One for ALW? +-12v preamp PSU

[RichardH]

Andy (and anyone else who can offer advice!!), been musing over a power supply for the Hart headamp I'm building, and had intended to build a 3x7 based jobbie. I'll be using the 8620 I have fitted to the Brown Dog (always get Led Zep's "Black Dog" thundering through the brain when I type that), so want to get +-12v.

The thought occurred to me however, that it might be possible to use a single superreg (in fact a standard 24v hicap one) and a rail splitter. Would this work, and be better than the 3x7 approach, or would the rail splitter negate the SR's superpowers?


Richard

 

[Andrew L Weekes]

It's certainly possible to use a rail splitter, but it has some disadvantages.

There's two ways to do it, one is to just add some DC bias at the op-amp input, taking the output (via a DC blocking capacitor) between ouput and ground.

The above is in effect what a headline does, so it can sound good, but personally a big electrolytic in the signal path is not my preferred choice. You'd have to have an input DC block too, as there would, byu neccesity be DC at the amplifier input.

Alternatively one generates a stiff 1/2 supply rail voltage using an active system, and take the o/p of the amp between the 1/2 rail and o/p.

This scheme requires that the 1/2 supply rail is generated by a wide-bandwidth, low impedance circuit, which tends to point towards power op-amps or a system not unlike a super-reg.

On that basis you may be better using two reg's!

I'd pick the LM317 / 337 route initially, it's simple, cheap and as John's found out sounds very good, compared to 78xx / 79xx reg's.

It's all I'm using in my headphone amp - it sounds so good, I've not been tempted to change it yet!

Andy.

 

[RichardH]

Thanks for that Andy - sounds sensible. Just been going through the parts bin, and have all I need bar the caps for the 3x7 route.

Am I best to follow the plain "Adjustable reg with improved ripple rejection" diag from the datasheet? Obviously I need some smoothing caps upstream of this - I'll probably just use Panasonic FCs there, as they're available from Farnell (who have a much more enlightened delivery charge scheme than RS!)

Any suggestions on caps to suit for the 10uF and 1uF positions? I have a feeling that Oscons might be OK for the 1uF position (or am I talking guff again?)

 

[Andrew L Weekes]

Richard,

FC's should be good for the input / smoothing side. I used Rubycon ZA for the input of John's, but large values are prohibitive.

I think Oscon could be good for the adj pin one, but not totally sure, but the output wants to be a more 'standard' component i.e. not low ESR.

Tant's are a good choice here, as could the solid aluminium types Les likes. Even bog-standard 'lytics work well. You want moderate ESR with low ESL.

I use Elna ROD, but these are RS only

Andy.