Behringer LM 3886t power problems

[wblakesx]

The main reason I registered was I found my Behringer LK1200H is widely seen as putting out about 30 watts when it's supposed to be 1220. The consensus at the B forum is that B used a different standard for watts. I opened mine and found a nice sized transformer 26.7vX2 @ 3.6 Amp 192 W and a pair of LM3886T s.

I suspect the preamp. There's lots of spare space in the chassis so I'm thinking some cooking might be in order. I'm sure the solution to this problem would be very welcome to a lot of disappointed people, how to get Behr up to speed is another issue that might be worth pondering.

[teemuk]

Couldn't find any reference to LK1200H. I assume you mean LX1200H? That's a 2 x 60 W to 2 x 8 ohms or 120 W to 1 x 4 ohms according to Behringer. (They don't quote THD or whether that figure is RMS, PMPO or whatever) I don't know about Behringer's different "standards" for such a precice unit as watt but 2 x 60 W to 2 x 8 ohms or 120W to 1 x 4 ohms (even in RMS) should be within the limits of two LM3886 chips. If the amp does not meet it's quoted output power then the problem is likely an underrated transformer. 192 VA does not sound much considering 120 W RMS output. If the supply capacitance is insufficient it is possible that the amp can't hold up a constant RMS power of 120 watts.

Anyway, your post is, to put it mildly, a bit ambiguos. You should know that numbers in model names usually have nothing to do with output power and I assume that 1220 must be a plain typo. If the output power is indeed only 30 watts then I assume that either Behringer made a design mistake, your amp is broken or you are simply using an incorrect speaker load. Your post leaves unclear which one is it...

I can't understand what you mean when you mention preamp, empty chassis space and cooking? Could you be more specific?

[wblakesx]

Thanx for the quick rely

Sorry, typo, LX12000H it is.

These chips are in a lot of guitar amplifiers.

My conservative rule of thumb: output power= i/2 mains power drawn; at any rate I would guess this amp puts out less than 15 w clean into my 25 w Jensen, surely 192 VA @ 26.7v should give it more.

(There are many types of watt ratings)

Supply capacitence? wouldn't that leave bass power?

The B forum has many convinced it is a 30 w head."The consensus at the B forum is that B used a different standard for watts. "


"You should know that numbers in model names usually have nothing to do with output power and I assume that 1220 must be a plain typo."
(true, it's 1200:?)


"If the output power is indeed only 30 watts then I assume that either Behringer made a design mistake, your amp is broken or you are simply using an incorrect speaker load. Your post leaves unclear which one is it..."
("I suspect the preamp.""The consensus at the B forum is that B used a different standard for watts. " I'm sure the solution to this problem would be very welcome to a lot of disappointed people, how to get Behr up to speed is another issue that might be worth pondering.")


"I can't understand what you mean when you mention preamp"("I suspect the preamp." the thingy before the amp:?)

"empty chassis space and cooking? Could you be more specific?" , (Lot's of room to play in Smuggle diamonds out of Africa with:).

[wblakesx]

I'm definitely going to have to learn  to touch type so's to warch me spelling

[wblakesx]

note to Behringer forum:

I bought this thing because it was reviewed so highly ( lots of shills?) , it was time for a modeling amp, and I could use a 120 watt ss with chorus. Now days it's not a big deal to pump out clean power, get a chip and a transformer and feed it a signal.
Since the reaction to my orig post seemed tangential I've been trying to get some info (no luck yet) at some other forums. The thing is the power trans is up to at least 100 watts and the chips capable of at least 50w at the voltage the ptrans puts out. The chips are LM3886t s, look them up, used in voxs, peaveys, all sorts of gear, read the specs. These  are great chips, audiophile stuff, 105 db noise figs at max. Last night I got an unexpected compliment on how great the amp sounded clean... it does, and nice reverb, +
echo, delay, chorus... and the power section that is there Should make over 100wats, only it looks like there's a design prob in the preamp or? (possibly voltage regs?)
There might be a very simple fix, if only I could look at a Behr circuit diagram or could get a real electronics nut to look at it.

They're largely too pissed at B  to be analytical and B seems to be bit too big fer itself.

They're selling post cards of the hanging...

[R.G.]

Watts are easy to measure. You put a resistive load to replace the speakers. Then you feed it a sine wave and watch the sine waves on the output as you turn it up until they just barely start to distort on peaks. When that happens, you note the voltage on peaks and you're done except for a little light calculator workout.

The power of a sine wave into a resistor is fixed mathematically. The RMS value of a sine wave with a peak of Vpk volts is Vpk*0.707. The power which a sine wave voltage of Vrms produces in a resistor R is
P = Vrms*Vrms/R.

60W into 8 ohms is Vrms = SQRT(60W* = 21.9Vrms, which is the same as 31V peak for a sine wave.

So go run a sine wave into it, and watch the output sine wave on a scope. If you can get to 31V peak without clipping into a load of 8 ohms, it's putting out 60W.

The same goes for 4 ohm loads, only the numbers vary.

[wblakesx]

Thanks RG, but the ears are enough. I'll file away the advice tho. I do basic calcs ok, the old e=i2r, e=ir, p=ei, etc. I just had a silverfaced 70's vibrochamp dropped of to fix. I may by a set of high poer out put transformers to form the basis for easy tube troubleshooting.
BTW, the Berhinger quietly went unlit while I was paying at 5 watts so back to ZZounds most likely. Some good crunch sounds, nice reverb some stupid things wrong.

[R.G.]

the ears are enough.

Your ears may be better than mine. I can't get mine to turn out numbers for comparison.

I'll file away the advice tho. I do basic calcs ok, the old e=i2r, e=ir, p=ei, etc.

P = E*I and I = V/R, so

P = (E*E)/R

But for AC, P, I, and E have to be RMS, and we most easily measure peak. For a sine wave, you only need one more equation, Vrms = Vpk*0.7071, or its inverse Vpk = Vrms*1.414.

Those last additions let you run back and forth, figuring sine wave power. So if you can memorize only one more (say, Vpk = Vrms*1.414) you get a whole new level of usefulness. That's also how it works with AC power from the wall, so you can calculate power supply voltages from RMS too.

Stretch! Think of it as mental Yoga.