Are solid-state amps subject to speaker load restrictions like tube amps?

[Mango Moon]

Hello All,

I've got a quick question regarding speaker loading, and the design and use of solid-state (so-called "modeling") amps like the Fender Mustang V head:

http://www.fender.com/products/mustang/models.php?prodNo=230005

This product is a relatively new and fairly high-powered stereo (75w:75w) solid-state guitar modeling amp that has a headphone jack which apparently bypasses the two (8-ohm) stereo speaker outputs for after-hours use, which raises a question about loading and potential damage to the amplifier.

I recently searched the Fender Instruments website and contacted Fender directly for some assistance, but they don't have the Owner's Manual posted on-line yet, so I have one basic, but as yet unanswered question:

Are these types of solid-state amps subject to the same mandatory speaker loading restrictions that apply to tube amps? In other words, when powered-up, do these solid-state amps need to be under a load (i.e., connected to one or more speakers) like a tube amp in order to prevent damage to the amplifier? Or does this principle not apply to solid-state amps?

The ability to use headphones silently (without speakers) would seem to imply that they DO NOT require such loading, but I'd like to know for sure?

Many Thanx

[DetroitDan]

Solid-state amps are, kind of, opposite of tube amps. They can handle too high an impedance, including infinite, fine. They suffer when connected to a load with an impedance lower than their rating.

Anyway, headphones jacks often break the signal between the pre-amp and power amp, without messing with the speaker circuit.

[Enzo]

And we can assume that Fender actually engineered the product and didn;t merely produce something easily blow up, even so.

I agree with Dan, here. Solid state amps don;t care what the load impedance is as long as it is above the minimum.

[Mango Moon]

Dan, Enzo . . .

Thanks guys. I really do wish that I understood this stuff better, but until I get better educated on some of it, I am most grateful for your assistance, as always.

Given the relatively low cost of the Mustang V amp (~ $300), I'll probably pick one up at some point just to evaluate it, and an Owner's Manual (likely to be enclosed) will probably clarify most of this stuff for me at that time . However, in the meantime, I do want to make sure that I'm hearing (interpreting) you both correctly here, as there does indeed appear to be a difference between the SS and tube varieties of amps, and also the 'stereo' business. So, are you saying that even though its relatively high-powered (150w = 75w per side) that the headphone jack itself (in the case of 'solid-state') provides the "minimum" required load or shunts the signal such that speakers are not necessary, like they would be with a comparable tube amp?

If so, then presumably, even though it has two 8-ohm stereo outputs, a guy should also be O.K. running a single 8-ohm speaker without any concern, a scenario which (incidentally) I have been told by Mesa-Boogie engineers that I should never do with my Mesa 50:50 stereo 'tube' amp, unless using a grounding/shorting plug in the un-used output jack.

[kg]

like enzo, i too agree with dan.

the solid state amps really have a minumum load impedance, and the tube amp a maximum one.

depending on how hard you're pushing them, these extremes may be extended a bit. ie, if you're playing at bedroom levels, it's not nearly as critical as when all the knobs are on 11.

[teemuk]

Solid-state amps are, kind of, opposite of tube amps. They can handle too high an impedance, including infinite, fine.

This is true in most cases, though not in all. I wouldn't trust it as an universal rule.

At least three (moderately rare) types of SS amps that make an exception are...

- Transformer coupled power amps powered from high voltage supply (e.g. the generic 50/60's radio -style single-ended design with one power transistor, output transformer and supply voltage of about 150V). They can fail similarly to tube amps if operated open load.
- Particular Class-D designs that produce higher output power to higher load impedances.
- Poorly designed amps and many class-D designs that begin to oscillate and overheat with high impedance or open loads.

So, though the particular Fender likely runs just fine, SS amps witstanding open or high impedance loads is not an universal rule. Not all of them are happy with such. It depends on design.

[g1]

"are you saying ...that the headphone jack itself (in the case of 'solid-state') provides the "minimum" required load or shunts the signal such that speakers are not necessary, like they would be with a comparable tube amp?"
No, this is not what is being said. There is not a minimum required load for SS amps. Aside from rare exceptions mentioned by teemuk,, solid state amps can run happily without any load, with or without headphones.
The minimum load listed on SS amps is not a requirement but tells you the safe minimum, below which the amp will usually overheat and fail. For example, running a SS amp rated for a minimum 8 ohms into a 4 ohm or 2 ohm load.

[Steve Conner]

Yes, those exceptions are hopefully rare. If the Mustang has a headphone jack, I expect that it should cut off the speakers in a safe manner, for silent practice sessions. That's what it's for.

Teemu, welcome back to the forum! I've got my new hybrid up and running, and it sounds great (to me at least!), please check it out. I promise to publish the schematic sometime soon...

http://music-electronics-forum.com/t23321/

I must admit I'm not sure what load impedances it can safely handle.

[Enzo]

I am not saying that the headphone jack itself provides any impedance either way. But I am saying that if Fender includes a headphone jack, they did arrange for any load requirements in designing for it. For example, the jack might disable the power amp completely and leave the headphones to be drivien by a separate driver.

[Mango Moon]

I am not saying that the headphone jack itself provides any impedance either way. But I am saying that if Fender includes a headphone jack, they did arrange for any load requirements in designing for it. For example, the jack might disable the power amp completely and leave the headphones to be drivien by a separate driver.

Enzo,

Thanks for your continued response here. Although I'm not completely clear on just how it does so, the headphone jack itself does indeed break the path to/from (?) the power stage (I know this from talking directly with Fender). However, a Fender tech also said to never run this solid-state amp (even when the headphones are connected) without one or more 8-ohm cabs connected to its Main Outs, which tells me (as an amateur to this stuff) that they believe damage to one or more output transformers might occur should (for example) the headphone connection become inadvertently broken, etc.

Anyway, thanks again to you and all of the others who took the time to post here. I appreciate it. In the end, it sounds like there are so many potentially different designs in the solid-state realm (just as there are with tube amp designs), that any number of scenarios are possible, so I'll try keep it simple. I'll always keep a pair of cabs connected to it (headphones or not), and if I decide to ultimately keep the amp, I will likely try to replace and relocate the current 1/8" headphone jack (which is oddly positioned on the rear panel, perpendicular to the chassis) to the very top control panel with a heavier gauge 1/4" jack.

Thanks again everyone!

[g1]

You mean output transistors, output transformers are used in tube amps.
Well, this is going to cause problems. The owners manual states "the Mustang V head amplifier provides stereo speaker outputs that must both be connected to a stereo speaker cabinet... or two 8 ohm speaker cabinets capable of handling 75 watts each". The keyword here is MUST. Yet the output jacks are NOT marked "do not operate amplifier without speakers connected". Do you know what percentage of users read their owners manuals? Unfortunately it is not a high percentage.
How many people are going to hook up cabs. when they connect this head to a computer? Will the head be damaged? Will the warranty be voided because of running with no load?
When they are out of warranty and being serviced, will they be damaged after repair because of checking to make sure they are ok without a load before hooking up speakers?
If anyone has a schematic for the power amp, I would sure like to see it. Otherwise, could any of you authorized for Fender warranty find out what the problem is with running unloaded? Thanks!

edit: I'm really hoping this is just a typo in the owners manual!

[Jazz P Bass]

Here is the phone jack statement from the manual.
Speakers are muted when headphone jack is used.
Therefore, the rep at Fender is mistaken.
The statement about "speakers must be connected" has to do with the "stereo" output.
Must, meaning a stereo cab, not must have to connect something.

[Mango Moon]

Gentlemen,

As the OP of this thread, I will acknowledge right up front that I'm not a trained tech, so I don't a lot of these specific design details (hence my transformer/transistor error above). Consequently, I've gotta rely on my fingers to do the dialing of the telephone and/or e-mailing to Fender reps (or to the MEF) in hopes of getting my questions answered. In so doing, here is what I was recently told about the Mustang V head and its use with speakers or headphone:

1) That the reference in the Owner's Manual about "mandatory" use of both (stereo) outputs was in error. In other words, I was told that it was perfectly acceptable to use either (not both) of the two 8-ohm output jacks to a single cab of 8-ohm or 16-ohm impedance (i.e., that using both output jacks simultaneously was not necessary). In fact, the gentleman I spoke with said that there was another Fender employee testing/using a Mustang V configured in just this way (one cab to one jack only) in the room immediately nextdoor to him.

2) That when using the headphone jack (whether to a set of stereo headphones or to a mixing board through DI Boxes), one or more speakers needed to be connected at all times.

Now, the gentleman I spoke with was admittedly NOT an engineer or a technical rep. He was a 'customer service' rep, and so unfortunately, the technical validity of the information stated in items 1 and 2 (above) is unknown. I hope this helps.

[g1]

Just to clear this up, I sent an email to Fender customer relations. The response was the opposite of 1) and 2) above. This makes the most sense. Either use no speakers or both left & right.
Here is the response I received:
"There is no problem operating the Mustang Amps without speakers attached. I read the manual and the point they are trying to get across is that if you are using speakers it A: Has to be connected to either a stereo speaker cabinet or B: Two separate 8ohm speaker cabinets. This is because it is a true stereo amp and if you use only one speaker connected to one output it will not sound right and the effects will not work. It is a solid state amp and is not affected by not having a speaker load on it."

[Jazz P Bass]

Thanks for clearing this up.
I would think they are using a TDA7294 IC, one per channel, which has an easily switched mute circuit in the chip.
The ic does not require a load to operate & will function just fine without a load.