why are we calling them "preamps"?

[wes480]

Maybe I am totally off the mark here, I am sure I am. But..my understanding of a "preamp" (in either recording..or playback i guess..most of my experience is in playback) is something that "colors" or changes the sound in it's own way. Like, "preamp" might give a sound that certain characteristic....and act as an interface between two different types of inputs....EQ...stuff like that.

In terms of actually boosting up the signal...thats the amplifier, no? So wouldn't mic preamps more correctly be called amplifiers?

Now, i am not saying there is no "preamp" phase to it...there most certainly is...and I agree that "mic preamp" in essence makes more sense..and sounds better. but, I guess technically what we always call a preamp is like a preamp/amp?

-Wes

 

[Mark H.]

Wes,

I think you've already got this, but the preamp brings a very faint signal from the microphone (or turntable) up to line level, right? Only when dealing with vinyl was it necessary to introduce an equalization curve to bring the groove signal back up to something resembling the original sound.

If you run a line-level signal into a pair of speakers, you're generally not going to hear much (exceptions in a moment). So you put the line-level signal through an amp to drive your speakers. Hence, preamp-amp-speakers.

Depending on your design philosophy, an ideal preamp adds no coloration at all (ditto the amp). A "processor," on the other hand, can be a preamp that also intentionally affects the sound (compression, etc.).

Exception in playback: many CD players and FM tuners put out enough line-level voltage to run an amp directly without preamplification. Thus, "line level preamps" are usually passive devices that actually attenuate the voltage of CD players and tuners. It is in this way that the word "preamp" is misused: a passive line-level control does not amplify the signal at all.

I hope this helps, and that I haven't missed something deeper in your question.

Best wishes,

Mark H.

 

[wes480]

no I think you hit it on the head Mark.

So, preamp brings it to line-level, as a general rule...and then amp makes it for speaker use.

So, in a home stereo reciever.....where you have a preamp and an amp - is the preamp still bringing it up to line level? and then the amp pushing it how much you want it it to?

I wasn't aware that a "preamp" by pure definition actually increased the sound level...and that is true with playback as well as recording?

thanks for the info.

 

[Mark H.]

Wes,

Yes, correct, especially true with playback.

In an "integrated amp" (preamp + amp in one box) or a "receiver" (tuner + preamp + amp in one box), the preamp section is actively involved in increasing the signal. The same is true of separates (separate components). (If it has "phono" inputs, then it also has the RIAA equalization curve correction).

In some large integrated amps and receivers, there are actually "preamp out" and "amplifier in" terminals that have to be bridged for the receiver to work. Sometimes this is done with a piece of heavy copper wire, sometimes with an 8" pair of interconnects.

The volume control increases the gain at the preamp stage. Amps generally don't have any gain control, just inputs and speaker terminals. It's the preamp that boosts (or at lower listening levels, attenuates) the line level signal.

Since I only had CD playback capability until fairly recently, I used to use passive stepped attenuators in lieu of any preamp stage for playback (I still do in my second system). However, after a couple of years, I found "straight in" not to be dynamic enough for my listening tastes, so I bought an active, tubed preamp gain stage, and I found it more enjoyable than the passive.

With microphones you're talking about a tiny signal, so the *quality* of microphone preamplification plays a potentially *enormous* role in the quality of your recording, because of how much it has to increase the signal.

Does this help? I hope I've made sense.

Best wishes,

Mark H.

 

[Harvey Gerst]

Back in the early days of "high fidelity" (late 40s, early 50s), you had separate pieces of gear for each source (record player, tape machine, tuner, aux, etc. Most wouldn't put out the 1 volt usually nneded to drive a power amp, and you needed some kind of control center to switch between all the sources, add special eq curves in the case of records (each record company had their own curve), and provide some simple eq to compensate for room or speaker deficiencies.

These were called "preamps" or "control centers". When integrated amps came along, we simply called the part with all the inputs and controls the "preamp section".

During the same period, adding more controls to channels became popular, and since the mics needed additional amplification to be useful, the whole mess became known as a "preamp", even though it usually only applies to the section that brings the signal up to usable levels, today, it's not unusual to include pads, phantom power, and polarity switches as part of a preamp.

 

[Mark H.]

Harvey,

You remember the old HH Scott receivers with at least three or four equalization curves to choose from?

Len Feldman writes, "One of the pre-amps that I helped to design had a total of 36 possible playback equalization settings, six for the bass end and six for the treble."

What a hoot! I toss off the RIAA reference as though none other ever existed, but I was just trying to keep it simple.

Mark H.

 

[wes480]

yeah mark...makes a lot of sense. just wondering about one thing:

"The volume control increases the gain at the preamp stage. Amps generally don't have any gain control, just inputs and speaker terminals. It's the preamp that boosts (or at lower listening levels, attenuates) the line level signal. "

Yeah...volume control on the preamp...but, I thought it somehow just "controlled" the amplifier...told it how much to amplify the signal by? I mean...whats the point of a power amp if the preamplifier is alreadying boosting the signal to whatever the listener wants?

Obviously - not the case, sense we have power amps - I just don't *quite* get it.

 

[tubedude]

It is true, though, that many people buy preamps specifically for the color it imparts on the sound, ie API, Neve, etc... they are as important for this aspect as any other, IMHO...

 

[Harvey Gerst]

The knobs on the front of most power amps are simple attenuators, designed to only lower levels, if the incoming signal is too loud. Most power amps in the old days didn't have any controls on them, except for power on/off. All control of levels was done from the preamp.

As high powered hifi amps became available, people started using them for PA. They asked for volume controls on the power amps to keep levels from getting too loud, and the ability to disconnect inputs while the power was still on, by turning down the input level controls.

 

[Mark H.]

Wes,

You *could* put the volume control on the amp (I think Parasound tried that early on to see if they could increase sales by eliminating the need for a preamp if the buyer only had one source device, such as a CD player).

As Harvey pointed out, though, most of us have multiple source devices, so it makes more sense to put the switching/routing capability together with the initial gain stage, so that the amp only ever sees one input: whatever is coming out of the preamp.

Another way to look at it is discussed on the Wadia website in the section on their digital volume control. That may make most sense of all to you. In brief, they attenuate the output signal by *reducing the number of bits* (aaaack!). They say you won't hear the loss, because they start with more than 16, but even they acknowledge that past a certain point, you're losing real, musical information. All the while, the amp is just looking at a signal (a couple of volts or less) coming from the CD player (or preamp). Obviously, it would be *better* to run the *full* ouput of the Wadia into a top-quality preamp rather than have it attenuate the signal digitally.

Ahhh, I just realized it all goes back to tubes, and Harvey will have to help me here. They used to use nice, small, inexpensive tubes for preamplification. In the "amp" section, they used huge, VERY expensive "output" tubes for amplification (to drive the speakers). In those days, it didn't make any sense to use big KT88s (or whatever) to bring a tiny signal up to 1 volt or so. AND, you couldn't use a little 12AX7 (or whatever) to drive a speaker. So the functions were separated into "preamp" and "amp."

You see, Wes, I think your question is deeper than you know. My small mind easily accepts convention, but I'm always delighted by people such as yourself who challenge those conventions.

Best wishes,

Mark H.

 

[Slackmaster2K]

Harvey and others, I've heard numerous times that the attenuator on a power amp should be left at pretty much full on for best sound quality. Can you comment on this?

Specifically, if I buy something like a Hafler P1500, it has attenuation knobs for both the L & R channels. Is using those knobs to control volume a "bad" thing?

The reason I ask this is that I'm using a soundcard, and I of course want to leave the output levels at 0db all the time because output level is a digital function. I've always assumed that some sort of volume control would be necessary between the soundcard and power amp (a little mixer or something).

How would the attenuater knobs on a power amp be different from a fader between the line source and the power amp?

Sorry to hi-jack the thread.

Slackmaster 2000

 

[Mark H.]

Efficiency of Speakers

Wes,

OK, another thought. Speakers vary greatly in their fundamental efficiency. You'll see this expressed in speaker specs as their being able to produce a sound pressure level of XXdB at 1 meter with 1 watt input at 1 kHz (just to bring some form of standardization to it). A sealed-box, two-way reproducer (like some British box speakers) or panel speakers (like some of the Magnepans) may have an "efficiency" of only 85dB or less. At the other extreme, a horn-driven behemoth like the Klipsch corner horns, which use physical properties to increase loudness, may have an efficiency of 108dB.

The former may sound best with an amp that produces 250 watts a side with lots of headroom. The latter can be driven with tiny over-priced esoteric tube amps that clip at 13 watts a side.

Most speakers fall somewhere in between. If you're only driving headphones, for instance, then a "preamp" may be all the amplification you need. But to drive large, relatively inefficient speakers, you need more power (hence the power amplifier, or amp).

The preamp increases (or decreases) what the amp sees. The amp does all the work of driving the speakers.

I hope this isn't just muddying the water further. We're about one more posting away from my point of "hell if I know!"

Mark H.

 

[Mark H.]

Slack,

It depends on the quality of the potentiometers and what (if any) alternatives you have. That's why when building passive attenuation devices, people carefully spec the pot and put extra money there. ("Noble" is one brand name of high quality pots that is revered, for instance.)

So yes, when the volume control is turned all the way up, you are closer to eliminating the pot from the signal chain than when you turn it down. In stereo playback equipment, it's generally eliminated altogether from the amp.

"Stepped attenuators" go further by simply putting a resistor in line with the signal (instead of a potentiometer). The "stepping" is moving the contacts from one resistor to the next. If done right, it's cleaner than any potentiometer. I believe one of the reasons for the huge difference in price between the Grace Design 101 and 201 is mainly that the 101 uses a high-quality pot while the 201 uses stepped attenuators.

A fader is a potentiometer. The contact either rotates or slides along something that provides infinitely variable resistance. Most potentiometers will degrade the sound -- certainly any potentiometer built into an amp as an afterthought will -- that's why (if you can) it's best to dial it all the way up and get it out of the way of the signal.

However, in practical terms, you're going to have to reduce your output somehow, so if the cleanest way for you to do it is to use the pots on your Haf, then use them. Perhaps a purist would see if he could replace those pots with Nobles and wire them in with silver wire and silver solder. It's been done!

Mark H.

P. S. It's perilous to leap-frog posts with Harvey and have no idea what he's said before my thoughts are already written and added to the thread. I'm going to bed! If I think of anything "brilliant" (hahahaha) in the middle of the night, I'll leap out of bed and share my thoughts. Otherwise, see you tomorrow.

 

[wes480]

Mark and harvey - thanks for the input! i definately see why they are corretly called preamps now. Though what you said harvey is true...people are quick to bundle anything into that name.

So, the way i understand it is this:

Microphone -----> Preamp (coloration if any, bring up to useable line level signal...determine through volume control how much the sound needs to be amplified for listening) -----> power amp (big dumb animal, just increases it by how much the preamp said)


Now,that makes perfect sense to me. maybe i misunderstood though.
I can "imagine" how that works with say, new digital equipment (like B&K separates) say...the preamp through software tells the amp "how much to increase" it by. My question is with older equipment (or hell, really newer to)...your power amp is just connected to the preamp via RCA cable generally? So how is the preamp communicating the necessary volume to the power amp? I don't see how a signal over an RCA cable can do that.

And...the preamp *only* brings something up to line level...not the rest of the way..right, so...yeah, I don't get how the power amp knows what to do with it.
i guess thats more of an EE question....but, something I'd like to know.

Can you guys shed some light on that?

 

[Harvey Gerst]

It's all about "voltage" in the analog world. Most power amps will put out full power with about 1.5 volts of AC signal going into it. Meaning it will be as loud as possible, just before its clipping point. Zero volts in means no output from the amplifier. As the voltage from the preamp increases, the volume goes up, till the preamp puts out about 1.5 volts and the amp is running at full output.

The amp in turn, takes that voltage and puts out an even higher voltage to the speakers (but at a lower impedance). It may put out enough to light a 100 watt light bulb.

 

[cordura21]

recording my turntable

can I plug the turntable to my mic/line pres?
Is there a place where I can get the eq-curves so I can load them in my plugins?
BTW: where does it sit in terms of sensitivity? More, less than a dynamic?

Cheers, Andrés