Richard Little

Amps, SS filters, phase? Please help

You have to help me; my eyes are crossing up looking at all the different options. To start with I have 2 AV15's and a BFD. Initially I only want to make one ported box for one of the drivers and power it with at least 500 watts of real power but would like to use the same amp to power an additional identical sub in the future. I also need to have a subsonic filter and a phase adjustment would be nice. For the subsonic filter, either like to buy constant variable or make a DIY low-cut filter so it can be made to match the enclosure. What do I need to do? I hate the idea of spending $400 on one component just to get the SS filter (rane pe15) use and another $140 for something to convert from a home audio AVR out to a pro amp in. Can anybody suggest something that might help with all this? Do DIY low-cut filters put in series with the sub amp work ok? The Mackie 1400i has the SS filter but is a little short on the watts. What else is out there? The SVS black box is great but what if I need more than at 15, 20, or 25 Hz filter. What can I do?

Dave Milne

Richard,
Are you comfortable building your own little electronics projects? Sallen-Key active filters are pretty easy to design and build. Variable frequency is a pain... but with fixed-frequency designs, you can put the frequency-setting resistors on DIP headers and plug in what you need. I believe there are some kits out there... probably something on Rod Elliott's site. Adding the phase adjustment is simple: just another op amp with a pot and a few passive components.

And no, you can't do this passively (in series with the sub). The component values would be HUGE and expensive.

Don't understand this. Home audio components use RCA jacks... most pro amps use 1/4" jacks. Adapters are available from Radio Shack for a buck or two. You can even get RCA to XLR adapters (for a few bucks more).

If you're not into DIY electronic projects, there are some beefy plate amps out there that might offer everything you are looking for... like the Hypex series.

Good Luck

Jack Gilvey

As usual, the DIY option is best, as Dave mentions. I don't DIY these type of things, so I grabbed an SVS/Marchand box...a great, well-made unit. It's a good buy if you need both of the functions it provides. It has something like 18dB of clean gain, so it'll allow just about any AVR to mate with the higher levels of many pro amps. Also, the SS filter is 24dB/oct, much steeper than others I'm familiar with. Largely due to this, your filter frequency doesn't have to be right at your Fb. The 15Hz setting should be good for anything below 20Hz or so. Since it's so steep, it can start the rolloff a little deeper and still have adequate filtering.

Also as Dave says, there are plate amps out there that will certainly do an AV15 justice. This one looks particularly nice:
http://www.acoustic-visions.com/~aco...s/keiga_52100/
Its infrasonic filter is something like -3dB @ 17 hz, second-order...I can e-mail you the .pdf on it if you'd like.

Of course, the pro-amp route will allow more flexibility in the future for another sub.

Richard Little

http://www.rane.com/bb22.html maybe this isn't really required but I have read one HTF members post where he was having quite a bit of trouble and thinks the mismatch of signal could have been the culprit.
Searching "Sallen-Key active filters" on goggle seems to have turned up a whole slew of new information. I had looked at Elliott's page http://sound.westhost.com/project99.htm but am a bit confused. I assume the power +-15V is DC but don't know if he meant +15 and a ground(0V) or if he actually meant +15V and a -15V which would be a +30V and a ground ( and not really sure on amperage required at any of the voltage scenarios). Also, under the Figure 1 (with all the 150nF caps it says that the -3dB frequency is about 17hz but a little farther down the page in Table 1, it shows that the -3dB point for 150nF caps is between 17.9hz and 22.3hz. So I don't know which he really meant, or I could be reading it wrong due to my crossed eyes? I do think that the DIY filter is the way for me because I can build if for exactly what I need/want. I have all the required tools, now I just need the connect the dot diagram:b Jack that Keiga 52100 does look impressive though, I would like to see the .pdf Thanks for the replies

Jack Gilvey

You got it.

Dave Milne

+15 V and -15V is not exactly the same as 0V and 30V. You need the bipolar supplies to avoid having to bias up the midpoint and capacitively couple the input and output.

I believe Rod has some easy bipolar supply designs on his site. The current requirements for an active filter are quite modest... a dozen milliamps or so are all that is typically required.

ThomasW

Color me curious, but unless one is playing vinyl why does anyone with these high excurson, high power handling drivers want or need any subsonic filtering?

Since I gave up on vinyl (1984), I haven't use an SS filter on any sub I've owned.

theAudioWorx Klone-Audio
Cult of the Infinitely Baffled (Resource for IB sub builders)

Dave Milne

I believe that vented systems have a nasty trait of almost completely unloading the driver below Fb. True, there's not much material in the sub-20Hz range... but some movies do venture down there. It may only take a handful of watts at 10 Hz to cause the VC of one of these 1000W (thermal) drivers to smack the pole piece.

ThomasW

I have Shiva's, DPL-12"s, BluePrint 1503's and HE-15 based subs in the house. Never had any problems letting them run sans SS filters.

theAudioWorx Klone-Audio
Cult of the Infinitely Baffled (Resource for IB sub builders)

Richard Little

" It's true, I learn something new every day.

In the notes at the bottom of the page( http://sound.westhost.com/project99.htm ) under construction, it says "Only a single channel is shown in Figure 1, the second channel uses the remaining opamp in each of the dual packages. It is imperative that this circuit is driven from a low impedance. The actual input impedance is greater than 47k at all frequencies, but the source impedance should be no more than 100 ohms or so. Question one is, what does single channel only mean (is that like the L or R channel or do I need two of the Figure 1’s for the filter to function)? And one last question, does source impedance come from the impedance of the subwoofer pre-out on the AVR and if so, isn't that typical impedance greater than 1k ohms? How would that affect this filter's use as a low-cut for subwoofers.

This type of stuff has scared me into finding a low-cut filter

Dave Milne

Yup. Figure 1 shows a left or a right channel. You need two for stereo It sounds like the way Ron's laid out his PCB, he uses the two "halves" of the dual TLO-72 op-amps for left and right channel, respectively, rather than for the two stages of his filter... no matter.

Right again. Fortunately it's usually quite low --almost certainly less than 100 ohms. If the AVR out is straight from internal an op-amp, it's probably below 1 ohm. Even configured as a line driver, it's maybe 50 ohms at most. A high source impedance --say, from the tap on a volume control, would alter the characteristics of the first stage of Rod's two-stage filter. Probably wouldn't be devastating... but could shift the frequency, introduce peaking, etc.