We have to thank Dr. John Meringolo in New York for this month’s tip. Besides his medical practice, Dr. Meringolo owns Dr. John’s Audio and specializes in home theater and high-end car audio installations. Last week John’s goal was to upgrade the factory sound system in a late model Cadillac Deville. The Navone Engineering, Inc. N-774V Line Output Adapter was selected to connect the speaker level output of the of the Front Left, Front Right, Rear Left, and Rear Right OEM speakers to the input of an aftermarket power amplifier. Upgrade speakers were also included in the system design.
The N-774V features full bandwidth, a maximum signal level of over 9 Volts, a low source impedance, full transformer isolation, and output level control from 0 up to 9 Volts. The input impedance of the N-774V is 10KOhms and this means that the effect of the Line Output Adapter on the factory stereo is minimal. In fact, when we compare the effect of a 10KOhm adapter load in parallel with a nominal 4 Ohm speaker load, the proverbial fly on the back of an elephant comes to mind. We’ll come back to this high input impedance.
Connecting to the correct speaker output leads can be difficult without a proper wiring diagram. Connecting to the wrong wires can lead to reverse polarity, crosstalk, and poor system performance. To insure that the wiring was correct, Dr. John connected directly to the + and speaker terminals of the factory speakers. In this way, the wiring is 100% verified. The factory speakers were then removed.
Since noise is always a consideration, Dr. John ran small gauge speaker cable from each one of the OEM speakers all the way to the car’s trunk. John chose a 4-channel Rockford power amplifier for this installation and installed the N-774V at the input of this amp. The idea here is to run as “hot” a signal as possible, for as long as possible. This keeps the noise relatively small in comparison to a speaker level signal. Why didn’t Dr. John run large size speaker cable to the input of the N-774V? The answer is because the input of the N-774V is 10KOhms and that means the current is some small number defined as around 10 Volts divided 10KOhms. Get it?
So What’s The Problem?
Dr. John did a great job in the installation. But there was a problem. One or more of the stock amplifier channels would periodically “cut out.” The dropped channels varied with no particular time of day or listening situation. Dr. John swapped out N-774V, checked all wires, and then re-checked everything twice. What could be the problem?
A quick call to Rob Putman, owner of Soundgate in Sheridan, WY yielded a clue to the source of the problem. Rob suggested the possibility of the stock GM power amplifier actually looking for its 4 Ohm load. Could the stock power amp be going into protection mode when loaded with a 10KOhm load instead of the normal 4 Ohm load? To check it out, Dr. John installed 4 Ohm, 30 Watt ceramic power resistors in place of the factory speakers. It worked. The stock amp remained active in all channels.
We don’t usually like to install power resistors in audio systems, but in this case, the power resistors tricked the channels in the Cadillac’s stock power amplifier into remaining active. We’d like to try 8 Ohm, 20 Ohm, or even 100 Ohm resistors in lieu of the 4 Ohm power resistors; however, time was short and the car had to leave. So the remedy here is to install power resistors in parallel with the input wires of the N-774V. At Navone Engineering, Inc., we stock the power resistors of all values. If you’re upgrading a new Caddy, be sure to include the resistors.
The system sounded great when complete and there was no noise. To optimize the sound, Dr. John set the system gain structure for around a 3:1 gain overlap in the Rockford amp. Thanks to the 9.5 Volt signal level from the N-774V, the 3 Volt input sensitivity at the Rockford amp yielded a very low “no hiss” noise floor, and a super punchy sound system. Thanks to Dr. John and Rob for this month’s Tech Tip.