Effects Loop Help

[Tbone55]

I would like to understand how an effects loop works in an amp and am looking for a schematic of an amp that has an effects loop built into it. I do know that there are serial and parallel fx loops depending on the amp and there is probably a serial/parallel version as well. Just looking to understand how the signal path flows between pre amp and power amp for effects that before the pre amp and those that go to the power amp side. Appreciate any advice or sources.

Thanks 🤗

[shooter]

Did you read through this?  might be crumbs to follow 

http://el34world.com/Forum/index.php?topic=10208.0

[Williamblake]

hth.

[Tbone55]

I did read through some of it. I'll read through some more. It just seemed more advanced than the basics I'm looking for. An active effects loop is new to me. I have a Marshall DSL20 that has an effects loop and it's the first amp I've owned that had one. I was trying to understand how the effects loop provides a clean signal to your timed based effects pedals (modulation, delay, reverb, etc) when you have your guitar signal and distortion effects going into the preamp. Isn't the signal from the send jack going to be dependent on whether or not you have a distortion effect turned on or off? Maybe I'm not looking at this correctly?

Thanks.

[shooter]

Here's my blackbox view;
take your pre signal, split out (parallel) it to effects stuff, bring it back and mix with original.
do same for series, except there is no mixing because you use the pre signal, effect it, then bring it back.
if all your effects are OFF, the original should just pass through unmolested (true bypass)

things like impedance, voltage amplitude, probably need to be considered

[2deaf]

I did read through some of it. I'll read through some more. It just seemed more advanced than the basics I'm looking for. An active effects loop is new to me. I have a Marshall DSL20 that has an effects loop and it's the first amp I've owned that had one. I was trying to understand how the effects loop provides a clean signal to your timed based effects pedals (modulation, delay, reverb, etc) when you have your guitar signal and distortion effects going into the preamp. Isn't the signal from the send jack going to be dependent on whether or not you have a distortion effect turned on or off?

Effects loops are almost always after the pre-amp and before the power amp (with the PI being part of the power amp).  Whatever goes into the pre-amp plus whatever the pre-amp does to that signal comes out of the effects send.  If you put distortion into the pre-amp input, distortion will come out of the effects send.

 

[2deaf]

things like impedance, voltage amplitude, probably need to be considered

You would think.  And yet there is a constant parade of FX Loop designs with no regard whatsoever for output impedance and output voltage.  Part of that parade passed right through this thread.

[Williamblake]

I have investigated fx loops and went with the simple solution, a breaking jack in front of the last gain stage that drives the MV pot. There is next to no difference to me if i use it or not but some effects might need a buffer driving them, for example the ditto looper when in bypass. So i put my tuner in the loop to as it is always buffering. Might not work for everybody but very simple.

[shooter]

Part of that parade passed right through this thread.

I like parades, but I don't like extra junk in my amp so I skipped it 

[pdf64]

It’s not the fx that benefit from being driven from a lowish impedance, it’s the cable (fx send to pedal). Or more specifically, the cable’s capacitance.
If the amp’s ‘send’ impedance is highish then the cable’s capacitance will roll off an excessive degree of high end.

[VMS]

I was trying to understand how the effects loop provides a clean signal to your timed based effects pedals (modulation, delay, reverb, etc) when you have your guitar signal and distortion effects going into the preamp. Isn't the signal from the send jack going to be dependent on whether or not you have a distortion effect turned on or off? Maybe I'm not looking at this correctly?

Thanks.

Yes, purpose of effects loop is not to give clean signal to time based effects. You put time based effects to loop so if you happen to have higain preamp it doesn't further more distort the repeats of your effects and turn them to mush. If that makes sense.


Hard to explain with my limited english :)

[2deaf]

I always figured a Line Out/FX Send should be able to drive a 9 metre cable with a cut-off frequency around 16KHz.  How much capacitance a 9 metre cable has varies from cable to cable, but 1nF is a pretty good average.  In order for the Line Out/FX Send to drive a capacitive load of 1nF, it needs an output impedance around 10K.  Some of the examples above have output impedances as high as 125K and some change.

A lesser problem with high output impedance is that some devices have input impedances as low as 25K and maybe even lower.  And they are frequently +4dBu types.  So now some designs create a voltage divider that is reducing the signal too much while the device wants even more signal.   

[pdf64]

Just to note that a 250k level level pot feeding the send jack has a max output impedance of only 63k 
Assuming a very low source impedance, eg cathode follower, at the pot’s midpoint, the 2 x 125k resistances are effectively in parallel.

[2deaf]

Just to note that a 250k level level pot feeding the send jack has a max output impedance of only 63k
Assuming a very low source impedance, eg cathode follower, at the pot’s midpoint, the 2 x 125k resistances are effectively in parallel.

Yeah, you're right.  But you said "level" twice, so does that bring it back to 125K?  Just kidding.

[shooter]

 

[jjasilli]

https://www.sweetwater.com/insync/use-amps-effects-loop/

{EDIT: removed tracking data from over-long URL - PRR}

[pdf64]

The Sweetwater page is good APART from there being no mention of the purpose and benefit of there being a lowish source impedance for the ‘send’ signal (ie to avoid cable capacitance muffling the tone).
Yes it mentions a buffered send, but thanks to the Dumblator kludge of a design, we see numerous ‘send’ circuits that put a buffered signal through a 250k ‘send’ level pot, thereby creating a source whose impedance varies with the ‘send’ control setting, from low to excessively high impedance.
Plus a buffer isn’t necessary to achieve a reasonably lowish ‘send’ source impedance; in many scenarios, a suitable resistive potential divider can achieve both signal level reduction and provide a output source impedance sufficiently low (eg <10k) to avoid excessive HF loss from cable capacitance.

[VMS]

So if one wants send control and low source impedance what would be best solution?


Using smaller pot?


In my forever taking project i was going to use KOC loop from the standard amp but it has 1MA pot on send. 

[pdf64]

But surely KOC’s 1M send level control is before the (cathode follower?) buffer? So the buffer will be, er, buffering that 
The issue with the Dumbleator is that the send level control is after the buffer, so the impedance of the circuit after that point is dominated by the send control pot setting. So with a 250k pot, that can be up to 63k around its midpoint. 
If driving eg 1nF’s worth of cable, it will create a low pass filter that’s 3dB down at 2k5Hz and rolling off at 12dB/octave above that, ie noticeably muffled.
http://www.learningaboutelectronics.com/Articles/Low-pass-filter-calculator.php#answer1

The crazy thing with the Dumble is that a 10k or 25k, even 50k pot could have been used after the buffer without it being a problem, as the low pass filter frequency would be much higher.
Presumably he did it like that for a specific tonal effect, but the problem is that it gets mistaken for good practice and folks just copy it

Please keep asking if the above doesn’t make sense 

[VMS]

Nope, it's after the cathode follower.
Could the reason for 1MA pot be that it's a parallel loop?

[pdf64]

Series or parallel, that seems 'less than ideal', and rather a waste of tube section for the CF buffer.
KOC is obviously a very knowledgeable chap, I'm surprised that got past QC.

I suggest that you provide your amp's schematic, that will facilitate then the most suitable fx implementations for it to be considered.

[VMS]

So much KOC stuff in that schematic that i don't want reveal all his secrets :)

[jjasilli]

See:  https://londonpower.com/electronics/tube-effects-loop/


TUT (1) Ch 7: Effects Loops is probably required reading.


Typical fx loops are 1V (0dB) signal into rack gear which is expecting lo Z, hi V.  Using guitar pedals in a typical effects loop is problematical.  They are expecting Hi Z and lo V, say 200mV = -20dB.  Using guitar pedals in an fx loop requires a send level control, and a 20dB boost for the return.  All this is covered by KOC.

[2deaf]

Presumably he did it like that for a specific tonal effect, but the problem is that it gets mistaken for good practice and folks just copy it

Ahhh.  No wonder I keep seeing a 250K send pot in the parade.
____________________________________________________

Attached is a schematic that somebody posted alleging that it is the KOC loop.  It uses a 100K send pot, which is still too large.

[Williamblake]

Sorry i posted so many distracting pictures, the last one is where i whent and i am still fine that way although i added some megs for reverb mixing to it but still don't feel the 4m cable in the loop. Maybe you can get lucky sometimes? And yes, in bypass that ditto looper needs a buffer in front of it so whatever rocks your boat.

[PRR]

> provide your amp's schematic

Yes, yes, yes!

The issue is that there are some FX which work at guitar-level, and some that work better at higher level (and often lower impedance), "line" or "studio" interface.

Guitar-level FXs can best be put before the amp. If used at line level they over-overload.

Studio FX (some marketed at guitarists) may give high hiss if not driven by line-level.

The classic guitar amps have no notion of "line level". Even if they have a "power amp insert point" the level is unclear and the impedances are often high.

Some of the simple amps don't tolerate any mucking-about-- they have just so much gain/level and impedance to "work", with no reserve for add-ins.

A particular point: several classic designs depend on "preamp tone". A late preamp stage is friven very hard to add distortion. In tube amps this will typically be far higher than any "FX level". We may have to pad-down to the insert and gain-up to match the original level structure.

So It Depends.