Multi-amp thought experiment

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diyAudio Member
Joined 2007
There has been no interest in my old Rotel 8 channel amp but it is too good to give away. What can I do with a small cheap DSP and 8 * 40 watts? At my first thought that isn't much for bass but that 40 * 4 is equal to most 150watt power amps I've ever owned and those never seemed shy of bass power. 3-Way or 4-Way? I haven't opened up the box for the DSP yet as too much going on at the moment but I need to get my mind organised.

Whatever is going to happen will be in the context of the drivers I have in the shed and on the shelves but that selection is pretty broad and most of the better, smaller drivers will cope with 40 watts. Let's brainstorm even if nothing gets built for a while, I am a terrible ditherer and prevaricator and I think I prefer to tinker than do straight builds but I have the need to build something big soon
 
40 * 4 is equal to most 150watt power amps I've ever owned and those never seemed shy of bass power.
3-Way or 4-Way?
I have been running active 3-way speakers for over a decade. I have found that the 2x10" woofers per side, take much more power than you might expect. In my 6.4x6.7x2.4 listening room, 200 watts rms is the minimum for adequate headroom. I tried a 6x Hypex UCD400 amp which clipped with ease on the 4 ohm woofers. I recall that Rotel uses ICE modules, which are more robust when abused, so they might be more tolerant but don't expect too much from 40 watts/channel. Otherwise, I am firmly in the active + digital crossover camp. The flexibility of DSP is a game changer in loudspeaker design. It will never be plug-and-play though. Expect a steep learning curve.
 
Or a TMWW and just use the one amp.

Tony.
This would be my preferred option. Better to drive each woofer separately rather than in parallel. This gives your woofer amps an easier ride. One thing I experienced is that clipping the woofers is easy to do in active mode but the sound stage does not collapse in a steaming pile as it would if a passive is clipped. With 2 woofers you could also try opposed side mounting with coupling rods. This really does effectively cancel the reaction forces into the cabinet.
 
diyAudio Member
Joined 2007
The alternative would be to use a mid/tweeter combination with a proven XO and well know performance and drive them with one channel. Nobody has claimed those Jaycar 50mm domes after all.
As for measuring on a baffle I thought the whole rationale for DSP was to mount according to good practice and then play?
 
Something big, you say? Can the channels be bridged? If so, that could give you some more power for bass. Failing that, don't forget that larger speakers tend to have higher efficiency anyway. And stereo bass output tends to be mono anyway, so that's amplitude summing at low frequencies, not power summing. IMO it makes a mono subwoofer problematic with most source material, and 2x 10" just sounds better than 1x 12".



In fact, you could probably go up to 2x15", or 4x10", have room-shaking bass for under 10W (OK, ymmv), and still cross over at 100Hz to any 7" mid of your choosing. Although at that point I'd probably start thinking about multiplying the mids for better efficiency in a line array, and then support that with a big tweeter.
 
I've been playing with the Dayton DSP. Nice toy and you can do some crazy stuff using it. Like passing one woofer at 300 but passing a parallel woofer at 80/120 etc


Is this the Dayton DSP408 ? If so I'd be interested in your thoughts as to how this compares to the Minidsp models (4x10 or two 2x4)?


A shame you haven't go the RMB1066 which can be bridged to make 3x150w (need two) rather than 6x60w.


I have been looking at a similar system using either the rmb1066 or three Quad amps (2x306 plus 1x606). My preferred option, when the drivers come back in stock, is for a TMWW using a SB26CDC (or ADC), Morel EM1308 (or mdm55) and a pair of Anarchy 7" (708 in parallel for 4 ohms) which will fit in 40l box nicely and drop into the low 30hz at f3. This is my choice as I need something fairly small (thin floor stander) for a smallish room (9ft x 9ft) so don't need stadium level of loudness or 20hz sub levels.


You may be able to get away with in stock 704 and run an channel per driver and as you suggest use the dsp to stagger the tow 7" to spread out the bass hump these have at around 200hz. As said it will not shake the earth but would get pretty loud even with 65w per driver as they should result in around 87-89db sensitivity for the bass depending on whether you use the 8 or 4 ohm versions. These drivers sim up fairly well with 24db slopes at 700hz and 4000hz with a notch/parametric filter in the mid bass around 200hz to drop the level down plus another couple to fully flatten the mid and tweeter plus sensitivities are not far out (2-3 db only).


For info the new version of Boxsim (v2.0) allows you to sim active filters and crossovers which is a really nice feature.
 
One thing I experienced is that clipping the woofers is easy to do in active mode
What I meant to say was, clipping the woofer AMPS is easy to do. The sealed woofers can handle plenty of power, it's the power amps that can't supply the grunt (Hypex UCD400). Bigger amps like my Emotiva XPA Gen3 are fine but are way overkill for mids and treble, and anyway you want to use your smaller Rotel.
 
diyAudio Member
Joined 2007
Yep Years ago I had the small Rotel stereo amp, I traded it in on the 1070 very quickly; but I did and still do have big and inefficient woofers, but yes I was looking for a solution that meant keeping using the Rotel 4 channel stereo amplifier
 
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What can I do with a small cheap DSP and 8 * 40 watts? At my first thought that isn't much for bass but that 40 * 4 is equal to most 150watt power amps I've ever owned and those never seemed shy of bass power.

Idea [1]
40 watts could be plenty depending on your goals.

Dude A wants to get a system that's -3dB @ 20Hz. Solution: high Xmax woofers and lots of power.

Dude B wants to get the most out of 40 watt amplifiers. Solution: more efficient woofers, higher -3dB point.

Idea [2]
lots of channels = lots of flexibility.

Dude X builds a simple 2 channel system. It is positioned for imaging, not for LF gain. Dude A uses 200 watts / channel to get satisfying SPL.

Dude Y builds a 2 channel system and adds distributed subs. Several subs are placed in high gain locations, getting a +9dB room boost. Dude Y only needs 25 watts / channel to get satisfying SPL

Idea [3]
use the best of both - flexibility and power..

Use multiple 40 watt channels AND a single large high powered unit, for a multi sub system.

2 x 40 watts --> L satellite
2 x 40 watts --> R satellite

big plate amp --> subwoofer

...and you'll have 4 x 40 watt channels left over. What to do with them?

Well, imagine you want your single big sub to be loud & smooth from 20-100Hz, but (after experimenting with sub positioning + room tweaks), you have deep nulls at 55Hz, 75Hz and 90Hz. Big nulls are hard to fill with EQ.

In this case, you could add four mini subs to fill in those nulls. 40 watts = no problem, since filling in a 55Hz hole is a much easier task then going flat to 20Hz.
 
Whatever is going to happen will be in the context of the drivers I have in the shed and on the shelves ... I think I prefer to tinker than do straight builds but I have the need to build something big soon

Can you create an inventory of your available drivers?

Does your DSP have 2 or 3 or 4 independent outputs per speaker?
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T-M-- --WW

DSP_woofer: drives both inputs to two amplifiers, each connected to one (4-ohm?)woofer. Ported woofers are more efficient than sealed woofers.
Amp1 --> woofer1
Amp2 --> woofer2

DSP_midrange: drives one amplifier connected to one (4-ohm?)midrange
Amp3 --> M

DSP_tweeter: drives one amplifier connected to one (4-ohm?)tweeter
AMP4--> T
 

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I think you might be surprised at what 40w will get you.

I tried bringing my PA system into the house one day and found you could make things rattle with a pair of 15"s being fed less than 50w each.

4x 15"s each with 40w would be rather loud, so long as you don't aim for 20Hz.

Remember Hoffman's Iron Law - small, efficient, low bass - pick any two.

In your case, I'd pick a LF cutoff of, say, 40Hz and mess around with the other two.

Chris
 
Can you create an inventory of your available drivers?


=======
T-M-- --WW
Great minds think...alike...sometimes. Drivers from top down; SB12NRX25-5, SB26ADC-C000-4, SB15NRXC30-8-UC, SB23MFCL45-8. Awaiting cooler weather to permit spray painting. Woofers are reaction-force-cancelling by threaded steel rods. Height approximately 1.3 m with tweeter at ear height in listening chair. Crossover is modified 10 channel MiniDSP through studio grade 192 kHz, 24 bit studio grade AD/DA converter. This is my attempt to downsize from 150 kg monkey coffins that I can no longer move unassisted.
 

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I suppose I should Insurance purposes and all that
A lot od cheap some good some very good but all older than 10 years in storage, it being a long time since I have made anything

Knowing what you have, especially wrt woofers, would really help to narrow down options. I really like the satellites plus small subs idea but if you don't have the drivers for it...

Knowing the intended use and the size/shape of the room would also be a big plus.
 
What's the sound quality like on that woofer (I'm looking at the 4 ohm version)?
Build quality is exceptional, huge double magnet and roll surround. Used as a sub-woofer (< 100 Hz) there is nothing of similar price/size to touch it. It sounds good even used as a front firing woofer up to 250 Hz. I did not know there is a 4 ohm version. Since I parallel them, 8 ohm suit me better.
 
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