Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Solid State Amplifiers => Preamps and Effects => Topic started by: franklin01 on August 19, 2006, 10:53:48 PM

Title: what exactly is a preamp
Post by: franklin01 on August 19, 2006, 10:53:48 PM
i am completely new to this whole guitar amp thing, and i have no idea how they work, for example, what is a preamp and what does it do? ive heard of thm but never really had thm explained to me, please help me!!
thanks
Title: Re: what exactly is a preamp
Post by: Crystallas on August 19, 2006, 11:21:31 PM
Its an amp that gives you a set of options and adjustments to add to the main amp.
Title: Re: what exactly is a preamp
Post by: joecool85 on August 20, 2006, 07:33:13 AM
More specifically it is a device with an amplification stage, it boosts the signal high enough for the power amp to use it effectively.  A guitar normally only puts out a few tenths of a volt, a preamp boosts it to several volts.  Some preamps are just that one part I mentioned, others have EQ sections as well (treble, mids, bass) and even effects.
Title: Re: what exactly is a preamp
Post by: franklin01 on August 20, 2006, 10:02:49 PM
ohhh, i think i get it now, so the preamp is the part usually near the top with all this nobs, and the power amp is the actual speaker? do i have this right??
Title: Re: what exactly is a preamp
Post by: joecool85 on August 21, 2006, 08:38:52 AM
Close.  This is how it works:

Input (in our case, guitar) makes the signal, it gets processed by the preamp to make it into a usable voltage for the power amp.  The power amp amplifies the signal to speaker level, the speaker hooks directly to the power amp.

Input         [-----------PREAMP--------------] [POWERAMP]
Guitar ----> Treble | Mids | Bass | Volume | Master ----> Speaker

Does that help?
Title: Re: what exactly is a preamp
Post by: franklin01 on August 21, 2006, 09:46:05 AM
ok so i get the preamp part, but please bear with me because now im having problems with the power amp part! so the power amp would be the part that has control over the speaker?(volume and such??)
Title: Re: what exactly is a preamp
Post by: joecool85 on August 21, 2006, 09:53:45 AM
Sort of.  You can control the volume several ways.  The "master" controls how much volume is going into the power amp, the more that goes into the power amp, the louder it will be in the speaker.  You can also cut the preamp (which means less signal to the power amp) and that does basically the same thing.
Title: Re: what exactly is a preamp
Post by: franklin01 on August 21, 2006, 09:55:00 AM
ok, so the power amp would be an actual cicuit board similar to the preamp right??
Title: Re: what exactly is a preamp
Post by: joecool85 on August 21, 2006, 10:01:46 AM
Correct.
Title: Re: what exactly is a preamp
Post by: franklin01 on August 21, 2006, 10:05:17 AM
ok so let me see if i have this right... in this schematic i found, , the power supply would be the bottom part, the preamp is the middle part and the power amp is the top part. do i have this right?? http://www.mif.pg.gda.pl/homepages/tom/files/cube60.gif
Title: Re: what exactly is a preamp
Post by: teemuk on August 21, 2006, 11:31:21 AM
Preamp is the top part excluding speaker/ headphone jacks at far right. Middle part is (from left to right): Jack for normal input, reverb driver, reverb tank output signal amplifying stage and everything right from "master volume" potentiometer is the power amplifier.
Title: Re: what exactly is a preamp
Post by: d4v1d5hu13r on May 25, 2008, 09:26:04 PM
Just to add to what he said, that's how Overdrive works. Well, in tube amps that is. If you have your preamp and master at say 5, and it's clean, and you pump that preamp to 10, it will 9.99/10 times clip. Thus you get anything from a light blues breakup to a heavy metal sound, depending on how many gain stages the preamp has.

Easiest way to explain tubes too it that they 'recreate the sound'. Like they take the sound and amplify it with rich harmonics that most SS amps can't get.
Title: Re: what exactly is a preamp
Post by: Jack1962 on July 02, 2008, 05:14:43 AM
Basically most amps are built like this:

Input-------(PREAMP(Gain,Volume and Tone controls live here))-----Driver(this stage either split the phase for push_pull power amps or just boost he current for power amps)-----POWER AMP-----the Speaks



                                           Rock On
Title: Re: what exactly is a preamp
Post by: katieburton12 on August 30, 2010, 05:20:52 AM
it takes a weak signal such as a microphone or guitar and brings it up to a line level signal. they can also include EQ and gain controls. then that signal goes into a poweramp that amplifies the signal for however loud it was designed for.
This should be in the guitar gear and accessories forum, unless you wanna build a preamp.
Title: Re: what exactly is a preamp
Post by: Guitarist on September 27, 2010, 07:09:30 PM
You have three basic types of amplifier. Where you draw the line is a little arbitrary but this is the idea. You have:
Pre amp
IPA Intermediate Power Amplifier
PA Power Amplifier
With audio, IPAs are not necessary. Neither is power transfer. With modern electronics, for audio we are concerned only with Voltage transfer. Hence, matching impedance is totally unnecessary unless you are making a telephone system with miles of wire.
So, given that we are talking guitar amps, the Pre amp purpose is to raise the voltage of the source (in this case a guitar pickup) to a leval useful to the Power Amp.  We have then raised the Voltage but have not yet given the ability to provide current at the voltage to develop useful power. It is easier and cheaper to add equalization and effects at low power so manufactures alway put equalization and effects in the preamp. It could be added in the power amp but would result in unecessary expense. Now that we have adequate voltage and all our equalizing and effects, we need power. So, we feed the output of the Pre Amp to a Power Amp. The Power Amp usually does not increase voltage but at unity voltage gain (ie no increase in voltage) adds the ability to supply current sufficient to drive a speaker (Typically 4 or 8 Ohm) If you try to drive a speaker with the output of a Pre Amp, the Pre Amp will be unable to supply the current to maintain the Voltage and hence the Voltage will drop and little or no sound will come from the speaker. The final Pre Amp stage may even overheat and destroy itself. This is why we have both Pre and Power Amps. Intermediate Power Amps are typically used in Radio where not only are we transfering voltage but indeed power.
Title: Re: what exactly is a preamp
Post by: Jack1962 on June 04, 2011, 07:30:24 AM
I have to disagree with Guitarist , Impedance is a concern , if they don't relative match you have loading problems which effect tone .

                                                  Rock On
Title: Re: what exactly is a preamp
Post by: Roly on July 09, 2012, 09:27:12 PM
Impedance matching is a rather vexed question - it depends a great deal on the situation.

Strictly speaking we match the impedance of the load to the source to achieve maximum power transfer.

In the case of valve (tube) amps the high output impedance of the power amplifier valves is matched to the much lower speaker impedance using a transformer, and in that case something close to an impedance match is sought.

With the line voltage level connection between preamp and main amp it is voltage not power that we are trying to transfer and therefore no attempt is made to match impedances.  The output impedance of a preamp will be low, hundreds of ohms to ohms, and the input impedance of the main or power amp will be very much higher, 10's of k ohms up.

These two impedances are effectively in series and divide the actual output voltage source, so making the ratio between them very high drops the least voltage.

Similarly where the output of a solid state amp drives a speaker the output impedance of the power amplifier will be a small fraction of an ohm while the impedance of the speaker(s) will be typically 8 ohms (but maybe 16 down to 4).

If we tried to actually match the impedance of the solid-state output stage for maximum power transfer, half the power would be dissipated (uselessly) in the output stage electronics as heat, and the other half in the speaker(s).  Typically this would also grossly exceed the current capacity of the output stage and destroy it.

Impedance mismatch is actually used all the time to limit the available power, and no attempt is make to match the impedance of your toaster or electric jug to the power station because, if it were, you would get half the power station capacity appearing in your kitchen - both spectacular and unhealthy.

So these appliances are constructed with a deliberate and gross impedance mismatch to the power station so that they only dissipate a few hundred watts of the megawatts the power station is churning out.

In radio systems, such as CB antennas, the object is maximum power transfer, voltage and current, so the antenna is matched to the impedance of the transmitter output stage.

@Onewatts - this is another case again, where the multi-effect is used mainly for signal conditioning, and PA system inputs actually have much greater voltage sensitivity than your typical guitar amp input.

You would consider this a preamp in the guitar amp sense of the term of lifting signal voltage level if it only fed a power output stage, say via Fx Return, but very generally most Fx units won't produce the sort of voltages required by most main amps for full output (e.g. 100mV v 1 volt).