Rear speaker hook up

[danoworld]

Hey Guys,
How difficult is it to hook up a set of rear speakers to the stock radio? I am planning on getting the radio reconditioned/with a hidden mp3 cable added. I cant see spending big money on a another stero/amp when I am just mostly cruising and I find myself just listening to the stock mild rumble anyway.   Has anyone just hooked up 6 x9's in a 68 charger? If so is it difficult to do? How is the power coming from just the stock radio?

Dan

[69RT3X9]

Hi Dan,

A "stock radio" may not work very well with a pair of rear speakers for a couple of reasons.  First of all, the Mopar thumbwheel radios from 1968-69 utilized an amplifier transistor output circuit.  For these amplifier transistors to operate within their designed voltage/current ranges, the total load impedance on the radio must be in the 8-10 ohm range.

So why does that matter?  Well, most modern speakers are designed with 4 ohm impedence.  A 4 ohm speaker will draw twice the current of an 8 ohm speaker thereby causing the amplifier transistor (the half dollar size device mounted to the black finned heat sink) to run very hot.  I am sure your radio's transistor will operate for a little while in this overcurrent condition, but it won't be long before the radio stops working due to the amplifier transistor overheating beyond recovery.  Ohm's Law is the scientific principle behind this overcurrent condition. As stated by Ohm's Law I=V/R  where I=current, V=voltage, R=resistance.  For the case of a car radio, voltage (V) supplied to the radio from your fuse box is constant, so as the resistance (ie speaker impedance) goes down the current (I) must go up.

Secondly, non-stereo "stock" Mopar thumbwheel radios only had 1 speaker output (Green/Blk wires).  In 68-69 Dodge Charger applications with the standard (4" x 10") dash speaker and optional single rear speaker installed, the speaker output from the radio was fed into a fader switch, which then allowed the driver to switch the radio between the front speaker, back speaker, or both.  This fader switch served 2 purposes, a splitter and an impedence controller.  As a splitter, it split the radio's single speaker output into two outputs, 1 set for the dash speaker, and 1 set for the rear.  As an "impedence controller" it maintained the combined speaker impedence at 8-10 ohms regardless of the switch setting.  If you plan to use the "stock radio" with multiple speakers you should either have some sort of "impedence controller" or hard-wire your speakers to result in a total combined impendence of 8-10 ohms.

I just completed a similar "radio project" on my 69 Charger.  I sent the original 5W factory thumbwheel AM/FM mono radio (option code = R21) to Ward's Classic Car Radio Repair for refurbishment work and MP3 auxillary jack installation.  The radio came back working and looking great!  I have it hooked up to the single 4" x 10" dash speaker (which I purchased from Greg at www.turnswitch.com).  This single speaker radio setup meets my occasional driver requirements for sound and iPod interface, but compared to a modern stereo system, it pales in comparison.  My desire to have a functional original radio in my car was more important to me than a high power, high sound quality system.  I am pleased with the results!  

-Jon

[twodko]

Hello Danoworld welcome to the site.
I too have a working stock AM radio in my 69. At some point I'm either going to upgrade my stock radio to AM/FM/Ipod or go with a "Hidden Audio" stereo of some kind. I have only 1 factory speaker that my radio feeds and these radios don't put out much power. I'll probably go with a "Hidden Audio" setup because adding more speakers to the existing radio causes problems. Speakers are electrically matched to their signal source.
The problem you'll face running another pair of speakers is impedance. Pardon me if this sounds condescending, not my intent. Impedance can be thought of as signal resistance in a circuit. Whenever you add more impedance (speakers) in a circuit you'll increase the amount of resistance in the circuit and decrease the available power to each load i.e. speakers.
I'm not sure what the impedance of the stock speaker(s) is but lets say its 8 ohms. If you add more speakers in the circuit you'll divide the impedance to 4 ohms and reduce the available power to the loads (speakers). I know it sounds like it should work the other way around but it doesn't. Its the same in any audio circuit.
If you're not looking for high power and loud volume from the stock radio - upgraded or not - stay with the stock speaker(s) you already have and find a another stereo system to drive some nice package tray ovals. Good luck and let us know what you end up doing. Cheers!

Tom

[69RT3X9]

One more thing:  The November 2009 issue of Hemmings Muscle Machines magazine has an article regarding installing an input jack in a vintage radio entitled "We've Got the Jack".  http://www.hemmings.com/mus/stories/2009/11/01/hmn_tips2.html

This story talks about Precision Stereo Repair in New Harford NY that installs an auxillary jack in 1968 Buick Riveria AM radio for $50.  The part of the story that caught my attention was that in addition to adding a aux jack, Precision Stereo Repair (Robert Pacini) can also "hot rod" the radio for $50 more.  They described "hot rodding" as modifying the AM radio with a few tricks (modern capacitors) to increase its output, resulting in better sound both on the AM band and through the MP3 player.

You might want to check with these guys to see if they do anything with Mopar radios?  It sounded interesting.  Like I mentioned in my previous post, I used Ward's Classic Radio Repair for mine and I was satisfied with the results.  Maybe Ward's can "hot rod" as well?

If you can't read the article, here is the contact information listed at the end of the story:

Precision Stereo Repair
8411 Seneca Turnpike
New Harford, NE  13413
oldstereo@hotmail.com
315-797-5219

[danoworld]

Hey guys, Thanks a lot for all the info. I was thinking of using Wards. I agree with you. I just really want a functional stock radio with the ability to play MP3's every now and then. I listen to mostly AM sports talk radio anyway.

Dan

[bull]

What about hooking a modern amplifier up to the stock Wards-rebuilt radio? Will that get you anywhere?