FBT Maxx 6A powered speaker limiter question

[Mick Bailey]

I've got a pair of these on test right now and the output on both appears to be on the low side - just 360W RMS into 4 Ohms on the LF at the point the limiter is fully active. The power amps are working fine and with the jumper removed to disable the limiter each one makes 676W. The amps are rated at 700W RMS (LF) and 200W RMS (HF).

I can't really see any problem with the limiters. They're pretty simple designs just rectifying a sampled voltage from the speaker terminals and using this to attenuate the signal via a vactrol.

This is the first time I've encountered this particular model. Because both speakers behave the same I'm wondering if this is intentional, or am I missing something?

fbt_maxx6a_sch.pdf

[Jazz P Bass]

I don't know if that is the way it is supposed to be, but I have seen it before on other equipment.

The limiter is not allowing full power but it most probably will let excursions through. (Peak Power)

Can you set it up to test it in some kind of burst mode?

[J M Fahey]

Just guessing, didn't analyze the schematic yet, but they might be using a (very reasonable) trick which of course can't be mentioned in the brochure:

700W RMS is a HUGE amount of power to be dissipated in a voice coil, even 300W RMS is a lot, but on the other side it's nice to have a lot of power in reserve.
Problem is that DJs (the worst offenders) and many brainless PA operators drive them 10dB above clipping all the time.

So a reasonable design decision is to offer a real high power amp (which with digital Tech is very easy) BUT the speaker must be protected.

One good compromise is to set limiters with relatively long attack time, so, say, a drum kick passes unmolested, but a continuous tone (such as you are using) or an endless string of strong Bass gets attenuated to a safe level.

Synthetic Bass as often used in House or Rave Music or those boring "Car speaker SPL contest" CDs are speaker murderers.

You might have something similar there; maybe testing with short bursts (say, 10% to 20% duty cycle) will allow the full power go through.

I use similar tricks in Bass amps meant for slappers.

[Mick Bailey]

I'll get them back on the bench later today and see how they respond to bursts. I did notice if I suddenly increased the signal gen level it would beat the limiter's response and it took a fraction of a second to kick in. Maybe testing with a mixed audio source would be a better bet.

[nickb]

I've seen the same thing before in powered speakers. I just did a Mackie where the LF channel is rated at 400W but on a continuous signal it's limited to 120W. I test using sine wave bursts frequently.

[Mick Bailey]

Further tests show up some interesting observations:

Injecting a line-level music source produces an output of about 400W with the limiter just operating and flickering to the beat. (Interestingly, increasing the level so that the limiter comes on fully then reduces the output). Peaks go beyond that - I measure occasional fleeting jumps to 52v RMS (equating to 676W). I set up my synth to do 10Hz gated pulses and the limiter flickers but the output is a reliable 52V RMS.

If a constant signal is applied so that the limiter lights continually, then the output reduces to 360W. If I then superimpose the 10Hz pulses, the output stays the same - the limiter 'locks down' the output once it's fully switched on.

So it would seem that Jazz P Bass hit on the reason for the apparent low output - the amp is set to allow the peaks through but the limiter controls the continuous power.

[Jazz P Bass]

Nice test setup with the synth.