Can I operate D-Class amplifier with disconnected power stage?

[sokolasty]

I play Fender Rumble Studio 40
I wonder if I can use it without speaker as preamp only

I know I can't simply disconnect speaker as I can in MosFet or Bipolar transistor amplifier.

I can use power soaker but these tend to be heavy

AFAIK I can use an 8Ohm resistor instead of speaker, as seen on DIY YT videos (correct or confirm)


But: can I simply disconnect power supply from amp stage without any risk?
I took a look inside and it seems extremely easy, see picture right center "apparent amp stage power connector"

 

[gtirard]

I play Fender Rumble Studio 40
I wonder if I can use it without speaker as preamp only

I know I can't simply disconnect speaker as I can in MosFet or Bipolar transistor amplifier.

I can use power soaker but these tend to be heavy

AFAIK I can use an 8Ohm resistor instead of speaker, as seen on DIY YT videos (correct or confirm)


But: can I simply disconnect power supply from amp stage without any risk?
I took a look inside and it seems extremely easy, see picture right center "apparent amp stage power connector"
View attachment 5314312

No, no, you can simply disconnect the cab!

 

[BoogieZK]

I play Fender Rumble Studio 40
I wonder if I can use it without speaker as preamp only
I know I can't simply disconnect speaker as I can in MosFet or Bipolar transistor amplifier.
I can use power soaker but these tend to be heavy
AFAIK I can use an 8Ohm resistor instead of speaker, as seen on DIY YT videos (correct or confirm)
But: can I simply disconnect power supply from amp stage without any risk?
I took a look inside and it seems extremely easy, see picture right center "apparent amp stage power connector"
View attachment 5314312

Just disconnect the speaker from the output of the amp.

Transistors amps are not like tube amps, if you don't connect a speaker to the power amp, there is no load = no current.

 

[sokolasty]

1.
From what I found:
Class D amplifiers should not run without load because the load is used to damp the LC output filter and prevent the filter-chokes from generating excessive voltages. Some class D amplifiers may survive running without a load, in particular if they are protected with catch-diodes.

More here
running with no load

2.
I asked Fender (specifically about Studio 40) and it's a no-go, answer was:
The Rumble 40 must always be connected to a speaker load.

 

[gtirard]

1.
From what I found:
Class D amplifiers should not run without load because the load is used to damp the LC output filter and prevent the filter-chokes from generating excessive voltages. Some class D amplifiers may survive running without a load, in particular if they are protected with catch-diodes.

More here
running with no load

2.
I asked Fender (specifically about Studio 40) and it's a no-go, answer was:
The Rumble 40 must always be connected to a speaker load.

1. Whoever FauxFrench is, he's just plain wrong: Class D amplifiers can run without (a speaker) load
2. Sorry, I thought you were speaking of a bass head. The Rumble 40 is a combo, it's hard wired to is own internal speaker, and I don't see a way to turn off the speaker out.
3. I wouldn't try to open the internals of the amp if I were you.

 

[Aloe]

uhm, what's your goal? make a speaker silent with line out still operating? just plug a 1/4" to 1/8" adapter to headphones out.

or turn master CCW, if the line out is pre-volume. on a regular Rumble it's post-volume, so the headphone trick is needed.

 

[sokolasty]

uhm, what's your goal? make a speaker silent with line out still operating? just plug a 1/4" to 1/8" adapter to headphones out.

or turn master CCW, if the line out is pre-volume. on a regular Rumble it's post-volume, so the headphone trick is needed.

I want to take electronic component out of combo and use it as a preamp/FX processor

 

[Aloe]

I want to take electronic component out of combo and use it as a preamp/FX processor

I suspected this and frankly won't suggest it. sell the Rumble and get a proper preamp/FX in a proper case.
until you hack the amp, you're able to sell it, get a preamp/FX and still have some money left.

for example, a zoom b1 four is under a hundred on thomann now, and a b2 four is still cheaper than a second-hand Rumble Studio.

 

[MultiScaleMale]

Get a preamp pedal and then you’re dealing with 9V instead of 110/220V.

 

[Tassieviking]

If you plug in any TSR jack into the headphone jack it mutes the power amplifier chip, see if the send jack or line out jack still works and you are good to go.
The amp uses a TDA7294 chip as an amplifier and it has a separate headphone amp, I personally would just disconnect the speaker but I can't guarantee it is ok to do so.

 

[sokolasty]

@Aloe

I want to keep the rest of combo and use it for home practice (with electronic component connected like a head). For rehearsal and gigging I would use electronic component only (provided there's PA and/or big bass amp), which fits in gigbag and is mere 2.2lb

 

[HolmeBass]

I have been running class D power stages with no speakers attached - infinite load impedance - for more than 8 years without issue. First on my little Eden, and more recently with a Mesa D-800+.

Tube amps require a stable load, non-infinite impedance, but transistor output stages not so much. Hopefully @agedhorse has/will chime in as you can absolutely trust what he says, and there maybe some special cases that I don’t know about.

 

[Tassieviking]

So you want to remove the whole head from the cab, no problems with that as long as the box is fully enclosed.
Disconnect the speaker leads and just take the metal box with you, make sure the speaker leads can not touch.
It might be worth it to install a speaker jack socket on the amp and a plug on the lead to the speaker, just plug the speaker back in when you slide the head back into the combo cab.
Also the Rumble 40 does not have a LC filter on the output of the power amp.

 

[40Hz]

Some bad advice being offered here.

Request @agedhorse and @Wasnex chime in on this one to get a straight answer.

 

[abarson]

Read the owner's manual from manufacturers that implement Class D: Gallien-Krueger, Mesa Boogie, Fender, Laney, Trace-Elliot, Traynor, etc. All of them make clear that the head can be used without a speaker load connected. This is by far more authoritative than some rando on a forum who won't even self-identify (FauxFrench).

The amount of butchery you are planning on a lowly combo really means you are pursuing the wrong solution. A preamp pedal would be a better solution.

 

[Tassieviking]

I don't think the amp is fully enclosed if you remove the head from the cab so I think you will have to leave it as it is for safety reasons.
If you want the amp for the inbuilt effects when you use FOH then you just have to bring the whole lot.
There are many preamp pedals around that would be perfect for what you want as well as many very small heads that will fit in the gig bag, it might be time to get something new.

 

[Kevin Hailey]

I want to take electronic component out of combo and use it as a preamp/FX processor

Don't do that! Put down the screwdriver.

 

[lug]

No offence, but it doesn't sound like you have the requisite knowledge to be attempting this kind of stuff yet. Patience and learning first. In the mean time, just buy a preamp.

 

[sokolasty]

For sake of pure curiosity I opened literally every amp and cabinet I ever had - it's pretty much safe as long as one pulls mains out and puts all things back together and has no remaining parts left


The amp is open from four sides so it's obvious I must make some sheet metal covers. I built five bass cabs, replaced speakers, soldered cables, I'll manage

 

[MultiScaleMale]

For sake of pure curiosity I opened literally every amp and cabinet I ever had - it's pretty much safe as long as one pulls mains out and puts all things back together and has no remaining parts left


The amp is open from four sides so it's obvious I must make some sheet metal covers. I built five bass cabs, replaced speakers, soldered cables, I'll manage

Good luck