I know mtx is known for its toughness

[hellohello]

http://www.crutchfield.com/S-RhtfiDV4npf/cgi-bin/ProdView.asp?a=&s=0&cc=07&g=120&id=review&i=236T282&pg=2&sortBy=3
Its a scratch and dent item, which is fine for me, but i wonder if anyone knows how well the "speaker level sensor" works. I like the idea, but does it work as advertised or do u have to crank the volume on the headunit to make it "sense" and then quickly turn it down before u go deaf? Im using the stock headunit because im stubborn and weird, so Im trying to think of all possible pitfalls before I go and make a mess, I mean decision. :rolleyes: :D

[exalted512]

www.davidnavone.com
use one of those LOCs
-Cody

[hellohello]

im guessing the n-902 is the one u are referring to, but what are the other two cheaper ones? (n-trig-lo & n-trig-lo-hd)

[PoweredByDodge]

the speaker "sensing" is shitty on just about every amp... i'm working on my own line out converter design that provides a 4 volt preamp output - assuming your factory deck puts out at least 7 watts RMS at 4 ohms. It's a work in progress... i'll keep ya'll posted, and it shall be pimpin. (and less than 5 bucks to make if i have it my way)


whole reason i'm gettin into it is that Jack has a 65 oldsmobile 442 that is COMPLETELY original down to everything but hte damn bolts... and he wants the factory deck to be kept, but also have 4 speakers in stock locations with say 20 or 30 watts rms per channel...

so -- gonna use stock radio (which has been rebuilt) and an fm modulated 6 disc changer or something with the control pad for the changer tucked under the sun visor or something... its a work in progress.

[hellohello]

you sir have my attention. I always enjoy building things, as long as i have a good schematic and parts list :)

[PoweredByDodge]

i'm probably going to sit down and build a simulation of it tomorrow - mainly your lookin at two SCR voltage regulators, caps, inductors, and a little box to put it in - like i said about a 5 buck cost.

[hellohello]

SCR voltage regulators... whassat?

[PoweredByDodge]

*scratches head* -- i think silicon controlled rectifier -- i forget though - i know what it does though :o) hehehe just an effective voltage regulator - dump in 9 volts, get 5 out. dump in 3 volts, get 3 out... 5.1 get 5 ... beautiful thingy. won't prevent clipping, but it will make sure that you don't overload your amp RCA input. find out where the deck clips (say 4.5 volts -- get an SCR that's 4 volt limited --- set your amp for around where you'd set it with a 4 volt RCA line out from a deck - plug it all together and forget about it).

[PoweredByDodge]

scratch tha actually - i think i got my part names mixed up -- i think a vreg is just a vreg --- what the hell is an scr then -- dammit i used to know this shit.. grrrrrr....

[MacLeod]

Youre both way over my head at this point. http://www.animation-station.com/smileys/images/spamafote.gif

[PoweredByDodge]

and i'm way wrong -- voltage regulators are semi-static, you can't just float up and down on them... bastards.

but - i've accomplished the same thing (at least in computer simulation) with an op amp. yay for the universally awesome little buggers. i'll smooth it all out later - its 1-30 in the damn mornin, i'm goin to bed... goodbye 0.25 volt LOC's... hello 4 volt Line out Converter.

[hellohello]

ooo opamps, i know what they are. I was always tempted to build my own amp either from a kit or from scratch, just because i figure i can make it smaller and fit into a tighter space. some of the commercial amps are kinda big, i understand that they use that big enclosure as a heatsink, but i could do without all the flashy logos, like the old rockford amps, plain and simple. :D

[PoweredByDodge]

son of a bitch - i had this all writen out and my browser clicked off...

grrr....


not bench tested yet - tested in computer simulation using Orcad PSpice and Multisym. and yes i know i draw like crap - but by the time i was done with the simulation my computer model looked even sloppier.


op amp is generic 741 (fairchild, TI).

RP1 and RP2 are 1/2 watt 500 ohm resistors.

Ri and Rf are 1/4 watt resistors - 10k and 3.9k respectively.

Cout should be 15 uF instead of 3uF as listed.


-------------


less than 5 degrees phase variance at high frequencies, none at low freq.

output...

head unit inputs ---->>> out to amp (RCA)
----------------------------------------------------------

56 watts max / 28 rms ---->> 6 volts peak / 4.25 volts rms.

15 watts max / 7.5 watts rms ---->> 3 volts peak / 2.1 volts rms.

5 watts max / 2.5 watts rms --->>> 1.8 volts peak / 1.2 volts rms.

what do we have?

good shit.

although with only a 5 watt max headunit - you're better off just running the speaker lines to the RCA's and getting about 4.5 volts rms to the amp.

this whole thing is adjustable ... change Rf to change the gain of this converter...

THIS ONLY WORKS FOR DOUBLE ENDED SPEAKER LINES ---- that means head units (factory or aftermarket) that have each speaker both a positive and negative... so the left speaker has a positive wire and a negative wire - and the right has a positive and a negative. THIS DOES NOT WORK FOR COMMON GROUND HEAD UNITS...

common ground decks are decks that have only the positive wire for each speaker... you're supposed to connect the negative speaker terminal to the sheetmetal of the car (chassis common ground). I'm still working on the design for that -- its going to be a little different - seems i get clipping when i try to run a common ground into this -- go figure.

[PoweredByDodge]

ps - this is by no means intended to be "gooder" than a real RCA out ... but it'll probably be a lot better than those damn 0.25 volt LOC's -- pieces of shit. as far as how you'd hook it up -- build it in a project box - hook it up in your dashboard... use the radio's switched ACC power and ground to hook to the box - then run RCA's to your trunk.

[PoweredByDodge]

for common ground systems...


this unit will only work if peak output power of the head unit is equal to or less than 18.5 watts, and rms output is equal to or less than 9.25 watts per channel.

you're only going to find common ground on OLD decks (classic cars, etc)... most dont even play cassettes... some have 8 tracks..lol... anyway - most of these decks don't put out more than 7 to 15 watts peak per channel, so it SHOULD work well.

.... same schematic as above -- BUT -- CONNECT SPEAKER WIRE + lead (the only lead there is) to Ri (on the op amp "-" input terminal). everythign else stays the same.

with a deck input of 18.5 watts peak / 9.25 watts rms, you'll get a preamp output to the amplifiers of about 3.3 volts ... rms output of 2.3 volts.


change feedback resistir, Rf = 7.5 k ohm... a 5 watt per channel peak common ground deck will get you a preout of about 3.3 peak... 2.3 rms. otherwise if you leave Rf alone it'll be way too weak.

[PoweredByDodge]

whole idea would be to tuck a changer away somewhere, keep your classic car deck - and then romp with 1,500 watts of bass out of your old 8 track.

[hellohello]

"and yes i know i draw like crap"
Nonsense, some of the best crayon work ive ever seen, this is definitely fridge-worthy my man. :)

[hellohello]

so if i understand this correctly, its basically a preamp... right?

[neomagus00]

basically, yep...

[PoweredByDodge]

yup...

that's all a normal LOC is, but i guess you could say this is an "active" pre amp instead of a passive one... passive components do not require a power supply, active ones do.

passive line out converters, like most that you see that give you a 0.250 volt preamp output - are great in theory - but a quarter volt is crap.

employing a little juice, you can get much more out of it.

you could actually just use a voltage divider ifyou knew for 100% sure what your particular amplifier's preamp input impedance was... but it changes for each manufacturer, and often between even models... so that's not an option.

[PoweredByDodge]

you'll need one whole circuit for each channel -- if your deck is just stereo 2 channel - the 2, if you have a 4 channel deck (front left, rear left, front right, rear right) then you CAN do 4 -- or you can jsut do 2 and leave the rears unhooked. depends what your application is... either way - its a very small design and should be able to fit into a pack of smokes like i said.

[PoweredByDodge]

attached is a diagram for your typical line out converter - just shitty little resistors - you can see why this creates such a problem - the design is garbage, the parameters used for design are even worse... and the cost is ridiculous - 15 to 25 bucks for something that you could make for 45 cents!!!

[PoweredByDodge]

ps --- if you'd like to make my design "adjustable gain" then you can simply remove Rf and in its place put a 10k potentiometer (variable resistor).

[hellohello]

ooo variable gain... instead of using a pot, i could try different values of resistance there right? Ill probably never mess with it once its installed

[hellohello]

or i could make a 12ax7 preamp instead... lol i have a feeling it would probably catch on fire if i tried it

[PoweredByDodge]

well don't "try" -- use your head man... find out what your factory deck is rated at. peak watts per channel... then, multiply by 4 and take the square root of that number.... thats the factory radio speaker voltage output.

then... say its i dunno - for example -- 6.2 volts... and you want to get down to a 3 volt pre amp ... well 3 is about half of 6.2... so you say "ok gain is equal to Rf divided by Ri --- so Ri has to be 10,000, what does Rf have to be ?" --- well that's easy -- 5,000. so you put a 5k resistor in place of Rf and you're done.

experimenting with values if you don't have some sort of a thought behind it will result in clipping, and possibly damaging both the little box you made as well as your amplifier and your head unit.

[PoweredByDodge]

12ax7 ? -- you're gonna build a tube preamp? dude... i'm not even goin there... lol

[hellohello]

i have rebuilt tube amps before, nothing from scratch tho... and i dont think id want fragile things in the car

[PoweredByDodge]

the real question is, why bother?

the purpose of tube equipment is an insanely high signal to noise ratio, good stereo separation, and all in all high quality audio.

now your chain is as strong as the weakest link - that being hte factory deck -- S/N on a factory deck is most likely crap - so is stereo separation... a good amplifier and good aftermarket deck will have a S/N of around 90 to 100 or better. you can get op amps that have S/N's of about 90 to 100... so why bother with tubes? -- espeically when they cost so much - you can build this whole thing for 5 bucks.

[PoweredByDodge]

shopping list attached - materials to build Left and Right stereo converter...

add $0.84 for Rf's of your choice... $2 for a printed circuit board, and $2 for a box to put it all in.

... if you would rather have speaker leads and power / ground (20 gauge) sticking out of the box in a sheath with spades on the end rather than a terminal block, then knock about $3.50 off of that.

all the parts are about 7 bucks... the box, board, and temrinal block bring you up to about $15 ... but i still think its a better option than a damned LOC.

[hellohello]

ok, that takes care of the preamp/rca situation, but I would still need a REM wire, i think u mentioned the cigarette lighter is a switched on 12v source, right?

[PoweredByDodge]

ehh... i'm not a big fan of just hooking it to switched 12 v... i mean, why leave your amplifier on when you're just toolin down the road with the radio off?

rather, i suggest this... get a small 3 or 5 amp relay (you'll only need about 1 amp, but i dont think they make relay's that small)... hook the normally open terminal #30 to the amplifier's REM trigger and also use it to power the pre-amp device. hook #87 and #85 to the cigarette lighter 12 V switched power... then hook #86 to a small toggle switch under the dash - the other end of the toggle switch should be hooked to ground... this will allow you to turn the amp and the pre amp on and off with the toggle, but still will shut them off if you turn the car off.

[hellohello]

there are very few times im in the car with the radio off... :)

[hellohello]

well, i managed to find a steal on the mtx, now i need to figure out what kind of fuse i need for the battery, ill be using 2 amps most likely in the near future, so how would i calculate that?

[neomagus00]

add up all the fuse values of the amps, and then find the closest fuse; round up rather than down

[PoweredByDodge]

figure out how much power you're using... this is probably the "best" way to do it.

say 75 x 4 at 4 ohms to high = 300 w
and 600 x 1 at 2 ohms on a mono sub amp = 600 w

divide by 0.78 for class AB amps... and 0.85 for D's...

300 / 0.78 = 384.6 / 14.4 v = 26.7 amps
600 / 0.85 = 705.9 / 14.4 v = 49.0 amps

49+27 = 76 amps... use an 80 amp fuse.


Or... go with the simpler, more commonlyused type of way...

add up fuse ratings for all your amps...

4 channel has say two 25 amp fuses.
mono sub amp has say three 25 amp fuses.

that's 125 amps... knock 20% off... and use that as a fuse.. 125 - 20% = 100 amp fuse.

[MacLeod]

I doubt youd need anything bigger than an 80 amp fuse anyway unless youre running a LOT of power. Im running nearly 1000 watts and only use an 80 amp.

The fuses in the amps arent necessarily the amount the amplifier will draw. In fact its doubtful the amplifier will ever draw anywhere near its fuse rating.

[PoweredByDodge]

not so true... mtx 500D's had 3 25 amp fuses = 75 amps...

- assume 85% effiency (rough)
- 500D's actually did between 775 and 850 x 1 into 2 ohms.
- rated at 14.4 V.

that's 69.4 amps (70A) at the 850 watt number. only 5 amps below the fuse rating.

[MacLeod]

Were they really that effeceint? I wouldve thought more around 65%.

[PoweredByDodge]

class D amplifiers are all betwen 80 and 85% efficient unless they're junk. that's the whole purpose of class D... efficiency.

class AB's can be anywhere from 60 to 75% efficient.

ex... mtx thunder 404 (newer model - circa 2 yrs ago or so)... 50 (2 25's) A fuse.
200 x 2 into 4 ohm load at 14.4...

say 45 amps (5 less than the 50 fuse rating) * 14.4 = 648 ...
400 / 648 = 62% ... it's probably better than that... but its around 65% ... some can be as efficient as 70 or 75 ... but it's not the norm.