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The Severe Beating of a Peavy Backstage II '04

Started by DJPhil, August 16, 2010, 06:47:53 AM

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DJPhil

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!

:duh

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.

J M Fahey

Hi DJPhil
1) Thanks for pointing to the spammer. (Sigh!)
2) You are not crazy, you are absolutely right.
That amp IS a piece of cr*p. :trouble
It is not a Peavey design. No way !!! :trouble
It's a "design" clearly made by a beginner, who has only received the 4th mail lesson out of a total of 50.
It also stinks of Korea or Taiwan (not Japan).
A 4558 with 50 volts !!! What were they thinking ??? :duh
A 4558 driving power emitter followers !!! (They do not swing rail to rail (wasted power) no can supply more than 5mA (wasted power) What were they thinking ??? :duh
Unbiased output transistors !!! (TONS of crossover distortion)  What were they thinking ??? :duh
It looks strikingly similar to products from a company called something like "Lee Duck Jeans" which took over Dean Markley.
I think they believe using the word Jeans in their name makes them sound very American !!  :lmao:
The preamp, yes, uses the Transtube ideas patented by Peavey.
My guess is the following:
Some friends which regularly visit Frankfurt Expo, Atlanta NAAMM , etc, told me that in those huge Expos you find impressive stands by the greats (Peavey, Fender, Marshall, etc,) but also booths from OEM manufacturers, generally Asian, which have the plants, the machinery and the (slave) labor to produce cheaply whatever you want. You just supply the blueprints, and in many cases, you only specify the product in a general way and leave the details to them.
My guess is that for some unfathomable reason, Peavey was heavily backordered with Backstages, and this mysterious (to us) Asian OEM manufacturer approached them and offered their services, or were offering everywhere a Ton of unsold "Jean Markleys" at very low price, and Peavey accepted, specifying new preamp boards with Transtube circuitry.
Front panel silkscreening, Tolex, knobs, etc. are elements that can be changed at will.
Please post some interior pictures, boards will tell us a lot.
I also suspect Taiwan speakers and not Eminences. Frames tell a story too.
I suggest you drop that power amp and install an LM3886 board , powered by the existing power supply.
You will have around 35W available  ;)
3) What are those Leprechaun jokes anyway ?  :trouble :trouble
My Father's family surnames were Fahey Ronnan Clavin Clarke, from Galway , Eire.  :grr

DJPhil

Quote from: J M Fahey on August 16, 2010, 10:39:39 AMHi DJPhil
1) Thanks for pointing to the spammer. (Sigh!)
No worries. It's like whack-a-mole, I just report them as I see em. :)

Quote from: J M Fahey on August 16, 2010, 10:39:39 AM2) You are not crazy, you are absolutely right.
That amp IS a piece of cr*p.
Woohoo! I'm not nuts! :D

Quote from: J M Fahey on August 16, 2010, 10:39:39 AMPlease post some interior pictures, boards will tell us a lot.

Board Bottom 1
Scratches are from me cleaning out flux residue from resoldering after component checks.

Board Bottom 2
Sharpie marks are mine, the brass standoff is my addition as the board was unsupported except by the pots and jacks. The nasty looking solder gob near the standoff is the chassis ground.

Board Bottom 3
The tacky looking stuff is electrical tape residue. The isolated solder pads in the upper left are for the fuse clip and blade terminals. I was sad to see this arrangement used and I don't really consider it safe.

Board Top 1
The electro that looks like it's in the way of the switch is on long enough leads to bend out of the way safely. This is the input jack and preamp section.

Board Top 2
There's the poor RC4558P. The lighting makes the chip look blank but it's clearly marked with a TI logo (though who knows these days). The three pin sockets go to the power transistors, which are TO-220 Isolated types with small breakout boards. They're screwed into the chassis with no thermal paste.

Board Top 3
Power and output section. There's the fuse holder I hate so much. The black and white wires are speaker output and the three pin connector top center is for the transformer.

Enclosure Back 1
They were very careful to make sure you know that this is patented pretty much everywhere.

Enclosure Back 2
I wish they'd have used an IEC connector, but that's asking a lot of a budget amp.

Enclosure Front 1
For the sake of completeness.

Enclosure Top 1
This is what makes me crazy about the fuse, there's a D punchout right there in the back! They seem to have cheaped out on a proper fuse and covered it up with the giant label. I've rigged up a temporary fuse holder for testing, and I've got some proper fuse holders on order. I'll feel a whole lot better when I can't see metal that's at mains potential.
The grommeted hole is for the speaker wires, and the washer scarred hole just down from the ground is for the board ground. Any two of the four holes in the center can be used for the power transistors, they're all threaded. There's a bit of spare room in this chassis, if you can imagine the board only extending to the fuse holder all the way across.

Transformers
This shows the label on top of the transformer. For a size comparison I put the 36VCT 180VA transformer I picked up a while back next to this one.
I mistakenly said in my first post that it was rated for 36VCT instead of it's marked 32VCT. I hooked up everything again (no speaker connected) and read voltages just to be sure. With the transformer attached to the board and no substantial load other than the rectifier and filter caps I was reading slightly over 18VAC RMS on each side of the secondaries. I tested the wall outlet (I've got a good, safe meter for this) at 121.8VAC RMS. The voltage across the power pins of the RC4558 was 48.02VDC with a little ripple.
I'd expect such a small transformer to regulate poorly at low load but I'd think that they'd factor that into the design. The AC output would have to droop all the way to about 26VCT before it was nearly safe for the opamp.

Quote from: J M Fahey on August 16, 2010, 10:39:39 AMI also suspect Taiwan speakers and not Eminences. Frames tell a story too.
The rest of the box is with my friend at the moment so I can't include pictures. I built him a LM386 baby amp to keep him going while I was working on this. The case is a mix of plywood walls and particle board front/back strip. The speaker is an unmarked 6.5" that actually sounds pretty good on the pocket amp.

Quote from: J M Fahey on August 16, 2010, 10:39:39 AMI suggest you drop that power amp and install an LM3886 board , powered by the existing power supply.
You will have around 35W available  ;)
My plan was to slap some zener's in for the opamp just to get things under control, add a proper fuse holder, and begin work on an entirely new amp. I've got a few TDA2030s on order to play with and I might just wire one up to take care of the power section on this one.

The new amp will likely use that monstrous 180VCT transformer from the picture and a LM3886. It's really tempting to go with a discrete build, as I've already got a bunch of TIP31/32Cs handy. Either way he needs a real speaker and cab as well, so I figure we'll take it slow. If we do it right, we'll have a good speaker cab (lots of good woodworking tools handy, my father's hobby) and an amp with enough power. Then the fun begins for me, and I'll spend all winter tinkering with preamps.  8)

Quote from: J M Fahey on August 16, 2010, 10:39:39 AM
3) What are those Leprechaun jokes anyway ?  :trouble :trouble
My Father's family surnames were Fahey Ronnan Clavin Clarke, from Galway , Eire.  :grr
Beannachtai! My family's from Cork originally (about four generations back). I'm half Irish and half French Canadian myself, which can be a bit confusing at family gatherings. When we moved out here to the middle of the US, by some quirk of fate we wound up living on Leprechaun Lane. It's the first thing I reach for when looking for a mythical creature.  :tu:

phatt

Sadly it's not just the cheap knockoffs.

I have an old Yamaha *Professional Series* PowerAmp, 2 x 45Watts.

most of the build could pass for military grade.
The input has an equally Pro setup.
Unbalanced line in 6.5 TR and a good quality cannon Balanced input, plus a switchable low pass filter (for use with BiAmping tricks)

I've had it for years, no real issues until I had a headscratching issue with a less than ideal mixer plugged into the bal input.

Upon lifting the hood I found to my horror that the cannon had pin 1 and 3 tied ???
Effectively rendering the bal input a Joke. 
So much for *Pro Series* equipment.
I learnt a lot about fake balancing (dirty little tricks) from that little conundrum.
Phil.

bry melvin

QuoteI've had it for years, no real issues until I had a headscratching issue with a less than ideal mixer plugged into the bal input.

Upon lifting the hood I found to my horror that the cannon had pin 1 and 3 tied

you were lucky...I've usually found those cheats when I plug my board into someone's amp and turn on the phantom power for my mikes.....POOF.....the smell of hot plastic...

J M Fahey

Very interesting detective work
I was looking at the pictures in order of appeareance .
The first boards didn't look like the typical Peavey stuff at all.
Besides they do not use phenolic paper boards but some impregnated textile material (no, not green fiberglass epoxy but a light brown one, where you still see the threads clearly, I think it's Micarta)
Then the green caps scream old Japanese or newer Taiwan/Korea.
The plastic transistors too, with their shape, not "D" but trsapezoidal as seen from above
Together with those encased .1" connectors and the switches it looks like it was made in a VCR factory or the OEM Hartke one.
The (pre-phosphatised sheet chassis and the abundance of un-necessary punching (which means low production costs and very cheap dies) hints the same.
In all, a very un-American(British?) amp.
I think *all* those patents are meant to scare, not the end user, but to avoid the OEM manufacturer building "by mistake" a few thousand extra and selling them under their own label.
I read about the OEM supplier of one of Korg pedalboards, which was offering its clone (really made in the same factory, with the same parts) for 1/4 the official price.
Do not waste time by lowering the 4558 supply voltage, because you will lose output excursion as well, the output devices are followers.
Replace it straight with the LM3886 and drive it from the transtube preamp.
What !!!!!!!!!!!!! Pins 1 and 3 are not joined ??????????????? :o :o