little help understanding a little more about amps please

---mario---

So I've been reading and doing a bit of research on amps and I think I've come to understand a bit of what's going on from guitar and pre-amp part with controls, switching, etc. as I could find allot to read about it, think I'll be able to start getting a grip on the more complicated aspects in that realm soon with some more reading.
Now, problemo, I couldn't find much on the power amp side of the mix and I only have a slight understanding of what happens here, so I've been hoping that you'll share your expansive knowledge with me ?

Particular questions I have:
-What happens in the power amp with what electrical components?
-How complicated procedure is it to match a pre-amp and power amp?
-Do you need a power amp in the first place (will you still be able to get an audio, though a low output, if you were to go straight from a pre-amp to speaker/ mixer/etc.) ?
-Am I missing the whole point to start with?

Any contributions will be much appreciated.

Regards,
Mario

Gearhead

It starts with speakers. Speakers need a relatively large amount of current to do their work properly. So even if the voltage you get out of a preamp is quite a bit, the speaker is not going to be happy with the current and will probably even damage your preamp.

Power amps have their own impact on Tone, but not nearly as much as preamps. The main thing to figure out is what size you need - if transistor, you need the biggest you can afford whereas with tubes you need to get the smallest you can get away with keeping up with the rest of the band.

Tonally, there are only small differences, but there are some. The difference you get from one speaker to the next will be far easier to hear and far more important to match to the style of music you play. Best is to go and try what works for you, speaker before power amp.

---mario---

So basically a power amp takes in from the pre-amp and transforms the signal to satisfy the speakers requirements, is it much more than a simple trasformer circuit then?
I guess then that things like Vox headphone amps is just a simple pre-amp as it wouldn't need much current to run headphone speakers and pc soundbords/-speakers that has their own power supply.

Karel-Mars

Hey Mario

I think Gearhead has answered your question, but maybe I can add.

Think of power as the product of current and voltage, thus P=V*I

Also keep in mind Ohm's Law: Resistance=Voltage/Current

A preamp has to deliver an output of about 0 dB which is about 1 volt. The input resistance of you power amp is about 47 kOhm, so the preamp only has to deliver 0.021 mA to get the signal through. Easy enough with low voltages (like a battery) and small transistors.

Now consider the power amp. If you have a 8 watt power amp pushing a 8 ohm speaker, you need about 1 ampere current. That is a lot more than any pre amp can deliver. And you need to manifest about 8 volts on the speaker terminals to effect a 1 ampere current through the speaker. Way out of range again. You need a preamp to amplify the signal (multiply it's strength) to the line level of 0dB and then you need a power amp to deliver the current to your speakers.

I hope this helps.

---mario---

yup, thanx guys, I'm on par now, basically put not that complicated, the real brain bugger is the pre-amp then to get what you want to achieve.

I've got a schematic or two now so I'll be toying around with small pre-amps to help learn, will use this thread if I run into a wall (or is it electric fence in this case?)

Thanx again for yer wit ?

aja

...mario... wrote: So basically a power amp takes in from the pre-amp and transforms the signal to satisfy the speakers requirements, is it much more than a simple trasformer circuit then?

Kinda - a transformer changes voltage for current - you can increase the voltage, but at a loss of current and vice versa. The power amp seeks to increase the voltage and current, without affecting the signal too much.

It might be interesting for you to read a little about impedance matching, many of your questions might be answered ?

---mario---

aja wrote: impedance matching, many of your questions might be answered ?

Thanx, will do ?

TomCat

All of the above are good answers.

Another simple way to put is is as follows:

A pre-amplifier's primary function is to increase signal voltage. (High Impedance)
A power amplifier's primary function is to increase signal current. (Low Impedance)

In Tube circuits the voltages are very high but are at higher impedances.
The role of the output transformer is to provide a transition from High Voltage/Low Current to Low Voltage/High Current to drive the loudspeaker.

Transistor/FET Power amplifiers already provide high current at low impedances so no O/P transformer is necessary.

Hope that helps.....

---mario---

Thanx again, I think I'm getting it now.

From everything I've read it looks like that impedance is just the resistance equivalent for AC circuits ??? and your basic function of the power amp transistor is to satisfy the impedance requirements for your speakers through phases high V/low I and inversely.
I think I did this when I was in school :? :-[ damn language gaps ?