Line Conditioners?

The DAC4800A and Cherry are designed to tolerate large AC mains fluctuations so you shouldn't need any line conditioning (save some money there). In fact, these amps even tolerate brief brown outs, playing through them without any noticeable effect.

About Speaker Cables --- They Don't Need to Cost a Fortune!

It comes down to resistance and inductance. If you can solder, buy 12 gauge or 14 gauge (if the run length is less than 15 feet) wire and get terminations from Digi-Key. By terminations, I mean banana plugs or spade lugs. I definitely recommend soldered connections instead of crimped for long life. I agree that this is a typical area for spending too much when it really doesn't make a difference to the sound or your system. This comes from an engineer with 20 years experience, not a marketing firm.

Audio Review Still Hasn't Responded

We tried to add the DAC4800A amplifier (see http://www.digitalamp.com/4800a_amp1.htm) to Audio Review so we can get customers to add reviews there. However, there is no selection for adding a new manufacturer. The listing shows up with "unknown" as the manufacturer. Here's the link:
http://www.audioreview.com/cat/amplification/amplifiers/unknown/dac4800a/PRD_419903_1583crx.aspx

We wrote to them, but no reply. We posted to the help board, but the post was removed.

Does anyone know somebody in charge at Audio Review? We would like to get on the manufacturers list. Locking us out is just not fair. Thanks.

Why Our Amps Sound Better

Some of our customers have told us that they love the sound of our digital (technically "Class-D") amps, but they don't like the sound of other digital amps (from other companies). It might be the output filter or feedback design. People often criticize me for saying that feedback wrapped around the output filter of a Class-D amp can have a negative impact on the sound. Actually, this could be your answer!

By the way, fake class designations like "Class-T" or "Class-Z" are actually Class-D. The definition of "Class-D amplifier" is "an amplifier that uses its output devices as switches". This is a pet peeve of mine.

Thanks for hearing me out.

Switching Amplifier Output Ripple Doesn't "Fold Down"

Regarding "folding down", this is called aliasing and is explained by sampling theory. Here is a basic explanation:
http://en.wikipedia.org/wiki/Nyquist%E2%80%93Shannon_sampling_theorem#Aliasing

Aliasing only happens when the waveform is sampled, which is not the case for a switching amplifier output driving a loudspeaker. The time domain output waveform of a switching amplifier (after the output filter) has switching ripple superimposed on top of the audio. The ripple is ultrasonic, so it is not heard. The mechanical limitations of loudspeakers do not allow this superimposed waveform to enter the room, not that it would matter anyway, as long as the switching frequency (Fsw) is high enough (>100KHz). Most switching amps change states (switch) at a rate of 350KHz or greater. Higher switching rates cause additional EMI and lower efficiency, and lower rates are harder to filter. The power dissipated in the loudspeaker by the ripple is typically a few milliwatts.

In order to accurately measure the performance of a switching amplifier, a filter that removes most of the ripple is usually placed between the amplifier under test and the measurement device. Such a filter requires flat response out to 20KHz or greater, but a rapid fall-off by the 100KHz range. Attempting to measure a switching amplifier without such a filter can drive the measurement equipment nuts as it tries to sync to the amplifier's output ripple instead of the audio signal.

Links to DAC4800A and Cherry Amplifier Information

Here's a link to the DAC4800A page:
http://www.DigitalAmp.com/4800a_amp1.htm

Here's a link to the Cherry page:
http://www.DigitalAmp.com/Cherry_amp2.htm

Here are links to reviews:
http://www.DigitalAmp.com/DAC_REVIEWS.htm
http://www.TrueDigitalPower.com/ (Amazon)
http://www.TrueDigitalAmp.com/ (Amazon)

Damping Factor

We received a question about damping factor. Both the DAC4800A and Cherry were tested for this at 1KHz, which is higher than normal for a damping factor test. Into 8 ohms, it measures around 200 (Rload/Rout). All the way down to 20Hz, the measurement was about the same.

There has been some controversy regarding the real meaning of damping factor, however. Tube amps are known for their sweet sound but usually have a low damping factor due to high output impedance. We believe the natural open loop performance of the amp comes into play as damping factor lowers. In most cases this occurs as frequency rises. With good power supply design and strong output stages, damping factor can mean less to the sound of an amp.

The proof is in the pudding, so there's no substitute for listening.

Output Resistance Measurement Trick

Here's an easy measurement trick to find output resistance, but you need an accurate audio analyzer to use it:

Load the amp with 24 ohms and change the input voltage (sine wave) until you get exactly 24 volts out of the amp. Remove the load and subtract 24 from the new output voltage measurement. This is the output resistance in ohms! 24 ohms was picked to increase accuracy, but you can do the same thing with 8 ohms and 8 volts. The voltage measurement should have 3 digits after the decimal point for this to yield an accurate measurement.

Knowing the output resistance, you can calculate damping factor (DF = Rload/Rout). Note that output resistance can change over frequency.

Preamp Output Impedance and Amplifier Input Impedance

We received some questions about DAC4800A (and Cherry) input impedance, which is 10K. Here was our response:

It's simple... You get more noise with higher input impedance. Going from 10K to 100K might cost as much as 3dB, but since stock SNR is over 114dB, there's room. We don't do this stock because most preamps have a 600 ohm or lower output impedance, more than 10:1 (the typical "rule of thumb") to drive the amp. We opted for better SNR. The current of the preamp's output driving 10K is VERY low (microamps), so that's not really a significant factor. We can modify amps for 100K input impedance when requested, but there is some rework required.

We don't recommend preamps with AC coupled outputs, although these are few and far between. AC coupling between the preamp and amp creates a high pass filter that can rob you of low bass response.

A Few Words About Output Current

The output devices in the DAC4800A and Cherry amps are capable of pulsed drain current of more than 150A! However, the output stage cycle-by-cycle current limit is set to about 30A instantaneous.

Our Amps are NOT based on Ice Power Modules

We DON'T make amps with Ice Power modules! I replied recently to someone's "just another ice power amp" email comment. It takes a lot more amp design experience (20 years for me) to design high performance, high efficiency, high power amps from the ground up than to put off-the-shelf modules in a box with a power supply. Something like 90% of the digital amps out there are like that, so we're special. We designed the guts. We build the guts. We are the guts.

Digital Amplifier Company amps are built with proprietary patented technology that took more than 10 years to develop. It's a novel approach to modulation and control that allows closer-to-the-rails output swing while holding noise down as far as 120dB from max output. Really low THD is also possible with this technology, down to 0.0006% on the bench.

We are customer centric because we appreciate having the privilege of serving them. They are the reason we exist, and there's nothing quite like having customers repeatedly tell you that your design is the best they have ever heard. That makes all the hard work that goes into each amp well worth it. Thanks again.

A Few Words About Buying Vintage Loudspeakers

Watch out for speakers with drivers you can't replace. Also, take a good look at the condition of woofer surrounds, dust caps, and the like. It may work well and sound good, but for how long? I burnt out one woofer each on two of my speaker systems, one DIY and the other a well known high end name, and couldn't replace the burnt drivers with exact replacements. One was luckily fixable with a "close replacement", and one lays dormant (Focal dual coil woofers). Of course, when one side blows out, you want to replace both sides. Crossovers are easier to find replacement parts for, that is, if the components are marked with the value (air coil inductors are notorious for no markings). Also check if the drivers are original. Good luck.