Impedance etc

[xgsjx]

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. 

[V2factoryman]

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?

 

[Bill Fitzmaurice]

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

[V2factoryman]

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.

[Quilly]

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. 

[RikkMalice]

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!

[barkin]

2 x 8 Ohm cabs will present a 4 Ohm load to the amp.

[RikkMalice]

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.

[xgsjx]

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

[chris_b]

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.

[Bill Fitzmaurice]

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.

[Quilly]

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

 

 

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

+1.

 

[Muzz]

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...

[Quilly]

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. 

[chris_b]

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.

[Huge Hands]

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? 

[chris_b]

23 minutes ago, Huge Hands said:

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? 

The switchs are fitted as a matter of course these days. It's now called the Two10S, I got BF to add the switch to my first 210.

[BigRedX]

4 hours ago, Quilly said:

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

Because IMO it doesn't give you any advantage. There's a whole thread on this already.

[Bill Fitzmaurice]

You don't even have to search out other threads, why it's not useful in general has been explained in this thread, repeatedly, including just a few posts before this one. The only reason to want a 4 ohm/8ohm 112 cab is to allow using it with a valve amp that needs a maximum 4 ohm load and with an SS amp that won't take a 4 ohm load. 

Edited February 27, 2019 by Bill Fitzmaurice

[Quilly]

I was running an 8 ohm valve amp into a 4 ohm cab for a while with no issues (the amp also has 16ohm outputs), I stopped because I'm just nervous of damaging the amp. I bough a 4/12 ohm switch for the cabinet in question (8 ohm isn't possible for the configuration of cab). My question is : Is it better to run 8 ohm amp into 12 ohm cab OR use the 16 ohm amp out into 12 ohm cab. My hunch is the latter is probably better for a valve amp to prevent damage to the transformer. Still not ideal for the valves but better than the current set up.  

[pintspiller]

@Quilly I'd say the latter too.

At our last gig I set my Carlsbro to 4ohm and ran my 8ohm cab and a 4ohm sub.

[Bill Fitzmaurice]

On 07/05/2019 at 06:10, Quilly said:

Is it better to run 8 ohm amp into 12 ohm cab OR use the 16 ohm amp out into 12 ohm cab.

The speaker impedance ratings for valves is the maximum, that for SS is the minimum. Use the tap on a valve head that's equal to or more than the speaker impedance, use a total speaker load on an SS amp that's equal to or greater than the amp rating.

[Quilly]

1 hour ago, Bill Fitzmaurice said:

The speaker impedance ratings for valves is the maximum, that for SS is the minimum. Use the tap on a valve head that's equal to or more than the speaker impedance, use a total speaker load on an SS amp that's equal to or greater than the amp rating.

I’m using a 4 ohm bass cab with an 8 ohm guitar head and it sounds sublime with bass . Just for home use mind. Is that because I’m driving the tubes a bit harder ? 

[Quilly]

I Cant understand why these arent readily available by multiple suppliers. 

 

[BigRedX]

2 minutes ago, Quilly said:

I Cant understand why these arent readily available by multiple suppliers. 

 

Because in order to work their "magic" they will dissipate a significant proportion of the amplifier's power as heat through the resistors required to match the input and output impedances. The more different the input and output impedances are the less efficient it makes your rig in terms of power being used to produce sound. Also the particular device pictured is only good for amplifiers up to 100 Watts, which makes it virtually useless for bass rigs.

A device rated for at least 300 Watts (which is what most bass players will need as a minimum) is going to be significantly bigger, heavier and probably require fan cooling. 

Most sensible people simply buy a rig where the amp and speakers are properly matched.