Ohms and amps

[Woodinblack]

This question was answered about 2 weeks ago. Since then it has got steadily less and less clear with every continuous post. I guess it was lucky we weren't discussing signal to noise, or that would be ironic!

[itu]

12 minutes ago, Bill Fitzmaurice said:

When you add a second identical cab in parallel you get a 6dB increase in both sensitivity and maximum output. That's the equivalent of quadrupling the power into a single cab, so it is most definitely a big change.

Sir, you are quite right, but many people think that the change would be in the same decade as watts (300 W to 600 W). I do not see that it is reasonable to emphasize power, but dBs.

This monitor speaker can be our reference to few calculations (not for you Sir F., you know this stuff):

https://products.electrovoice.com/emea/en/elx200-12/

sensitivity 93 dB / 1 W / 1 m

continuous power handling 300 watts / 8 ohm (1200 W peak)

max theoretical SPL: 128 dB / 1 m

The amp mentioned can produce up to 300 watts per speaker. 1 watt produces 93 dB from this speaker (see sensitivity), 2 watts 96 dB, 4 W 99 dB, 8 W 102 dB, 16 W 105 dB, 32 W 108 dB, 64 W 111 dB, 128 W 114 dB, 250 W 117 dB, 500 W 120 dB (threshold of pain), 1 kW 123 dB, 2 kW 126 dB.

From the previous list we can take the numbers 250 W (the amp was 300 W) and 117 dB. With two speakers we can get +6 dB and this will be equal a total of 123 dB with two speakers.

A good thing is to understand that if the speaker was capable of eating lots of watts to produce decibels, the amp should be able to push 1 kW to reach the mentioned decibel level. Here we see that some extra watts do not automatically equal much more loudness.

[Downunderwonder]

The usual situation runs like this.

One powerful amp. One cabinet. The rig only gets so loud before the amp overdrives the cab.

At that point you are all out of headroom.

If that's only just enough to be heard in a loud band it's not at all dynamic unless everyone respects that limit and adjusts themselves down.

More usually the bass player adds another cabinet.

Now if the amp is powerful enough to drive both cabinets to their limit you have the full +6dB experience in the low end.

Going from zero headroom to 6dB of headroom is quite fun, especially if the band becomes responsive to bass dynamics.

[Chienmortbb]

15 hours ago, Bill Fitzmaurice said:

When you add a second identical cab in parallel you get a 6dB increase in both sensitivity and maximum output. That's the equivalent of quadrupling the power into a single cab, so it is most definitely a big change.

 

That would only happen if the two cords weren't of like polarity. The reason why they say not to do so is because it won't add to output, and you might have two cords that aren't of like polarity.

There is also the possibilty that your amp has two power amps, each feeding one of the speaker sockets. In that case it would be inadvisable.

[Bill Fitzmaurice]

That's the case with my Superfly. The manual is specific on that fact. Of course that does little good if one doesn't bother to read it.

[BassmanPaul]

RTFM is always a sensible option!

[NancyJohnson]

The amp/cab impedence thing comes up frequently, doesn't it and we, as a group, seem to be obsessed with headroom/amp-power/wattage.

You know, and perhaps I've been lucky, I've been in this game for over 30 years and I've never experienced gear damage due to mismatching or over-egging things with my backline.  I'll admit here that I still don't fully grasp the whole ohms/watts thing and don't let it worry me.

Take comfort from this.

[Woodinblack]

13 minutes ago, NancyJohnson said:

The amp/cab impedence thing comes up frequently, doesn't it and we, as a group, seem to be obsessed with headroom/amp-power/wattage.

I think there is more rubbish quoted on this subject than any other in here!

in total contrast to the number of "I blew my amp / speaker" threads.

 

[Downunderwonder]

Long before internet forums I must have picked up on the essential truth from one of the old pros I got to jam with as a beginner.

In my first rowdy band with my pride and joy Trace rig being hard to hear over the stupid loud guitarist I drew a line in the sand and the guitarist was forced to pull his head in.

The line I drew was where the speaker didn't get any louder despite increasing the master and backing off a tad for good measure.

Ten years later I learned about ohms and watts and volts and sensitivity and it made little difference to operations aside from the addition of a high pass filter.

[NancyJohnson]

1 hour ago, Woodinblack said:

I think there is more rubbish quoted on this subject than any other in here!

in total contrast to the number of "I blew my amp / speaker" threads.

 

I've played in loud bands, quiet bands, tiny venues up to medium sized festivals.  I honestly believe the rule of thumb is to buy headroom, know your frequency, know the room you're playing, know where your amp sounds best (ie how far away from walls etc.) and not run everything at 10.  Gear now is way more reliable than the heavy old stuff I inherited in the 1980s and with that comes better tolerances.  If it plugs together, it'll work.

I know when I post here I will put in caveats that tone is subjective, I doubt anyone would be able to tick a box and say with any authority that they recognise a 50watts hitting their ears over 100watts.  The secret is to ensure - irrespective of what style you play - that the bass sits well in the mix with the drums and then roll the guitars/keys/BVs up to compliment that; if you're playing in a room with a sound guy, he should be able to identify where things are going wrong and Identify that.  Nobody runs their gear at 10 all the time, do they?

My gear right now is a Darkglass A/O head - I only utilise the power side and prefer my tone to be shaped by a @Tech21NYC dUg stompbox.  It's supposed to be 900w.  I played with a very loud punky/rocky band a few times in 2019 and I never took the DG output over 40%, the cabs (I tried a couple) didn't melt down or anything.  I couldn't have been any less concerned about the ohm-age of either.

Edited May 31, 2021 by NancyJohnson

[Chienmortbb]

29 minutes ago, Downunderwonder said:

The line I drew was where the speaker didn't get any louder despite increasing the master and backing off a tad for good measure

That was likley to be power compression. As the speaker warms up, the coils increases in impedance so you lose volume. turn up and the speaker heats up even more and you lose more volume. Backing off would not help immediately as the heat chnages lag the That is why you need "headroom" in cab power.

[Chienmortbb]

On 18/05/2021 at 18:28, Bill Fitzmaurice said:

That would only happen if the two cords weren't of like polarity. The reason why they say not to do so is because it won't add to output, and you might have two cords that aren't of like polarity.

Or if the two outputs were from two separate power amps.

[Bill Fitzmaurice]

1 hour ago, Chienmortbb said:

That was likley to be power compression. As the speaker warms up, the coils increases in impedance so you lose volume. turn up and the speaker heats up even more and you lose more volume.

That's one possibility. There's also mechanical compression, where the speaker has run out of excursion, so additional input doesn't give additional output. The same can happen with the amp. In practice all three can occur simultaneously, especially with guitar amps, in which case it's often intentional, although the player probably has no idea why he's getting the result.

[Woodinblack]

2 hours ago, NancyJohnson said:

Nobody runs their gear at 10 all the time, do they?

Wouldn't really matter if they did - bass is a transient thing anyway, a few spikes of full power and then most of the time next to nothing. Not like keys where you can actually be providing a continuous signal.

[barkin]

3 hours ago, Chienmortbb said:

Or if the two outputs were from two separate power amps.

Have I travelled back in time, to May 19th...?

[BassmanPaul]

7 hours ago, NancyJohnson said:

I

My gear right now is a Darkglass A/O head - I only utilise the power side and prefer my tone to be shaped by a @Tech21NYC dUg stompbox.  It's supposed to be 900w.  I played with a very loud punky/rocky band a few times in 2019 and I never took the DG output over 40%, the cabs (I tried a couple) didn't melt down or anything.  I couldn't have been any less concerned about the ohm-age of either.

Oh come on Nancy love Ohmage?? The correct word is impedance!

It's obvious that you know what you are doing. You use your amplification wisely. All too many people just crank the amp to get louder without giving a damn about whether the speaker systems are capable of handling that extra power. 

The only time I damaged drivers since 1962 is when I left my amplification in the van over a cold winters night. Stupidly I failed to allow the stuff to warm up to room temperature before dropping a nice low B into the pair of Acme B2s. I  cracked a surround on one driver in each cabinet. Let me tell you THAT was a long night. <moan!> Luckily Andy Lewis the builder of the cabinets shipped me a replacement for each very quickly. 

 

Edited May 31, 2021 by BassmanPaul

[NancyJohnson]

9 minutes ago, BassmanPaul said:

Oh come on Nancy love Ohmage?? The correct word is impedance!

I blame half a bottle of red and three very large Bourbon & Cokes.

[Chienmortbb]

4 hours ago, barkin said:

Have I travelled back in time, to May 19th...?

Yes it's Grounddog day. (chien is French for dog)

[agedhorse]

On 18/05/2021 at 10:07, abarson said:

Never connect both outputs of the amp to one cabinet. Doing so shorts the output of the amp

This is true under 2 conditions:

1. if one of the two cable is wired reverse polarity.

2. if each of the two speaker jacks is connected to a different power amp channel. This is common in some amps, using stereo or 2 channel power amps internally.

[Phil Starr]

Just a thought but Mesa make valve amps (should that be tube as a USA brand?) If so there is an output transformer on the other side of those jacks and impedance matching is done by the transformer. So, for this particular amp connection this way is correct and avoids a mismatching load. The Ashdown is solid state so using both the parallel output sockets would be fine and so would this, but not with the Mesa. It's just unfortunate that this image was used.

I do feel for the OP the first advice was fine and he only has one cab. I hope he plugged it in and just got on with playing.

I did find one blurry picture of the back of a Mesa 

 

Edited June 2, 2021 by Phil Starr