Leaping Ohms!!

[lareplus]

I found one reference to using a 4 ohm speaker in a Vox AD30VT, which is a 30 watt amp rated for 8 ohms. It was said that the amp has a chip that could handle that load move to 4 ohms.

Is that a wild and reckless comment??

Can anyone comment on this? Or if it doesn't work straight away is it possible to mod the amp by replacing a chip or two so that the 4 ohm speaker works?

The great thing is that if it does work, you have a 60 watt amp instead of a 30 watt amp, without any weight increase!

. . .whoopee!! ( I like loud amps that don't weigh very much )

[printer2]

The power output stage is only one part of the amp. Even if you do put two chips in the amp the power supply is only sized to put out 30W and you will not be getting more. The chip is rated for about 60W at 4 ohms already but that is for normal music, not distorted guitar.

The chip has been run in parallel but it is a little more complicated than wiring them up together, not for your average person.

[lareplus]

so could an amp tech make it work?

[LarsOS]

By rebuilding the whole thing, yes. It's probably better and cheaper and easier to get an amp that is actually built for 4 Ohm.

Or you could wire two 4 Ohm speakers in series to achieve 8 Ohm.

[lareplus]

The sounds of the amp are great and the weight is perfect. . .but it's not loud enough to play with the band. I can't mike it through a PA for practice because we don't have one and I don't want a heavier amp.

The tone from this little amp is pretty marvelous, if you haven't heard one.

[Singin' Dave]

How do you guys practice without a PA of some sort? Particularly if you are as loud as you say.

[lareplus]

we've never had a PA. .

[Wally]

Thsiis a solid state amp, right? DO you have the manual? IS there a 'minimum ohms' posted on the back of the amp near the speaker outputs? A solid state amp will work with any speaker load as long as one doesn't go under the designed minimum. With each chagne, the output power changes....decreased output with higher impedance loads, increased output with lower impedance loads. AS long as you don't go under the minimum, that mp is good to go, as far as I understand solid state amplification.
As always, when in doubt, refer to a user's manual.

[waparker4]

http://www.valvetronix.net/docs/AD15_30_50_100VT_E4.pdf

says max 30W @ 8 ohms.. i guess that implies 8 ohms minimum.

[Wally]

IN that manual that waparker linked us to, it states that no other speaker impedance other than 8 ohms is to be used. Only the 50wt version has an external speaker jack. IT states that when the ext speker jack is utilized, theinternal speaker is disconnected. This, to me, means that this 50 watt amp wants to see nothing other than an 8 ohm load. Since the other amps....like the AD30VT....has no ext spkr jack, I take it that those amps do not want to see anything other than an 8 ohm load,also.

Don't take this as written in stone, but that is what I read there. What people do versus what the designer intenedd are sometimes at great odds.
I take it that these amps are different from most other SS amps in this respect.

[printer2]

Originally Posted by printer2

The power output stage is only one part of the amp. Even if you do put two chips in the amp the power supply is only sized to put out 30W and you will not be getting more. The chip is rated for about 60W at 4 ohms already but that is for normal music, not distorted guitar.

The chip has been run in parallel but it is a little more complicated than wiring them up together, not for your average person.

Should not have written the part where you can run two together, gives people the wrong idea. But it was early when I wrote it and I was running out the door.

Just to repeat myself, the chip is rated to put out 30W in 8 ohms and 60W into 4 ohms. You do not have to run two chips 'if the amp was sized to put out 60W'. That mean if the designers put in a transformer that can put out double the current than needed for running at 8 ohms. Also if they have adequate heat sink area to keep the chip cool at 4 ohms. The chip will put out 60W given these two things. The chip will drive 4 ohms with no harm to it as it pretty much protects itself if it gets out of its happy place.

The transformer does not know how to protect itself. If it gets overloaded it can fry. It is probably the weak link. If I had one of these amps and I wanted to determine if it could run 4 ohms I would measure the supply rails and see what they do at 8 ohms and at 4 ohms at full output. I would also measure the transformer and internal temperature looking for trouble.

Not to say I am recommending others to do the above, but being the type of person that could fix the above amp it would not bother me to push it outside its rated performance.