DVD Subwoofer Preamp Output Works Better?

monkeyphant

I own a Sherwood Newcastle R863 Receiver and two Fosgate Audionics FA12.0 Subwoofers: http://www.fosgateaudionics.com/products/FA12_0.asp

The Sherwood Subwoofer trim setting ranges from -14 to +14.
When I use the Sherwood sub preamp output, I set the subwoofer trim to -7 and the subwoofer gain to about midway (as recommended by the SVS setup guide to prevent distortion). The bass response overall was not very good for movies, even though the Fosgate tech support said that the FA12.0 could compete with Velodyne, SVS, and virtually any other subwoofer in the same apporximate price range ($699.00 Retail, but I was able to get two for $299.00 each!). The sub low pass crossover was set to the max setting to prevent cascading.

I then decided to try an experiment out of sheer frustration. I connected my Panasonic RP-91 subwoofer preamp output to an old Yamaha DSP E492 subwoofer input. I connected each FA12.0 to the sub preamp output on the Yamaha. The E492 is a Pro-Logic receiver/ preamp with a 5 channel preamp inputs and outputs. The RP-91 bass management settings were all speakers set to small and the subwoofer trim was set to neutral. The low pass crossover of the RP-91 is 120Hz. To prevent distortion I set the Yamaha sub trim setting to -15 (the range is -21 to 0). I left the FA12.0 settings for the gain and low pass crossover the same level and set the E492 master volume to about 1/4 and put in U-571.

The depth charges made the basement walls, ceiling and foundation shake. I played the depth charge scenes over and over again because I could not believe the bass response! I could feel the pressure in my ears. My question is why the difference between the Sherwood and Panasonic RP-91 sub preamp output levels?

monkeyphant

John K.

MP, if I follow, you arbitrarily set the 863 sub trim at -7 and the sub level about half way and left them that way. But those are just starting points; if an 863 sub trim at say 0 is necessary for enough bass, then do it. You repeated "to prevent distortion", but there's no reason to expect distortion from that sort of setting on the 863. Different sub pre-amps differ in their output at the same control setting and it's nothing to be concerned about; use as much as necessary. Set all speakers small, the 863 crossover filter at 80hz and set the crossover on the sub at max, as you did previously.

Dr. Spec

The overall subwoofer output is a function of both the sub out voltage and the gain provided by the sub plate amp.

It is possible that the Sherwood simply has a fairly weak sub out voltage signal that needs to be amplified more than another brand of AVR. You could also try leaving the Sherwood at -7 and upping the Fosgate plate amp volume and see what happens.

You really didn't mention anything about sub level calibration and that's really where I think you need to go here.

Use your AVR test tones and a SPL meter and balance all the surround channels to 75 dB at the listening position (use C-weighted slow on the meter). Then run the sub test tone and increase the Fosgate volume control until you are getting about 78-79 dB on the meter. It will bounce so take an average, and using the 80 dB scale on the meter (as opposes to the 70 dB scale) will reduce bouncing even more.

I'm betting you will need more than 1/2 setting on the Fosgate with the Sherwood left at -7 to obtain this calibration level.

The distortion you are referring to occurs when the sub pre-out level is set too high. This pre-out seems more prone than the other channels to suffer an increase in THD as the level control is increased. For example, the recently tested Denon 2803 has a 0.91% THD at the 0 setting of the sub level. That is acceptably low, but higher than one might expect. If you cranked it up to the +5 or +10 setting, the THD will climb significantly - to a total of as much as 3-4%. Conversely, if you set it to -5, the THD will drop considerably lower than the 0.91%. So it is a good idea to run the sub pre-out level at the 0 or lower setting to keep THD in check, and adjust overall volume at the plate amp.

Doc

monkeyphant

John K
The -7 setting on the 863 was not arbitrary. According to the SVS
sub calibration recommendation:

"Keep the receiver's subwoofer output control to about 25% up or lower than "0 dB" (or say -6dB given a typical receiver channel limits of –12 dB to +12 dB) . This will allow your amp to work with the cleanest signal possible from your receiver, while still leaving plenty of downward adjustment you can use from your viewing position (using the remote)."

This is why I chose -7db: (-14db + 14db)*.25 = -7db for the 863.
I chose -15 for the Yamaha: (-21 + 0)*.25 = 5. Then (-21 -5) =
-15.

Dr. Spec

Then run the sub test tone and increase the Fosgate volume control until you are getting about 78-79 dB on the meter. It will bounce so take an average, and using the 80 dB scale on the meter (as opposes to the 70 dB scale) will reduce bouncing even more.

I used an SPL meter initally, but I didn't use the FA12.0 sub gain to adjust the sub volume and I think that's the problem. SVS suggested to start with the sub gain at about midway and I should have adjusted the FA12.0 volume accordingly.

I do see one advantage to using the E492 as the master volume
control. To adjust the sub trim on the 863 remote, you have to select each speaker from the LF to the LRS before you get to the sub trim.

Dr. Spec

Let us know how you make out. There is nothing inherently "wrong" with setting the Fosgate to 3/4 or wherever it needs to be.

The fact that the Fosgate was much louder with the Panny/Yammie combo just means that the combo was sending a stronger signal (in millivolts) than the Sherwood was at the same sub volume level setting (1/2). In fact the Panny subwoofer pre-amp might very well have its own gain stage.

Anyway, each individual setting (AVR Master Volume, AVR sub level, and sub volume setting) is pretty much meaningless unless taken in context with the other two. And the only way to do that is with speaker and sub calibration.

Unless the Sherwood is faulty, you really should be able to get a proper calibration at -7 by fiddling with the Fosgate volume setting.

Remember to keep the Master Volume the same for all speaker and sub calibration. Most AVRs default to Master Volume 00 when the test tones are activated, but not all. Sometimes you have to manually increase the Master Volume level until you get 75 dB on the meter with the test tones and then note that specific volume reading.

The individual AVR speaker levels will vary depending on their relative efficiency and distance to the mic, but you should try to keep them all around a 0 baseline, and some of them will end up at 0, some will be -2 or -3 and some might be +2 or +3; that sort of thing.

The reason you can keep the sub level at -7 is because you have the plate amp at the Fosgate to make up the difference. When you run the sub test tone, leave the Master Volume the same and just jog over to the Fosgate and crank up the amp until you are getting 78-79 dB. That is about 5-6 dB hot in actuality since the RS meter reads a bit LOW on the sub tone, and 78-79 is a good starting point anyway. If it seems too hot, drop the plate amp a bit. If it's not not enough, wick up the plate amp a bit. But I wouldn't run the sub much hotter than about 6 dB over the other speakers, regardless.

Doc

monkeyphant

Doc

I used in 863 internal test tones and I followed your instructions and calibrated the speakers and the FA12.0 together. This is the procedure I had followed originally, except for the plate amp adjustment. Turns out, even the sub gain setting was close to 1/2. The bass repsonse was identical as before (nowhere as good as the Panny/ Yammy combo). I checked the 863 preamp output, 180 mV, and compared to the RP91 audio output, 2Vms. I suspect at this point I might be splitting audio hairs. I'm half tempted to just raise the 863 sub trim setting, get a bag of raw unshelled lightly salted peanuts, a bunch of 1/2 ripe bananas (the basic diet of the near extinct monkeyphant) and watch a few George of the Jungle cartoons...and be done with it!

MP

John K.

MP, you're right, you're making this a bit too complicated. First, when I used "arbitrary" before, I meant that you were using the arbitrary settings SVS suggested at that one point(in some other replies Tom Vodhanel has suggested starting with the receiver bass trim about half way). Some receiver might have a distortion problem with the sub trim more than about 1/4 up, but I doubt that a receiver of the quality of your 863 would fall into that category. For example, although they haven't tested the 863, S & V tested the Sherwood 7108(far lower priced) in their January 2003 edition and reported only 0.2% distortion at the sub output with the trim at 0.

So, unless the sub out on your 863 happens to be defective, just turning the trim up closer to 0 should give you enough bass without distorting , since the 863's output level is apparently quite low at the -7 point.

monkeyphant

Thanks to John K. and Dr Spec. I appreciate the help!

MP

Dr. Spec

No problemo!

Like I said above, the 0 level or lower is usually OK with any sub pre-out. Less than 0 is ideal because it tends to lower the THD a bit. I certainly wouldn't run it at +5 or higher under any circumstances, and if you find the need to you should look at some type of clean gain stage on the pre-out.

Without a doubt 180 mV is a very wimpy signal at -7 with the Master Volume at reference calibration level, and it might not be enough juice to drive the Fosgate amp to full power. That is unusual, but it happens. You might want to write/call/email Fosgate and ask them what is the minimum pre-out voltage required to drive the amp to full power. If they say something like 2 or 3 volts, then you have your answer.

Using the 2803 for comparison again, its maximum unclipped subwoofer output at reference volume setting, with the subwoofer trim at 0, was 7.9 volts. That's plenty of juice to drive any typical subwoofer plate amp to full power.

I'd measure the Sherwood at the 0 level with the Master Volume at your reference calibration setting and see how many volts it is. If its not a few volts, then you have a problem.

Doc

monkeyphant

Doc

I'll send Fosgate Audionics an email tomorrow and ask them about the minimum pre-out voltage required to drive the FA12.0 amp to full power. Their response should be interesting. The FA12.0 is a domesticated version of the Hafler TRM 12 recording studio monitor sub. Maybe that's the reason it may need additional power to drive the amp. The only difference is the TRM 12 has XLR balanced and unbalanced RCA phono connectors. I can't do any testing with the 863 tonight because there is a severe thunderstorm watch in the forecast.

MP

monkeyphant

Dr. Spec

I didn't get a response from my email. According to the FA12.0 Manual, the Input Sensitivity Range for RCA Input is 160mV to 5V RMS. With the 863 preamp output at 180 mV, and the RP91 audio output at 2Vms, I think that would explain the difference in bass response.

MP

Dr. Spec

No doubt. You are very close to the minimum - too close for comfort. I would up it to 0 and check it again. It should be considerably higher. Good luck.

Doc