Axiom Home Page Forums General Discussion Home Theater bi-amping with m80s

it's called a biasing signal. to forward bias a transistor....

here is a 57min lecture on the topic

http://www.youtube.com/watch?v=PSdHf6yozyc


i skimmed though the lecture, it seems to answer most of the amplifier questions i have been seeing on the board as of late.

I studied transistors as a teenager and from what I remember, it's a 3 wire gadget where one wire is for the control voltage which is a very small current. The 2nd wire is for a higher voltage. The 3rd wire is a common wire for both, which is also the output wire from the transistor.

If no current flows in the control wire, then no current flows through the 2nd wire to the output. When current flows through the control wire, the transistor causes a copy of that current to flow through the 2nd wire to the output wire, but at a much higher voltage, thereby causing amplification.

I'll watch the video when I get some spare time Dakkon, thanks.

This is starting to make more sense to me, but I'm still fuzzy around the relationship between the amplifier and the transistor(s).

Which one gets the signal from the pre-amp section; the amp or the transistor? Or, in this case (channel output controlled by transistors) is the 'amp' just used for volume / gain control?

If I were to draw a block diagram with the ins / outs, what would it look like from the sine wave representing channel frequency to the speaker driver?

I searched "mosfet" at Wiki and am trying to get through that.

Thanks for the clarification on the assignable channels. I have not read the manual yet, but kept seeing "assignable amps" in the big AVS threads for these AVR's.

OK, you've made me curious.... Why would a teenager do this?

Me thinks we all should have studied this as a teenager (in high school) and then there'd be more than 3 or 4 people on this board who have a clear grasp of the way it all works! Lol

I think amp is probably the total unit of power supply+output transistor, not a specific component in relation to the output transistor. Am I correct in that?

So what JohnK has been saying is that power amps (ie, not integrated or receivers) have one power supply per output transistor?

Some power amps are designed with a supply per output device, but others (like the A1400-8) aren't. There's nothing inherently wrong with a design based on a single power supply. But more often than not, the single supply isn't big enough, and dividing it among several output devices isn't ideal. But then some multi-channel amps, use one power supply per channel, but in an effort to squeeze that many transformers in one chassis, they are all under-sized.

It's all about the over-all design, and there are many ways to do it wrong.

Ken - I had the same epiphany a little while ago while I was recovering from a near death experience trying to run a mile under 6:30 (didn't make it either, dammit; 6:34).

The epiphany being that the "amp" per John's description, is the whole process: power supply, transistors, power rails, etc.

But regardless of that, a multi channel system will still need a controlling device to regulate power to each channel's output, dependant to its own discrete input signal.

Study? As a teanager? You mean like with, gasp...books and stuff?

michael_d,

The controlling device is your (stereo or multichannel) preamp--either a separate one or the preamp section inside your AV receiver.

The level control in the preamp meters out the low varying voltage to its pre-outs, depending on the level setting, then to the amplifier inputs. Where you set the "volume" control adjusts the preamp circuitry to send a signal that will vary (with the dynamics of the music signal) from a few millivolts to perhaps 1 volt or more. As JohnK mentioned, most amplifiers have 28 to 30 dB of gain, which means that if the amp section in your outboard power amp or inside your AV receiver is getting a 1-volt input signal, it will be close to delivering its full output to the speakers.

Alan