dc voltage at speaker

[cigarbox2]

I have a peavy studio pro 112 transtube red stripe model. when the tdynamics control is turned to 50% it has a weird and loud oscillating hum there is also dc voltage at speaker wires. the voltage varies between 5 and 8 volts on my dmm. i was wanting some suggestions on where to start troubleshooting i would like to fix the amp myself. i got the amp cheap and thought it would be a great way to learn.

[joecool85]

Sounds like the poweramp IC is shot.  Do you have a schematic for this amp?  If not, contact Peavey and see about getting one.

[cigarbox2]

I dont have the schematic i will contact peavy customer service. I will check the part you suggested and let you know what i find.

[cigarbox2]

peavey customer service is out for the holidays wont return until jan 2 will get schematic then and post.

[DrGonz78]

Maybe this StudioPro112 TT schematic will help??

[cigarbox2]

thanks for the schematic doc i will check tomorrow. mine is a usa made amp maybe its close enough to work.

[Roly]

Thanks @DrGronz78, but WTF am I looking at here?  A chassis symbol on the output half rail?  I mean, I can dig the current feedback, but the grounding scheme around the output has got me baffled for the moment.  :headscratch:   

Afterthought: Oh and @cigarbox2 - disconnect your loudspeaker if you value it.

[DrGonz78]

@Roly I think this is what you are referring to...? I have no idea here... I don't think I have seen something like that before! :lmao: >>>reference WTF.jpg

Edit: I think this is a strange version of this schematic and it even says "Trade Secret--Patent Pending"

Double Edit: I wonder if the output transistors are really bad yet. I think there could be something pushing a smaller amount of DC on the output here. Or JoeCool is right and one or both of the power amp IC's are shot! Not quite dead but if they are headed to dead then they will heat up good n hot. So turn on the amp (speaker disconnected as Roly said) for a minute and then see if one or both output power amp IC's are really hot.

[phatt]

Yep a curly one, had me scratching my head for quite a while.
I don't think Peavey invented the idea as I'm sure you will find this topography on those obscure hifi pages somewhere.

There is a name for this output setup,,, but brain fade???
The only word in my head is *Strickland* or something like that??
I'm sure some googling will find out more,,, or better still,, where is Teemu?
I'm fairly certain he makes mention of it in his book.
Phil.

[Roly]

Oh, you too eh?  That's a worry.  I thought it was just me.

It would make more sense to me if the output just went down to the 0.33r resistor in the current feedback gubbins below.   :loco

{By the same author...}



Had a look at the DIY Audio discussion and the Patents, and I don't think so Phil, that seems to be switch mode FET's, and this is neither.

The output stage proper starts at C69, and it's really conventional until we get to the chassis symbol where we would expect the output.

This implies that the way the signal is getting out is via the supply rails, wobbling the entire power supply up and down.  I find that a bit hard to swallow, yet that appears to be what they are doing.

R122 and C81 are a Zobel network, so that is the effective output; R125 is to develop ground reference current feedback (R123), and the voltage feedback is going via the compensated attenuator to the left of the Zobel (R119 etc).

Pardon me if they are welcome to their trade secret.  I certainly wouldn't clone this.

So what we have is just a normal fully-comp stage, but the output comes from the PSU mid-point, which is now effectively the output half rail.


I'd first check that all the supply voltages are correct (across each cap, not to ground!).

Then I'd look at pins 5 and 6 of U5B and see if they were equal, and if not if they were driving the output on pin 7 in the right direction for the imbalance.

If that was okay I move on to check the drivers and output pair.

HTH

[cigarbox2]

I'm  going through the amp and checking all of your suggestions and enjoying the comedy and artwork. Hopefully these will turn up tthe problem.

[Enzo]

Oh relax, no WTF at all, this is a VERY common arrangement, often called a "flying rail" amplifier.   Many Peavey solid state amps are built this way, most QSC power amps built this way, and others.

Let us do a thought experiment.  Imagine you built this amp on a block of wood, no chassis to ground to.  "Ground" would just be another wire.  Now you have the amp circuit, they all look about the same, a pair of rails and push pull transistors connected to a "output" bus.  Then you have power supply, a center tapped split rail supply.  And a speaker.  Normally, you would connect the speaker between the "output" bus and the power supply center tap, which you think of as ground. But in our thoughts, there is no ground, just a plank of wood.   Now step back, your speaker is normally grounded at the power transformer center tap, and hot to "output.  This flying rail approach grounds the speaker at the "output" end and the hot to the center tap of the transformer.  In both arrangements, the speaker is connected between the exact same two places, "output bus" and center tap.  Only difference is which end you connect to ground.

Look at the schematic, see the output bus is right in line with the center tap to its right?  Other than the fact the lines go down to the feedback stuff, is thatr not where the speaker goes?  Between those two points.  And if the center tap were grounded insted of the output, wouldn;t it look about the same?

The two rails will always be the same voltage apart, but they will vary with respect to ground as your signal.  That means your center tap will be moving with them.  That is why they call it flying rails.

Trade secret on the drawing does not imply anything odd about the circuit.  It just means they consider it their intellectual property.   I find it amusing that Fender puts a big do not show this schematic to anyone warning on their drawings, yet they throw a copy in with each amp they sell.

If your T-dynamics is setting up an oscillation, it is after all in the amp feedback loop.  I'd first check the health of those two 15v rails.  And look to see if the main rails are symmetrical and clean.

[Roly]

Oh yeah, I get it okay now.  I just don't see why.

Rod Elliot managed to include current feedback without flapping the power supply up and down;



http://sound.westhost.com/project27.htm

[Enzo]

Sure, but they can use current feedback in either configuration.   "Flapping the power supply up and down" sounds perjorative.  One could as easily say this method drives a speaker without flapping the output common bus around.  It's not like the amp works any harder.

[cigarbox2]

Sorry about the delay did not get home till late yesterday. I also want to say my goof for not understanding your terminology before i posted. I checked the voltage and it is 15 on both sides. The schematic is not close enough to mine i have the 2nd version transtube model. Also the weird loud hum i described should jbe called motorboating i learned. I also chkd for any loose or broken solder joints and could not find any.