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

[RDV]

http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=LM3886TF

RDV

[petemoore]

  Whew, that's Steep !!! 
  inexpensive clean power in seems. Very hard to resisto.

[RDV]

  Whew, that's Steep !!! 
  inexpensive clean power in seems. Very hard to resisto.

Oh mama me-o mama me-oh!!

Couldn't resist-o

RDV

[RDV]

Just finished my LM3886 stereo version, and fired right up. No smoke. A month of planning and work finally coming together perfectly.

A thank you to Brian at chipamps.com for the PCBs. They were brilliant!

Pictures forthcoming.

...but. What do I do now?

RDV

[R.G.]

The LM3886TF is $5.99 each at Digikey in ones.

[RDV]

...test the damned thing, that's what!

I ran the summed output of a Sony Discman into one channel, the output of the Thomas/Vox preamp into the other side and played Won't Get Fooled Again and BaBa O'Reilly using the backing tracks I purchased from a nice gentleman from the north of England. Cranked. With the top cover on the amp.

I opened it immediately afterwards to check heatsink warmth and the results were barely. Warm, that is. The side running the backing tracks was considerably warm-er, but still nowhere near hot, and that's with a 5-piece band playing through it!

Long Live Rock!!(I need it every night)

RDV

[RDV]

...did it again, but twice as loud, at least loud enough for a restaurant/lounge gig(that would tolerate the Who). The bass was vibrating my guts if you know what I mean. I've gotta stop before the po-lice come to shut down the band(tee hee). Took the cover off again and the backup track channel heatsink was hot but nowhere near shutdown hot. The guitar side was warm, but not hot. If I were to use it gigging, I'd use both channels for backing tracks and vocals only, so I think it would be no problem. At worst, I've got a serious fan if needed that runs off straight mains power.

RDV

[RDV]

Okay, okay... back to the limiter. I hope you're all able to follow along through the distractions. This thread is similar to my life!

I decided to give the limiter another go, taking the 25vdc coming off my 18vac rectified by way of a 1000µF cap. This voltage goes to the limiter pot.

This changes the ballgame considerably. Now the formerly treble-heavy amp becomes a bass-heavy amp that's about to kill the 12" I've got it hooked to. I don't know why, perhaps the .001µF to ground at the input of the limiter circuit does take some treble out. I think I've got to replace the linear bass pot with an audio which should help a little.

No longer does the output squash out all the way anymore which I do believe means that's what the problem was(the voltage differential that is).

I hope I've made myself unclear.

The jury is totally still out on this limiter because when you turn the thing all the way down so you can hopefully get your clean headroom back it starts an audible hum. I probably need to isolate that 25v somehow more from the regulated 18v that comes after it. I don't have a clue how to do this or if it can be done.

I'LL BE BACK.

My wife's out of town this weekend which is why I'm so full time into this.

RDV

[RDV]

Sigh.

The clipping of the limiter is quite harsh.

I need a drink.

+

RDV

[Joecool85]

You should just do a different preamp/eq and be rid of the limiter.

[PenPen]

No, keep the preamp. That thing sounds great. Honestly, if you aren't clipping the power stage, I say screw the limiter.

[Threefish]

Darn. Don't you hate that. You've got it working and ....
....this isn't what I wanted!

Ditch the preamp/eq just because of the limiter? After all this work?? No way!!!
The setup without limiter sounds great doesn't it? The samples sound great to me. I can totally understand the challenge, but do you really need the limiter?

This is a great thread RDV. Thanks for the regular updates. I'm sure I'm one of the many following this with great interest.
Keep at it.

[RDV]

Thanks guys.

I've reached another of those 'let it set a few days and think about it' periods. I'm thinking of building a cab sim in place of the limiter. I want something to tame a bit of that SS skronk.

I shall return.

RDV

[R.G.]

The Achilles heel of the LM3886 is that it sounds ...really... bad if you ever trip its protective circuits by overdriving it. Practically any limiter you put in front of it to stop it from doing its protective dance will sound better than listening to it save itself. The Thomas Vox limiter isn't the final word on limiters by any means, but do use something other than the innate overload of the LM3886. Scale the input to three or four diodes or something.

[RDV]

The Achilles heel of the LM3886 is that it sounds ...really... bad if you ever trip its protective circuits by overdriving it. Practically any limiter you put in front of it to stop it from doing its protective dance will sound better than listening to it save itself. The Thomas Vox limiter isn't the final word on limiters by any means, but do use something other than the innate overload of the LM3886. Scale the input to three or four diodes or something.

If I could somehow not get that rather harsh clipping that's occurring WAY before the LM3886 would even consider clipping, it wouldn't be a problem.

I haven't scrapped it yet. I may try a bit lower voltage on the preamp while still maintaining the 24.1v I've got across the limiter pot. It may or may not have an effect.

I also don't know if I should have gain matched the two transistors in the limiter. Another thought is I used an MPSA18 in the gain stage before the limiter which may have been a poor choice considering everything else is 2N3904 or 6.

This is pretty new territory for me, and I'm not a smart(Re: Educated) man. I'm curious like a cat, or is that stupid?

I'm going to listen with fresher ears tomorrow.

RDV

[RDV]

Hi:

I reduced the preamp voltage to 15v and may go lower. It seems to decrease the harshness of the clipping.

Further testing is needed.

Oh, and I did have a drink.

Vodka.

Absolute.

RDV

[PenPen]

Oh, and I did have a drink.

Vodka.

Absolute.

RDV

I prefer Grey Goose.

Vodka tonic always gets the brain going for me too

[RDV]

LM3886 @ Futurlec for cheap

Not the insulated version though.

RDV

[Joecool85]

I'll pay the extra couple bucks for the TF. 

Can we have some pics of your amps?   

[RDV]

I'll pay the extra couple bucks for the TF. 

Can we have some pics of your amps?   

I keep forgetting to bring home a camera from work. They're really nice digital Kodaks and better still, I know how to use them.

RDV