Frequency responses series/parallel, tweeter needs etc…

Question:

>Sounds fun… come on – who’s bright idea was it to put the bass through >that little rig?

Presumably the teachers I know they have been practising with bass through this small PA (because the drummer is too loud for the small combo’s they have). It probably doesn’t help with my daughter using her new 5 string bass either . Funnily enough she killed a bass combo a few weeks ago (a Carlsboro Bassline 80) – where she goes doing drumming!. It’s a kind of musical youth club, run by the Police – originally for young offenders, but opened last Christmas to the general public (in case you think my daughter might be a young offender!). She meets some very interesting people there!. Anyway, in between drumming sessions she strolls round doing a bit of guitar, a bit of bass etc. While she was doing a ‘bit of bass’, smoke started coming out of the combo!. When I picked her up later I was presented with the combo to take away and look at!. The fault was actually very simple, and is probably worth explaining here if anyone has a Bassline 80 combo. In the event it was nothing my daughter had done, but a design fault in the amplifier (at least in my opinion). On the output of a transistor amplifier there’s a capacitor and resistor in series (across the speaker), this is called a Zobel Network. The capacitor was the cause of the smoke!. Now these NEVER fail, but this was so bad I couldn’t even read the value off of it, so I was going to fit something like a 0.1uF – which is what I would use in an amplifier I design. But seeing as I only live about 8 miles from the Carlsboro factory I gave them a ring, they were extremely helpful, and told me that the value had been changed three times during production, and told me that the final production value was 1uF. I replaced that and it worked fine – so if you have a Carlsboro Bassline 80 and it starts smoking (but still works) this capacitor sounds like a likely cause. It’s a very simple job to do, and should only take about 10 minutes, the capacitor itself is cheap as well – something like 50 pence. — Nigel Goodwin C.Farmer Ltd. Matlock

Response:

> writes >I’m confused chris.  What are we hooking in series vs parallalel? >Components in a speaker cab?  Speaker cabs?  Buttered toast? > I’m also very confused as well!.

Shouldn’t be… The data’s comparing series and parallel pickup wiring. > As I understand it, this was all to prove that a bass guitar puts out > less treble than a complete band through a PA system.

I wasn’t out to prove that – since we knew that already – but to investigate how much it’s possible to under-rate the tweeter/midrange by and still get a satisfactory response from the cabs. > But I think we all knew that anyway!.

Yup. > The basic thing you really need to know about tweeters is that their > power handling specification is usually quoted for "average music". > There are charts available which give the percentage of power at > different crossover frequencies for this "average music". It’s important > to choose your crossover frequency high enough to reduce the amount of > power to the tweeter to a level it can handle.

Yup – that’s where the 4/5 1/5 comes from in the calculations for 1.5kHz crossovers. > With a bass the fairly low levels of treble aren’t really a problem for > tweeters, as Chris’s figures show (and we all knew anyway – but actual > tests are always nice to have). Nice graphs by the way!.

thanks. > Where you may have problems is with things like keyboards, where you may > have a high frequency, high power, sustained note – this no longer > conforms to "average music" and may well exceed the tweeters ratings and > blow it.

Two words: Feedback. That’s the main reason why bass cab tweeters blow when used with a mic. I’ve looked at more tracks since and the -6dB on my first track is actually quite high. the other two tracks had 1.5kHz down 11 and 17dB respectively. > It’s obviously very hard to know how "average" your music is, I suspect > the only real way is to try it and see what happens – if you have > tweeter problems, you know to try uprating them

It’s not got much to do with "average" really. Here we;re talking about pickups just going through a DI and into the soundcard for measurement. Sure, there will be pickups with a higher high frequency output (I’m using smiths rock masters so few strings will be brighter but some pickups will…)  but how much are we talking about? maybe 6dB – I don’t think people would boost more than that… > Talking of which, my daughter was doing a couple of small gigs at school > last week (as part of a school concert) – everything was plugged through > a small PA system – 80W Carlsbro mixer/amplifier, and a couple of 1×12 > speakers. The gigs were Wednesday and Thursday evenings, but at > rehearsal on Tuesday the speakers started smoking!. > They eventually did the gigs using small combo’s, and I’ve offered to > look at the speakers and amplifier for them. I’m presuming the amplifier > has blown and put lots of DC across the speakers!.

Sounds fun… come on – who’s bright idea was it to put the bass through that little rig? cb

Response:

writes >I’m confused chris.  What are we hooking in series vs parallalel? >Components in a speaker cab?  Speaker cabs?  Buttered toast?

I’m also very confused as well!. As I understand it, this was all to prove that a bass guitar puts out less treble than a complete band through a PA system. But I think we all knew that anyway!. The basic thing you really need to know about tweeters is that their power handling specification is usually quoted for "average music". There are charts available which give the percentage of power at different crossover frequencies for this "average music". It’s important to choose your crossover frequency high enough to reduce the amount of power to the tweeter to a level it can handle. With a bass the fairly low levels of treble aren’t really a problem for tweeters, as Chris’s figures show (and we all knew anyway – but actual tests are always nice to have). Nice graphs by the way!. Where you may have problems is with things like keyboards, where you may have a high frequency, high power, sustained note – this no longer conforms to "average music" and may well exceed the tweeters ratings and blow it. It’s obviously very hard to know how "average" your music is, I suspect the only real way is to try it and see what happens – if you have tweeter problems, you know to try uprating them Talking of which, my daughter was doing a couple of small gigs at school last week (as part of a school concert) – everything was plugged through a small PA system – 80W Carlsbro mixer/amplifier, and a couple of 1×12 speakers. The gigs were Wednesday and Thursday evenings, but at rehearsal on Tuesday the speakers started smoking!. They eventually did the gigs using small combo’s, and I’ve offered to look at the speakers and amplifier for them. I’m presuming the amplifier has blown and put lots of DC across the speakers!. — Nigel Goodwin C.Farmer Ltd. Matlock

Response:

– Hide quoted text — Show quoted text ->Hi All, >I just got round to doing something I’ve been meaning to for a while… look >at plots, do some math make some comparisons etc… >Anyway, I recorded the same scale played as close as I can to the same way >with the same bass etc… and jotted down some numbers: > (all these numbers and stuff snipped because I have no idea what they > mean) > Chris, I respect the fact that you know so much about electronics.  I > can never hope to know as much about electronics as you do.  And > absolutely there is a need in the world for guys like you.  But damn, > you sure go way over my head!

How about this: In my example, you’d need an 80W tweeter at the efficiencies stated for a PA compared to a 2.5W tweeter needed for bass. Make sense? It also means that the eminence PSD2002 compression driver is more than enough for an 8×10" in it’s new 80W version. Here are the frequency responses: http://www.geocities.com/leahsut/series-parallel.jpg cb

Response:

– Hide quoted text — Show quoted text -> Hi All, > I just got round to doing something I’ve been meaning to for a while… look > at plots, do some math make some comparisons etc… > Anyway, I recorded the same scale played as close as I can to the same way > with the same bass etc… and jotted down some numbers: > -10dB > =580Hz Series > =530Hz parallel > -20 dB > = 821Hz Parallel > = 750Hz series > -30dB > =1.8kHz Series > =1.5kHz Parallel > – > -50 > S+P 3.3kHz > I’m confused chris.  What are we hooking in series vs parallalel? > Components in a speaker cab?  Speaker cabs?  Buttered toast? > I’m interested in hearing a bit of intro to digest the numbers.

Whoopsie… the p/j pickups in series/parallel. cb

Response:

>Hi All, >I just got round to doing something I’ve been meaning to for a while… look >at plots, do some math make some comparisons etc… >Anyway, I recorded the same scale played as close as I can to the same way >with the same bass etc… and jotted down some numbers:

(all these numbers and stuff snipped because I have no idea what they mean) Chris, I respect the fact that you know so much about electronics.  I can never hope to know as much about electronics as you do.  And absolutely there is a need in the world for guys like you.  But damn, you sure go way over my head!

Response:

- Hide quoted text — Show quoted text – > Hi All, > I just got round to doing something I’ve been meaning to for a while… look > at plots, do some math make some comparisons etc… > Anyway, I recorded the same scale played as close as I can to the same way > with the same bass etc… and jotted down some numbers: > -10dB > =580Hz Series > =530Hz parallel > -20 dB > = 821Hz Parallel > = 750Hz series > -30dB > =1.8kHz Series > =1.5kHz Parallel > – > -50 > S+P 3.3kHz

I’m confused chris.  What are we hooking in series vs parallalel? Components in a speaker cab?  Speaker cabs?  Buttered toast? I’m interested in hearing a bit of intro to digest the numbers. —   /"  ASCII Ribbon Campaign                | Todd H   /                                       | http://www.toddh.net/    X   Promoting good netiquette            | http://triplethreatband.com/   /  http://www.toddh.net/netiquette/     | "4 lines suffice."

Response:

Hi All, I just got round to doing something I’ve been meaning to for a while… look at plots, do some math make some comparisons etc… Anyway, I recorded the same scale played as close as I can to the same way with the same bass etc… and jotted down some numbers: -10dB =580Hz Series =530Hz parallel -20 dB = 821Hz Parallel = 750Hz series -30dB =1.8kHz Series =1.5kHz Parallel – -50 S+P 3.3kHz I took a normal music track and did the same analysis on it – needless to say, the lowest peak power up to 10kHz was -16dB – above 3kHz… at 1.5kHz -6.6dB (remember this is peak over the entire passage) Now some assumptions: If this track is typical and the DI bass output is also typical then here’s what you can assume… There’s a slightly greater midrange (500Hz) roll-off when pickups are wired in parallel. Upper mids roll off faster, catching up in series though Compared to "program", bass signals at 1.5kHz have 15dB less upper mids and even less treble – meaning that if your midrange/tweeter need only have a power rating of 1/32nd of what would be needed in a PA cab. How tweeters are rated is a bit of a mystery because you have to look at power/bandwidth curves and such. Above 1.5kHz though with program music (whatever that is…) you’ll find only 20% of the power. Here’s an example: Driver = 92dB 600W below 1.5kHz Mid/tweet = 104dB 40W above 1.5kHz First thing you need to do here is calculate the attenuation needed to match the drivers (roughly) – 9dB The real power handling of the driver is 4/5 x 600W = 480W Conversely, the real power handling of the midrange is *only* 8W… the 9dB difference in efficiency gives us attenuation of 1/8th the original power which means the equivalent of 64W rms is the maximum rms power that the high pass circuit (including Lpad) can cope with (I’m working with theoretically "ideal" crossovers which have a sharp cut-off… not real world) Now in a PA application, the high pass section inclusive of attenuators would have to cope with 1/5 x 600W = 120W – far roo much for our 64W capable tweeter assembly. – we’d need an 80W rms tweeter of the same efficiency to cope with that application. According to the measurements I did comparing music to bass guitar, the 40W driver is massively over-rated even though it’s only capable of 8W rms in real terms…  1/32 of the power requirement of the PA cab is all that’s needed – so at that efficiency, it actually only needs to be rated at 2.5W!!!! Remember that’s IF my measurements are right and based on a few assumptions. It does prove to me that a 40W compression driver has more than enough power for a 4×10" rig – probably even an 8×10" rig for bass guitar.  Just to be on the safe side, go with something like 1/8th of the PA equivalent for a tweeter/midrange in a bass rig operating above 1.5kHz and do some of your own math… I could be wrong… Feedback welcome (actually feedback’s a pretty shitty idea with a bass cab)… and yup, I’m no expert so corrections welcome too! cb

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