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PaX amplifier: invitation to discussion

HiFi, LoFi, Tubes, Transistors, ICs, Commercial, DIY

Postby janneman01 » Sat Jun 07, 2008 12:00 am

I'm not sure it is so critical to keep it ultra-short. What IS critical is to connect it at the right point. The top of the feedback divider should be connected as close as possible to the speaker outpout as possible. Remember, it is the top of the feedback divider that will have the cleanest sound, that is where the distortion is nulled out.


Also, connect the bottom of the feedback divider to the same point as the input ground, and to this point all other signal grounds like the load resistor of the AD844. Don't share these tracks with anything else. The star ground for the supply, where the local supply decoupling and the speaker return comes together, should be connected to the signal ground star via a small resistor like 1 ohms. See my layout as one way to do it.

Jan Didden
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Postby janneman01 » Mon Jun 09, 2008 12:00 am

I have changed the protection curve on the XL preadsheet on my website. The amplifier can now deliver a bit more power with complex loads.
If you want to implement this change you need to change one zener diode value (two zeners) and some resistor values.

Details here: http://www.linearaudio.nl/pax-5.htm

Jan Didden
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Postby kioseusznook » Wed Jul 02, 2008 12:00 am

Hi Jan

I too would be interested in adding an XLR balanced input to the amp, mainly because there'll be a big distance between the pre-amp and the PAX (driving surround speakers). What is your recommended design for this?

Thanks
Znook
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Postby janneman01 » Wed Jul 02, 2008 12:00 am

Hi Znook,

There are several ways to make the paX amplifier to accept a balanced input.

The easiest is to use a balanced cable right up to the amp and then use an adapter from XLR to RCA. That way, you will keep the advantages of balanced operation - cancellation of common mode hum and noise - to a great extend.

Second best is using an adapter cable as I describe on my website here: http://www.linearaudio.nl/bal2unbal.htm.

The best in terms of performance would be a balanced cable and then a balanced to single ended conversion circuit either inside the paX box or as an external box close to the amp. This can be based on one of those well-known balanced line receivers from ADI or TI.

My advice would be to check how options 1 or 2 works with some listening tests. Option 3 would be better for common mode supression but does introduce additional active circuitry. If CM noise is not a big problem, you could introduce more problems then you get rid off.

Jan Didden
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Postby kioseusznook » Wed Jul 02, 2008 12:00 am

Jan, thanks for your input.

Yes I've seen those XLR to RCA adapters and they are an option but I intend to make the amp/s the best that I can and would rather have everything 'built in' so to speak.

To that end I'll probably use THAT's 1280s rather than the TI or ADI options as I believe these are a tad better.

Anyways at the moment I'm just researching everything so once again thanks for your valued input.

Znook.
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Postby janneman01 » Thu Jul 03, 2008 12:00 am

Yes the THAT's will certainly work well here. You could use the exisiting supply voltage with a regulator like I use for the AD844's in the voltage amp.

Jan Didden
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Postby kioseusznook » Thu Jul 03, 2008 12:00 am

Jan

I note your post above regarding the changing of some component values with respect to the paX driving complex loads; could you clarify exactly which components these are with regard to the circuit diagrams printed in the Elektor magazines, and also to what value/s they should be changed to. Thanks.

Also regarding L1; the article states 3 layers of 8 turns of 1mm5 magnet wire but at what start diameter?

Other than that everything's looking good (bar that the Sanyo 2SA1208s appear to be getting in short supply...)

Znook
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Postby janneman01 » Fri Jul 04, 2008 12:00 am

Hello Znook,

The changed values can be seen in the spreadsheet if you compare them to the diagram in the article, but I have just now also documented them here:

http://www.linearaudio.nl/SOA-2.htm .

For L1, I use a screwdriver as a core to wind it, and then afterwards slide it off. The inner diameter is about 5mm. This is not critical.

Jan Didden
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Postby hadihf » Sat Jul 05, 2008 12:00 am

Hello Jan,

Can you please point to a good and cheap source of suitable enclosure for pAX.

Regards
Farooq
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Postby janneman01 » Sat Jul 05, 2008 12:00 am

Hi Farook,

Hah! Cheap and good? Difficult .
I am also looking at one, I made my prototype myself but that takes too long and is also not cheap.
I am looking at these:

http://www.hifi2000.it/default.asp?id=104&mnu=104

The pesante' are models with heatsinks. Not really cheap but look very good.

Jan Didden'
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