Soundcraftsmen amp help

cdm418

Hey all! Noob question here. I just picked up a Soundcraftsmen 300X4 amp. This is a 4 channel amp, but I’ll just be using 2 channels for a stereo setup (left and right). The speaker connections are labeled A, B, C, D. No mention of left/right. Would I connect my left Speaker to A, and right Speaker to B. Or Left speaker to A, and right Speaker to C. Here’s a pic...

https://photos.app.goo.gl/2HRz34xxB9r7H3Bi7

I assume:
A is front left
B is rear left
C is front right
D is rear right

is that correct?

mhardy6647

Not familiar with that specific amplifier (and leery of clicking on a blind link, sorry!)
but my guess would be that if there are four channels of amplification -- the amp should have four inputs to go with the four outputs. Assuming all four channels are rated the same, I'd say pick any two and just make sure you have the right INs and OUTs!

EDIT: OK, check that That amp is interesting. Google did turn up a test report courtesy of the amazing document resource at www.americanradiohistory.com

https://www.americanradiohistory.com/Archive-DB-Magazine/80s/DB-1989-03-04.pdf
See page 48 of the magazine (which is pg. 50 of the scan PDF). There's also an ad for the unit (imagine that?! ) on page 37 (scan page 39).



Even though not a hifi amp per se, it might be a decent sounding amplifier, given the use of MOSFETs.

It essentially contains two separate stereo (two-channel) power amplifiers in one box. Each stereo amp can be operated in bridged mode to make is a very powerful two channel amplifier. The above-mentioned report gives at least rudimentary explanation of how to so that.
Photos of the back panel available on-line appear to identify hookups for the various configurations, but the resolution is too poor to tell for certain, and I haven't found a manual on-line. Read the test report, look carefully at the back panel (or post a big ol' photo directly to this thread) and we can likely suss it out.

Heck, maybe someone else here actually knows.

mhardy6647

ahh, I gritted my teeth and looked at your link



Just hook it up exactly as it says For bridged stereo operation (two channels, each, apparently rated for 600 watts into 8 ohms!), connect one stereo channel input to "A" and the other input to "C" and flip those little slide switches to "Bridge". The only tricky part is the speaker hookup -- just do what it says, on the "A" side, connect one speaker to the A red terminal and the B red terminal; on the "C" side, connect the other speaker to the C red terminal and the D red terminal. Make sure both speakers are connected with the same relative polarity -- i.e., if you hook the "+" wire of one speaker to the "A" red terminal, you'll want the "+" wire of the other speaker hooked to the "C" red terminal (and the "-" wires for the two speakers will go to the "B" and "D" red terminals, respectively). Do all hookups with the amp turned off (!) and, ideally, unplugged -- and don't use the bridged mode if you're not comfortable doing so!

Note that, if you want to, you could just use any pair you wish of the four power amplifier channels to power a pair of speakers (and just kind of pretend that the other two aren't even there). This will still get you 210 watts into each of the channels (with an 8 ohm load)

HTH, as they say.

EDIT: Do you know whether the amplifier was/is working properly? An amplifier like this has massive potential to damage loudspeakers if it has any pre-existing issues and/or if connected improperly! Treat it and your speakers with much respect -- and/or test with a pair of "cannon fodder" loudspeakers first!

Do you know how to test for "DC" (direct current voltage) offset at the speaker terminals? If so, you should absolutely do so before risking a pair of speakers to that behemoth, I'd opine.

cdm418

Thank you both for the replies! Great info. I’m powering a pair of Polk Audio Lsi15’s. I’m told they are very power hungry! The previous owner used this as a DJ to power his subs. He plugged it in to show me it powers on, but I didn’t listen to it. I opened it up and cleaned it well. I do not know how to test for DC offset. Can you explain? Thanks!!

mhardy6647

Amplifiers for music should have little or no "DC offset" (voltage) at the speaker outputs. There are certain failures (typically shorted driver or power output transisors) in solid state, direct coupled amplifiers that can lead to large DC voltages being present on the speaker terminals. Such voltages, if high enough, can more or less instantly destroy speaker drivers -- typically woofers (by burning the voice coils open), and/or damage crossover parts.

Do you have and/or know how to use a voltmeter (VTVM, VOM, or DMM)?
If so, just set the meter to measure DC voltage and check for DC voltage across the speaker terminals for each channel (bearing in mind whether the amps are BRIDGEd or not) with the power amp on but with no speaker load connected. There should be little or no DC present. In fact, most direct coupled (fully complementary) ss power amplifiers have a "DC offset" spec (typically tens of millivolts DC) to which they should be adjusted and if they're in good working order. If there are volts of DC, you've got troubles.

The other, but more dangerous, way, to look for DC on the outputs is to observe the woofers when the amp is on and connected to them. If they move (either out or in) and stay displaced, there is probably DC that's polarizing the voice coils (making them work as regular old electromagnets) -- the heat generated by such a DC 'bias' on the amp output could damage the woofers (slowly or quickly).

Many amplifiers have 'protection circuits', fuses, or circuit breakers meant to protect
speakers in the event of DC issues... but in practice, it's not uncommon for the speaker itself to act as the fuse!

Not trying to be an alarmist, just trying to protect your speakers. If you're not worried about replacing the speakers (in a worst case scenario), just plug and chug.




Here's a primer on DC offset:
https://audiokarma.org/forums/index.php?threads/amplifier-distortion-dc-offset-and-you.5634/

cdm418

Thanks for your help. I’m new to all this! Definitely don’t want to damage the speakers, I’m Loving them! I’ll try and grab a voltmeter from Lowe’s tomorrow and check it out.