Determining an older bridgable amp?

[jol50]

Is there an easy way to figure it out if it does not say on the amp? Most you can find the reversed channel by sight or testing, but I have one that may not be. It is an old alpine and the speaker grounds (-) are hooked together inside, not good...but strange part is the positives do not seem to go to the mosfets and have a lot of resistance to them. What does this mean? I'm sure I hit every leg on the outputs, or is it a class A? It runs hot and is 2x30, is assembled strangely so I can only see the bottom of the board unless I tear it all apart. It is the alpine 3518 on ampguts.

I looked for a thread about bridging stuff but could not find anything. Can you change an amp, or is it pointless/not possible to make one side out of phase...

[I Am An Idiot]

If the speaker negatives are connected together, It is not a bridgeable amp. (Without an external inverting module of some sort)

[Perry Babin]

Looking at the 3518 on the ampguts site, it appears that the amp has a speaker protection relay. A relay would open the connections between the transistors and the speaker terminals.

[ppia600]

/\ sounds like he'd need to turn the amp on and with no rca's connected to the inputs then test the speaker leads with the ohm meter. Whichever two are connected (less than 1 ohm) aren't the bridge terminals. You would use the other two.

[Perry Babin]

ppia600:
The relay only breaks the connection for the 'active' speaker terminals. He said that the negatives were directly connected. That would mean that the amp was not bridgeable without inverting one input (as was mentioned previously).

With the amp on and the speaker relay engaged, he should be able to find continuity between the positive terminals and the output transistor emitters.

jol50:
The outputs in this amp are not FETs. They are BJTs.

[jol50]

Originally posted by Perry Babin

The outputs in this amp are not FETs. They are BJTs.

Just glad I never said I knew what I was talking about.

I like the sound of it, old ancient thing. Doh, I see the black box relay. I looked at the pic after I looked at the amp from the bottom of the board only. This amp is a little different than the current run of the mill. I will test the other relay pins to the outputs, bet they go right there. Guess that is one way to get rid of on/off thump as well.

Seems like someone would have made an input gizmo to mono and phase the L/R to use on these old amps...or is there more to it? Does it have to be done at the OP amp/etc? I think this is 2x30 at 12v, it would be perfect to bridge two...30w is not quite enough, but it is very clean IMO. And, I just bought a second one for $2. lol.

I have to read about BJTs more, don't know if there is a significance in using them. Just bought some to218 for an audiobahn that were 4.25 each, kind of pricey.

[ppia600]

Wow that thing must be ancient. I never knew alpine sold un bridgeable two channels. The first JVC amp I had worked like that and it sounded great. Since it wasn't bridgeable I used the two positives to run a center channel speaker in the dash of my z. I used a rheostat to control its volume, and the left and right channels ran kick panel speakers. It actually sounded very cool.

[jol50]

Well, I have a pair of these now even though the (insert many slang descriptive words here) I bought three amps from tossed them in a box bare and now two amps are totally chewed up. Might have to figure out how to paint a heatsink. At least I didn't pay much, but what a nice guy. He took forever as well.

So, is it possible to build a box with one RCA set in, and two inverted RCA out that will mono each amp one left ch and one right ch? Or can I mod the amp internally...or would that likely be a giant pita to do? Then again maybe it would be nicer to have a separate box I suppose and not mess with the amps. Will this hurt the amps at all (like extra heat, because they run hot now at 2ohm) or change how they run? Yes the grounds are tied together as I recall, had to put that amp away and do other stuff. I'd like to try this if not too complicated, otherwise I will look for a different amp for now. Oh, would be nice to have some kind of gain or two trimmers for gain on it too since these amps are preset to 500mv input.

I did some searching and saw some op-amp circuits that kind of do this type of thing, but not how I want to do it. They are mostly about sound cancellation, in DIY headphones.

BTW, I also have a manual for the 3518 and it is rated at 40wrms x2 @ 4ohms both channels driven at 1kHz, no more than 8% THD...no voltage but 14.4 required. 30wrms x2 30-20kHz at .1%THD. S/N 100dB IHF A weighted. Pulse regulated power supply. +/- 3 dB 20Hz to 60kHz.

[djQUAN]

http://sound.westhost.com/bridging.htm

http://sound.westhost.com/project14.htm

http://sound.westhost.com/project20.htm

[tsmith1315]

So, is it possible to build a box with one RCA set in, and two inverted RCA out that will mono each amp one left ch and one right ch? Or can I mod the amp internally

jol, you can do it with a couple of op amps. Build an inverting follower for one channel, and a non-inverting follower for the other. Set them both for a gain of 1 (unity), and be sure the op amps you use are stable at unity gain.

You can power the op amps with 12V by splitting it up with a voltage divider and capacitor coupling the output to block the DC.

I've done this a few times with fine results.

Preferrably, you could tap into the power supply rails for the amp, and drop that to a usable level with regulators or zeners. In that case, you could add a mild gain stage.

Or cheap and greasily, you could use a pair of small transformers, and simply wire the output of one side out of phase. I've done that too, with surprising results.

Tim