LM3886 Amp Build Report

[ragtime8922]

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 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/SuperSimplePre-Amp.gif
You could then use pedals for your breakup without being limited to an internal built in OD.

RDV

Thanks, once again, Ricky. I'm gonna hit that link right now.

[RDV]

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

I've got another amp chip, which I'm gonna get a bigger transformer for and do it up right. It's a really good sounding clean amp on it's own.

My preamp hummed even through my battery driven amp. I thought I'd try a dual supply for preamp, but I think I'll have to rebuild it with a single supply.

RDV

[RDV]

I just cut the ground from the ct to the preamp board and it stopped the hum. So I'm playing and gradually the volume slowly goes out to nothing (replaced by hum) like misbiasing. I hooked the ground back up to see if that was it, so I still had no sound along with the ground loop sound back.  I don't know. I was sure hoping to least accomplish this before I get off vacation. It's not looking so good.

RDV

[Nasse]

Glad to hear good single sided 3886 layout is available. I have one made from a kit, there is something not star grounded and it had slight hum all the time. I have another one done by Elektor layout which is better I believe, not any audible hum.

I´m sure your hum problem is something small mistake or solder blob in where it should not be. Are you sure you connected the center taps of both transformers? There was something mentioned about star grounding in 3886 datasheet

It would be easy to test your preamp without that unregulated powersupply, use batteries and listen if the hum goes away. Maybe you need a shielded cable that has one end of shield disconnected. Does it hum when feeded direct from mixer?

My chips are LM3886T´s, the heat sink is at supply potential, so be aware that nothing touches it. It is recommended not to insulate the chip from heatsink if you have 4 ohm load and power supply is near max. I read yesterday that Elektor article again and it suggests +/-30 max supply with 4 ohm load, +/- 35 volts with 8 ohms. I used 2x24 volt center tapped toroidal transformer

[RDV]

I seem to have it worked out. The first stage was not bypassed with a cap, I did that and it came back with no hum at all, and a very sweet clean sound. I still don't have the preamp transformer's ct hooked to ground on the preamp board, though it is hooked to the other tranny's ct. Very full range and the Marshall EQ trims and boosts in just the right way. At least I can finally rest now. I gits obsessed.

RDV

[RDV]

I rigged up an LM317 regulator, and set it up for voltage between 11v - 13v and hooked it across the positive and negative rails(20 volts there cause of the split) of the dual supply for the preamp for the purpose of running a small salvaged 12v CPU fan aimed at the small heat sink on the amp chip. I then used the 470 trim to set it to 12v exactly, and I have a cooling fan.

My next issues are hardware oriented. I've got to secure the wires going from the preamp to the amp, and the fan to the side of the amp. I want to be able to use it on a gig if I have to, and that will require some more work.

I've got the whole thing in a small Dean Markely combo(a K-20 I think) cabinet that was a 15watt 8" practice amp sort of thingy, but with nice fawn colored tolex and the option of adding an 8" speaker back in there.

I'm also going to add preamp out/power amp jacks so I can run my Digitech RP-12 in there if I need to also.

RDV

[RDV]

This is a preamp I came up with by studying a number of different hi-fi and guitar-amp preamps. As usual I wanted to keep it simple, but still sound good, using the Burr-Brown OPA2134 in a cascaded format. Much to my surprise, the distortion acheived by cranking the gain up and master down sounds damn good, without a diode or BJT in sight! I hope it still sounds as good to me tomorrow as it does today. This wouldn't be a half bad pedal actually. The other preamp I came up with sounded good clean but didn't respond well to distortion pedals, so I worked a few days(and nights) and came up with this:


I think it works well with the LM3886 amp.

RDV

[HellStorme]

RDV: Do you have a PCB for that preamp? I'm very interested in building it but i don't have the skills to translate the schematic to a PCB, so if you have it and you want to share it with the rest of ou mortals it would be really nice.

[RDV]

RDV: Do you have a PCB for that preamp? I'm very interested in building it but i don't have the skills to translate the schematic to a PCB, so if you have it and you want to share it with the rest of ou mortals it would be really nice.

Quite mortal here actually, I perfed it.

I've done some re-thinking of the whole concept, and that preamp as shown is dirty only, almost no clean headroom available and would really need a 3rd stage to acheive any kind of real volume. It's almost more of a fuzz pedal.

http://sound.westhost.com/projects-5.htm is a really good place to start looking for a good preamp design. I came with this by looking there, which probably would work better as a clean preamp, though I haven't tested it as yet.

RDV

[HellStorme]

Thanks, looks very promising.
I was thinking to use the preamp from the Marshal Lead 12, but i think it would be pretty dirty as well.

[ryanscissorhands]

How many steps past "way too far" did I go?

. . .not enough. You need a mic'ing simulator, spring reverb w/tank, and then you'd have a POD in a box. A very big box. . . hey, wait, isn't a POD supposed to be all of that in a box? Damn. Well, if you put all of that togethre and put it on a gokart chassis, you could drive it to the gig--SO convenient.