Im BAAAACK! Marathon amplifier question, won't work bridged mono!

[240z4u]

Alright, well the soundstream was confidence inspiring. I pulled out a pair of Marathon amps that I have that my buddy told me were "dead". I am not sure what he did, but he said he attempted to bridge them (maybe connect two amps together). God only knows.

I was lead to believe they were dead. Not so, they work fine in stereo and have dirty distorted output bridged mono.

Odd, what would cause this? If I flip the input switch from mono to stereo there is no change when its bridged. When in stereo, it works fine in either position. No changes when crossover turned off/on either.

You guys have any thoughts? I can take a couple snapshots of the boards if needed. The amps say MARATHON on them and 1 ohm stable. There is NO model name/number. The shell looks like a Massive amp.

Evan

BTW, I was doing my testing with an 8 ohm full range home audio speaker. Not sure if that would be a clue..

[Perry Babin]

Post a few photos.

If you set the volume so that the output is loud but clean in stereo and then bridge it, does it produce distorted output when bridged?

If you use a y-cable to send the same signal to both channels, does it still produce distorted output in mono?

With the RCA cables unplugged, do you have continuity (~0 ohms) between the secondary center-tap of the transformer and the RCA shield? If you don't know where the secondary center-tap is on your amp, post a photo of the power transformer.

[240z4u]

Originally posted by Perry Babin
Post a few photos.

If you set the volume so that the output is loud but clean in stereo and then bridge it, does it produce distorted output when bridged?


If you use a y-cable to send the same signal to both channels, does it still produce distorted output in mono?

With the RCA cables unplugged, do you have continuity (~0 ohms) between the secondary center-tap of the transformer and the RCA shield? If you don't know where the secondary center-tap is on your amp, post a photo of the power transformer.

Yes, output gets distorted and is much lower when the amp is bridged.


Not sure on the transformer thing, and my damn camera just went dead!! I am gonna let it charge, then take some pictures.

I assume the "transformer" is one of the toroidal transformers in the chassis. There is a pair of them. I was able to get continuity from a couple leads on the transformer to the RCA shield. Ill have to show you what exactly I mean after my camera charges.

I, surprisingly enough, do not have a Y cable! Ill have to get one. The amp does play clean when mono is selected and running the amp in stereo.

Evan

Perry, your a saint. Thank you so much.

[Perry Babin]

The transformer is the toroid. There are two groups of wires on it (generally on opposite sides of the core). There are 4 wires (or bundles of wires) in each group. The 'primary' side has more copper than the secondary side (where they enter the board). Of the 4 wires for each group, the two in the center are the center-tap. The primary is typically nearer to the B+ and ground terminals. The secondary is generally nearer the audio section of the amplifier.

You don't need to buy a y-cable. I wanted to know if the amp would have output (when bridged) if exactly the same signal was fed into both channels.

Which terminals are you using for bridging?

Do you read 0 ohms between the two terminals that you're not using for bridging?

If so, you should also read 0 ohms between those terminals and the RCA shields (no RCAs plugged in).

Does it make a difference in the output when bridged if you push/pull on the RCA jacks?

[240z4u]

Perry, moving the RCA connectors had no effect. I am using the outside terminals (its labled as "bridged" on the outside of the amp) I can't for the life of me remember which ones it is.

Ill check for 0 ohms tonight or tomorrow after work (I got called into work tonight for a service call).


Thanks! - Evan

Perry, am I correct in understanding from your description that when you bridge an amp it makes one channel of the amp push and one pull? I know my terminology sounds wierd, but I am use to push pull vacuum tube amps.

[Perry Babin]

I'm not sure how you define push/pull.

When bridged, one speaker output temrinal is driven either positive or negative. The other speaker output terminal is driven with the opposite polarity. If one swings positive, the other swings negative and vice-versa. The two channels are 180 degrees out of phase (one normal channel and one inverted channel).

If you connect a speaker to an amp that's not bridgeable (doesn't have an inverted channel) and you connect the speaker to the two output terminals that have signal (the positive terminals for most non-bridgeable amps), you will get relatively little output when you feed the channels with a stereo signal. If you feed both channels precisely the same signal, you will essentially get no output until the amp is driven to clipping. When clipped, the output will be badly distorted.

[240z4u]

Okay, thats exactly what I meant. Thanks for clarifying. At least I learned something productive today.

Thanks.

[240z4u]

Okay I do have continuity from the rca shield to the two unused terminals on the amp. I also have continuity between those two terminals that are unused.







In this particular photo, you can see the toroid. I have continuity at the leads that are on the upper right. One is a single lead and another is a smaller bundle.







Left positive, right negative is what the shell says for bridging. I get extremely (almost non-existant) output. I do, however, get output if I connect both positive wires to the speaker.

[theAnonymous1]

Originally posted by 240z4u
I do, however, get output if I connect both positive wires to the speaker.

That is because one of the + outputs as labeled on the amp is actually just a ground. This intentional "mislabeling" is done because of how a bridged amp works (out of phase outputs).

[240z4u]

Interesting, I figured that this was the case and that result was normal.

Pretty cool actually, because I don't understand the precise circuits but I am getting a good handle on how these things work.

I am reading the basic amp repair tutorial right now.

Thanks! - Evan

Another oddity, as the power ramps up in the amp it buzzes a bit. Then it stabilizes and audio comes on.