Budget T-amp system sub?

wakibaki

Banned
2008-01-08 11:51 pm
I recently acquired a gen2 Sonic Impact T-amp, which I am running in using the TV lineout into a couple of 40-year-old Wharfedale Dalesmans. The sound is great (apart from the TV which buzzes faintly through the supplied interconnect). Anyway it kind of rekindled my interest in budget hi-fi.

I have a pair of Audio Nirvana Super 8's coming whch I intend to build into the Commomsense Audio 2.8 cft enclosure. I figure this is a restrained and efficient way to spend money, considering the anticipated spl. The amp cost ~$100 US shipped and the drivers $250. Hmmm... budget.

For music listening I intend to run a T-amp into these speakers using an M-audio 2496 souindcard which I've got in a desktop computer. This soundcard has a good reputation as an entry-level tool for digital recording studios. I got a 4-channel Behringer mixer I can always interpose if I feel I need some EQ.

Now I'm thinking about building a subwoofer to complement this arrangement. It needs to be a genuine musical asset, and cost should be proportional to the rest of the system. I can build pretty much anything. The room is about 6x7x3 metres and fairly live.

I also need to connect it to the existing system and I'm not quite sure about how best to go about that.

I like efficient, I don't want to expend a lot of watts. I don't want to spend a lot of personal energy redesigning the wheel.

Any suggestions as to where to start?

w
 

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi wakibaki,
Efficient = largish box. I doubt you want a high order box, so a B4 would seem to be reasonable unless you stick with a smallish woofer and high cutoff frequency. Some 8" Peerless woofers will get you into the 35 +- Hz range. One older Peerless 8" ran about 28Hz, -3dB. Impressive. Vifa might get into the low 40s. Much higher than this and I would be thinking a damped port / sealed box. A 6.5" pretty much requires a sealed box and maybe a damped port.

If you want to reach lower, a good 10" woofer would work well. Again, I'd be thinking a B4 type box and expect a little over 20 Hz -3dB. That's lower than your room would support I think. You would really feel that.

Run it with something like an LM4780. I'm not into class D technology, and a chip amp would probably run less money and complexity.

Did you define your requirement beyond what you posted originally? I'm sure box size may dictate changes (possibly). A box built to the optimal size will be more efficient, higher order boxes are more difficult to get and keep tuned. Especially as the voice coil temp goes up.

-Chris
 

MJL21193

Disabled Account
2007-03-10 1:20 am
As Chris says, higher efficiency usually means a big box.
A driver I was looking at a while ago might suit your needs. It's a Peerless 830668 and it's fairly cheap ($60.00 here).
Put this in a 60 litre ported box, and you'll have an F3 of 30Hz.

Output with 50 watts max would be ~106db. An LM3886 chip amp could be enough power for moderate use.

Response is slightly peaked, but this is the trade off for the smaller box. If you can accommodate a bigger box, lower F3 and a flatter response would result.
 

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MJL21193

Disabled Account
2007-03-10 1:20 am
anatech said:


Thank you for doing the homework. ;-)


I had this driver modeled already, plus a number of other ones, so not much work.

The LM4780 bridged would be the perfect choice for a mid sized sub like this - possible to get about 120 watts into an 8 ohm load with a +/-30 V supply. Not sure if wakibaki is into doing his own electronics though.
 

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi wakibaki,
Comments?
How close are we to your mark?

Hi John,
You know what? I suspect this would be a decent sub woofer. A front firing woofer would have impact that tends to get lost firing into the floor. I might be interested in this or some variation.

-Chris

Edit: John, I don't think you've had enough time to actually read much, but did anything in those books strike your curiosity?
 

MJL21193

Disabled Account
2007-03-10 1:20 am
anatech said:


Edit: John, I don't think you've had enough time to actually read much, but did anything in those books strike your curiosity?


They have been light bedtime reading :). You're right though, I haven't had much opportunity to sit down with them for any amount of time.
Scimming through them, I can see that I really should have had these about 3 years ago when I seriously started in this hobby. Research on the internet has given me much of what is in those books, from one source or the other.
Great reference though.

PS: I've sent you a email.
 
kristleifur said:
If you find a reasonably efficient low-impedance driver, you can use this Class-D amp, 41Hz Amp9:

http://41hz.com/main.aspx?pageID=130

Sounds great, and you can parallel the outputs for quite a bit of power into 1-2 Ohm loads. Energy efficient and small.
They are class T but, thats almost the same

Check Amp2, 2x 400w @ 4, bridgable,... good for 8 ohme speakers
 

wakibaki

Banned
2008-01-08 11:51 pm
With regard to defining my requirements more precisely, I can only say that I appreciate that I am posing a question that requires a judgement call on the part of the reader, but perhaps I can tell you something about how I make my judgements.

I like things that do a very good job and preferably the best. Kind of balls-out engineering. Sometimes I can afford them and I buy them.

I also like things that do a good or adequate job at the price. That’s value engineering. Sometimes the best value is also the best. Of course value is a much more complex call because people’s tastes vary from those who feel they want the very best to people who insist that anything more than a boom-box is wasted on them.

I can extract a lot of pleasure out of music without the necessity for a high quality reproduction so I aim to strike a balance in what I spend on myself, and my 56-year-old ears cannot possibly be as revealing as they once were although everything sounds exactly the same to me.

On the other hand, I have played a number of instruments over many years and have a reasonably developed musical sensibility and still do appreciate a quality system when I hear one.

Anyway, when I became aware of the existence T-amp comparatively recently I quickly formed the view that there was a consensus that it was good value. Not only good value, but I could buy a SMT board with one, shipped, and a complete SI Gen2 at prices that were a no-brainer for a tinkerer like me (I’m a design engineer).

Very shortly after I plugged the stock SI amp into my existing system, I started to become aware of things I hadn’t been aware of before. Not so much that I hadn’t heard them, more that I hadn’t realised they sounded so nice.

Anyway I was forced to the conclusion that the amp was probably outperforming the veteran Wharfedale Dalesmans which are all I’ve managed to sneak into the room. These are nice old infinite-baffle boxes only about 8 inches deep with 12in and 5in drivers and a kind of ‘very classy gramophone’ sound. I don’t think they’re tremendously efficient.

Now I’m going to take the PCB amp, modify it according to the consensus I found on this forum, build a power supply and connect it up to a PC I do some recording with in another room. I wanted to build some high-efficiency speakers to go with the T-amp for general listening, as opposed to near-field monitors. Obviously 5 watts or so is not much to play with and a FR speaker avoiding a crossover is starting to look good for some reasonable SPLs. The Audio Nirvanas are rated ‘very loud’ in the one independent review I found and it was this and the phase plug that pushed me towards the ‘super 8s’ rather than the few dollars difference in price with the Fostex 206s. The T-amp is happiest when not driven very hard, but I should get ~106dB (per channel) @ 4 watts which is probably enough to make me worry about bothering the neighbours.

I ordered a pair of ANs as some kind of compromise between price and performance given the low cost of the amplifier, the spending rules of how to split your budget of former years have had to go out of the window. Not wanting to enter into any great complications I have elected to build the speakers into the 2.8 cft enclosure for which free plans are provided, as the effort and even cost involved in building an enclosure does not change much with size. I have seen some criticism of the enclosure design, but there is no way to know how valid this is, if at all, and I figure I have some manoeuvring room with the speaker placement and vent size.

Unfortunately due to a failure on my part to return a customs declaration promptly, the drivers have been stuck in transit for a month. That’s another story tho’.

With the computer dragged in to serve as a source I think the system will provide an overall sound quality unimaginable for the price just a few years ago, and with comparatively little effort on my part. Now I am thinking about whether it is possible to get a readily perceptible improvement in quality while still keeping the cost in proportion with what I have spent already. The drivers and shipping were by far the most expensive item @ ~$280. A subwoofer seems the most obvious thing, and maybe if the FR enclosures are a bit coloured, then with luck the worst will be below the crossover.

Looking at systems on the market and the size of the room (12.5x11.5x10ft), I was looking to have ~200W available into a 12in driver aiming for max extension rather than max volume. I’m more interested in organ music than special effects, and I’d prefer the amp to be loafing. I’m a bit vague on room effects, so comments about that were helpful, perhaps you could elaborate a bit?

HSUs VTF-2 Mk 3 http://www.hsuresearch.com/products/vtf-2-mk3.html retails at ~$500 and looks a very good way to go buying off-the-shelf, but is probably overkill in volume terms, and would more-or-less double the overall spend. If I could spend half that I would consider it a success, and any less would be a bonus. I could afford the HSU, I just get a kick out of cheap-but-good with some diy. OTOH if somebody says ‘buy the HSU’ I may go that way, although shipping may be a problem. Reading some of the reviews, it may be difficult to reliably match the price/performance ratio without access to test facilities and without a lot of personal effort, but they’re probably too good.

There’s no wife acceptance factor as these are going in my room but I don’t want much more than about a 2ft cube. I could certainly build any of the T-amps or chip amps mentioned. In fact I was probably going to build a minimalist 3886 and PSU to try against the 2024 T-amp. Maybe realistically the power would be sufficient for a sub.

The last thing is that while I can probably design a way of getting a mono bass signal out of a stereo output and leave the stereo unaffected apart from some bass cut, it’s usually not a bad thing to ask around if an elegant solution exists. Anyway I haven’t messed with audio for some time. I guess a couple unity-gain buffers on each channel, one pair to a summer and then 3 active filters. 2 * 14-pin DIL packages, if you’re not too fussy about xtalk, but it needs a PSU.
 

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi wakibaki,
One comment.

If you build a box large enough, it disappears and becomes part of the room. At this point in time you also have maximum efficiency and the lowest cut off frequency.

I've found 12" woofers to be an unhappy size. Too slow for "quick" bass (no snap) and too small to really grab the air and move it.

Want to have fun? Build your single sub as a large box. Use a pair of 15" woofers (not for stereo). You should be able to get into the sub 20 Hz -3 dB cutoff region now.

I've done this over 20 years ago for an organ lover and it worked out just lovely. Even had WAF simply because it didn't look like a speaker box. It was built into a nook (wasted space), floor to ceiling. Today's speakers are different. However, physics and trusting the math will bring you extreme pleasure at reasonable dollars.

Now you only need to upgrade your sound card. :D

-Chris
 
For a look at my approach to value see the speaker project section of my website.

Tubby the tubie's place

The sub is driven by a plate amp that was salvaged (slightly modified) from a small AR subwoofer. For real value in high efficiency mains you might check out the Hawthorne Audio Silver Iris drivers. The guys at the forum are very helpful.

Hawthorne Forums

The drivers are not cheap (much more expensive than what I did) but there is definitely value there.

mike
 

wakibaki

Banned
2008-01-08 11:51 pm
anatech said:

Now you only need to upgrade your sound card. :D

-Chris

Get thee behind me Satan!

I looked at the Hawthorne open baffle driver, this looks like an interesting item. Also having seen Mike's 'fireplace' I started to wonder if it could go in the unused fireplace, which actually looks not unlike Mike's, but is marble. The chimney still exits to the open air but maybe with a bit of loose stuffing it would give me an effective infinite baffle.

Anyway, as is the way of these things, a colleague offered me the carcase of a Mordant-Short MSW 20 subwoofer with 4 Ohm driver, minus the amplifier (blown up), for a not-unreasonable sum. So now I'm looking for an amplifier to drive this. It was a fairly respectable sub that I'm sure I can find a use for. Does anyone know if the original 200W amp is likely to have had a lot of EQ?

As for the DVC, looks like I'm committed to this for the moment, with maybe the Hawthorne to follow

w
 

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi wakibaki,
That's hard to say. This gift item is about opposite to what you really wanted. A smaller box than reasonable that is compensated by a high powered amp. EQ ing the amp is actually very normal practice.

Since you are heading that way, either repair the original amp. This ensures it will run as originally designed. You can also confirm any EQ from examining the preamp section. Plus, it fits the hole!

-Chris