New amp seems quieter than I expected.

Haych

I acquired a Jet City JCA50H recently, used of course, but was a decent price and I've wanted one for a while so I thought why not.

I don't have a speaker cabinet for it yet though, so borrowed a friends' Blackstar 4X8 just to mess around with.  

The amp works as expected, all dials do what they should and there's no bad noises from the amp and it sounds pretty good even through the toy cab.

However, I only turned it up to 5 on both channels, but it didn't seem very loud.  It was making quite a din but I was thinking it's never loud enough to gig with, it'd never keep up with a drummer, for example.

Is it possibly the toy cab, tired valves or maybe the taper on the master volumes?  Most 50 watt valve amps I've played are usually at volume 1 you can't hear really it, at volume 2 the windows have just fallen out.  Or have I got a dud?

ICBM

I would guess the cab. @ecc83 may have an accurate spec for it, but I’d be surprised if it’s over about 94dB - which would mean the amp is only as loud as a 12W amp into a 100dB cabinet.

Haych

Ummm, yeah. I’d love to pretend I knew what you just said but I haven’t a clue lol. 

Just plugged it back in and wound it all the way up and while it does make the floorboards rattle it’s certainly not gigworthy by any means. 

ecc83

TOY! Cab? Cheeky bstd!

Heh! Yes well I guess it is a bit of a 'cosmetic exercise? No worse than the other 'mini-stacks'?

The speaker chassis are 'Blackbirds. Yeah, WTF! I would hazard a guess at the ~ 25 quid Celestion Eights and they are specced at 95dBW (don't think there ARE many 'guitar 8s much hotter than that?)  but even 95dB gives you 112dB SPL at a mtr if you hit them with 50watts and that is blinking loud so yes, maybe the amps is a bit poorly?

In fact the cab is rated at 60W which is only JEEUST about safe on a good 50 watter IF they are indeed Celestions!

Haych, just noticed your 'boggledness"!? I can walk you through the minefiled of decibels and watts if you want to come back about them?

Dave.

ICBM

Haych said:

Ummm, yeah. I’d love to pretend I knew what you just said but I haven’t a clue lol. 

I don't know if this will help but I'll try to get there in stages .

Basically the final volume is not just dependent on the amp power - the efficiency of the speaker in turning that electrical power into acoustic energy is at least as important. The less efficient the speaker, the quieter the volume will be for a given amp power.

Typical guitar 12" speakers are in the 97 to 100dB @1W, 1m range - that means that if you put in one watt and listen from 1 metre away, you will hear between 97 and 100dB... which is quite a lot.

Every 3dB up or down is equivalent to doubling or halving the power of the amp - that sounds odd because the difference between 97dB and 100dB doesn't look like much - but it is, because the scale is logarithmic. So a 25W amp into a 100dB speaker will be exactly the same volume as a 50W amp into a 97dB speaker. (Ignoring speaker thermal compression etc for now!)

Speaker efficiency is also related roughly to size, because a larger cone moves more air. (Again it's not quite as simple as that, but it will do for now...) As a rough rule of thumb, each 'standard size' of speakers - 6.5", 8", 10", 12", 15" - is about 3dB more efficient than the one below and 3dB less than the one above. So an average 8" speaker will be about 6dB less efficient than an average 12", all other things being similar. Thus I would expect a typical 8" speaker to have a sensitivity of about 91-94dB. (Dave quotes 95dB for the Celestion Eight, which is at the very top end.)

With me so far?

That's for one speaker. In a cabinet with two speakers you get some extra efficiency in air movement by coupling them together, but the power is then divided between both speakers, so you don't double the efficiency (ie add 3dB) - you normally gain about 1-2dB depending on the cab design and how close the speakers are to thermal compression. The same is true but a bit less so if you double again, so typically a 4-speaker cab is around 2-3dB more efficient than a single speaker of the same type.

All this means that a typical 4x8" cab is probably going to be around 93-94dB assuming fairly standard speaker and cab design. (Give or take a dB or two.) ie 6dB less efficient than a single 12" 100dB speaker like a V30.

Going back to the beginning, this means that in order to get the same volume from the 4x8" as the 1x12", you would need four times the amplifier power. So a 50W amp into a 4x8" is going to be around the same volume as a 12.5W amp into a single V30 cab.

For most players this would probably be at about the absolute lower limit of power for a gigging amp to compete with a full band, and probably wouldn't work if you need a completely clean tone and/or aren't mic'ed up.

On top of that, the 4x8" probably won't have the low-end thump of even a single 12" - because for typical guitar-type speakers that's limited by how far the cone can move, which is less in smaller speakers - and which is where you most notice the lack of power in my experience.

Does that explain it a bit better?

I also agree with Dave that thrashing the little cab with a 50W amp is probably at the upper limit of what's safe for the speakers, and basically I wouldn't do it.

Haych

ecc83 said:

TOY! Cab? Cheeky bstd!

Heh! Yes well I guess it is a bit of a 'cosmetic exercise? No worse than the other 'mini-stacks'?

Haych, just noticed your 'boggledness"!? I can walk you through the minefiled of decibels and watts if you want to come back about them?

Dave.

Hey Dave, sorry no offence intended, I just meant it in the sense that I doubt it is a cab that would be useful for gigging. For bedroom or home studio use I’m sure it’s a very capable cab. 

If if you’d like to explain the nitty gritty around watts and decimals etc then I’ll be a willing student?

cheers. 

Haych

ICBM said:

Haych said:

Ummm, yeah. I’d love to pretend I knew what you just said but I haven’t a clue lol. 

I don't know if this will help but I'll try to get there in stages .

Basically the final volume is not just dependent on the amp power - the efficiency of the speaker in turning that electrical power into acoustic energy is at least as important. The less efficient the speaker, the quieter the volume will be for a given amp power.

Typical guitar 12" speakers are in the 97 to 100dB @1W, 1m range - that means that if you put in one watt and listen from 1 metre away, you will hear between 97 and 100dB... which is quite a lot.

Every 3dB up or down is equivalent to doubling or halving the power of the amp - that sounds odd because the difference between 97dB and 100dB doesn't look like much - but it is, because the scale is logarithmic. So a 25W amp into a 100dB speaker will be exactly the same volume as a 50W amp into a 97dB speaker. (Ignoring speaker thermal compression etc for now!)

Speaker efficiency is also related roughly to size, because a larger cone moves more air. (Again it's not quite as simple as that, but it will do for now...) As a rough rule of thumb, each 'standard size' of speakers - 6.5", 8", 10", 12", 15" - is about 3dB more efficient than the one below and 3dB less than the one above. So an average 8" speaker will be about 6dB less efficient than an average 12", all other things being similar. Thus I would expect a typical 8" speaker to have a sensitivity of about 91-94dB. (Dave quotes 95dB for the Celestion Eight, which is at the very top end.)

With me so far?

That's for one speaker. In a cabinet with two speakers you get some extra efficiency in air movement by coupling them together, but the power is then divided between both speakers, so you don't double the efficiency (ie add 3dB) - you normally gain about 1-2dB depending on the cab design and how close the speakers are to thermal compression. The same is true but a bit less so if you double again, so typically a 4-speaker cab is around 2-3dB more efficient than a single speaker of the same type.

All this means that a typical 4x8" cab is probably going to be around 93-94dB assuming fairly standard speaker and cab design. (Give or take a dB or two.) ie 6dB less efficient than a single 12" 100dB speaker like a V30.

Going back to the beginning, this means that in order to get the same volume from the 4x8" as the 1x12", you would need four times the amplifier power. So a 50W amp into a 4x8" is going to be around the same volume as a 12.5W amp into a single V30 cab.

For most players this would probably be at about the absolute lower limit of power for a gigging amp to compete with a full band, and probably wouldn't work if you need a completely clean tone and/or aren't mic'ed up.

On top of that, the 4x8" probably won't have the low-end thump of even a single 12" - because for typical guitar-type speakers that's limited by how far the cone can move, which is less in smaller speakers - and which is where you most notice the lack of power in my experience.

Does that explain it a bit better?

I also agree with Dave that thrashing the little cab with a 50W amp is probably at the upper limit of what's safe for the speakers, and basically I wouldn't do it.

Thanks @ICBM even I understood much of that. I’ll probably have to re-read it to get the gist of it properly but I think I’m starting to grasp the principles involved. 

Really appreciate you sharing your knowledge. Thank you. 

ecc83

I am not going to argue with ICBM because all he says is true IF we just keep with 'guitar' speakers. This is because they are all built on much the same basis, lightweight cones and stiff suspensions (the edge of the cone) So, for a given cone size, sensitivity* will increase because it usually goes hand in hand with a bigger magnet and thus for a given power input the air 'displacement'  is greater (the output volume of a speaker is proportional to cone area AND peak to peak displacement.... Think "pistons" you can have a large bore and a short stroke or long stroke and smaller bore. The net swept volume is the same. The choice of these ratios  becomes important 'at the limit' that is the maximum power a speaker or engine can deliver.)

Low frequency performance for speakers in general is not related to cones size  but it is generally easier to have a big'un moving a little rather than a little'un moving a lot!  Before the development of 'roll surrounds' the only choice for decent bass was huge speakers and fridge sized cabs!

You will find a lot of conflicting information about speakers, amplifiers and audio systems in general. This is because, although they all have to obey the laws of physics the APPLICATION of those laws can vary depending on what you want to achieve and very often on how much you can spend!  Modern, active Digital Signal Processing can do wondrous things! I do not PRETEND to understand any of that!

*I prefer to stick to "sensitivity" because "efficiency" is the ratio of POWER out to power in and it is very difficult to measure the acoustic power output of a speaker. Sorry! Just me being a pedantic old fart.

Dave.

ICBM

Yes, this is definitely just for guitar speakers. For 'proper' speakers with better-designed suspensions the 'swept volume' of the cone can bear almost no relation to the diameter! And low-frequency performance even less...

Sensitivity is the correct term but for someone trying to make the link between amp power and acoustic volume, 'efficiency' makes more sense I think .

ecc83

ICBM said:

Yes, this is definitely just for guitar speakers. For 'proper' speakers with better-designed suspensions the 'swept volume' of the cone can bear almost no relation to the diameter! And low-frequency performance even less...

Sensitivity is the correct term but for someone trying to make the link between amp power and acoustic volume, 'efficiency' makes more sense I think .

Yes, all cool. P.O.F. out!

Dave.

p90fool

The JCA50H is loud enough to gig with over a rock drummer without breaking a sweat, it's a very loud amp. 

I don't even know what a 4x8 cabinet is, but I'm pretty sure it's not meant to be anywhere near a stage. 

ICBM

p90fool said:

I don't even know what a 4x8 cabinet is, but I'm pretty sure it's not meant to be anywhere near a stage. 

Unless it’s a bass cab...

http://media.guitarcenter.com/is/image/MMGS7/Little-Henry-4x8-Bass-Cabinet/000000111049871-00-500x500.jpg