Seeing as my ETI 480 100 watt sub amp disgraced itself several months ago I had to replace it. People are constantly RAVING about Gainclones, so the time has arrived to give it a go. I got an old PC power supply box because it has a fan, an IEC mains inlet connector, and is a nice tidy size. Got a ~150VA 25-0-25 v tranny from a discarded Sony plastic thing and fed it to a bridge and +/- 14,000 approx. Gives about +/- 32.8V at no signal.
Gotta have a good low pass filter of course, so I "went for the doctor" and made an 8th order (48dB per octave
) low pass filter using unity gain Sallen-Key stages with a -3dB point of 90Hz. Actually I measured the exact capacitor values and twiddled the resistors to suit because in a filter that sharp the component values have to be right-on. Should work good hopefully.
Will fire it up later tonight and maybe get some pictures posted too! If I can stop listening to it that is...
Gotta have a good low pass filter of course, so I "went for the doctor" and made an 8th order (48dB per octave
) low pass filter using unity gain Sallen-Key stages with a -3dB point of 90Hz. Actually I measured the exact capacitor values and twiddled the resistors to suit because in a filter that sharp the component values have to be right-on. Should work good hopefully.Will fire it up later tonight and maybe get some pictures posted too! If I can stop listening to it that is...
Nearly, but not quite
Well, it worked just fine. Only trouble is, the subs sounded a bit boomy and muddy. The previous drivers that I used (burnt out by amp ) had added mass in the form of 6 coins glued to the centre of the cone and this gave a pretty good rollof starting at about 60Hz. The drivers I have now are showing up the deficiencies in the box. It has only a layer of carpet on the inside faces so maybe that is not enough. Also, I am going to lower the low pass filter cuttoff frequency from 90Hz to 60 Hz and see how it goes.
These are the drivers: http://www.wescomponents.com/Speakers/Woofer/sp8-100/sp8-100.htm mrfeedback take note!
I took the amp to work today and gave it a rev up on the bench and it measured 44.25 watts with one channel driven into 8 ohms. I didn't have time to rig up a second load to run both channels together.
One problem I am having with it though is a slight oscillation on both upper and lower peaks of the waveform when it is approaching maximum output. I have tried a 10R/100nF in series across the output, also a 100k+10nF in series across the 220k feedback resistor (10k input resistor) but to no avail. The supply rails are bypassed with high-frequency type electrolytics about 10-15 mm from the chips so that should be ok I would think.
The problem is identical on both channels, and I just remembered I noticed it as well on a 'current-dumping afterburner' I was playing with too, also with an LM3875. (Got 140W rms BTW...)
What should I look for next?
Well, it worked just fine.
Only trouble is, the subs sounded a bit boomy and muddy. The previous drivers that I used (burnt out by amp ) had added mass in the form of 6 coins glued to the centre of the cone and this gave a pretty good rollof starting at about 60Hz. The drivers I have now are showing up the deficiencies in the box. It has only a layer of carpet on the inside faces so maybe that is not enough. Also, I am going to lower the low pass filter cuttoff frequency from 90Hz to 60 Hz and see how it goes.These are the drivers: http://www.wescomponents.com/Speakers/Woofer/sp8-100/sp8-100.htm mrfeedback take note!
I took the amp to work today and gave it a rev up on the bench and it measured 44.25 watts with one channel driven into 8 ohms. I didn't have time to rig up a second load to run both channels together.
One problem I am having with it though is a slight oscillation on both upper and lower peaks of the waveform when it is approaching maximum output. I have tried a 10R/100nF in series across the output, also a 100k+10nF in series across the 220k feedback resistor (10k input resistor) but to no avail. The supply rails are bypassed with high-frequency type electrolytics about 10-15 mm from the chips so that should be ok I would think.
The problem is identical on both channels, and I just remembered I noticed it as well on a 'current-dumping afterburner' I was playing with too, also with an LM3875. (Got 140W rms BTW...)
What should I look for next?
Steep-o-matic low pass filter.
Today I changed the low pass filter from 90 Hz cutoff to 60Hz. Big improvement. The bass rolls off quick smart before it reaches the boomy frequencies. Then I measure the frequency response of the amp at 5 Hz intervals from 20 Hz to 150 Hz with (at below cutoff) 40 v p/p output into an 8 ohm load. The graph has been normalised to 1 volt. In 1 octave the output voltage drops by a factor of 250, power output by a factor of 250x250 = 62,500 =48dB
Today I changed the low pass filter from 90 Hz cutoff to 60Hz. Big improvement. The bass rolls off quick smart before it reaches the boomy frequencies. Then I measure the frequency response of the amp at 5 Hz intervals from 20 Hz to 150 Hz with (at below cutoff) 40 v p/p output into an 8 ohm load. The graph has been normalised to 1 volt. In 1 octave the output voltage drops by a factor of 250, power output by a factor of 250x250 = 62,500 =48dB
Attachments
I Hear Ya....
Out of the packing box the roll surround is a bit stiff and that can lead to boomyish bass, but a bit of clean, gentle and powerful excursion should sort them out.
On reflection, the boxes that I put them into were rather smaller than your big cabinets, and in this condition they worked great.
With running in I think these should work fine, and especially for the price.
Eric / - You can call me Eric.
Hey Graham, you have not given them a chance to bed in yet - connect them up to a really big amp and give them a big smooth flogging that they will not forget.
Out of the packing box the roll surround is a bit stiff and that can lead to boomyish bass, but a bit of clean, gentle and powerful excursion should sort them out.
On reflection, the boxes that I put them into were rather smaller than your big cabinets, and in this condition they worked great.
With running in I think these should work fine, and especially for the price.
Eric / - You can call me Eric.
Re: I Hear Ya....
Will go try and take some pics. Hmmm.. it's 11:20 pm Friday night here.
You're right, they haven't had much use yet. But I did set them on the bench and apply enough low voltage AC to flex them ~1cm p/p for about 1-1/2 hours. Actually it's a pity they have foam surrounds instead of rubber. If they did I reckon they would be even better.mrfeedback said:
Will go try and take some pics. Hmmm.. it's 11:20 pm Friday night here.
are there different sizes of PC PSU's because the one in my family's PC just broke and so i took it, and i don't see how i can fit the transformer and the circuit in this little box. if i don't use a toroid maybe? what is this black thing that's inbetween the power switch and the IEC connector?
From the parameters of the speaker unit I would say that this unit does'nt like what youre doing to it. The unit doesn't like small boxes, you covered that problem I can see from the picture, but when you put it in a large box it results in a relatively big group delay and out of phase operation. This might be resulting in the boomy sound.
It might be the speaker and not the amp that is the cause for the boomy sound.
Did the manufaturer made recommendations regarding the box volume?
It might be the speaker and not the amp that is the cause for the boomy sound.
Did the manufaturer made recommendations regarding the box volume?
It's a bit of an ongoing project really. I'm quite prepared to half fill it with bricks if necessary. Maybe even put a partition inside it and make it a 6th order [badass] bandpass system. Next thing to do though is to open the port up a bit to retune it from the current 31.6 Hz to 35Hz.indoubt said:
The other day when I had one of the drivers out, the cat got inside it for a look-see. If I was quick enough I could have reduced the volume and increased the internal sound absorption in one go!
The whole story is here: http://www.diyaudio.com/forums/showthread.php?s=&threadid=9714&highlight=245
Circlotron said:
Doesn't have a power switch.Which picture? There is a black cable clamp (unused) blocking off a hole next to that grey cable gland with all the wires passing through it...
oh, i was talking about mine. i think its for selecting 115 or 230 volts? those are the numbers on this little switch thing that i just noticed.
Inspiration
Hello,
Graham, you sir really have a touch for building electronics. Very nice construction. In fact, I am inspired with what you did. I have a friend who has a pc repair shop and when I told him I am into diy he gave me 7 pcs of busted pc psu that at the time I intend to use as chassis for bench power supply since I don't have an adjustable one. But because of your project, I now have an excuse to my friend to ask for more busted pc psu.
Best regards,
JojoD
Hello,
Graham, you sir really have a touch for building electronics. Very nice construction. In fact, I am inspired with what you did. I have a friend who has a pc repair shop and when I told him I am into diy he gave me 7 pcs of busted pc psu that at the time I intend to use as chassis for bench power supply since I don't have an adjustable one. But because of your project, I now have an excuse to my friend to ask for more busted pc psu.
Best regards,
JojoD
I used a PC PSU box for my power only, as all of it won't fit in one of them. I plan to use another for the chips and heatsinks but I haven't found another PSU enclosure yet. They can be either bolted together side by side or on top of each other.
As far as I know, they are a standard size for all ATX units. I am guessing that the older AT style are either the same size, or smaller.
As far as I know, they are a standard size for all ATX units. I am guessing that the older AT style are either the same size, or smaller.
hi tmd1,
i am into pc psu repair myself, i got a boxfull of psu casings that i gave away to the garbage man for recycling....anyway, you can look for older AT psu, from branded (Compaq, Dell, etc) desktop pc, for 486 and pentium 1's, if you are lucky you may find one that is bigger than the standard atx psu's...good luck!!!
i am into pc psu repair myself, i got a boxfull of psu casings that i gave away to the garbage man for recycling....anyway, you can look for older AT psu, from branded (Compaq, Dell, etc) desktop pc, for 486 and pentium 1's, if you are lucky you may find one that is bigger than the standard atx psu's...good luck!!!
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