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Active & Passive Effects Loops

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Active & Passive Effects Loops

Postby Jay Menon » Sat Apr 15, 2017 1:30 pm

On the subject of effects loops, my understanding is that:

A buffered loop: allows the use of reasonably lengthy cables, whilst protecting to some extent against signal degradation. This does not however, affect the level / volume of the signal.

Effect loops may be configured to work with for instrument level effects (-10dBV) or line level (+4dBV) effects. And some loops may well be switchable between the two levels.

If I were to therefore use a boost pedal or a headphone amp in my passive loop (Ceriatone Dumble clone), I could use it with Boss type reverb / delay floor pedal?

Expert opinions deeply appreciated.
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Re: Active & Passive Effects Loops

Postby DC-Choppah » Sun Apr 16, 2017 10:18 pm

-10 dB is still line level. Instrument level is lower by 20 dB. +4 dB is pro line level.

Guitar floor pedals use instrument level. If you run a line level signal into most pedals they will be overdriven so you need a reamp DI box.

If you try to use line level gear with instrument level signals it will be noisy and thin. You need an amp that puts out a line level signal to do that.

I run line level effects - Lexicon MX200 and others in the guitar chain and have to use the special 'to power amp' line level output of my preamp to drive these line level effects. So I have two effects chains. One is instrument level the other line level.
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Re: Active & Passive Effects Loops

Postby CS70 » Wed Apr 19, 2017 7:38 am

Jay Menon wrote:On the subject of effects loops, my understanding is that:

A buffered loop: allows the use of reasonably lengthy cables, whilst protecting to some extent against signal degradation. This does not however, affect the level / volume of the signal.

Effect loops may be configured to work with for instrument level effects (-10dBV) or line level (+4dBV) effects. And some loops may well be switchable between the two levels.

If I were to therefore use a boost pedal or a headphone amp in my passive loop (Ceriatone Dumble clone), I could use it with Boss type reverb / delay floor pedal?

Expert opinions deeply appreciated.

Yes, it's a good trick to use clean boosters or any other clean preamp after an effect in the loop to bring the signal back to line level, provided the effect can work with a line level in. Some pedals do, some sound bad, a few may fry (I guess they're often designed to be attentuate line voltage in the input, but each design may be different of course). Start with zero preamp gain on the amp and get carefully up from there.

For example, my Carbon Copy works fine with the output of my Mesa amp FX loop send, but its own output is too low for the FX return so I need a clean boost if I want to use it there.

In general the easiest is to use either a rack effect or a digital pedal which often is designed to work with a variety of input and output voltages.
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Re: Active & Passive Effects Loops

Postby Wonks » Fri Apr 21, 2017 3:40 pm

Your Ceriatone FX loop is far from passive as it's taken from after the pre-amp. A passive FX loop can only really exist in the signal chain from the guitar to the amp.
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Re: Active & Passive Effects Loops

Postby Jay Menon » Sun May 14, 2017 12:30 pm

So the loop operates at line level rather then instrument level...

I need to use only one pedal (a graphic eq) in the loop and so I was looking at an MXR M108 ten-band graphic eq.

Specs:
Input impedance 470 kOhms
Output impedence 5 kOhms
Max input level: +14dBV at 1kHz
Max output level: +14dBV at 1kHz

Would that work please?
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Re: Active & Passive Effects Loops

Postby Sam Spoons » Sun May 14, 2017 8:11 pm

Jay Menon wrote:So the loop operates at line level rather then instrument level...

Not necessarily

I need to use only one pedal (a graphic eq) in the loop and so I was looking at an MXR M108 ten-band graphic eq.

Specs:
Input impedance 470 kOhms
Output impedence 5 kOhms
Max input level: +14dBV at 1kHz
Max output level: +14dBV at 1kHz

Would that work please?

I would have said so, but, given it's a guitar pedal...
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Re: Active & Passive Effects Loops

Postby CS70 » Sun May 14, 2017 9:34 pm

Jay Menon wrote:So the loop operates at line level rather then instrument level...

I need to use only one pedal (a graphic eq) in the loop and so I was looking at an MXR M108 ten-band graphic eq.

Specs:
Input impedance 470 kOhms
Output impedence 5 kOhms
Max input level: +14dBV at 1kHz
Max output level: +14dBV at 1kHz

Would that work please?

Yes, the MXR is made for going both in front or in the effect loops, so it will work.
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Re: Active & Passive Effects Loops

Postby ef37a » Mon May 15, 2017 5:03 am

FX loops on many guitar amplifiers are often poorly implemented.
They are often put at a less than optimal point in the signal route and often at odd levels and impedances all over the shop.

There really is no excuse for this these days with cheap, very high performance op amps such as the NE5532. The problem is, some people get agitated by the concept of ANY 'Silicon' in a 'pure tube' guitar amp's signal path. The fact that the 'effect' it is looping to and from could be at least ten times worse for noise and distortion than an NE cuts no ice.

One answer is of course an entirely valve powered loop and Merlin Blencowe's book 'Tube Preamps' shows several designs. Would however add considerably to the cost of an amp in production.

Re 'instrument' and 'line level' ? Yes, the former could be said to be around -30dBV but then a good many AI instrument inputs crack over even with passive guitars!

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Re: Active & Passive Effects Loops

Postby Hugh Robjohns » Mon May 15, 2017 9:17 am

ef37a wrote:Re 'instrument' and 'line level' ? Yes, the former could be said to be around -30dBV but then a good many AI instrument inputs crack over even with passive guitars!

I think that's pretty optimistically low, Dave. Things are generally a lot hotter in both camps these days -- nominal signal levels across the board have quite definitely risen over the last three or four decades.

When I were a lad, the typical microphone being spoken into put out something around -55dBu. Now it's not unusual to find -40dBu with many capacitor mics.

Instrument-level was a nominal -20dBu (average) with peaks around -10dBu at best -- noticeably lower than semi-pro line-level which was nominally -8dBu peaking to +2dBu.

But modern keyboards and other electronic noise-makers seem to produce something much closer to professional 'line-level' these days, and with the advances in magnet and winding technology guitar pickups have generally become a lot louder over the years too.

It's not unusual to find guitars that can hit peaks of +6dBu or more (as some owners of early Focusrite interfaces discovered to their frustration!).

At the same time, in the 1970s professional line-level British gear was a nominal 0dBu with peaks to +10dBu-ish, while Americans and pro-studios worked at the slightly higher +4dBu (with peaks around +14dBu).

Now though, because the meters on digital gear reveal the headroom space, and commercial music has the headroom deliberately stripped off anyway, it's disturbingly common to find people routinely working with average digital levels hovering around -10dBFS with peaks to 0dBFS, which gives signal levels in the analogue devices averaging around +15dBu and peaks of +24dBu. (No wonder, then, that it all sounds a bit strained and harsh... :-( ).

In short... analogue signal levels are commonly a lot hotter these days than might be expected! :D
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Re: Active & Passive Effects Loops

Postby ef37a » Mon May 15, 2017 11:06 am

Agreed Hugh but I was basing that level on 'our' test specs (and I never* had a complaint of lack of pre amp headroom!) .

-30dBV (note 'V', the guitar amp industry is more logical and less arcane!) is not fixed in stone, neg 40 and other levels are used where appropriate, just an example.

Guitars MAY have got hotter (keyboards are not my thing!) but the pedal industry is largely wedded to the infernal PP3! That means a best possible headroom of around +10dBV.
Guitarists are SO conservative and moan much about any other form of juice!

Of course the FX loop debacle is easily solved. Have a nice low output Z and two level choices, -10dBV and +4dBu, and put it in the 'right place'. Simples! Why doesn't every amp maker do that?

Re AI high Z ins? I have had many guitarists on another forum say they can overload most AIs in the £100-£250 area. IMHO they just have too much gain.

*Was one hiccup, an amp would make nasty noises on its OD channel if hit with a really hot humbucker or active guitar...AND the mains had to be close to top tol'!

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Re: Active & Passive Effects Loops

Postby DC-Choppah » Tue May 16, 2017 2:15 pm

Jay Menon wrote:So the loop operates at line level rather then instrument level...

I need to use only one pedal (a graphic eq) in the loop and so I was looking at an MXR M108 ten-band graphic eq.

Specs:
Input impedance 470 kOhms
Output impedence 5 kOhms
Max input level: +14dBV at 1kHz
Max output level: +14dBV at 1kHz

Would that work please?


No. The pedal will clip all the time (LEDs will light up).

I am assuming your FX loop is really a line level loop as you say.

A line level signal is the full output of your preamp and is meant to go to a power amp, or line level FX unit, but NOT a guitar pedal.

The input controls on the MXR will allow you to reduce the input signal coming from your loop but this is not enough attenuation and you will still clip. You will also have a thin and noisy sound because you will have messed up the gain structure of your signal chain. They tell you in the manual for the MXR to set the levels as 'high as possible' without clipping. But you will be doing just the opposite and will be trying to set the input level as low as possible. But it won't go low enough.

Instrument level and Line level effects do not mix. I have played with all these combinations (unfortunately!)

If you want to use this EQ just put it in front of your amp, not in the FX loop.

If you want to use it inside of an FX loop then you will need a buffered FX loop pedal in front of your amp.

Then you will be like me and have 2 FX loops. One at line level and one at instrument level.
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