[bgewaudio]

Can anybody tell me where I can get an in depth artical on pre amp functionality?

[bgewaudio]

Anyone else's input would be much appreciated

Thanx guy's

[JoshK]

I don't know of any article, but what do you want to know?  Apart from source switching and volume control, an active preamp's role is to provide a high impedance to your sources while providing a low impedance to your amp (impedance matching) and in some cases provide some additional gain and/or drive (current output).  Passive preamps work when impedance matching and gain are not needed. 

Ideally, you want as few gain sources as possible that provide adequate gain for your system. 

[bgewaudio]

Joshk, basically what I wish to learn, Is a step by step process in which the audio signal travels, and what components in the preamp perform which task.  Sorry I guess I wasn't very clear, I do understand what a preamp function is, I would just like to know the more technical side of things.

[bgewaudio]

Actually, Josh, even if you know any good tutorial sites would be a help?

Thanks

[Steve]

Hi BGE,

     I agree with Josh, that the fewer stages used, the better off one is. Some amps integrate the preamp section into them, called integrateds. I would rather see the volume control installed directly into those amps, thus eliminating an IC and buffer stage/transformer.

     On the other hand, there are problems with most integrates which is overcome by using separates.

     A separate gainstage active preamplifier offers both high impedance/low impedance matching, gain, and elimination of feedback through the power supply, at least for that stage. Thus an active buffer stage/transformer isn't needed, which in itself adds to the cost (just like a separate active preamplifier would) and which adds extra stages that aren't needed.
So one effectively eliminates an active buffer stage, or transformer, from the audio system.
   

[avahifi]

I am going to repeat here what I know about this subject again, as it seems a worthwhile place to do it.

A passive preamplifier actually is not a preamplifier at all, inasmuch as it does not amplify.  Well - there are some that use some kind of step up transformers, but that is another can of worms.

Essentially a passive preamp is a volume control in a box with some switching functions.

The advantage musically is that it eliminates all active preamp circuit functions, and if the active preamp circuits are not of very high quailty they deserve to be eliminated.

The disadvantage muscially is that is eliminates all active preamp circuit functions.  Those functions should be designed to provide a easy pure resistive load for the sources to drive, eliminate out of band garbage, drive the sh--- out of all downstream loads no matter how goofy they are, and not screw up in and of themselves.  Lots of designers drop the ball here.

A passive preamp essentially connects your sources directly to the power amp, thru the likely significant amount of distributed capacitance of the cables between the source and the passive preamp, its internal load, the distributed capacitance of the cables between the passive preamp and the power amp, and the internal distributed capacitance of the amplifier's input circuits.  Whew!!!  If you wonder what a capacitive load does to the music signal, see all the test results done over the years showing this, they cause a leading edge spike and ringing on the signal, the bigger the load the worse the results.  In addition now that load makes demands on the drive current capability of the source that it likely does not have.

So with a passive preamp you have the distortion of the additional capacitive load on the source, and from taxing its drive current capability.  With an active preamp, you have the distortion of the preamp's active circuits themselves.

Which is worse?  It depends on the quality of the preamp active circuits, the load driving capacity of the source, and the amount of distributed capacitance of the cables.  Your results may vary.   

In general we would suggest short, low capacitance well shielded interconnect cables, sources with excellent current drive capability (a spec not usually talked about) an amplifier with a pure resistive input impedance, and a very very low distortion active preamp line stage providing a resistive input load and high output drive current.

Hope this has not confused you too much.

Frank Van Alstine

[DSK]

Frank,
Thanks for an informative post. Every now and again we get people trotting out the old argument that a passive (or no) preamp MUST be better than an active one, as there is less circuitry (and signal cables) involved. People that have experimented more, usually admit that it is not so clear cut.

A couple of years ago, at the home of Hugh Dean (AKSA), we listened to his system with his (stock) AKSA GK-1 pre-amp in place. The music was beautiful, emotional, engaging, natural, dynamic ...  We then removed the GK-1, running the CD player direct to the amp (AKSA 55w N+). The life and emotion of the music just drained away, leaving a much more flat and bland presentation that was far less convincing and did not entice us into listening for very long.

Late last year I replaced my Sony SCD-XA777ES SACD player with a Bolder modded SqueezeBox2 (with Bolder Deluxe Power Supply). As I had heard so many good things about running the SB2 direct into the amp, I initially ran it directly into my AKSA 100N+ amp. It sounded very good, in fact so good that I left the system this way for several months. Then one day, for interest, I re-installed my AKSA GK-1 pre-amp between the SB2 and 100N+. Again things improved, more body, lifelike dynamics, quieter, more engaging, etc.

 For completeness I should mention that my GK-1 is modded. It has a single input (no source switch), the stock volume pot is replaced with a modded TKD stepped attenuator (as used in CTC BlowTorch and Lamm Ref2), and the stock tubes are replaced with NOS Siemens 7308 gold pin tubes. The first 2 of these mods mean there is less circuitry in the signal path than a typical active pre-amp. I have heard it argued that, perhaps after circuit design, the volume pot has the most significant impact on the transparency of an active pre-amp. So, by replacing the stock pot with a more transparent one, the feared 'colourations' of an active pre-amp can be reduced while keeping the electrical benefits described by Frank above.

HTH, YMMV, FWIW .... 

[bgewaudio]

Thanks alot guys, this is actually the kind of info Im looking for, Know of anymore sources I can read myself?

[Steve]

"Essentially a passive preamp is a volume control in a box with some switching functions.

The advantage musically is that it eliminates all active preamp circuit functions, and if the active preamp circuits are not of very high quailty they deserve to be eliminated."

Yes, the passive isn't really a preamp at all. That is because the preamp gainstage is simply built into the amplifier for the necessary gain to be present, and then renamed as an amp stage. The preamp stage is not really eliminated, just renamed. (That is what an integrated amp is.) Installing the volume control and switching into the amp (hopefully minimal hum pickup) eliminates the need for an IC, simplifying things.

Some time ago, one company was advertising on their site that they used a passive preamp in their amp, I guess seemingly to indicate that they had eliminated a preamp stage, when in fact, the stage was simply installed in the amp and renamed as an amplifier stage.

Cheers.

[WEEZ]

Isn't there usually (3) stages of amplification?

1) input stage (pre-amp)
2) driver stage
3) output stage

All three can be in one box; or split into two boxes (the traditional 'pre-amp' or stage (1) in one box; and the driver and output stage in another box; (stages (2) and (3) )

The problem I see with so called 'passives' is, that they don't provide a low output impedence (or buffer stage) between the source and the amplifier. But an 'active' zero gain buffer pre (Superphon, for example) accomplishes that function and should work just fine. (Provided that the amplifier has sufficient gain).

I think that most systems today have too much gain, however. Today's CD players and DAC's output 2 volts or more. Couple that to the traditional pre-amp with 20db of gain; and an amplifier with the tradtional 26db of gain and the system is screaming loud with the volume control barely cracked open.

My pre-amp has 12.5db of gain and my amp has 26db. Even that is too much with a digital source most of the time. But I live with it.

Most systems would work just fine with 32db total. Maybe less. But I'll 'pass' on the 'passive pre-amp' idea. (pun intended )

WEEZ

[Steve]

Isn't there usually (3) stages of amplification?

1) input stage (pre-amp)
2) driver stage
3) output stage

All three can be in one box; or split into two boxes (the traditional 'pre-amp' or stage (1) in one box; and the driver and output stage in another box; (stages (2) and (3) )"

Right on. I prefer the separates as explained below.

"The problem I see with so called 'passives' is, that they don't provide a low output impedence (or buffer stage) between the source and the amplifier. But an 'active' zero gain buffer pre (Superphon, for example) accomplishes that function and should work just fine. (Provided that the amplifier has sufficient gain)."

I agree with the passives being poor. I also refuse to use a gainless stage or tranny when one can simply install the volume/switches in the 3 stage amp and avoid an extra stage (and additional expense), and an IC?
 
Or, just separate/outboard the first stage from the 2nd and 3rd (typical preamp/amp combo). Again we need no extra impedance matching device, and we eliminate any frequency dependent sonic feedback between the first stage (outboarded preamp) and 2nd/third (basic amp) that would normally occur with all three stages in one chassis with one typical power supply. I prefer 3 total stages rather than 4 (with a buffer/tranny).


"I think that most systems today have too much gain, however. Today's CD players and DAC's output 2 volts or more. Couple that to the traditional pre-amp with 20db of gain; and an amplifier with the tradtional 26db of gain and the system is screaming loud with the volume control barely cracked open."


I agree. But a typical 2 stage amp could be designed with only 15db or less gain, depending on the tube compliment. Of course, a higher gain single tube stage means a lower volume control setting, resulting in higher frequency response. Of course one also needs some range of adjustment of the volume control.


"My pre-amp has 12.5db of gain and my amp has 26db. Even that is too much with a digital source most of the time. But I live with it."

Yes, I agree again. I have yet to see a CD player with an op or tube analog section that was really very good. I think this is a weak link in the audio chain.

"Most systems would work just fine with 32db total. Maybe less. But I'll 'pass' on the 'passive pre-amp' idea. (pun intended )"

  agreed. Take care my friend.

WEEZ

[OTL]

Here's a good explanation on the why's and how's of a true passive volume control.  http://www.tweakaudio.com/Ultimate%20Attenuators.html

Good examples of buffered pre's are McCormack TLC and Superphon Revelation III. http://members.cox.net/smcaudio/tlc1.pdf        http://www.superphon.com/flashpages/revelation3.html

Also, there's a very complete explanation of the buffered pre from Cory Greenburg 
http://www.stereophile.com/amplificationreviews/54/

Lastly, no RIAA EQ with these puppies.

Hope the references help.

[bgewaudio]

Thanks a lot OTL, very helpful indeed