CWelsh Posted March 10 Share Posted March 10 The specs for the Forte III say the impedance is "8 Ohms compatible" and I'm curious what that means. I just saw something from Audio Research saying it pays to experiment a little with which impedance tap you choose when using tube amps because a "nominal impedance rating" of 4, 8, or 16 Ohms may be misleading. Apparently not the case with SS. I have an old Fisher integrated that I have tried with my Fortes and I wasn't overly impressed. The Fisher is rated at 28 wpc, and my 7 wpc Class A ACA just blows it away in terms of output volume. I haven't done any experimentation using different output taps on the Fisher yet, but expect I will. I just wondered if anyone else has any experience/thoughts? Quote Link to comment Share on other sites More sharing options...
Dave MacKay Posted March 10 Share Posted March 10 1 hour ago, CWelsh said: The specs for the Forte III say the impedance is "8 Ohms compatible" and I'm curious what that means. I just saw something from Audio Research saying it pays to experiment a little with which impedance tap you choose when using tube amps because a "nominal impedance rating" of 4, 8, or 16 Ohms may be misleading. Apparently not the case with SS. Impedance changes with frequency. An 8 ohm rating is a nominal figure — the actual impedance can vary significantly over the frequency range by quite a lot. Some crossovers (networks) try to normalize the impedance presented by a speaker but most don’t. Consider the K-33E woofer used in La Scala. Its impedance ranges from about 4 ohms (at around 150 Hz) to around 27 ohms (at around 45 Hz). The DCR (DC resistance) measures just under 4 ohms, but that’s a resistance figure, not impedance. Some time ago, the late (and much missed) DJK posted information that “the measured magnitude of impedance which the lipschorn presents to an amplifier … The lowest value within the audio range is 4.5 ohms and occurs at 55 Hz, while the highest value is almost 10x that amount, 42.3 ohms at 2155 Hz.” Even so, the Klipschorn is rated at 8 ohms. All amps — both tube and SS — will adjust (within reason) to the differing impedance presented by the speaker. However, tube amps may struggle to push around the low impedances more than SS. Unless you’re pushing the limits of your system, you should feel free to experiment with different impedance taps. You might try one speaker on one tap and the other on a different tap for testing. Dave 2 Quote Link to comment Share on other sites More sharing options...
82 Cornwalls Posted March 10 Share Posted March 10 Tube amplifiers use the output transformer to match as closely as possible the impedance the output tube likes to see with the impedance of the speaker. Some tube amplifiers use multiple output taps and it is good to try them as AR suggests, other tube amplifiers use an output transformer that only has one tap like 6 ohms knowing that it will work reasonably well with a wide range of speakers. There is more to it of course like voltage vs current etc. I have actually experienced a tube amplifier with a well designed power supply and circuit outperforming solid state amplifiers with weak power supplies (cost saving) driving very low (below 4 ohms) impedance. This is the exception rather than the norm though and as with most things we won't know for sure how an amplifier and a pair of speakers will work together until we try them. The "nominal" rating of a speaker generally is not very useful. There are guidelines, but many manufacturers have their own way of doing things. 1 Quote Link to comment Share on other sites More sharing options...
CWelsh Posted March 13 Author Share Posted March 13 The experiment was successful! I tried the Fisher using the 4 ohm taps and it made an amazing difference. The output volume is significantly increased. Surprisingly, still not equal to the ACA. However, the Fisher gives more oomph to the bass. Other than the bass, though, I prefer the ACA. It's fun to learn new things! 1 Quote Link to comment Share on other sites More sharing options...
82 Cornwalls Posted March 13 Share Posted March 13 I wonder if the Fisher is working 100% since it should play at least as loud as the ACA. How old are the tubes? Quote Link to comment Share on other sites More sharing options...
derrickdj1 Posted March 13 Share Posted March 13 It is Ok to connect a nominal 8 ohm speaker to either the 4 or 8 ohm tap on the amp. You can't do the reverse. Never connect a nominal 4 ohm speaker to the 8 ohm tap.... Quote Link to comment Share on other sites More sharing options...
Dave MacKay Posted March 13 Share Posted March 13 24 minutes ago, derrickdj1 said: It is Ok to connect a nominal 8 ohm speaker to either the 4 or 8 ohm tap on the amp. You can't do the reverse. Never connect a nominal 4 ohm speaker to the 8 ohm tap.... Why? Both 4 ohm and 8 ohm are “nominal” values and the actual impedance will span quite a range. Unless one is running the amp at its maximum (never a good idea) it shouldn’t be harmful. 1 Quote Link to comment Share on other sites More sharing options...
CWelsh Posted March 13 Author Share Posted March 13 17 hours ago, 83 LSIs said: I wonder if the Fisher is working 100% since it should play at least as loud as the ACA. How old are the tubes? I suspect it may not be working 100%, but I don't have the ability to test it. I've had this amp for more than 25 years and haven't done anything to it. I'm pretty sure it was in storage for at least 5-10 years before that. All of the tubes except the rectifier are Fisher brand, so it is possible they are original. If not, I will bet they are at least 50 years old. Both channels are working and, to my ears at least, sound even. In fact, the sound stage sounds really nice. Quote Link to comment Share on other sites More sharing options...
geezin' Posted March 13 Share Posted March 13 If you're happy with it and there's no smoke it's all good. 2 Quote Link to comment Share on other sites More sharing options...
82 Cornwalls Posted March 13 Share Posted March 13 Edit: read @CWelsh 's posts in the pictures of your tube amplifier thread so this is a moot point. I would disagree since he feels a 7 watt ACA solid state is louder than the 27 watt tube Fisher and he is not completely happy with it. No guarantee a refresh will change it a lot, but from its history I would want to find out what it is truly capable of. The use of I above obviously indicates my opinion and thoughts. 1 Quote Link to comment Share on other sites More sharing options...
derrickdj1 Posted March 14 Share Posted March 14 Speakers are given ohm rating because the design work optimally within a certain range. Many speakers will fluctuate +/-2.5 db. An 8 ohm speaker can be used with an amp designed for 4 ohms. This will deliver a bit more power and play a tad louder. Doing the reverse, putting a 4 ohm speaker on 8 ohm amp will draw more current and overwork the amp. Also, higher distortion. If the speaker is 4 ohms, it will most likely be louder than an 8 ohm speaker and the reverse also holds as a general rule. 1 Quote Link to comment Share on other sites More sharing options...
Dave MacKay Posted March 14 Share Posted March 14 6 hours ago, derrickdj1 said: If the speaker is 4 ohms, it will most likely be louder than an 8 ohm speaker and the reverse also holds as a general rule. Agreed. 1 Quote Link to comment Share on other sites More sharing options...
backlog Posted March 14 Share Posted March 14 Impedance, damping factor, etc… I was looking for the electrical spec that determines speaker ratings but found this instead. Bass control is important. https://mynewmicrophone.com/the-complete-guide-to-speaker-impedance-2ω-4ω-8ω-more/ 1 Quote Link to comment Share on other sites More sharing options...
backlog Posted March 14 Share Posted March 14 It’s in the article…. The IEC (International Electrotechnical Commission) standard for rated speaker impedance is as follows: the minimum impedance shall not fall below 80% of the nominal (rated) impedance over the defined frequency range of the speaker. For example: 4 Ω speakers have a minimum impedance no less than 3.2 Ω 8 Ω speakers have a minimum impedance no less than 6.4 Ω 2 1 Quote Link to comment Share on other sites More sharing options...
82 Cornwalls Posted March 14 Share Posted March 14 Unfortunately many speaker companies don't follow that. Just one example from a well know company "Impedance: 8 ohms nominal, 3.1 ohms minimum." And the summary from independent measurements "Although technically this is an 8 ohm design, I think it should be used with amplifiers that are comfortable with 4 ohm loads." Where minimum impedance is and the phase angle are also important considerations. 1 Quote Link to comment Share on other sites More sharing options...
DirtyErnie Posted March 21 Share Posted March 21 With tube amps, a 4-ohm speaker on the 8-ohm tap will run the tubes out of current before they hit their minimum voltage. This won't give you max power, but also won't put the tubes in a fundamentally bad state. Load line comes out the top of the curves. an 8-ohm speaker on the 4-ohm tap will (at full power) run the tubes down to their minimum voltage with current capability to spare. Load line comes out the left side of the curves. This is when the screen grids start to conduct heavily. It's possible to torch your tubes like that. Again, this is in a max-power situation, probably (hopefully!) more applicable to guitar amplifiers than HiFi. I feel a good rule-of-thumb is to match the tap on the amplifier with the actual lowest impedance that happens in the bass region. If that means a 4-ohm woofer is being used, use the 4-ohm tap and it should keep your amp happy. Power drops off as frequency goes up, so the midrange impedance peak on your speakers would still keep the actual load line in the middle of the map. 2 Quote Link to comment Share on other sites More sharing options...
82 Cornwalls Posted March 22 Share Posted March 22 Must depend on the design of the amplifier; read what Roger M. posted here regarding light loading in reply #9 https://www.audiocircle.com/index.php?topic=98785.0 I just hook things together and listen, if it sounds good I don't worry much and have never experienced any problems. It doesn't seem very useful to compare guitar amplifiers to ones designed for and used in a home system. Quote Link to comment Share on other sites More sharing options...
DirtyErnie Posted March 22 Share Posted March 22 Doing a little reading about the amplifier mentioned, sounds like a high-voltage, lower-current implementation on that amplifier. But physics is physics; unless he has a good current-limiting circuit on the screen grids, plugging a 4-ohm speaker into a sub-4-ohm tap will force the load-line lower, will increase your Class-A power, and will push the load-line out the left side of the chart, and that will cause lots of current to want to flow out the screen grids if you push the tubes to the limits. You can't cheat physics, but you can mold it to your advantage if you're willing to accept the consequences (and maybe design around them). Quote Link to comment Share on other sites More sharing options...
CWelsh Posted March 22 Author Share Posted March 22 26 minutes ago, DirtyErnie said: Doing a little reading about the amplifier mentioned, sounds like a high-voltage, lower-current implementation on that amplifier. But physics is physics; unless he has a good current-limiting circuit on the screen grids, plugging a 4-ohm speaker into a sub-4-ohm tap will force the load-line lower, will increase your Class-A power, and will push the load-line out the left side of the chart, and that will cause lots of current to want to flow out the screen grids if you push the tubes to the limits. You can't cheat physics, but you can mold it to your advantage if you're willing to accept the consequences (and maybe design around them). I don't understand most of what you're saying, but I'm glad people are engaged with the question. I have the Fisher amp listed for sale on Reverb thinking I could use the funds for other enhancements to my system. However, I'm starting to rethink. We have a shop in town that refurbishes vintage audio equipment and I may take it in for an evaluation. @83 LSIs question of whether it is operating at 100% has me thinking. 2 Quote Link to comment Share on other sites More sharing options...
82 Cornwalls Posted March 22 Share Posted March 22 DE Maybe I'm misunderstanding what you are saying??? But, You keep mentioning pushing the tubes to their limits, that sounds to me like more of a guitar amp thing than sensible design/use for music reproduction with a tube amp in a home setting. If someone is regularly pushing their amp to the limits when listening to music I hope they would notice the bad results/sound and decide to move up a notch (or two) in power output. Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.