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Question about Preamps

Hi All,

I'm reading Morgan Jones' "Valve Amplifers" book 2nd Ed.
On page 384, he talkes about the perfect preamp. fyi, no preamp.
He makes the point that most formats, CD DVD, DACs, etc. already output 2 VMS, so that a preamp is a unnecessary component in the way that will add cost and possibly sound degragation to you system.

I was wondering what people's opinions on this forum have to say about that?

Eternalightwithin
 
Currently I have the same opinion. If you don't need the gain why use preamp. I integrated volume at the output of my DAC and it's the only source I'm using at the moment because I believe that source switching degrade sound too.;)
Probably the better place for volume control would be at the input of the amp but I didn't get to it yet.
Some people claim that by using preamp they achieve better dynamics but if your stages are properly matched I don't think it should be an issue. Although there were times I didn't believe AC cords make difference either.;)
 
Eternallight,

If the output and inputs are all direct coupled and will not be affected by any external variation in load, then yes, a potentiometer or a switched attenuator might be the perfect preamp.

However, The devices involved will be affected to a degree by the variation in load of a potentiometer/attenuator. So sound will change some throughout the range.

Also, that 2 volts is at maximum output. If the amplifier's inut sensitivity is 2 volts for full output, fine. But there are many amps that need more input. And most music is not recorded with that full a range.

As for preamps degenerating the sound... some preamps may actually enhance the sound. Some preamps are so good they do nothing for the sound but amplify it (inaudible distortion and coloration).

A preamp gives you more flexibility. For example, unless all you are going to do is listen to CDs on a CD player, the audio sources will vary. For instance, I have satellite, a VCR, and a DVD. The DVD of all devices is low volume output. I guess so as to give the biggest dynamic range impact. Without a preamp I would hardly be able to hear it. The Satellite OTOH sounds ten times louder.

It is up to you. If the input sensitivity of your amp is low, then do not use a preamp. Of course, you will need some sort of attenuation, otherwise your neighbors and ears will not like you for long.


Gabe
 
Gabevee said:
Some preamps are so good they do nothing for the sound but amplify it (inaudible distortion and coloration).


Gabe

If a foot of wire has its own sound signature, input jacks have different sound, PCBs material influence the sound, volume controls sound different, different power supplies sound different, a perfect preamp will never exist. It will always change the original sound; in case of an inferior source or an amp it may be sometimes for better.;)
 
Harry Haller started a huge thread on a shunt passive attenuator -- or passive-pre if you prefer. For pocket change you can slap together a prototype and give it a try.

Everyone's system is different, and everyone has different needs, expectations, and ability to hear so the final arbiter is you listening to your system.

It is all about enjoying the music...

dave
 
I think preamps are necessary for technical and practical reasons. Practically, it is a pain in the *** if you have to switch cables to listen to your SACD instead of your FM radio. The preamp is a natural place for source switching. The preamp is also a handy place to convert single-ended signals to balanced for transmission to the amplifier.

On the technical side, I think it is handy for an amp to take a 20Vrms balanced input. Less gain in the amp means less transmission noise gets amplified.
 
I'm using five different systems. The reference doesn't have source switching because I use only one source. If I listen to radio or tape or DAT or turntable I'm using other systems. If I decide to use balance connection to an amp the main sources would be balanced too.
Don't get me wrong, I too used to be once under the impression that preamp was necessary. Even my first DIY project was a preamp . But it was long time ago. ;)
 
In the case of high output sources (CD etc.) me too thinks that a preamp is not necessary and does rather harm in cases where the extra gain isn`t needed.
To me it does not make much sense to amplify a signal let`s say 20dB or even more, just for attenuating it some inches later down under the amplifiers input level.
I do not believe that a preamplifier can enhance performance - as an amplifier does what it is made for, namely amplifying, all its distortion and noise is amplified too and therefore the quality of an audio amplifier output can never be "better" than the input, so better to avoid gain where it is not needed .
What concerns power amplifiers sensivity, I guess 2V input is enough for full power output in more than 90% of applications.
In cases where the source has too less of current driving the load and the necessary attenuator, a buffer can be used instead of an amplifier. The attenuator also could be placed inbetween two buffers in order to make this configuration more flexible in using it from different signal sources.
Where some gain is unavoidable it can be limited to much less value than that of preamps in the common sense.
For example I use a combination of the two foregoing variations in a combined active crossover/ "pre"-amp: a buffer before the attenuator and thereafter an amplifier with a gain of about x1,5 only. At the crossover outputs there is another x1,5 amplification for an overall voltage gain of slightly more than only x2 (or 6dB). That`s more than sufficient to drive the power amplifiers. More voltage gain actually is rarely needed.
And:
a.) not to use preamps in the common sense does not mean automatically to do without their "practical" features as input-source switching (at least not for us DIY`ers).
b.)for the vinyl lovers - a phono cartridge preamp is better to be made by its own and placed outside a preamp anyway - that`s no requirement for using a line preamp in the common sense.
On the technical side, I think it is handy for an amp to take a 20Vrms balanced input
??? I wouldn`t like to hear what Your speakers will say to this.
Less gain in the amp means less transmission noise gets amplified.
The power amplifiers voltage gain (and to some extend therefore its noise) is not any lower because You feed its input with a higher voltage from a preamp.
Fact is that almost any preamplifier in reality overall does not at all amplify the signal , but rather attenuate it and the power amp usually overtakes the main job (if not the hole) in voltage ("pre-")amplification as well. An advantage regarding the noise issue (and maybe others too -I´m not an amp-designer) could be true if power amps were designed for current amplification only and the preamp would do what could be supposed it should do (deliver the voltage gain). Obviously this appears not to be the case in most common gear and therefore I wonder for what at all
a preamp in the classical sense is good for (besides being handy switching inputs and regarding profit of the hifi business)!?
 
Cocolino,

If you have never read it, it may help for you to understand what I meant to read the paper written by Russel O. Hamm:

http://www.dwfearn.com/tvst1.htm

There is some enhancement of sound quality when certain types of distortion are introduced into the music. As a matter of fact, many of my own customers have said (including the gentleman whose site has the article) that the edginess has been taken away from CDs. The proof is in the pudding.

The volume control ahead of the preamp accounts for much, also. So one may not necessarily have a problem if the control is set right.

Also, many high end amplifiers are not necessarily made for gain. They are made to amplify power. Power is added not necessarily by amplitude amplification but also by current addition.

I have seen both commercially available amplifiers and DIY schematics where the amp needs up to 30 volts to put anything out, making a preamp a must. This is because it takes the 30 volts and adds current to it so that the net output is higher power. In fact I made an amp using tubes (a prototype to my 300B amp) that had a gain of about 3, but the sound was very good and the linearity virtually boundless. From about 1-100kHz. Normally it would have been 40-20kHz. I was experimenting with different forms of NFB. I am saving that one for another project.

The above amplifier might also be known as a buffer or a unity gain amp. In digital electronics buffers are used where long data lines or multiple connections for data flow are necessary. You yourself mention having them in an active crossover and attenuator.

Also, most amps I have seen have not much more than a voltage gain of 10 (20dB). If one has a high power amp, that is not close to full output.

Of course, I don't know or have seen everything.

Try it either way. Whichever sounds and works best for you, do it that way! ;)

Gabe
 
As absurd as the use of preamp might seem, I think that the best way is to compare the sound of the system with it and without it. Although in my reference system none of the preamps ever impressed me, on my other system in a basement I noticed that a preamp really improved the sound. It was BOZ with SOZ and Marantz CD 94 transport with Technics X-1000 DAC (CAD$8,000). Since SOZ doesn't have much gain I thought that connecting output of a DAC directly to SOZ would sound better (without volume control). With a preamp the sound was however more dynamic and 3-dimentional and overall better. I was really surprised.

Another thought on phono preamp. Probably the best way would be to build it as a module which plugs directly to the output jacks of your turntable. With SS designes it would not be hard to make it really small. By plugging it directly one might avoid running extra sets of cables which are not really recommended here with such low signals.
 
It's your thing, do what you wanna do.

I enjoy the purist approach when I don't need the gain. When I listen to my tube amp though I have to use a preamp due to the 1.7 input volt requirement of my tube amp. I could use a passive with it but will only get about 2 watts out of it instead of 10 to 15 with the preamp. Passive, IMHO is cleaner, and truer to my source. What drives the argument is we all hear differently as well as listen to different types of music. It will eventually be up to YOU to decide what you like best and then, the arguments and discussions on the subject will be moot. One things for sure, we all agree to disagree. Ain't it fun:D
 
HPotter said:
Another thought on phono preamp. Probably the best way would be to build it as a module which plugs directly to the output jacks of your turntable. With SS designes it would not be hard to make it really small. By plugging it directly one might avoid running extra sets of cables which are not really recommended here with such low signals.

I have a buddy who has taken this to extremes with the basic elements of his FET based phono-pre sitting on the tone-arm (his own DIY) inches from the cartridge.

dave
 
Since EternaLightWith asked our opinions, I'll give mine, using no preamp doesn't suck. I won't say it's better then using a preamp, because it really depends on the rest of the system. But very often a passive-pre sounds good. Try it and see if YOU like it (who cares if others do), it really don't cost much. If you've already built yourself a preamp, just add a switch after the pot to bypasse the active circuitry (I've done that). Flick the switch, preamp, flick it again, passive volume control. Best way to compare, I find.
 
Gabevee
I have read this article a while ago by chance and I can understand the conclusions about advantages of tube gear (in particular in recording application) versus SS in overdrive condition. The soft-clipping behavier of tubes in opposite to hard clipping SS and the sonic advantage in this condition is almost common sense meanwhile, I believe. That a Mic needs a preamp in recording is mandatory is not in question also. But I cannot see what this can have in common with the basic question here of neccesity of preamps in Hifi systems and their operation in normal and "linear" (not overdriven) operation. Anyway thanks for the link!!
Of course some power amps will ask for voltage preamplification but I do not know a single commercial product demanding 30V input (but of course me too don't know or have seen everything) but yes some DIY circuits do (power followers). Those as well as high-power amps needing more than a couple of volts for full power ouput are in absolute minority and my previous guess that 2V output of a signal source will do the job in 90% should be pretty close to reality.

Originally posted by HPotter
With a preamp the sound was however more dynamic and 3-dimentional and overall better. I was really surprised.
Are You sure the improvement came from the additional voltage gain of the preamp and not because of eventually better current drive ability of the pre?
Maybe a simple buffer stage (no voltage gain) with better current handling than the DAC`s output could have had the same (or even better) effect?
 
A comment on that tube article

Cocolino, took the words out of my mouth. Beat me to it you did :D

Question though, the article came at it from a recording studio angle. It seemed to say that tube amps and pres are superior when used for guitar amps and mic preamps.

I don't see how this carries over to HiFi?

In my mind the only way you could justify a preamp would be if you wanted the particular sound coloration that tube bring.
But could you not achieve that goal in a tube power amp as well?

Also, another reason to invest in a preamp might be to increase the the voltage (gain) of the signal, so that you would not have to cmpromise on other parameters in the design of the power amp. ie. distortion, impendance, etc.

Another thing I find a little superfluous is that talk about overdriving. I mean the idea is to have plenty of headroom so that this doesn't happen, along with high efficiency speakers, why would you ever go into distortion in the first place. The only time I could see of that happening is on a guitar amp, for obvious reasons.

If I had my way, I'd probably go with a tube pre, for input selection and tube sound and have an solid state power amp.

But then again I heard that this was not usually done because for impendance differences between tubes and SS, amoung other things. Morgan Jones describes mixing the two together like trying to race on a bike over ice. ;)

So what you think?

Eternalightwithin
 
I'll just clarify what I said earlier. If your amp can swallow a 20Vrms input signal, it doesn't need a voltage gain stage. Suddenly your amps are reduced to the simple duty of directing the electrons from the wall into the speaker. A current follower. Simple.

To get from a 2Vrms line signal on the source equipment to audible sound requires voltage and current gain. I simply prefer to let the preamp deal with the voltage gain. I don't think that is a crime. And I stand by my belief that high voltage transmission to the amp is superior to low voltage transmission.

The great thing about DIY audio is that you can arrange things however you want :)