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Topic: 4 ohms or 8 ohms? What determines which? |
Billy Carr
From:
Seminary, Mississippi, USA (deceased)
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Posted 6 Feb 2013 2:17 am |
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| I've often wondered what determines if an amp is 4 ohms or 8 ohms? I can replace speakers easily but I've never known exactly what separates 4 ohms from 8 ohms. Maybe some of you guys that are techs on such things can share a little info. One other question. An amp has 4 10" speakers rated at 8 ohms and you want to remove the 4 10's. Ok, will replacement speakers, let's say 2 12's need to be rated at 8 ohms like the 4 10's? Maybe a 15" replacement. 4 or 8 ohms? I really don't know the answer. Thanks. |
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Billy Carr
From:
Seminary, Mississippi, USA (deceased)
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Posted 6 Feb 2013 2:20 am amps, etc.
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| Forgot this. What is a good lightweight 15" speaker to place in a Session 400? Thanks. |
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Jack Stoner
From:
Kansas City, MO
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Posted 6 Feb 2013 2:44 am
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The rating for the amplifier determines what is the correct impedence (ohms) load for the amplifier.
For example the Nashville 400 is rated at full power out on a 4 ohm load. If you used an 8 ohm speaker the rated output power would be a little over half the rated power output.
The amp manufacturer will specify the minimum ohms for a solid state amp. Going below the minimum could cause damage to the amp. On a tube amp, matching the impedence to the impedence of the power output transformer is key to full power.
The Emminence EPS 15-C 4ohm neodymium speaker (7 lbs) is designed to be a direct replacement for the Peavey 1501-4 speaker used on many Peavey steel guitar amps. And, the Peavey 1501-4 was designed to be a high power replacement (clone) for the JBL speakers that were in the original Session 400's. The JBL speakers couldn't handle the power output of the Session 400 and would fail. |
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Bill Myers
From:
Camp Hill, Pennsylvania, USA
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Posted 6 Feb 2013 2:46 am
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The output transformer in the amp is generally what determines the output impedance of the amp. The output transformer is a coil and the coil can be tapped at different locations to determine what the output impedance will be. A good example would be a Twin Reverb versus a Vibrosonic. It is the same basic amp but a Twin has a 4 ohm output transformer and a Vibrosonic has an 8 ohm output transformer. In the days when Twins and Vibrosonic's were new...most speakers were 8 ohms. So when you hooked a pair of 12's together in series you get a 16 ohms load...in parallel you get 4 ohms. 4 ohms makes more volume so I am guessing this is why they used 4 ohms. A Vibrosonic uses one 8 ohm speaker...so they use an 8 ohm output transformer.
As far a 4 ohm speaker...I have heard good things about the EPS15C. My buddy who is also a great amp tech has one in his nashville 400 and he is very happy with the sound.
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Carter D10 9 x 5, Nashville 112, Goodrich L10K |
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Helmut Gragger
From:
Austria
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Posted 6 Feb 2013 3:15 am
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| Bill Myers wrote: |
| The output transformer in the amp is generally what determines the output impedance of the amp. |
...if there is one. Practially all contemporary non-tube amplifiers are direct coupled. Nevertheless, the manufacturer determines a minimum load impedance. Going below that may destroy the amplifier. Going above will not do any harm, but limit the acoustical power somewhat.
Note that the ear is logarithmic and half the power does not mean half the loudness!
Also, the given speaker impedance figure usually presents its DC coil resistance. Over the useful frequency range, the speaker´s impedance may well go below (or way above) the rated DC resistance - that´s why it is called "impedance".
Since this happens in a small frequency region, the amp survives it.
Be careful how you wire multiple drivers.
-helmut
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feel at home at: http://me.aquataur.guru |
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Stephen Cowell
From:
Round Rock, Texas, USA
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Posted 6 Feb 2013 4:38 am
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| Helmut Gragger wrote: |
| Bill Myers wrote: |
| The output transformer in the amp is generally what determines the output impedance of the amp. |
...if there is one. Practially all contemporary non-tube amplifiers are direct coupled. Nevertheless, the manufacturer determines a minimum load impedance. Going below that may destroy the amplifier. Going above will not do any harm, but limit the acoustical power somewhat. |
Important to note exceptions... tube amps hate higher impedances, note the shorted speaker jacks of Fender amps (playing into a short is safer for the amp than playing into an open). Also, the Roland XL80 specifies 'no more than 8ohms' be connected, even though it's a solid-state amp.
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Too much junk to list... always getting more. |
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Helmut Gragger
From:
Austria
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Posted 6 Feb 2013 4:52 am
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| Stephen Cowell wrote: |
| Important to note exceptions... tube amps hate higher impedances, note the shorted speaker jacks of Fender amps (playing into a short is safer for the amp than playing into an open). |
Tube amplifiers have a selectable output impedance matched to the load. This means, a resistance measurement of the output transformer tap would yield 8,16 Ohms or whatever selected. An impedance mismatch is not optimal, but an open output may produce arching due to the high off load voltages, thus the output short circuit. This is not as dramatic as it may sound, because there is always the wire resistance of 4,8 resp. 16 Ohms left. I believe this is not the best mode to run a tube amp, but it protects it.
| Stephen Cowell wrote: |
| Also, the Roland XL80 specifies 'no more than 8ohms' be connected, even though it's a solid-state amp. |
I never stop learning... 
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feel at home at: http://me.aquataur.guru |
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Bill Myers
From:
Camp Hill, Pennsylvania, USA
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Posted 6 Feb 2013 5:23 am
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Tube amp are very sensitive to the amount flow of power going out to the speaker. If you think of a speaker like an antenna...the load is designed to match the flow of current going out of the amplifier section. This is the reason that tube amps have output transformers. It tunes the load (speaker) to the output resistance to minimize the amount of resistance coming out of the amplifier. Solid state amps are not near as sensitive to the match of the load to the output section...and therefore do not generally have an output transformer.
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Carter D10 9 x 5, Nashville 112, Goodrich L10K |
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Jim Hussey
From:
Reno, Nevada - USA
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Posted 6 Feb 2013 6:52 am
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Shorting unused outputs?
I use a Mossvalve stereo amp. When rehearsing in a small room, I use just 1 speaker with the output turned all the way down on the unused channel. Should I be shorting the unused output with a resistor?
Don't mean to hijack this thread, but I find this output topic very interesting.
Thanks |
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Jack Stoner
From:
Kansas City, MO
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Posted 6 Feb 2013 7:02 am
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Most solid state amps will run OK with no load attached. Most power amps, such as the MV500 (I used to have one) have a Bridged mode that uses both channels into one as another option.
I wouldn't put a load resistor accross an unused output unless it was one that had a high enough power rating and was a non-inductive load resistor. If you put a small wattage resistor across the unused output you chance it shorting and if the resistor shorts you will blow the output transistors (at a minimum). |
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Rich Hlaves
From:
Wildomar, California, USA
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Posted 6 Feb 2013 9:09 am
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Jim, never short the output of a solid state amp. It will go into protection mode and if not protected die very quickly.
The reference above to Fender shorting outputs is for tube amps only.
A solid state amp channel should run fine with no load. I do it all the time with my PA gear depending on how it's configured.
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On man....let the smoke out of another one. |
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Helmut Gragger
From:
Austria
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Posted 6 Feb 2013 9:41 am
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We should make this clear for casual readers...
Looking into a typical valve amp´s schematic, you may find that they use a metal output jack with a switch contact. The contact goes to ground and opens as soon as you plug your speaker in, without you noticing it.
There is nothing you have to do to this. Fully automatic protection.
As mentioned, doing this on a solid state amp would very likely damage the output stage somehow. A protecion mechanism like that is not necessary for solid state.
-helmut
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feel at home at: http://me.aquataur.guru |
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Dave Grafe
From:
Hudson River Valley NY
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Posted 6 Feb 2013 10:24 am
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| Quote: |
| This is the reason that tube amps have output transformers |
WRONG! The only reason for output transformers in tube amps is that the very high plate voltage (300 - 600VDC) required for their operation must be isolated from the speaker and other external wiring. Solid state amps for the most part (solid state digital amps are another issue altogether - see below) need no such isolation and thus no output transformer.
To simplify the discussion, vacuum tube 'valve' amplifiers and control circuits operate by charging the tube's plate with a high positive voltage, heating the cathode to produce a negatively charged electron stream, and passing this stream through a grid of varying polarity controlled by the originating AC voltage (the input signal). The output signal must then be separated from the plate voltage before it can be output to the speaker or the next stage of signal processing. At lower signal levels and plate voltages this is accomplished via smallish DC voltage blocking capacitors, but at the higher voltage and current levels transformers are used.
Like all electro-magnetic coils there exists within the transformer, in addition to the DC resistance of the wire itself, a complex pattern of inductive and reactive internal voltages known as impedance. This impedance changes constantly depending on the frequency of the AC signal passing through the transformer, but for purposes of clarity a 'nominal' impedance is identified which is generally at or near the lowest point in the impedance curve for a given device. A properly designed amplifier will match the tubes' output impedance to the input side of the OT, and will match the resulting output impedance of the OT to the intended speaker's nominal impedance, the 'load' impedance.
If this load impedance is too high the flow of current through the transformer is inhibited and the pile up of electrons on the input side leads to overheating and failure of both transofrmers and tubes. If the impedance is too low it may affect frequency response but will cause no damage - this is why an open load will kill a tube amp while a short does no harm. However, contrary to another post in this thread, not all tube amps have OT taps for different load impedances, thus the ongoing confusion and discussion about what does or does not work when replacing the OEM speaker with one of our own preference.
Solid state 'transitor' amp output sections essentially operate like controlled resistors, using a small current to control the flow of electricity from the DC power supply to the load. If this load is too high, the flow will simply be reduced, yielding lower volume, higher fidelity and less heat in the output devices than the specified 'optimum' load. If this load impedance is too low, the increased current flow will cause the output devices to overheat and fail. If the amp is connected to a dead short the entire power supply will dump in a big hurry and your output devices will be literally toast!
As has been pointed out here in many threads before, if it is a solid state amp, the danger is in too low a load impedance, while for a tube amp the danger is in the load impedance being too high.
As for calculating the load presented by multiple speakers, for speakers connected in series the total impedance is the sum of the parts; when connected in parallel the total is the reciprocal of the sum of the reciprocals. Thus four 8-ohm speakers in series would present 8+8+8+8 = 32 ohms impedance, the same four speakers connected in parallel would present 1/(1/8+1/8+1/8+1/ = 2 ohms total impedance. These formulae will work for any number of speakers of any impedance, and can be adapted for various series-parallel configurations in order to determine the correct impedance matching for your amplifier's output section.
Digital amps (classes D, H, etc.) use various schems to chop the AC line voltage into little slices and then control the release of these slices via digital means, using a large transformer as a choke to smooth out the slices on the output side. If one of these amps fails you will have line voltage passing through the output and through your speakers and everybody goes home sad. |
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Robert Leaman
From:
Murphy, North Carolina, USA
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Posted 6 Feb 2013 12:53 pm Impedance Matching
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For everyone's information. only when the load impedance matches the source impedance will maximum power be transmitted. In other words, if source impedance is 4 ohms, then load impedance must be 4 ohms for maximum power transmission. Any impedance mismatch results in a power transmission loss. This holds for any frequency range when impedance must be considered.
MSEE |
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Dave Mudgett
From:
Central Pennsylvania and Gallatin, Tennessee
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Posted 6 Feb 2013 2:15 pm
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I'm sure Robert knows this and is trying to keep the technical level straightforward: but for complex impedances, maximum power transfer occurs when the power source and loads are complex-conjugate-matched, i.e., the load complex impedance is the complex-conjugate of the source complex impedance. Complex impedance of a device is the resistance (which is generally a real constant at all frequencies) plus capacitive and inductive reactance (which are generally both imaginary functions of frequency). An impedance and its complex-conjugate are the same if and only if they are purely resistive, not the situation with a speaker or output transformer. In fact, at many frequencies of interest, the inductive reactance is often dominant.
| Quote: |
| The only reason for output transformers in tube amps is that the very high plate voltage (300 - 600VDC) required for their operation must be isolated from the speaker and other external wiring. |
This is a good practical reason to use an output transformer in a tube power amp, but as Robert states, it is not the only reason, nor are output transformers absolutely necessary. It is possible to build a tube power amp without an output transformer - they are called Output TransformerLess amps, or OTL for short. Since at least the 50s, designers have messed around with OTL amps for the simple reason that output transformers typically introduce a fair amount of distortion into the system. More in the links:
http://www.audiodesignguide.com/otl/otl.html
http://www.audiodesignguide.com/otl/
http://www.audioasylum.com/forums/otl/bbs.html
I'm not arguing for OTL tube amps, not at all. The main practical reasons for using an output transformer are impedance matching AND decoupling of the high DC voltages from the speakers. I also happen to like the character of the distortion of a well-designed tube amp with an output transformer. But output transformers are not absolutely necessary.
Billy - this subject of amp/speaker impedance matching has been discussed pretty exhaustively several times - here are a few links:
http://bb.steelguitarforum.com/viewtopic.php?t=228139
http://bb.steelguitarforum.com/viewtopic.php?t=170054
where I referenced this page at weberVST.com
http://www.webervst.com/sptalk.html
I personally like the SICA neodymium 15" speaker in my Nashville 400 and LTD 400 (Session 400 in a smaller box). These amps want 4 Ohms - I would never use a lower impedance than that, and using 8 Ohms cuts the power and, to my tastes, the tonality of those amps. You can get 4 Ohms in the following ways:
1. 1 4 Ohm speaker
2. 2 8 Ohm speakers in parallel
3. 4 16-Ohm speakers in parallel
4. 2 pairs of 4-Ohm speakers in series/parallel or parallel/series
Series/parallel means you take pairs of speakers and hook them up in series, then take the two pairs and hook them up in parallel. Parallel/series is the opposite - take pairs of 2 speakers and hook them up in parallel, and then take those two pairs and hook them up in series.
Yeah, this can get complex if you want to get into it. But I tend to just run amps the way they were designed unless there's some very good reason to change. |
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Helmut Gragger
From:
Austria
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Posted 7 Feb 2013 2:12 am
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| Dave Grafe wrote: |
Digital amps (classes D, H, etc.) use various schems to chop the AC line voltage into little slices and then control the release of these slices via digital means, |
Dave,
when I was building switchmode psu´s those switchmode amps were just on the advent...
If you have a primary switched device, is there a chance of building up dangerous static voltages if the load impedance is wrong?
The earlier message above on the Roland impedance demand annoys me. To tell purely from the footprint, the Roland must have a digtal output stage, I cannot imagine that they are able to cram a linear stage into that small enclosure.
On the other hand, I have never encountered a solid state linear output stage that is annoyed over a too big output impedance.
-helmut
_________________
feel at home at: http://me.aquataur.guru |
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Dave Grafe
From:
Hudson River Valley NY
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Posted 7 Feb 2013 8:49 am
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Perhaps Mr. Mudgett will correct me here, but increasing the load on a linear solid state output will do no damage, but simply reduce the amount of current flow; also the damping factor will go up, at least theoretically reducing distortion and spurious resonances due to speaker over-excursion.
I am aware of - and at one time owned - an OTL console radio, but it used windings at the rear of the speaker itself to double as a power choke and simultaneously to create the necessary magnetic field for the voice coil to operate in. To my knowledge this is not a practical matter for a guitar amp using an off-the-shelf musical instrument speaker (400+ volts on my detachable speaker lead, no thank you!  ), and thus my use of the word 'only' which is true within the context of common instrument anmplifiers. My apologies for any confusion.... |
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Dave Potter
From:
Texas
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Posted 7 Feb 2013 4:21 pm
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| Dave Mudgett wrote: |
| Yeah, this can get complex if you want to get into it. But I tend to just run amps the way they were designed unless there's some very good reason to change. |
I'm certain it CAN get complex - that's why universities offer degrees in electronics engineering. But, IMO, it's not necessary to have such a degree to stay on the right side of things.
"Imaginary functions of frequency" and "complex conjugates" notwithstanding, all that's really necessary is to make sure the load impedance matches the source impedance, as defined in the documentation for the amp.
It's also worth mentioning that "measuring the ohms" (resistance) of any circuit is NOT the same thing as that circuit's "impedance". This is not something one accomplishes with an ohmmeter. |
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