Non Linear Volume

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In an attempt to understand why my OB bass produces so little movement at the cone tb46 suggested I measure some output voltages. After a few quick measurements some answers came into focus.

Using a test tone disk I set the track on repeat, and played a 1kHz test tone. Using the left channel analog output of my Oppo BDP83-SE I measured a constant 1.4v every time it played.

Next I wanted to measure the output from my Ground Sound DCN28 active crossover which I had been accusing of having low output.

Typically I listen to music anywhere between -50dB and -30dB, wanting to give it a fair go I decided to start the measurements at -30dB. Because I wanted to test the bass channel for output voltage, I tested the 4th channel and set the track on 63Hz and repeat.

At -30dB I measured 0.0v. I repeated the test at -20dB and got a reading of .1v, next test was at -10dB where I got a reading of .7v and finally a test at 0dB and it measured 2.7dB.

So what does this tell me? Firstly I was wrongly accusing the DCN28 of low output, but what I did discover was the rate at which the voltage rose.

Being that I haven't played it at anywhere near the volumes I just tested it at, explains why I have never seen the kind of movement I expected from the bass driver cone that everyone keeps telling me is common in OB bass.

So what I might need to do is attenuate each of the channels in the active xover and turn the main volume up a bit to get it closer to -10dB where the action is taking place.

While on -30dB I took the opportunity to check the Watt Output meter in my new bass amp and play it through the various tones starting at 20Hz below are the results.

Because I only had a few seconds between each track I had to look fast at the falling numbers but I think these are fairly right.

20Hz = .3w
25Hz = 8.9w
31Hz = 38w
40Hz = 63w
50Hz = 76w
63Hz = 78w
80Hz = 75w
100Hz = 44w
125Hz = 4.4w
160Hz = 2.4w
I think your table of wattage is showing that the active filter + amplifier has roll-off frequencies near 31Hz and 100Hz.
The numbers outside these frequencies show a very steep roll-off slope.

As to the lack of output at your "normal" listening volumes, I'd suggest that maybe you don't have sufficient gain in the active filter feeding the bass only amplifier.

How do the sensitivities of the mid/treble drivers compare to the bass driver? If the bass driver is more sensitive, then that is usually compensated by turning down the pot on the bass only active filter.
If, as is more usual, that the bass driver has lower sensitivity, that would require the active filter pots on the mid and treble to be turned down.
You pretty close.. HP - 25Hz, 36dB Butterworth, LP - 100Hz, 36dB Butterworth.

Currently I have no gain or delay applied to anything. I was previously mucking around with the gain on the bass but I found it made the bass muddy sounding.

Perhaps this is the area where I need some more work in understanding. If I up the gain while the output voltage is low then it is an increase on a lower number, whereas if I up the volume so the output voltage is higher and decrease the gain, then I might wind up with more?

The speakers are all over the place the tweeters I think are around 93dB and are less efficient than the non dipole version of the RAAL 140-15AMs which are about 94.5dB

The mids are in the mid 80's I think but there are two of them. I should also mention that the bass are all pretty much baffleless at the moment. This is going to change shortly.

The mid woofers are similar to the mids I think.

The TC Sounds Pro 5100 woofers are probably up around the tweeters, and the new Ultra LMS 5400 I just bought is about 89dB, but I currently have a bigger amp powering it then the 5100s.

So they are all over the place at the moment. I should mention that all of the bass are pretty much baffleless at the moment but that is going to change shortly.

Now that I have come to realize where the power goes up on the DCN28 I might just nudge up the volume a bit past where I am used to and see if it moves any further than the mm or two I am used to.
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Even though the mids are lower sensitivity, there are more of them as there are the mid woofers (4).

If you look at the attached you can see from about 400Hz up is quite alright with a taper off towards the higher frequencies so its not too harsh. The problem is in the lower bass where it falls away under 100Hz.

I'm not looking to optimize the curve at this time, as it is ok for the rough baffles they are in, and these are not finished designs. I am just looking to get more power to the woofers rather than improve the graph.

The way I think I need to do this is to increase the volume on the DCN28 to increase the voltage output.


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Hi Oliver, Yes considering how poorly I have the DCN28 setup up (and that is mostly due to not having properly made up matching speakers, or the time to learn the software) it does sound remarkably good which is testimony to how well the DCN28 actually works.

With time, a much better room, and some time in the shed to build some better baffles, I expect this result will only get better.

I think most of the tapering off is due to baffleless bass drivers sitting on the floor (one has a small baffle), but in part some of it is due to not getting enough power to the bass drivers, to really punch out those low notes.

Based on what I measured above, I think I need to decrease the top end, mids, mid woofers, while leaving the woofers at +0 and turn the volume up on the DCN28.

In suitable H frames I might just get a bit more bottom end out of those Pro 5100's.

The two PRs I just bought I can use on the Ultra LMS 5400 and make a smaller cabinet.
Started doing a bit of tweaking in the x-over tonight and I think I have found the problem... it is either the wrong driver or the wrong amp.

Currently I am listening to the one LMS 5400 I have with the bridged Crown I-Tech 9000 amp with no baffle on the left channel, and the Pro 5100 with a small flat baffle and the 1000w Rotel amp on the right channel. With only one amp on at a time I started with the left channel.

Playing a bit of Pink Floyd. (Track 8, Echos, Learning To Fly), at the start of the song where it starts with the fade to boom of the first note, with the 5400 it is a nice even balanced with the rest of the speakers presence. With the right channel and the 5100 it sounds like some small girly fart!

I had to back to the x-over back from 100Hz to 72Hz as the 5400 is a bit too thumpy, but I think it sorts out the question of needing more power. The 5100 is definitely a smoother sounding driver but it lacks the oomph of the 5400 in an OB configuration.

On the weekend I might switch the second 5100 over to the Crown amp and see how the Crown stacks up against the Rotel.
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