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Topic: amp head ohm setting ? |
Delvin Morgan
From:
Lindstrom, Minnesota, USA
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Posted 4 Jul 2009 6:56 am |
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| I have 2 tube amp heads (Epi Jr & PV VK100), they both have 4,8 and 16 ohm speaker settings. Question is, what is the advantage or disadvantage of each ohm setting? |
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Jon Light
From:
Saugerties, NY
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Posted 4 Jul 2009 7:06 am
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| No advantage/disadvantage. You match the output impedance to the marked impedance of the speaker you are connecting to. |
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Delvin Morgan
From:
Lindstrom, Minnesota, USA
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Posted 4 Jul 2009 7:14 am
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| OK, then why are speakers made with all the different impedance values? |
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Jon Light
From:
Saugerties, NY
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Posted 4 Jul 2009 7:34 am
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I don't know what other technical reasons might be involved but one primary reason is that if you have a two speaker cabinet with, for instance, two 8 ohm speakers, then depending on whether they are wired in parallel or in series, the only impedance options for these two speakers combined is 16 ohm or 4 ohm. Two 8 ohm speakers together cannot be 8 ohm. Older amps did not have output options. The transformer was fixed at...whatever. 8 ohm, let's say. So you would need one 8 ohm speaker. Or two 4 ohm speakers. Or two 16 ohm speakers.
The short answer comes back to the lack of switchable output impedance. If the output options existed back then, I don't know that it would have been necessary to have different choices in speaker impedances. But you need to match whatever the amp transformer says it puts out.
In reading this over, it is a mess but it is accurate, as far as I understand. Rather than edit into intelligeability (sp) I will let it be. |
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Dave Mudgett
From:
Central Pennsylvania and Gallatin, Tennessee
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Posted 4 Jul 2009 7:51 am
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The short answer is that those amp heads are designed so they can be used with different kinds of speaker cabinets with different impedances. It's a very useful design feature for a tube head, since (IMO) they run best when the output impedance of the head is approximately matched to the input impedance of the speaker cabinet, as Jon said.
There are lots of reasons why different speaker cabinet impedances are used. First - each speaker in the cabinet has its individual impedance, which depends on how the speaker is designed and how it's intended to be used. Secondly, there is the number of speakers in the cabinet and how they're wired together - how they're wired affects the overall impedance of the cabinet. If you have 4 8-Ohm speakers in an old Fender amp, they're almost always wired in parallel for 2 ohms. But if one was to split them up into 2 pair, wire each pair in parallel, and then wire those two pair in series, 8 ohms would result. Similarly, one could wire each pair in series, and then wire the pair together in parallel, to also give 8 Ohms. Or one could wire all 4 in series to get 32 Ohms. Some individual speakers are 4 Ohms, some are 16 Ohms - many Marshall 4x12" cabs do this. With different impedances and wiring options, you can see that there are a lot of possible speaker impedances one could contend with.
I think most of the time, the overall cabinet impedance is a matter of convenience, but that impedance choice - transformer tap + overall impedance - affects the overall tone of the amp. The difference is subtle, but present. A Super Reverb run into 4 speakers in parallel for 2 Ohms sounds different than the same amp run as a series pair of 2 speakers in parallel for 8 Ohms.
This isn't overly complex if you understand basic electricity or electronics, but if not - the short answer is enough. Just match the amp impedance to the speaker impedance, and you're good to go. |
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Dave Grafe
From:
Hudson River Valley NY
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Posted 7 Jul 2009 9:48 am
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Just as with simple resitance, the total load of speakers wired in in series is the sum of the individual parts (8+8=16) and the total load of speakers wired in parallel is the reciprocal of the sum of the reciprocals (1/8+1/8=1/4).
Unlike solid state amplifiers, which have a minimum acceptable impedance but work very well with any load higher than the minimum, tube amplifiers use an output transformer and lose efficiency and/or overheat badly when the load is not properly matched to the output windings. This is why the better tube amp designs feature impedance selectors and why it is important to set them properly. |
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Delvin Morgan
From:
Lindstrom, Minnesota, USA
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Posted 7 Jul 2009 11:16 am
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| But I don't believe anyone has answered why speakers were of so many impedance values in the first place. Is a 16 ohm speaker better tone than, say an 8 ohm speaker? If you have a combo amp with built in speaker the impedance shouldn't matter, right? Then why are some 4, 8 or 16 ohm design? |
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John Gould
From:
Houston, TX Now in Cleveland TX
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Posted 7 Jul 2009 7:33 pm
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The reason that speaker are made with different voice coil resistance values is to match the different values of output devices. Not all amps can operate at different impedance values. Combo amps that use only one speaker need that one speaker to be the right value for the output of the amps final stage. The value has no bearing on tone, the paper the cone is made from and the weight of it has the most to do with tone. You can thump the cone of a speaker with your finger and tell a bunch about how a speaker is voiced.
I'm not saying the voice coil doesn't effect tone, I'm saying the same make and model of speaker one a 8 ohm and one a 16 ohm should sound the same.
_________________
A couple of guitars
Fender GTX 100 Fender Mustang III Fender Blues Jr. Boss Katana MKII 50
Justice Pro Lite and Sho Bud Pro II |
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Larry Bressington
From:
Nebraska
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Posted 7 Jul 2009 7:46 pm
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An ohm reading is a variable resistance, basically, there is no such thing as 'fixed ohms'
Its an average that the amp likes, it's a very variable signal and ohm reading, if it's a solid state amp she maybe more consistant.
If you are daisy chaining two 8 ohm speakers, you are running 4 ohms.
8 ohms DIVIDED BY 2 speaker in series = 4 OHMS
8 Divided by 2 = 4
If you are running parallel, then it's 8 ohms + 8 ohms
Very few people run that unless they are electronically versed and have a pre-wired system.
Daisy chaining at the gig is ' series'
Always; Ohms divided by speaker amount.
EX;
Three 8 ohm speakers = 8 divided by 3 = 2.6 ohms on average, your amp will struggle under high power if it's an 4 ohm load on average, you will get away with it up to 50% power, then it's all down hill.
Be carefull, output transistors can be exspensive, and they will get hot with an average load less than 4 if if that's the rated.
Some are a bearcat to replace too, meaning you have to remove heatsink and many other componants.
It's a complete teardown, and you will also need an In circuit transitor tester for NPN and PNP
Traditional plug in and play is 'series"
So it's speakers divided by ohms!
Good luck me lad!!
_________________
A.K.A Chappy. |
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John Gould
From:
Houston, TX Now in Cleveland TX
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Posted 8 Jul 2009 5:54 am
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I'm sorry Larry but I think you got it backwards.
When you daisy chain cabs its parallel.
Series Wiring - For speakers in series you add the impedances of the speakers together. To determine the wattage you add the wattage of the speakers together.
If you were using (2) 8 ohm speakers you'd have a 16 ohm cabinet. If you were using (2) 200 watt speakers you'd have 400 watts of power handling.
Parallel Wiring - Speakers of equal impedances you divide the impedance value by the number of speakers. To determine the wattage you add the wattage of the speakers together.
If you wire (4) 16 ohm speakers in parallel you'd have a 4 ohm cabinet. (2) 16 ohm speakers in parallel would give you an 8 ohm cabinet. If you were using (4) 100 watts speakers you'd have 400 watts power handling. If you were using (2) 100 watt speakers you'd have 200 watts power handling.
Series/Parallel Wiring - All speaker's should be the same impedance & wattage. The impedance of the cabinet will be the same as one speaker. The wattage will be 4 times the individual speaker wattage.
If you're using 8 ohm speakers the cabinet will be 8 ohms. If you were using 100 watt speakers the wattage for the cabinet will be 400 watts power handling.
_________________
A couple of guitars
Fender GTX 100 Fender Mustang III Fender Blues Jr. Boss Katana MKII 50
Justice Pro Lite and Sho Bud Pro II |
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Larry Bressington
From:
Nebraska
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Posted 8 Jul 2009 6:23 am
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Sorry guys, i did do that backwards!
Thanks john for bring that to my attention. 
_________________
A.K.A Chappy. |
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Dave Mudgett
From:
Central Pennsylvania and Gallatin, Tennessee
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Posted 8 Jul 2009 8:02 am
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| Quote: |
| But I don't believe anyone has answered why speakers were of so many impedance values in the first place. |
Actually, I think I did, but in a general way. One important thing is how the speakers will be used in one or more cabinets. Tube amps to like have a load matched to the output tube + transformer configuration, and lots of amps don't have multi-impedance output taps like the ones you have. But different people may want to run different types and/or numbers of cabinets. So if you want to run a specific type of cab at a particular impedance, the speakers' impedance matters.
For example, if you want to run two 4x12" cabs in parallel at a total 8 Ohm impedance, then probably the most common way to get this is to wire 4 16-Ohm speakers in series-parallel to make each cab 16 Ohms, and now run those two 16 Ohm cabs in parallel for 8 Ohms total. If each speaker was 8 Ohms, the total would now be 4 Ohms, and so on.
The other thing is that there is a performance and tonal difference between using, let's say, an 8-Ohm tap into an 8-Ohm load vs. a 2-Ohm tap into a 2-Ohm load. That difference is more subtle than actually mismatching the amp and load impedances, but it's there. Remember that true complex impedance is frequency-dependent, so the transformer, voice coil, and nominal impedance figure in.
| Quote: |
| If you have a combo amp with built in speaker the impedance shouldn't matter, right? Then why are some 4, 8 or 16 ohm design? |
For example, most old Fender combos used 8-Ohm speakers because they were (and are) more common, so the overall cabinet impedance was a function of the speaker circuit topology - how many speakers and how they're wired. |
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Jon Light
From:
Saugerties, NY
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Posted 8 Jul 2009 11:02 am
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| Delvin Morgan wrote: |
| But I don't believe anyone has answered why speakers were of so many impedance values in the first place. |
| Dave Mudgett wrote: |
Actually, I think I did, but in a general way. |
I did too. In days before multi-taps the only way to match up various speaker configurations with the ouput impedance was by mixing & matching different speaker impedances. |
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Donny Hinson
From:
Glen Burnie, Md. U.S.A.
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Posted 8 Jul 2009 3:48 pm
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Amplifier manufacturers are constricted by design factors, as well as by component availability and price. Many solid state amps use 4 ohm outputs because it's very economical and easy to design a 4 ohm direct-coupled output stage. Tube amp manufacturers have the same dilemmas, but they also have to be able to procure output transformers to match both the tubes and the speakers. A design meeting may go something like this...
"We can save $10 per amp if we use 8 ohm transformers!"
"No problem, we'll just switch to using two 16 ohm speakers".
Individual speakers generally range in impedance from 2 ohms to 32 ohms, and they generally pick whatever they can use to save money. It's a business, after all. |
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Dave Grafe
From:
Hudson River Valley NY
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Posted 14 Jul 2009 9:10 pm
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"Always; Ohms divided by speaker amount. "
Sure, you could do it that way IF AND ONLY IF the individual impedances are all equal, but this simple formula breaks down when they are not. The correct formula that I have already posted above remains the one taught all beginning electronics students: ...the total load of individual loads wired in parallel is the reciprocal of the sum of the reciprocals (1/8+1/8=1/4) or (1/8+1/8+1/8=3/ or 1/8+1/8+1/4=1/2), etc.
It is also significant to note that the impedance of a given speaker will vary widely from the listed NOMINAL impedances as the actual impedance is constantly changing with frequency and acoustical loading of the cone structure. |
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Dave Mudgett
From:
Central Pennsylvania and Gallatin, Tennessee
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Posted 15 Jul 2009 11:27 am
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Dave's correct: the proper formula for n different impedances in parallel, Z1, Z2, ... , Zn, is
1/Ztotal = 1/Z1 + 1/Z2 + ... + 1/Zn
You can see that if the impedances are all equal (call them simply Z), this reduces to
1/Ztotal = n/Z, so Ztotal = Z/n
Impedance definitely depends on frequency, as I said in my earlier posts. That dependence is a function of many parameters in the output transformer and speaker(s). This is one reason why using a 2-Ohm tap into a 2-Ohm load may sound different than using an 8-Ohm tap into an 8-Ohm load, even if the basic speaker design is identical. |
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