preamp

Maximum output voltage of cdplayers in unbalance mode is between 1.9-2.2 volt.
Maximum input voltage of power amplifier is between 1.2-2.2 volt.
Do you know what's duty of preamplifiers?!!!!
I thinked preamp have voltage gain and power amp have current gain and now i don't know what a preamp do?

 

Firstly, they allow you to switch between various sources: cdp, tuner, dvd, vinyl, etc.

Secondly, they may have a second set of outputs or a subwoofer amp output.

They may also incorporate a balanc control, which can be handy.

Finally, they increase the gain where it's needed and take some of the weight off those cdps that don't have a top quality output stage.

That's my take on the subject. I look forward to hearing the comments of some of the forum's wise elders, particularly regarding the point about 'taking the weight off the cdp output'.

 

Err...aren't we missing the obvious here? 2.2V whacked straight into your power amp is going to sound awfully loud. I think you'll need a volume control, Sir :drum:

If your CD player has variable output then you're laughing. That is of course if the output stage sounds any good , which I believe was Steves point.

From there we get really complicated and enter the DAC side :duck:

Regards

Hex

 

Its more involved than that as always

I am trying to get my head around the rationale behind things designs and its not easy, owing to the lack of really good writing on the subjects.

A power amp has a sensitivity, which confusingly is the voltage required IN to give the power out in watts.

eg, an amp of 100watts will give a voltage out of V squared divided by load, say 8 ohms.

which is 100=V Sq/8

800 is v sq., volts is around 30 say, but this is RMS, so you need to x 1.4 to get the peak, say 44volts.

This will be the power rails of the amp which limits the output volts.

So 2 volts in which is a typcial sensitivity gives max out, of 44 volts.

You can calculate gain=out/in =44/2 =22, in dbs is 20log (22)

So if a cd of 2 volts out drives a power amp direct, it will give full power of 100 watts....DEAFENINGLY LOUD, may damage your ears and your speakers.

So you need a volume control to cut the volts.

Power amps consist of 3 stages, the output stage supplies high current to the load, the speakers, and you also need those 40 volts, so the rest of the amp takes those 2 volts and makes upto 40 volts. Sure enough they are called the voltage amplifying stages, and always work in class A.

It is the output stage that is class A, AB, or B, used as a marketing ploy by makers. so amps are a combination of class A and class B, or A, or....

So power amps also really are both voltage AND current amps.

As steve said, you need a few sources, so you have a switch, you may want balance, and tone, and....

Pre-amps means 'before', so you put if 'before your main or power(volt and current big brute) amp.

They may be passive...just a volume control, or active, which means they amplify.

Active can use various means, they can be

1/ valve
2/ transistor
3/ chips ICS

Now we know from above amplifiy means volts OR current, so a current amp. is a buffer, and is used to drive a nasty load like a long cable or a speaker.

So some pre-amps may be current amps AKA buffers and they don't have VOLTAGE gain.

the other variety of active preamps amplifiy voltage

The reasoning behind pre-amps goes back to pre digital.

tuners and cassettes give out voltages of 0.3 volts, vinyl, typically 0.5 volts, after it goes through the phonostage, so you need more voltage to fully drive the power amp which wants 2v.

so We use an active preamp to give a gain of what? around 10x, which will give lo and behold, 3 volts, enough for our insensitive power amp.

It can be done with valves, ICs, transistors.

So with cd, all you need is a vol control, but sometimes to my ears you get more bass, more punch, more life with an active pre, tho you don't really need it.
Don't ask me about headroom and overload, cos I don't understand it yet!

OK? simple eh?

 

Now you've covered the very basics Ian, prehaps a intermediate lesson Prof ?

 

Nice post, Ian.

 

I agree you but you don't think replacing [CDplayer+pre+power] by [CDtransport+pre(DAC+pre)+power] is more logical?

 

amir,
most dacs with variable output (volume control) do not have analog inputs as the volume is done by right shifting each data word in the digital domain. this means that anyone with analog sources (vinyl, tape, sacd, dvd-a, etc.) is sh*t outta luck if they want to hook up to their pre-less system. also the right shift is seen by some to reduce quality - true if you are dealing wiht raw 16 bit words however most dacs upsample to 24bits which means you can loose 8 of them without losing any origional data.
even in my simle system i use one analog input on the pre to connect my home cinema to my main rig. this would be impossible if i was using a variable out dac.
cheers


julian