Probably a dumb question but I won't sleep if I don't ask....

[sonolink]

Hi everybody,

I apologize if this is a stupid question but I really would like to know why this is like that:

Guitar pedals PSU are usually center negative. BUT some designs are center positive. The reason doesn't really matter right now since it's just a fact...it can be because of many reasons, transistors or whatever...the thing I DO NOT manage to understand is: why the final connection between the PCB and the DC connector cannot be wired so that ALL pedals are i.e. center negative regardless of the inner design?

Can someone PLEASE shed some light on this and try to get me to understand this?
Please?

Thanks in advance!
Cheers
Sono

 

[JohnRoberts]

Because there are lots of small companies designing products and there is no law making them all the same.

JR

 

[hodad]

There's also relatively few devices besides guitar pedal that are center negative. How that came to be the guitar pedal standard (relatively speaking) is a mystery to me.

 

[volker]

There's also relatively few devices besides guitar pedal that are center negative. How that came to be the guitar pedal standard (relatively speaking) is a mystery to me.

That's because of the battery switching via the stereo input. It wouldn't work the other way around with the common switching jacks.

why the final connection between the PCB and the DC connector cannot be wired so that ALL pedals are i.e. center negative regardless of the inner design?

Can you show an example where it wouldn't work? I can't see no reason why the leads and polarity of the jack could not be exchanged as long as no battery switching is involved.

 

[Youngwhisk]

If you are powering more than one device with only one power supply, where those devices have opposite polarity there would be a problem. It shouldn't be done but someone surely will. Using something like a Pedal Power with multiple isolated supplies solves that problem.

 

[JohnRoberts]

Not only swapped polarity but they also come in different pin diameter sizes. I had to sort through this back in 80s while at Peavey specifying wall warts and the like.

I even developed a rack mount power transformer with 6x1A windings so you could neatly power a rack full of wall wart gear without the messy wall warts. This was neat, but too pricey for Peavey customers who were cheap like me.

JR

 

[sonolink]

That's because of the battery switching via the stereo input. It wouldn't work the other way around with the common switching jacks.


Can you show an example where it wouldn't work? I can't see no reason why the leads and polarity of the jack could not be exchanged as long as no battery switching is involved.

Well that's the reason of my original question
Why aren't they all just center negative to avoid accidents?

 

[JohnRoberts]

Why aren't they all just center negative to avoid accidents?

That would be too easy. When designing new products there is little motivation to be compatible with other manufacturers.

The wall-wart I designed in at Peavey was AC (so no negative or positive lead). There should be a lower cost from sourcing a common design but larger companies can start with a blank sheet and order whatever they want/need.

JR

 

[hodad]

The wall-wart I designed in at Peavey was AC (so no negative or positive lead).

Is that the one for the LM8 rackmount mixer? Are you also the one who put the power connector on the side of that rack unit, necessitating a right angle plug on a fairly odd value AC adapter?
I have exactly one wall wart that will work with that mixer--it didn't come with the original Peavey wall wart, unfortunately--and I have a pretty hefty collection of wall warts.

Nice mixer though.

 

[JohnRoberts]

If that is the line mixer I did back in the 80s/90s I have a story.

today's TMI:

At Peavey high level management signed off on the pricing of every SKU checking it for profitability before releasing it for sale. I heard through the grapevine that the president of Peavey (Melia Peavey, RIP) kicked that mixer pricing back to cost engineering because she didn't believe the mixer could be that cheap to assemble.

If you open it up you will find one PCB that breaks apart into three sub panels. One section of the PCB has all the front panel pots and switches. There is a middle PCB that carries the signals from the front panel board to rear panel input/output jacks board. The three PCBs are connected together with machine inserted wire jumpers.

The three sections run through the machine and hand insertion, then the wave solder line while flat. At final assembly the line worker simply breaks the three sections apart and folds them up to fit inside the chassis. Hows that for low assembly labor?

I suspect the power jack was on the side because we did not have tall enough power jack in the system. IIRC the "Hi-D" series of jacks used for audio I/O are 0.950" high (for use in flat mixers).

I love it when upper management can't believe how good one of my designs is, of course she never told me. I heard about it from friends in cost engineering. Not the first or only time she second guessed me, but that's life in corporate politics.

JR

PS: Peavey probably used hundreds of thousands of that one wall wart in tens of SKUs.

found one on EBAY for $25 but I'd ask Peavey customer service because we probably bought them for like $2 each by the boatload back in the day. ebay ww Peavey accessories charges more like $27 now that's inflation. peavey accessories

 

[Newmarket]

I think you can blame BOSS !

 

[Matador]

The three PCBs are connected together with machine inserted wire jumpers....At final assembly the line worker simply breaks the three sections apart and folds them up to fit inside the chassis. Hows that for low assembly labor?

That exact same assembly was used in the Peavey Classic 30 and 50 guitar amps (three PCB's with wire jumpers, folded into a U shape). A great idea for a mixer....

Unfortunately it doesn't seem that mechanical stress was considered on guitar amps, and the fact that two of the sides of the U are on opposite sides of the chassis (and rigidly bound by the pots on the top, and tube sockets on the bottom), with the center "floating" to absorb the mechanical differences across the chassis, means that I have had to repair fatigued/broken wire jumpers in at least 6 amps over the last year.

On the upside, it did seem to work for about 20 years.

 

[JohnRoberts]

Mechanical packaging design is all about optimizing for the tools or process you have. The machine inserted wire jumpers were crazy cheap probably less than a penny each. We made our own single sided PCB so those were cheap too. I don't know if the guitar amp guys copied me, or I was inspired by them.

My mixer design did optimize the cost saving benefit.

More expensive PCB might have resulted in smaller PCB and ribbon cables from front to back.

JR

 

[Tubetec]

I probably wouldnt take on any more Peavey classic series amp for repair at this stage , if its been in and out of its box a few times and the link wires are already fatigued you can easily end up fighting a loosing battle.

While the sound of a classic 30 isnt too bad when it does work I always found them a bit unreliable , heavy on tubes and the power transformer /chassis ran hot enough to cook eggs on. I did one time install a large mains driven fan from an old power amp on a friends classic 30 , it definately helped keep things at a proper temperature and prolonged tube life a bit at the expence of some extra acoustic noise, which didnt matter too much in a live gig situation .

 

[boji]

Why aren't they all just center negative to avoid accidents?

Consumer-grade network equipment, laptops, IT-related peripherals modify the brick/wallwart's center pin and sleeve diameters, but majority have pin-positive outputs.

 

[Newmarket]

Yes fx etc are the exception in electronics here.

 

[JohnRoberts]

I probably wouldnt take on any more Peavey classic series amp for repair at this stage , if its been in and out of its box a few times and the link wires are already fatigued you can easily end up fighting a loosing battle.

Indeed, machine inserted solid wire jumpers are not robust over multiple bend cycles (I'm sorry). Ribbon cable would be more flexible, but limited wrt spacing high impedance signals physically apart. If some subassembly needs to be repeatedly removed and reinstalled the solid wire jumpers are not the best choice. That was not my engineering group, while my mixer engineers shared a common lab space with the guitar amp design engineers.

While the sound of a classic 30 isnt too bad when it does work I always found them a bit unreliable , heavy on tubes and the power transformer /chassis ran hot enough to cook eggs on. I did one time install a large mains driven fan from an old power amp on a friends classic 30 , it definately helped keep things at a proper temperature and prolonged tube life a bit at the expence of some extra acoustic noise, which didnt matter too much in a live gig situation .

I'm not a guitar amp guy, but seem to recall the classic 50 (?) being pretty well regarded.

JR

 

[Tubetec]

I do know one guy who played a classic 50 for many years without trouble .
I looked back at the circuit of the classic 30 and all 4 output tube heaters run in series ie 24 volts AC , it does bring in a saving on the PT but as I said it runs hot and any variation in heater current might cause the neighbouring tubes to drop more or less voltage , Idont recall testing the actual AC voltage at the EL84 sockets in the classic 30 , as you can see its a little awkward to fault find unless you take the board out ,interestingly the Classic 50 runs the same 4 EL84's also fixed bias mode with all powertube heaters in parralel , and ribbon connections .

 

[JohnRoberts]

I can see a lot more hand assembly labor ($$) in the 50 than the 30... IIRC those cheap flat ribbon cables on the right hand side of the 50 can get fatigued and break from too many cycles, but they get inserted before the wave solder operation so don't require the more expensive headers.

JR