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DACT Step-Attenuator: Can newbie build passive pre-amp?

post #1 of 10
Thread Starter 
Could more experienced DIY'ers give me some sense of how difficult it would be to use a DACT step-attenuator as a volume control (passive pre-amp)?

The thought is to use an iBook as a cd sound source from the headphone out port/jack. It's an bedroom/computer workstation system and I'd like to keep cost low. In fact, if someone told me it would be no problem, I'd run to iBook's audio out right into Antique Sound Lab's Wave and control volume from the virtual cd-player on the MacOS desktop.

On the other hand,

DACT's Allan Isaksen suggests I could wire the step-attenuator into monoblocks or build a separate pre-amp.

Embarrassed to ask, is the supposition that I know enough about circuitry to design a pre-amp? I don't. Do people think I'd be better off with Dr. Bottlehead's foreplay?

Now I'm really embarrassed, but how does someone find a chasis to passive pre-amp?

I'll appreciate any pointers and advice.

Richard Moss
Washington, DC[/I]
post #2 of 10
Sure. You can do it. But first you need to make sure that your source has enough output voltage ( 2vrms is a good number) to drive an amp directly. If it doesn't then a linestage is what you want. There are a lot of very simple linestages out there so either way I'm sure that you can find something that will appeal to you. First find out how much voltage your sound card is putting out. Hell hook your sound card output up to an amp and see how it sounds. Be warned though. You may not like the way it sounds. Check it out and post a reply.

post #3 of 10
good advice.u also want to watch the impedance matching
10:1 from card output/10:1 to amp input

Another way of saying if the sound card has a 100 ohm output and the amp a 100 K ohm input then a 10 k attenuator would work

post #4 of 10
I don't mean to diss your ibook, because I've never heard one used as a CD player, but if it's anything like typical computer CD player outputs, it's not going to be primo, to say the least. Since you have the volume control on the computer, you don't need extra attenuation. Just hook it up to the amps and use the computer to control volume.

A DACT attenuator is probably more than you need to spend, anyway. You could get by cheap with two pairs of female RCA jacks and a dual audio taper potentiometer from Rat Shack for about $7 USD, and wire them up in an Altoids box. Unless, of course, I'm dissing your computer and it sounds better than one might expect.

If the computer opamp doesn't swing enough voltage to make the AV8's loud enough for your tastes, I'd suggest you use this as a good excuse to get the Bottlehead Foreplay. You CAN build it, the sound is great, and it will change the way you look at audio equipment forever (probably). Just one thing -- go ahead and build Voltsecond's filament snubber into the project from the get-go. Otherwise, AV8's and sensitive speakers will hum, almost assuredly, with a stock Foreplay. Mine did.

Of course, this is all just my opinion and experience, which could be wrong.
post #5 of 10
Thread Starter 
Videoshield, thanks—there's no attachment to the iBook as a cd-player. Obviously Apple Tysons & Apple Tech support say it's a great CD transport. My digital music composer friends said, 'Just replace the sound card. (put it where, with what, at what cost?).'

A $100 discman style unit seemed a better idea and maybe Teobj 4000 if space weren't an issue. I probably didn't know enough to name what I need at RS & didn't get very far talking about step-attenuators and sound-pots. Much rather spend $7 + altoids, than >$100 for DACT, although the DACT people are really helpful.

But I wonder what's going on in general. If computer drives, either IDE harddrives or CD-RW or CD compromise sound so much, why do people walk around as if they have bragging rights about their 500 MP3 files? Generation-X'ers have told me they've just given away/sold off all their CD player and run their entire music system off their G4's, whatever.

cc: via e-mail to Videoshield
post #6 of 10
Most generation Xers have no idea what a good sounding audio system sounds like. So don't listen to them. Check out Doc bottleheads wares. Also check out the Decware kits. Decware makes a tube buffered attenuator that has peaked my curiosity. Also you may want to check out a Pioneer DV-440 DVD player. 24bit 192Khz sampling. They sound nice for 180.00 USD plus they play mp3's. Have a good one.
post #7 of 10
Actually, I believe someone once said that the current gen iBooks have a pretty good headphone output so it may be a decent source... that sound ouput is a hybrid port for video as well, I believe.
post #8 of 10
'Just replace the sound card. (put it where, with what, at what cost?).'
You could use a USB DAC, any of which should work on a Mac running a recent version of MacOS. (i.e., one that understands USB audio.) The stereo-link is 9.75" wide x 8" deep x 2" high at the crest on the front. (Closer to 1" at the back.) It'll cost you about $130.

I did a review of it here on head-fi a few months back. The short version is, "very good for the price". It won't beat a $200-300 DAC, and though the headphone jack is better than most consumer equipment's headphone jacks, it can't beat a basic cmoy DIY amp. None of this is surprising, given the cost of the thing. I use mine in my work system, with a hotrodded cmoy headphone amp hooked to the stereo-link's RCA outputs. It works well.

There are other USB DACs out there which may be better, but I haven't tried any of them yet.
post #9 of 10
the Wheatfield USB DAC I listened to at last years HE01 will be available from headroom at some point.Sounded pretty good to me
post #10 of 10

Why not use a simple remote control

Perhaps you can consider this simple setup: keyspan (http://www.keyspan.com) makes a great little credit-card sized remote control for any USbB-Mac. You van choose which key controls what, and it is a good way to control volume and tracks on >CD, DVD or MP3. Probably the cheapest solution to your problem too.

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