effects loop idea

[daz]

This is either a stupid idea or one i am stupid for NOT doing when i installed a passive insert point for a loop. Typical cathode driven tone stack into master volume ala jcm800 style circuit. What i did is install it by taking the master's output and inserted the loop between the master and PI. It just hit me how stupid that is if instead it was possible to take the signal from the treble pot's middle lug and the return would be the masters input lug. This way i could set the effects processors input level to the proper signal and it wouldn't change as i turn the master up or down which has been a problem as it is now. Might even help the issue of my DSP not putting out enough level. So i'm definately stupid one way or the other, but help me determine whether i'm stupid because i didn't do it like that in the first place, or because it's a stupid idea. Or in simpler terms, is there any reason putting it between the treble output and the master wouldn't work?

[Enzo]

First, FIRST, have you put a reasonable signal level into the amp and determined the signal level at any of hte points you are considering for your loop? If you have say 30v of signal entering teh PI, then it makes little sense to send that out to some rack FX. And likewise, the returning ONE volt of signal would barely nudge the PI. How much signal is at that treble control?

Ever notice in real amps that the loop send is often from a cathode follower after a signal drop, and the return has a recovery gain stage?

[Guitarist]

Better äter the Vol pot and that can work well going out. You could also add another resistor somehow for an semi-isolated send/slave out at whatever lower level you like.

For recovery this schematic approach is hard to beat. There are other combo-tubes like this too.

[daz]

First, FIRST, have you put a reasonable signal level into the amp and determined the signal level at any of hte points you are considering for your loop? If you have say 30v of signal entering teh PI, then it makes little sense to send that out to some rack FX. And likewise, the returning ONE volt of signal would barely nudge the PI. How much signal is at that treble control?

Ever notice in real amps that the loop send is often from a cathode follower after a signal drop, and the return has a recovery gain stage?

The DSP has a lot of leeway in both directions in and out with levels for both and a +4/-20dB switch. So i don't think the signal before the master will be too low. But i was wondering what other issues may be a problem in thaat location. I can't think of any reason it wouldn't be fine aside from levels which as i said should be good with the DSP's flexibility. But i thought i'd ask b4 i try it.

[Enzo]

Compare signal levels at +4db and then again at 30v of signal. Not even close. That is why I suggested FINDING OUT.

[daz]

I just went ahead and tried it. The results were not spectacular as far as tone. It didn't sound good. What i find odd tho, and i suppose theory will explain it, is that the signal going into the DSP was very hot and i had to turn the input level very low yet even with the DSP's out level all the way up (200%) the output was very low. Couldn't even reach small stage level with the master on 10. Yet before the feed was a lot lower level than now. Is there a better way than this or how i had it b4 (between master and PI) for passive? I don't want to go active for several reasons so if not i'll just use a second amp as effects return, but if this is possible it would be preferable.

[eschertron]

Compare signal levels at +4db and then again at 30v of signal. Not even close. That is why I suggested FINDING OUT.

What i find odd tho, and i suppose theory will explain it, is that the signal going into the DSP was very hot and i had to turn the input level very low yet even with the DSP's out level all the way up (200%) the output was very low..

Enzo strikes again!

[daz]

Enzo strikes again!

Yeah, and i think he loves making me look stupid, but thats ok because 1-i am, and 2-the guy knows this stuff inside and out, so he has carte blanche as far as i'm concerned.

I guess it;s obvious the problem is the DSP just doesn't have enough output because the input level is fully adjustable to any signal barring maybe a very very tiny one. And once set to max w/o distortion the output isn't enough.

By the way, in the schematic guitarist posted, i see no loop. What am i missing?

[eschertron]

I guess it;s obvious the problem is the DSP just doesn't have enough output because the input level is fully adjustable to any signal barring maybe a very very tiny one. And once set to max w/o distortion the output isn't enough.

I am certainly no Enzo

I looked up +4dB and it corresponds to roughly 1.6vrms, so a little less than 5vpp. If the signal feeding the MV is 30vpp, then you'll need to reduce it by 6x to feed the loop, and then increase the gain back by 6x post-loop. Got half a tube available? Or maybe a few cents worth of silicon and a small handful of components. I'd have to do some research for a recovery stage design, probably something I will do when I get far enough into a future project.

Hopefully soon, I love a challenge!

[daz]

I am certainly no Enzo

I looked up +4dB and it corresponds to roughly 1.6vrms, so a little less than 5vpp. If the signal feeding the MV is 30vpp, then you'll need to reduce it by 6x to feed the loop, and then increase the gain back by 6x post-loop. Got half a tube available? Or maybe a few cents worth of silicon and a small handful of components. I'd have to do some research for a recovery stage design, probably something I will do when I get far enough into a future project.

Hopefully soon, I love a challenge!

Yeah, i thought about that for about .01 second which was enough to say, uh uh. Don't even have the real estate for a tube and don't wanna bother with SS devices in it. Too much work/trouble. I just really don't want to bother. I'll just buy a tiny "effects return" amp. A little 20 watt modeler with effects built in and a 10" is all i need. Probably better off that way anyways because the amp's dry tone stays pure. I just figured if there was a way to insert it where it'll get me enough volume i'd try it, but past that possibility i'd rather just go the other route.

[Enzo]

daz, the last thing I want to do is make you look stupid. The thing i do want to do is make you learn something. The signal level structure in a tube amp is totally different from that in a solid state amp. That is the reason tube amps with FX loops have the output buffer and the recover stage.

There is nothing wrong with breaking the signal path at the points you suggested. And it brings up important discussions like before/after the volume control. But we still have to keep the signal levels compatible with the external equipment

[daz]

daz, the last thing I want to do is make you look stupid. The thing i do want to do is make you learn something. The signal level structure in a tube amp is totally different from that in a solid state amp. That is the reason tube amps with FX loops have the output buffer and the recover stage.

There is nothing wrong with breaking the signal path at the points you suggested. And it brings up important discussions like before/after the volume control. But we still have to keep the signal levels compatible with the external equipment

There are amps with passive loops tho aren't there? If so, how do they implement them to get around my issue?

[Enzo]

Show me an example and I will explain it.

I can look at the PV Classic 30. Not totally passive, they added a little emitter follower stage to drive the thing - but that adds no gain. So the signal level is already low leaving the tone stack. And the return jack, look where it goes, right into a gain stage before the PI.

The PV5150 looks passive at first glance, the schematic is kinda scattered. But if you look at it, the send comes off the tone stacks and post controls so is at low level there by design leaving the cathode follower at V3b. the return goes into V3a, the gain stage, to boost it back up to the PI levels.

[daz]

Show me an example and I will explain it.

I can look at the PV Classic 30. Not totally passive, they added a little emitter follower stage to drive the thing - but that adds no gain. So the signal level is already low leaving the tone stack. And the return jack, look where it goes, right into a gain stage before the PI.

The PV5150 looks passive at first glance, the schematic is kinda scattered. But if you look at it, the send comes off the tone stacks and post controls so is at low level there by design leaving the cathode follower at V3b. the return goes into V3a, the gain stage, to boost it back up to the PI levels.

I can't think of any offhand but no matter. I'm just going to use a second amp as effects return, or possibly build a small external preamp box.

[daz]

I figured out how to make it work ! I never thought of this, but then even when i did i figured theres no way it'll likely work and i still can't figure out why it does. The unit is a DSP thats made to use as either a guitar processor or a line level processor. So while it has in and out jacks on the back, it also has a input on the front for hi impedance. It's labeled "guitar hi Z". I never used it in the past aside form some recording i did with it years ago because it has some amp models built in. Other then that i always used it as a line level effects. Now it works perfect and the input is perfectly adjustable and the master doesn't make it go from nothing to nuclear within 2 numbers on the master and the output is loud as hell ! Incredible !! so tell me....why does the pre PI master work perfectly in the guitar input? Wouldn't the signal coming out of it be much too hot for the processor's guitar input?