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Question about opamp, resistor and capacitor? Answered

Hello i have some questions.

1.Im going to build a ROG Tonemender (http://www.runoffgroove.com/tonemender.html).What the 10M resistor do in this schematic ? and its ok if i use a smaller value like 2M or 4M7 ??
2.If a opamp rated output 10W with 4 Ohms speaker, how many W i can get if i connect it with a 8 Ohms speaker ?? can i have the formula for this please.
3.My amp have output power is 100W , can i connect a 8 Ohms 26W speaker to the amp ?
4.I want to find an opamp like LM386 but use lower supply voltage (5v) can you give me some suggest please.
5.What type of capacitor in the picture bellow ?

Thanks for advance.

sorry for my bad English!



Best Answer 6 years ago

1--the initial 10M resistor is just a ground reference. A somewhat lower resistance won't matter. Too low, and it will effect the input impedance...

2--There probably aren't any "opamps" with a 10W output. It's a power amp chip.

General rule--for solid state amps, not for tube amps--is that the halving the load (4 to 8 ohms) leads to 1/2 the output wattage. Use the Ohms Law equations for this-- P = V^2 / R (wattage = voltage squared / ohms). You don't need to know the output voltage to plug in different resistances and test the concept...

But it's best to look to the specs of the amplifier itself as to the load vs. output...

3-- You can connect a 25W speaker to a 100W amp, but it won't survive if you feed it more than 25 watts.

4-- there's a low voltage LM386, with a supply V range of 4V-12V. And the 386 is a power amp, not an op amp.

5-- probably a metalized polyester cap. Not a big deal, polyester, polystyrene, metal film--whatever. So long as it's voltage rating is sufficient...

Thanks you very much !
But can u tell me what i can use instead of LM366 ? because i cant find any low vol LM386 in my country .Thanks!

I'm not sure where you're using the LM386. Are you building more than the Tonemender?

ROG has specific opamp recommendations. If you need a basic opamp, then the TL07x is a good choice for most audio preamp apps (the dual version, TL072 is in your schematic). Maybe the other listed choices would be better, but the TL072 should work. They are easy to find.

You don't need a "low volume" 386, you need a volume control !

Its good that you are trying to understand the schematic. So many people find circuits online, try to build them, don't have a clue and it doesn't work. Of course, then they don't know what's wrong! I've been learning all this stuff at http://www.diyguitarpedals.com and it seems pretty sweet, might be worth a look. There's only so much you can learn from trawling forums.

What gmoon said... but I believe the 10M resistor is there to prevent "thump" or "pop" when the circuit is either turned on or plugged into. For more info on that, search for "bypass" on GeoFex's website.


1.) Gives you a high DC input resistance, of 10 Meg.

2.) P=I^2 x R

3.) Yes, but at reduced volume.

Look for amps that have bridged outputs, or make one