Op-Amp Pre for Sonic Impact? - diyAudio
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Old 18th August 2006, 07:11 PM   #1
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Default Op-Amp Pre for Sonic Impact?

I've been building a boombox around a pair of SI boards (one will power external speakers if needed). Starting to sound pretty good, but I find that when I go through a preamp I get louder sound and perhaps a bit more richness, especially running off a portable CD player.

I can flip a switch and the thing will run on either a 12v SLA or one of those 14v monitor SMPS that have been on eBay.

I'd like to build in a low-gain preamp/buffer using an op-amp such as the OPA 134 or whatever will give me decent sound and a few dB gain. There is a circuit shown on a current Charlize thread (the one used in the Decibel Dungeon Propod) seems to be on the right track, but details about power connection are missing, and the OPA 134 seems to want a dual supply.

Most of what I've read says that op-amps just aren't at their best w/o a dual-rail supply, although I know there are single-supply versions out there. Would it make any kind of sense to use a DC-DC converter (15V+ to 5-12v + and - ?), or would that just add trouble?

I'm a much better metalworker than I am a circuit designer, so I need something with a complete circuit, no loose details to wrap up, although I am not really looking for a kit or even a pre-etched board. I've been searching all over and haven't been able to come up with something that's perfect for my application, although I feel it is probably right under my nose. Anybody solved this problem before?

--Buckapound
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Old 18th August 2006, 07:18 PM   #2
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Can't help much on your problem, other than by mentioning that there are "rail splitter" chips available that create +/- rails w. a middle ground. The TLE2426 is one. They're used quite a bit in headphone amps. Don't know if the virtual ground would create problems though.

Mainly, I wanted to say:
DAMN that is a nice boombox! There's nothing as cool as custom .
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Old 18th August 2006, 09:16 PM   #3
Pano is offline Pano  United States
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You can run most opamps single rail, no big deal. You will need a DC blocking cap on the output. About 2uF, minimum.

Have a look at these headphone amps. Any of the single rail type will work as your preamp. Or you can split the rail if need be.
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Old 18th August 2006, 09:21 PM   #4
ekaerin is offline ekaerin  Sweden
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Backapound,
Not what you where asking for but I agree, DAMN good looking boombox.
Pretty far from the plastic crap they throw in your face at the local low-fi outlet.

/ Mattias
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Old 18th August 2006, 10:22 PM   #5
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I've gone through a lot of trouble with solutions that ought to have worked--as I said I'm a better metalsmith than an electronics guy.

I tried Greg Ball's GB-BX bass extender. This was absolutely what I wanted in there, but after a lot of work with him, and a lot of soldering and fiddling, we never could get it running on a 12v single rail, so it went back to Australia.

I also worked with one of the CMOY amp guys on eBay and he customized an amp (2 x 2227P) for my purposes. When I plugged it into the box's power supply, I got a lot of dangerous cone movement along with some coarse hissing/whooshing and a little audio. There were blocking caps between it and the t-amp. He eventually got tired of me asking silly questions and stopped answering my email. Sounds great with headphones, though.

So I'm gun-shy of the whole thing and am looking for a circuit diagram beyond the usual app sheet ones where they assume you know how to finish the loose sketches.

As for the cool-looking box, it's actually made of fiberglas-carbon fiber-nomex honeycomb core aircraft panel. Molto rigid. Welded all the stainless, turned the knobs, feet and flared port, which now has the power LED in the center of it. I've got a hi-tech battery conditioner in there, and a regulated 5v USB port on the back to recharge the iPod, lots of other goodies. One of those projects you just keep piling on, out of control, really.

--Buckapound
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Old 19th August 2006, 02:09 AM   #6
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You could buy a JEC TC-754 and bury it in there somewhere - it's only $90 and even gives you a phono stage. Works well with my Charlize and Super-T. TNT have a good review of it
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Old 19th August 2006, 06:39 AM   #7
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Default Preamp question

Buckapound,

Since no one answered your question I will give it a go. Yes using a good opamp circuit to its fullest potential requires a dual supply. This can be achieved rather painlessly by using a low power DC/DC converter package to power it. You want one with an input spec of 8-16volts and dual isolated, regulated outputs of +/- 12v. This is enough for powering the amp to its maximum with no problem. I personally would recommend the LM6172 chip for the best sound. Offset will not be a problem as you still will need the amps input to be capacitively coupled. Look through some National opamp app notes on how to build a gain of 4 amp circuit. I would not recommend more gain than this unless you plan on using some real low output sources. The DC/DC converters inputs and outputs will need to be filtered as close to the pins as you can get. Use an input cap of 1000uf and output caps of 220uf all @ 16V. I recommend the Panasonic FM series available from Digikey.com They also sell a line of converters.
Someone on the forum might even do a circuit board for you if you ask nicely. It would be nice to have all the parts on one board.
Roger
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Old 19th August 2006, 07:20 PM   #8
Bearman is offline Bearman  United States
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Here is one option for ya. It's a 5.1 channel 12v auto preamp with sub out. I have bought several and have used them on one of my chipamps. They sound pretty good for $38.

http://www.overstock.com/cgi-bin/d2....496501#reviews
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Old 19th August 2006, 07:52 PM   #9
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Not sure if this is any help but you may find something useful here:

http://electronics.dantimax.dk/Kits/index.html

Check out the RemPre board.
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Old 21st August 2006, 10:55 AM   #10
Pano is offline Pano  United States
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My questions would be this:

Why the preamp?
Do you really need the extra gain stage? The SI boards have a higher than unity gain, just right for most low output portable sources.

Your cable run to the amp is short, right? You don't really need a buffer to push the signal down a long cable.

The input stage of the Tripath chip IS an opamp. If your cables are short and gain is enough, why bother with a preamp? You can usually drive the SI into clipping with a portable CD/MP3 player. What more do you need?
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