incompatible source grounding between amp types??

[LazySteve]

Well first the good news - having fixed my Punch 60ix (with Perry's excellent assistance) here:

Rockford Fosgate 60ix FETs blown

I acquired a second broken 60ix and found I had already learned enough to fix that one too without even having to post any questions

So, on to my install - I modified the stock RF x-over card and used one of the RF amps on a low pass to my rear 6x9s, which sound great. I have 5 inch coaxials in the front and first I tried feeding these straight off the front outputs of the HU which all sounded OK, however, I wanted more control over the front speakers to be able to boost the mid range, so (and I know this next bit is going to sound a bit hack) I thought I would try a decidedly low-end graphic eq I happen to have with a slave amp inside just on the fronts.

I wired it up but now connecting either of the front channels to the graphic/amp generates a bump from the rears and cuts off all sound.

I suspect that I am applying an incompatible input ground reference between the two amps, does that sound right? and is there a work around for it?

I should add, I am feeding everything from speaker outputs - I have no HU RCA outputs and no LOC.

Also my cheap little graphic/amp takes a common source ground for both L and R channels.

So I'm wondering, is there a fix for this or is it just a case of my trying to drive a square peg into a round hole ?

Thanks in advance,

Steve

[Perry Babin]

The output of the EQ likely has 6v of DC on them. If that's true and you want to use the speaker level to drive the amps, you'll have to ground the RCA shield going to the amps to the case of the EQ and use the positive speaker wires from the output of the EQ to feed the center conductor of the RCA cables feeding the amps. The gains on the amp will likely have to be set at the minimum.

An LOC would be a much better option.

[LazySteve]

Thanks Perry,

Maybe I haven't explained it very well, I don't want to go from the eq into an amp. The eq has a little slave amp inside which I think is quite adequate for what I want the front speakers to do. The eq is designed to take a high level input from speaker wires.

I want to run the rear speakers purely through one of my RF amps and the front speakers purely through the eq/amp. I have the RF set on a low gain and it runs fine off the speaker outputs with the HU settings I am using so I'd rather not get into a LOC if I don't have to.

Maybe after the wife has gone to bed I'll go into "paint" and try to do a little schematic to explain what I mean.

[Perry Babin]

If the input of the EQ has a common ground input (especially if it's connected to chassis ground), it's likely causing the speaker level amp from the head unit to go into protect. You could connect the common ground input to the head unit's case and use the positive speaker wires from the head unit to drive the positive speaker level inputs.

With nothing connected to the speaker level outputs of the head unit and the EQ, do all have ~6v on them?

[LazySteve]

Yes I just checked and the speaker outputs (both +ve and -ve) on the HU and the eq. are all just a little under 6v as compared to chassis ground.

I've drawn a little schematic of what I think you're proposing (I've mostly drawn just one channel). Have I got that right?

[LazySteve]

Apologies are in order, I have given a bum steer with my schematic; the output from the eq. does have a common wire but it is common between front and rear on the fader, there isn't a common wire between right and left channel eq. output.

Anyway, I played with this some more tonight. I think the eq. common input wire is indeed internally connected to chassis ground. I get exactly the same results whether I ground it or not.

I can get the whole system to work by running either the +ve or the -ve speaker wire from the HU into the +ve input of the eq. It doesn't seem to make any difference which I use, and that way I can adjust the eq. to get the overall sound image I was looking for (the front speakers are way too "toppy" without the eq.)

The only problem now is that when I switch it all off and then on again it comes back on with no sound. However, if I then simply interrupt the signal from the HU to the eq. and then reconnect it again everything is fine.

So I'm still a bit in the dark about exactly what's going on, but at least I've made some progress. All suggestions gratefully received.

[Perry Babin]

The DC on the speaker wires is likely the cause of the problem. When you switch it on, the DC voltage goes from 0v to 6v. That could be overloading the input of the crossover.

If you connect a capacitor in series between the head unit and the EQ and connect a resistor between the EQ input wire and ground, it may prevent this. A 100 ohm resistor and a 10uF cap should work if there is no load resistor at the input of the EQ. This combination will give you about a 150Hz crossover which should make the EQ sound a bit better.

[Damqik]

The problem you have here is the eq is designed with a common ground as often done before the mid 80's, on almost any modern HU the speaker negitive is above ground, i.e the speaker ground wire is not the negitive rail of the power supply or in this case the car battery,
so the head unit power amp is going into protection until you break the connection between it and the eq. You will eventually kill it running this way!!

If you really want to run an eq it must be low level - RCA connections from HU and amplified after that,
personally I would change to a HU that offers the control your after, forget the eq as its internal amplifiers will no doubt add distortion into mix.

if your only after a quick fix remove the EQ all together, filter the output of the HU using a speaker crossover to remove some of it's toppyness. You would no doubt have to play with component values but should be able to get sound your after.

[LazySteve]

Thanks guys,

I've decided this eq. was probably a bit too much square peg in round hole so I looked at another one I had buried away that I used successfully a few years back.

Although they look quite similar this other one doesn't have a common ground input (so a bit more modern I guess, though amazingly the first one was still new in the box, which is why I used it thinking it would be in better order).

I checked all the inputs to make sure they are divorced from chassis earth, which they are, and I installed it this a.m. It all works fine and the overall sound is now pretty much what I was looking for.

I can't detect any issues with the sound quality but it's all going in a '70s muscle car I've been restoring so I think the level of road and engine noise is going to mask any minor distortion in any event, even though I did go quite heavy on sound proofing when I put the car together.

[ppia600]

I remember there was a common ground adapter you could build with a few components, we had to do it a few times when running aftermarket decks into factory common ground amplifiers. They worked well considering. I believe it was comprised of a capacitor and a few resistors per channel. It supposedly didn't affect frequency response. I'll try and find the diagram