Product Review -
Audio Electronic Supply AE-1 and SE-1 Class A Tube Preamp and Power Amp - January, 1995
By John E. Johnson, Jr.
Audio Electronic Supply AE-1 Single Ended Triode Stereo Preamplifier -- Pure Class A tube preamplifier, input impedance 82 kOhm, output impedance 800 Ohms, size 5 9/16"H x 14"W x 10"D, weight 15 pounds. $599 with Cary 6SN7 tubes. Audio Electronic Supply, 111A Woodwinds Industrial Court, Cary, North Carolina 27511, (919) 460-6461.
Audio Electronic Supply SE-1 Single Ended Triode Stereo Power Amplifier -- Pure Class A tube amplifier, 7 watts per channel output, no negative feedback, frequency response 15 Hz - 20 kHz + 0.5 dB, input impedance 150 kOhm, size 7 5/8"H x 14"W x 10"D, weight 31 pounds. $950 with Cary 300B output tubes. Audio Electronic Supply, 111A Woodwinds Industrial Court, Cary, North Carolina 27511, (919) 460-6461.
Tubes are back, and in a big way. Most audiophiles have, at one time or another, considered purchasing a Class A amplifier, but the cost was prohibitive. Only two years ago, a single ended pure class A tube power amplifier, with less than 10 watts per channel output, was over $3,000. All that has changed in the last year, with several models becoming available at $1,600 or less. The Audio Electronic Supply SE-1 (a subsidiary of Cary Audio Design) is one of these, and it sounds unbelievable - or, rather, totally believable. Together with the Audio Electronic Supply AE-1, which is a pure class A preamplifier, you have the makings of platinum sound, through and through.
For those of you too young to know what a tube looks like, or have succumbed over the years to discarding the old tube amp for solid state, your ears are in for a pleasant surprise. The single ended variety of amplifier is actually the oldest design there is. It was first used in radios more than half a century ago, but because of its low power (only a watt or two), manufacturers changed the output design to incorporate push pull triodes, then push pull tetrodes and pentodes, and the single ended triode was left in the dust. Now, of course, power amplifiers of the push-pull type, both tube and solid state, are available with hundreds of watts of output per channel. The pure class A triode has been just waiting there for rediscovery. The Europeans and Japanese have been tube- aholics for a long time now; perhaps they never really stopped appreciating their sound qualities. But here in America, a renaissance is about to happen, partially because of affordability, and the AE-1 as well as the SE-1 will be right there when you are ready.
Before we go any further, it is necessary to understand how tubes work. There are four basic types: diodes, triodes, tetrodes, and pentodes. All tubes have at least two components, installed within a glass envelope (the tube) under vacuum. These are the cathode and plate. The cathode is like a filament in a light bulb. It is heated by electric current, and in doing so, the cathode becomes hot, so warm that it emits light, just like the light bulb. Because of the heat, electrons that are normally bound tightly to the atoms in the filament, move away from the atoms and form a cloud around the filament. The electrons are negatively charged. A metal plate near the cathode has a positive voltage applied to it by the power supply, making it positively charged, with respect to the cathode. The negatively charged electrons are attracted to the positively charged plate (often called the anode), and current (movement of electrons) thus flows. The diode has only these two components (cathode and plate), and because current can only flow in one direction (from the cathode to the plate), diodes are used to "rectify" alternating current to direct current.
The triode has an additional component, called a control grid, which is basically a mesh of wires placed between the cathode and plate. If a negative voltage is placed on the grid - and it does not take much - current flow between the cathode and plate will be impeded (the cathode electrons are repelled by the negatively charged grid). By applying the varying musical signal voltage to the grid, current is allowed to flow from the cathode to the plate in time with the music. The input to the grid is small, but the plate voltage is large, so the current flowing from the plate in time with the music is much larger than what was applied to the grid, and we have - voila - amplification. Tetrodes have two grids, and pentodes have three, with correspondingly more output capabilities than the triode, but unfortunately more distortion.
Single ended designs have one tube (or solid state device) connected to the output, and push-pull designs use one or more pairs, each member of the pair handling half of the signal. While single ended tube amplifiers produce primarily even order (second, fourth, etc.) harmonic distortion, push-pull tube designs, as well as most solid state designs, produce odd order (third, fifth, etc.) harmonic distortion. Even order distortion is consonant (non- irritating to the ears), while odd order distortion is dissonant (very irritating).
Class A amplification means that the voltage applied to the grid is such that current is always flowing between the cathode and plate, even when the amplifier is at idle and no music is passing through. Of course, the energy flowing at idle has to go somewhere, and, in fact, it is dissipated as heat. The amount of energy dissipated (at the plate) during idle is about 4 times the amount of power that the amplifier is capable of delivering to the output when a musical signal is applied. Thus, class A equipment not only runs hot, but is rather inefficient. But the sound quality is worth it, and difficult to put into words. You will need to hear it for yourselves. The main reason it sounds so good is that, because current is always flowing in the circuit, this current can be instantly sent to the output when a musical signal is applied to the grid. (Of course, we are talking about tubes here. The same principles can be applied to solid state transistors, where the "source" is equivalent to the cathode, the "drain" equivalent to the plate, and the "gate" equivalent to the grid.) In class B, the grid voltage is at a level such that there is no current flowing at idle. Therefore, the musical signal applied to the grid starts and stops the current flow between the cathode and plate. This tiny fraction of a second difference in having the current which is already flowing instantly diverted to the output in class A mode, compared to turning the flow on and off in class B, makes the difference in sound reproduction.
The AE-1 is single ended, and pure class A, which means that all of the amplification stages are operated in class A. It uses four dual triode 6SN7 tubes, two for each channel. A dual triode has two triode configurations contained within a single glass envelope. In the case of the AE-1, the individual elements of each triode are strapped together. The plates of the first tube are connected to the grids of the second, while the grids of the first are connected to the cathodes of the second, which then leads to the output connector. This results in a very straight forward, but sophisticated circuit, with very little in the way of the signal path and a frequency response that is flat throughout the audible spectrum.
The AE-1 is a very attractive package, consisting of a black chassis, with the tubes symmetrically arranged on top, for good heat convection to the surrounding air. The control switches on the front panel are all round knobs, including the power, which turns on the filament voltage (standby) and then the plate voltage (operate). Two knobs in the middle control the volume separately for the left and right channels, and a fourth knob on the right side of the panel is the source selector (CD, Aux 1, Aux 2). A red LED indicates that the unit is turned on. The rear panel has gold plated RCA connectors for three stereo inputs, as well as one set of outputs. Two natural finish wooden panels on the sides of the chassis give the AE-1 a very unique, and elegant appearance.
Before we get into the sound characteristics of the AE-1 preamplifier, I want to introduce you to the AE-1's matching power amplifier, the SE-1.
The Audio Electronic Supply SE-1 is a single ended design, employing two 6SL7 dual triodes as the input stage (one per channel), and the voltage thus generated is used to drive the output tubes, which are the classic 300B triodes (one per channel). The SE-1 is also pure class A, with all stages of amplification operated in class A.
The SE-1 is neatly arranged on a black chassis, with the power and output transformers at the rear, and the tubes at the front. Two switches on the front panel activate AC power, and the other supplies power to the cathode (standby) and plate voltage (operate). Connections on the back include gold plated RCA input jacks and 5 way binding posts for the speaker outputs. There is also a jack to measure the cathode current for bias adjustments to the grid of the 300B's. You can purchase the SE-1 (and AE-1) as a kit, but we received ours factory wired, with the outputs connected to the 8 Ohm taps. Two wooden panels are attached to the sides for aesthetic purposes, and the SE-1 looks very attractive, even without chrome or brass plating anywhere, matching the AE-1 in appearance.
Because its 7 watt per channel output is a limiting factor, we decided to test the SE-1 using mini-monitors and self-powered subwoofers in a bi-amplified configuration. Therefore, we requested, from the factory, an input filter (capacitor) that would rolloff the signal being fed to the input stage below 100 Hz. The formula for calculating the proper capacitance value is as follows: Crossover frequency = 1 divided by (2 pi times the input resistance times the capacitance value). Cary Audio Design (which owns Audio Electronic Supply as a subsidiary) graciously complied by wiring the input of each channel in series with a Kimber Kap 0.01 mF 400 V metallized tubular polypropylene capacitor. This resulted in the frequency response being normal at 100 Hz and down 8 dB at 50 Hz, followed by a very sharp decline below 50 Hz. Thus, most of the amplifier's energy could be devoted to frequencies other than the power hungry deep bass. Audio Electronic Supply offers several options on both the AE-1 and SE- 1. Therefore, we would suggest that, if you are considering the purchase of either of these units, you should contact the factory and request the catalog so that you can have the options installed at the time you place an order, rather than having to send the unit(s) back for installing the options, or installing them yourself.
We were truly surprised at how shockingly good the SE- 1 sounds. Pure and sweet, without a trace of harshness, are terms and phrases that abounded in our listening laboratory. Of course, this is to be expected with a pure class A single ended triode amplifier using the 300B as an output tube, but for less than $1,000, to maintain such marvelous sound is astounding. We were able to obtain volume that was satisfactory for not only listening to music, but for home theater applications as well! The input filter did its job, and coupled with two powerful subwoofers, even the sounds of Jurassic Park came through nicely. Of course, we are not talking about the kind of volume that one hears in a typical home theater demo, but it was enough to be comfortable with. We had to use a different preamplifier (solid state passive buffered) for home theater listening, because the AE-1 does not have a tape loop for connecting a surround sound processor. The AE-1 and SE-1, when paired together, had a warmer mid-range than when the SE-1 was used with the passive preamplifier. Sibilants (the "s" in the human voice) were crystal clear, and oh my goodness, the Chopin and Rachmaninov!
The beauty of the AE-1 and SE-1, as well as any other well built single ended class A designs of this type, is that they distort gracefully, producing essentially even order harmonics (non- irritating). We turned up the volume, and although the power amplifier was obviously clipping, it did not bother our ears. This is in sharp contrast to other amplifier designs, where clipping makes you grit your teeth.
Only one half of the input dual triode (6SL7) is used in the signal path, with the other half connected to power for stabilization purposes. The number of resistors and capacitors is relatively small, making the route from input to output travel through very few components.
The 300B output tube employs a directly heated cathode. This improves the sound compared to indirectly heated cathodes, and provides for more efficient electron emission. The life expectancy of the 300B's supplied with the SE-1 is about 2,000 - 4,000 hours.
The SE-1 employs no negative feedback. This fact is very important since the use of such feedback, necessary with tetrodes or pentodes, as well as solid state amplifiers, can result in increasing one type of distortion while reducing another. With triodes, even in push pull configuration, negative feedback is not necessary. One of the primary reasons for using negative feedback is to lower the output impedance. Because negative feedback is not used on the SE-1, the output impedance is rather high, 3.2 Ohms. An amplifier using negative feedback might have an output impedance of 0.1 Ohm. The lower the output impedance, the less sensitive the amplifier is to the load (speaker cable and speaker). Typically, then, amplifiers with high output impedances do not perform extremely well in the low frequencies (lack of bass "tightness"). This is one other reason we decided to filter the input, and use subwoofers for the bass. The input filter (capacitor) does result in phase shift (90 degrees at crossover, with decreasing amounts as the frequency increases), but since the shift is identical in both channels, we did not find it perceptible. Nevertheless, the input filter does degrade the purity, no matter how you look at it. The high end of the spectrum was a bit soft, but this is typical of single ended triode amplifiers in spite of a flat frequency response. We invite readers to comment on their own experiences with this phenomenon.
The effective increase in power available to the mid- range and high frequencies, by filtering out the low frequencies, allowed us to use speakers that are rather inefficient by normal standards (88 dB/ 1 watt/ 1 meter) with very good results. We were amazed, in fact, at how much volume can be produced with 7 watts per channel when employed in this manner. The drawback, of course, even with increased power to upper frequencies, is dynamic range, but this tradeoff is worth the benefits of sound so clean, and yet at such a reasonable price, that we suggest changing the term "High End Audio" to "High Definition Audio". The Audio Electronic Supply AE-1 and SE-1 are a must buy for any serious tube loving audiophile.
John E. Johnson, Jr.
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