Sizing a Resistor for a Kenwood Car Amplifier

Are you sure it isn't 12V at 0.7A?

Hmmm, you can't work out the resitor value unless you know the current draw.
You could make a voltage divider (2 resistors)...but...

0.7 v is a suspicious value as that's the voltage agross the BE junction of a transistor.
I'd suggest trial and error, starting with a nice 'safe' value which will only allow 1mA to flow. 1mA @ 12v is 12k ohm, anything around there would be a good start point.

If you insist on a value you could be really safe and start at 100k.
If it was me (bear in mind I'm a Cat and, by nature somewhat impetuous I'd start with 10k )
If it doesn't work just half the resistance until it does.... check the voltage with your meter as you try each resistor...you can do some arithmetic if it makes you feel good .

Del

Where did these voltages come from? is this what you measured, or is this what the instructions tell you?

On all the car amps ive seen, you have three wires as you have stated. One goes directly to the battery with a heavy gauge and a fuse or circuit breaker, one goes to the head unit, and one is ground.

The one that goes to the head unit is the wire that switches the amp on or off, but I have always seen these use 12 volts.

I have run amps independently many times by simply connecting a jumper wire from the power wire to the sense wire, internally its just a relay.

I have never used a kenwood amp, so perhaps they are different, so I would find the instructions that came with the amp, but I have done this with at least 3 brand name amps without an issue.

GA for your answer.

The OP has misunderstood something, a car radio or amp will only be connected to either 12 volts or ground....this is a really weird voltage.......something has been measured or understood wrongly.

There may be an output from the Radio to raise and lower an electric aerial, also an input for a built in mobile phone.....

My personal take is to STOP right there, get some professional help, before something gets burned......

This doesn't sound right. The standby power input is usually 12V for a car amplifier and resistors are not needed.

Download the manual for the car amplifier off the internet for a correct wiring diagram.

Kita,
I assume the blue wire is a lot smaller than the yellow and black power connections.

The blue wire seems to be simply high = off, low = on.
Like another contributer said, the 0.7V is probably as low as the solid state switching component operates.

The on-off device in the amp can be an electo-mechanical relay with 12.X Volts applied to its coil and the blue wire is to complete the path to ground.

It is possible that the head provides a delay before going to .7Volts and turning on the amp. This allows the Amp to stablize before blasting heavy metal and hurting itself.
See how long it takes for the amp to operate after being turned on.

If there is no delay you could tie blue permanently to Ground or remotely operate the amp using a toggle switch.

Jon

unless your kenwood is from the pre 1990 it will take 12.v on the blue turn on wire to get it to power up. i have seen car stereo's with a blown blue lead turn on circuit that would output 12 v when off and then go low when turned on. this is caused by exceading the out put capacity (.7a) by having a excessively large fuse in the blue lead. however some brands do not fuse the turn on wire. as far as i know or unless thigs have realy changed in the last couple years you can not cascade car audio amplifiers as the output from the first amp will overdrive the second amp most of the kids that thought they could do this ended up with blown junk as a result.

thanks everyone. don't have the model number in front of me but will add later. I know that it is from the early 1990's. To clarify, the voltages read were taken by myself using a multimeter. The positive lead of the multimeter was kept on the yellow wire, while the negative lead was moved from the blue to black, with the ignition on and the head unit switched between on and off. There is indeed a delay between the click of the amplifier being turned on and sound coming through the speakers. I will try cats suggestion of using a 10K resistor off a 12V feed and seeing if it has the desired effect.

Kita,

I see where the error came from in your voltage measurements. The negative lead of your multimeter should always be referenced to "ground". This is the equivalent of the "-" or minus side of the battery. It's also tied to the chassis of the vehicle. When you are working with automotive electronics/electrical, your measurements should always be referenced to ground/- battery/chassis. The easiest way to do this is to connect your minus lead to a metal section of the vehicle. The black wire you talked about was most likely "ground" as well, and could have been used for the minus side of your voltmeter. You would keep your voltmeter minus on the black wire, then measure the voltage on the blue and yellow wires with the +/Red/positive lead of the voltmeter.

If you cannot find or use a grounded metal section of the vehicle for your ground reference, you can use a "grounded" wire. You can determine if a wire is ground by using the ohm meter function of the multimeter, and measuring the resistance from the car chassis to the wire. If the resistance is very close to 0 ohms, it's probably a ground wire, if the wire is black in color, it's probably a ground wire. There are exceptions to these, though, and your best bet is always the vehicle chassis.

As for your amp, you do not need to use any resistor in series with the enable voltage, just apply +12V to turn on the amp. The amp has a series resistor built into it, and it will draw very little current through this enable line. Keep in mind, you do not want to have the amp enabled when the vehicle is not on/running. Even with no audio input, the amp will draw at least an amp of current, and would run the battery down over time.

One last item, as someone earlier stated, I'm not sure what you meant when you said "I would like to use independently of the head unit in my car to act as a preamp for a busking setup", but if you mean trying to use one amp to drive another, to get "more out of it", the only thing you will get more of is smoke. The amps are setup to be line-level inputs (< 2V audio input), and speaker-level outputs. Trying to use them for any other setup is not going to get you anything but burnt-up amps. Perhaps you could explain what you are trying to do, and we might be able to offer some suggestions.

Tom

F.Y.I.

Busking, from Wiki....

Busking is the practice of performing in public places for tips and gratuities. People engaging in this practice are called buskers

I didn't know what it was either.....

Aha. Well I will check the measurements while keeping the negative lead grounded (will check resistance of black amp wire). In the meantime, to give some background.... I do indeed intend to use the amp as part of a rig for busking, that is, playing music in public places for tips. However busking is usually done by live performance over a small PA system, wheras I am using a mixture of live performance and recorded music that is broadcast using a small FM transmitter, that people can listen to using headphones with fm radios embedded in them. Its mostly for fun but I charge a small fee for the headphones. For the system to be mobile I run everything off a 40Ah 12v battery. So far I have been unable to find a 12V preamp/ mixer that gives sufficient gain to produce a nice clear signal to the headphones. Of course if I use an inverter then I can use a normal 230V mixer and that produces sufficient gain, but I would like to keep it safe and simple. And I know that 9V powered mini mixers are available but have found that they are either not powerful enough or are a little pricey for a hobby project. I realised that I had this spare amp, which has adjustable gain, from 0 upwards. Obviously I would blow the transmitter if the amp was set anything much above 0, but having experimented with it seems to be quite finely adjustable. I can feel everyone telling me already not to do it.....

Kita,

You would indeed be risking your FM transmitter by driving it with the audio amp. Even if you could adjust the level down low enough, you would be introducing a lot of noise into the signal, the amp is just not designed for output levels so low, driving such a high impedance (the FM transmitter). I'm not sure why a battery powered mixer would have different drive characteristics, they should all conform to the "line level" output standards for audio signals. You really need to go with the other options you mentioned, a DC powered audio mixer, or use an inverter to get 230V AC to power a regular plug-in type mixer.

If you are not getting good sound quality with your setup, I would suspect your FM transmitter as well. Does it have a gain setting? If you think you are not getting enough drive level from your mixer into the FM transmitter, try using the headphone output from the mixer, instead of the line-level output. That will give you more drive level, but will still be a lot less drive than trying to use the audio amp to drive the transmitter. The sensitivity and range of your headphones are going to determine how good things sound, too.

Not sure where you're located, but Ebay is a good place to find used audio gear at a much better price than retail. Good luck and have fun

Tom

Wild guess here, but he may be using a very cheap, Walmart type FM transmitter or similar, I have one I bought years ago, its great, but it needs some quite serious input to get the range and quality needed.......unbelievable I know.....but true.

I also feel that this car amp is over doing it and will empty his battery far faster than a small custom built amp or kit to put together yourself, that you can buy in many places......

What needs to be done (at home) is to find out how much "drive" voltage is needed to cause the FM transmitter to work as designed......then go and buy a simple amp or dual amps if stereo is required, to do the job. A simple amp with no extras that also need to be powered up.....is how I would have done it......

ok thanks for everyone's input. I will try to find a kit or used 12v preamp/ mixer. For the record the kenwood amp was kac 424, and the transmitter is a 1w output model from aareff (http://www.aareff.com/1wtx.htm).