We don't all get it right the first time (1 Viewer)

[Ronnie Ferrell]

Allen-- Chris did a great job documenting his build of the bass traps. Take a look at his site! I did not take any pictures of mine when I built them because I knew Chris was doing a bang up job, documenting his!
Info on the retail versions can be found here:
ASC
but a 13" by 47.5" trap is $500
Jon Risch was kind enough to reverse engineer them and come up with his own clone of them. His clone trap has been tested and is as good or better than the retail versions!
His site can be found here:
John Risch's DIY page
Here are some build pictures from his site.
Here is a picture of one of mine before I covered it with burlap.

I have now covered them with the burlap and made some end caps for them. I used bleached burlap from WalMart. They blend in really well with my wall color and you do not notice them!
Hope that helps!

 

[Aaron_Smith]

This thread got me thinking about a few things.
- Whether it's better to turn up the subwoofer gain at the receiver or the plate amp. My feeling is that it would be better to have the level high at the reciever and lower at the plate amp. Think about it: you are sending a low level signal, usually less than 2V, through a long run of (generally unbalanced) cable to the subwoofer. The transmission noise in the cable from interference from other equipment, and especially 60 Hz noise from appliances, flourescent lights, etc. is going to be amplified as much as the signal itself is amplified at the plate amp. Therefore, I would think you would want the pre-amp sending a higher level signal, and amplify it less at the speaker end.
- Bass traps and their ability to affect peaks vs. nulls. In theory they could have the same effect on either-- their purpose is simply to absorb bass energy in a given bandwidth. Peaks and nulls are caused by the same thing- strong room reflections. Peaks occur at the frequencies where the reflected waves are perfectly in-phase with the original signal, and nulls occur when the reflections are 180 degrees out of phase with the signal. Both are strongest at the fundamental (1st harmonic) frequency. Anyways, Chris, it sounds more likely that your particular bass traps are affecting a frequency range that tends to be a null rather than a peak-- different bass traps may affect the peaks more.
I'm not an expert on either of these topics, just trying to apply a little engineering logic... would like to hear other opinions on the matter.

 

[Ronnie Ferrell]

The reason I run my amp at 1/3 gain, the widely used Parts Express plate amp, is exactly the same reason Brian and Chris Carswell do. I like using the auto-on feature! If I set the amp gain high and the receiver gain low, the auto-on feature does not get a strong enough signal to actually turn the amp on. Or as Chris stated, the amp will turn itself off during parts of the movie that has little to no bass. I am sure it would be best to have the amp gain set to max and adjust it at the receiver, but I am lazy and enjoy the auto on feature!
Ronnie

 

[Brian Bunge]

Aaron,

I also agree that you want as strong a signal coming in from the receiver as possible. It increases your S/N ratio. This is precisely why many higher end car audio "head units" come with 4V pre's. Car audio environments can be quite noisy so the higher output voltage gives you a cleaner signal back to the amps.

 

[Greg Monfort]

If I'm understanding the various comments, there appears to be a lot of misunderstanding WRT sensitivity (gain) matching. I know Mackie has owner's manuals on-line that do a good job of explaining it.

Anyway, based on various remarks, at least some of these plate amps are poor designs at best and aren't good matches for at least some of the receivers, keeping some folks from realizing all their LFE channel's potential.

GM

 

[MichaelAngelo]

[I seem to remember a thread in which this subject was discussed]
Engineers: is the output of the amp roughly equal to the input signal x the amps wattage? If this is the case, then the amp will only "put out" X amount of watts, whether its high receiver gain/ low amp gain, or vice versa. But it seems to sound better with the receiver set high, and the amp set to 1/4-1/2.
With the gain on the amp set high,it will just reach max power output sooner, ie., with less receiver volume.

 

[Ronnie Ferrell]

Anyway, based on various remarks, at least some of these plate amps are poor designs at best and aren't good matches for at least some of the receivers...

What can you expect for a $119 plate amp? IMHO, At this price point, we are getting more than we payed for...
rf

 

[Aaron_Smith]

Michael, to respond to your question-- it doesn't exactly work that way. The goal of any amplifier is to increase the magnitude of the signal (in voltage)-- this is what creates the power. The two things that limit an amplifier's ability to do that are:

- they can only "swing" the voltage so fast. This is called the slew rate; if the magnitude of the signal becomes too great, the amp can't slew the voltage fast enough, therefore it doesn't accurately reproduce the waveform-- and distortion is the result.
- they can only deliver so much current. The load (speaker) will require a certain flow of current through the coils to accurately reproduce the waveform-- the speakers impedance tells you that. If the amp can't deliver enough current to keep the speakers happy, the wave form flattens out at the tops, becoming a sqare wave. This is bad for the speakers- it causes heat buildup in the coils and can eventually fry the speaker. That's why crappy amplifiers don't work well with low-impedance speakers- their cheap power supplies can deliver enough current.

Now... pre-amps are designed to swing voltage quickly and accurately; however, they can provide very little current. This is why the input impedance of the downstream component needs to be high (and is designed that way). A power amp stage can provide abundant current-- the cost is that they have limited gain and can only magnify voltage so much. So most amplifiers are designed with a voltage amplification stage, and a power stage.

The last thing is that I would bet that the parts express amp does not have a voltage amplification stage... it's just a power amp with a set gain (of say 24dB), and the volume knob attenuates the input signal to suit the output needs. This means that you're not really dealing with any more or less amp headroom with the knob turned all the way up, or all the way down. The headroom is limited by the amount of current that the amp can provide, invariant of the knob setting.

I hope my explanations provide more clarity than confusion. Brian's elaborated the point much more simply- high signal level between the receiver and the plate amp keeps the signal to noise ratio high.

 

[PaulDF]

As it is, I have my receiver output AND my amp gain set to at least 3/4. Even then, the auto on doesn't click on until the volume is turned up more than should be required.

I think I have done most of the adjusting and measuring and moving that I can, or want to do. I seem to have good output with my sub corner loaded, phase set to 90 deg., receiver out and amp gain set to at least 3/4. When I turn on my little 150 watt 10", I gain 2-3 decibels in my listening position. With a phase of 0 deg, it seems to have no adverse effects. Is it okay to use this?

As for the bass traps, what do you all recommend? Tube type traps? Located in the corners of the room? What about the side walls, do bass reflections there need to be tamed too?

WAF is an issue for me. I am thinking columns with plastic on the front, but more open to the rear would look better for us. Closed ends would be a plus too, as they could double as stands for trinkets etc. My best option to please the wife is to make them look like roman columns.

Would two traps in the rear corners of my room be adequate? My sub is in the front corner, and the left corner opens to a large hallway. Big flat panels might be out of the question for me, though I'm not sure they are what I need anyhow.

Paul

 

[Aaron_Smith]

There are several types of bass traps: simple absorptive (very thick foam basically), resononant tubes, panel/moving membrane, hemholtz resonator, etc. They all do the same stuff but in different ways. I would do a google search for bass traps and you'll find more info than you can read. Unfortunately almost any bass trap design will be pretty big, so good luck getting it past the wife!

 

[Ronnie Ferrell]

As it is, I have my receiver output AND my amp gain set to at least 3/4. Even then, the auto on doesn't click on until the volume is turned up more than should be required.

This sounds fishy to me. Setting you receiver to 3/4 should easily turn on your sub amp. Quick question: Do you also have your other channels gains boosted way up?

Try this test. Set all 5 channel gains to their lowest setting. Then set your subs level at 3/4. Now how much bass do you have when you turn you receiver up to listening levels?

The friend that I built the other sealed tempest for kept complaining he had no bass at all. I finally figured out he had all of his receivers channel gains maxed out and his sub/LFE gain set about half way... :b

Ronnie

 

[MichaelAngelo]

Aaron,
Thanks for taking the time to respond. You answered several questions I have had, clearly and concisely. Thanks.

Michael Angelo

 

[Greg Monfort]

>What can you expect for a $119 plate amp? IMHO, At this price point, we are getting more than we payed for...
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Well, I didn't know what it cost, but if it keeps the speaker from performing to its peak capability when that's what I need it to do then it's only worth its resale value to me. Seems 'penny wise and pound foolish' to me, but as always, YMMV.
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>This means that you're not really dealing with any more or less amp headroom with the knob turned all the way up, or all the way down.
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Correct.
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> The headroom is limited by the amount of current that the amp can provide, invariant of the knob setting.
====
The amp's, but as my suggested reading points out, the higher you set the receiver's gain, the less headroom it has, IOW it acts as an adjustable compressor, ergo the amp has less peak to average SPL capability. Of course if that's the goal.....
As a matter of the correct level setting procedure, my original advice is the way to do it to get the widest LFE system headroom. If the receiver and amp are seriously mismatched or have other signal issues, then of course you do what you have to do to make them work together as best you can.
Obviously, I made the erroneous assumption that there was a standard for the LFE out and/or the plate amp was designed for something else and/or you get what you pay for so I'll shut up on the subject.
GM

 

[MichaelAngelo]

Is somebody's Tumult late ??

 

[PaulDF]

I too must thank everybody for all their input on this thread. There are some very knowledgable people here.
Ronnie, my mains are set to -2 on a scale of -10 to +10. I had thought of lowering it more, but figured it might lessen the sound quality. Of course I could be wrong!
I will try it though.
Another question.... Will leaving the plate amp gain set to max cause it to heat up more? Mine certainly gets warm, just wonder what would happen after perhaps 6 or more hours of poundage??

 

[Chris Tsutsui]

I don't think it will heat up more. Even when the amp is off, the transformer likes to stay warm.

Think about the majority of amps out there that don't have adjustable gains.

 

[Ronnie Ferrell]

Ronnie, my mains are set to -2 on a scale of -10 to +10. I had thought of lowering it more, but figured it might lessen the sound quality. Of course I could be wrong!

RATS! I thought I was on to something there...
Just for comparison, I have my mains set to 0 on a scale of -10 to +10 and my sub set at +8 for DD 5.1 and to +4 for stereo music. I have not calibrated my system at all because it is just a 2.1 system right now. X-mas money will be used to get the center and surrounds. Anyway with the above settings, my sub amps auto-on turns the amp on even if the receiver is set to a low start-up volume.
Lower the mains even more and leave the sub set around +6 to +8 just for kicks to see what happens. Let us know!
Ronnie

 

[PaulDF]

Okay, I lowered my mains setting as much as possible. It may have given a little more bass, but not a lot. It actually sounded kinda sickly with the mains turned right down.
But I think I hear my walls giving way... When I REALLY crank the volume with the bass turned high, I have always gotten a horrible noise which I figured was the walls (or my previous amp clipping? It was just a 2 channel receiver). I just tried not to crank it that loud so much. With this experiment it really made a lot of bad noise.
I have slid the sub out and away from the wall to see exactly what the noise was, and with the sub 3-4' away it still made a bit of the noise but not nearly as much(hope its not the enclosure!!).
As I stated earlier, when I leave the room, there is a much more defined bass, and louder in some areas even.
This leads me to believe some sort of room treatments will really help me out...