No load OK, who knew!

scottfeldstein · #1 11-20-2012, 07:57 AM

So I'm up late last night and I get an urge to noodle around. I usually run my pedal board into my mixer, my mixer into my headphones. But last night I thought maybe I'd just plug my headphones into my amp and play with my amp's EQ for a change.

Surely the MB500 had a headphone out. It did!

But when I played, my cabs still made noise. Not what I had in mind for a silent jam in the middle of the night. I consulted the amp's manual and to my surprise it says 4 Ohms, 8 Ohms and no load are all OK. That is, I can unplug both cabs from the amp and still run it. That way, I can get the headphones working but have nothing come out of the cabs.

Am I the only one who didn't know this was a thing, no load being OK?

DirtySix · #2 11-20-2012, 08:00 AM

Nope. I did not know this. I'll have to take a look at the manual for my amp. Might be a game changer for my little apartment!

Mykk · #3 11-20-2012, 08:03 AM

The MB500 is class D isn't it? I thought the only amps that could not function without a load were tube amps.

MysticMichael · #4 11-20-2012, 08:05 AM

Quote:

Originally Posted by scottfeldstein

Am I the only one who didn't know this was a thing, no load being OK?

Couldn't say whether you're the only one, but AFAIK it is safe to run without a speaker load on virtually any solid-state amp (not the case with tube amps, of course).

MM

davidjackson · #5 11-20-2012, 08:07 AM

I thought the only time when no load was a danger was when you had a valve power amp stage?

I thought this:

Solid state pre-amp and solid state power-amp is OK with no load.
Valve pre-amp and solid state power amp is OK with no load.
Valve pre-amp and valve power amp with no load = BANG!

Apologies if I have completely misunderstood.

scottfeldstein · #6 11-20-2012, 08:16 AM

Page 7 of the MB500 manual clearly says: "Delivers 500w into 4 Ohm loads. Lower impedances such as 8 Ohms or 16 Ohms and no load are acceptable."

Primakurtz · #7 11-20-2012, 08:18 AM

Quote:

Originally Posted by davidjackson

I thought the only time when no load was a danger was when you had a valve power amp stage?

I thought this:

Solid state pre-amp and solid state power-amp is OK with no load.
Valve pre-amp and solid state power amp is OK with no load.
Valve pre-amp and valve power amp with no load = BANG!

Apologies if I have completely misunderstood.

No apology needed - you are completely correct.

**bass4worship** · #8 11-20-2012, 08:21 AM

Good to know, thanks.

KJung · #9 11-20-2012, 08:31 AM

Virtually all (maybe 100%) of solid state amps (i.e., solid state power sections) can run safely with no speaker attached. Preamp tubes don't matter.

Only tube power sections run into trouble running with no speaker or load from a power soak or whatever.

scottfeldstein · #10 11-20-2012, 08:33 AM

I'm a complete ignoramus! Or I was. I hope there are others who can be enlightened because of this thread. (Also, I hope I wasn't the only one who didn't know this. Heh.)

KJung · #11 11-20-2012, 08:37 AM

Quote:

Originally Posted by scottfeldstein

I'm a complete ignoramus! Or I was. I hope there are others who can be enlightened because of this thread. (Also, I hope I wasn't the only one who didn't know this. Heh.)

You are not an ignoramus

Impedance in general, and the differences between tube power sections and solid state power sections regarding how they deal with different impedance levels continues to be asked about daily on this site.

davidjackson · #12 11-20-2012, 08:39 AM

Quote:

Originally Posted by Primakurtz

you are completely correct.

There truly is a first time for everything.

Thanks.

JTE · #13 11-20-2012, 09:02 AM

Think of it this way. With a solid state output section (power amp) one can run it into a HIGHER impedance. No load is NOT zero ohms, it is INFINITE ohms. 0 ohms is a dead short, not an open circuit.

Jogn

johnpbass · #14 11-20-2012, 09:28 AM

Quote:

Originally Posted by JTE

Think of it this way. With a solid state output section (power amp) one can run it into a HIGHER impedance. No load is NOT zero ohms, it is INFINITE ohms. 0 ohms is a dead short, not an open circuit.

Jogn

Now that's something I did NOT know!

KJung · #15 11-20-2012, 09:32 AM

Quote:

Originally Posted by johnpbass

Now that's something I did NOT know!

+1 Impedance represents resistance. Think of it like a garden hose. The lower the impedance (with a solid state power section), the more the faucet is opened up (i.e., less resistance). The higher the impedance, the more resistance, hence the more the faucet is closed. If you have '0' impedance (no load), the faucet is closed all the way.. no water gets through... the 'hose' (i.e. amp) is at 'rest'.

hdracer · #16 11-20-2012, 09:35 AM

Quote:

Originally Posted by davidjackson

I thought the only time when no load was a danger was when you had a valve power amp stage?

I thought this:

Solid state pre-amp and solid state power-amp is OK with no load.
Valve pre-amp and solid state power amp is OK with no load.
Valve pre-amp and valve power amp with no load = BANG!

Apologies if I have completely misunderstood.

Quote:

Originally Posted by Primakurtz

No apology needed - you are completely correct.

Not ALL tube amps.

You can run the Fender Bassman 100T with no load.

KJung · #17 11-20-2012, 09:36 AM

Quote:

Originally Posted by hdracer

Not ALL tube amps.

You can run the Fender Bassman 100T with no load.

Yeah, that amp, from what I've read about it, is kind of designed for the studio, and seems to have some sort of 'power soak' )or something) built in to allow you to do that. Pretty cool. That is a VERY rare exception.

Edit: just looked at the spec. Yes, per above, it does have an internal power soak. Nice for studio use.

Primakurtz · #18 11-20-2012, 09:38 AM

Aye, there's the rub. Some tube amps do have an automatic load built in, so that you can unplug the speaker(s) without harm. The new Fenders feature this. Others?

iiipopes · #19 11-20-2012, 09:39 AM

There is confusion here, especially since tube amps are the opposite.

Tube amps usually have an output transformer, so there has to be some load to keep the amp from trying to push too much current through the primary windings of the transformer.

Most solid state amps, however, do not have an output transformer; it's straight out into the amp. So to open up the load, meaning not plugging anything in, is no different (in practical terms -- techs, don't get on to me here) than simply not having the light switch turned on.

This does mean that there may be concerns about DC voltage when clipping, but very little.

Go with the owners manual. In this case, no speakers plugged in is acceptable for this particular model amp. All other amps, depending on their circuitry, may differ.

KJung · #20 11-20-2012, 09:39 AM

Quote:

Originally Posted by Primakurtz

Aye, there's the rub. Some tube amps do have an automatic load built in, so that you can unplug the speaker(s) without harm. The new Fenders feature this. Others?

None that I am aware of. Again, that head seems like it was designed for studio work. I don't think (could be wrong) the 300 watt version has that feature.