Onboard preamp and ground question

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Will grounding the output of an onboard preamp (active bass) hurt the preamp?

I believe it would, and shouldn't be done. Goes against all practices I've ever learned.

But an answer from D'Addario concerning their Circuit Breaker Instrument Cable (pic), the type for muting a cable for quick instrument swaps, has me confused.

They told me that internally, the switch shorts the signal to ground when muted. That answer is fine for a passive bass, but doesn't seem right for an active one. Consequently, the answer seems like a blow-off or it's incomplete.

I wrote back in response asking the same question at the top of the post. Waiting for answer.

Just thought I'd parallel off and ask here at the same time.

 

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subbing to find out

 

[Lo-E]

I can’t think of any way that would damage a preamp. You’re simply shorting an audio signal to ground. If you have a volume pot after yor preamp it would do exactly the same thing.

 

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I can’t think of any way that would damage a preamp. You’re simply shorting an audio signal to ground. If you have a volume pot after yor preamp it would do exactly the same thing.

I didn't know if a preamp's output section had to be specifically designed to withstand a sustained ground at the output or not.

Of course, with passive, the small Instrument Level output of the pickups get shorted to ground in many circuits (volume pots, switches, blends, etc). But an active preamp is a Line Level output, so I figured that couldn't happen.

I found a Bartolini Buffer/Preamp wiring diagram from the Bart site that shorts the outputs (dual preamp) directly to ground, as you said, when the Blend pot is turned to one extreme.:

 

[bowthing]

Almost any preamp (especially if it runs on a single battery) will have a capacitor that blocks DC on the output, so shorting it out won't hurt anything.

 

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No reply from D'Addario yet, but it's the weekend.

From the replies so far, it seems our onboard preamps, no matter the brand, can withstand sustained grounding (0 ohm load) at the output while still powered by battery. Here's a visual showing 1/2 of a switch shorting the main signal to ground thru contacts 4 and 5, muting the jack:

So this should be no problem for any brand preamp, correct?

 

[Crater]

Every preamp I've ever seen a schematic for has a DC blocking/coupling capacitor, and almost all of them have a series resistor on the output as well. Op-amps are usually designed to limit their output current to safe (for the chip) level, so you should be fine.

 

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It just never seemed right to me to short a preamp output. Livin' and learnin' LOL.

But if D'Addario confirms their Circuit Breaker Instrument Cable only shorts the entire signal to ground, then that's gonna cinch it for me. That's because their product works with any brand preamp. Well...I don't know of any reports of their product, or any manufacturer's muting plug cable, destroying someone's preamp.

Hopefully Monday or Tuesday I'll hear something. They were good at responding fast to my first query.

FWIW, my first drawing opened the preamp output while grounding the input (tip) to the bass amp. This seemed the way to go (especially the preamp disconnect). But a TB member said, and I agree with, it's generally not good practice to run audio signals thru switch contacts when possible. Only other option was to short the preamp output, which is why I wrote D'Addario and started the thread.:

 

[Jerry Catanescu]

Momentarily shorting the preamp's output should be perfectly OK. I would not worry about it at all.

 

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My area of interest is long-term, sustained shorting.

 

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D'Addario responded to my second email. Didn't answer how the internals of their Circuit Breaker plug worked.

Thank you for reaching out! The circuit breakers can be used on passive or active systems. We have not received any known complaints about any issue with active systems since the cables release over a decade ago!
I hope this helps. Please let me know if you have any further questions.

However, I think I found my answer on the front of every new cable's packaging LMFAO! Accidentally saw a pic of the packaging online and started laughing.:

Full package:


Closeup:


Anyway, does it look to anyone else like the switch plunger pushes a conductive element down against the ground/shielding shell, grounding the entire signal path of the plug?

Just wanna make sure I'm interpreting the drawing correctly.

Wish I could get my hands on one and meter it.

 

[Crater]

Anyway, does it look to anyone else like the switch plunger pushes a conductive element down against the ground/shielding shell, grounding the entire signal path of the plug?

That's exactly what it looks like to me...

 

[Passinwind]

My area of interest is long-term, sustained shorting.

What’s your use case?

 

[E2942]

If it's set up like a switch, then it could accidentally sustain a Ground for the output of the preamp, but at the same time they'd have gotten a figurative shedload of reports back about it causing failures, it it were a problem.

 

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What’s your use case?

One of the D'Addario muting cables has a latching function (as opposed to momentary operation). The intended use of this cable was for quick instrument changes on stage. I wondered if players latching the cable mute and walking away for awhile would hurt an active bass's preamp output by continually being grounded. In other words, the preamp seeing a sustained 0 ohm load.

If the preamp could handle that, it led me to reconfigure a circuit for active basses that requires the preamp output and main amp input to be grounded during a portion of operation.

Seems to be yes, onboard active preamp output can be hard grounded with no ill effects. Good to know info for circuit modders and makers.

Unless everyone else knew but me... LOL

 

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If it's set up like a switch, then it could accidentally sustain a Ground for the output of the preamp, but at the same time they'd have gotten a figurative shedload of reports back about it causing failures, it it were a problem.

Not an accident having the preamp output with a sustained ground. That's what the plug was designed to do, intentionally ground the preamp output.

All the reports I've seen have been cable plug failures, not preamp failures. I don't use one, 'cause I haven't had a need to.

 

[E2942]

The switch does wear out, so you either chop it off & put on a new plug or toss the whole thing.

Accidentally leaving the switch to Ground is what I meant, or if someone was using it & didn't know about the switch.

It definitely happened to me at the local D'Addario dealer!

 

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The switch does wear out, so you either chop it off & put on a new plug or toss the whole thing.

Fortunately, the plug, itself, is a non-solder type. Undo the side screw, pull the cable out. Order new plug, trim the end of a cable per manufacturer, shove it inside the open end. Then secure with a screw.

 

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I was asking about hard grounding an onboard preamp output because I had a circuit idea where a portion of it would have to ground the preamp output.

The circuit's purpose is to mute an active bass and stop battery drain while muted. Mute/unmute without that damnable POP! All without unplugging the bass.

Just requires a 2P2T on-on-on switch. The switch keeps proper sequencing to ensure jack is always muted (preamp output grounded) when battery is switched on or off, thus preventing the pop from being heard.

Based on the automatic 'power on/off delay' built into audio equipment (bass amps, PA's., etc) to prevent popping in speaker output connections during power up or power down.

Instead of automatic, the delay in this circuit is manually selected by the player in middle MUTE position. Important note: Player must pause in the middle MUTE position before continuing to ON or OFF. MUTE position is the manual delay where the 'POP' would happen while output is muted. Pause in the middle for 1 second or less when switching to ON or OFF, else possible equipment damage and pissed-off people can occur.

Anyway, tested on two basses (alligator clip leads).
- When battery was kept on, there were no pops or sounds when shorting/unshorting preamp output. No change in battery current. Long term shorting of preamp output was fine.
- When preamp output was kept shorted (muted), there were no pops, no sound at all, when connecting/disconnecting the battery.

This circuit...:
- ...won't work with any form of bypass (EQ bypass, preamp bypass, passive mode). It might be able to, but I haven't looked hard into it.
- ...won't allow you to disconnect the instrument cable plug without noise. To unplug instrument cable, use external muting like silent plugs, pedals, amp mute, etc.

Sucks not handling bypasses for now. But if you want to silently (no pops) mute/unmute your active bass and stop battery drain without pulling the plug, this is a simple and effective solution.

 

[Jerry Catanescu]

I use a 3PDT toggle to both turn off the preamp and do a true bypass, and just deal with the pop.
If the pop is an issue, it can usually be significantly alleviated by adding an RC circuit to the preamp that ramps up/down the voltage at power on/power off. Something like the one in this unrelated, but useful circuit I built recently: