Horrible Microphonics/Hum Issue with Gainclone Amps

[Tlarwa]

I've bern running my new (to me, I bought it built) Smash with my 1.5W PSE amp and amp Super Pensil speakers with great results.  No noise at all, and it sounds great.  I just bought a pair of DALAudio gainclone mono block amps (http://www.dalaudio.com/Studio-3875-Mono-Block-Chip-Amps-p/dals3875mob-r2.htm) to use with my Klipsch Forte II's and have had completely different results.  As soon as I power the amps on I instantly get a loud hum and what I assume to be microphonics (I'm new to this, so be gentle with me).  But since it an echoey noise like you'd hear through a microphone I assume that's what it is.  Anyway, as a test I ran the DAL amps into my passive pre, and everything sounded great.  So it's clearly an issue of the Smash and the DAL amps not playing well together.  My first thought is that it's a gain issue, is that true?  The DAL amps do not have adjustable gain.  If it's something else, I'm all ears!  Again, I'm new to this, so I appreciate any and all help and suggestions.  I am capable of soldering and understand basic circuitry, so if theres a fix through adding or subtracting something in the circuit, I can give it a whirl.

Thanks!

Tom

 

[Doc B]

Yeah you probably need to unclone some of that gain. The gain is usually ridiculous, like 30dB. Try inline attenuators, maybe -18dB.

 

[Tlarwa]

I put my single input/single output passive preamp (attenuator) in-line between the amps and the Smash.  That allowed me to dial down the gain.  Now I just need to decide how much gain I want to supply the preamp.  I can't decide if it sounds better with the gain dialed way back and more volume control on the Smash, or vice versa.  Or does it even make a difference?

 

[Doc B]

The point is turn down the gain at the amp as much as is feasible, so you can turn up the volume on the Smash. That will reduce the microphonics.

 

[Paul Birkeland]

If you look in the datasheet for the LM3886, there is some mentioning of the lowest possible gain that you can use with the chip itself.  I would start by adjusting the resistor divider on each channel to achieve this gain. 

Next would be a little padding at the input as well.

When you have very sensitive speakers (like your Fortes), and a very sensitive amplifier (high power and/or high gain), you will run into these issues. 

-PB