Need Advice on designing a budget active speaker

[mertol]

Hi All ;

I am in need of some help. At last, I seem to have enough time to star building my self small active desktop monitors.

Here are my design goals :
1) Keep the total cost under 400 USD including everything.
2) Keep the size as small as possible,F3 of 70-80hz is more than adequate for me

I have vey basic knowledge on diy audio, and therefor need some help on making decisions.

Here are my questions :
(I am not after building the ultimate thing, as it's my first %100 diy project needs to be simple enough)


After reading a lot I decided

2-way setup wit a 4inch/4.5inch woofer + 1 inch tweeter

I will be using a LM1875T based stereo power amp to drive each unit.
An opamp based simple preamp

(I have 2x Integrated amp PCB's where 2x1875T is used with a NE5532, I will be glad if I can use those PCB's)

It's better for me to use line level passive filters (max 12db) as both active filters and passive speaker level filters are complex and will require too much time. And I think it won't be very hard to XO a small woofer to a decent tweeter using a 12db slope.

I will use an passive external box for input source switching.


Here are my questions :

1) Can I use line level filters between the source and line input of preamplifier?

2) Can I use the volume pot ( a stepped attunater) at the external passive line selector? It will be like :

Source --> Selector --> Volume pot -> Line Level Filter --> PreAMp--> Power Amp-->Driver


3) Torroidal transformers are not very expensive here. So will using 2x transformers for +/- supply rails bring me any advantage, or should I spare this money for better drivers?

4) Any recommendations about woofer/tweeter selection?
I prefer them to have similar sensivity. Max I want to pay is 140USD for 1xWoofer + 1xTweeter. (I prefer transparent sound with good imaging)

best regards
Mertol

[Inductor]

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It had to be funny... (LOL)

[DougL]

I prefer them to have similar sensivity.

This is probably not going to happen. 4" mid-base drivers tend to have lower sensitivity. True mid-range drivers have the sensitivity, but roll off at 400 Hz.

[steph_tsf]

Originally Posted by mertol

It's better for me to use line level passive filters (max 12db) as both active filters and passive speaker level filters are complex and will require too much time. And I think it won't be very hard to XO a small woofer to a decent tweeter using a 12db slope. I will use an passive external box for input source switching.

For 400 USD you can buy not one but two pairs of Creative Labs Gigaworks T40. Use one pair as reference, don't modify them. Carefully measure them using SpectraLabs 4.32. Open the second pair, reverse-engineer them (draw the schematic), modify them using better biamplification like your LM1875 amplifiers and better power supply, install a phase-accurate crossover, time-adjust the woofers with the tweeter, add a kevlar membrane high frequency notch, add a Linkwitz Transform for linearizing and extending the bass. Believe me, only this way you'll learn how to get good sound.
Don't undervaluate the benefits of carefully measuring the "untouched" Creative Labs Gigaworks T40. Pay attention to this. Determine where there is room for improvement, on an objective basis.

[steph_tsf]

When you are done and satisfied with your "improved" Creative Labs, take them with you in a Bose shop and ask a direct comparison with the Bose MusicMonitor.

[mertol]

I think reverse engineering and carefully measuring T40's abilities is not the correct route for me. And T40's are more expensive in this part of the world due to some tax's.

My integrated amp boards are on the way to home right now, and I will appreciate if anyone can help me on driver selection.

br
Mertol

[steph_tsf]

Originally Posted by mertol

1- It's better for me to use line level passive filters (max 12db) as both active filters and passive speaker level filters are complex and will require too much time.
2- I think it won't be very hard to XO a small woofer to a decent tweeter using a 12db slope.
3- Any recommendations about woofer/tweeter selection ? I prefer them to have similar sensivity.

1+2+3 indicate that your intellectual openness and commitment are on the thin side for such a project. Do you know the differences between closed box, closed box with passive radiator, and bass-reflex ? Are you fluent in using LTspiceIV for drawing schematics and simulating crossovers ? What measurement gear are you using; are you fluent with SpectraLab 4.32 ?

Originally Posted by mertol

F3 of 70-80hz is more than adequate for me.

Are you aware that a frequency response like Butterworth with a 70-80Hz -3dB frequency corner is going to subjectively translate into a dry, dull sound ? Is there a subwoofer available ?

Originally Posted by mertol

I have 2x Integrated amp PCB's where 2x1875T is used with a NE5532.

If you want to get some productive advice, you need to post the schematic of those PCB's.

[steph_tsf]

Why building from scratch ? Buy a pair of Realistic Minimus-7, add some equalization in your LM1875T-based amplifiers, and you are done. This is a closed-box design, easy to equalize in the bass using a Linkwitz Transform circuitry.
See here : REALISTIC MINIMUS-7 SPEAKERS MADE IN JAPAN GREAT SOUND | eBay
Later on you may bi-wire them.
This way you can use an external crossover like a 4th-order Linkwitz-Riley, with an impedance compensator for the woofer and a RLC notch (-8 dB at 4kHz) for the woofer.
This way you also can apply biamplification, introducing an active crossover.

[Inductor]

Originally Posted by mertol

My integrated amp boards are on the way to home right now, and I will appreciate if anyone can help me on driver selection.

Hi Mertol,
Sorry for the joke. I think you need to be a little more assertive. So you don't want to buy a new pair of speakers but the drivers individually.
Just to say, for the sake of information for others, that good drivers/speakers in that/good price point are a hand full.

Wharfedale Diamond 10.1 http://www.stereophile.com/content/w...01-loudspeaker
Q Acoustics 2010
Cambridge Audio S30

Now you know what to expect, approximately.
So, what brands or shops you have access to in Turkey/others if you know, or you just ask from any in Europe/Germany/others? Do you want also to buy kits or not, for/if the price of the kit sent to you being a good one.
There are good brand names like Visaton, Seas, others, what are you into? or that is not so important.
Into the design, we, here in diyAudio, have been doing a different 2-Way with a full-range and woofer with very good results, or do you prefer a standard tweeter? Others will help with the electronics, because I only work with passives, but I think the questions I ask you, because of locations for delivery and taxes are of no less importance for you. Regards.

[steph_tsf]

Originally Posted by Inductor

Just to say, for the sake of information for others, that good drivers/speakers in that/good price point are a hand full.
Wharfedale Diamond 10.1
Q Acoustics 2010
Cambridge Audio S30

Fully agree. Those speakers represent excellent value for the money. If their built-in passive crossover are not ill-designed, adding a dedicated amplifier embedding a Linkwitz Transform and some careful medium/high equalization should result in a pretty good listening experience at moderate listening levels, obviating the need for a subwoofer. Would be nice to start three distinct threads, each about one of the above speaker, carefully measuring them, reverse-engineering them, and proposing various upgrade paths like : 1) designing a Linkwitz Transform for linearizing and extending the deep bass, 2) bi-wiring them for allowing an external crossover, 3) designing a 4th-order Linkwitz-Riley passive crossover with the woofer impedance corrector, the woofer hi-freq RLC damping, and the tweeter time alignment, 4) designing the corresponding 4th-order Linkwitz-Riley active crossover and twin power amp.