Impedance Question....

[Ron]

<FONT face="Comic Sans MS">Though others have said otherwise, at least in 1996 GTS's, each rear subwoofer has 6 ohms of impedance. Given that most upgrade subs are available in either 4 ohm or 8 ohm impedance, which is the better way to go with the stock amp, up or down?

Second question. Where is the crossover for the rear subs? Does the head unit just send low frequency signals to the rear amp, is the crossover integrated into the amp, or is there an external crossover somewhere?

It seems to me that the rear subs get too limited of a frequency range and that even with a new set of JL Audio 6W0's, the sound is still muddy.....

Thanks</FONT f>

 

[toddt]

It would seem to me that there should be a transformer out there to match 6-8 or 6-4.

Perhaps you should ask your stereo dealer.

 

[SmokinV10]

Ron,
I thought this was a more personal question. Replace the "ed" in Impedence with "ot". Was gonna send you to the pharmacy for ******.

 

[Ron]

<FONT face="Comic Sans MS">Toddt,

I will of course, however at 11:00pm on a Sunday night, only the VCA is open.

Smokin,

Fortunately, my problems lie only in the automotive world, though if you don't quit that nasty habit, you might be needing that little blue pill yourself.....</FONT f>

 

[Ulysses]

Ron, a question that you should ask is "what is the output impedance of the amp?" Most likely, it's a 4 ohm system.

Impedance matching ala transformers is a good thing, but usually only for low frequencies and for power. They are usually only good for matching at the frequency that you match to and not for a spectrum of frequencies passed to the sub.

It is usually best to go down in impedance. You want to transfer as much power to the sub as possible, but there are limitations. Most amps will limit out when they see a total load impedance of less than 2 ohms.

The crossover for the rear subs are in the amp itself.

 

[SmokinV10]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>
Fortunately, my problems lie only in the automotive world, though if you don't quit that nasty habit, you might be needing that little blue pill yourself.....
<HR></BLOCKQUOTE>

Ron,
How'd ya know what color it was?

 

[joe117]

Don't you get maximum power transfer when your source and load impedance are matched? That's the way it works with RF/microwave components. I'm a RF/microvave guy not an audio guy.

 

[Gerald]

Buying speakers lower in impedence will yeild more power, but you will sacrafice a couple things. If your amp is lets say 8 ohms at 50 watts per channel and you buy 4 ohm speakers, that should make the amp put out 100 watts per channel. Your amp will now be working harder.
It will get hotter. In normal listening it shouldn't matter but if you start to turn it up, you MIGHT (although I haven't done it yet) burn it up (very unlikely). Might blow the amp fuse or it might shut down again, because of thermal load.
Doubling the impendence will double the power, it will also, double the distortion, heat, and all the negatives also.
So check your AMP and see the recommended impendece. If your only running capped bass off of it, you should be ok because bass doesn't distort very much and your only running 6 1/2" speakers.
For example, in my viper I bought a 8" woofer and it didn't sound good hooked up 8 ohms at 100 watts (sounded terrible, underpowered). So I bridged it so it would be at 4 ohms. Bass is MUCH better now. Amp has been running VERY hot (can't even touch it). It's been like that for about a year now, now problems with it shutting down, etc..


Just my opinion..

Gerald

 

[joe117]

Gerald, like I said in before, I'm not an audio guy but I do know electronics. Please write a little more about how you double the power by cutting the load impedance in half or doubling the impedance. Power is Volts x Amps, tell me how the volts and amps shake out in your scenario with the various loads.

 

[Gerald]

I looked this up for you. It's a great explaination....
You can essentially buy a much less expensive amp for a single sub and bridge it and it becomes a VERY powerful little amp..

Question:
How do I bridge my amp?

Answer:
Bridging a stereo amplifier essentially turns two channels into one mono channel with greater power output. In order to bridge an amplifier it must have at least two channels and it must be capable of being bridged. Check your amplifier''s manual to determine if you can bridge your amp''s channels. If you do not have a manual, check your amplifier''s back panel for a bridging switch. Amplifiers capable of bridging will have a switch somewhere on the rear panel labeled ''bridge'' or ''bridge mode'' or ''mono'' or something similar.

Once you have established that your amplifier is capable of bridging consider the speaker load you will present to it. By bridging an amplifier you essentially halve the impedance it sees from a given speaker so an 8 ohm speaker looks like a 4 ohm speaker to the bridged amp. Be sure your amplifier is capable of delivering consistent power into the lower impedance without overheating or distorting before going through with the bridging process. In most cases you are better off to have a single more powerful amplifier than two bridged amps due to the excess heat and strain that can be induced.

Assuming your amplifier is capable of being bridged and you have decided it will work with your equipment, the actual process of bridging your amp is fairly simple. Turn off the power to your amplifier and disconnect your speakers. Most amplifiers will use the left input as the bridged mode input. Check the back of your amp for this information - the input to use for bridged operation should be marked in some fashion. Connect a single RCA cable from your pre-amp to this input. Run a strand of speaker wire from the positive (red in almost all cases) input on your speaker to the positive speaker-level output on your amplifier corresponding to the bridged input you just connected. Connect the negative (black in almost all cases) input on your speaker to the second POSITIVE (red in almost all cases) output on your amplifier. In most cases the wiring will work like this: a speaker wire connection will run from the positive (red) output on your amplifier''s left channel to the positive (red) input on your speaker and a speaker wire will connect the positive (red) output on your amplifier''s right channel to the negative (black) input on your speaker. Once you have made your connections, flip the bridging switch from stereo mode to bridged or mono mode and power the amplifier back up.

The important points to remember in bridging an amplifier are to switch the amp to bridged mode, use ONLY the amp''s positive speaker outputs, and be sure your amplifier and speaker will work well in bridged mode based on the speaker''s nominal and minimum impedance (halved) and your amp''s capability to operate into low ohm loads.

 

[Ron]

<FONT face="Comic Sans MS">Thanks Gerald!

Now this naturally brings up the question, is the factory amp bridgable?

Further question. If you run full rear fade on a factory GTS system, you hear all of the music, highs and lows, through the subs.(quite poorly of course).

Wouldn't a proper crossover prevent anything above a specific frequency from being sent to the subs? Would the overall system sound better with a full range speaker rather than a sub since the system is sending all frequencies back to the rear of the car anyway?</FONT f>

 

[HP]

V=I x R or voltage=current x resistance
in a series connection
V=I x (R1 + R2)
in a parallel connection
V=I x Rt
These are the formulaes for resistance, I think someone was
looking for.

 

[Gerald]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Ron:
<FONT face="Comic Sans MS">Thanks Gerald!

Now this naturally brings up the question, is the factory amp bridgable?

Further question. If you run full rear fade on a factory GTS system, you hear all of the music, highs and lows, through the subs.(quite poorly of course).

Wouldn't a proper crossover prevent anything above a specific frequency from being sent to the subs? Would the overall system sound better with a full range speaker rather than a sub since the system is sending all frequencies back to the rear of the car anyway?</FONT f>

<HR></BLOCKQUOTE>

Umm, I don't have a stock system anymore so I don't know what the configuration is. I just looked at the stock amp that came with my 98 GTS. I dont' see anywhere that it's bridgeable. I would guess they are NOT because they are the "plug type" connectors and low end models.
Would the system sound better with full range? It might.. With the different power outputs in amps and products, it's really hard to say what would sound best. But in my opinion, if you want to get by cheap, just run the mids and highs off of the stock amp and crossover with caps (50 cents a peice). And buy another smaller amp that is bridgeable and hook the two rear speakers together so they are one, then bridge the amp. Did I confuse you now?

Gerald

 

[Ron]

<FONT face="Comic Sans MS">Gerald;

Yes.

Would it be possible to sketch out a crude schematic on the second amp / cap scenario?

Thanks.....</FONT f>

 

[Jerry Horton]

You may actually be better off getting a new 4 channel amp. If you spend an extra hundred to two hundred dollars, you will be able to control all of your speakers, and be able to tune your system. Not only that, but probably a little more power, since that stock one puts out 25-30 watts per channel. If you want the rear to be stereo, you shouldn't bridge the amp. Actually, unless you got anything but normal 4 ohm JL's, you wouldn't be able to bridge the amp. What I am trying to say, is that you might be better off with a new 4-channel amp. If you have any questions for me, feel free to email, as I have extensive knowledge on this subject.

 

[Ron]

<FONT face="Comic Sans MS">I understand the formulas and logic here, (thank you to the contributors, both public & private) but really what I want to learn now is how to modify the stock amp to eliminate it's low frequency filter and perhaps tighten up it high filter. It appears to me that in order to prevent damage to the cheap stock subwoofers, Dodge / Alpine spec'd the amp to filter out not only the highs (I question the effectiveness since I still hear highs trying to be reproduced out of the subs) but also the very lows. Now that I have a proper set of subs installed, I want that low frequency range back at a minimum.

I know I could buy a new amp, which ultimately I might have to do, but I prefer to stay stock if possible for appearance, cost and wiring reasons.

So the question now is:

Is the stock amp frequency range modifiable, either externally (crossover perhaps) or internally?</FONT f>

 

[Ulysses]

You cannot get back the lows externally since the assumption is that they are already filtered out by the amp. Once they are out, they are out.

Modifying the internals will be more trouble than it is worth. You would have to get a hold of the schematics at the very least and they are most likely proprietary.

For example:

Let's say you did get a hold of the schematics and that you isolated the filter network. You want to modify this network to fit your specs. What does this network do? You would have to find out it's characteristics. What is it's frequency response? (cutoff frequencies and steepness of rolloff). Let's say you figured out the which part of the network was the highpass and you could eliminate that part. Then you have to design the network that you want it to respond to. How steep do I want the lowpass filter to rolloff and what frequency does it cutoff at? Will it even fit on the existing board layout? Let's say it doesn't fit on the board so you fib the board so that the signal comes out at the ports after you have eliminated the network all together. Now you want that lowpass filter externally so you will most likely need an inductor. Inductors are not available in a wide variety like capacitors are. So it's a "wind your own" situation. Or if it becomes too complex, you end up buying an active crossover anyway.

Easier to buy an amp that is already setup for what you want it to do.

 

[Ron]

<FONT face="Comic Sans MS">Ulysses;

Pretty "sound" logic. Looks like it's time to upgrade.

Any recommendations that fit these qualities:
<UL TYPE=SQUARE>
<LI>Reasonably priced
<LI>Mounts like OEM
<LI>Uses OEM wiring - (optional)
<LI>Improved sound, (but no need for window breaking capability)
<LI>Adjustable
[/list]

I assume the amp only powers the rear subs and the head unit powers the door speakers, correct?

Thanks</FONT f>

 

[joe117]

Steve340, I know the formula for power and if your amp would deliver the same voltage across a 4 ohm speaker as it would across an 8 ohm speaker then you would double the power. I wonder why the foolish marketers would pass up the chance to advertise their amp as having twice the power? Gee, we could put six or seven speakers across the amp output and get a whole bunch of extra power.....we could power the whole world if we only had enough speakers.... Or perhaps there is something wrong with your theory?

 

[Ulysses]

Ron,

Someone installed an Alpine amp along with the JL speakers and posted it here not too long ago. It was an OEM fit for the amp, more power.

I'm not sure about your year car, but the amp in my '00 drives all the speakers.

 

[Jerry Horton]

Ron, You may be best off getting a new alpine of the same size (I think the model number is MRP-F200). It may be the same size as stock. I think it retails for $200. The only thing is that it is only 40 watts/channel @4 ohms. You also might want to check out the Kenwood Kac-849. It puts out more, but is a little bigger in size, and I think is more flexible with crossover. I want to try and simplify the whole impedance thing. Most car speakers are rated @ 4 ohms. Therefore most car amplifiers are rated for RMS power @ 4 ohms. Car amplifiers don't need to match impedance, their output is dependent on the impedance. With a very good amplifier, if you double the impedance, you get half the power, and if you half the impedance, you get double the power. If you bridge 2 cannels into one, you get 4 times the power. Take the Kenwood, for example. It puts out 50 watts/channel @ 4 ohms. It puts out 75 watts into 2 ohms, and 150 watts/channel bridged. Theoretically, it would put out around 30 watts into 8 ohms. While this isn't doubled and quadrupled, you see what I mean. I think the best way for you to run your subs is to wire them in parallel-bridged(positive of both speakers to left positive of amp, and negative of both speakers to right negative).

 

[Ulysses]

joe117 said:
<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>I wonder why the foolish marketers would pass up the chance to advertise their amp as having twice the power?<HR></BLOCKQUOTE>

Ah!, joe, but some unscrupulous manufacturers actually do just that.

There is some confusion here. For the sake of simplicity, if we model an amp as a simple voltage source, we would see this:

Where Rl = the load resistance and R0 = the internal resistance of the system.

Vl = V * Rl/(R0 + Rl)

If Rl=R0, V across R0 would be equal to Vl across Rl. The power, P0, across R0 would be exactly equal to the power, Pl, across Rl (which, as joe stated, happens to be the max power). For a DC circuit, as Rl gets smaller Vl gets smaller. Therefore
Vl*I=(Vl*Vl)/Rl, the power across Rl, gets smaller. The rest of the power being dissipated across R0. Here is probably where the confusion is made.

A good amp will try to keep Vl the same by increasing the amount of current to the load accordingly. This implies that R0 is not a constant or that V is not a constant. For a load half of Rl the power becomes (Vl*Vl)/(.5Rl)= 2*(Vl*Vl)/Rl, double the power. There are, of course, limitations on how much current the amp can source, which limits the amount of power that can be supplied to the load. One cannot keep adding speakers in parallel to lower the load and raise the power.

This is all recollection, so don't take my word for it.

 

[joe117]

And Mr. Ulysses is exactly right. If the amp says in it's specifications that it will deliver double the power when you cut the load impedance in half, then it will probably do just that. However, I would bet that most amps are rated at their full power by the manufacturer and you are not going to get something for nothing just by lowering the load impedance. If you drive one speaker with both sides of a stereo amp by bridging, you will, of course, get the amp's full power into one speaker. That's not the same thing as just cutting the impedance in half to get double power out of a single channel.