What do these Amp Effects Loop Specs Mean?

[.adamo]

Hello TGPers—

Lurk all the time but post very little (so far) -- thanks always for everything I learn and enjoy from your input on countless topics. A simple (I hope) question this time about guitar amp effects loop specs.

What do these mean? I can't find a clear answer anywhere online, about dBV specs for effects loop and my amps in particular.

Send Jack: 250 ohm, max. +6 dBV
Return Jack: 500K ohm, -3 dBV

I get the resistance/impedance part: the send jack is buffered so it's sending a low-z signal. Input impedance is high on the return jack so it doesn't load pedals.

What I don't get are these dBV measurements! Why +6 dBV (max?) on the way out but -3 dBV on the way back?

I'm building a fairly complex wet/dry/wet rig using the fx loop on the two wet amps (two H&K Tubemeister 18s) for a stereo effects loop. My loop is getting crowded with stereo effects and I want to make sure I'm not overloading the return jack or exceeding pedals' input ratings.

I have 5 devices in the loop, all of which are configured to "line level", but their max input level specs vary.

• My Meris pedals are all set to "synth/line level" listed as max input at +12.5 dBu, but can be switched to "instrument" at +9 dBu.
  
• My EHX 720 looper lists max input level at +6 dBu
• My SA Kingmaker says max input level is +6 dbV = 8.2 dBu

I guess +6 dBV = 8.2 dBu?

So far, running the Tubemeisters clean, everything seems to be working well enough! On the lead/dirty channel, sometimes I feel like I'm getting some upper register distortion with the wet effects (dly/verb) on and cranked, but it could just be the volume piling up in my practice room, reflections, room modes, etc.

Again tho, my concern is just making sure these specs line up and everything has the headroom to handle everything else. I guess what's getting me worried is the spec on the Return Jack: -3 dBV.

Thanks for your clarification on some or all of this!

 

[HotBluePlates]

... What do these mean? I can't find a clear answer anywhere online, about dBV specs ...

Send Jack: 250 ohm, max. +6 dBV
Return Jack: 500K ohm, -3 dBV
...
What I don't get are these dBV measurements! Why +6 dBV (max?) on the way out but -3 dBV on the way back?
...

Short Answer:

- Your H&K's Master Volume controls (both of them) affect the size of the signal sent out the loop to your pedals. If you think you might be overdriving your pedals, turn the Master Volume control(s) down.

- Your H&K has a make-up gain stage following the FX Return jack, and it can only handle a signal of ~0.71 volts; if you think you might be overdriving the Return, turn the output level of your pedals down.​

_______________________________________________________________

"Decibels" describe a ratio, generally of signal- or sound-level as compared to a reference level. That ratio is logarithmic, because human hearing responds to a logarithmic change of sound pressure, rather than a linear change (10 times the pressure seems roughly "twice as loud" to our ears).

There are a lot of different "reference levels" out there for use in different applications, and dBu & dBV (a "capital V") are two of them.

- "dBu" is related to "dBm" where the latter is a reference level of 1 milliwatt. For old audio circuits where the standard impedance of transmission lines connecting gear was 600Ω, 1 dBm was 1 milliwatt into a 600Ω load which implied 0.775v (because Power = Volts^2 / Impedance, and 0.775v^2 / 600Ω = 1 milliwatt).

- In cases where the power level wasn't the critical factor to watch but voltage was, dBu was used as a reference voltage of 0.775v.

- dBV simplifies things by assuming a reference level of 1 volt, regardless of the circuit's impedance.​

... I guess +6 dBV = 8.2 dBu? ...

I think you had already found much of the above info on your own. And it looks like you figured out how to calculate +6 dBV = +8.2 dBm = ~1.99v (call it "2 volts").

Or maybe you just copy/pasted from the Kingmaker's manual.

... What do these mean? I can't find a clear answer anywhere online, about dBV specs for effects loop and my amps in particular.

Send Jack: 250 ohm, max. +6 dBV
Return Jack: 500K ohm, -3 dBV

... Why +6 dBV (max?) on the way out but -3 dBV on the way back?
...
I have 5 devices in the loop, all of which are configured to "line level", but their max input level specs vary.

• My Meris pedals are all set to "synth/line level" listed as max input at +12.5 dBu, but can be switched to "instrument" at +9 dBu.
  
• My EHX 720 looper lists max input level at +6 dBu
• My SA Kingmaker says max input level is +6 dbV = 8.2 dBu

....
Again tho, my concern is just making sure these specs line up and everything has the headroom to handle everything else. I guess what's getting me worried is the spec on the Return Jack: -3 dBV. ...

Let's keep it simple. +6 dBV = ~2 volts, +12.5 dBu = ~3.27 volts, and +9dBu = ~2.18 volts, and +6 dBu = 1.55 volts. Looks like your EHX 720 looper is the limiting factor, as it has the lowest maximum input level.

The H&K offers a serial loop with no external Send/Return level controls. After some Google-Fu, the service manual shows the FX Loop Send level is dictated by the Master Volume setting. If you're driving the outboard effects too hard, turn down the Lead/Clean Master.

- The "+6dBV max" spec is not a signal level that will necessarily come out of the Send jack, but the max signal level the opamp driving the FX Send jack can throw before it starts distorting.

- There is make-up gain at the FX Return jack, because players might be using pedals in the loop that would normally be at instrument level and ahead of an amp's input jack. So H&K warms you not to apply more than "-3 dBV" which is 10^(-3/20) = ~0.71 volts.​

"Less than 1 volt back in the Return jack??!? What do I do??"​

Your effects almost certainly have some control of their output level. If you think you're overdriving/distorting the FX Return amplifier stage, turn your pedals' output down some.

 

[.adamo]

HotBluePlates, thank you for this detailed and informative answer, I really appreciate it!

I think I managed to find the same service schematic for the amp you took a look at, showing the Master controls of each channel driving the effects loop send, and the return stage with some kind of buffer, mixer and passing on to the plate of the PI (I think?) and then the power stage... I'm not at all literate in reading schematics like this, but think I see what you're pointing out.

Makes working with the effects loop a little challenging. I've definitely put some pedals in there in the past that didn't like the hot signal coming out of the send; no problem, they work fine out front. But that's why I've been placing strictly digital effects with higher headroom options in the loop.

But then, as you calculated, you run into headroom issues in the amp because H&K put make-up gain on the return jack. The manual makes it seem like the loop is for line-level rack type effects, not guitar pedals, so you'd think make-up gain would not be desirable....

The solution you offered—turn down the effects output—of course makes perfect sense. It's harder in my application since the harmonizer, delay, verb and looper are all effects that add wet signal and mix it in parallel with the analog dry through. They all have mix controls but don't really touch the level of the dry signal, so presumably the signal at the end of the effects in the loop is larger than what came out of the send—and then the amp adds in some more gain on the return jack!

I wish I knew what the dBV/dBu output of the pedals actually is, but I can use my ears. The Kingmaker would be a good test. Set it up as a clean boost and crank the volume to see if I can actually overdrive the loop return. Don't people try to coax power stage saturation with boosts in the loop like that anyway? Iow, maybe overdriving the return could sound good?

The pedals don't seem to be distorting at their inputs at all -- looks like they can handle some very large signals!

As you said though, the +6 dBv is just a max, not what the send is actually throwing -- wish I knew what it was actually throwing (best sound out of these amps comes with master control cranked to about 2-3 o'clock).

My idea—curious if you have any thoughts—would be to place a simple passive volume device at the end of the effects in the loop chain. Reading around seems like a 25k pot would be the ideal choice given the signal level? Or should I go lower/higher? (Apologies if I've strayed from amp talk to pedal talk in the amp forum.)

This has been really informative and thanks again for digging through the spec sheets with me. (Fwiw yes I did copy the conversion out of the Kingmaker manual but bells went off when you reminded me about the equations for the reference levels. I'd seen them before but now know how to use them!)

 

[HotBluePlates]

Take everything below with a grain of salt, as I'm not an "effects loop guy." The few occasions when I use effects, I typically run them in front of the amp.

... the harmonizer, delay, verb and looper are all effects that add wet signal and mix it in parallel with the analog dry through. They all have mix controls but don't really touch the level of the dry signal, so presumably the signal at the end of the effects in the loop is larger than what came out of the send—and then the amp adds in some more gain on the return jack! ...

You mentioned having your Meris pedals set to Synth level, so you'll probably want to try Guitar level instead. My assumption is if the pedal is set for guitar level, it will also amplify less, or otherwise deliver a smaller output signal.

I also thought about the possibility of adding a "volume control" or similar after your pedals & before the Return jack. You might want to avoid that if possible (though maybe amps with a "Return Level" control are just doing the same, but inside the amp chassis), especially if lowering the output level of the Meris pedals is as easy as switching to Guitar level. If it's hard for you to tell when trying that, maybe shoot Meris' Support an email asking how to control output level.

... you reminded me about the equations for the reference levels. I'd seen them before but now know how to use them! ...

I had to re-teach myself how to use them for the post! Let's use an example of, "What voltage is implied by +4.3 dBu?"

For voltage, the format is:

___ dBu = 20 * log (Voltage / Reference)

- Reference level for dBu is √(0.6v), ~0.775v, so we have:
+4.3 dBu = 20 * log (Voltage / 0.775v)

- Divide both sides by 20, for (+4.3 dBu / 20) = log (Voltage / 0.775v).

- 4.3 / 20 = 0.215, so 0.215 = log (Voltage / 0.775v)​

- Get rid of the "log" by using the stuff on the left of the equal-sign as the exponent of 10.

- 10^(0.215) = Voltage / 0.775v​

- Multiply both sides by 0.775v to get the Voltage implied by +4.3dBu:

- Voltage = [(10^0.215) * 0.775v] = ~1.27v
​

CHECK:

20 * log (1.2714v / 0.775v) =
20 * log (1.64) =
20 * 0.2148 = +4.297 dBu

​

 

[.adamo]

Good old algebra! Thanks for the breakdown. This info is helping me make better gain staging decisions in an amp with a design that makes gain staging tougher than most. Much appreciated!