ark (09/27/82)
I recently finished building Heathkit's AP-1800 preamp. This is one of the most complicated preamps I have ever seen, both in features and internal construction complexity. I am presently using it to drive the power-amp section of my receiver while I wait for the return of my ailing power-amp circuit board (see my previous netnews articles on the subject). Since it is not in its final configuration, I have not done any really critical listening. I have also not done any lab tests yet. With these limitations, I am almost entirely pleased with the preamp. I will mention my negative impressions first, since they are so heavily outweighed by my positive ones. First, I don't like the design of the headphone circuit. It works by demanding that you feed the power amp output back to appropriate terminals on the preamp. These are connected to a switch that lets you select which speakers you want to use from the preamp panel and also to a voltage divider that drives the headphone jack. While I have no objection to switching speakers from the preamp, I do feel that a separate headphone amp should have been incorporated. Reasons: (1) there could then be a separate level control for headphones. (2) you could listen to phones while your power amp is in the shop. (3, and most important to me) my speakers are Bose 901s, which demand the use of a special-purpose equalizer between the preamp and power amp to match the frequency response of the amplification chain to that of the speakers. The system used by this preamp makes it impossible to get a flat signal to the phones. I intend to modify the preamp to incorporate a separate headphone amplifier. Second, the tape switching arrangement, while very flexible, does not suit my taste as much as it could. There is the usual source selector switch that picks the input, and then there is the tape monitor switch, with three positions: "off", "tape 1", and "tape 2." There is a third switch that controls what goes to the tape recorders: "off", "source", "1->2", and "2->1". Now, this arrangement lets you do almost anything you might want without recabling, but it could be made simpler. Several amplifiers and receivers recently on the market do it this way: there are two identical selector switches marked "source" and "tape". The first controls what you listen to; the second what you record. With this arrangement, you can record from any source while listening to any source, including what you are recording or the output from the tape. The only advantage I see to the first scheme is that it makes it harder to set up a feedback loop inadvertently when you have your tape recorder set to play back the source. My third complaint is even more petty. There are two sets of outputs on the preamp and a front panel switch lets you pick "1", "2", or "both". When you are sending the output to only one set of terminals, the other set is left open rather than being shorted. If your power amp has a high input impedance and you are using longish cables, this will result in hum. This is not really important, because it only happens when the power amp is disconnected. General summary: The preamp is a large unit (perhaps 5 inches of EIA rack space) that weighs about 20 pounds. It is nicely finished in black anodized aluminum, and all the switches and knobs have a nice feel to them. The left section of the panel is devoted to high-level switching: power, output selector, and speaker selector. Keeping these together (the output is not really high-level, but it is low-impedance, and thus relatively immune to hum pickup) tends to keep hum away from the rest of the elctronics. The power transformer is in this section too, which has its own shield. There is a LED to tell you when the preamp is switched on. Moving to the right, you find the tone controls. There are separate bass and treble controls for each channel, with click stops (which I think are trendy but otherwise have no strong feelings about). Despite said click stops, these controls are variable resistors, so intermediate settings are possible. Each pair of tone controls has a three-position switch between them offering a choice of two different turnover frequencies or flat. Near the center are filters and switches. There are high- and low-frequency filters, each with a choice of flat or two different turnover frequencies, an "adapter" switch to let you interpose something like a graphic equalizer or compander ahead of the volume control, and the aforementioned source selector and tape switches. There are also two unusual switches: phono preamp sensitivity and capacitance. The sensitivity switch has three positions marked "1 mv," "2 mv," and "4 mv," which values are apparently supposed to be purely nominal, and the capacitance switch is marked A, B, and C. You decide what values are supposed to be there during assembly. These two switches are accompanied by a LED which is supposed to light if the phono preamp is overloaded. I have not been able to get it to light yet, except by disconnecting the turntable and poking a small screwdriver into the phono input to pick up hum (this test is actually recommended during initial checkout). The input selector lets you choose from "aux 1," "aux 2," "phono 1," "phono 2," "phono mc," or "tuner". Yes, Virginia, there is a moving-coil phono preamp, but, since I don't have any MC cartridges, I haven't really tested it. There is only one moving-magnet phono preamp. Over on the right are the volume, balance, and loudness controls, and switches for mode selection and muting. The mode switch has positions for L, R, L+R, L-R, stereo, and reverse. The most unusual one is L-R, which is useful for balancing channels on, say, a tuner or tape recorder with output level controls. The muting switch lets you reduce output level by 20 or 40 dB. The -20 setting is useful when the phone rings. I can't see much use for the -40. The loudness control is very well designed. It is a knob, not a switch. When it is fully clockwise, the loudness circuit is fully bypassed and frequency response is flat. When you turn it counterclockwise, it reduces the output level, and at the same time boosts the low and high end somewhat to compensate for our ears' nonlinearities. It should be used this way: turn it full clockwise and adjust the volume control for a level that sounds realistic. This will usually be quite loud. Now turn the loudness control counterclockwise until you get a comfortable listening level. The music will get quieter, but will not become lifeless. The preamp seems to be well designed, with careful attention given to shielding and crosstalk where it matters. The tape switching, for instance, is all done on a board mounted on the back panel, with very short wire runs and long shafts extending from the switches to the front panel. There is a relay that shorts the outputs for something like ten seconds after the unit has been switched on, to give the internal circuits time to stabilize. All the switching is completely noiseless. There is no audible hum and the phono preamp appears very quiet. The signal sounds very clean. There is a second relay that controls the switched outlets on the back panel, so they can actually drive 1200 watts worth of accessories. The preamp worked the first time I switched it on, with two minor exceptions: (1) a solder bridge I made by accident on one of the boards cause it only to play in mono (debugging time: one minute), and (2) a defective blocking capacitor in the MC phono preamp caused one channel not to work at all in MC mode (debugging time, 15 minutes). As I mentioned before, this is a complicated preamp. I expect there are over 1,000 components in all. The component count and the close quarters for some of the connections are such that I would not recommend it for the novice kit-builder. However, if you're already handy with a soldering iron, give it a try.