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

[agedhorse]

I just looked at the docs because something didn't seem right based on my memory (which isn't perfect because it's been quite a while)...

1. The Rumble 40(V3) does not use a class D power amp, therefore the OP's original assumption is simply incorrect.

2. The power amp in the Rumble 40 can be safely operated without a speaker connected.

3. The part that the OP is assuming is the power amp is actually the switchmode power supply and not the power amplifier.

Besed on the above, the OP does not have the knowledge or understanding to be modifying this amp IMO.

 

[eddododo]

Some bad advice being offered here.

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

I suspect the response will be “you have no business poking around in that chassis”

 

[trevcda]

3. The part that the OP is assuming is the power amp is actually the switchmode power supply and not the power amplifier.

In the OP's defense and per his attached picture, it is labeled as an amplifier module. Lacking a heat sink which I would expect to see and an amp board and that was present on the board to the left, I was also thinking it was a power supply board. But I didn't look up a schematic to verify.

 

[SAlcott]

Dunning-Kruger effect in operation.

 

[logdrum]

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

Everyone or a majority of TB folks will say no and the default answer is to discourage. MAybe your best bet is to DM folks like passinwind.

 

[agedhorse]

In the OP's defense and per his attached picture, it is labeled as an amplifier module. Lacking a heat sink which I would expect to see and an amp board and that was present on the board to the left, I was also thinking it was a power supply board. But I didn't look up a schematic to verify.

The picture is wrong, or was taken out of context.

 

[agedhorse]

Everyone or a majority of TB folks will say no and the default answer is to discourage. MAybe your best bet is to DM folks like passinwind.

Or he already got his best answer.

 

[DJ Bebop]

Arc flash injury only hurts while you are conscious. You are very, very fortunate to have survived without even worse injuries imo.

 

[40Hz]

I suspect the response will be “you have no business poking around in that chassis”

Probably with good reason too.

In the IT world there’s a saying which has saved the job of many a tech. And probably would have done the same for a few overly confident laypeople too if they heeded it:

If you don’t know what you’re doing - ask.​

***​

If you do know what you’re doing​

it never hurts to ask anyway.​

 

[L Anthony]

In my previous life as a power plant/drydock mechanic, I used to work with large steam, compressed air and water lines. I knew what they could do to me if I didn't respect them, but they usually would give me a little warning before disaster struck if something wasn't right. I also worked with electricians, many of whom told me about close calls they had had, because electricity, unlike steam, compressed air and water, doesn't usually warn you before it strikes.

Watch your butt, or you won't have one.

 

[Artman]

Luckily in Europe we pay our health insurance in tax (me: 5% of income) and do not co-pay

Lucky indeed.

 

[seamonkey]

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

I don't even see an output filter coil. (I could be blind )
I've seen other powered speakers with internal class-d made without a filter also. If there's no external speaker output, then I bet a class-d can be made without a filter, and to also comply with FCC (US) requirements. if it's all self-contained, the amp and speakers could be shielded.

 

[Artman]

 

[smoothbrain]

I usually joke that electricity only hurts till you pass out, but I can only still see because I wear glasses with safety lenses in them.
I have been blind and deaf once for 15-20 minutes when I opened a large electric panel and something shorted out in the cabinet, I had burns on my face and arms but only slight and left no scarring. I had holes in my shirt from melted copper from the cables melting and exploding, all because another electrician had done something stupid.
It was a fairly large electrical explosion with only 3 phase 415volts, the 415v is only 3 lots of 240v like we have at home. 220-240v can be fatal if it gets you, I think I am lucky having worked as an electrician my whole life with live machinery and not having any permanent injuries.
Just be careful and work safely, you can't see electricity but it has a nasty bite.
Make sure it is safe for other people who might touch the amp as well.

Seriously, be aware and careful and have actual and clear knowledge about the risks involved with whatever it is you are attempting to do.

Idiotically, I once vaporized about 6 inches of a very large screwdriver/pry-bar and the contact electrodes I was probing because I was ignorant. I would have been blinded if I had not had thick glass safety lenses in my eyeglasses. I would have been better off with plastic lenses. There were globular spheres of copper/steel/silver embedded halfway melted through the glass lenses. I had burns.

 

[agedhorse]

I don't even see an output filter coil. (I could be blind )
I've seen other powered speakers with internal class-d made without a filter also. If there's no external speaker output, then I bet a class-d can be made without a filter, and to also comply with FCC (US) requirements. if it's all self-contained, the amp and speakers could be shielded.

It's possible at lower power levels, it's called "filterless class D topologies" but it gets very hard as the power levels increase.

 

[logdrum]

The picture is wrong, or was taken out of context.

It's a web page update issue, I did not see the reply. No need to bust my balls

 

[agedhorse]

It's a web page update issue, I did not see the reply. No need to bust my balls

I wasn’t busting anyone’s balls. That wasn’t my intent either.

 

[Paul New]

It's a web page update issue, I did not see the reply. No need to bust my balls

Too late! Balls can not be unbusted.

 

[seamonkey]

Fender is totally ambiguous about the output section of the Rumble 40 studio
https://www.fmicassets.com/Damroot/..._Rumble_Studio_40_Stage_800_Rev_A_ENGLISH.pdf
States: "Rumble Studio 40 and Rumble Stage 800 use class-D power amplifiers"

While the product page:
Invalid Link Removed
States:

• 40-watt WiFi-equipped digital bass amplifier"

As everyone, except Marketeers know, Class-d is not digital. They could mean the pre-amp is digital.

In anycase, Class-D is fine. with an infinite impedance load, pre or post filter negative feedback path, except when they haven't tested it. And they don't support what they don't test.

It sure seems like this is a great pre-amp to use it by itself if you don't care about what their support says. Hacking these Rumble 40's could be a good way to go.

Since Fender came out with their Tone Master Pro, I highly doubt they will ever offer any lower level "all digital" pre-amps.

 

[logdrum]

I wasn’t busting anyone’s balls. That wasn’t my intent either.

I meant to quote this:

Or he already got his best answer.

How else are we going to have people interested in engineering and electronics if we discourage folks from tinkering or advise them to just buy another preamp. If I told my son that he just needs to be finance a new car rather than attempting to replace the engine by himself, he would not have acquired new skills. If the OP contacted some of our members who actually have the knowledge it will be more productive than everyone else piling on the bandwagon of discouragement or even ridicule at times.