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New pair of power amps for the my subs

Posted 19th March 2011 at 08:48 AM by googlyone

In the process of upgrading the subs in the play room. The aim was to deliver in the 1kW region into 8 Ohms. Discretion (and common sense - if that ever entered the equation) led me to shoot for something closer to 650 watts (measued 73vrms, or 666 watts on a sine wave into an 8 ohm load, which brought a wry smile to my face).

I chose to use a bridged configuration, as this allowed the use of supply rails at sensible levels (+/-65V) odd. It also allowed me to use a heap of capacitors that I had laying around.

The transformers are ANTEK units, which much to my disgust cost less to buy and ship from the US than to buy locally made. (Sorry about being parochial, but shipping a two 20lb lumps of steel to Australia is expensive - what is with our local businesses???)

No surprise that I was worried about:
- Power supply - needs to be big
- Cooling - an awful lot of heat to get rid of
- Safety - don't want to be melting my speakers, myself, wiring fuses etc
- Weight and volume

The power supply uses a 1KVA torroid in each amplifier, and 72000uF supply rail caps in each amplifier.

Something that does not get a lot of attention in discussions is the ripple current rating of bulk storage in higher powered amplifiers. I think that people get away with ignoring this because most music is not a sinewave, and consequently while there are some significant current peaks, the average current a supply rail delivers is 10-20dB down on the peaks.

Given these amps are going into an active XO system, which at times has had me drive it pretty damn hard, I did want to be quite sure that contiuous output would not exceed the ripple rating of the caps. Thus there are a few of them.

A positive side effect is that the rail droop, even on continuous output is quite moderate. Putting 75VRMS into an 8 ohm load saw the rails droop from +/-66V to +/-59V.

Gotta say that the dummy load was toasty warm - I had to build my own "room heater" dummy load to test the amp!

I love a big heatsink. I have a stack of power amps, but to dissipate the sort of power that these ones were going to generate needed fored air cooling. I aproahed this by using a fan driving air into the case, with a plenum formed by the heatsinks running down ach side of the case, with air exhausted our of each side of the front panel.

This has proven very effective.

For protection (read safety) I am atually not that fussed about melting speakers due to power - it would just give me an excuse to build some more! And experience has shown me that the speakers I have will likely take this in their stride on music. That said the amplifier includes:
- Slow start
- DC protection
- turn on thump delay, fast disconnect at turnoff (a thump from this amp would be nasty!)
- Thermal control of the fans
- Over temperature cutout of the speaker (80 degc)
- Over temp cutout just above 80degc for the mains power
- Amplifiers held of from turning on while slow start is operating

The amplifier architecture is very conventional - and as described in detail by D Self.

If you have any interest in thermal control, schematics, PCBs etc just ask.

Well - there you go for now.
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  1. Old Comment
    wintermute's Avatar
    Have you tried Harbuch transformers at Hornsby NSW? They make antrim transformers under licence, and the quality is very good. I purchased a 300VA from them in 2005, Id be surprised if it would cost more to buy from them than the cost of buying from the US once shipping came into it but I could of course be wrong! Monster amps by the way

    Posted 22nd March 2011 at 06:20 AM by wintermute wintermute is offline
  2. Old Comment

    Re Harbuch transformers

    Yep - have used them in the past. Their transformers have been good. The quote for 1KVA toroid from Harbuch was twice the cost of sourcing one from the USA. I spent several hours storming around the house cursing - before buying from overseas - as I do prefer to support local industry. That said, local industry must be competitive.
    Posted 30th March 2011 at 08:46 AM by googlyone googlyone is offline
  3. Old Comment
    wintermute's Avatar
    That much! I guess since the aussie dollar is at parity US stuff is looking really cheap now! Probably a whole bunch of stuff at work, but one thing that really affects our prices down under is the lack of volume. Our population just isn't big enough to get the economies of scale that a bigger market allows

    Posted 31st March 2011 at 08:33 AM by wintermute wintermute is offline

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