litte gem mkII, ruby, bridged 386 build-output wiring?

[suprleed]

I'm still in the planning stages of my 386 based amp.  I'm considering a bridged design like the gem mkII.  If I wanted to wire the output to a line out jack so I can use it with various speaker cabs, do I simply wire the positive output to the jack tip and the negative output to the ground sleeve?  The ROG schem shows the outputs directly wired to the speaker below rather than to an output jack and I'm a little confused.



Would the same wiring go for a headphone jack?  I would imagine that I may need some way to attenuate the volume as described in the ROG FAQ (for non-bridged amps) so I don't go deaf from using headphones.  Any suggestions on how to do this?

Another question just came to mind.  If I wanted to build 2 or 3 different buffer/boost "preamp" sections with a switch or two to select between them, could I have them all running to a single 386 power amp section or would I have to have the switching system totally remove the unused buffer/boost section from the circuit.  For example, if I want to use the stock gem II buffer and one of the AMZ boosters as my two "preamps."  Can i set up a spdt switch at the input to select one of the two the preamps, but have them booth connected to the same point on the power amp (right after the .22 cap on the schematic above)?  Or would I have to use a dpdt switch to remove the unused buffer completely from the power amp?  Not sure if there would be problems with the signal flowing back up into the unused preamp section or not.  I'm still pretty green when it comes to this stuff.

Any advice from someone more experienced than myself would be greatly appreciated, thx.

[suprleed]

BUMP...

[raulgrell]

I'm sorry that I don't come with an answer to any of your questions, but instead another question;

Would stacking another LM386 on each existing one give a 2 watt output?

[suprleed]

I've thought about the stacked approach as well but I think I remember reading a thread somewhere here on the forum that said it wouldn't work.  I've done so many searches on ruby/gem amps I can't keep them all straight. 

I would be worried about heat issues in a stacked setup, but then again I'm no expert and that would be an easy way to get higher wattage if it would in fact work!  Someone with more experience will have to address that one.

Anyone have any tips on wiring bridged chips as per my questions above?

[chilecocula]

I recently built a bridged LM386 amplifier (bridged bassman ruby with IC input buffer) , and did the wiring just as you ask ,the positive output to the jack tip and the negative output to the ground sleeve, and there was no problem.

[Jobet]

I'm sorry that I don't come with an answer to any of your questions, but instead another question;

Would stacking another LM386 on each existing one give a 2 watt output?

From memory, the LM386 is rated at about 0.5 watts. Bridging two of them will give you appx 1 watt.

If you want to get to 2 watts straight, go for the LM380 instead.

[Jobet]

It might be useful to revisit the configuration of a bridged amp for better understanding.

If you notice, the input is split and one of them feeds into the non inverting input of the 386 while the other feeding into the inverting input of the other 386.

What this does is that you are amplifying the same signal, but the output of each IC will be 180 degrees phase-reversed from each other. This will mean a greater voltage swing where they meet (e.g. the speaker).

Now if you tap at one of the outputs and reference that to ground, you will be effectively getting only 1/2 of the power that the circuit can dissipate. And since both signals at both IC's are identical except for the phase reversal, then doing this and feeding into further device will still yield you the original signal (again only 1/2 of the power that the circuit is meant to push out). Which is fine for headphone amps. However, when used with a cabsim, you will get only 1/2 of the power you built for once you start feeding it into a speaker.

My advice then is to lay off a bridged power amp circuit if the final output is not a speaker. It will get darn complicated and possibly messy if you short the output of one of the IC's to ground.

[suprleed]

Now if you tap at one of the outputs and reference that to ground, you will be effectively getting only 1/2 of the power that the circuit can dissipate. And since both signals at both IC's are identical except for the phase reversal, then doing this and feeding into further device will still yield you the original signal (again only 1/2 of the power that the circuit is meant to push out).

I just want to make sure I'm following you here.  So this idea would essentially negate the purpose of the bridged amp because 1/2 of the signal is going to ground and only 1/2 will be output?

My advice then is to lay off a bridged power amp circuit if the final output is not a speaker. It will get darn complicated and possibly messy if you short the output of one of the IC's to ground.

So if my final output is intended to be a "line out" jack to be used with a speaker cab, a bridged amp is not the way to go (without getting too complex)?



 

[Jobet]

Okay. This is actually what's going on in my head as I wrote that.

The outputs of the stages are going into the speaker and neither of them are ground, unlike say if you use only one 386. Aight? One lead of the speaker goes to the output and one lead goes to the ground ?

With the bridge, neither of the leads that go to the speaker is ground.

What I'm thinking is that if you connect it to the input of another external device, say a cabsim (I'm thinking of an active one like a condor) one of the leads going into that device will be ground.

And since there's a chance that these devices will share the same power supply and hence the same ground, what you will be doing is you will be shorting the output of one of these 386's to ground.

So yeah, if you'd ask me, it would be better to go with an amp that has a single 386, so you won't have any compatibility problems, wrt ground, so to speak.

[tiges_ tendres]

I think you might want to take out some extra insurance if you are going to use it for headphones.

If you check the wiring diagrams at general guitar gadgets you will see a way to make a zoebel network.  The relevant .pdf is listed in the Ruby file on that website.

[suprleed]

With the bridge, neither of the leads that go to the speaker is ground.

What I'm thinking is that if you connect it to the input of another external device, say a cabsim (I'm thinking of an active one like a condor) one of the leads going into that device will be ground.

And since there's a chance that these devices will share the same power supply and hence the same ground, what you will be doing is you will be shorting the output of one of these 386's to ground.

So yeah, if you'd ask me, it would be better to go with an amp that has a single 386, so you won't have any compatibility problems, wrt ground, so to speak.

Makes sense.  I think I'll put the bridged design on the shelf for now and just go with a single IC design.

If you check the wiring diagrams at general guitar gadgets you will see a way to make a zoebel network.  The relevant .pdf is listed in the Ruby file on that website.

Great link.  I assume the zoebel network attenuates the volume on the headphone jack so you don't go deaf/bleed out your ears? 

[tiges_ tendres]

I think it stops it from blowing out the speakers in the headphones.  If my memory serves me correctly (probably not)  when chip amps first came out and were the cats meow of audio technology, people were finding that after a while there speakers would start to sound bad.  Something like that anyway.

Do a wikipedia for zoebel network and it should tell you all about it.