Strange DI box / amp volume drop

Kerem Koseoglu

Kerem Koseoglu

Aug 27, 2018
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Istanbul, Turkey
www.keremkoseoglu.com
#1
I searched around, but couldn't find an answer, so here I am with a new thread.

My simplified setup; nothing unusual:
  • Bass goes into DI box
  • Thru output - TRS cable - goes into amp (Aguilar AG 700)
  • XLR output - XLR cable - goes into audio interface
Observation:
  • If I unplug the TRS cable from the amp, I hear a nice high volume from the audio interface. No issues here.
  • If I plug the cable to the amps "Input"...
    • ...but don't turn the amp on, the XLR volume drops significantly.
    • ...and turn the amp on, XLR volume increases back to normal.
  • If I plug the cable to the amps "Effect return", XLR volume stays on normal level in all scenarios.
I tried several active / passive DI boxes, but got the exact same result.

Is the described volume drop normal / expected?
  • If yes;
    • What is the logical reason behind it? Impedance?
    • How come that the effects return doesn't behave the same way? Impedance?
  • If no;
    • Do I have a faulty unit?
    • Is this behavior specific to Aguilar AG?
I appreciate your answers!
 
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#4
Kerem Koseoglu said:
  • If yes;
    • What is the logical reason behind it? Impedance?
    • How come that the effects return doesn't behave the same way? Impedance?
Click to expand...
Yes, and yes -- impedance. Your amp's input is a very high impedance when it's turned on; it's likely to be a very low impedance if the active device in the preamp isn't activated by power. This low impedance loads down the output of bass itself through the transformer in a passive DI, or the output device in the DI if it's an active one. My guess about why it doesn't happen with the FX return is that it is isolated from this effect by passive, resistive elements that are there to help protect the power amp from potential external weirdness. That's my theory anyway. Don't use the DI with the amp not powered up -- that's my suggestion to resolve the issue.
 
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#6
Redbrangus said:
Yes, and yes -- impedance. Your amp's input is a very high impedance when it's turned on; it's likely to be a very low impedance if the active device in the preamp isn't activated by power. This low impedance loads down the output of bass itself through the transformer in a passive DI, or the output device in the DI if it's an active one. My guess about why it doesn't happen with the FX return is that it is isolated from this effect by passive, resistive elements that are there to help protect the power amp from potential external weirdness. That's my theory anyway. Don't use the DI with the amp not powered up -- that's my suggestion to resolve the issue.
Click to expand...

Wasnex said:
FYI, most Thru connections on DIs are simply parallel to the input. So if you connect the Thru to something with a low input impedance, it will load down the signal from your bass.

Sounds like the input impedance of your amp is low when it is turned off.
Click to expand...

Correct, and this is not uncommon either.
 
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#9
Kerem Koseoglu said:
I searched around, but couldn't find an answer, so here I am with a new thread.

My simplified setup; nothing unusual:
  • Bass goes into DI box
  • Thru output - TRS cable - goes into amp (Aguilar AG 700)
  • XLR output - XLR cable - goes into audio interface
Observation:
  • If I unplug the TRS cable from the amp, I hear a nice high volume from the audio interface. No issues here.
  • If I plug the cable to the amps "Input"...
    • ...but don't turn the amp on, the XLR volume drops significantly.
    • ...and turn the amp on, XLR volume increases back to normal.
  • If I plug the cable to the amps "Effect return", XLR volume stays on normal level in all scenarios.
I tried several active / passive DI boxes, but got the exact same result.

Is the described volume drop normal / expected?
  • If yes;
    • What is the logical reason behind it? Impedance?
    • How come that the effects return doesn't behave the same way? Impedance?
  • If no;
    • Do I have a faulty unit?
    • Is this behavior specific to Aguilar AG?
I appreciate your answers!
Click to expand...

I think that it’s great that you took the time, in a methodical way, to understand your DI and your amp. Even if it was out of necessity to determine if it was working properly.

All bets are off when experimenting with your amp not powered up.

You AG 700 has a bridged output. Some DI’s can be connected in parallel with the speaker output and a cabinet. Be very careful to understand what you shouldn’t do in this case.

With some amps, usually older ones, placing a DI at an effect send or pre-amp out can load the signal going into the power amp. The amp may have not been designed to drive a device such as a DI. The result is a decrease of volume level out of the power amp. Again an impedance matching issue. It’s a result of the amp’s design. So performance can vary depending on the amp.
 
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#10
beans-on-toast said:
You AG 700 has a bridged output. Some DI’s can be connected in parallel with the speaker output and a cabinet. Be very careful to understand what you shouldn’t do in this case.
Click to expand...

I haven't seen any DIs that would be suitable at speaker level with a bridged amp. I would assume all class-D bass amps operate in bridge mode, unless I saw some sort of documentation that clearly states otherwise.


Why connecting a DI to a bridged output is a problem. DIs use a Tip Sleeve (TS) connector. The Sleeve connects the - speaker out to ground. The - speaker terminal on tube amps and non-bridged solid-state amps is also grounded. So you can use a speaker level DI on tube amps and old led sleds. Note: some led sleds have two output sections that can be bridged. Don't connect a speaker-level DI when using bridge mode.

Bridged amps put a signal on both the + and - terminal of the speaker outs. So if you connect a DI the signal on the - terminal is shorted to ground. This should be considered an illegal connection.


This is a simplified schematic of an amp that is operating in BTL mode.
upload_2019-12-15_1-50-4-png.3631441

Note A1 is non-inverting amp, and A2 is an inverting amp.

This shows the signal polarity:
02-how-to-use-the-amplifier-btl-model-jpg.3631459


As you can see, both outputs produce a signal, so the negative speaker terminal will have voltage when the amp is making power.
 
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