Battery vs Conventional PSUs

[Gopher]

I've been contributing to a discussion of the use of battery power supplies for preamps in another thread but I'd like to start a wider discussion here about the pros and cons of battery vs conventional capacitor smoothed & regulated active PSUs.

My understanding is that batteries are almost ideal low noise DC supplies when idle but are slower than active PSUs and produce higher levels of transient noise when supplying real life current because of the chemical reaction times and processes involved. Their output impedance may also be relatively high for some battery types compared to active PSUs.

It also seems to me that the major problem with conventional active power supplies is the wideband RFI generated by the sharp current pulses and diode switching noise of the capacitor smoothed diode bridge circuit which can be transmitted to sensitive preamp circuits via wiring and by radiating. Batteries do not suffer from this problem.

Is not the simple answer to combine the best of both types and regulate battery power supplies with high performance low noise regulators?

Gopher

[AndrewT]

Hi Gopher,
glad to see someone trying to reason with the pros and cons.
That other guy seems to be blinkered (in all his threads).

I don't know where to find it, but there is a very full expose on regulated supply noise (and impedance?) and the site includes comparison with various battery types. A search might find it. It is referenced in this Forum.

[Gopher]

Andrew

The link is

http://www.tnt-audio.com/clinica/reg..._noise1_e.html

and subsequent parts of the article.

My point with the regulated battery PSU is that the battery effectively replaces the mains, transformer and diodes with the advantage of almost zero RFI while the regulator supplies the stable, low noise, low impedance power that the battery can't on its own.

I don't think I've ever seen battery supplies regulated anywhere before. Has anyone else?

[AndrewT]

Hi,
you might be on to something,
now the question becomes rechargable or throwaway?

[owen]

Please dont entertain the idea of regulating battery supplies - they limit the current available - and sound 'squashed' as a result.

Firstly, a battery supply needs to be designed in the same manner to a regular PSU. To inhibit chemical noise, I strongly recommend the use of a choke in each 'leg' of the supply (one for + and -, and a third on the Gnd leg - it doesnt need to be huge in value (0.5mH is more than enough), but it will work a treat.

Secondly, use a bank of small caps, as if you were using a normal AC supply. This will provide an initial reserve of current to 'cover' the slight lag in the chemistry.

Done properly, a battery supply is utterly silent. Sometimes they can sound 'slow' - in the main, but from experience, that is because there is no additional unwanted noise between each single note - the silences are more apparent, and so sound longer.

In terms of current delivery they have huge ability to deliver without the strangulation of regulators, from a very,very low source impedance at low frequency. This rises slightly because of the chemistry at higher frequencies, and that is why you need to design the PSU to improve current delivery in those circumstances, but current demand drops rapidly, so its influence is less anyway

However, this is the exact opposite of an AC supply, as the source impedance rises dramtically as you approach DC (to the resistance of the transformer windings) that you ameliorate using all sorts of band aids. This does however, mean that at high frequency, the impedance is low, and the coupling (how well a transformer works) is very very good - ideal circumstances for propagation of RF from the main supply itself.

Using a battery supply, a humble LF353 opamp, and 6 or so other parts, I built a preamp that slaughtered a numbver of very, very expensive comercial pre-amps (my Meridian included). It had imaging to die for, huge dynamics (and I mean HUGE), and a lovely relaxed 'listen to me' manner.

I liked it so much, I will be building another when time permits.

Owen

[Gopher]

Owen

I was going to suggest filtering the battery output with C-R-C network using large electrolytics in the same way as a conventional PSU. The more chemical noise can be removed the better.

As I see it, the overriding reason for using a battery to replace the mains, transformer and diode bridges is that they won't have the broadband RFI out to several MHz that conventional active supplies have.

Others have reported opposite results to your experience with the sonics of battery PSUs, namely they sound good initially but ultimately lack the dynamics of active supplies.

Whether regulators following a filtered battery will give the best of both worlds is surely something worth trying.

Gopher

[AndrewT]

Hi Owen,
thanks for sharing that experience.

If you were to start building this ideal (battery/choke/cap) supply, what would be your specification/component values?

[owen]

Ordinary supplies can sound more dynamic - and I think thats an artifice created by the crud floating on the supply, and poor implementation.

Nail an ordinary supply (without regs) to kill the RF, and have a battery supply (properly designed) to compare to, and they sound very, very alike.

All I have are rules of thumb - upto 0.5mH choke (speaker supply house, air core, high current, must have a low DCR) is fine, and I use between 5 and 10k uF of capacitance - either as banks of 100uF caps or bypassed 1000uF caps.

If you were going to be bold, a large supply could be realistically built with polypropylene caps (and audiofool grade if you so wish). The Battery needs to be a cyclic duty type - as you'll be using it, and then recharging, not topping it up as you go along. These however are relatively easy to find.

One of the best bypassing tips I was given by Nelson Pass - put a bypass cap between the + and -rail, as well as betwen each rail and ground.

Definitely a case of 'try it and see', but in my view, you will be astounded by the results, even with low budget parts.

Have fun, and report back




Owen

[richie00boy]

It is simply not worth regulating a battery, as the main shortcoming of the battery supply - that of high impedance and thus poor burst current delivery, is only worsened by placing a regulator after it.

Also, to cope with the other shortcoming and reject the noise, a regulator would have to be extremely fast.

I think the way forward would be to simply tag a conventional regulated supply for AC onto a battery - bridge and smoothing caps and all - and let the caps do the work for low impedance drive and also some noise reduction as well.

[owen]

Rechargable Batteries (unless they are *disposable* carbon types) do not have a high output impedance - infact it is the other way around, by usually an order of magnitude or more.

A rechargable battery can deliver a burst current that is huge (a 1.3V NiMH 1300 mAH can deliver > 3 amps as a burst), and once again, it is the other way around. The impedance of a standard transformer limits the current deliverable, as a product of the magnetic flux capability of the core.

Both of these facts are grounded in simple physics - the burst current capability of a large SLA battery is high enough that a spanner isnt a big enough fuse (yes, the spanner will explode apart!). Not to be recommended, as the battery usually bursts too due to shock. .. electrolyte everywhere!!!!!

Hybrid supplies can work, but there is something quite magical about a good pure battery supply implementation.


Owen