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Topics - DJPhil

Sure, there are dozens of places to start learning electronics. Everyone has their favorites.

There's a series of interactive tutorials hosted by the University of Texas at Austin that really surprised me.

I stumbled on this one while looking for something that would appeal to a friend of mine who's very visual and hands on. It seems like a good intro just to get the terms and ideas rolling around in the mind. I think it'd be excellent for those without a strong math or science background, or perhaps those who work better through analogy than abstraction.

Just thought I'd mention it in case anyone found it useful. :)
I think I've finally figured out the problem with my friend's amp after five months of on again, off again head scratching.  xP

I'm going to try to resist the urge to turn this into a fifty page writeup on how slow I've been to see the obvious. I'm posting this to get a sanity check on my conclusions and also to leave a record of this issue somewhere for others on the internet to find if they have the same problem.

The original problem was excessive hum. I had no trouble getting the schematic from Peavey, they were quick to help. I did have to specify that I was after the Backstage II schematic, also known as the Backstage '04. I've included the schematic in attachment for review. The difference between the two is striking. The Backstage II is in my opinion a cheap knockoff, cheaply and possibly dangerously constructed. More on that by request. The important detail is that the Backstage II is rated at 10 watts and has an 8Ω speaker. Though I can't find any info on whether that's peak, rms, or leprechaun watts I'm pretty sure it's RMS.

Over the course of testing the amp I verified almost every component, many out of board, that was not specifically contained in the overdrive circuitry. All of the obvious things were slowly ruled out. The first problem that really tripped me up was finding ~50VDC across the poor RC4558 opamp's power pins. That's way over the max rating of 36VDC, and the chip would heat up to about 70C at idle. I'd guessed that such an overvoltage would very likely cause poor supply rejection, and that I was lucky that hum was the only problem!

Going back to the schematics showed that there was nothing in the design stopping the Vcc and Vee rails from delivering their full voltage to the RC4558. The part number on the schematic matched the part number printed on the (clearly labeled, yay!) power transformer, which is rated at 36VCT. This The opamp is essentially configured as a summing and voltage gain stage for the output transistors.

My first thought was that they left out (among other things) some way to drop the rail voltage down for the opamp. I cursed them for being cheap enough to skip a pair of zeners and a pair of resistors that would have made this amp less of a complete lemon, especially after seeing odd value E24 1% resistors all over the board. As my understanding of amplifiers grew I realized that the output transistors aren't providing any voltage gain, so it occurred to me that there was no reason whatsoever to use the transformer they did. They could have used a 24VCT transformer from the start and wound up with the same output power without baking the opamp!


My conclusion now is that either by design error or supplier miscommunication they wound up with the wrong transformers for the job. However it happened, they let at least some of the amps out the door like this, and the results are strewn all over the review sites. Many complain of loud hum, noise, and early failure. One poor fellow was told he blew out his guitar cable. People can be jerks sometimes.

So that's the short version. I've got a half finished spice circuit, pictures galore, datasheets, and measurements if anyone needs them. If anyone has questions ask away.

My question to the more experienced is: are my conclusions sound? I'd really love to stamp 'Solved' on this mystery.
Amplifier Discussion / Salvage transformer
July 06, 2010, 04:52:16 AM
I've been reading and reading, so much to know!

I had a quick question about this transformer I picked up on the cheap. I think it may be a candidate for a medium sized amp but I'm getting dizzy swimming through all the datasheets. I'm on the brink of being sure of this, but I figured I'd better check my math.

This amp will be for a friend who's likely never to gig with it, but will occasionally be practicing with a band. He's been getting drowned out with his current 15W practice amp so we thought we'd build something a bit better.

The transformer is an EI with bell shielding and was listed as 36VCT 3A with a shipping weight of about 3.5Kg. Once it arrived I tested it at 21.7VAC RMS on each secondary with no load. I'm guessing that it'll settle down nicely to about +/- 25VDC or so once I get the power supply built. As a bonus, it's got a 5.73VAC RMS winding on it tempting me to add some freaky digital circuitry for no good reason. :)

The voltage seems just about right for a LM1875, and I think there may be (more than) enough current to do a two channel build. I like the idea of using the 1875 as opposed to most of the other chips in this range I've found on Mouser. They seem to try to do everything for you to cut component count and the 1875 seems to keep things fairly simple.

Does this seem like a good way to go?