Changing speaker impedances? Possible?

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Mrjones2004x
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Changing speaker impedances? Possible?

Post by Mrjones2004x » Sat Nov 20, 2010 3:35 pm

Wondered if it was possible to change the value of a speakers resistance?

Following on from the vox ac4tv thread i tried my amp with an 8 ohm speaker from my 2x12cab and the sound was really nice, more bright more articulate and louder. Anyhow the combo comes with a 16ohm speaker is it possible to switch it down to 8ohms?

Ive read on the net that if you have a 8 ohm speaker and you want to run it as 16ohm you can use a resister in series. An 8ohm resister in series rated at 10 watts would be suitable for a 4-9watt amp apparently.

If this is right is it possible to do the reverse and run a 16ohm speaker at 8ohms? Id like the option of swapping over by a switch when i fancied to changed the sound and overall volume of the amp. If it means buying an 8ohm speaker then i would do it if it was safe to run it as 16ohms with a resister.

Any comments??

If i run a 16ohm resister in paralell with a switch it will take the load of the speaker to 8ohms yes? Is it safe to do so??

This is the type of resister i was thinking of http://uk.farnell.com/jsp/displayProduc ... 2-00001000" onclick="window.open(this.href);return false;
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Re: Changing speaker impedances? Possible?

Post by TheOrangeJuicer » Sat Nov 20, 2010 6:17 pm

Mrjones2004x wrote:Wondered if it was possible to change the value of a speakers resistance?

Following on from the vox ac4tv thread i tried my amp with an 8 ohm speaker from my 2x12cab and the sound was really nice, more bright more articulate and louder. Anyhow the combo comes with a 16ohm speaker is it possible to switch it down to 8ohms?

Ive read on the net that if you have a 8 ohm speaker and you want to run it as 16ohm you can use a resister in series. An 8ohm resister in series rated at 10 watts would be suitable for a 4-9watt amp apparently.

If this is right is it possible to do the reverse and run a 16ohm speaker at 8ohms? Id like the option of swapping over by a switch when i fancied to changed the sound and overall volume of the amp. If it means buying an 8ohm speaker then i would do it if it was safe to run it as 16ohms with a resister.

Any comments??

If i run a 16ohm resister in paralell with a switch it will take the load of the speaker to 8ohms yes? Is it safe to do so??

This is the type of resister i was thinking of http://uk.farnell.com/jsp/displayProduc ... 2-00001000" onclick="window.open(this.href);return false;
A 16 ohm resistor of sufficient wattage in parallel with a 16 ohm driver will result in an 8 ohm load. Don't do it though. It just sucks the tone.

There are a lot of reasons that one speaker might sound different from another. Just because it was a different impedance doesn't mean much. Do you know what type of speaker it was that you liked the sound of? What is in the amp now? It might be possible to find the speaker type that you preferred the sound of in a 16 ohm version. It might also be possible to find a similar or just a better sounding speaker for the amp if we knew what was in there now and what you'd rather have it sound more like.
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Re: Changing speaker impedances? Possible?

Post by Mrjones2004x » Sat Nov 20, 2010 8:48 pm

not sure if you followed the other vox thread but when you run the 8ohm speaker it plugs into the 16ohm jack as that is the only one available but the transformer is of high enough spec to be able to cope with it and the amp will push out more wattage and sound completely different.

with the 8ohm speaker the amp became more articulate and the tone was brighter overall, the tone knob all the way off way really bright but nice. the whole amp sound changed so thats why i wondered about doing the mod so i can run it as standard 16ohm which is sort of crunchy and fuzzy to me and the swap to 8ohm by switching in a resister to change to other style.

if you havent read the other thread then you'll understand what i mean by trying out this mod.

thanks.
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Re: Changing speaker impedances? Possible?

Post by TheOrangeJuicer » Sat Nov 20, 2010 10:09 pm

I understand what you are saying, but that's not why it is happening. The other speaker sounded different because it was different. Simply changing the impedance of the same speaker will only make it sound a bit more compressed, the opposite of what it sounds like you may want. I build amplifiers and have worked on them for years, so I have learned a few things along the way. Changing the loading of the same speaker isn't going to help, it'll just suck tone from your amp. If you answer the questions that I asked, perhaps others could make some suggestions that will help. I'm just trying to help but I can't if you insist on telling me what my answer should be...

If you really want to find out what a resistor will do, then go buy a big 16 ohm one and strap it across there like I said and you'll then have first hand experience. I can't recommend it as being productive for improving your tone though.
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Re: Changing speaker impedances? Possible?

Post by Mrjones2004x » Sat Nov 20, 2010 10:43 pm

the stock 16ohm speaker is an 8inch celestion seventy i think and the 12inch i used was a seventy as well from a cheap laney solid state.

i think there both the same type jus

t diff sizes.

i tried my marshall cab at 16ohms and that has a vintage 30 and a heritage speaker and it sounded the same as the combo just bigger and bolder with more punch still had the fuzzy type gain just more larger overall.

hope this helps.

the 8ohm speaker sounds very diff, the tone knob did nothing really and had to be in the full counter clockwise position and it was stilll pretty bright. the brightness was insane when maxed.
the marshall cab still sounded similar to the stock speaker in regards to tone being fuzzy.

hope this makes sence.
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Re: Changing speaker impedances? Possible?

Post by baytamusic » Sat Nov 20, 2010 11:48 pm

TheOrangeJuicer wrote:
Mrjones2004x wrote:Wondered if it was possible to change the value of a speakers resistance?

Following on from the vox ac4tv thread i tried my amp with an 8 ohm speaker from my 2x12cab and the sound was really nice, more bright more articulate and louder. Anyhow the combo comes with a 16ohm speaker is it possible to switch it down to 8ohms?

Ive read on the net that if you have a 8 ohm speaker and you want to run it as 16ohm you can use a resister in series. An 8ohm resister in series rated at 10 watts would be suitable for a 4-9watt amp apparently.

If this is right is it possible to do the reverse and run a 16ohm speaker at 8ohms? Id like the option of swapping over by a switch when i fancied to changed the sound and overall volume of the amp. If it means buying an 8ohm speaker then i would do it if it was safe to run it as 16ohms with a resister.

Any comments??

If i run a 16ohm resister in paralell with a switch it will take the load of the speaker to 8ohms yes? Is it safe to do so??

This is the type of resister i was thinking of http://uk.farnell.com/jsp/displayProduc ... 2-00001000" onclick="window.open(this.href);return false;
A 16 ohm resistor of sufficient wattage in parallel with a 16 ohm driver will result in an 8 ohm load. Don't do it though. It just sucks the tone.

There are a lot of reasons that one speaker might sound different from another. Just because it was a different impedance doesn't mean much. Do you know what type of speaker it was that you liked the sound of? What is in the amp now? It might be possible to find the speaker type that you preferred the sound of in a 16 ohm version. It might also be possible to find a similar or just a better sounding speaker for the amp if we knew what was in there now and what you'd rather have it sound more like.
-Bill
Actually, there is some weird thing going on with the Vox AC4. I guess if you use an 8ohm speaker the amp puts out about 8-9 watts instead of the normal 4-5w. I think he just wants to know if he can change the resistance of a 16ohm speaker to 8ohms using some type of resistor to see if it makes a difference with the stock speaker running at 8ohm and getting more output from the amp.

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Re: Changing speaker impedances? Possible?

Post by Mrjones2004x » Sun Nov 21, 2010 1:03 am

exactly as above. want to have it switchable so can use it as normal 16ohm with attenuater at home the switch to 8ohm on 4watt setting to get max from it with a slight different sound. the 8ohm sounded very different for a similar speaker even the size difference was big
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Re: Changing speaker impedances? Possible?

Post by TheOrangeJuicer » Sun Nov 21, 2010 3:12 pm

Mrjones2004x wrote:exactly as above. want to have it switchable so can use it as normal 16ohm with attenuater at home the switch to 8ohm on 4watt setting to get max from it with a slight different sound. the 8ohm sounded very different for a similar speaker even the size difference was big
OK, I went to the other thread and read it about the amp in question. It is definitely not designed to be efficient but was deliberately hobbled by Vox. That sounds pretty goofy to me in itself. I thought you were writing the amp description at first because there were no quotation marks and it appeared that you were the author, even though you began with "I read this on another forum" or some such. So I was even more confused at why if you knew how the amp worked that you'd be asking the question. Now it all makes more sense. The part that doesn't make much sense is Vox's design. It goes to show you just how cheap Chinese amps are, they can waste components due to the economy of doing business there as well as the economy of scale.

What the fellow is saying is that it is indeed an 8 ohm amp with only the one output. So they intentionally misloaded the thing with the 16 ohm speaker (or the Chinese factory loaded it incorrectly). Either way, it's not an efficient design. It sounds to me like it was intentional due to your comments about the tone control. They must have also wired it up to be excessively bright so that it still sounded fairly bright with the little speaker they chose to use. That makes it hard to work with without rebuilding the whole thing. They didn't want it to be efficient so that it could be marketed as a "bedroom amp", yet they didn't want to design another one, so they just modded an existing design.

The amp will not put out 10w, no matter what you do to it as it's a single-ended design, so you don't need to worry about the transformer if its truly sized for the full output. That I cannot verify based upon his speculation but it does sound correct. You have a couple of things going on there with the speaker swap. You do have some power transfer differences due to the impedance mismatch but you also have a couple of other issues to address. For a straight-forward answer, yes you'll get more power output from the amp with an 8 ohm speaker connected. The amp sounds like it only has the one output, no taps so you cannot switch speaker loads or parallel without changing the amps loading or changing the speaker impedance to keep it consistent. The differences in level that you are hearing are more due to the speaker size than the speaker impedance. yes, by increasing the size and lowering the impedance in this case, you'll get even more volume due to the increased power output. That's because the amp has the wrong speaker in it stock. That still will not address the tone stack issue that you experienced.

Now, back to the original question about adding resistors to the speaker outputs. Even though the amp really wants to see an 8 ohm load, you still can't really make good use of a resistor there as it'll still effect the tone. it will waste power and so the power gained by using the proper speaker loading will be lost by using the resistor. To better explain that, the amp does just see the load as an impedance but your ears hear it as an output. You could safely load the amp with a purely resistive load and it would not fail but it also would not have any audible output. it's output would be only heat. So you'd have a very expensive, very inefficient space heater... The function of the speaker is to convert the amps output power to sound. So using a pure speaker load, you get a more complete transfer of power to acoustic output. By using resistors to form a portion of that load, you again change the output from sound to heat but this time only by a percentage, so let's say you'll lose 25% efficiency. That means that you've not really gained any power as your 5w amp is now only a 4w amp, even though you've increased the driver size a bit, the result is no net gain. It will change the tone though. Typically, adding resistive elements and lowering impedance causes HF roll-off. Now that you've expressed how bright the amp sounds with a proper speaker connected, maybe that's not such a bad thing tonally and it'd be an easier fix than going after the actual problem, which is in the front end of the amplifier. So what I'm saying here is that you are better off by selecting speakers that are tonally to your liking and that are rated at 8 ohms in order to take full advantage of the amplifiers power output. Short of that, I'd swap amps. That's way too much hassle. I mean, if you got the amp for free, I guess that's one thing but why put up with that crap if you don't have to, just get an amp that works for you.
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Re: Changing speaker impedances? Possible?

Post by Mrjones2004x » Sun Nov 21, 2010 6:55 pm

Great reply, sorry for the confusion. All the info I had was 2nd hand and not my knowledge as I'm new to electronics in amps.

Anyhow, I liked the 8ohm sound. Volume isnt important really. I'd like to use the attenuater for quiet practice times so from what I've read this circuit will need modding to deal with any higher wattage and also some other mods required for the tone stack. Would prob want to do a 3 position switch taking it down in brightness overall. Also to prolong the life of the el84 I would need to change the grid resister for another value.
Does this sound right to you? Or is it all wrong?
I like modding so want to make amp as good as possible. Also the speaker is a vox own brand a vx8 not the seventy 80.

Thank for your help so far, any further help is always appreciated as I'd like to learn what I'm doing and why
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Re: Changing speaker impedances? Possible?

Post by TheOrangeJuicer » Sun Nov 21, 2010 8:08 pm

Mrjones2004x wrote:Great reply, sorry for the confusion. All the info I had was 2nd hand and not my knowledge as I'm new to electronics in amps.

Anyhow, I liked the 8ohm sound. Volume isnt important really. I'd like to use the attenuater for quiet practice times so from what I've read this circuit will need modding to deal with any higher wattage and also some other mods required for the tone stack. Would prob want to do a 3 position switch taking it down in brightness overall. Also to prolong the life of the el84 I would need to change the grid resister for another value.
Does this sound right to you? Or is it all wrong?
I like modding so want to make amp as good as possible. Also the speaker is a vox own brand a vx8 not the seventy 80.

Thank for your help so far, any further help is always appreciated as I'd like to learn what I'm doing and why
I don't think it needs any mods. Just connecting an 8 ohm speaker of your choice to it will change the tone. Find a speaker you like, and you're done. It's not going to put out twice the wattage, he's wrong about that. You might get maybe another watt at peaks. I think the stock resistors will be fine. I don't know where the talk of changing grid resistors came from. The EL84 is a cheap tube and it sounds good when run hot. I'd leave the thing alone. If you are not happy with the tone, I'd get something else. If it's a new amp, you might be able to do a trade-in without loss. If it's a used amp, it'll still have more value unmolested.

All they did was slap a couple of resistors on the output to form an attenuator. Now that I think about it, they might have used a 16 ohm driver because of that. If they load down the secondary with resistors, an 8 ohm driver might create a 2 ohm load or something. It's hard to say just how it's wired to the speaker through those resistors. That would make sense from Vox's standpoint as it's a super cheap way to make an attenuator. you just start out too high with the load impedance and then you drop it down to too low with a couple of click from the power switch. That would make the middle position perhaps already the 8 ohm load by way of 16 ohm speaker and resistor that you were originally asking about.

Bottom line is that these things are super cheap and no matter what you do to them, they are one trick ponies. You can make them brighter or darker, more muted or more dynamic but they are still 4w amps with nothing but the basic circuit, your guitar and a volume control. The power switch is just changing the load and sucking power from the speaker.

Again, you're better off with another amp if it's not doing what you want. If you like it's one trick, then hang with it but it's never going to be a do-it-all amp. there isn't a trick to add a switch and get Vox, click - fender, click - Marshall, click -Orange. Fuggehtaboutit...

I make a great sounding little Fender Princeton clone and it's a 5-6w amp. It does cleans very well and you can crank the volume and it gets a little raunchy sounding but not way louder. It's only got 4-6w output depending on what guitar/pedal you throw at it. You buy the amp for its basic tone and build quality and then you add pedals to taste. Modding it is not a good idea. It is a simple amp with limited range and is meant to be just that. For guys that want bigger amps, I make them too. Not one amp covers everything. If they could, no one would need a different one and there would be way fewer choices. Why do you think Vox makes the AC15 and the AC30?!
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Re: Changing speaker impedances? Possible?

Post by sidvicious » Sun Nov 21, 2010 9:25 pm

i don't know as much as bill but i know a bit.

what i'm not following is why you'd really want to mod an amp like that - the vox4. if you like the vox thing then i'd recommend getting a night train and switch between them. or, get an orange dual terror. both are reasonably priced. i had to do a gig for a friend's wedding reception recently. using a nt and the dt, it went off great. it was a large crowd but the amps handled it perfectly. plenty of volume and tone. if i need more then i'll use my vox50 and tv50.

i don't intend to criticize what i detect is your curiosity; not at all. indeed, curiosity and tinkering leads to discoveries. however, as old as tube circuitry is, its pretty much been done.

i tend to give props to the designer of the amp. as an analogy i drive bmw's and own several. on the forums the fanatics are always talking mods from oil to intake, etc. i figure bmw knows what its doing so i use the recommended oil weight. i also see no need to increase air intake volume as the cars i have already run like scalded-ass apes.

same thing for amps. bill is right about economic decisions affecting the product, but the basic electrical circuitry will be pretty much on spot.

bill [and andy] is also right that output and impedance is a fixed concept. surf-style players of sufficient age know about all there is to know about transformers and matching speakers, etc. you might try some of those forums provided the players are still alive.

good luck, however, whatever your approach.
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Re: Changing speaker impedances? Possible?

Post by Mrjones2004x » Sun Nov 21, 2010 11:13 pm

I already have a blackstar ht5 for my main style of music I like and I wanted something in a small package that did the vox sound. I just want to improve it to the best it can be. I liked the sound that came from it with an 8ohm speaker so hence the mod questions.
Simple as that really. Had a epi valve junior before which didn't get voxy sounding at all, the class 5 doesn't sound voxy. It's the only thing close to what I wanted so if I tweak it I get what I want.
The amp I want doesnt exsist; an ac30 in 5 watt combo form. If I can mods this and get a sound I like them I'm happy. The 8 ohm gave me a bright voxy sound in my opinion and wanted to make sure what I needed to make the rest of the circuit ok with an 8ohm speaker
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Re: Changing speaker impedances? Possible?

Post by Mrjones2004x » Sun Nov 21, 2010 11:25 pm

If mount a lpad like normal would this be ok as then from what people have said I can keep on the 4 setting and use the lpad to attenuate instead of the built in one? Still using an 8 ohm speaker of course.
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Re: Changing speaker impedances? Possible?

Post by TheOrangeJuicer » Mon Nov 22, 2010 12:23 am

Mrjones2004x wrote:If mount a lpad like normal would this be ok as then from what people have said I can keep on the 4 setting and use the lpad to attenuate instead of the built in one? Still using an 8 ohm speaker of course.
An external, 8 ohm, 1x12 or 1x10 would be the best option for tonal versatility. The L-pad is actually a good idea for such a low wattage amp. It'll suck tone too but it's pretty much infinitely adjustable, so you should find a couple of places that are good to go there. That can either be a separate control in it's own little project box (like a pedal sized box) with 1/4" in and 1/4" out. That would make it easy to insert in line aith any small amp and speaker cab.

If you want to keep it in it's compact, combo form, then it's going to be hard to do much more than a 1x10 due to space restaints, no? If you can wedge a 10" speaker in there, you could mount an L-pad on an L shaped bracket and screw the bracket to the inside of the cab, so that the only mod is a couple of tiny holes on the inside of the combo cab. And yes, keeping the amps built-in attenuator on the maximum (off) setting.

One of the "secrets" to the AC30 sound is its hard nose, near class A bias of the output tubes. That gives what otherwise might be an overly bright sound, a very liquid and "sweet" sound. So you go from what might have sounded like breaking glass to ringing chrome bells. If Vox has run the tube rather hot in the little amp, then that's not a bad thing as long as the circuit board can handle it. Like I said earlier, the EL84 is a cheap tube. Buy a couple of Sovteks and you'll be stocked for years.
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Re: Changing speaker impedances? Possible?

Post by Mrjones2004x » Mon Nov 22, 2010 2:40 am

I just modified the cab so it's open at the bottom, I could squeeze a 10" in width wise but I'd have to check depth wise as the tubes and transformer are close to the speaker already. Prob stick with 8" but get better quality one. If I could move the attenuater board on the amp already and place the lpad there that would be good but not sure if possible.
We'll see! Thanks so far
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