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 Post subject: Impedance Mismatch
PostPosted: Thu Jan 13, 2011 8:20 pm 
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Mark II

Joined: Fri Sep 14, 2007 7:48 pm
Posts: 64
I know the amp can tolerate certain mismatches, but I have two 8 ohm cabs and no parallel box. Can I safely plug one into the 8 ohm output and the other into the 4 ohm output?

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 Post subject: Re: Impedance Mismatch
PostPosted: Fri Jan 14, 2011 12:07 pm 
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Mark II
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Joined: Mon Mar 23, 2009 8:10 am
Posts: 83
Location: Joburg, South Africa
Get a second opinion but in theory I think that's ok: I'm no expert but my understanding is that as long as the ohms load on the speaker isn't less than the output on the amp you don’t damage the output stage of the amp. 4 ohm output into an 8 ohm speaker is fine as far as I understand it.

However, someone way cleverer than me needs to clarify whether or not it’s safe to use both the amp’s speaker outputs at the same time.

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 Post subject: Re: Impedance Mismatch
PostPosted: Fri Jan 14, 2011 12:14 pm 
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Mark II

Joined: Fri Sep 14, 2007 7:48 pm
Posts: 64
Well, I tried it anyway and actually preferred the sound of the Thiele only so that's what I'm going to go with. ;-) I still may need the answer for other speaker cabs.

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 Post subject: Re: Impedance Mismatch
PostPosted: Sun Jan 16, 2011 2:37 pm 
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Mark I

Joined: Sun Oct 19, 2008 4:48 pm
Posts: 49
Location: England
Sebber wrote:
Get a second opinion but in theory I think that's ok: I'm no expert but my understanding is that as long as the ohms load on the speaker isn't less than the output on the amp you don’t damage the output stage of the amp. 4 ohm output into an 8 ohm speaker is fine as far as I understand it.

However, someone way cleverer than me needs to clarify whether or not it’s safe to use both the amp’s speaker outputs at the same time.


Its been a while since I learned this stuff but I thnk if the amp is perfectly matched with a single 4 or 8 ohm speaker in the appropriate output then using both at the same time will result in a mismatch (reflect back a combined lower load to the primary winding of the OP transformer) and cause more power to be dissipated in the output tubes - potentially reducing the life of the tubes.

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 Post subject: Re: Impedance Mismatch
PostPosted: Mon Jan 31, 2011 1:23 pm 
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Mark I

Joined: Wed Oct 27, 2010 11:42 am
Posts: 31
Location: Tampa, Florida
can't you just chain the cabs together and run them out of the 4 ohm output? (most cabs, in my experience, allow you to do this)

(1/R equivalent) = (1/R1) + (1/R2)

So you would have 1/8 + 1/8, which is 2/8, which is the same as 1/4. So your total resistance would end up being 4 ohms.

http://www.1728.com/resistrs.htm

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 Post subject: Re: Impedance Mismatch
PostPosted: Mon Jan 31, 2011 2:06 pm 
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Mark II

Joined: Fri Sep 14, 2007 7:48 pm
Posts: 64
Sacausey wrote:
can't you just chain the cabs together and run them out of the 4 ohm output? (most cabs, in my experience, allow you to do this)

(1/R equivalent) = (1/R1) + (1/R2)

So you would have 1/8 + 1/8, which is 2/8, which is the same as 1/4. So your total resistance would end up being 4 ohms.

http://www.1728.com/resistrs.htm


That works if one of the cabs has two jacks. In this case, both have only one. But it turns out I liked the sound of the Thiele by itself better than both together so the issue became moot.

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 Post subject: Re: Impedance Mismatch
PostPosted: Sat Mar 05, 2011 5:55 pm 
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Bottle Rocket

Joined: Sun Feb 27, 2011 5:44 pm
Posts: 10
BoogieJem wrote:
Sebber wrote:
Get a second opinion but in theory I think that's ok: I'm no expert but my understanding is that as long as the ohms load on the speaker isn't less than the output on the amp you don’t damage the output stage of the amp. 4 ohm output into an 8 ohm speaker is fine as far as I understand it.

However, someone way cleverer than me needs to clarify whether or not it’s safe to use both the amp’s speaker outputs at the same time.


Its been a while since I learned this stuff but I thnk if the amp is perfectly matched with a single 4 or 8 ohm speaker in the appropriate output then using both at the same time will result in a mismatch (reflect back a combined lower load to the primary winding of the OP transformer) and cause more power to be dissipated in the output tubes - potentially reducing the life of the tubes.



Help !!! Im ashamed that I do not know this stuff, but it is what it is, and I dont want to hurt my new equipment.

My MB TA15 has a 4 ohm and a 9 ohm out jack. My new REC series MB 2 x 12 Cab (that was a floor model and did not come with a manual)
has two input jacks......one for 4 and one for 8. From reading on here, I understand I can do the 8 ohm out on the amp to the
4 ohm in on the cab and it will sound different, but no harm will come to my equipment. So I guess its also ok to do a 4 out on the amp into
the 4 in on the amp, but never a 4 out on amp into the 8 in on cab.

WHat are the differences I will experience in sound when I do any of the above and what is safe and the recommended ?? I have only done the 8 out and 8 in so far.

Thank you for your help and info!!!


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 Post subject: Re: Impedance Mismatch
PostPosted: Mon May 02, 2011 5:18 am 
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Mark I

Joined: Sun Oct 19, 2008 4:48 pm
Posts: 49
Location: England
guitarguy56 wrote:
BoogieJem wrote:
Sebber wrote:
Get a second opinion but in theory I think that's ok: I'm no expert but my understanding is that as long as the ohms load on the speaker isn't less than the output on the amp you don’t damage the output stage of the amp. 4 ohm output into an 8 ohm speaker is fine as far as I understand it.

However, someone way cleverer than me needs to clarify whether or not it’s safe to use both the amp’s speaker outputs at the same time.


Its been a while since I learned this stuff but I thnk if the amp is perfectly matched with a single 4 or 8 ohm speaker in the appropriate output then using both at the same time will result in a mismatch (reflect back a combined lower load to the primary winding of the OP transformer) and cause more power to be dissipated in the output tubes - potentially reducing the life of the tubes.



Help !!! Im ashamed that I do not know this stuff, but it is what it is, and I dont want to hurt my new equipment.

My MB TA15 has a 4 ohm and a 9 ohm out jack. My new REC series MB 2 x 12 Cab (that was a floor model and did not come with a manual)
has two input jacks......one for 4 and one for 8. From reading on here, I understand I can do the 8 ohm out on the amp to the
4 ohm in on the cab and it will sound different, but no harm will come to my equipment. So I guess its also ok to do a 4 out on the amp into
the 4 in on the amp, but never a 4 out on amp into the 8 in on cab.

WHat are the differences I will experience in sound when I do any of the above and what is safe and the recommended ?? I have only done the 8 out and 8 in so far.

Thank you for your help and info!!!


Sorry I've been away from this board for a while so did not see this - but you have it the wrong way around. It is safe to plug a higher Ohms speaker input into a lower Ohms output on the amp (eg 8 Ohm speaker into 4 Ohm output) the result will be a little less volume and sometimes a warmer tone. A perfect match is naturally 8 Ohm to 8 Ohm or 4 Ohm to 4 Ohm this is how the amp was designed to operate and gives the loudest output. A potentially harmful mismatch is to have the amp drive a lower Ohm speaker than the rated output this causes the tubes to dissipate more power as internal heat.

Hope that helps

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Mesa Mk IV, Mesa LSC 2x12, Mesa TA-15 Mesa Thiele 1x12 C90, Peavy Classic 30, PRS Swamp Ash Special, PRS Custom 24, Gibson ES-135, Fender Baja Tele


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 Post subject: Re: Impedance Mismatch
PostPosted: Mon May 02, 2011 10:37 pm 
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Bottle Rocket

Joined: Mon Feb 28, 2011 12:37 pm
Posts: 19
For anyone with doubts on this topic I put an Excel sheet on another forum that will help you calculate the total impedace, even using the different outputs of the amp.
http://www.marshallforum.com/attachment ... hookup.zip

Always check what you are doing and read the manual :) The sheet is just a calculation tool and does not replace checking the connection and feasibility.


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 Post subject: Re: Impedance Mismatch
PostPosted: Tue May 03, 2011 5:48 pm 
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Bottle Rocket

Joined: Mon Feb 28, 2011 12:37 pm
Posts: 19
By the way: One 8 Ohm cab into each output is not OK. It's like plugging in an 5,33 Ohm cab into the 8 Ohm output.
Good you didn't like it.


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