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mental2k 19th July 2011 11:31 AM

Ambitious Noob looking for advice.
Hi, first off I am a noob, and I realise the best advice for this whole idea may well be "don't do it" but I'm gonna put myself out there anyway. BTW before I start, this post could get long. Also I'll apologise up front for any nonsense I may speak, go easy I've probably just misinterpreted something I've read.

I have a bit of a hankering to build an amp/pre-amp/speakers/entire hifi setup. While I realise this is not a cheap enterprise I do do intend to build it over a period of time and build up an entire system. First off, if possible I would prefer to avoid using kits, I'll get much more satisfaction building the entire thing myself. (Though I don't mind using schematics from others lol).

The source is going to be an audigy2 zs souncard in my HTPC, and eventually I want to build the equipment to amplify all 6 channels of a 5.1 surround setup. However I'd like to start smaller and build a 3 channel amp for 2.1 and then add a second 3 channel amp at a later date for the 3 rear channels.

The idea at the minute is to build 3 gainclone (2xLM1875 + 1xLM3886) circuits connected to a 450VA toroidal transformer. Initially I'd build one of the L/R channels, then the second then the bass, then put in enclosure. So far, so vanilla gainclone.

This is where the tomfoolery comes in. I would like multiple inputs, for 2 iphones, an ipad, tv, and xbox360 as well as the audigy and maybe a spare one, I don't want to have to unplug and replug cables when switching sources as I can often do this several times an hour. A switch to change them is more than tolerable, Is this possible without introducing massive detriment to the amp's quality? Should this be dealt with at the preamp, or maybe even before this?

Some of the sources above are stereo, I do realise this is how they're meant to be, however I was thinking it might be nice to have the option of a virtual 2.1 surround. Again as a switchable option at some point along the line (probably at the same point as the source switch).

The way I envisioned doing this was splitting the L/R channels coming in (so we have 2 left and 2 right channels) putting the first set through a high pass filter and then passing them on to the L/R channels of the amp and let them be amplified there. The second pair are passed through a low pass filter, and then some kind of crossover (I'm a little hazy on this aspect) to combine them into one channel and pass this to the bass channel. Is this just utter stupidity?

Finally is there a good DIY preamp which I could build (and modify if required) that is a gainclone equivalent for preamps?

Thanks for reading if you've got this far, and thanks for any advice you dole out.

sreten 19th July 2011 12:17 PM

1 Attachment(s)

You just need a very simple mixer circuit to combine the low passed 0.1 channels.
(The low pass is the crossover, typically 2nd order L/R, but sometimes 2nd order
L/R on the high pass to small speakers with their roll-off combines best with using
4th order L/R for the low pass, other 2.1 alignments are possible.)

Input switching can be a multipole switch, controlled relays, or a switching chip.

You really need to research pre amp chips, I'm sure there is something to suit.

Though I would tend to buy something to suit and than turbocharge it.

rgds, sreten.

How to avoid electrolytics in the signal path below :

mental2k 19th July 2011 12:45 PM

Hi Sreten, and thanks for some advice on the 2.1 channel. I am a noob so I'm still getting down with the lingo. By electrolytics you mean electrolytic capacitors? Also what do you mean by 2nd and 4th order L/R?

Since I posted that I've had the idea of having an input stage, have this as a separate module which takes all the inputs and just 1 bank of outputs, possibly do all the filtering and mixing at this point. Then pass the outputs to the preamp, and then on to the amp, would this be a logical method of construction? This would allow me to build the amp first and use it without preamp and switching inputs then improve the entire setup as I go along. The whole thing is very conceptual at the moment, though I am learning as I go and enjoying it too :-)

Also can anyone recommend a good glossary of terms and basic reading that I should endeavor upon?

sreten 19th July 2011 01:48 PM

Elliott Sound Products - The Audio Pages (Main Index)


The above is as good a place to start as any.

By electrolytics I did mean electrolytic capacitors.

Building a preamp is a major design task, way beyond building a gain clone.

Throw in a SD card reader, USB stick reader and DAC inputs and it gets
overwhelming, especially as you ideally want full multichannel decoding.

I'd seriously consider the used market for an AV preamplifier.

rgds, sreten.

mental2k 19th July 2011 02:07 PM

You might be on to something there sreten, buying second hand, although my HTPC can deal with the SD card and USB, and I'm planning on using the analogue outputs on the Audigy card for the 5.1 surround, and all the other inputs are planned as analogue. Although the whole thing is organic at the moment and as with all my projects is likely to morph over time.

PS.- Thanks for the link, it is a great looking resource. I enjoy learning about this stuff as much as actually doing it.

eccdbb 22nd July 2011 04:47 AM

Specify your filtering goals more clearly. Filtering is a whole monster topic on its own.

Perhaps you should build in modules- mixer/pre-amp, filter, power amp. Build the entire 1ch first. If it works well, duplicate.

You need to specify your desired power output (into 4 or 8ohms). Then you can work backwards and figure out what power supply is required.

Oh and noob x giant transformer...I hope you know about safety.

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