Effect loop level

[sbirkenstock]

Hi everybody,

I build my own effects loop and my challenge is the enormous difference in peak to peak voltage at the effects loop (right before the PI).
I apply a .5V peak to peak sine wave at the amp input.
With the amp volume at about 3, I get about 3 volts peak to peak at the effects loop.
With the amp volume at 10 about 40v peak to peak.
I got an effects send potentiometer. Some amps do have that, several have a -10 dBv or a +4 dBu switch instead.
So if I take the maximum, the effect send level at amp volume 3 will be very low.

Here are my questions:
a) do you have to adjust the effects send level to your amp volume level?
Means if you change your volume (more than a bit) you have to change your effects send level as well? (Reducing your flexibility?)
So is this a challenge for all amps?

b) most reverb pedals (that´s what I want to use in the effects loop) are not very specific about the max input.
It seems save though to use -10 dBv, which is 0.8944V peak to peak max if I understand it correctly.
Would that mean that the effect send has to be reduced to that value at amp volume 10?
So the effect level can get very low?

c) I could not find any shematics for a -10 dBv effect loop. Are you aware of any?
The Marshall JCM 800 Reissue has an effects loop that can be switched between -10 dBv or a +4 dBu.
Any schematics for that?

Thanks a lot,
Stephan

[Gaz]

I think the point you are missing is the recovery stage. You do need to drop the level way down to instrument level for FX pedals, but then you need to boost it back up to where it was (this can be done with a tube gain stage or more transparently with a transistor). The loop in the 800 reissue is good (Iron Sounds copied it for their kits actually), but it would be hard to shoehorn in a tube design. I recommend Merlin Blencowes Preamp book. He has a whole section on FX Loops that would explain everything you need to know.

[sbirkenstock]

Hi Gaz,
thank you very much!
I actually do own Merlin Blencowes Preamp book, just found it again and it does include a chapter on FX Loops! Thanks a lot for that tip!
My effect loop has a tube recovery stage with volume control, as well as a cathode follower tube to feed the effect send.
Both sides have potentiometer to adjust the volume.

Is there any effect loop that will work with a reverb pedal, where I can change the amp volume from 3 to 8 without adjusting the fx send level (and likely the FX recovery volume)?

Stephan

[pdf64]

It tends to be that amps with fx loops usually have a channel gain control to set preamp signal level, and a master vol to set how loud it is.
The channel gain is then set to drive the last stage of the channel either a bit below clipping, or clipped, depending on the channel’s intended application. Hence the signal level to be passed on to an fxloop is in a more consistent and known range.

From your description it doesn’t seem that your amp has a master vol and you’re expecting a channel gain to control both preamp signal level and the amp’s volume?
And if so, what benefit an fx loop?

[sbirkenstock]

Hi pdf64,
thank you for your reply.
I did not describe my amp fully of course...
I do have a distortion channel with master and a clean channel without a master.
The volume for the clean channel is working in regards to the fx loop like the master of the distortion channel.
If you want to change the volume you hear you turn either the volume on the clean channel or the master of the distortion channel.
So same issue.

There are some other reasons why an effects loop make sense:
Most reverb pedals do sound a lot better in the effect loop than in front of the amp.
Especially if you can make them 100% wet, and you can keep the dry signal tube driven (parallel fx loop).

Well, that said, it works very nice if you do not change the volume (too much).
It seems like you have to readjust the fx loop in any amp if you change the volume too much.

Stephan

[TUBEDUDE]

In one amp, i sourced the signal for the effects loop from the second stage, before the volume control. This will keep the effects level constant. It does mean you won't get any distortion from your hot channel in the effects send. With some effects this may not be optimal. I use mainly an Afterneath or an Avalanche Run in the loop, and find the clean verb/delay sounds great on distorted tones, giving them some added clarity on top of the distortion. It can be a subtle effect that adds ambience that's not obvious. I like that sound a lot better than distorted verb. It has a sparkle that is very 3D. It also makes larger amounts of distortion more palatable for a given situation. Long discrete delays will of course be more obvious, but that's an effect in itself, distorted stab clean repeat etc.

[TUBEDUDE]

In Merlin's preamp book, fig. 12-10 shows a nice loop that ingeniously keeps the return level constant while the send level is adjusted.

[Gaz]

There are lots of good ways to do an effects loop. Even passively buffered ones (using a simple voltage divider to knock down the voltage and get a low-ish impedance) like in Cornford amps seem to do a fine job. If you have any volume control before the loop then it's acting like a send level control and you don't need an additional send control- it's redundant. In any case, set the maximum send level (with say a 200mV 1khz sine way at the input) to be 1Vrms or less. Most pedals will be fine with that, and during most playing conditions it will be less. That way if you turn everything all the way up in a moment of glory you won't overdrive the input of your pedals. Hope that makes sense.

[jjman]

I recently checked my pedals that “should” be used in a loop. My Digiverb doesn’t clip until it sees 3.8Vp-p. Boss DM-3 at 3.6Vp-p. I then recalibrated my “send” which is actually a ‘wet” on mine since it is parallel. With the tube recovery stage, the wet side can now be stronger than the dry side into the PI. I added a dry-kill switch at the max-wet setting of the send control so it can now go into series mode with the dry off and the wet very wet.