Op-amp question

jovial_cynic · 2008-03-20 4:06 pm

Hey folks -

I've been dabbling with some DIY audio schematics, and managed to cobble together the LM386 Little Gem amp, and I'm onto my next project.

I've got a KA4558 op-amp chip handy. All the schematics I've seen show similar op-amp chips used in pre-amp circuits, and not in actual amplifier circuits.

Can the KA4558 or similar op-amp chips be used to make a small practice amp, or are they only useful in the pre-amp circuits?

And yes, I know that if op-amps can be used for the pre-amp, the pre-amp circuit can be coupled with a power-amp circuit, which is a sideways way of answering my question. But that's not what I'm asking. I want to know if an op-amp can be used to build a small stand-alone practice amp, or if I need to also build the power-amp side to make it function properly.

jovial_cynic · 2008-03-20 5:28 pm

More specifically...

Is this KA4558-based circuit appropriate for a guitar amp? Or does this appear to be a pre-amp?

[IMGDEAD]http://newprotest.org/projects/audio/images/20080319_canAmp/KA4558.gif[/IMGDEAD]

xiphmont · 2008-03-20 5:50 pm

jovial_cynic said:
Hey folks -

I've got a KA4558 op-amp chip handy. All the schematics I've seen show similar op-amp chips used in pre-amp circuits, and not in actual amplifier circuits.

Can the KA4558 or similar op-amp chips be used to make a small practice amp, or are they only useful in the pre-amp circuits?

I can't easily find a data sheet for this chip... So I can't answer specific to the KA4558.

The big reason op-amps are not used as power amps is because they can't provide any substantial amount of power. Most can only source between 10 and 30mA. Many even less. They're intended to drive into high impedences only.

jovial_cynic · 2008-03-20 6:10 pm

Datasheet: http://www.fairchildsemi.com/ds/KA/KA4558.pdf

Can you help me see where the output power is listed?

I see that the LM386 that I used before is apparently a "power amp," and it has an output power of 700mW. I know that watts = volts * amps, but I'm not sure what voltage to use to do the formula. The operating supply voltage (9v)? In which case, we're near 78 mA. Is 78 mA typically considered high enough to drive a set of speakers?

If the op-amp output power is too low, that certainly *does* answer the question for me, and I'll have to find another use for my scavenged parts.

cuibono · 2008-03-20 6:38 pm

Yes, opamp power is too low to drive a speaker directly. I'd guess a small practice amp uses about 20W minimally.

KSTR · 2008-03-20 6:39 pm

Can you help me see where the output power is listed?

Actually it isn't listed in the datasheet, neither is the (more important and more useful) max. ouput current. This is a very old-school opamp, and it won't put out much current. You can see this from the curves labeled "Output Voltage Swing vs. Load Resistance", anything below 1kOhms or so will cause the chip to starve output current.

So without a current buffer circuit this chip won't even be capable to drive a typical headphone adequately, let alone a speaker.

- Klaus

richie00boy · 2008-03-20 6:39 pm

If you look at the datasheet for the LM358 as you have done for the KA4558, you will see what load impedances they recommend. op-amps will not drive speakers, some special ones may drive headphones but nothing more.

jovial_cynic · 2008-03-20 7:00 pm

cuibono said:
Yes, opamp power is too low to drive a speaker directly. I'd guess a small practice amp uses about 20W minimally.

The LM386-based Little Gem practice amp is a 9v-powered 1/2W amp, and it's plenty loud for just messing around.

So without a current buffer circuit this chip won't even be capable to drive a typical headphone adequately, let alone a speaker.

Good to know. So... what would the diagram I posted be used for? Is that just a proof-of-concept diagram that doesn't really amount to anything?

You mentioned a "current buffer circuit." That could be done with another op-amp, right? In the case where you're using a dual op-amp chip... could both functions be done with a single chip?

xiphmont · 2008-03-20 7:50 pm

jovial_cynic said:
The LM386-based Little Gem practice amp is a 9v-powered 1/2W amp, and it's plenty loud for just messing around.

The LM386 is more like 1/8th watt, 1/4 if you want 5%-ish distortion. Still, the average op amp can supply only a fraction of that much power.

An LM386 isn't really an op-amp (at least, it makes a pretty lousy op amp by modern standards ;-)

Monty

jovial_cynic · 2008-03-20 7:54 pm

xiphmont said:
The LM386 is more like 1/8th watt, 1/4 if you want 5%-ish distortion. Still, the average op amp can supply only a fraction of that much power.

An LM386 isn't really an op-amp (at least, it makes a pretty lousy op amp by modern standards ;-)

Monty

Hrm. Everything I've read calls the Little Gem a 1/2 watt amp. Regardless, I get what you're saying about op-amps not supplying enough power.

As for the LM386 -- I know it's not an op-amp.

cuibono · 2008-03-20 7:56 pm

Your chips are using are not buffer chips, so they cannot be used as such. This is a chip commonly used for headphones: http://focus.ti.com/docs/prod/folders/print/buf634.html

Opamps have tons of different uses, the circuit you posted is a voltage amplifier.

jovial_cynic · 2008-03-20 8:11 pm

cuibono said:
Opamps have tons of different uses, the circuit you posted is a voltage amplifier.

Does a voltage amplifier function properly as a pre-amp? The site I pulled the schematic from said that this was for amplifying audio.

:xeye:

richie00boy · 2008-03-21 11:28 am

LOL audio signals are voltage signals.

Op-amp question

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