It is currently Mon Mar 16, 2020 1:44 pm

All times are UTC - 7 hours



Post new topic Reply to topic  [ 8 posts ] 
Author Message
Post subject: Ohm ratings
Posted: Wed Jan 16, 2008 2:59 pm
Offline
Hobbyist
Hobbyist

Joined: Sat Aug 11, 2007 4:49 am
Posts: 31
Feel free to call me an ignoramus, but can anyone explain to me the difference between a 4 ohm load and an 8 ohm load? I know that ohms are the unit of resistance, but how do the different values affect real world guitar tone? I'm sure it's very simple, but i've never understood it.

Marge


Top
Profile
Fender Play Winter Sale 2020
Post subject: Ohm Ratings
Posted: Wed Jan 16, 2008 3:29 pm
Offline
Professional Musician
Professional Musician
User avatar

Joined: Fri Nov 09, 2007 7:46 am
Posts: 1019
Location: State of Confusion
Has nothing to do with tone. The simplist explanation is that it has to do with the power amplifier. Every power amplifier (a part of your amp) has a maximum "safe" output, above which you risk burning up the amp. This maximum output is based on a given electrical load (such as a speaker) and the load is defined by its resistance (ohms). If the load is less, it allows the power amplifier to produce more than its maxium rating, risking it's burnup.

For example, if the amp is 15watts with an 8 ohm speaker, and you disconnect that speaker and attach one with a 4ohm resistance it can cause failure (it will produce more than 15watts). If you replace the 8 ohm speaker with a 16 ohm speaker, the amp won't burn up, but cannot reach its 15watt output because the load is too great. It would function OK, however with the 16 ohm speaker, just couldn't get quite as loud.

I don't want to confuse you, but if you connect two 8ohm speakers in parallel connection, the total resistance is 4ohms, and this is not a good thing for a power amp designed for an 8 ohm load. If the two 8 ohm speakers are connected in series, the total load is 16ohms, which would not be harmful. So you need to understand thoroughly what you're doing when adding/swapping speakers to a system!

_________________
The quintessential sound of 60/70's R&R:
Fender Tube Amps
Gibson Guitars


Top
Profile
Post subject:
Posted: Thu Jan 17, 2008 5:17 pm
Offline
Professional Musician
Professional Musician

Joined: Tue Jan 01, 2008 1:51 pm
Posts: 2503
Location: Bucks County, Pennsylvania, USA
Thanks RVM lead I've often wondered about this.


Top
Profile
Post subject:
Posted: Thu Jan 17, 2008 6:01 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Wed Jun 13, 2007 7:02 am
Posts: 8609
Location: Vacaville, CA USA
And if you use 2 16ohm speakers wired in parallel they would be 8ohms.

_________________
Chet Feathers

Authorized TonePros Dealer
Authorized WD Music Products Dealer
F/A Official Southpaw Compliance Certification Tester http://faamps.com/

http://www.facebook.com/cafeathers


I didn't Lose my mind, I traded it for this guitar.


Top
Profile
Post subject:
Posted: Fri Jan 18, 2008 6:00 am
Offline
Hobbyist
Hobbyist

Joined: Mon Jul 23, 2007 6:53 pm
Posts: 71
In the case of tube amps, you don't increase power when you use a lower-impedance load (it's not resistance, because it's AC/audio, not DC). Instead, you're looking for an ideal match to the impedance of your output tubes. The manufacturer specifies the load that will do this, but tube amps typically have 100 percent leeway without causing problems. So a tube amp rated for 8 ohms can drive a 4 ohm or 16 ohm load without problems, but at a slight loss in efficiency and perhaps a reduction in maximum loudness.

When you add a second, identical speaker in parallel, say another 8 ohm speaker to a Blues Junior, you get a slight (3dB) increase in maximum loudness, better bass, and a wider sound field.

Solid-state amps are entirely different. They don't have output transformers and will put out more power, as RVM Lead describes, with lower-impedance loads. A load with too little impedance will cause them to burn out or blow a fuse.


Top
Profile
Post subject: Ohm ratings
Posted: Fri Jan 18, 2008 8:24 am
Offline
Professional Musician
Professional Musician
User avatar

Joined: Fri Nov 09, 2007 7:46 am
Posts: 1019
Location: State of Confusion
This question is directed to Billm, or anyone else who may know the answer:

Anyone who has both tube and solid state amps knows that you cannot compare the rated wattage output of the two. It has been my "gut feel" (with no real scientific backup) that to get an approximate comparison, multiply the tube wattage by a factor fo 3 to 4. In other words, the Blues Jr. rated at 15 watts would compare to a solid state amp of 45 to 60 watts. This is my comparison to assess how the volume of the two may compare.

Is there a scientific means of comparing class A versus solid state power outputs, or is it just too different to do so other than some "gut feel" method as I have done?

_________________
The quintessential sound of 60/70's R&R:
Fender Tube Amps
Gibson Guitars


Top
Profile
Post subject:
Posted: Fri Jan 18, 2008 9:42 am
Offline
Hobbyist
Hobbyist

Joined: Mon Jul 23, 2007 6:53 pm
Posts: 71
When solid state amps pass their peak output they go into severe distortion, so you get the rated watts and not a bit more. When tube amps pass their peak output, they go into distortion gradually. For guitars, at least, the distortion they produce is pleasant and musically useful. So yes, as a practical matter, you do need at least twice the solid state watts to give you the same music power as a tube amp.

Amp vendors base the power rating on how much output they get at a given distortion rating. Fender rates the Blues Junior, for instance, at 15 watts and 5 percent distortion. Another vendor might take the exact same circuit and rate it 20 watts at 10 percent distortion. With solid state, you don't have that smooth onset of distortion--it just hits a wall.


Top
Profile
Post subject: Ohm ratings
Posted: Fri Jan 18, 2008 11:38 am
Offline
Professional Musician
Professional Musician
User avatar

Joined: Fri Nov 09, 2007 7:46 am
Posts: 1019
Location: State of Confusion
Thanks Billm

_________________
The quintessential sound of 60/70's R&R:
Fender Tube Amps
Gibson Guitars


Top
Profile
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 8 posts ] 

All times are UTC - 7 hours

Fender Play Winter Sale 2020

Who is online

Users browsing this forum: No registered users and 2 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum

Search for:
Jump to: