BillFitzmaurice.info

Hmmm - been messing around with the PA2. I've set a patch (full range using just the Jack Lite 12's) at 55 hz as suggested. And the 'LR 12dB /octave' option (which I don't understand..)

The Limiter works in dBs with a "softer" soft knee option called OverEasy, which sounds like the way to do it. The default on the Limiter is 0dB. Is that the way to leave it - having set the gain structure by the book (channel then desk outs, PA2 ins then power amp?

It offers three EQ 'patches' - speech, flat, and music & speech. Selecting the latter, there's "DJ" and "MyBand" - the latter looking like it's right. But I take it the auto EQ RTA over-rides that.

I'm not sure where Compression comes in. The default on the Fullrange patch that I've modified seems to have everything turned off.

I'm enjoying much of the usual computer network fun - so all of you with the PAx etc, pre-networking, can enjoy my inability so far to get the PA2 onto the Wi Fi network - which is working fine!! The PA is resolutely refusing to take on an IP - beyond a string of zeros. But so far I've been trying to get it to network with my desktop Mac. If I have the same trouble with my iPhone, I won't be happy.

Apologies again if these are dumb comments and questions. I'm looking to be able to have this unit do my EQ'ing for me as I'll be playing, plus not having to worry about feedback and clipping (or speaker damage). I shall gradually do things like add a pre-fader 'backline" delay (another option) and think into the need for alignment delay when I get the subs.

But this does look like an easy way to do some seriously sophisticated stuff - which for me as a guitarist is a vertical learning curve. I just need to get my head round things enough to get familiar with the menus, then to experiment in a couple of weeks when I'm setting it all up in a large empty hall to try things out.

I would not use "Over-Easy" on a system limiter. If I recall this is DBX's trademark for soft knee or program dependent compression, which implies that the attack time may change based upon program material. This is fine when compressing vocals or instruments, but could prove disastrous when limiting voltage to your drivers. You should use an impenetrable brick wall protecting your drivers from excessive voltage, this means zero millisecond attack time.

I run a brick wall with zero attack time in my DCX. I also use a 2:1 system compressor with a knee about 6 db below that wall on my X32 mixer. I set up my gain stages so I can see with my mixer output meters how far away I am from my system limit. This combination gives me an audible and visual clue that I am close to what my system can safely deliver.

How do you decide where to place that wall - and what in terms of settings actually is the wall?

Oh... sir, sir... please sir, can I say it???

IITP
The 'wall' is the the 50V (or whatever the plans say) limit. I think that a T39 lab 12 is 50V

See the section on protecting your loudspeakers for details of how to set the limit. This has to be done in conjunction with your amp that you plan to use with the subs, or the setting is meaningless.

HugoMack wrote:How do you decide where to place that wall - and what in terms of settings actually is the wall?

Turn off the soft knee. Limits are based on the speaker you are using. It in the plans toward the back....unless it's moved.

oh dear. Is it all explained perfectly somewhere that I've failed to find? I can't find anything about voltage limits in the Jack 12 Lite plans.

Trouble is, being on vertical learning curve means comprehension overload, and reading things but not realising their significance. Every single bit of this BFM adventure has been like this.

There must be a maximum voltage for each speaker; and then regarding the spec of your amp, the need to sort out the gain structure to prevent it clipping - which with a lower powered amp, it could do at less voltage than the speaker's max?

HugoMack wrote:
It offers three EQ 'patches' - speech, flat, and music & speech. Selecting the latter, there's "DJ" and "MyBand" - the latter looking like it's right. But I take it the auto EQ RTA over-rides that.

.

I use the music/band curve. Used to go flat, but I was having to boost the subs and cut the low mids....the band curve does that for you....

HugoMack wrote:oh dear. Is it all explained perfectly somewhere that I've failed to find? I can't find anything about voltage limits in the Jack 12 Lite plans.

Trouble is, being on vertical learning curve means comprehension overload, and reading things but not realising their significance. Every single bit of this BFM adventure has been like this.

There must be a maximum voltage for each speaker; and then regarding the spec of your amp, the need to sort out the gain structure to prevent it clipping - which with a lower powered amp, it could do at less voltage than the speaker's max?

Voltage limits are for the subs. I'm guessing that with the 3012HO and highpassing at 55, you should be able to hit those with 35 volts (maybe more) without damage - but I'd try to get Bill's input on that.

Run your amps wide open and let the DR control how much it gets.

Thanks Bruce. I was reading a link about gain structure that explained how by ordering it from channel, desk outs, DSP then amps (getting the pink noise just into the amber at each stage, you'd end up with the power amps at around 1-2 o'clock - so that the surges that would otherwise clip are within the headroom of the power amp.

Would this have been written pre-DSP? ANd so how do you get the DR to 'know' when the power amp will clip? (Sorry in advance. There's probably something I ought to know, as that sounds really smart-arse and isn't meant to be )

HugoMack wrote:Thanks Bruce. I was reading a link about gain structure that explained how by ordering it from channel, desk outs, DSP then amps (getting the pink noise just into the amber at each stage, you'd end up with the power amps at around 1-2 o'clock - so that the surges that would otherwise clip are within the headroom of the power amp.

Would this have been written pre-DSP? ANd so how do you get the DR to 'know' when the power amp will clip? (Sorry in advance. There's probably something I ought to know, as that sounds really smart-arse and isn't meant to be )

First of all, have enough power to give yourself headroom. Then set the driverack to the proper limit for you speaker......there will be no clip.

You don't want anything in the chain to be readily and easily manipulated. Power amp knobs are the easiest thing for someone to turn up. So, you get your system all set with you limiter keeping the voltage right where you want it, but your power amp is turned to 1 o'clock. Some yahoo walks by and turns the amp to 10 - there goes you speaker. If it's turned up all the way, it can't be turned up louder, right?

The Driverack doesn't know when the amp will clip, nor does it care. There is no anti-clip feature in the driverack. Only a limiter. Once you set the limiter with the amp all the way up - you can't get any louder.

The deltalite 2510 in those cabs it is suited for has a voltage limit of 40V.
Bill has said in the past though, that, approaching that 40V, those cabs will be distorting.

Extrapolate that. 40V should be buckets...

Grant Bunter wrote:The deltalite 2510 in those cabs it is suited for has a voltage limit of 40V.
Bill has said in the past though, that, approaching that 40V, those cabs will be distorting.

Extrapolate that. 40V should be buckets...

He says he's got the 3012HO in those.....but, running full range I'd be a little conservative with too much voltage on that small xmax. But then, I'm a careful guy (says the guy who toasted two diaphragms).

55v for the LAB12. I couldn't find a number for the 3012HO, but you'll know when the Jacks are distorting.

I've been trying to find out the voltages that various power amp options produce - they not being listed alongside AES in the ratings. It seems to me that the usual doubling of a speakers RMS output to decide what amp to buy, ought to relate to voltage limits.

Somebody suggested this formula:

1000w into 4r can be worked backwards using p= v ^2 /r to give v= root 4000, = 63 v

This person also said, relating to my enquiry for an amp to drive LAb 12 loaded T39s: "I have absolutely no clue where you got the "my speakers maximum voltage is 55V" from ... is that RMS or peak to peak? Whatever, either way it is incorrect. The Lab12 maximum peak voltage will be around 97V RMS , sine wave, for brief duration. But this figure is also frequency dependant and depends on the cab, Fs etc. Peak power is usually 4 times the AES value, unless mechanical liits are reached first."

I'm not arguing with this guy who makes power amps, and I'm completely happy with Bill's voltage limits. But it seems this guy has got things the wrong way round.

Also, if your limiters are going to stop power at 55 volts, what's the point of an amp that delivers more? Or rather how much headway above that, should the amp be capable of producing? It shouldn't clip, but the limiter is going to catch that pretty close to the clipping point....

Hmmm (I'm probably being stupid).

HugoMack wrote:I've been trying to find out the voltages that various power amp options produce - they not being listed alongside AES in the ratings. It seems to me that the usual doubling of a speakers RMS output to decide what amp to buy, ought to relate to voltage limits.

Somebody suggested this formula:

1000w into 4r can be worked backwards using p= v ^2 /r to give v= root 4000, = 63 v

This person also said, relating to my enquiry for an amp to drive LAb 12 loaded T39s: "I have absolutely no clue where you got the "my speakers maximum voltage is 55V" from ... is that RMS or peak to peak? Whatever, either way it is incorrect. The Lab12 maximum peak voltage will be around 97V RMS , sine wave, for brief duration. But this figure is also frequency dependant and depends on the cab, Fs etc. Peak power is usually 4 times the AES value, unless mechanical liits are reached first."

I'm not arguing with this guy who makes power amps, and I'm completely happy with Bill's voltage limits. But it seems this guy has got things the wrong way round.

Also, if your limiters are going to stop power at 55 volts, what's the point of an amp that delivers more? Or rather how much headway above that, should the amp be capable of producing? It shouldn't clip, but the limiter is going to catch that pretty close to the clipping point....

Hmmm (I'm probably being stupid).

Nope, you're not being stupid.

Yes, impedance in a driver is variable with frequency.
Yes, the plans give a voltage limit. We often forget to include the statement that the voltage limit is a "displacement" limit at a certain frequency (to prevent over excursion). While other factors are important, Xmax and impedance are the two most important criteria in Bill's designs in setting a displacement limit. Another is thermal limit.
IIRC, it turns out, a lab12 reaches Xmax and thermal limit in a T39 at 50V and 45 or 40Hz dependin upon the number of cabs used.

There is a need to define the type of enclosure the driver is in!
Sure, a lab 12 might be briefly capable of 97V in a direct radiator. Never, in a T39.
And the respondent hasn't stated at what sine wave frequency. So their answer may only hold true at say 1K. That doesn't at all tell you what the brief peak capability is at say 40Hz, in any cab.
As frequency lowers, excursion increases.

To help you understand more power in your amp than the voltage limit required from it, think of an ordinary car. It might redline at say 5000 revs. If you continually drive that car to 5000 revs, all the time, you will likely blow the motor sooner rather than later.

Same goes for an amp. If you continually push it to it's operating limits it may stop working. That's a disaster for your performance.
Being an amp, it may go into thermal shutdown, or, at least, reach clipping.

What you want is an amp with more voltage output capability than the cabs you are trying to drive with it, simply so you are not flogging the poor thing to death.
You need to take into account impedance load.

So, tins tacks, if you were to run 2 x T39's channel, with lab 12's, to 50V (damn that would be loud), you might want an amp that is capable of say 57V/channel @ 4 ohms. That's about 810W/4ohms.
Ideally your amp would be 2 ohm load capable as well, for a little added safety in trms of impedance.

You also don't want to get to large an amp. Most limiters have a defined amount of limiting. To powerful an amp means maximising limiting, meaning you're into the limiter all the time to keep the voltage down with to powerful an amp.

Hope that helps...