After extended break in I gave the AQ-2004 a first listen. The results were not good. I found the dynamic range extremely compressed, the top end harshly abrasive, and detail utterly missing. I am not saying it failed in comparison to my favorite line amp, but that it failed in comparison to consumer-type solid-state gear of the 1970s. Frequency extension at both extremes seemed rolled off, and in particular the bass sounded weak and loose. The image was vague and confined to between the speakers. Sense of space was seemed artificial in the midrange and nonexistent in the treble region. On recordings that I know to have a less-than-wide sound stage, imaging sounded almost monophonic. In general, this line amp lacked most of what I enjoy about tube sonics. I expected the unit to at least present warmth, so that it could be used to take the edge off of CDs when used with solid-state gear. Instead, it added an edge of its own. I listened to the AQ-2004 over several days and the situation did not improve. A careful examination of the schematic revealed most of the sources of most of the problems. It took a good deal of restraint not to grab a soldering iron and dig in. But, the unit was sent to me for an honest evaluation, not a complete redesign.
I usually start at the power supply when I look for problems , or strengths, within any design. The supply is where the first problem revealed itself in the form of 1N4007 rectifiers. The 1N4007 is a member of a family of silicon diodes which I call, slow hard-recovery types. All members of this family have serious switching noise at high frequencies, in this case, audible frequencies. They are certainly the cause of the harsh treble. There are two inexpensive ways of solving this problem, one is to bypass the diodes with .01 uF caps, the other is to use faster recovery diodes of modern design. The current price of 1N4007 diodes is 3.9 cents each in 500 lots, and bypass caps could cost as little as 10 cents each in similar quantities. Fast-recovery diodes, which will at least move the switching noise to a higher frequency, can be had for 5.3 cents each in suitable values. With costs such as these, I find it inexcusable not to use a better approach. One positive feature of the supply is that it does use a DC supply to the heaters to reduce hum. This circuit should also use, at the very least, bypass capacitors across the rectifiers. It doesn’t.
Having completed my journey through the power supply, I began my look at the circuit itself. The circuit begins, conventionally enough, with a volume control, but what’s been placed right behind the potentiometer mystifies me. It is a cathode follower. The purpose of a cathode follower is as an impedance matching device, in other words it has a very high input impedance, and a very low output impedance. Line level devices, such as CD players, have very low output impedance, usually around 500 Ohms, so there is no good reason for a cathode follower on the input of a line amp. Cathode followers are also notorious, when misused or improperly designed, for robbing the music of dynamic range. In this case that is exactly what happened. Once dynamic range has been lost, it cannot convincingly be brought back. Part of the life of the music has died.
The actual gain stage of this line amp is a good, simple, honest design which has been hampered by the input stage. Unfortunately, by the time the music has reached this stage, it’s too late. The final stage is another cathode follower, which under normal circumstances I wouldn’t have a problem with, provided it was properly designed. When using a power amp with high input impedance, a cathode follower can make the power amp easier to drive, and make life for the interconnect cables much better. In this case the addition of a second cathode follower further robs the dynamic range. This is like pouring acid on the corpse. The worst part of this line stage output is that it could prove fatal to the power amp it is feeding. The output has been left free-floating. Without getting too technical, the output follower, under certain conditions, could suddenly unload around 70 Volts into the power amp, causing the death of more than the music.
From power supply, to input, to output, everything I heard was confirmed by the circuit design. This is a real shame. This line amp could have been acceptable, for LESS money than was spent, by eliminating the existing input stage, and fixing the power supply. What really irritates me is that my first product review for publication has to be so negative.
The unit was also full of solder flakes and bits of wire just waiting to short something out. The pilot lamp circuit simply wasn’t there, there was no LED in the hole drilled for it on the face plate. The only good thing I can write about it is that it doesn’t hum. My review partner, Catcher, howled and had to be exiled whenever the unit was in use. This bargain is no bargain.
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