Mind Blower Speakers were out in the 70's. Each one had there own amp built onto the back of the magnet. I am restoring a old hot rod truck and I want to keep it real in detail that was available in the 70's. Any help would be helpful.
2007-02-19 01:03:56 · 3 answers · asked by slickasaonion2 1 in Cars & Transportation ➔ Car Audio
http://www.speakerplans.com/
2007-02-19 07:50:53 · answer #1 · answered by Indiana Jones 6 · 0⤊ 0⤋
The pair of Tenna Mindblowers that I had had a standard 6X9 8 ohm speaker with nothing special about it. I dissected the output transformer of one channel and found a double winding on it, where each wire was laid side by side next to each other and then wound to 72 turns. One wire at the end of the winding was attached to the end of the other wire at the beginning of the winding, which is the center tap that connects to the negative.
The two free ends were each connected to the emitter of one of the two output transistors. It was at this junction where the free ends of the output transformer connected to the output transistor emitters that the two speaker voice coil leads were connected.
In an output voltage test I did on one channel with the speaker connected, and measured with a true RMS Fluke DVM, I had a reading of up to 25 volts AC across the output, which is 78.125 Watts RMS. For a wattage rating of 30 Watts True RMS per channel, Tenna was being very conservative in their output rating.
I found that the control box, which Tenna included, knocked out the upper range when it was switched into boost mode. I had a Pioneer KP500 cassette deck with separate bass and treble controls. I had noticed that the upper end was there when it wasn't in boost mode, but as soon as I put it in boost mode, the upper end disappeared.
I found that by eliminating the Mindblower control box, there was no longer any loss of the upper end.at full output and I then had the entire range. They were making it seem more bassy by eliminating the upper end. I had absolutely no loss of bass with it eliminated from the circuit.
The Mindblowers that I had did not have the darlington pairs as is shown in the schematic that I downloaded. Each output transistor had the emitter of a separate small signal transistor connected to its base to function as a driver transistor.
You can find it under US Patent number 4,130,725.
I've often thought I would like to copy that amplifier design and wind 3 to 4 more times the number of turns that they used and use a 14 gauge wire size for it. Then possibly using mosfets for the output drivers. You would have to run several in parallel to handle the additional amperage and with a high enough voltage rating to accommodate the increased voltage. It would be great to have this design of an amplifier that was able to drive 4, 2, or 1 ohm impedance loads.
Unlike bipolar silicone transistors, which are a current controlled device, requiring a bias voltage at the input, mosfets are a voltage controlled device like tubes. You can copy any tube circuit with mosfets, except you don't need the heater voltage or the high plate voltages. You can directly parallel mosfets in a circuit whereas you have to pay attention to the input bias voltage of each bipolar transistor so that it amplifies the entire signal at the final output, otherwise, the first 7/10 of a volt in the signal won't be amplified and will be lost.
I hope this answers all of your questions and clears up all of the circuit design mysteries of the Tenna Mindblower systems.
2014-03-28 07:22:57 · answer #2 · answered by Carl 1 · 0⤊ 0⤋
They used a special loudspeaker with a center-tapped voice-coil, not available today (unless you want to pay through the nose for a pair).
http://i56.photobucket.com/albums/g196/dkleitsch/Mindblower.jpg
Amplifier schematic from the patent.
2014-02-25 00:36:32 · answer #3 · answered by ? 1 · 0⤊ 0⤋