chipamp.com 3886 amp power supply questions

[phatt]

Here I go again,,, whinger me!
This one bugs me because in the old days it was RED=Hot, Black=Neg and Green was EARTH.
Really simple and really obvious but these days it's seems anything goes.

It would be great if the international bodies got together and made up there mind and settled on a standard.
In Australia it's now Brown for Hot Blue for Neg and green/yellow stripe for earth
but some OS gear comes with black and white power cords %^&*

Re fuse;
Normally the fuse is on the HOT wire as this is the one the wall switch disconnects when switched off (most power point switches only switch the HOT)
but in the case of a power cord wired out of phase then it makes little diff. (can get confusing ay?)

In all the books I've read the power cord runs,, Hot to Fuse > then to main switch of Amp > then to transformer.
That makes perfect sense to me but I've seen some kit builds showing the switch before Fuse,,, go figure??

I do like the IEC power connectors that have the input cord connector/fuse/powerswitch all in one unit.
The fuse can only be removed after the power cord is removed,, very clever and very safe.

You are correct in thinking the mains input phase is of no concern BUT in your case
the dual primaries should be wired *IN Phase*
i.e. The two reds tied and then the two blacks tied,,,(though I would double check the specs).

A simple single primary would of course be so much easier to wire up,, winky.
Phil

[joecool85]

Phil's got it right.  Connect the black wire to the fuse, then that fused lead to the power switch.  Connect the switch from the wire as well as the white neutral from the power cord to your red and black on the transformer.  Doesn't matter which goes to red or black.  In fact, a lot of times on the primaries it will be just two wires, both the same color - that's how much it doesn't matter.  I think they only did colors due to the 4 wire setup that allows this to be wired for 110v or 220v input.

[Top Top]

Ok, so far so good. Why they excluded the color red for hot is beyond me.

Is there any major safety concern that I am not thinking of for not having a power switch? My rig is all in a rack/shelf thing with a main power switch anyway, so I was actually just going to exclude it to have one less place where 120V AC needs to be exposed/soldered.

Also, how do you know what the calculation of the fuse rating should be? I bought a holder and a fuse, but I think the fuse I got is rated extremely high.

[joecool85]

Ok, so far so good. Why they excluded the color red for hot is beyond me.

Is there any major safety concern that I am not thinking of for not having a power switch? My rig is all in a rack/shelf thing with a main power switch anyway, so I was actually just going to exclude it to have one less place where 120V AC needs to be exposed/soldered.

Also, how do you know what the calculation of the fuse rating should be? I bought a holder and a fuse, but I think the fuse I got is rated extremely high.

2 amp slow blow fuse would be ideal.  I always figure go 1.5 times what the amp can put out in power.

LM3886 puts out a max of 120w PEAK.  120w/110v means it would draw peak 1.09 amps.  However, due to thermal inefficiencies etc I multiply by 1.5 to make sure it won't blow.  This puts you at 1.6, round up to the next available fuse size and you're at 2 amp.  I recommended a slow blow fuse because that's what Brian at chipamp normally recommends, but to tell you the truth, I'm sure a regular 2 amp fuse is fine.

Safety wise no reason to not include a power switch.  I like having one because it's handy, but thats up to you entirely.

**Also, I didn't mean to edit your post TopTop, hit the button by accident lol.

[J M Fahey]

Try to fit a power switch anyway, so if necessary you can turn it off independently of the rest of the racked devices.
The power fuse is not there to "protect" your amp from harm, (they are too slow for that) but to avoid the damage spreading too much and specially to prevent it catching fire and burning your house down, loss of life, etc., and I'm not kidding or exagerating.
That said, you need a fuse that's *somewhat* larger than the maximum "normal" power consumption, so it neither blows needlessly (and annoyingly) at turn-on, nor "never" blows letting your transformer melt or catch fire.
SS amps need around 50% more than rated power output, so a 100W amp needs around 150W, divide that by wall voltage to get Amperes.
150/120=1.25A ; next higher value is 1.5 or 1.6A .
2 A might be used if the only available, but not higher or it may not do its job.

OOPS !!! Looks like we simulposted !!  0:)

[LJN]

Juan is right. You don't want to have the amp burn your house down, or worse. A regular 2 amp fuse should do nicely. Just watch and listen very closely when you turn the power on. When I fixed my old amp, The first few times I used it, I mostly watched for any signs of smoke or odd smells. Sometimes you'll be able to tell if something's going wrong, and if you're lucky, you may get to kill the power before it kills your amp, or you. I came close to killing an amp once, the speaker wire had shorted out, and it got really hot, then it shut down. I'm not sure what saved it, but it still works. I got lucky, though, It's a peavey. I don't think Juan, or Joe will tell you the wrong thing. They seem to know what they're talking about. Their advice has helped me with a few things. I hope the amp works for you, and I hope what I've said helps you out.

[Top Top]

Alright. Got heatsink and all ins and outs set up and first plug in!

It works, no smoke, no sparks, I am still alive!

Generally impressed at how little hiss/hum this amp puts off!

Now, I have no preamp yet and no input vol control, and just plugged a guitar straight in. I expected more volume, but I am assuming the input is looking for a much hotter signal than I am giving it.

Looking at the 3886 datasheet, I think I don't know the proper term to look for for the spec on what is the upper limit of input signal voltage before distortion.

I am thinking of tacking a simple mixer on the front of the amp -- probably the GGG mini mixer, which you can see the schem for here:

http://www.generalguitargadgets.com/diagrams/mixer_sc.gif?phpMyAdmin=78482479fd7e7fc3768044a841b3e85a

(sorry says not to post schem, so I am just linking)

Thinking I might have to up the gain on the last opamp stage in order to get the most volume out of the amp. Maybe just replace that 100K feedback resistor with a pot for a master vol control?

[J M Fahey]

Yes.
As shown, that mini mixer has no gain (or rather, gain=1).
You can replace either R9 or R13 with an Audio (Logarithmic) 1M pot, which becomes a gain control.
Or you can build a TubeScreamer clone (I guess GGG has a board and layout ) and use it as a preamp, if you add a small switch in series with the clipping diodes, so it can be either a distortion pedal or a clean preamp.
This project will be of the same complexity as the mini mixer but far more useful to you, not to mention it will sound better.

[Top Top]

Yes.
As shown, that mini mixer has no gain (or rather, gain=1).
You can replace either R9 or R13 with an Audio (Logarithmic) 1M pot, which becomes a gain control.
Or you can build a TubeScreamer clone (I guess GGG has a board and layout ) and use it as a preamp, if you add a small switch in series with the clipping diodes, so it can be either a distortion pedal or a clean preamp.
This project will be of the same complexity as the mini mixer but far more useful to you, not to mention it will sound better.

This isn't strictly a guitar amp and having a mixer built in would be handy for handling the multiple sources I would be plugging into it, which is why I was going for the mixer idea. I recall from the past that the mini mixer is unity.

I'll probably go with a gain pot on the mixer as built in distortion is not what I am looking for.

[joecool85]

If I recall correctly it wants something like +/-2v.  I remember that my LM3886 (same circuit as this but different PCB) was pretty quiet without the preamp and then when I hooked it up to my K-20X lineout it just about blew my head off because I had the volume pretty high and wasn't expecting much.  I also had it hooked up to a fairly efficient 10" Dean Markley cab and as we all know, the more efficient the speaker - the louder the amp!

[phatt]

Hi Top top,  Not sure where to direct you but the mixer is going to be a potetial problem.

It will work with powered units (i.e. a drum machine or synth as these have onboard preamps already built in)
BUT the 10k pots on the inputs will just kill any  guitar  plugged in direct.

Quite honestly a small 4 or 6 input mixer can be picked up cheap and that would save a big headache trying to build a mixer that will give good results.
Hint! if you are not up to speed on electronics,,, stay well away from mixers .
Phil.

[Top Top]

Hi Top top,  Not sure where to direct you but the mixer is going to be a potetial problem.

It will work with powered units (i.e. a drum machine or synth as these have onboard preamps already built in)
BUT the 10k pots on the inputs will just kill any  guitar  plugged in direct.

Quite honestly a small 4 or 6 input mixer can be picked up cheap and that would save a big headache trying to build a mixer that will give good results.
Hint! if you are not up to speed on electronics,,, stay well away from mixers .
Phil.

I'm actually fine with regular audio electronics. It is the power supply/120V AC stuff that is new to me (and much more dangerous hence all the questions). I've built several mixers before and a lot of other audio circuits, even designed some basic audio and synth circuits.

I will not likely be plugging a guitar straight into this amp/mixer, though I do have a mixer design that combines basic JFET pre-amps with the mini mixer I linked there (and 100K pots). I could throw one of those on one of the input channels, but I do not have a huge front panel, so I can't have too many pots.

Any proper mic/guitar pre-amping will probably be done outside of the amp (right now my guitar runs through a rangemaster clone before my current amp).

[Top Top]

Finished it!



It has a two input op-amp based mixer with a gain control on one of the op amp stages as mentioned earlier... I put an LM317 on the mixer board and regulated the DC off the amp board down to 15V for the mixer.

Works well, minimal buzz. It even sounds decent with a guitar plugged straight in, though that is definitely not the aim here.

I had thought about doing more than 2 inputs, but I was running out of room as the enclosure is pretty tight with the transformer, amp board, and mixer in there now. 2 inputs is actually fine and is what is on the amp that I currently use.

The enclosure is plastic, yes, but the bottom is lined with a metal mesh which acts as a ground plane.

I haven't been able to turn it up all the way yet (it's the middle of the night), but it definitely gets much much louder with the mixer.

Thanks everyone for being patient with my questions.

[joecool85]

Looks good.  Is that a power button all the way to the right?  If so, does it turn or push in?

[Top Top]

Oh, no, it's a power indicator lamp. (LED off the regulated 15V)