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Old 13th September 2008, 02:29 PM   #11
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Neil

Thanks for the link to the spreadsheet

> I don't know of any subwoofer plate amp that doesn't have a subsonic (aka "rumble") filter. The Jaycar amp almost certainly has one, since it has a boost at 35Hz. The boost is usually added by making the subsonic filter have a high Q. This results in a peak at the boost frequency followed by a 12-db rolloff for the low frequencies.

Yes the Jaycar amp has a boost at 35 Hz. While a high Q gives a boost at the filter frequency, how do we know the amount of rolloff below that?
It probably is enough for most sealed alignments; some vented alignments up to a watts input, down to a Hz, etc; but safe EBS requires a very steep rolloff, dependent on the driver used and output needed.

To avoid risk of blowing the sub driver, I believe its bets to implement an appropriately designed subsonic filter
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Old 13th September 2008, 02:47 PM   #12
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"This is pretty typical of plate amp performance"

Those smooth curves to me look to me like modelled performances of a plate amp - combined with mid level sub drivers, at different upper XO frequencies.

While I dont think plate amps are generally that great, I dont believe that the amp alone drops as shown on the graph - between 20 and 10 hz by about 10! dB
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Old 13th September 2008, 02:49 PM   #13
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HK26147

The link isnt working from here right now
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Old 13th September 2008, 03:01 PM   #14
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Quote:
Originally posted by otto88
Neil

Yes the Jaycar amp has a boost at 35 Hz. While a high Q gives a boost at the filter frequency, how do we know the amount of rolloff below that?
Every plate amp that I have looked at uses a 2-pole Sallen-Key rumble filter. The two poles give you a 12db rolloff. If you make the filter Q greater than .7, it will give you boost before the rolloff, but it will still result in the 12db rolloff. So you know what the subsonic filter is and the spreadsheet helps you change the Q and cutoff frequency.

Quote:

To avoid risk of blowing the sub driver, I believe its bets to implement an appropriately designed subsonic filter
Usually 12db/octave is considered an appropriately designed subsonic filter. Also, there are usually coupling capacitors in the circuit that provide additional rolloff. So you are likely to get good protection for your sub from any subwoofer plate amp. Whether or not you want additional rolloff is your design decision.
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Old 13th September 2008, 07:41 PM   #15
rabbitz is offline rabbitz  Australia
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Jaycar kit KC5452 (15Hz at 18dB/octave)

Altronics kit K5568 (20Hz at 18dB/octave)

Fits between the LFE and the plate amp input.
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Old 13th September 2008, 08:18 PM   #16
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Quote:
While I dont think plate amps are generally that great, I dont believe that the amp alone drops as shown on the graph - between 20 and 10 hz by about 10! dB
I didn't draw that myself, it came straight from the amplifier manufacturer's spec sheet. It has nothing to do with the drivers that the end user may or may not connect, it simply shows the amp response.
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Old 13th September 2008, 09:01 PM   #17
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I thought I'd add my two--cents.

Hard for any manufacturer to make and to advertize an amp with any diminution in the 20-20k band.

But, in my fairly long experience, you really want a really really sharp cutoff (like 36dB/8ave) and you really need it to start somewhere north of 20 Hz, like 25... since there is likely to be a little bump at that point (good) and no real cut till below 20 Hz. If you start at a fantasy point, like 18 Hz, you end up cutting too little.

I guess it would be good to keep those useless subsonics out of the amp chain right from the start and again right before the amp. I bet there's lots of overloading going on inside those DC-to-llight amps from junk you can't hear anyway.

Given the inaccuracies of filters esp. LF cut, best to have a single bass line. Otherwise, maintaining two (or more) woofer channels, you are creating separate bass pseudo-phases, and pseudo-sounds where none existed before. All this and many more reasons are real good rpersuasion to mix your base below, say 100-140 Hz. I think you should do so even if you choose to use two independent woofers for some odd reason.
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Old 14th September 2008, 05:13 AM   #18
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Neil

> Every plate amp that I have looked at uses a 2-pole Sallen-Key rumble filter

Thank you, I didnít know that.

12 db/octave could be an adequate subsonic filter in most situations, but not for EBS. If I knew how to post a graph of the excursion requirements from Unibox, I would. While (in one alignment Iím considering) with a 100 watt amp, excursion required is comfortably under Xmax at 13 Hz; by 10 Hz its nearly double Xmax.

In the vast majority of material, thereís no signal below 13 Hz. But a 1 in 100 (?) soundtrack could easily blow the sub driver. So 12 db/octave isnít adequate.


rabbitz

Thank you, I didnít know that Jaycar and Altronics had kits.

So if Neilís general comment that most plate amps has a 2-pole Sallen-Key rumble filter of 12 db/octave does apply to the Jaycar AA0508 (do you know?) and an 18 db/octave filter is added between the LFE and the plate amp input, thatís 30 db/octave, which I think would be ok for EBS.
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Old 14th September 2008, 05:27 AM   #19
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(I wasnít suggesting that you drew the graph) Even if they ďonly for low bassĒ Iím amazed that *any amps can have such a highly limited bandwidth.
If youíre doing a 0.1 LFE sub on a budget, and want say 15 - 150 Hz +/- 3 dB, would a PA amp be better?

Ben

We agree about the need for a sharp cutoff (like 36 dB) the 18 Hz I mentioned wasnít so much a starting point, but as an example point by which the cut needs to start Ďkicking in, to protect below 13 Hz, where its needed.
Thatís what I like about http://sound.westhost.com/project99.htm - itís configurable.

> best to have a single bass line. Otherwise, maintaining two (or more) woofer channels, you are creating separate bass pseudo-phases, and pseudo-sounds where none existed before. All this and many more reasons are real good persuasion to mix your base below, say 100-140 Hz. I think you should do so even if you choose to use two independent woofers for some odd reason.

Not sure what you meant by ďa single bass lineĒ. Being an 0.1 for HT, itís a single channel, only for LFE, below 120 Lz. Maybe youíre referring to eg a two way + sub(?)

Cheers
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Old 14th September 2008, 01:11 PM   #20
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Quote:
Originally posted by otto88
rabbitz

Thank you, I didnít know that Jaycar and Altronics had kits.

So if Neilís general comment that most plate amps has a 2-pole Sallen-Key rumble filter of 12 db/octave does apply to the Jaycar AA0508 (do you know?) and an 18 db/octave filter is added between the LFE and the plate amp input, thatís 30 db/octave, which I think would be ok for EBS.
I have no idea if the Jaycar has a filter. It's very similar to my Parts Express plate amp (300-793) and there's no mention of it in the blurb. The other plate amp I have is a S200W (WES and Stones Sound Studio) has 24dB/octave at 17Hz plus and audio/video setting.
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