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 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...
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.
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.
Perry Babin said:
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.
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.
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.
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.
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
Thanks for the great links djQUAN, the Project 20 looks the best initially. Now I am not sure if I should build a box or mod the amp. My only issues are keeping the signal clean and I'd like a gain pot but not required. However I'd like to understand what I am doing and do not have a full understanding of this yet...and naturally don't want to let any smoke out of the amps, but maybe more so not have an issue of distortion or feedback/etc.
Tsmith, what would you do? I want to run the front stage with these is my ideal use, one on each channel. The bridged power on a high pass should be plenty and if it sounds the same, it will sound good.
Tsmith, what would you do?
I want to run the front stage with these is my ideal use, one on each channel. The bridged power on a high pass should be plenty and if it sounds the same, it will sound good.
I'd pull the bottom off the amp and look for rail voltage, and empty space.
A single quad op amp will do everything you want to do and more, and for a simple pair of followers and gain would need few external components. The whole thing may fit on a perfboard inside the amp, with a minimum of connectors, etc. They pull virtually no current, and won't add an appreciable load to the amp.
You could also tap into the supply and run a small cable out of the amp to power the external inverter, like the Arizona amps did.
If you don't want to fool around in there, I'll send you a simple 12V divider schematic (Rod may have one on ESP), and you an put it into a little project box.
Then, I'd clean up that third amp, use the two bridged amps on midranges and the third amp on tweeters for a bi-amped front stage. (Hey, you asked!)
But then again, I ain't normal.
Tim
A single quad op amp will do everything you want to do and more, and for a simple pair of followers and gain would need few external components. The whole thing may fit on a perfboard inside the amp, with a minimum of connectors, etc. They pull virtually no current, and won't add an appreciable load to the amp.
You could also tap into the supply and run a small cable out of the amp to power the external inverter, like the Arizona amps did.
If you don't want to fool around in there, I'll send you a simple 12V divider schematic (Rod may have one on ESP), and you an put it into a little project box.
Then, I'd clean up that third amp, use the two bridged amps on midranges and the third amp on tweeters for a bi-amped front stage. (Hey, you asked!)
But then again, I ain't normal.
Tim
What about just tying the two positives together (after adjusting the bias until there is virtually no difference in voltage between the two channels). That way you could just Y the two rcas at the input from one rca cable. Would that work to allow running a lower impedance load?? Just curious
The bias adjustment sets the idle current through the output transistors and should have virtually no control over the DC offset.
Even if the DC offset was precisely the same, connecting the two channels together would likely lead to instability/oscillation and the amp would self destruct.
If you REALLY wanted to do it, you could insert a low value resistor in series with each output. That 'may' allow the outputs to remain stable but you'd have to test it thoroughly to make sure the amp remained stable with various loads. You would also have to insure that each channel received precisely the same signal (y-cables would work) and have the gains set to match the output levels.
Any defect in the amp that would cause one channel to drop out would cause extreme stress on both channels. Even something as simple as a mismatched level on the equalization or crossover frequency (doesn't apply to this amp) would cause a difference in the output level of the individual channels and could cause the amp to fail.
Even if the DC offset was precisely the same, connecting the two channels together would likely lead to instability/oscillation and the amp would self destruct.
If you REALLY wanted to do it, you could insert a low value resistor in series with each output. That 'may' allow the outputs to remain stable but you'd have to test it thoroughly to make sure the amp remained stable with various loads. You would also have to insure that each channel received precisely the same signal (y-cables would work) and have the gains set to match the output levels.
Any defect in the amp that would cause one channel to drop out would cause extreme stress on both channels. Even something as simple as a mismatched level on the equalization or crossover frequency (doesn't apply to this amp) would cause a difference in the output level of the individual channels and could cause the amp to fail.
Ok, so messing with the bias wouldn't help. But the rest of what you said is pretty much what I was talking about. Surely adding the two resistors would help some, but yeah the differences in the two seperate preamp circuits may conflict at certain points throughout the frequency range and power levels. Their differences would be multiplied by the time the signals reached the output transistors and power would be wasted as well by using the extra two resistors. I was just thinking out loud though...
Perry Babin said:
I just procured a manual for this amp and it says nothing about running it at 2 ohms, even though I have done it for years at a time with one. Unless I try to slow it down a little with larger output resistors or something, but don't run it full tilt that often anyway.
So I can do it inside or outside of the amp? I'll drag that ampguts picture in here...
So I can do it inside or outside of the amp? I'll drag that ampguts picture in here...
An externally hosted image should be here but it was not working when we last tested it.
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