Amp vs. PreAmp - how different?
Mar 28, 2006 at 3:09 PM Thread Starter Post #1 of 10

jpelg

Needs a regular fix of 'Fi
'06 Nat Meet Co-Coordinator
Joined
Jan 19, 2002
Posts
8,675
Likes
26
Location
The Elm City
I've read it many times here that a headphone amp is basically an active pre-amp, in terms of it's amplification.

So going back to standard active pre-amps vs. speaker amplifiers:

1. What is the difference in how each amplifies the signal?

2. Can an active preamp perform as well as a speaker amp with a headphone jack coupled directly to the main amplifier circuit (stepped down, of course)?

*Realizing that there are probably many design differences between models, perhaps specific specific pre/amps can be cited.

**If we could keep this in layman's terms, and not delve into too much tech-speak, that would be great
wink.gif
.

TIA.
 
Mar 28, 2006 at 5:27 PM Post #3 of 10

jpelg

Needs a regular fix of 'Fi
'06 Nat Meet Co-Coordinator
Joined
Jan 19, 2002
Posts
8,675
Likes
26
Location
The Elm City
Integrated, I guess.
 
Mar 28, 2006 at 6:07 PM Post #4 of 10

replytoken

500+ Head-Fier
Joined
Jul 6, 2004
Posts
753
Likes
10
I only have a moment for a short answer, and it is not going to be technical since I am not an engineer.

It depends.
biggrin.gif


In all seriousness, I can only partially answer your question, but I can call to light some of the issues that are related to your question. An integrated amp, as its name implies, has both, or acts as both, a preamp and a power amp. If designed properly, these two parts are optimized for working together. We turn them on and assume that everything works well.

As you have said, a preamplifier has very similar duties as a headphone amplifier, and properly designed, one unit could serve both functions. Assuming for a moment that the headphone function is well designed, we need to focus our attention on how this unit functons as a preamp, and how it relates to a power amplifier. This is where things can get tricky, and this is where somebody else will hopefully provide a better technical explanation.

There is an impedance associated with the output signal of the preamp, and there is an input impedance that the power amplifier uses, and the relationship between these two can be somewhat complicated as there are no set standards for either. So, when I said it depends, I was not kidding. If you are not good at matching equipment, or do not want to go down that road, an integrated amp makes lots of sense. Regarding an integrated amp having a headphone amp, it also depends. The Portal Panache integrated amp is supposed to have a very good headphone jack, but I believe that this was designed this way, and was not an afterthought. When you buy equipment, to a degree you are buying into the desinger's audio philosophy, more so when you buy from small companies.

I know this does not fully answer your question, but hopefully it will help get this thread rolling. I hope others who can better explain the technical will fill in my gaps.
 
Mar 28, 2006 at 6:32 PM Post #5 of 10

jpelg

Needs a regular fix of 'Fi
'06 Nat Meet Co-Coordinator
Joined
Jan 19, 2002
Posts
8,675
Likes
26
Location
The Elm City
Thanks, replytoken.

I guess my question is how does the amplification that an active pre-amp provides to a heaphone differ from that of an integrated amplifier, assuming both have a headphone jack that is fed by each unit's gain stage (I hope I used that term correctly)? Is there a difference in the way each amplifies a signal, other than how much?

And, can the best headphone jack on an active pre-amp compete with the best headphone jack on an integrated amp (or receiver, for that matter), again assuming the above conditions.
 
Mar 28, 2006 at 6:55 PM Post #6 of 10

00940

Headphoneus Supremus
Joined
Nov 6, 2002
Posts
4,493
Likes
44
As a common rule, a standard integrated amplifier has no voltage gain section independant of the current gain section. The manufacturers usually only put a sources selector and a potentiometer in front of a power amplifier. The power amplifier usually has its gain set so that an input signal of 2V (the common output of cdps) will drive it to maximum with a 8ohms load. In this case, the headphones out will be fed from the speakers outputs, with resistors networks to match impedances.

If they put a dedicated preamp inside an integrated amp (for various reasons such as controlling the input impedance or isolating the potentiometer), it will usually be only powerfull enough to drive the power amp (a few mA of outputs), because more isn't needed.

The difference in between a dedicated preamp and a power amp is mainly in the quality of the gain devices (the preamp will usually use lower voltage, lower noise devices) and power supply (the power supply lines of the preamp will not be polluted by the heavy draw of the current stage and will be better regulated).
 
Mar 28, 2006 at 7:22 PM Post #7 of 10

rickcr42

Are YOU talkin' to me?
Joined
Jun 21, 2001
Posts
13,874
Likes
14
Quote:

I guess my question is how does the amplification that an active pre-amp provides to a heaphone differ from that of an integrated amplifier, assuming both have a headphone jack that is fed by each unit's gain stage (I hope I used that term correctly)? Is there a difference in the way each amplifies a signal, other than how much?


both are really no more than impedance converters/matching devices with one adding a power stage to the gain stage to get the drive requirements and the other dumping power from the driving section.

where it gets a bit snarly is the actual interface between the output drive section and the low impedance (relatively speaking using a line iput as reference) headphone.

If you tried to drive an average headphone load directly from a preamplifier output it needs to satisfy two requirements

1-needs to have enough current reserves to handle the additional load.When you increase the load on an output stage you ease the drive requirements of that stage while at the same time reducing its current delivery or power (just like when you use a 16 ohm speaker on a 100wpc/8 ohms amp you now have a 50wpc amp).
Reduce the output load impedance and you increase the demands not just on the circuit itself but the power suppy feeding the citcuit and again using a speaker amp as an example why many can do 8 or 16 ohms (even 32 grado fans
wink.gif
) they totally fll apart and have LESS power into at 4 ohm load.Voltage delivery into a low impedance load means more current must be delivered and this being a stressful state,one that many designs are not up to for reasons of cost,the power supply sags,the output section clips and the sound sucks.
so if you want a line stage to drive a headphone it needs help in the output section and why you see buffers or discrete stages added for more drive.
The reverse is a typical headphone amp can not only drive headphones but a true 600 ohm input after a very long interconnect.the classic "line driver" is really the same as a headphone amp.

2-The second part of the equation is low end rolloff.ALL audio gear has a bass cut or it would be flat to DC and DC being a voltage as well as a frequency means it will be passed along to every stage following and if thise stages also pass DC the final result is blown speakers or blown headphones ! They HATE DC and were not designed to withstand any but a miniscule amount.
So knowing there needs to be a method to cast this off or block it most preamps have a DC blocking capacitor and depending on the quality is not detrimental to the overall sound (BTW-DC servos eliminate the cp but have their own "sound" and DC blance controls drift so choose your poison).

Where this can be a problem is in what was the assumed load impedance of the next stage when the designer chose the part ?
for example :If the data sheet for the preamp says "20-20,000hz +/- 0.5dB into a 10K load or above" and you load the output with a 5K load you just shifted the -5dB bass rolloff point from 20hz to 40hz ! 2.5k and it becomes 80hz and worse,100 ohms (some headpones ) and it is 200hz !!!!!!
No way anyone settles for that and calls it music so the capacitor needs to be taken into consideration if one is present and willbe the over riding factor in ultimate sonic quality.If none is present on the output the above part above output section drivew holds precedence.

Power amplifers:

Whole different beast and already designed to play into a low impedance load,even lower than headphones,and with serious damping and serious VA capabilities so why is this a bad thing ?

If your headphones were designed to play off a power amp (K1000,AKG "M" series,some KOSS.MAYBE the Sony V-6) they will be voice matched to play from the superior damping and power available,will in fact sound exactly as they were designed to sound on the test bench but use a headphone meant to be driven from a less muscular output stage,one with less damping and you could lose all by from overdamping the headphones !
The fix is to play around with matching resistor values to get the matchups "just so" but otherwise you took the bass away and will proclaim the experiement a failure.

Botom line is ALL power amps can drive EVERY headphone while SOME preamplifiers can drive SELECTIVE headphones as long as you pay attention to the specs and do your matchups accordingly

hope this rambling response helps a bit
wink.gif
 
Mar 28, 2006 at 8:34 PM Post #8 of 10

jpelg

Needs a regular fix of 'Fi
'06 Nat Meet Co-Coordinator
Joined
Jan 19, 2002
Posts
8,675
Likes
26
Location
The Elm City
Wow. Thanks, guys. Great responses. Still "chewing" on 'em
wink.gif
.

So a low-impedance device, like a headphone, presents a more difficult load on a typical active preamp, while a higher impedance can is actually easier to drive (not taking into account 'efficiency')?

But, a quality preamp often has higher-grade components that may translate to higher quality sound to your headphones?

Quote:

DC being a voltage as well as a frequency


I'm a little lost on that. Is there an easy way to dumb that down in some way?
 
Mar 28, 2006 at 8:45 PM Post #9 of 10

rickcr42

Are YOU talkin' to me?
Joined
Jun 21, 2001
Posts
13,874
Likes
14
I Quote:

'm a little lost on that. Is there an easy way to dumb that down in some way?


icon10.gif


"0" hertz is like plugging your headphones into a car battery
very_evil_smiley.gif


A DC voltage is ruler flat without peaks and valeys and why it is called "direct" current (since current is the actual power and voltage the means to convey it)

AC voltage varies according to frequency and why it is called "Alternating Current",because the peaks and valleys alternate according to frequency

Music is an alternating current,a battery DC so 0hz=DC
icon10.gif
 
Mar 28, 2006 at 8:54 PM Post #10 of 10

rickcr42

Are YOU talkin' to me?
Joined
Jun 21, 2001
Posts
13,874
Likes
14
Quote:

So a low-impedance device, like a headphone, presents a more difficult load on a typical active preamp, while a higher impedance can is actually easier to drive (not taking into account 'efficiency')?



Pretty much.Unless the design is specifically meant to be driven by a true power amp such as some of the AKG cans.The reason they are such a b*tch to drive is they are not only a higher medance but inefficient as well and you combine the two (higher load=less curent available from the output stage/high drive voltage required because of low efficiency) you have a nightmare for a "muscled up" line stage which is what most headpghone amps are.

Quote:

But, a quality preamp often has higher-grade components that may translate to higher quality sound to your headphones?


That does not track at all.My preamp IS my headphone amp with the line stage being a "tap" after the gain stage and using a WOT/CF as the line driver/headphone driver.

Quality parts is not the domain of what the device is or what its primary function happens to be but purely at what price point it is to be sold at.

so a dual use preamp/headphone amp such as the Welborne Labs Ultrapath is identical in quality no matter which output is actually used.
 

Users who are viewing this thread

Top