lm3886 no mute

[rtill]

hello,
curious if I don't want the mute option with the lm3886, do I just connect pins 4 and 8 together to the V- rail?

 

[Redshift187]

I think you should use a current limiting resistor. It's common to use a resistor and capacitor to slow down the turn on, which will (usually) get rid of turn on thumps.

 

[rtill]

thanks, works now! I do have a question in regards to using it as a guitar power amp coupled to a tube preamp. If I turn the volume past 2, it starts to break up/cut in and out. I am assuming my load is too big. What can I do? couple it with a 470k resitor?

 

[pacificblue]

What can I do? couple it with a 470k resitor?

Couple what with a 470k resistor?

Is the heatsink big enough? What is the supply voltage? What is the load impedance? What value is the mute resistor? Is volume setting 2 already pretty loud?

Schematic? Photos?

 

[AndrewT]

why are two threads discussing the same query?

 

[jaycee]

Use a 22K resistor between MUTE and -V. Having none will burn out the chip.

 

[rtill]

found my mistake, using a 30-0-30 tranny. puts it at the edge of it's limits. Will downgrade to a 20-0-20 tranny.
One more question....
I don't want stereo - can I connect the inputs and outputs of two chimpamps and get double the watts? Stupid question, but I'm here to learn. And do they both need seperate power supplies or can they use just 1 or will there be sag unless the tranny can supply enough amps?

 

[Redshift187]

Yes you can, you can parallel, bridge or both (with 4 or more chips). Why don't you want stereo?

 

[rtill]

why not stereo? - cuz I am running a mono 4x12 4 ohm cab.
so I can use the same power supply?

 

[AndrewT]

if you need the power of

a mono 4x12 4 ohm cab.

then you need the power and heat dissipation ability of a discrete output stage.
An alternative would be to power EACH driver from it's own dedicated chipamp. This too will need a big sink/s if you plan sustained SPLs.

 

[rtill]

how big of a heatsink will I need for 2 of them? I currently am using a 6inch by 2 1/2 inch heatsink from an old pentium II processor.

 

[AndrewT]

Hi,
what's the sensitivity and recommended power input for each of the 12inch (4ohm?) drivers?

As a first guess for PA duty, you'll need a lot more than two of the CPU coolers to keep 4, 6, or 8 chipamps cool.

A pair of parallel chipamps set up for 130W maximum output, driving a single 12inch will probably need at least one of those cpu coolers in passive mode. You might get away with two sets of parallel chipamps to one cooler if the sink has a big effective fan on it.

 

[rtill]

each speaker is 25 watts. Don't know the sensitivity.

I hooked them up parallel and they motorboat...sounds like a heavy tremolo. What is that about?
power supply is pretty standard 4700uf on each rail, running a 24-0-24 3 amp transformer.

 

[Redshift187]

Are you saying you just hooked up two or more 4 ohm speakers in parallel to an LM3886?

 

[rtill]

I built 2 chip amps and hooked them up parallel like this... http://www.shine7.com/audio/pa100.htm
and it motorboats.

Seperately, they sound fine, but in parallel they motorboat.

 

[AndrewT]

do you realise that you can series connect a pair of 4ohm drivers to give an 8ohm load and then drive that from a 60W into 8ohm chipamp?
If you build up two of these systems then you get the full 120W into your speakers.
No need to parallel any of your chipamps.

A simpler alternative;
Power each 4ohm driver with it's own 40W into 4ohm chipamp, giving a total input capability of 160W.

 

[pacificblue]

Did you select the feedback resistors with less than 0,1 % tolerance as specified for the PA100?
Did you separate input (clean) ground from output and power (dirty) ground, e. g. with a small resistor 1-10 Ohm?

 

[rtill]

Did you select the feedback resistors with less than 0,1 % tolerance as specified for the PA100?
Did you separate input (clean) ground from output and power (dirty) ground, e. g. with a small resistor 1-10 Ohm?

the .5 ohms, yes

each circuit has a 10 ohm on each of their outputs, could this be the issue?

 

[akis]

I used a 2.4C/W heatsink per chip, as seen here

http://uk.farnell.com/jsp/search/productdetail.jsp?SKU=4621505

no mica and no glue, drilled and bolted with thermal silicone grease
and it is inadequate, the Lm3886 really cooks it.


I am adding a 12 cm 12V fan as a last ditch effort.

 

[Redshift187]

Do you have those resistors mixed up? It should be 1-10 ohm for input ground isolation and 0.5 ohm or less on the output. You should not have 10 ohm resistors on the output.