386 amp question

[Marcos - Munky]

A bassist friend asked me to build him a clone of the Yamaha NE-1, with included power supply and with a little bedroom amp. He want the enclosure to have a bass input and speaker output (of course ) but also a effect output, so he can use it with a "big" amp when he wants to. And when he uses it with a "big" amp, there won't be any speakers connected to the amp inside the enclosure.

My question is about the amp part. I know I can't turn the LM386 (and other amps) on with no speaker connected to it, because it will burn. I need to find some way to do the connections, so when he connect a cable at effect output jack the amp is safely turned off, without burning the LM386. How can I do this?

[FiveseveN]

I know I can't turn the LM386 (and other amps) on with no speaker connected to it, because it will burn.

That is not true. Problem solved.
Your information probably comes from tube amps with transformer output.
But I doubt the 386 is enough power for a bass amp, even for bedroom rehearsal. Try a 10 W chip.

[Mark Hammer]

Many smaller power-amp chips anticipate including a "Zobel network" on the output, which usually comes in the form of something like a 10R resistor in series with a smallish-value (e.g., 100-220nf) capacitor.

[GibsonGM]

An LM1875 might be a chip you'd want to look into for this. Pretty flexible, not hard to work with. You'd probably need to mount it on a heat sink, but for few parts you could make a decent practice amp.    Lots of schematics using it on the net! 

[Marcos - Munky]

Thanks a lot for all the info. I did a search for the LM1875, and it's quite too loud for what he's looking for. What about headphone amps, any suggestion? Since he'll be using it just for bedroom practice, headphones could do the job.

Your information probably comes from tube amps with transformer output.

Yeah, actually it came from some guys who works with tube amps, they told me transformers will surely burn, and if the amp uses a solid state output it MAY burn. So it's better to be safe than sorry.

Many smaller power-amp chips anticipate including a "Zobel network" on the output, which usually comes in the form of something like a 10R resistor in series with a smallish-value (e.g., 100-220nf) capacitor.

Seizing the opportunity, what about the TDA7498? I have one of that ready to use boards, but it have 2 outputs, and I'll be using only one. It isnt a small power-amp chip (100W+100W), so how can I protect one output when it's not used?

[Mark Hammer]

I'm not sure how we went from a sub-1W 386 to a 2-W 1875 to a 200W TDA7498.  Did you originally mean LM3886?

[Marcos - Munky]

Sorry, I didn't explained it very well, my bad . My friend wants a small practice amp, so it was really a 386. I've asked about the TDA7498 just because we are talking about the zobel network, I have one here and it's for an future build.

[reddesert]

A bassist friend asked me to build him a clone of the Yamaha NE-1, with included power supply and with a little bedroom amp. He want the enclosure to have a bass input and speaker output (of course ) but also a effect output, so he can use it with a "big" amp when he wants to. And when he uses it with a "big" amp, there won't be any speakers connected to the amp inside the enclosure.

My question is about the amp part. I know I can't turn the LM386 (and other amps) on with no speaker connected to it, because it will burn. I need to find some way to do the connections, so when he connect a cable at effect output jack the amp is safely turned off, without burning the LM386. How can I do this?

I don't think you're going to fry a solid state amp by running it without a load attached. That's usually considered to be a problem with tube amps. However, it would be cleaner to disconnect the preamp from the power amp when you have an external cable plugged into the effect out. You can use a switching jack to do this.  An example of a normally closed jack is here: http://smallbear-electronics.mybigcommerce.com/1-4-in-nc-tip-switchcraft-12a/ 

This is how amps with an effects loop (pre amp out, power amp in) work. You run the signal path through the NC connectors of the jacks: tip - NC - NC - tip. When a cable is plugged into either jack, the pre to power connection is interrupted.

[Marcos - Munky]

Thanks for the suggestion. Actually I'm using switching jacks, so that'll be easy to do.

[Jdansti]

A 386 amp with a 12" speaker can get dang loud. I imagine a 15" bass speaker would get pretty loud too. I'm sure it would be loud enough to annoy other people in the house. The chip is so cheap and the circuit is so easy to make, it might be worth your while testing it with a decent speaker.

[boppy100]

You may want to consider the bass boost shown on the data sheet as well.  It consists of a 10K resistor and 33nF cap between pins 1 and 5.

[Marcos - Munky]

Thanks. I'll try the LM386 this week. If it's not enough, I'll try the LM1875.

[slashandburn]

The jacks reddesert  posted are not the ones I'd use for a speaker out that should bypass the internal speaker. That's not to say it can't be done (I don't know, someone else should be able to advise) but I'd tend to use plastic jacks before  the internal speaker, disconnecting the speaker when a Jack is inserted.

These ones, with four lugs, speaker wired to the break contacts. https://www.google.co.uk/search?q=6.35+mono+jack+socket&client=ms-android-htc-rev&prmd=sivn&source=lnms&tbm=isch&sa=X&ved=0ahUKEwi06Y-72NjSAhXrA8AKHYqOBOgQ_AUICCgC&biw=360&bih=511#imgrc=jWMVrJejBf0LkM:


Either way, do it! I love the little 386 amps.

[Marcos - Munky]

Actually, we're using plastic jacks. My friend picked them for the black nuts, he liked them more than the metal ones.

Yesterday we've built the NE-1 board and tested it. Everything worked fine, and we've put it into the enclosure. Next step is to etch the power supply board and amp board (I've already did the layout). It may take a week or two before next step, since my friend is from another city, and he wants to see how I build an pedal from the beggining to the finishing.