zahid@neptune.AMD.COM (Zahid Ahsanullah) (03/22/90)
>From: chowkwan@aero.org (Raymond Chowkwanyun) Posted-Date: 21 Mar 90 17:25:30 GMT > zahid@neptune.AMD.COM (Zahid Ahsanullah) writes: >is colored due to cone momentum. Also 4 Ohms don't have much midrange >response anyway. **Why does mid-range performance correlate with the impedance of the **speaker? Don't some highly regarded home speakers such as **electrostatics (martin logan) or ribbons (apogee) have very low impedances? **Yet these are speakers reknowned for their midrange. When I said 4 Ohms I meant a conventional voice coil driven speaker. What I think happens is that a 4 Ohm coil having less turns does not land the impedance at the place where the amplifier is normally designed for. There is a range of impedances that happen from 20-20Khz and a good amp should be able to dissipate the exact same power for this range. The lower this impedance is the harder it is for the amplifier to maintain the same output. In other words, you are trying to almost short the outputs out at 2 ohms. Not many amps can handle this situation. Also to get a good drive from the speakers at mid frequencies, the coil has to cover a good portion of the magnet to get a sufficient force. This is because at mid freq there is not a whole lot of current being driven by the amp because of the increased load impedance. At the same time the coil should be light so as to keep the total weight of the acoustic elements low. A 4 Ohm coil has generally smaller number turns than an 8 ohm coil and thus covers less area. You could however use a thicker wire for 4 Ohms and get the same numbers of turns but you'll also increase the weight. Electrostatic speaker have a difference set of problems and their dynamic impedances vary quite differently from a coil type speaker. I really can't talk much about them since I've never researched them enough. > The tape deck is next being powered only by 12 volts. I have >designed enough amplifiers to know that 12 volts aint never gonna >cut it as far as frequency response is concerned. You need atleats 20 >volts to get anything decent out of a preamp. Something about Bipolar **Your point about lack of volts is taken. Can this problem be gotten **around by stepping up the voltage inside the amplifier? I've **recently seen an ad for a tube amp for cars in TAS. I assume he **has to step up the voltage to operate the tubes properly. Generally a car stereo preamp is designed to operate at a maximum of 16 volts. This figure I got from blowing up a couple of preamps from my waste bin. You could go above 16 volts but the unit will only work for a few weeks before the small signal transistors gets destroyed by the heat. Stepping up voltage is never easy and is almost always wasteful unless you use the new switching invertors. They are small and truly amazing. Maybe this is what can be used if you just wanted prop up the voltage of the amp available in a tape deck. But be aware though, you will have to separate the motor drives, digital and other stuff from this supply. **Your idea of putting a home system in a car is an intruiging one. **I'd like to try it out (I have a spare NAD amp). Can you email **me more details: like how did you get the amp to fit in your dash? **Was it just because you made it yourself that you could get it **small enough? I'll Email the details soon, but here's a brief description. My amp was home designed and was only a power amp. There was however only one stage preamp for input matching and the loudness control. The loudness switch was the only control on it and the wole amp was 6*4*4 inches in dimension. Most of the dimension was due to the heat sink. The power supply was quite large and was located in the trunk. I don't know how big your NAD is, but I suspect it is quite large. If you can, fit it in trunk and leave the tone control to your taste. Use the tape player's volume as the volume control. For a supply, use atleast a 200 Watt switching supply or invertor. You may notice that you cannot go above a certain volume due to the limitation of the supply. But let me tell you, the sound will be awesome and you'll probably never go back to the old system. **-- ray Regards Zahid@neptune.amd.com