| SBob |
Hi, I have just finished building a 3886-based Gainclone. However, I suspect that it is oscillating since it is getting warm, but not hot, even when there is no music playing. My old Gainclone, with a 3875, has always been very cool when it is not playing.
So, my question is: is it normal that a 3886 GC is getting a bit warm? Or should I start searching for oscillation? Would be nice to know to avoid going on a wild goose chase...
Some data: Voltage is +/- 27 volts. No zobel or LR (yet). Point2point. Star grounding with separate signal and power stars (the same set-up as my previous GC)
/Robert |
|
|
| SpittinLLama |
It should be similar in temperature to the LM3875 under the same conditions of voltage, heat sink, and input signal. If you suspect there is some oscillation then add the snubber, R+C, and that should cure it.
-SL |
|
|
| SBob |
The conditions are almost identical, the only thing differing is the chip, the 10k mute resistor and the PS has two 2200uF caps/chip instead of one 10000uF/rail. But the wiring is of course a bit different, which could explain that it picks up something to make it oscillate.
I tried looking at the output with the oscilloscope, but could not see anything apart from a little noise, ~1 mV. I also tried building a simple RF detector with a diode, condenser and a resistor, but the voltage over the resistor was zero when applied to the output.
I will try adding the R+C network (there are already 100nF caps over the PS caps) and see if it helps, but it would have been fun being able to actually see the oscillations.
/Robert |
|
|
| jaycee |
Add decoupling close to the IC as well, never hurts...
My own LM3886 amp (with a PCB) is stone cold when idle, and it only has the output inductor+resistor, no zobel... but I have 100nF on each rail close to the IC, and 330uF on the board (with 2x4700uF per rail off board). |
|
|
| AndrewT |
Hi,
3875 datasheet shows 30 to 37mA quiescent current.
The 3886 datasheet specifies Iq<85mA
That difference alone can account for the temperature difference. |
|
|
| SBob |
I have now tried adding an R+C network (4R+100nF), but the chip is still getting warm. I measured the quiescent current to 52 mA, which apparently is within normal range.
I'm wondering, if it were oscillating, would it not draw more current? Is there any way of actually "seeing" the oscillations? |
|
|
| Nordic |
Yep, I wouldn't say my lm3886s run cool, and they are on 10 x 10 x 4 cm sinks each.... Are both equaly hot...? If not, look at the bad channel's speaker with an ohmmeter...
Likely your scope isn't good enough to see HF either... so you cant rule out oscillation....
I think if it doesn't go into thermal protection during normal use, you are OK. |
|
|
| tamasic1 |
| Each of my 3886s are on an AMD heatsink. Warm if under half volume, hotter than the bad place if above. I re-attached the original PC fans as I use this amp in my garage and CRANK-IT-UP! I run it at near top volume as all the way will hurts your ears in just a few seconds - but there is no distortion! |
|
|
| Conrad Hoffman |
| A handy way to find HF oscillations is to use an ordinary transistor radio tuned between AM stations. Bring the antenna near the DUT and see if you hear anything. Don't short anything out! Maybe cover the antenna with some shrink tubing or similar. This can work to find RF coming from power supplies as well. It's not definitive, but another tool to add to the arsenal. IMO, you probably don't have oscillations, or you would have seen it on the scope. It's probably just the quiescent current of the device heating things up. |
|
|
| jaycee |
| Ah, the smaller heatsink might explain why mine is stone cold. Mine is 160mm wide, 25mm deep and 75mm high :) that's for both channels though. |
|
|
| SpittinLLama |
I think Andrew may be right depending on the heat sink size and the supply voltages. If you can't see any oscillation on a scope then there is probably not enough to worry about.
-SL |
|
|
| SBob |
OK, I am feeling re-assured that there is no oscillation going on. It is only using 50mA (which seems about right from the data sheet) and I cannot see anything on the scope. I will try the AM radio thing though, but since other peoples 3886s also gets warm I think mine is normal.
I just found it odd that it gets warm both with and without any load, but the heat sink on my test rig was a lot smaller than jaycees. (In fact, it was a flat piece of aluminium 25X12 cm for both channels. I guess the fact that my 3875 never even gets warm made me over confident...)
I was looking at the data sheet and +-27V into 4 ohms will produce about 40W, which will need a heat sink with 1.9 C/W. So I will have to get a big heat sink, which will hopefully keep it cool.
Thanks everyone for your help!
/Robert |
|
|
| AndrewT |
| quote: | Originally posted by SBob
It is only using 50mA (which seems about right from the data sheet) ....I just found it odd that it gets warm both with and without any load, but the heat sink on my test rig was a lot smaller than jaycees. (In fact, it was a flat piece of aluminium 25X12 cm for both channels. I guess the fact that my 3875 never even gets warm made me over confident...)
I was looking at the data sheet and +-27V into 4 ohms will produce about 40W, which will need a heat sink with 1.9 C/W. So I will have to get a big heat sink, which will hopefully keep it cool.
|
sounds like you built it before comprehending the datasheet design procedure!
Might as well jump in with both feet. The worst is I'll only sink. |
|
|
| jaycee |
you might want to check for good thermal coupling with the heatsink. In doubt, sand the heatsink with some fine sandpaper to make sure it's flat. If you are using isolated heatsinks (a good idea!) Mica will do but make sure it's quite thin, and you have to use paste thinly.
Personally, I used the grey "sil pad" things with mine (the TO3P ones fit just fine) and i dont have a heat transfer issue. Saves the mess of paste, too. |
|
|
| EAH |
Please:
Can you tell me what is :
grey "sil pad" things
EDUARDO AH. |
|
|
| EAH |
TY
I will try
EDUARDO AH. |
|
|
| peranders |
| quote: | Originally posted by SBob
OK, I am feeling re-assured that there is no oscillation going on. It is only using 50mA (which seems about right from the data sheet) and I cannot see anything on the scope. I will try the AM radio thing though, but since other peoples 3886s also gets warm I think mine is normal.
In fact, it was a flat piece of aluminium 25X12 cm for both channels.
I was looking at the data sheet and +-27V into 4 ohms will produce about 40W, which will need a heat sink with 1.9 C/W. So I will have to get a big heat sink, which will hopefully keep it cool.
|
Your cooling isn't much. You've better try a serious heatsink. Check ELFA how 1.9 W/C heatsink look like. |
|
|
| SBob |
I finally got around to finishing the amp. As you can see in the image I built two heatsinks from scrap pieces of aluminium. They seem to be enough since they only get luke warm when playing at fairly high volume.
I used the approximation formula 50/sqrt(Area) = C/W to get an idea of how big they shold be.
It seems that the 3886 requires a lot more heatsinking than the 3875 in my previous GC.
How it sounds? Much better, I think, than the NAD 7240 it replaces. (I use it to drive the woofers in an active system. Another GC is driving the tweeters, which might explain why I prefer to match it with another GC) |
|
|
|
