Quote from: J M Fahey on June 23, 2011, 07:38:46 AM
Nice. Congratulations.
Any gut shots?

Sorry, missed your post somehow.

Should have snapped one before closing it up. I'm sure I'll find some reason to open it up again and take one then... Pretty much looks like the pics I posted before but with more wires running around and a small perf board with the mixer op-amp on it.

The yellow lamp lens I got from an electronics surplus store. I have no idea what it's really supposed to be, but it works well as a lamp lens.

Behind it I just have a normal yellow LED to light it up.

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

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.

Quote from: phatt on June 15, 2011, 08:33:52 AM
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).

Quote from: J M Fahey on June 14, 2011, 04:51:58 AM
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.

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?

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.

Ok got all this so far.

I am on the fuse now. the 3 prong power cord that I am using has black white and green wires. I know the green is ground (it goes to the center ground prong). I looked up online and it says black is usually hot and white is neutral. Does that mean I should hook the black wire from my plug to the fuse terminal?

And then from the fuse to the input of the transformer - do I hook the black or red wires to it? Or perhaps it doesn't really matter because ultimately it is just rectified to DC anyway?

EDIT: did a little research and found info that says you should connect hot to the fuse... so I did.

The main question then is from there into the transformer - does it matter if the "hot" from the wall goes to the red or black wires into the transformer? I would think the only difference will be that the AC is 180° out of phase one way vs the other, but shouldn't matter if it is being made into DC anyway, right?

So let me make sure I have this correct -- few questions:

1. the "AC1" & "AC2" with the line over them on the board stay unconnected? (the ones that currently have the green wires going to them)

2. I would want to take a blue and a green from separate secondaries (one from output 1 and one from output 2) to connect together, right? I assume so otherwise I am shorting the transformer, if I understand correctly. And they connect to that middle "GND" connection on the PSU side of the board?

3. The grounds can all go to the same chassis ground connection? (ie: Mains ground (prong 3 on 3 prong power plug), AC secondary ground (the green and blue tied together) and DC/audio/speaker ground)? Will it be the same for when I add a small preamp/mixer running at a lower DC voltage? (tapped/stepped down with a regulator off the DC voltage going to the amp chip)

I think that's all for now, I am sure I will have more questions later.

ok... need to step back a second here.

The board was assembled by JoeCool and was tested by him as well to be working (hope he can jump in here and throw down a word about how he connected it).

My problem is that I do not know how to hook up a power transformer to it. It was my impression that all I needed was a power transformer.

Several people have suggested the power transformer I have bought (see earlier posts in this thread), so I am wondering why that is the case if it will not work.

Thanks for the tip about the chasis. I may install large metal plates inside of it to act as a ground plane.

OK... going to prod this one on a little bit with some pics.

After looking at the power supply diagram from that other thread, I wired up the secondaries (?) to the board in what I think is the right configuration.



PS diagram:


Got a box to put it in, still need to pick up a heat sink tomorrow.



So the wall AC part, I am still a little stumped about. Four connections on my transformer, two in the diagram.  Do I tie something together? Leave two of them unconnected?

Not wanting to die, set my house on fire, or kill a $20 transformer, I have not touched them yet. Also there should probably be a fuse wired in there somewhere as well I assume.

Edit: Actually, looking at that diagram again, I notice those two connections crossed to opposite coils. So I am assuming this means I tie something together... but still not sure how. Do I tie the black from one coil to the red from the other, on both sets?

Here's the thing: I already have the amp kit. Asking for a transformer to power it was not misleading at all. I am going to start by just building it regardless (not planning on building a double 3886 at the moment). So I am happy for the info provided so far   Also there is a 100 watt PA head/mixer sitting at my favorite mom and pop music store for $60, so there is a limit I am willing to spend on transformers and amp kits to make it myself.

Moving on...

My transformer arrived at my door today. I am going to check out the wiring to get it up and running. This is where I am going to need some help - I looked at RG's post in the thread linked by joecool in the beginning of this thread, but the description is a bit confusing to me because it refers to everything by terms I don't yet know, ie: "You can connect two identical secondaries into one centertapped secondary."

There's two sets of blue wires, two sets of green (two pairs outputs according to the label), and two sets of red, and two sets of black (inputs according to the label).

I've got this thing about death (not really my thing) so I want to make sure I understand how to wire it correctly even at the risk of appearing completely dense.

That's 8 wires that all have to go somewhere. Is there a diagram somewhere for what to do with these 8 wires?

Quote from: J M Fahey on June 03, 2011, 03:15:56 AM
From what you say, may I guess you don't have a Bass player either? (that part being played by a Synth?)
In that case the "PA" amp would carry only 2 Keyboards (one of them sequenced, as a background track) plus a single mic.
Well, that lightens the load a bit.
Sending the guitar to its own amp, even a 15W one, will help anyway.
Pity you told us so late, or we would have suggested the 160VA transformer instead, which would have allowed the future addition of another 3886 driving an extra box.
That would *sure* increase clean headroom.

My setup is not exactly "normal." It's not really rock music at all. My setup is nord micromodular with trigger-able sequences/sounds stored in it, guitar w/some FX, guitar to midi converter into a synth, home made synth/sequencer thing, home made sampler, banjo, microphone.

Usually the nord plays "backing" sequences and I play guitar or banjo parts on top, whistle, and guitar->midi into old yamaha FM synth for some other sounds, and then the other stuff for between song transitions. There's some bassy sounds in there and occasional synth percussion parts, but it's not a standard drums/bass/guitar kind of thing. More melodically based.

I have my home made TDA2005 based 2 output channel space-heater amp (which landed me this 3886 kit when I won the contest way back a couple months ago). I could start putting my guitar through that again, and keep the 3886 for the synths stuff. My main concern is keeping equipment hauling from getting totally out of hand.

This is kinda what this whole mess looks like at the moment:
http://www.youtube.com/watch?v=g2WVlSKIbLg

Quote from: J M Fahey on June 02, 2011, 09:48:58 PM
Ok, now I understand you.
Rather than a regular PA, you are talking about a multi-instrument amp.
That's an easier task.
The main problem is, do you play with or without a rummer?
If without, you can barely make it, considering your amp serves 4 instruments, meaning you'll have 12W at most for each of them.(4x12=48)
If "with", he alone sets a certain volume standard or level you won't be able to match.
Simple Physics.

No drummer.

At any one time, I usually have two instruments going at the most. One is a synth with a sequencer and several voices, so it is a little hard to say really how many instruments are playing at any one time... I have been thinking of splitting off my guitar to it's own amp at a miminum in addition to the "PA." I used to do it that way for a while, but then started using FX on a whole submix which includes my guitar.