Bench testing system w/scope and fn.gen

Hi folks,

It's been a while since I'v visited these boards; similarly I'v been out of the audio arena for some time also. I did a quick search on oscilloscope, and couldn't find a whole lot of details on bench testing... I use the term 'bench' loosly: I mean the components installed in a vehicle, but using bench equipment to test/set properly. I am pretty well versed in electronics, own a Tek. 465B scope, and 15Mhz arbitrary function generator, and some other goodies. Admittedly I don't have any electronic experiance with audio 'tuning'. Anyways, I'm working on getting my audio system in my Nissan 240sx back up to par, and would just like some tips, hints, or just general knowledge of what you folks do to fully utilize similar test equipment. To start, here is my intended setup(h/u only installed as of writing): please don't mind the small-powered setup, I just need something for now.  I think this will actually help the purpose of this thread(to help educate others as well), because it won't be cluttered with the confusion of 5 other in-line amp/processing devices. I'd like to keep this as basic and rudamentary as possible, for my sake and others.

Kenwood Excellon KDC-X569 headunit

Infinity Reference 6020CS 6.5" in doors(undecided on amp to power these)

Infinity BASSlink 12" all-in-one.

Now to the nitty-gritty, with everything in place, RCA's, power, etc. Do you measure your noise floor/ceiling before proceeding(with car on)? Is it better to use a true(ie not burnt) test tone CD, or inject a signal directly into the RCA's for amp gain settings? If this is the case, I would obviously want to use the line-level pk-pk that my Kenwood outputs. Any other factors, ie should I assume an ideal 50ohm termination at the amp inputs?

For a sub I imagine tweaking gain is a simply matter of increasing gain on the amp untill clipping or distortion is present on the oscilloscope. General rule is to keep gains as low as possible untill right before final amp stage, right?(to keep the signal-to-noise ratio low untill then).

Now in regards to the Infinity cross-overs: is the procedure similar? Will measing rolloff rates and cuttoff frequencies of each x-over be of any benefit?

I'll stop there for now, rather than trying to ask too many questions at once. Thanks for reading, any insite is greatly appreciated.

That's fair, it might not seem like it's worth the effort for me to take... or is it not worth your effort to help me out on a small-sized system? Which ever the case may be, I just wanted to learn how to do it right. I have setup a dual 12" blaupunkt system before(in a custom box that I built), and I just always thought I could have always squeezed more out of them.(now I know I was severely limited by their digital amps).

Anyways, my intentions were to learn the basics now while the system at hand is simple...

With your gear you could do both the listening for the clipping signal and also look at it on the scope.  The volume shouldn't be overwhelming for listening, although it is common to put ear plugs in when you're driving any system to full power.

You can use a burned CD if you make it right.  Do a search of this forum for "tone generator" and you'll see a thread I wrote that links to a good online program you can download.  The key is to use sine waves recorded at 0 db.  Use the tone to set the head unit output and also to set the amp gains to match.  Other searches:  "test tones" and "setting gain" or "gain" will yield more info written in the recent past on this subject.

The general idea is to use a tone recorded at the highest possible level of signal strength (0 db) and set the units so that they are at full clean output at the same point.  With the head unit, this will be a number on the volume level that you will know is the maximum you can ever turn it after you find the point of clipping.  With the amp gain, it is to match the amp output to the head unit...so that when you turn volume up on the deck the amp will respond on a scale so that both peak simutaneously. When you get the deck volume up to that known limit you will know that anything beyond that will introduce clipping into the signal.  The clipping would come from both the deck and the amp.

You'll find more here:  http://www.bcae1.com/

For just the pair of front components?  Use a two-channel amp rated at about 2 X 50 watts RMS.  You could go higher to something like 2 X 65 watts if you find a good deal on something, but you'll most likely find the best selection in the 50 watt range.  Check with your local installer shops and see if they have something they can hook you up with.  Otherwise, there's a plethora of 2 channel amps on the web.  Some common names to avoid are Pyle, well....this will take too much typing.  Try this:  go to this website http://www.ikesound.com/  Look at the list of name brands along the left side.  Now, pick out Alpine, Blaupunkt, Clarion, Directed Audio, Hifonics, Infinity, JBL, JL Audio, Kenwood Exelon, Kicker, Lightning Audio, MB Quart, MTX, Orion, Phoenix Gold, Planet Audio, Precision Power, and Rockford Fosgate.  Toss all the others, and I'm not sure all of these brands chosen from the list make amplifiers.  There are, of course, other brands not seen here in the mid-price range that you could get a recommendation for... but you get the idea.  There are probably twice as many you wouldn't want to part with your money for. 

If I left anybody out please accept my heartfelts.  Not an endorsement of the above linked website.

2ch, right... I don't know what I was thinking... for whatever reason I assumed that each component had its own cross-over, but it's one per set... will each channel of the amp see a 2 or 4-ohm load(the tweeter and mid are 4ohm each, but do they get paralleled?).

Thx again.

With the supplied (infinity) x-over , the amp will see a 4 ohm load per channel.

There are some infinty copmonents that have a net load of 2 ohms, but I do not think you have that model.

A crossed-over component set (or a coaxial unit with a cap on the tweeter) works as a single 4 ohm unit.  The crossover divides the frequencies between the components, so in effect there is only one 4 ohm component playing at a time.  Mind you, these are very short time durations!  But overall, the crossover maintains the 4 ohm impedance.  The components are fed from the crossover, so there is no parallel or other wiring to be done between them.

Additionally, the set of components is rated for their total power handling capacity as a set, not individually.  This distribution of power is also a responsibility of the crossover.  http://www.speakerworkshop.com/ is a great crossover design tool.  Save this link for more study about the way these work.

Hey thanks for all the great responses everyone, I appreciate the attention. I read practically all of the bcae1.com site(with the exception of the basic electronics stuff)...wow, lots of good info there. One part in particular has me, for lack of better terms, worried:

This is stated because typical music recordings are lower than 0dB, and compressed, correct? I do agree with the suggestions of 'tuning' with music *and* test tones... that being said it seems that setting the system up first with test tones is a mute point(haha, no pun intended. ), because I'll likely end up gaining up to make up for the lower-dB recordings. Of course I could look at the music on the scope, but I'm not sure how well clipping would be seen(as it is not a periodic signal, easy to trigger on).

Also from reading the 'Setting gains with a scope' section(heh, why didn't I find this earlier!), my 2V pre-out may be quite limiting for bass/trebble adjustments. They suggest using the fader to reduce the amplitude a bit, but I think my pre-outs are all non-fading(?).

There seem to be some nifty features on this Kenwood deck, things I'm not used to seeing: Center frequency and Q adjustments(bass/mid/treb), Front high-pass filter, Rear high-pass filter, non-fading low-pass filter. I'm hoping those will be of some help when I get those Infinity's in(believe me, it'll be an upgrade from $19.99 4x6 Pyles)

Regards,

'slip