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Impedance etc


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#241 Dad3353

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Posted 10 June 2016 - 07:21 PM

View Postthelowend77, on 10 June 2016 - 06:36 PM, said:

...
Cheers in advance for any help guys...

Yes, you're confusing a couple of issues here, but let's clear them up.
Firstly, the twin sockets on the cabs (and, indeed, on almost all amps...) are wired in parallel. One can use either; it makes no difference. If an amp has two outputs, one would usually plug a cab into each, and the amp would 'see' the total impedance in parallel, just as if there was only one cab. For the two 8 ohm cabs mentioned, the amp would see 4 ohms.
Your amp has only one output, so you would plug instead one cab into that, and the other into the second output on the first cab. The result, electrically, for the amp is identical; it will see 4 ohms just the same.
The power handling question is also slightly askew. Each cab has a rating of 400w. Its impedance is of no importance for that; it just 'is'. The amp has a power output dependant to some extent on the impedance presented. It will give a certain power at 8 ohms, and a bit more into 4 ohms. The power rating of the two cabs combined is 800w. The amp will be able to supply 500w into that load (4 ohms...). The cabs are therefore 'safe'. One single cab can handle 400w, but the amp, into 8 ohms, will be able to supply only around 300 or so, so, again, the cab is 'safe'.
A word of warning, just the same. The manufacturer's rating of 400w doesn't mean that much in real terms; it basically means that the speaker (probably...) won't burst into flames before that wattage. In reality, it may very well sound bloomin' awful well before then. Use your ears to judge if the cabs are being over-driven (and wear ear protection, if you value your hearing...). Don't wait for the magic smoke to appear.
There; does that help at all..?
Subject to completion, correction and/or contradiction from others.

Edited by Dad3353, 10 June 2016 - 07:21 PM.

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#242 thelowend77

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Posted 10 June 2016 - 09:19 PM

Brilliant. That really clears it up for me. Thank you.

#243 Dad3353

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Posted 10 June 2016 - 09:21 PM

Attached Image: monthly_07_2014/post-11876-0-11131500-1406229196.gif
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#244 sebowden

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Posted 29 June 2016 - 06:57 PM

I have just bought a Selmer PA 100, not actually picked it up yet, but it has an Ohm Selector, and you can chose between 3.75 (4), 7.5 (8) and 15 (16) Ohm speakers. There is also a 100V option, but I don't know what that means. I have a Peavey 410TX which is 8Ohm, does anyone know if the selector enables the amp to run at 100W with any selection? i.e, if I wire it through my cab, will it be running at 100W?

I understand that most amps halve the wattage if they are 8Ohm but are being played through a 4Ohm cab, but does the Ohm selector change this?

Edited by sebowden, 29 June 2016 - 07:05 PM.


#245 obbm

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Posted 29 June 2016 - 07:30 PM

The PA100 is a valve amp and uses a multi-tap transformer to match the output stage to the speaker load. It always gives the full output. You must never run a valve amp without a load connected.

The power output of Solid State amps however varies depending on the connected load as you say in your last line. Solid-state amps can be run with no load connected.
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#246 sebowden

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Posted 29 June 2016 - 07:53 PM

View Postobbm, on 29 June 2016 - 07:30 PM, said:

The PA100 is a valve amp and uses a multi-tap transformer to match the output stage to the speaker load. It always gives the full output.

Cheers!!

#247 Dad3353

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Posted 29 June 2016 - 07:54 PM

View Postsebowden, on 29 June 2016 - 06:57 PM, said:

I have just bought a Selmer PA 100...
...There is also a 100V option, but I don't know what that means....

The power of an amp is partly dissipated in the cabling resistance, which is why speaker cables are better if they're quite 'chunky'. When cabling over greater distances than just to a cab, however, it's very expensive to use chunky cable. Your amp, you say, is a PA amp; as such, it was designed to be able to power speakers laid out across a field, or all around a village hall. For these distances, it's more efficient to 'up' the voltage (and thus reduce the current, for the same power...), which enables long cable runs of lighter cable. The downside (there's always a downside...)..? The speakers have to be able to cope with this high voltage, either by having special windings, or having a 'drop-down' transformer built-in.
Do not use this 100v tap, ever, unless doing specialised, long-distance cable runs with adequate speakers, such as those quaint Tannoy horns that used to be dotted around village fairs.
Hope this helps.
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#248 Chienmortbb

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Posted 29 June 2016 - 08:02 PM

View PostDad3353, on 29 June 2016 - 07:54 PM, said:

The power of an amp is partly dissipated in the cabling resistance, which is why speaker cables are better if they're quite 'chunky'. When cabling over greater distances than just to a cab, however, it's very expensive to use chunky cable. Your amp, you say, is a PA amp; as such, it was designed to be able to power speakers laid out across a field, or all around a village hall. For these distances, it's more efficient to 'up' the voltage (and thus reduce the current, for the same power...), which enables long cable runs of lighter cable. The downside (there's always a downside...)..? The speakers have to be able to cope with this high voltage, either by having special windings, or having a 'drop-down' transformer built-in.
Do not use this 100v tap, ever, unless doing specialised, long-distance cable runs with adequate speakers, such as those quaint Tannoy horns that used to be dotted around village fairs.
Hope this helps.
100V line systems ate high impedance. A 100W speaker at 100V is 100R (ohms note the the international symbol for ohms has been R for years). If you try to put 100V across a 4R or 8R speaker it could be very expensive.

Chunky cables (if the conductors are chunky) are better but if the cable length is from Head to cab, where the Head is say 1 metre for less does not need a massive cable. PA cables go much further and can do with being quite big.

Edited by Chienmortbb, 29 June 2016 - 08:07 PM.


#249 icastle

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Posted 29 June 2016 - 09:11 PM

100V line systems were dependant on two transformers.

The one at the amplifier end stepped the voltage up to 100V.
In a speaker designed for 100V usage, there is another transformer in there to drop the 100V back down to a usable level.

The 100V is used purely as a 'carrier' for the audio signal.


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#250 jeigenbass

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Posted 04 July 2016 - 01:37 PM

Apologies for a question that's probably already been asked 100 times but at least this is the right thread...!

Will a 350W 4ohm Aguilar DB210 cab work with a Markbass 500W @ 4ohm head?


Thanks!

#251 chris_b

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Posted 04 July 2016 - 02:48 PM

Yes it will.

You'll be capable of putting the full 500 watts into it which, at 4 ohms, means you can't add another cab.

I wouldn't turn the amp up to the point where it could damage the cab or run the bass frequencies flat out but with those limitations you can run this cab with this amp.

I prefer 8 ohm 210 cabs because then you can add another cab if you need more volume and IMO more speakers gives you a better tone.

Edited by chris_b, 04 July 2016 - 02:49 PM.

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#252 jeigenbass

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Posted 04 July 2016 - 09:00 PM

Quote

Yes it will.

You'll be capable of putting the full 500 watts into it which, at 4 ohms, means you can't add another cab.

I wouldn't turn the amp up to the point where it could damage the cab or run the bass frequencies flat out but with those limitations you can run this cab with this amp.


I prefer 8 ohm 210 cabs because then you can add another cab if you need more volume and IMO more speakers gives you a better tone.

Thanks!
So does that mean I would never be able to add another cab safely? not even another 4 ohm one or in parallel (my amp has two outputs)?

#253 obbm

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Posted 04 July 2016 - 09:55 PM

View Postjeigenbass, on 04 July 2016 - 09:00 PM, said:

Thanks!
So does that mean I would never be able to add another cab safely? not even another 4 ohm one or in parallel (my amp has two outputs)?

If the amps spec says no lower than a 4-ohm load then that is it. You should not any lower regardless of how many spare sockets there are.
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#254 bigjimmyc

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Posted 09 November 2016 - 04:29 PM

I'm looking at buying an Ampeg Svt-2 all-valve amp. I believe the output selector is 4R or 2R only. However my main cab is a bf410 at 8R.

Other than buying a second cab, will I be able to match this head with the cab without causing damage?
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#255 AndyRoo

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Posted 06 January 2017 - 02:23 PM

Hi all,

So I think I've got most of this and apologies if this has been answered already but there's one piece that's not quite clear/I haven't found the answer yet on this thread/the web.

My setup (picked up through the BC for-sale boards) is as follows:

Yamaha BBT500H head, 500W at 2ohm, 250W at 4ohm, class D amp.
Cab 1 is a matched Yamaha BBT210S (250W handling at 4ohm). So far so good and one happy customer.

I'd like to add another cab for bigger venues and the obvious thing is to keep it simple and get a 250W (or greater) 4ohm cab (so 500W total at 2ohm = happy days). However let’s say I want to [hypothetically] use an 8ohm 250W cab instead...

The overall impedance is going to be 2.667ohms (still fine for the class D head).
I understand the power will be split unevenly between the two cabs with 2/3 going into the 4ohm and 1/3 going into the 8ohm cab.

My question is: is this one thirds/two thirds power splitting based on the max head power (i.e. 500W, which would put 160W into the 8ohm and be fine but 333W into the 4ohm cab and ultimately shred it subject to “Alex’s first rule”) OR is this power splitting based on the power at the overall impedance (i.e. 500 * 2/2.667 = 375W) in which case the power nominally going into each cab is essentially fine (250W into the 4ohm and 125W into the 8ohm)?

Thanks!

Edited by AndyRoo, 06 January 2017 - 02:24 PM.


#256 Bill Fitzmaurice

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Posted 06 January 2017 - 03:08 PM

View PostAndyRoo, on 06 January 2017 - 02:23 PM, said:

is this one thirds/two thirds power splitting based on the max head power (i.e. 500W, which would put 160W into the 8ohm and be fine but 333W into the 4ohm cab and ultimately shred it subject to “Alex’s first rule”) OR is this power splitting based on the power at the overall impedance (i.e. 500 * 2/2.667 = 375W)

It's based on the impedance. The problem here is that the 210 will be doing most of the work. Depending on what you're adding it might not be all that worthwhile. A second identical 4 ohm 210 would be the best option.
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#257 AndyRoo

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Posted 06 January 2017 - 03:16 PM

Hey Bill, that's great, thank you!

Yes I'm keeping my eyes on the boards for a 4ohm 210 (or even something like a 110 or 112) for when payday comes around but good to know that I can play with 8ohm options (caveat it might sound awful) but without risking instant cab damage.

Cheers!

Edited by AndyRoo, 06 January 2017 - 03:21 PM.


#258 Bill Fitzmaurice

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Posted 06 January 2017 - 05:07 PM

An 8 ohm 110 would keep the same power in all three drivers. If you were going to a 112 or 115 it would probably be better for that to be 4 ohm.
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#259 Chienmortbb

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Posted 31 January 2017 - 10:59 PM

View PostBill Fitzmaurice, on 06 January 2017 - 05:07 PM, said:

An 8 ohm 110 would keep the same power in all three drivers. If you were going to a 112 or 115 it would probably be better for that to be 4 ohm.
Bill is quite right here but there are no easy answers. I have rewritten this answer several times as it is complex. In my opinion the best way forward is for you to find a second identical cabinet OR find two other identical cabinets whether 4 or 8 Ohm. The reasons are complex (as are the impedances but unless you want a physics lesson we will leave that alone).

Most Class D heads have a tuned circuit on the output that takes into account the speaker impedance. The speaker is part of the amp's output circuit. Change the speaker impedance and the amp behaves differently. I suspect that the Yamaha amp is optimised for 4 ohms so two 8R cabs would work well. Two identical 4R cabs will also work well although I don't like running at 2R. The reason is that amplifiers with short circuit or current limiting protection (most class D and class A/B, solid state amps have this) can get confused with 2R operation as it is getting close to zero ohms. So in a nutshell either get a second identical 4R Yamaha BBT210S or buy two identical 4R or 8R cabs.

Edited by Chienmortbb, 31 January 2017 - 11:01 PM.


#260 Deep End

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Posted 21 February 2017 - 07:06 PM

Same question alert -
Sorry about this but it's a new consideration for me and I cannot (as yet) wrap my head around this.

Can anyone help me out with this possible head/cab logic -

Aguilar TH500 (500 watts into 4 Ohms, 250 watts into 8 Ohms) into Ashdown Bentley 4x10 cab (600 watt 8 Ohm)
Am I right in thinking this isn't a great marriage? The cab will drain LOTS of power, really the head needs to be bigger/more powerful or the cab needs less impendence?

Heid is fried on this one.
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#261 chris_b

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Posted 21 February 2017 - 11:49 PM

A TH500 is 350 watts into 8 ohms and you cannot damage an amp by running it into a cab with a higher watts rating. Cabs don't drain amps. Amps power cabs so IMO you would be fine with this gear as long as 350 watts is all you need. If you want more volume add another 8 ohm cab.

I have no idea what this Ashdown cab sounds like but I've used a TH500 for the last 3 years through several Bergantino cabs and several Barefaced cabs. It has sounded perfect to my ears in every situation.
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#262 Powertripper

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Posted 11 March 2017 - 02:55 PM

Hi guys, really useful thread here, nice one. I have read through and found some answers to a similar question but I'm just wondering if anyone could give me an answer on a specific scenario, I have recently acquired an SWR Goliath III 410 4-ohm cab that I was hoping to try out in series with my Ashdown 410 8-ohm, is this possible or not recommended? I know I can't run them in parallel as neither of my amps have 2ohm outputs, but if I ran from amp > cab 1 (4ohm) > cab 2 (8ohm) in series, would that create a total impedance of 12 ohm therefore could I run it from the 8ohm out of my amp?

Hope that question makes sense and I'm frustrated at myself for still not completely understanding all of this stuff. Maths isn't my strong point so I find it easier to get a second opinion. I'd also be interested to know if people think it's unwise to match different types of cab anyway, regardless of impedance.

Thanks guys.

#263 barkin

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Posted 13 March 2017 - 04:14 PM

 Powertripper, on 11 March 2017 - 02:55 PM, said:

...if I ran from amp > cab 1 (4ohm) > cab 2 (8ohm) in series...

Unless you use special/custom speaker cables, the connections will be parallel.





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