Will this Octo 18 build plan be suitable? - Page 3 - AVS Forum
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post #61 of 69 Old 04-03-2013, 05:20 PM
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Originally Posted by noah katz View Post

true for biamping, but bringing up the bass (or any other freq) to parity with others means the waveforms can add, boosting the peak input signal voltage and requiring up to double the amp voltage.
missed that part, thanks

I think you misread Rod's point. He's saying that bringing it up to parity (boosting +12dB when it's recorded at -12dB) is not boosting the signal any higher than it already is.

The additive effect is negligible in his scenario. As I pointed out, these types of articles predate the 5 discrete channels +LFE+10dB redirected to the SW output scenario we deal with regularly and so are not relevant to this discussion. Thus my caution regarding the outdated suggestion that +20dB of gain is OK.
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post #62 of 69 Old 04-03-2013, 05:43 PM - Thread Starter
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My plan was to come off the A/V Receiver's RCA LFE jack and into the L/T. Then from there to an Aphex 10/4 Interface model 124a. This unit's transformerless balanced outputs have a peak output level of +24dBm. From there I was going to run xlrs into my CV5000s. I shouldn't have to use a lot of amp gain under that scenario and the Aphex is wide bandwidth (5Hz-100kHz) and dynamic range so it shouldn't be raising the noise floor on the signal too much either, right?
I realize that I still need to keep the L/T's gain at no more than 10-12 dB due to the limitations of the 15 volt supply, correct?
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post #63 of 69 Old 04-03-2013, 06:30 PM
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Quote:
Originally Posted by bossobass View Post


OK, let me ask you this; what amp do you have that has any setting that doesn't result in severe clipping with 20V input???

Input sensitivity is defined as the amount of input voltage required to drive the amp to clip (full power). Variable gain is irrelevant in this scenario of 20V input. . Maximum input sensitivity with a Lab FP14000 is under 10V with any combination of gain limit and gain attenuation setting.

And please with the EP4000 even being mentioned in this discussion.

No one has said anything about clipping the input stage of the amplifier that I read in this thread. The discussion is about clipping the supply rails of the L/T and/or driving the amp into clipping.

When most people calibrate, they use the rumble tone which doesn't even play any of the boosted bandwidth. You use that method and play WOTW at reference level with a 20dB boost L/T (or shelf, or whatever signal shaping method) and there is gonna be severe clipping, or worse.

No pro sound equipment is designed for calibration with a 30-100 Hz pink noise rumble at -20dBRL when the signal is boosted 20dB below that band and WOTW is played through the system at reference level. Sorry but no gear is designed to operate with those signals.



Of course the signal will be clipped all to hell and back if you go dumping 20v in. My point which was missed is that having tons of voltage headroom in the LT device past what the amps need to produce full power is not good for anything other than keeping down the SNR. Most pro amps can be set to accept anywhere from around 1.5v all the way to around 5-10v input prior to producing clipping in the output stage meanwhile the input stage will still have some headroom. If your LT or signal shaping unit can produce 5 volts or whatever it is more than enough to work and pull full power with any modern amp let alone 15-20v levels which as you say no amps can even make use of. The amps don't need anymore than a few volts to do their thing and they are already able to be driven into heavy clipping. The rest of the extra voltage headroom in the EQ device is irrelevant to the amplifier completely other than as a means to keep the noise floor as low as possible.
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Originally Posted by bossobass View Post

No pro sound equipment is designed for calibration with a 30-100 Hz pink noise rumble at -20dBRL when the signal is boosted 20dB below that band and WOTW is played through the system at reference level. Sorry but no gear is designed to operate with those signals

Of course no equipment is designed for this niche. And this is exactly correct. Guy plays rumble noise to calibrate SW level, with 20dB of boost dialed in below 20Hz, then goes to playback a movie at REF and finds that the peaks required from the SW system are potentially +40 hotter than the rumble tone + an extra 8dB potential from redirected bass + the 20dB from the boost if the peaks fall in that frequency range = Clipped amps, signal chain or driver cones launched. Absolutely. However if you attempt this and you do encounter any of the following (Clipped amps, signal chain or driver cones launched) you either A: do not know how to set your gain structure and it is out of whack or B: (And this is far more likely) You don't have enough gear for the gig. The above is a brutally demanding challenge. Once you set up your signal chain decently so that the amp output stages always clip first before anything else in the signal chain and it just comes down to amplifier and driver muscle. It only takes a handful of volts in the majority of cases, not 15 or 20 volts from the signal shaper. The rest of that voltage past what is needed to produce output clipping in your amps is just not needed to prevent signal clipping unless you just don't know how to properly setup your gain structure.
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post #64 of 69 Old 04-04-2013, 07:24 AM - Thread Starter
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A quote from Rod Elliot's L/T circuit build intructions:

The board is small, and can be installed into almost any existing preamp or power amp as desired. It requires ±15V at no more than ~10mA (opamp dependent).).

Is that something to be disregarded because of your statement in post #44? My Power Supply outputs almost 2 1/2 times the current that Rod is specifying.
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post #65 of 69 Old 04-04-2013, 08:38 AM
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Quote:
Originally Posted by Linkwitz Riley View Post

A quote from Rod Elliot's L/T circuit build intructions:

The board is small, and can be installed into almost any existing preamp or power amp as desired. It requires ±15V at no more than ~10mA (opamp dependent).).

Is that something to be disregarded because of your statement in post #44? My Power Supply outputs almost 2 1/2 times the current that Rod is specifying.

I looked at that circuit, and the only active element is a dual op-amp, which at the worst case load might draw 15 mA or so (peak value of DC + AC). Your supply is 240 mA max, so you are fine.

The value of C4 in Figure 4 of the project page causes the circuit to have a -3 dB frequency of 15.6 Hz (relative to its ideal shelving value), so you may want to increase C4. One possibility might be to use the same 10uF non-polar electrolytic capacitor type that you use for C8.

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post #66 of 69 Old 04-05-2013, 07:31 AM
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Quote:
Originally Posted by Linkwitz Riley View Post

A quote from Rod Elliot's L/T circuit build intructions:

The board is small, and can be installed into almost any existing preamp or power amp as desired. It requires ±15V at no more than ~10mA (opamp dependent).).

Is that something to be disregarded because of your statement in post #44? My Power Supply outputs almost 2 1/2 times the current that Rod is specifying.

Current vs voltage.

An op-amp can theoretically provide infinite boost. The circuitry takes little current but it boosts the voltage of the signal coming into it according to the circuit design. I guess all I can say here is what I already have, except to clarify the obvious; when I say 'clip the PS', I mean to say that if you exceed the stable voltage of your power supply, the signal will be clipped and/or otherwise lo-fi.

In measuring the SW out of a typical AVR, if you play music at 0dBMVL with the SW trim calibrated at 0, the voltage measured might be somewhere between 1-2V. If you then play the bridge collapse scene in WOTW, you might measure a jump in voltage of 4 or 5 times that. Obviously a big difference.

As Josh and I touched on, there are many people who calibrate with a rumble tone and many of them pay little heed to where the SW trim ends up. This is a hugely compromised method if the user has a MiniDSP or DCX and has implemented a "L/T' of 'x'dB boost in the bandwidth that is omitted by the rumble tone. Then, after the fact, they "bump the SW trim by + 'x'dB to run the subs hot".

As Josh points out, setting the gain stage is everything when it comes to optimal performance from whatever subwoofer system you've built. Well, if you're designing your L/T, you have the opportunity to have control over one link in the signal chain. And, regardless of the boost method used, the assumption should be that the signal being sent to the amplifier will cause the amplifier to clip (exceed its capacity of full power).

I'm just saying, FWIW, that the heavy hitters of soundtracks that we all build subwoofers to enjoy will send an already hot voltage (several times the AVRs specification) to your L/T which will then increase that voltage according to your design ( or according to your fiddling with a MiniDSP or DCX or whatever device to affect a boosted signal) and send that to your amplifier. I've measured the voltage and the spikes can be huge at reference level playback.

So huge that no system that will reasonably fit our spaces and mains capabilities can handle it. So, some sort of limiting will be required. As Josh mentions (and I wholeheartedly agree), the limiting should be at the amplifier output stage and it should be of a level of sophistication that does not horribly distort the signal we all cherish.

If you have to attenuate the boosted signal and then use your amplifier to bring it back up, you'll raise the noise floor of an already noisy signal (this is basically how the LFE+10dB channel gets its headroom). That's even more incentive to use caution in designing the signal shaper. My solution is to create a dozen circuits, each with a different specific curve that acts as a limiting system when higher playback levels are desired. This way, the altered signal remains intact throughout the listening session vs being constantly and infinitely distorted to an unknown degree during the listening session. But, it's still a method of limiting.

When you build a single circuit with no limiting, use caution to center that circuit between too little and too much, leaning slightly toward the too much side and utilizing an amplifier that will deal with the result from there (if that makes any sense).
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post #67 of 69 Old 04-05-2013, 01:17 PM - Thread Starter
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OK...thanks for all of your patience and assistance. I will start building the first L/T using the numbers you provided earlier. I guess I'll start the cabinets too, now that I've decided on 3.5 cubes per driver. Just have to wait until PE gets more HO18s in next week. The SI HT18 deal looked really sweet...but 4 months? I already own the Dayton HF series, which I thinks sounds very "musical" so hopefully the HO series won't be too far of a departure from that biggrin.gif
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post #68 of 69 Old 04-23-2013, 11:57 AM
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ok, so im just trying to get a little bit o' knowledge up in me.. Would it be ya'lls recommendation to not have the gain on the amp all the way up.. As that may cause more noise or distortion? I tried to follow what all was said here, but im still a newb, and some of it went right over my head, cuz im just too cool for school cool.gif , naw not really.. Im just curious. PS, ill be powering 2 of the 18" HO's with an Ep4000 via an onkyo 809. I dont even really know what an LT is, but im hoping i dont need one, because it sure got confusing from that point on smile.gif Guess i need to go read a book or two. I just want some quality bass biggrin.gif
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post #69 of 69 Old 04-23-2013, 12:11 PM
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Quote:
Originally Posted by NickThatStands View Post

ok, so im just trying to get a little bit o' knowledge up in me.. Would it be ya'lls recommendation to not have the gain on the amp all the way up.. As that may cause more noise or distortion? I tried to follow what all was said here, but im still a newb, and some of it went right over my head, cuz im just too cool for school cool.gif , naw not really.. Im just curious. PS, ill be powering 2 of the 18" HO's with an Ep4000 via an onkyo 809. I dont even really know what an LT is, but im hoping i dont need one, because it sure got confusing from that point on smile.gif Guess i need to go read a book or two. I just want some quality bass biggrin.gif

Yes that will amplify the noise; which is not what you want....
Yes, you want the signal level to be the maximum dynamic range possible. High voltage for loud signals, and low voltage for not.
In this way, you will be amplifying the signal, not the noise.

Amplifying a signal will always increase noise no matter what, be it a pre-amp or amp. Each boost is a conversion process that isn't lossless (laws of physics).
But you can at least maximize the situation by optimizing the input signal.

It's always better to cut than boost a signal, be it 1volt or 1000volts.
If ALL amps accepted 24/192 digital signals and had quality 120+SNR DAC's inside with low EMI, the world would be a better place; but alas there is nothing that can be done to avoid it in the current implementation of audio equipment that exists today.
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