like, but it's because the technology of early op-amps was not very good. But that has changed. They are very good now, and Classé has put them to use in the CP-800. (Of course, inexpensive components still use inexpensive op-amps that don't perform very well.) The CP-800 is fully differential (balanced) from input to output, and two OPA827 op-amps are used for each channel output (left and right), one for the + side of the signal and one for the - side. Of note here is that another set of op-amps are used for the RCA single-ended outputs, so you can use the XLR and RCA outputs at the same time.

The remote control is of routed anodized aluminum, with easy-to-read buttons that are tactile and shaped differently for easy distinction in the dark. Its rounded edges make it feel nice in the hands . . . very comfortable.