Choosing & Wiring Sub with Speaker-Level Inputs for Camb.Audio AM-10 Amp (No Sub-Out)

drumtheater

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Hi all,

This is my first post on hifivision, although I have been reading and lurking for some time.

I have recently upgraded (I hope) from a borrowed NuForce uDAC - Topping amplifier - CA S30 system (the system reviewed on this forum by my friend psychotropic) to the following setup:

(1) Cambridge Audio Topaz series CD-10 CD Player
(2) Cambridge Audio Topaz series AM-10 Integrated Amplifier
(3) AudioPro Avanti A10 bookshelf speakers
(4) Q Acoustics speaker stands

Components (1)-(3) were bought by me in Hong Kong in early November 2012 and carried back personally (travelling as a family so there was sufficient weight and customs allowance). The speaker stands were bought from Decibel in Chennai.

Prices in INR: Rs 26,700 for the CDP and Amp together (each cost 13k-odd)
Rs 18,700 for the bookshelf speakers (pair)
Rs 8,200 for the speaker stands (pair)
Rs 900 for 6 metres of Belkin speaker cable

I got close to 50% discount on the CDP, amp and speakers since they were display units and the shop was having a clearance sale. (One big reason for buying them. Second big reason for buying them was to just get it over with-- I can be indecisive and didn't want to spend months checking out options in Chennai without getting closer to buying anything.)

The speakers don't seem to be well known in Chennai (AudioPro is a Swedish company, apparently). I am a recent convert to the audiophile world and my ears are nowhere near as trained as I'd like them to be, so I don't think I can accurately describe the system sound. As a 2.0 bookshelf system it has definite limitations-- every floorstander system I've heard so far has sounded better than this. But it was the best system in the store in my budget, and for the price, I think the sound is quite clear and bright, especially in the mid and high ranges and vocals. In fact, it's a little too bright; bordering on harshness sometimes. A lot seems to depend on the source recording-- Donald Fagen's "Morph The Cat" and psychotropic's Usher demo CD really brought the system to life, with responsive bass lines and imaging.

Unfortunately, I don't spend my time listening to Donald Fagen and (that particular) Usher. I listen to a lot of rock and metal, especially progressive metal, as well as a large amount of what would be termed pop and a fair amount of EDM (house/trance/whatever). And while playing the CDs I regularly listen to, especially Dream Theater CDs, I feel there is a marked lack of bass response. Quite often, it sounds like an entire part of the recording is missing, because the kick drum and the bass guitar are reduced to pitiful shadows of what they're supposed to be.

Although I've always been partial to heavy bass, I had decided while researching this system prior to buying it that my budget for my entry-level system could not extend to a subwoofer. But given the sound I've described above, a subwoofer is absolutely imperative for me now.

Unfortunately, audiophile subwoofer prices seem quite ridiculous compared to the cost of my system. I started off with a budget of Rs 30k, ended up spending almost Rs 57k, and now a subwoofer will cost another 20-30k rupees? My 30k budget will have tripled by the end of it.

So, priority A in my purchase: Low Price. I know that I'll get only what I pay for, but I'm willing to compromise now to keep the budget as low as possible. I will definitely upgrade from this system one day, so I don't need to buy a sub that will last me my whole life. Ideally, I would like to find a used sub in good condition.

Priority B: This is more of a condition precedent, really. The AM-10 amp does NOT have dedicated subwoofer pre-outs. Therefore, any sub that I buy HAS to have at least speaker-level inputs. I would like it if it had speaker-level outputs as well, but it doesn't seem to be essential.

Listening Area: One of the worst-designed rooms in the history of mankind. It's a small bedroom, about 12 feet x 15 feet. Stone floor. One wall has THREE doors (two lead to bathing areas). Another wall has a built-in wooden cupboard. A third wall has a window AC 1 foot from ground level. There are windows above the window AC and in the fourth wall. Into all of this are crammed: a wooden bureau; a work table with my laptop, desk phone, set-top-box, router, etc; a single bed, a shoe rack, and an electronic drum kit (Roland TD-4K). Therefore, the sub needs to be as small as possible.

I visited a couple of audio stores in Chennai today, looking for sub options. The only one I've found so far with high-level inputs is a Q Acoustics 2070 at Decibel. Somewhat unusually for a sub, it's a long and thin enclosure with twin 17-cm front-facing long-throw drivers stacked vertically and a rear port. It's quoted at Rs 33k new but being offered at around Rs 22k used.

My Questions (General)

(1) Are there recommendations for a no-brainer small subwoofer at a price point of Rs 12-15k that I can explore in Chennai? Rs 20k price point? 25k? 30?

(2) Will the Q Acoustics sub be a good buy for my requirements, or should I keep looking for something better?

(3) Will the Q Acoustics sub be too large and powerful for my small listening area? I do hope to get a home demo before buying it.

(4) Are there recommendations for subwoofer specialists in Chennai? Or even a DIY guy who could build a cheap but hifi sub for me?

My Questions (Wiring)

(1) The amp has a set of line-level outputs called "Rec Out". There are no other line outs. The manual says that these are for connecting to "the recording input of a tape deck, CD recorder input, etc". The manual has absolutely nothing else to say about these outputs. Can I use these outputs to connect to the line-level inputs on a subwoofer? Would there be any adverse consequences? I would imagine that similar to connecting to pre-outs, the sub would receive a full-frequency signal and the sub's crossover would have to filter out the high frequencies (as opposed to an LFE Out from a receiver). I would also imagine that the biggest drawback of this approach would be that the amp volume control would not control the subwoofer volume, since it wouldn't affect the strength of the Rec Out signal (drawing analogies to the Record function on old cassette decks).

(2) The amp has four binding posts for its speaker-level outs (one pair for the left channel and one pair for the right channel). Right now, speaker cable connects these posts to the binding posts on the bookshelf speakers. I have read that if a sub has speaker-level inputs as well as outputs, it is possible to wire a setup in "series" (it may not be technically 'in series', but I think of it as that), as follows:

Amp High-Level Outs -> Subwoofer High-Level Ins
Subwoofer High-Level Outs -> Bookshelf Speakers

I have also read that it is also possible to wire a setup in parallel, as follows:

Amp High-Level Outs -> Subwoofer High-Level Ins
Amp High-Level Outs -> Bookshelf speakers

Thus there would be two wires connected simultaneously to each of the four binding posts on the amp: one wire for the sub and one wire for the bookshelves.

But what about the impedance, especially when wiring in parallel?

My bookshelf speakers have an impedance of 4 ohms (each, I suppose). The lowest impedance which the amp's user manual permits is 4 ohms and the highest is 8 ohms (I suppose this is per channel?). The amp's power output is listed as 35 watts into 8 ohms.

The subwoofer's technical specifications do not list an impedance at all! I read somewhere that active subwoofers accepting high-level inputs must first reduce the high-level signal to a line-level signal which they then re-amplify, and that in order to effect the reduction of the signal, the sub high-level inputs present a very high impedance to the incoming signal. "Very high" here is in comparison to the 4 ohms presented by the bookshelf speakers. Therefore, the argument went, the total impedance presented by the sub connected in parallel with the bookshelves would be only marginally less than the impedance presented by the bookshelves on their own.

(This follows from the equation 1/R = 1/r1 + 1/r2 which I seem to dimly remember from the days when I was taught such things.)

I have had a look at the Q Acoustic sub's user manual. In the context of a system such as mine with no subwoofer pre-outs, it recommends wiring the sub in parallel to the bookshelf speakers as described above. It is completely silent on impedance aspects and seems to assume that nothing will go wrong as a result of that.

At many places on the internet I have seen people take this same stance, ie that a sub when being connected via its high-level inputs can be wired between the bookshelves and the amp, or in parallel, or even from the bookshelves directly (without being connected to the amp/receiver at all), and that all of these options will work and none of them will make a difference to the sound.

The absolute last thing I want to happen is for my new speakers/amp to get fried as a result of impedance issues. Can anybody shed some light on this for me? Am I worrying about a non-issue?

Thanks so much in advance. I have pictures of my setup which I will post at the appropriate place after figuring out how to upload photos to the forum.

The amp's user manual is available here: http://www.cambridgeaudio.com/assets/documents/AP259381TopazAM10UsersManual-01English.pdf

The sub's user manual is here: http://www.qacoustics.co.uk/themes/qacoustics_new/pdf/2000i_Manual.pdf
 
Re: Choosing & Wiring Sub with Speaker-Level Inputs for Camb.Audio AM-10 Amp (No Sub-

As you understand, you need a line level(non amplified signal for your sub). You can use a Y splitter from your CD player that will allow two sets of signals to be used by two amps. I use it for an integrated which doesnt have a sub out. The other option is to drill holes and mount an extra set of RCAs on the amp( from the line level inputs) from which you connect your sub -I've done that as well.
 
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