LM3886/Vox SS Amp Thread(w/ Samples!)

[R.G.]

Every solid state power amp needs a Zobel network.
Every solid state power amp needs inductive catch diodes.
Every solid state power amp needs a damped series inductor.
Every solid state power amp needs power supply decoupling right at the circuit.

These are things, just like the wrapper on an effect, that are not particularly interesting or exciting - until it doesn't work.

stm's rignt - ten ohms and 0.1uF is the standard Zobel, and it seems to work for almost all speakers. And that wire arrangement is important.

However, he's incorrect about loading. Solid state amps and tube amps have opposite preferences about loading. Solid state amps work fine into arbitrarily high load resistances, including no load resistance connected at all. They tend to have trouble with too-low load resistances, including dying. Tube amps get unstable and oscillate, sometimes to death, when they are unloaded, but cope with overloads (too-low a load resistance, right down to dead shorts) just fine. They're different. However the conventional wisdom about musical instrument amps is not to unload them. It's a legacy of the tubes-only era.

[RDV]

I'm running a 4ohm load, but I have been all along.

I'm going to install a Zobel tonight, as I have a 2watt 10ohm resistor handy, it can't hurt, and hopefully will help.

What I don't get is that it had been idling cool and not getting really HOT unless fully cranked.

It happened last night as I was playing at low volume. I turned it off and felt the chip and it was very hot.

I know there's the negative supply running through the heatsink and all, but it's not touching anything.

I know I'll have to make sure the big heatsink I'm getting doesn't make contact with chassis ground.

RDV

[R.G.]

I know there's the negative supply running through the heatsink and all, but it's not touching anything.
I know I'll have to make sure the big heatsink I'm getting doesn't make contact with chassis ground.

Do yourself a major favor - isolate the chip from the heatsink with a thermal insulator pad and thermal grease. Leaving heatsinks electrically hot is like leaving a rattlesnake inside your cabinet for the next time you open it up. You can get heatsink insulators at Mouser and Digikey. The Cho-therm and Sil-pad insulators are silicon rubber loaded with thermally conductive gook and work almost as well as mica and grease. You can also get the "TF" or "T" options on the LM3886 (I forget which it is) with the heatsink tab covered in epoxy so that an external insulator is not needed, only thermal grease.

[Steben]

By the way that limiter with tubes: why use a limiter if you have tubes?...(for guitar purposes of course)

[RDV]

I see why they offer an insulated version(LM3886TF) because a standard TO-220 sized insulator is gonna be way too small.

Guh.

RDV

[Caferacernoc]

Hi, don't mean to hijack the thread. Just a quick question on these discussed limiters. I'm still at the 9 volt stompbox level but I'm interested in these soft diode limiters since my goal is always soft, amp like distortion. Anybody have a simple schematic of something similar scaled for a pedal? Thanks.

[RDV]

I registered for a sample roll of Sil-Pad 1500ST. 

RDV

[R.G.]

Anybody have a simple schematic of something similar scaled for a pedal?

That's a problem, and it is a problem of scaling, as you mention.

In a 9V stompbox, the signal levels are smaller than they are at power amp inputs by about a 3:1 to 10:1 ratio. Diodes are used for switching in the softening, gain changing resistors. But diode drops don't scale down, they're fixed by the physics of the semiconductor in the diode. So you can do a good job of soft clipping with a waveshaper approach like this on bigger signals, but it's much harder at stompbox levels. There are ways to do this, but they involve all the normal machinations of our attempts at stompboxes.

I have an affinity for very soft distortion too. One of the best simple ways is with a CMOS inverter distortion, like the "Hot Tubes" run at quite low gains.

[RDV]

I got the heatsink in. My god this thing is HUGE. I can make 4 out of this at the LEAST. The backplate is thick and the fins are at least 1.75". I'll take it to work tomorrow and work it over with a sawzall. 

RDV

[RDV]

Added Zobel, still getting hot at low volume. I think that the preamp/gainstage/limiter is zapping the little 300ma tranny that's running them cause the fan is not putting out much air at all. Heatsink next.

RDV

[R.G.]

HUGE is a virtue for heatsinks 

[RDV]

Now that I think about it, I believe Mouser made a mistake and sent me 100ma transformers instead of the 300s I ordered(about a year ago). I thought they were just mis-labeled cause they said 100FG on them, but no. I should've sent em' back when I got em'. That means my pedalboard supply is only 100ma too(though it's enough for the stuff on it).

Grrrr...

RDV

[RDV]

Boy, it's hard to cut an 8" x 6" x 2"(fins) piece of extruded aluminum w/out a bandsaw! 
I used a chopsaw instead and it worked, but made ugly cuts.

But I've got my heatsinks.

RDV

[RDV]

Some more thinking out loud; I wonder if I should've used ceramic PS bypass caps(.1µF) on the rails(next to the chip) instead of the films I used. Perhaps that is causing high-freq oscillations resulting in chip running hot...

I may replace them tonight as I just happen to have a bunch of little blue ceramics that were being sold as films on eBay.

RDV

[RDV]

Kids, I didn't design this with a 5 lb. heatsink in mind. I had to move the output, and the mute stuff is on yet another daughterboard, but it's on and it's not smoking. I'm gonna let it idle a while and see how warm it gets. I know it's quieter without that fan anyways. I cut a 3" X 4" piece out for it, and as heavy as this stuff is should be more than enough.

RDV

[RDV]

I unhooked one of the 10"s raising the load to 8 ohms and it's running much cooler and quieter, and hasn't shut down since installing the bigger heatsink. Unless something else bad happens I'll let the thread rest till I start on the next amp. I'm going to buy PCBs(either from Chipamps.com or GGG) for this one and a power supply PCB from Chipamp. I've got a couple of the insulated versions(LM3886TF) coming and will be using those.

Late

RDV

[R.G.]

I think it may simply have been going into thermal shutdown. 4 ohms loads are hard on many chip amps.

Here's a thought - what happens if you feel rich and decide to put ...two... LM3886's in there, one for each speaker? This would cost you another $5.00.

If your power supply can take it, you now have a double-powered amp and neither one gets too hot. The switch to half-power is as simple as pulling the mute on one amp.

I have this recurring fantasy about building modular amps; modules of one enclosure/one speaker/one power supply driven by whatever drives the amp. Repairs are easy - just grab another one - and you can use as much or as little power as you want.

[RDV]

I think it may simply have been going into thermal shutdown. 4 ohms loads are hard on many chip amps.

Here's a thought - what happens if you feel rich and decide to put ...two... LM3886's in there, one for each speaker? This would cost you another $5.00.

If your power supply can take it, you now have a double-powered amp and neither one gets too hot. The switch to half-power is as simple as pulling the mute on one amp.

I have this recurring fantasy about building modular amps; modules of one enclosure/one speaker/one power supply driven by whatever drives the amp. Repairs are easy - just grab another one - and you can use as much or as little power as you want.

That's sort of what I was planning for the next one as I've ordered a couple more chips. I may splurge on that 160va toroid to do that. Sort of a dual mono/stereo thing. I left half of the heatsink I bought intact for such a purpose.

Kinda fun in'nit?

RDV

[RDV]

I have this recurring fantasy about building modular amps; modules of one enclosure/one speaker/one power supply driven by whatever drives the amp. Repairs are easy - just grab another one - and you can use as much or as little power as you want.

I wonder if any commercial versions of this exist?

Or at least, is anyone using LM3886s in a currently produced guitar amp?

Hmm...

RDV

[Johan]

Or at least, is anyone using LM3886s in a currently produced guitar amp?

Hmm...

RDV

..Line-6 uses them extencivly....
johan