speaker question

[flora]

Is it possible to convert a 4-ohm speaker to an 8-ohm load by running a 4-ohm resistor in series with it ?

[sluckey]

yes

[flora]

For the resistor, does wattage rating matter ?

[sluckey]

yes

[billcreller]

 

[John]

http://www.horrorseek.com/home/halloween/wolfstone/Audio/spkimp_SpeakerImpedanceMatching.html#ChangingTheImpedanceOfSpeakers

Google is your friend. ;) If I'm reading that page correctly, if you have a 25 watt amp/speaker, you need a 25 watt resistor as well.

[flora]

http://www.horrorseek.com/home/halloween/wolfstone/Audio/spkimp_SpeakerImpedanceMatching.html#ChangingTheImpedanceOfSpeakers

Google is your friend. ;) If I'm reading that page correctly, if you have a 25 watt amp/speaker, you need a 25 watt resistor as well.

Really... I thought the forum is my friend.

[John]

http://www.horrorseek.com/home/halloween/wolfstone/Audio/spkimp_SpeakerImpedanceMatching.html#ChangingTheImpedanceOfSpeakers

Google is your friend. ;) If I'm reading that page correctly, if you have a 25 watt amp/speaker, you need a 25 watt resistor as well.

Really... I thought the forum is my friend.

It is. That's why I posted the link and gave my (hopefully correct) interpretation. :)

[flora]

A "global moderator" should have the patience to answer a question , even if he thinks it's somehow too obvious and below his consideration, instead of grandstanding to his forum followers ,with cutsie one word replies .

[sluckey]

A "global moderator" should have the patience to answer a question , even if he thinks it's somehow too obvious and below his consideration, instead of grandstanding to his forum followers ,with cutsie one word replies .

You asked two simple yes/no questions and got two correct replies. If you want details, give us some more info to go on.

[flora]

Yeah..yeah, sure Steve... you win.. "correct answers".

[sluckey]

You win... both replies were certainly "correct".

It's not a contest. I really thought those were the answers you were seeking. Do you have a particular application in mind that needs more details? For example, I have a push/pull 6V6 amp that only has an 8Ω output tap. I want to use my 20 watt 4Ω speaker, but don't want the impedance mismatch. I think I can do this with a 4Ω resistor. What are the precautions or downsides I should be aware of? Something like that.

I sure didn't mean to be cute or piss you off with my direct answer. That's just my style. I've been told I should hone my communications skills before.   

[skyclad]

Okay - I'll bite. My 25 watt amp only has an 8 ohm output tap but I want to use my 20 watt, 4 ohm speaker - don't want the impedance mismatch. What are the precautions or downsides I should be aware of?

[sluckey]

Okay - I'll bite. My 25 watt amp only has an 8 ohm output tap but I want to use my 20 watt, 4 ohm speaker - don't want the impedance mismatch. What are the precautions or downsides I should be aware of?

If you absolutely don't want the impedance mismatch, then read the info on the link that John posted above. And if you specifically want to use a 4Ω resistor connected in series, get one big enough to handle 1/2 the power of the amp (precaution), since it will be burning off half the amp's power as heat (downside). Actually, you better double the resistor power rating for a safety margin.

Lot's of people run 4Ω loads on 8Ω taps all the time with no ill effects. I'd be more concerned about putting a 20W speaker on a 25W amp. I'd probably just put that 20W 4Ω speaker on the 8Ω tap and just not crank the amp wide open.

[RicharD]

>Is it possible to convert a 4-ohm speaker to an 8-ohm load by running a 4-ohm resistor in series with it ?

Seems like a bad idea to me.  A speaker is inductive, not purely resistive and it's impedance changes with frequency.  The fixed resistor in series should sorta act like a current limiter.... I think.  Seems like that would mean the speaker can't work to it's fullest potential.  Can you = yes, should you = no IMO.

-Richard

[alerich]

Another reason for over rating the power rating on the resistor is that when they get really hot sometimes resistor values drift in the downward direction. Learned that caveat while building attenuators. Since this is essentially a mini attenuator it would be good to over rate the power rating. Can't hurt.

What about a high current cap across the resistor calculated to allow a bit of the high end shimmer to pass to the speaker while still allowing the resistor to dissipate most of the power it is intended to? I use large 10uf caps in my attenuator and it adds a little high end back to the attenuated signal.

[bigdaddy]

Webervst makes an impedance matching output transformer just for this application.

https://taweber.powweb.com/store/zmatch.htm

[woolly]

way to go Steve, you managed to run off the only young lady we got on this board.

[sluckey]

I sincerely hope not. I still don't see how anyone could find offense in my replies to the original yes/no questions.

[J Rindt]

Flora -
I understand your question. You are using ohms law to solve a problem, and you are correct on paper. But as some of these guys have pointed out, with a speaker that is "reactive" and operating at AC not DC, the resistor is a bad idea. In addition to whatever information is in the link(s) that were posted for you to read, that resistor will be wasting all the power it consumes as heat. So to get an impedance match, which is what you are trying to do, you are giving up a lot of power by adding that series resistor. It would be better to just get an 8 ohm speaker. As a guitar player, it is good to have a few different speakers in your arsenal 
Good Luck

[john_t]

Ok I have the same issue in reverse. I have a champ style amp (aka Hoffman schedule 40 build) I installed the 4 ohm output transformer. I have a Jensen c10q 8 OHM speaker I would like to use. Is there a simple fix to avoid the impedance mismatch. I know I could use two 8 ohm speakers.  I would like to use just one speaker. I have also had some feedback it is not a big issue on this amp.

John

[sluckey]

To avoid the mismatch, put an 8Ω/25W resistor in parallel with the 8Ω speaker. But really, just plug the speaker in and play it like you stole it.

[jhadhar65]

>But really, just plug the speaker in and play it like you stole it.

Second.  Any other solution is way too much work for too little result.

[john_t]

Thanks for the reply's. I had read the mismatch could tax the output tube. I guess I wont worry about it.

jt