LM3886 Amp Build Report

[RDV]

Yeah, it's me again.

I stayed up all night and built the one from GGG. It sounds good at first, then gets hot and starts distorting. I'm too tired to mess with it anymore and I'm gonna catch a couple hours sleep. I will say that I'm giving it a 4 ohm load rather than 8. Two 10"ers wired in parallel. I also just used a small heatsink(about 1" X 1 1/2" with little rib thingys), apparently it needs one considerably bigger.

Nitey Nite

RDV

[black mariah]

Well, hurry up and fix the problem. I'm looking into building one and need to know what to expect. So wake up and quit being a wuss. Sleep is overrated.

I'm off to bed. Goodnight.  :wink:

[RDV]

All right! Wake up and go to sleep!!

Har Har!

I had neglected to ground the PS at the CT and between the divider caps. It's a wonder I didn't burn up the chip(or me)! It works really well now driven by my new Mackie dumpster board.

RDV

[MartyMart]

" I was dreaming last night that I was awake, then woke up and found myself to be asleep"   !!!!!

Yeah, I know,  thats WIERD  !!

Marty. :wink:

[petemoore]

Yes that is wierd...ya know what they say' about when thappens...
 


                Go
                   Back
                          To
                             Bed
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!]
 THere WILL never be a substitute for sleep !!!

[smashinator]

RDV - What transformer did you use?  I'm looking to build one too...

[RDV]

RDV - What transformer did you use?  I'm looking to build one too...

According to R.G., this one isn't big enough to get all the power out of the chip, but I'd already ordered it:

Hammond 183K36
36CT at 1.56A/18V@3.12A

http://www.mouser.com/index.cfm?handler=displayproduct&lstdispproductid=416095

RDV

[RDV]

I've been testing ever since I got up and I really like it a lot. Very, very clean with a ton of headroom. It really lets you hear what your pedals sound like.

What must be said is that some sort of preamp w/EQ or @ least a buffer is needed because it sounds just like plugging into a straight poweramp(I guess cause that's what it is?); a bit dull. I'm leaving mine raw because I have a Mackie 1202VLZ mixer that I'm using it with. It really does shine though because whatever you put into it, you get out of it. I thing it would make an excellent hi-fi amp. I'm extremely enjoying my modified DoubleD into it right now!

RDV

[The Tone God]

It sounds good at first, then gets hot and starts distorting.

When something like what you describe above happens it could be the overheat protection circuitry kicking in to save the amp. Be sure to heatsink the 3886 properly. There are formulas in the spec sheet to calculate the heat dissapation needed which will help you in selecting a appropriate heatsink.

Andrew

[RDV]

It sounds good at first, then gets hot and starts distorting.

When something like what you describe above happens it could be the overheat protection circuitry kicking in to save the amp. Be sure to heatsink the 3886 properly. There are formulas in the spec sheet to calculate the heat dissapation needed which will help you in selecting a appropriate heatsink.

It was getting hot, but it never did shut down. I had neglected to ground the PS at the CT and between the divider caps. It's a wonder I didn't burn up the chip(or me)! It works really well now driven by my new Mackie dumpster board.

In fact I've left in on now for 5 hours and counting with no problem.

RDV

[Marcos - Munky]

How big the heatsink need to be? I'm building one of this too, but need to find a power supply and a speaker for it yet.

[RDV]

How big the heatsink need to be? I'm building one of this too, but need to find a power supply and a speaker for it yet.

For a heat sink, the bigger the better. If I ever want to try to use this amp live I'll have to get a bigger heat sink. I've just got the one from Radio Shack. Here's the one I used:


For a supply, unless you luck out and find a 32v+/- 4A bipolar supply ready made you'll have to build your own. I used a 36v 1.58A tranny which is really too small. It gives me about 25v +/-. You really need a 50vct 4A tranny if you want the full pull from this chip, that'll give you about 35v+/-.

RDV

[Dean Hazelwanter]

When I did that pcb layout (complete with star ground) I 'torture-tested' a poor LM3886 to see how robust it would be. With 51 volt supply (|V+| + |V-| = 51.1v), into a 4 ohm resistive load it put out a *continuous* 40 v P-P without clipping. Unless my math is bad, this is 400 watts peak to peak continuous!!! The bank of load resistors made a great space heater!  After 5 minutes or so it started to overheat because the heatsink wasn't big enough. It finally shut itself down. I waited until it cooled down and it seemed to be fine when I fired it up again, although that *can't* be good for it. This beats a lot of designs with transistor outputs - look at them wrong and you're replacing outputs (and maybe drivers etc).

As for how big of a heatsink to use, bigger is definitely better. It also depends on how hard you will be working it. You don't *have* to run it at the maximum voltage. You can use a much lower voltage, down to  (|V+| + |V-| > 12v)

BTW, another cool feature is the 'mute' switch input - you can give your solid state amp a 'standby' switch just like tube amps!    :wink:

[RDV]

I've put a salvaged CPU fan in front of the heatsink, now it doesn't even get warm.

RDV

[ragtime8922]

Did you build this to have a small amp on or very near your test rig for pedal building? I needed one too so I built a small amp with the LM1875T. I just modified the ap note a bit and to avoid doing the heat sink calculation I just used one that was way big. I slavaged it from either an old VCR or an old computer power supply (both in same box of salvage parts). For the speaker I used a little rear surround speaker cabinet. It's a little Sony, 8 Ohms.
      Anyway, it does it's job but it sounds like this LM3886 would be good one as well. Is that chip stereo capable? My next test rig amp is going to be stereo (or have a switch to toggle from stereo and mono) just to be able to test stereo modulation stuff.
      BTW, the LM1875 amp ain't too bad for a test rig application.

[ragtime8922]

I've been testing ever since I got up and I really like it a lot. Very, very clean with a ton of headroom. It really lets you hear what your pedals sound like.

What must be said is that some sort of preamp w/EQ or @ least a buffer is needed because it sounds just like plugging into a straight poweramp(I guess cause that's what it is?); a bit dull. I'm leaving mine raw because I have a Mackie 1202VLZ mixer that I'm using it with. It really does shine though because whatever you put into it, you get out of it. I thing it would make an excellent hi-fi amp. I'm extremely enjoying my modified DoubleD into it right now!

RDV

RDV, I once asked you if the ROG stuff was to be used as OD/Distortion pedals or as pre-amps and your answer was "both". In light of this do you think any of them would make a good front end to the 3886 amp? If so then I think I'll add an effects loop with adjustable input and output levels to test effects in both series and parallel.
      Hey, maybe we can get real crazy and put like 6 ROG amp sims on a rotary switch and have our own analog modeling amp....test rig. Then add another rotary for tone stacks including "none" as an option. See, I take it 9 or 10 steps to far. By the time I'd be done adding all the stuff that comes to mind I'd end up not needing my half stack anymore. How about throwing in a rotary for power amps? Tube Class A, Tube Class AB, Solid State. And if nobody tells Mesa we can add yet another toggle for rectifier type........How many steps past "way too far" did I go?

[RDV]

RDV, I once asked you if the ROG stuff was to be used as OD/Distortion pedals or as pre-amps and your answer was "both". In light of this do you think any of them would make a good front end to the 3886 amp? If so then I think I'll add an effects loop with adjustable input and output levels to test effects in both series and parallel.
      Hey, maybe we can get real crazy and put like 6 ROG amp sims on a rotary switch and have our own analog modeling amp....test rig. Then add another rotary for tone stacks including "none" as an option. See, I take it 9 or 10 steps to far. By the time I'd be done adding all the stuff that comes to mind I'd end up not needing my half stack anymore. How about throwing in a rotary for power amps? Tube Class A, Tube Class AB, Solid State. And if nobody tells Mesa we can add yet another toggle for rectifier type........How many steps past "way too far" did I go?

I'm glad you asked. I think that anything you use as a preamp for it should have a tonestack of some sort. That said though, I'm really digging the modified DoubleD through it cause it gives me essentially a 3 channel amp. The chain is DoubleD, Mackie 1202VLZ(for the EQ with a Digitech PDS2002 in the aux for some wetness), then the LM3886 amp. With the DoubleD bypassed I get a terrific warm clean Jazz tone ala Wes Montgomery or Johnny Smith(no, I'm not kidding!). With the DoubleD on the Jiggle channel I get a more cutting trebley sound with some warm tube-like breakup. With the Bounce channel I get a great dynamic(cause I lowered it's gain) lead sound.

I think my (Not So)Super simple preamp w/ 3band tone control(Marshall or Fender, you pick) would be a good choice for a front end.
http://img.photobucket.com/albums/v165/rickydon/NotSoSuperSimplePre-Amp.gif

RDV

[RDV]

I know that designing a SS amp is probably a pretty lame topic for tube purists, but hey, I'm having fun with it.

I built my little Super-Simple Preamp to use with the LM3886 amp. As drawn, it didn't have enough gain, so I made the first stage a gain stage rather than just a buffer, and that helped a lot. The link to it above will go to the new schem.

I also built a 10v bipolar power supply to run the preamp with a 12.6vct 300ma tranny. I didn't regulate it, I just filtered the crap out of it. It doesn't seem to me that you need regulation with a bipolar supply, as long as you don't go over the voltage rating for the opamp. It's humming a bit, but I think I need to add another cap or two on each side of the supply to ground. It's not a big hum, but a bit more than I'd like. Of course it could be cause I've just got it all laid out and not in a case.

It sounds really nice to my ears, but I'm still refining the whole thing. When(if) I'm finally satisfied with the whole thing, I'll post a comprehensive schematic.

RDV

[RDV]

Ugh, why does my preamp hum more as I turn it down?

RDV

[Johan]

Ugh, why does my preamp hum more as I turn it down?

RDV

...groundloops...probably something being doublegrounded somwhere.ecpecialy since you use two transformers. perhaps it would be better if you used the higher voltage transformer for the whole amp, lowering the higher voltage with regulators before sending it to the pramp section..

Johan