Passive effects loop question

[daz]

I went thru the whole loop design thing about 6 or 8 years ago and many here gave me suggestions but none worked well for my amp at least. But since that failed effort i was using SS amps and now i resurrected that old tube build and i want to try to add a simple passive loop. I have done this before in other amps many years ago and it worked ok. I have a DSP thats got level indicators and level controls for in and out. But heres the thing...it;s a typical marshall style tone stack into a pre PI master and if i put the loop after the master then changing the master setting can overdrive the DSP. So heres the question before i go ahead and trying it, and please, lets not get into active ways of implementing it because if i can't do it passively i'll just pass on it and use a second small amp as effects. I just need to know the answer to this. So, if i use the tone stack output for the send and return the effects to the master volume input, what problems will i run into? I assume it would be a impedance issue? Whatever the issue would be is there a solution? Thanks all, and again, (posted in the Enzo thread) RIP Enzo....wish you were here to reply....end of an era....

[loudthud]

How does putting the loop before the Master prevent the DSP from overloading ? It won't unless you run the Drive or Gain control so low there won't be any preamp distortion. If you place the loop after the Master Vol, at least you CAN turn the Master down so the DSP is not overdriven. If the DSP can't drive the power amp, that's a different issue. It should, but not as hard as the amp without the loop.

[pdf64]

Surely it’s simple and quick to try it both ways, to see which if either give usable results?
It’s probably going to depend on the capabilities of your particular fx unit.
A key thing to mitigate for high frequency roll off is to keep the send cable as short as feasible.

FYI I just use an echo pedal in the passive loop of my 2204 type build, I put the loop after the master volume as there’s no way the pedal would cope with the signal level between the tone stack and master vol.
It works fine at low master settings. Much above halfway the pedal gets unhappy, but with a 4x12 the rig is drowning out the rest of the band at that point anyway, so it’s not an issue. With a 1x12 medium magnet speaker, it’s a bit more marginal, as the master needs turning higher.

[daz]

How does putting the loop before the Master prevent the DSP from overloading ?

I didn't say that. What i am saying is if i put it after the tonestack i won't have to adjust the DSP input every time i decide to change the amp master setting. If it put it AFTER the amp master then every time i adjust the the amp master the DSP input will have to be adjusted or it will then overload it or if i turn down the master it will not be an optimal signal to the DSP. So my question was what issues i will run into putting it between the TS and master.

[daz]

Surely it’s simple and quick to try it both ways, to see which if either give usable results?
It’s probably going to depend on the capabilities of your particular fx unit.
A key thing to mitigate for high frequency roll off is to keep the send cable as short as feasible.

FYI I just use an echo pedal in the passive loop of my 2204 type build, I put the loop after the master volume as there’s no way the pedal would cope with the signal level between the tone stack and master vol.
It works fine at low master settings. Much above halfway the pedal gets unhappy, but with a 4x12 the rig is drowning out the rest of the band at that point anyway, so it’s not an issue. With a 1x12 medium magnet speaker, it’s a bit more marginal, as the master needs turning higher.

Thats why i want it between the TS and master. But you know theory a zillion times better then I, so since you put it after the master i assume there's a reason not to put it between the TS and master. My DSP has a +4/-20 switch plus in and out level control and lever meters for both. But what about the impedance? Will that present issues and if so is there a solution? As to lead length, send and return would be front of amp to rear directly across from each other and thats about6-7 inches. I would use shielded cable.

[daz]

By the way, amp has a cathode follower fed tone stack.

[Greg Robinson]

The biggest issue is that the signal out of the tonestack can be up to around 100Vpk, way more than your interface can handle. You need some attenuation before the loop, plus ideally some recovery gain in the return (if you want to maintain the preamps ability to overdrive the output stage), which you won't get with a passive scheme.
IMO, passive effects loops always require too many compromises to be worth it.

[daz]

The biggest issue is that the signal out of the tonestack can be up to around 100Vpk, way more than your interface can handle. You need some attenuation before the loop, plus ideally some recovery gain in the return (if you want to maintain the preamps ability to overdrive the output stage), which you won't get with a passive scheme.
IMO, passive effects loops always require too many compromises to be worth it.

Thanks, that makes sense and may have been why i abandoned that idea long ago. Guess i should take notes but in any case i have tried passive with several amps and likely with this. I can attenuate the send of course but if i DID try this years ago i likely did that and found the DSP's out level wasn't enough. But memory has faded. I do seem to recall having a problem getting enough output at some point but don't recall if it was having tried it this way. Seems like it should work, as the DSP had a out level that goes to 200%. I will just have to try it.

[pdf64]

The input impedance and the max signal level it can tolerate are the unknowns here.
Although my echo pedal has a high, instrument compatible input impedance, eg 500k or higher, it can only cope with maybe a max signal of 2Vpk, roughly 1.5Vrms.

A 200% output gain on your fx is only 6dB gain, whereas it’ll probably need more than 20dB attenuation on the signal from the tone stack.

[uneumann]

... What i am saying is if i put it after the tonestack i won't have to adjust the DSP input every time i decide to change the amp master setting. If it put it AFTER the amp master then every time i adjust the the amp master the DSP input will have to be adjusted or it will then overload it or if i turn down the master it will not be an optimal signal to the DSP. So my question was what issues i will run into putting it between the TS and master.

I think you may have a misconception here... You don't need to drive the FX unit to 0 dB for it to work properly or optimally - especially for a DSP unit. It will work fine with lower signals too. Think of 0 dB not as "optimal" but as the MAX level it works at before clipping. It's to be avoided IMO. Once you come around on that notion, you'll find it probably works best to put the FX after the master since at that point the dynamic range of the output stage is fixed. If you set the FX 0dB to be just above output stage clipping, you will operate the FX system in a safe zone no matter what the Master is set to.

[daz]

I think you may have a misconception here... You don't need to drive the FX unit to 0 dB for it to work properly or optimally - especially for a DSP unit. It will work fine with lower signals too. Think of 0 dB not as "optimal" but as the MAX level it works at before clipping. It's to be avoided IMO. Once you come around on that notion, you'll find it probably works best to put the FX after the master since at that point the dynamic range of the output stage is fixed. If you set the FX 0dB to be just above output stage clipping, you will operate the FX system in a safe zone no matter what the Master is set to.

No, i get that. But I may then not get as much volume as i would now w/o a loop if the input drops very low. I will be using it for very low home volumes to pretty loud band volumes and everything in between, and i don't want to have to gain stage the thing every time i use it at different volumes. I just want to be able to use the master. In any case, when i have time to get to this i will try both ways.

Also, while i said the DSP has a adjustable out level, thats a master control. I don't know if they mean *with the patch level maxed* or what. So i may be able to get a lot more volume maxing the patch's master level. I will only be using one patch with delay and thats it.