to build a Pre-amp, or maybe an amp?

[Mysteriphys]

hello, I'm kinda of new to electronics, and I've been messing around with some ideas for making a pre-amp to use with a home stereo, and possibly an fx pedal or two depending on the kind of chips/bits I can get my hands on.

I had an old univox amp that came with a couple of 4558D op-amp chips from the axl-20wea board that from looking at bad review after bad review seems to be garbage anyway, (the pots on the original amp were braking and falling apart from use, kinda sad for a practice amp i would think)

so instead of trying to rebuild the board again i decided I want to do something fun and build a pre-amp setup, possibly even a new amp using the old components, though I'm new to actually figuring out the ohms/voltage/wattage stuff, I'm not exactly new to soldering and de-soldering, I like to fix old broken r/c cars, so I possibly have an upper hand there.

my question would be, I've seen the lm386 pre-amp deal (lil-smokey amp) that people have been throwing together with that chip, but I can see a slight difference in the pinout between it and the 4558D, if I wanted to substitute the lm386 with the 4558D what would be the difference in the setup of the pinout wiring?

here are the pinouts:

http://www.datasheetdir.com/LM386+Audio-Power-Amplifier - LM386
http://www.alldatasheet.com/datashee.../NJM4558D.html - 4558D

link to a thread in this forum about the lm386 model with schematic:

http://music-electronics-forum.com/s...ead.php?t=8046

Also I have a bunch of power supplies so I'd like to try and make this run at 12v as opposed to the schematics 9v, and I'm not sure how to calculate the compensation for the capsor thinking maybe I should just use a resistor something to drop the voltage.

if any of this sounds wrong like I said I'm new

[loudthud]

The 4558 is a general purpose dual op-amp. The LM386 is intended to be a one chip power amp. The two are not compatible. Circuits intended for the LM386 will not bias properly with half of a 4558. Look for some circuits that use the 4558.

[Mysteriphys]

hey thanks, i didn't want to start anything without knowing something like thta, I picked up a good audio circuitry book and I'll be leafing through it, as well as looking up some schemes. I'll post back if I work something out. thanks again

[armstrom]

The LM386 can be used just fine as a preamp. I used one as the preamp of my first amp build: http://music-electronics-forum.com/s...ead.php?t=9777 (pics and sound clips)

The trick to using it as a preamp is to keep the supply voltage down (assuming you want to get some distortion out of it when cranking the gain). If you run the preamp at 12V it will just get louder and louder as you crank the gain. Drop that down to about 6-7V and you will start to get some nice distortion with higher gain settings. Take a look at the peak-to-peak output vs supply voltage chart on page 4 of the LM386 datasheet. The different curves represents how the output voltage varies with different output loads. You can see that if you're driving an 8 ohm speaker directly with the LM386 a 12V supply will make the amp clip at ~6.5V peak-to-peak output while using it as a preamp (the top curve with a load of inifinity) will deliver peak to peak output of 10V! Following the curve down you can see you need about 7.5V supply in order to get the amp to clip at the same voltage it would when driving an 8 ohm speaker. This is what I did with my preamp. If you want to add some extra flexibility to your preamp you could add a switch to change from 12V supply to 6-7V. This would in effect give you a clean/dirty switch. Just use a low resistance voltage divider. I used 150ohm and 220ohm 1/2W resistors.

Since the output impedance of the LM386 is very low you can drive just about any power amp stage you can imagine with optimum signal transfer. In my case I added a big muff tone control between the output of the preamp and the power amp (a TDA2005 in my case).

The 4558 will make a fine preamp as well. As an opamp it needs either a bipolar supply or the non-inverting input has to be referenced to half the supply voltage (neither of these are needed for the LM386 as it is internally biased). The general opinion of various posts I've read is that the LM386 sounds better when clipping than a regular opamp does. Some call it "tube-ish" but I won't go that far (especially on a board this tube-centric ) Just listen to the clips in my post and decide for yourself. If you want an easy schematic for an LM386 based amp (that can be used as a preamp also) take a look at the Ruby amp. http://www.runoffgroove.com/ruby.html

[Mysteriphys]

well definitely confirmed it won't work the same, but also confirmed it should work, I wired everything up directly off the chip, it's making some noise, just not very much.

the speaker I'm using is 8ohm .5W

gonna try wiring up the other half and see if I can make sparks now. :-P

one problem i noticed it that i technically don't have a gain circuit :-s

[armstrom]

well definitely confirmed it won't work the same, but also confirmed it should work, I wired everything up directly off the chip, it's making some noise, just not very much.

the speaker I'm using is 8ohm .5W

gonna try wiring up the other half and see if I can make sparks now. :-P

one problem i noticed it that i technically don't have a gain circuit :-s

Oh, I realize now that I misread your post. Sorry You want to use the 4558 for a preamp, gotcha! Did you confirm your circuit is properly biased? Take a look at the circuits posted in this thread for ideas: http://www.ssguitar.com/index.php?topic=562.0 Keep in mind that the circuits being discussed in that thread use JFET input opamps while the 4558 has a BJT input. While this does not affect the way the opamp functions in general (using negative feedback to control gain, half supply biasing, etc...) it can affect how the opamp interacts with other elements in your circuit due to the lower input impedances. I know you want to reuse some of the parts you already have but you can pick up a new JFET input dual opamp like the TL082 at your local radio shack for $2. Go ahead and grab an LM386 for $1.50 while you're there... they're fun to play with and you can build all sorts of neat circuits with them and just a few other discrete components.

-Matt