Andrews Amp Lab Blog

Gain, Headroom, Volume and Power

Let’s start with GAIN. There are several ways to measure gain but in guitar amplifiers, when gain is discussed, we usually mean voltage gain. In this case, gain is the ratio of input voltage to output voltage. For example, if a signal of 1 volt is connected to the input of an amplifier with a gain of 25, then the output voltage would be 25 volts (if the amp has enough power to produce 25 volts). Voltage gain can also be converted to dB gain but let’s not complicate this too much. If an amp doesn’t have very much gain, then gain can also be thought of as volume. Voltage gain can often mean an increase in volume, but not always.

In reality, most guitar amplifiers have much more gain than just 25. The input signal from a guitar pickup could be as high as 1 volt (1V) and this signal is multiplied by the total gain of the amp but the amplifier’s output limits how much voltage can be produced at the output jack. For example, a guitar amplifier has a total gain of 500 but because of it’s modest output power, it can only produce 15V at the output to drive the speakers. In this case the one volt input multiplied times the gain of 500 would be 500V so what happens? The amp overdrives or clips. Clipping is when the loudest parts of the audio signal are clipped off by the limitations of any of the various amplification stages in the amp. This causes the sound of the guitar to be distorted while having increased sustain. The clipping can occur in the preamp, phase inverter, power amp or in all three areas. All guitar amps have a volume control that adjusts the gain of the amp. Many amps also have additional gain, volume or master volume controls that allow adjustment of the gain in each section of the amp. These additional controls can allow the player to introduce clipping tones in various amp stages in order to get more variations of tone, touch sensitivity, sustain and volume.

Now we’ll move on to POWER. Power is the amount of signal available to drive the speakers. Output power is measured in watts. Tube type guitar amps can have power output ranging from less than one watt to more than 200 watts but most range from 5 watts to 100 watts. Amps are traditionally rated at maximum power before the output tubes clip (clean power). However, most tube amps can produce quite a few more watts beyond their rated output power if driven hard enough to allow the output tubes to clip. For example, the Andrews A-22 produces about 22 watts before clipping and more than 40 watts when driven to full clipping. The maximum available volume is proportional to the maximum available power (given identical speakers). However the relationship is not linear. We’ll discuss that in another article.

Speaking of VOLUME, this is where gain and power come together. As stated above, more power equals more volume. However, some amps of the same wattage ratings seem louder than others. Why is this? Often it is because of speaker efficiency but that is for another article too. For now, let’s assume the speakers are the same. There are a couple of reasons that one amp might sound louder than another. One reason is that the louder-sounding amp might produce more of the frequencies to which the human ear is more sensitive. The other reason is that the louder amp might have more gain. High gain amps can sound louder for various reasons. First of all, the amp will need enough gain to drive the power tubes to full clipping in order to achieve maxium output power. A very low gain amp might not be able to do so without some kind of signal booster between the guitar and the amp. Also, higher gain amps have more apparent volume. What I mean is that a higher gain amp will achieve maximum volume at lower settings of the volume or gain control. If it has the same output power, it really doesn’t get any louder, it just seems louder because of the extra sensitivity to the guitar and volume control settings. Even lightly picked or hammered notes come through at full volume.

Now let’s discuss HEADROOM. This one is a little more difficult. This is a term that’s almost always misused by guitar players. Customers often ask if I can give their amp more “clean headroom”. Headroom is actually defined as (I’ll paraphrase) the ability of an amp to produce transient or instantaneous volume beyond the nominal output capability. When customers ask for more headroom, what they usually want is more clean volume, but they often don’t realize it. Actual headroom can be increased by replacing transformers with upgrades that incorporate more metal, adding a choke, converting to solid state rectifiers, or adding more filtering to the power supply. However, these changes will not produce much increase in clean volume, just actual headroom, and it will barely be noticeable to most guitarist. Adding more clean volume can be achieved by increasing the output power of the amp, changing to more efficient speakers or reducing distortion in the amp. When I explain this to customers, they often say, “I don’t want more volume, just more clean headroom”. The only way to achieve that goal is to reduce distortion in the amp without increasing power. One place to start is by substituting preamp tubes with lower gain tubes. By using lower gain tubes, the overall gain will be reduced and distortion MAY also be reduced slightly at the same time. If that doesn’t give the desired results, short of completely re-designing the amp, modifications to reduce gain can be tried. The results might or might not be satisfactory because some of the rich harmonics of the amp may be lost in the process. Reducing gain in an amp by any means will most likely only change the range of the volume and gain controls and rarely give you andy real boost in clean volume.  An upgrade to a more effecient speaker can usually provide the most improvement in clean volume.  Most of the time, if tube or speaker substitutions don’t achieve the desired result, (assuming your amp is working properly) it’s time to start shopping for a new cleaner-sounding, more powerful amp. 

 Preamp Tube Substitution - It is generally ok to substitute any tube number that begins with “12″ and ends with “7″ with any other that begins with “12″ and ends with “7″. The exception being the reverb driver which should not be substituted. I prefer the sound of 12AX7s and 12AU7s in preamps. Compared to 12AX7, a 12AU7 will have much less gain and a sweet clean tone with more definition. The 12AX7 has more gain and a softer sound that overdrives more smoothly. 12AT7s sound fine for reverb drivers and phase inverters but not so great for any other preamp positions in my opinion. 12AY7 sounds good if you can find a good one but I really don’t care much for any of the ones being made these days. The 5751 and a few other industrial types can also be used but be sure to check substitution tables before using. 

I’ve come to realize that there is a lot of confusion and misinformation floating around regarding effects loops.  First, what is an effects loop?  Generally speaking, an effects loop (aka FX loop) is a set of connections on an amp that allows the user to connect effect units such as reverb, delay, vibrato, chorus etc. after the preamp and before the power amp.

There have been countless on-line discussions and articles written on how and when to use an FX loop, what kind of effects should be connected to them and so on.  The purpose of this article is to explore the different types of loops and the advantages and disadvantages of each type.

The most basic loop is achieved simply by disconnecting the internal connection between the preamp and power amp and installing jacks on the amp so that effect units can be connected to the signals.  This type of loop is still being used in many amps today because of the ease and low cost to install it.  However, it often provides very disappointing results for a couple of reasons.  #1 A typical power amp needs around 7 volts RMS to drive the power amp (phase inverter) to full power.  That means your effects units need to be able to provide that much signal to drive the amp since the effect’s output now provides the signal to drive the amp.  Most effects run on 9 volts DC and are only capable of outputting around 3 volts RMS (9 volts peak to peak).  Also the effects unit may easily overdriven by the large signal coming out of the amps preamp circuit.  #2 - The impedance of the internal circuits and the effects are usually not well matched.  That means there will be additional looses when long cables are used.  This is clearly the worst type of effects loop.

A better type of loop is one that is buffered.  Buffering means the loop circuit can correct for mismatched impedances will and not create incorrect loads on the circuits of the amp or the effects.  In addition to buffering, signal cutting and boosting can be added.  That means that the loop circuit can reduce the level of the signal coming from the preamp down to a proper level  so the the effect unit is not overloaded and then boost the low signal coming from the effect back up to the proper level for the power amp if necessary.  The best loops have a switch or variable control to accomodate both low level (stomp boxes) and high level (rack mount) effects.  Keep in mind however that some 9 volt units will still be overloaded when playing at high volume.  Even with a buffered loop, best results are achieved with effects units that run on supplies higher than 9 volts.

There are two main types of buffered loops that can be used in tube amps.  They are tube driven loops and solid state loops.  Let’s look at the advantages and disadvantages of each type.

Tube Driven Loops
Advantages
1) Maintains a pure tube signal path for all signals in the the amp?  Remember however that when a loop is used, you will be connecting effects that are solid state devices so you really won’t have a pure tube signal at all.  Most effects that would be connected to your loop have many solid state gain and buffer stages your signal will pass through.  Adding just two more clean solid buffer stages will not change the fact that your effects signal path is already solid state.  Therefore, there is no real benefit to having a tube buffer over a solid state one.

Disadvantages
1) Will cause more noise such as hiss and hum.
2) Will alter the tone somewhat.
3) Will take up space necessitating a larger amp chassis
4) Higher cost
5) More heat

Solid State Loops
Advantages
1) Lower noise
2) Less distortion
3) Can be made extremely “transparent”
4) Smaller footprint
5) Lower cost

Disadvantages
1) No longer a pure tube signal path? See above.  A switch can be added to allow the loop to be completely disconnected from the circuit when not needed.  That switch also allows an easy A/B comparison of the affect the loop circuit has on the tone and noise of the amp. 

I highly recommend buffered solid state effects loops with a level switch and a bypass switch for any application.

Jeff Andrews