Inverted Amplifier Channel (?)

[Kevin Mullins]

I've been working on a jukebox project off and on now for some time and have one stumbling block I'm not clear on, so some enlightenment would be of help.
http://forum.arcadecontrols.com/index.php?topic=58692.0

The question is about an "inverted" channel on amplifiers.

Apparently the original 1978 amplifier was a two channel 125 watt or so amp.
But ONE channel was inverted. (?)
Then this was fed into a crossover network and divided to the corresponding speakers.
The Two larger bass speakers in the base of the unit were apparently run in a MONO fashion utilizing all the amps power. (kinda like bridging the amp)
The smaller midrangers were in typical STEREO fashion.

Coming from the original amp there are four wires leading down to the crossover section.
Green , Yellow, and two Blacks. (the two blacks do have continuity between them)
You would have thought that the two colored wires were "positive" and the two black ones were "negative". But that is not the case when trying to use the original crossover due to the fact that one channel from the original amp was somehow "inverted".
So this is what the crossover is needing in order to split the frequency properly.
I'm trying to utilize as much original equipment as possible, so I don't really want to modify the crossover. I'd rather figure out more about the "inverted" channel from the amp.
And NO, I do not have the original amp to investigate further on how they originally did it.
And it's not as simple as switching a positive and negative around obiously. (the negatives are not isolated on the new amp)

I'm trying to use a standard amp, or "invert" one of the channels somehow, and still use the original crossover network as it was designed to be.

I know this is a bit confusing and I don't have any pics at the moment because the crossover is in the base of the unit still. It is easy to get to, but the wiring is tucked up pretty good and hard to trace.
I can get some pretty impressive sound out of this thing, but by only using one channel from the amp. So it's really still only at half it potential.

Has anyone had any audio experience with any of this type of thing?

[richms]

Use a car amp then, one of the channels is inverted and the -ve terminal of that side is driven, with the +ve connected to ground. Sounds like the inbuilt crossover is driving the wooffers in tri-mode operation.

http://www.bcae1.com/trimode.htm

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

incidentally the 2 best explanations I could find were the top 2 when googling for trimode amplifier...

If you invert the signal into one channel of a proper home amp (with an op-amp inverter) then you can achieve the same result. You cant usually use a crap amp since they are already internally bridged to allow them to use a cheaper powersupply and a single chip amplifier device.

Car amp is sure to work since thats how they are designed to operate.

[Kevin Mullins]

You cant usually use a crap amp since they are already internally bridged to allow them to use a cheaper powersupply and a single chip amplifier device.

Looks like this may have been part of my problem all along, and the reason why no combination I tried would work.

I'm glad you brought up the car amp idea, I intentionally left that part out when asking my question. I was actually trying to use an older one I have had laying around for years now. After reading up on the links you gave me and understanding it a bit better, I decided to do what tinkerers do best..... open it up and take a look. Because after checking all the continuity combinations I could think of, nothing seem to equate to it having an inverted channel.

Sure enough, all I found inside was a few resistors, a few capacitors, and one lowly dual channel IC. KIA6220H
http://www.tranzistoare.ro/datasheets/270/249873_DS.pdf
If you look at the block diagram and such it's pretty obvious that this one won't work unless I modify it.

I'm still contimplating using a home stereo setup to have the amp/radio option with remote control, so I may still go that route since I will have to do some work to get either option to function with the original crossover network.

Great information you gave along with the links.
Really helped clear things up and make it a bit more understandable to work with.
"Learn something new every day."

[Ken Layton]

First suggestion is to buy the jukebox service manual so you'll have the amplifier schematics and the cabinet/crossover schematics. Rowe had a very well designed sound system and was the best of the 45 rpm record jukebox manufacturers.

Rowe inverted the audio signal polarity of one channel in the preamplifier section of the jukebox amplifier. One channel is out-of-phase with the other to improve stereo seperation/imaging. In a simple nutshell, many jukeboxes are installed on location with external speakers connected. To take advantage of the best stereo sound, when the speakers on the left wall push the speaker cones outward, the speakers on the right wall pull their speaker cones inward. This results in both beter bass response and stereo effect.

In essence the plus and minus speaker connections of one channel are reversed to the other channel.

[Kevin Mullins]

Yeah, I've learned that jukebox documentation is not readily available on the internet like most other stuff. But majority of the places I have found to purchase the service manuals want up in the neighborhodds of around $50.00 for a copy. I really only need a few tidbits from it.(there is very little original left in this machine) But I may still look at buying a copy, I'm a stickler for proper documentation, just have to cough up the dough.

I've gotten alot of great help from the Jukebox Madness forum, but came to a brick wall on this particular issue.

I'll probably look at playing with the idea of adding an inverter to the preamp section of whatever I decide to use as shown in one of the web sites richms linked to. Especially if I go with the "radio" option. Because it seems this will need modified anyways. I'd really like to have the authentic sound effect that this juke had before.

Thanks for the laymans explanation of the push/pull concept. Really makes sense and easy to visualize how it functions.

[Ken Layton]

Sometimes you can find an original jukebox manual on ebay for around $20.

Most jukebox manuals have been reproduced by the licensed Victory Glass Company (www.victoryglass.com).

Some jukebox manuals are available from www.jeffwager.com

[richms]

Hmm, with no voltage stepup in that amp, I wonder where the hell they are getting that 120 watt figure from

I am surprised that there was not a giant steel plate inside it to give the illusion of quality. I have seen that on many a cheap car amp.

Just remember if you do go sticking an inverter in one of the channels of your propper amp to swap the output jacks around polaritywise.

[Kevin Mullins]

Hmm, with no voltage stepup in that amp, I wonder where the hell they are getting that 120 watt figure from

I am surprised that there was not a giant steel plate inside it to give the illusion of quality. I have seen that on many a cheap car amp.

Yes, it was indeed a more cheaply built piece. There were resistors all sticking up and cockeyed components everywhere. (well, what few components there really was in it)

From looking at the datasheet on that amp IC....
Left - 30 watts
Right - 30 watts
Bridged - 60 watts
TOTAL - 120 watts !!

Hell, I dunno. I think it was just an impressive figure to slap on the thing in its day.
(It's several years old)

[richms]

I asked my mate who does car audio where they pull those figures from. Its 14.4 volts, with a 3.2 ohm load since they allow for a 20% deviation on speaker impedances and still are able to call them 4 ohm speakers.

Gotta laugh at that.