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Koch News blog

Effects Loops Explained by Dolf Koch – Series vs. Parallel Effects Loops

12/9/2013

4 Comments

 
Foto
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!

The History

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.

The Solution: 

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.

Foto
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.


4 Comments
Cadavre link
12/6/2014 11:29:55 am

I use a stereo mixer with a mesa TriAxis and 2:90 to attenuate the output from a chorus and a delay (both effects are analog stereo outputs to the triaxis stereo returns). The mixer is a so called analog passive mixer / splitter.The stereo main outputs on the mieer router to the A/B FX return ports on the router. The mono effect send from the triaxis routes to a separate hard wired splitter.

The idea is to avoid "hard connect" cascading effects such that each effect gets a clean signal from the Triaxis,

Used to use 3 y cables.

The chorus (ROLAND CE), goes into the red when the master volume on the Triaxis is set above 4. I have tried to use 2 channels on the mixer to attenuate the send levels to each effect, to help assure a less than red level on the chorus input level control, but am naive as how decibels translate to watts or some optimum FX level on the TriAxis.

The A/B output from the chorus and delay are mixed by channel levels and the main out level on the mixer, back into the FX return on the TriAxis.

The mixer level controls go from -20 db to +20 db.

Trying to understand what zero db means. I was entertaining the prospect that zero db meant the signal was exactly the same. The chorus return is typically set around 12 oclock and the delay about 9 oclock. Would appreciate any confirmation or defamation regarding the setup and my current understanding (or misunderstanding) on my interpretation of "db". The mixers master output is typically at 9 oclock.

Also, when switching the FX off on the TriAxis, the sound output bumps up by orders of magnitude. I had read that the soud output should be the same with in FX on as it is when the FX is off.

I like the tone - and since both effects use a mono in and a stereo out - and eliminated the y cables.

Other Configurations not testes was to shut the loop between the Triaxis output to the 2;90. There are also recording port outs on the TriAxis.

Reply
michael spanos link
6/7/2015 09:26:40 am

hi there !
i am about to buy a used koch twintone II.

i was thinking if i can use a stompbox pedal, through the effects loop, to detour the preamp, so i can use the volume knob of the clean channel as a master volume knob.

as you can see, i want to loose all the drive from the clean channel of the amp.
i googled it but couldn't find much.

thank u very much for your help !

Reply
Jim Ott
6/11/2015 08:24:42 am

I have a Twintone II and use it with my tuner and Wah pedal in front of the amp and a compressor, chorus and delay in the serial loop. I love the amp but if I use an overdrive pedal or my GT 6, the amp gets real muddy. Any suggestion on setup. I would love to keep using this amp. It has such depth and it was quite the investment. Thanks, Jim.

Reply
Sttefaun link
7/6/2015 07:36:00 am

What is referred to as a series effect loop is really called an insert point, such as on some mixers, placed just after the channel input, to go out to maybe a preamp, compressor, effects or EQ in series and come back into the same stereo in and out jack to continue on thru the channel strip.
A parallel loop really is a loop. I have one amp that will do either or both series or parallel. Most amp companies do not tell if it is series or parallel.
I use signal processing in series, and time based effects in parallel to get the cleanest, clearest sound quality.

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