Koch Guitar Electronics 2019.
Effects Loops Explained by Dolf Koch
Nowadays, an effects loop is an integral part of almost any musical amplifier. Although it just looks like it’s no more than just two jack inputs, a “Send” and a “Return”, there is so much more behind it!
Tube amps made in the 50s and 60s have no effect loops simply because there were no effect units. The only two effects available were reverb and tremolo and top of the line models had one or both effects built in. Later, when effect units became available (wah-wah, delay, chorus, flanger), they were always connected between instrument and the amplifier input. Because amps didn’t have distortion channels this worked very well. However, when the first amps with overdrive channels came on the market, it turned out that having effect units in front of an overdrive channel increased the level of hissing noise rather drastically. Besides that, chorus type effects almost dissapeared and became inaudible when the distortion level was turned up high.
In recording studios, mixing consoles had so called “Inserts” in each channel as well as in the master section. This is a stereo jack with a combined input and output which made it possible to insert an effects unit in the signal path (serial connection). If you wanted to have better control over your effects, the unit’s output could be fed back to a (free) channel input (parallel connection).
In the 70s, these Inserts became a feature on guitar amps as well and got the name: “Effects Loop” or “FX Loop”. The output was called “Send” and the input “Return” and they are always placed between the preamp and the power amp section. This is the so called serial effects loop.
It might seem simple to build an effects loop in an amp, just cut the wire that connects the preamp with the power amp and place two jack plugs instead. Yet nothing could be further from the truth, especially in tube amps. I’ll try to explain why.
Problem 1: Tubes operate on very high voltages from 300-500V. Therefore, signal levels inside a tube amp are very high.
Effect units, which are mostly battery operated, operate on 9V. Therefore, signal levels in an effects unit are very low. So if we would connect both by feeding the very high signal of the tube amp through the effects unit, it will be completely overloaded.
Problem 2: Tube stages have very high impedances while solid state stages have rather low impedances. This means that if an effects unit is connected, the signal level of the tube amp will drop drastically.
Problem 3: Most effect units in combination with the connecting cables will affect the sound of your amp, mostly because they have low quality components inside.
A good Effects Loop has to include an electronic circuit that:
1) brings the high signal level of the tube amp back to the low signal level of the effects unit which goes to the Send input on the amplifier
2) brings the low signal level of the Return jack back to the high signal level of the tube amp
3) matches the impedances of the tube amp to those of the effects unit.
4) does not affect the sound of the amp (!!!)
Such a circuit can be made with tubes, with opamps (solid state) or with a combination of both. All our amps have a combination of both, except for the Powertone-III which has an all-tube driven Effects Loop.
In my Effect Loop designs I always pay much attention to all four design criteria.With the right combination of good design and the use of quality parts our Effect Loops do not affect the sound of our amps. All our Effect Loops operate on a signal level of -10dBV which perfectly matches the signal level of all battery operated effects units. I always use a “buffer” circuit to drive the Send output. This buffer circuit brings the high impedance of the tube amp down to a very low output impedance. Therefore, our Send outputs will drive any effects unit on the market with minimum signal loss due to cable influences.
Serial vs. Parallel Effect Loops
In a Serial Loop the signal of your amp is completely routed through the effects unit, which passes the signal through while adding an effects signal to it. It is obvious that the effects unit, if it is not of the highest quality, could negatively affect your sound. The advantage of a serial loop is that you can connect several effect units (in serial) to one loop.
In a Parallel Loop the signal of your amp is NOT routed through the effects unit but it stays inside the amp. The effects signal on the parallel input (Return) is mixed by a (Level) potmeter with the signal of your amp inside the amp. Therefore, the sound of your amp can not be affected by the effects unit. The disadvantage of a parallel loop is that you can connect only one (multi) effects unit to each loop.
The Applications & Final Thoughts
In our Twintone II &III, Powertone-III and Supernova amps we offer combined effect loops with two return jacks, which can be used to connect your effects unit either serial or parallel.
Apart from your choice of effects units, all our amps have effect loops which get the maximum out of these unit(s).
We hope this article helps you with your quest to find your dream sound, please leave a comment with your thoughts or questions and be sure to check out some of our other articles.
Voltage, Current, Impedance of guitar speakers
In this article I will explain the basics of guitar speakers such as the Voltage, Current, Impedance, Series and Parallel connection of speakers.
Voltage is the height of the audio signal that an amp offers to a speaker. If you compare it with a tap water installation, voltage is the height of the water in a tank or the water pressure in the water system.
An amplifier sends this audio signal as a current to the speaker. This current flows through the speaker and pushes it to create soundwaves. In a tap water installation, the current is the water flowing from the tap.
Impedance or resistance is the matter in which the speaker resists the audio signal. In a tap water installation resistance is determined by the tap. The more you open the tap, the lower the resistance gets for the water and the higher the water current becomes, and vice versa.
So remember: higher resistance = less current and lower resistance = more current.
Series & Parallel Speaker Wiring
If you connect two speakers in series (see diagram below), both speakers will resist the audio signal more than one speaker and the total current will get less. In a tap water installation if you place a second tap behind the first tap (in series), the second tap lets only a part of the water through that comes from the first tap. So the total water current gets less. As we saw above less current is the result of a higher resistance.
So if you connect two speakers in series the total resistance or impedance becomes higher, it will increase to double the value.
On the other hand, if you connect two speakers in parallel (see diagram below or, by plugging the second speaker in the extension output of you amp or speaker box), both speakers will draw current from the amp. So the total current will increase to twice the value. In a tap water installation this is the same as using a second tap next to the first tap. Both taps together will supply twice the water current. As we saw above more current is the result of a lower resistance.
So if you connect two speakers in parallel the total resistance or impedance becomes lower, it will decrease to half the value.
Since the introduction of the first “Radiola” moving-coil loudspeakers in 1926, we use the word “impedance” to define how much a speaker loads an amplifier. The value of this impedance is specified in “ohms”. But why in ohms? Isn’t ohms used for resistors? And isn’t a speaker a totally a different thing than a resistor?
This is all true… let me try to explain.
Ohms & Frequency
A resistor has the same ohm value throughout the whole frequency area, this ohm value is called resistance (R). A speaker on the other hand has an ohm value that depends on the frequency, therefore this ohm value is called AC resistance or impedance (Z).
The picture to the left shows how the impedance of an average 4 ohm speaker varies with the frequency.
In the graph you can see that the 4 ohm (3.99) impedance of this speaker is an average (or nominal) value. The lowest value is 3.47 ohm (at 400Hz) and the highest is 12 ohm (at 20 KHz). Also, you can see that there’s a peak value of 6.5 ohms at 100Hz, this is called the resonance frequency of the speaker.
Impedance vs. Output Power
As you might know, the output power of solid state amps varies with the load resistance of the speaker. A solid state amp generates a constant ouput voltage and therfore if the impedance of the load increases the output power decreases, this happens in a lineair way. Normally, full power is given at 4 ohms, half power at 8 ohms and a quarter power at 16 ohms. This means that when a speaker loads the amp, the power which the amp generates depends on the frequency, because the impedance of the speaker varies with the frequency.
However, with a tube amp this works differently. Especially tube amps for guitar do not generate a constant output voltage. The output voltage increases when the load decreases. If a tube amp gives full power at a 4 ohm load, it might generate even 70% or more of it’s power at 8 ohms (instead of 50%). This is one of the reasons that tube amps sound different than solid state amps, they react different to the speaker’s dependence of frequency.
Tube Amps & Speaker Output
As described above solid state amps lose their output power in a lineair way: the higher the impedance, the less output power. If the speaker is disconnected, there’s no load and the amp’s output power is zero.
With tube amps this is totally different. A tube amp tries to maintain its output power and as a result of that it raises its output voltage if the impedance of the load gets higher. So if you play a tube amp and disconnect the speaker the load gets unendlessly high and the amp will raise its output voltage to very high levels (thousands of volts inside the amp).
Parts such as output tubes and output transformers are not able to withstand these high voltages which often results in severe damage to those parts. So keep this in mind if you have a tube amp; always connect a speaker or a dummy load to your tube amp, never play it without speaker or power attenuator/dummybox connected.
Our ATR series of hybrid amps the Jupiter, Startrooper and ATR4502 (see also the blog article about these amps), are designed in such a way that they react the same way to speakers as tube amps. Although they operate with mosfets to drive the speaker, their ouput power does not fall back to 50% if the impedance is doubled. This is one of the reasons why our ATR series of hybrid guitar amplifiers sound like real tube amps.
Although their speaker outputs behave similar to tube amps you can safely disconnect the speaker and play them without a speaker or load connected. If you do that you will not damage anything.