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Impedance etc

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The question was if the full power of an amp can be drawn into a 8ohm single cab, without the need for further cabs.

Could for example a split cable be produced that uses both speaker outs at the amp end but with just one speakon at the other end?

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13 hours ago, Deedee said:

I’m sure this may have been covered somewhere, but can you ‘trick’ an amp with a min load of 4 ohms into thinking it’s putting out to a 4 ohm speaker even if it’s actually an 8 ohm speaker?

 

There's no point. You may think that there's an advantage to the proverbial 'getting all the watts out of my amp', but there isn't. If you did have a 4 ohm cab versus 8 ohms the greatest potential difference in maximum output would be 2dB, which is inconsequential. That's best case, with an amp that's rated no more than 1/4 the cab power rating. With a less skewed amp to speaker power rating, where the maximum output is likely limited by the driver excursion, there would be no difference.

Could for example a split cable be produced that uses both speaker outs at the amp end but with just one speakon at the other end?

The speaker outs are already connected within the amp. That cable would accomplish nothing.

Edited by Bill Fitzmaurice

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30 minutes ago, Bill Fitzmaurice said:

There's no point. You may think that there's an advantage to the proverbial 'getting all the watts out of my amp', but there isn't. If you did have a 4 ohm cab versus 8 ohms the greatest potential difference in maximum output would be 2dB, which is inconsequential. That's best case, with an amp that's rated no more than 1/4 the cab power rating. With a less skewed amp to speaker power rating, where the maximum output is likely limited by the driver excursion, there would be no difference.

 

The speaker outs are already connected within the amp. That cable would accomplish nothing.

Aha, that’s put that one to bed then.

Many thanks for clarifying for a certified technophobe 👍

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Question: I have a Trace Elliott 500W cab (500AH-7) and I have 2 x  4 ohm cabinets. The speaker outputs consist of a speakon connection and a 1/4"  jack output. If i daisy chain the cabs they will have 2 ohms right? . If I plug this combination into my head will I fry the output transformer?

Secondly as there are 2 separate outputs on my can I plug either one into each cab (i'e will it be 4 ohms into each head as normal). 

On the back of my amp it reads 'Minimum load 4 ohms'

 

      

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4 ohms on the amp means 1 4ohm cab or 2 8 ohm cabs. 2 ohms will probably damage the amp.

Either speaker output socket is fine as is daisy chaining the cabs.

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2 hours ago, Quilly said:

 If I plug this combination into my head will I fry the output transformer?

On the back of my amp it reads 'Minimum load 4 ohms'

If it says 'Minimum Load 4 ohms' that indicates an SS amp. Modern SS amps do not have output transformers.

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Class D amps will usually go into protect mode which is designed to prevent and damage to the amp. Current SS amps, I'm not sure if they have protection circuits.

Older amps?? Not sure, but probably don't have protection built in. If that's the case you'll be running them beyond their spec so I'd guess you're almost guaranteed to damage something.

Just be safe and never go below the stated minimum load for the amp.

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Having read through most of this thread and trying to understand how this works, am i right in thinking that all these combos we see which run at 300 watts at 8 Ohms and 500 Watts at 4 Ohms, will in all liklihood have two internal 16 Ohms speakers wired in parallel, or two 4 Ohm speakers in serial,which run at 8 Ohms, and then when the 8 Ohms extension cab is connected it runs as two 8 Ohm speakers in parallel at 4 Ohms giving the full 500 watts... ?

Edited by Creeper

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On 19/10/2018 at 12:56, Creeper said:

Having read through most of this thread and trying to understand how this works, am i right in thinking that all these combos we see which run at 300 watts at 8 Ohms and 500 Watts at 4 Ohms, will in all liklihood have two internal 16 Ohms speakers wired in parallel, or two 4 Ohm speakers in serial,which run at 8 Ohms, and then when the 8 Ohms extension cab is connected it runs as two 8 Ohm speakers in parallel at 4 Ohms giving the full 500 watts... ?

If it’s a 2 driver combo or cab, then it’s usually 2 16 ohm drivers to make 8 ohm. But then it could be just one 8 ohm driver or 4 32 ohm drivers. 

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Honestly, just when I think I've got my head round this impedance thing something gets said which makes me doubt myself. I've just put a head in for investigation/repair at a leading tech shop in the UK, I won't say which. When it came back from repair I tried it and it failed again. They took it back again and put it on test and can't find anything wrong. They've had it on load test for a week and it's fine. But that's beside the (this) point. Because the tech himself couldn't find a fault, he thought it may be the way I'm deploying my amp which is making it cut out. So he asked how my set up worked. So I explained it's a head with two channels, each of 8 ohms/500 watts, or a third bridged channel at 4 ohms/1,000 watts. I use the bridged option with two 8 ohm cabinets daisy chained. That's 4 ohms, right? He thinks it's 16 ohms? Am I right or have I just embarrassed myself again?

 

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Typically when channels are bridged the minimum acceptable impedance load is doubled.  You have two 8 ohm cabs, you have two 8 ohm capable channels, use each cab with one channel. This comes down to the unfortunately seldom debunked notion that more watts equals more output. It doesn't.

https://billfitzmaurice.info/forum/viewtopic.php?f=10&t=19292

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Well you learn something every day!  Thank you Bill for this insight, I hadn't appreciated the difference between the straight outputs and the bridged mode. I will rearrange the configuration accordingly.

 

Best regards,

Andy.

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At the risk of starting a never ending chain of opinions I will ask a question (that’s probably been asked and answered before many times) . I have a valve head that only has 8 and 16ohm outputs. Will I damage it by using a 4ohm load on the 8 ohm tap. The concensus seems to suggest a 1/2. - 2x mismatch is ok with valve amps actually preferring the 1/2 option Over the 2x option. I really love the sound that m getting from the head and I don’t want to fry it. 

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Apologies for the incoming question - as much as I try to understand this concept it always confuses me!

900w head at 4Ohms or 450w at min load of 2/8Ohm

Can I run this into two 2x8 cabs, 200w rated at 8Ohm?

Thanks!

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So to clarify, would two 8ohm 200w cabs presenting a 4 ohm load handle 400w total? Even though they are rated to handle 200w, would they begin to clip and distort? Not that I've ever run an amp head at full volume so I imagine it's not a full 900w I would be running into the cabs anyway.

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The watts from the amp is an “up to” amount & not continuous.  Use your ears when playing & if it sounds bad, turn down. 

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An amp rated at 900 watts at 4 ohms will put a maximum 450 watts into an 8 ohm cab. 2 x 8 ohm cabs look like 4 ohms to the amp.

2 x 200 watt cabs will be fine with your amp, but the amp has the potential to put 900 watts into 400 watts of cabs. You should be fine if you keep the volume down to a level the cabs can handle.

The system you mention seems unbalanced to me. I see a 900 watt amp as a gigging-in-a-loud-band amp. 2x8" cabs are not loud-band cabs IMO.

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3 hours ago, RikkMalice said:

So to clarify, would two 8ohm 200w cabs presenting a 4 ohm load handle 400w total?

Before the voice coils burn out, yes. Before they reach xlim, the excursion before mechanical damage occurs, probably. Before they reach xmax, which can cause evrything from moderate low frequency compression to high level harmonic distortion to full on farting out, no way to know unless the manufacturer reveals that information. The number who do so can be counted on the fingers of one hand, with at least two fingers unused.

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1809085148_4_8ohm.PNG.14b87d011b464e88f74a042d4d001c27.PNG

Why don't more speaker manufacturers do this?...seems like such a no-brainer. 

 

 

On 27/10/2018 at 20:44, Rich said:

+1.

 

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Looks like an accident waiting to happen to me - a surface switch, easily flickable while plugging in in the dark on a stage; with two of those into a traditional 4ohm-minimum amp, you could easily ping the switches to present a 2ohm load to the amp...it'd put me off buying one immediately. At the very least they should make the switch recessed, or harder to flick...

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56 minutes ago, Muzz said:

Looks like an accident waiting to happen to me - a surface switch, easily flickable while plugging in in the dark on a stage; with two of those into a traditional 4ohm-minimum amp, you could easily ping the switches to present a 2ohm load to the amp...it'd put me off buying one immediately. At the very least they should make the switch recessed, or harder to flick...

Actually Epifani also do this with their DIST range of cabinets. Not cheap. 

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The Barefaced Two10 also has a switch on the back. It is just another step in the set up. Plug in the speaker cables and then make sure the switch is correctly set.

I use 4 ohm, 2.67 ohm and 2 ohm amps. It's just a simple thing to check I'm using the correct amp with the cabs I want to use that night.

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55 minutes ago, chris_b said:

The Barefaced Two10 also has a switch on the back. It is just another step in the set up. Plug in the speaker cables and then make sure the switch is correctly set.

I use 4 ohm, 2.67 ohm and 2 ohm amps. It's just a simple thing to check I'm using the correct amp with the cabs I want to use that night.

Must be on later models - I have two early ones (were known as the Retro 2x10 back then) and there is no switch.  You had to get a loom fitted to change them from 4ohm to 12ohm apparently? 

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