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Old 7th November 2006, 04:22 PM   #1
Alcaid is offline Alcaid  Norway
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Default 2x44VAC 300VA: What sub amp to build?

Hi all,

Previously I have built several gainclones, active filters, headphone amps ++ (no kits, from scratch incl. PCBs) but now I feel like building something more complex.

Would like to build my first solid state amp, and was thinking of a subwoofer amp.

Got this nice toroid (2x44VAC 300VA) doing nothing, so any designs recommended? The voltage vs. VA rating would suggest a MOSFET design wouldn't it?

Was thinking of the ESP P101, but please come with suggestions.
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Old 7th November 2006, 04:29 PM   #2
Pasi P is offline Pasi P  Finland
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Gregs GB300D can be one good one.

http://www.diyhifi.org/amplifierguru...u/guru_003.htm
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Old 7th November 2006, 04:48 PM   #3
Alcaid is offline Alcaid  Norway
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Quote:
Originally posted by Pasi P
Gregs GB300D can be one good one.

http://www.diyhifi.org/amplifierguru...u/guru_003.htm

I've looked at those for my main system, but isn't the BG300D overkill for a sub amp? The 300VA toroid I already have would propably be underrated, wouldn't it?
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Old 8th November 2006, 10:44 AM   #4
AndrewT is offline AndrewT  Scotland
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Hi Alcaid,
The Vac determines the PSU supply rail voltage.
You will have about +-64Vdc.

That voltage then determines the output into a specified load.

Let's try 8ohm as a starter.
Assume for the moment that the AC output voltage will be about 55Vpk (=39Vac) then the power into 8r is about 190W.

If you apply the usual recommendation for transformewr rating then VA = 1.5times maximum output power.
The 300VA is just about ideal for an 8ohm load. But not for anything lower!!!!!
You will compromise performance if you use 6ohm or 4ohm loading.

On that basis you are looking for amp kit or module or schematic that suits +-64Vdc supplies when driving an 8ohm load.

I think that both P101 and GB300 fall into this category.
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Old 8th November 2006, 11:11 AM   #5
paulb is offline paulb  Canada
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Quote:
Originally posted by Pasi P
Gregs GB300D can be one good one.

http://www.diyhifi.org/amplifierguru...u/guru_003.htm
The link is wrong. Try this one:
http://www.diyhifi.org/amplifierguru/guru_003.htm
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Old 8th November 2006, 08:41 PM   #6
Alcaid is offline Alcaid  Norway
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Quote:
Originally posted by AndrewT
Hi Alcaid,
The Vac determines the PSU supply rail voltage.
You will have about +-64Vdc.

That voltage then determines the output into a specified load.

Let's try 8ohm as a starter.
Assume for the moment that the AC output voltage will be about 55Vpk (=39Vac) then the power into 8r is about 190W.

If you apply the usual recommendation for transformewr rating then VA = 1.5times maximum output power.
The 300VA is just about ideal for an 8ohm load. But not for anything lower!!!!!
You will compromise performance if you use 6ohm or 4ohm loading.

On that basis you are looking for amp kit or module or schematic that suits +-64Vdc supplies when driving an 8ohm load.

I think that both P101 and GB300 fall into this category.
Ok, I said I was new to SS, not new to diy hifi and the easy calculations needed (Ohm's Law etc. I'm not an EE, but I have an Engineering degree )

Wouldn't 2x44vac equal +/-60V DC after rectification and voltage loss in rectifying diodes (with load)?

I was aiming for a 200W RMS design, subwoofer will be chosen to suit impedance wize. I agree that if voltage drop in output stage isn't too high, it would be called for an 8ohm driver. The P101 design has a 12V voltage drop over the output Mosfets. Also counting in some voltage sagging on the power rails at heavy loads, would't that count more in the direction of a 4 ohm driver?
((60V-12V-5V)^2 / (4 ohm * 2) = 230 W RMS

- Am I way off here?
- Is the estimated rail voltage drop of 5V over/underestimated? (stiff unregulated powersupply is planned)
(This calls for simulations with PSUD2 )

The GB300 doesn't drop that much voltage over the output Mosfets, so that is a sure shot for 8 ohm drivers.

Thank you AndrewT for writing a thorough reply
Any one with any Class D designs?
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Old 8th November 2006, 09:18 PM   #7
Pasi P is offline Pasi P  Finland
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UCD is great sub (or woofer) amp. I use 4ch UCD180AD amp for my Orions.
(Mid and treble amps are Greg's GB150D:s. Great combination IMHO )
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Old 8th November 2006, 09:21 PM   #8
Alcaid is offline Alcaid  Norway
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Quote:
Originally posted by Pasi P
UCD is great sub (or woofer) amp. I use 4ch UCD180AD amp for my Orions.
(Mid and treble amps are Greg's GB150D:s. Great combination IMHO )
UCD amps are not DIY in my head. It's plug and play
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Old 9th November 2006, 07:26 AM   #9
AndrewT is offline AndrewT  Scotland
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Hi,
do not even think about 4ohm loading with a 300VA 44-0-44 transformer.

I arrived at the PSU voltage by adding in most of the regulation that will apply to that size of transformer.

The regulation is the percentage rise in voltage from the transformer when the loading is reduced from the rated output current to effectively open circuit (infinite impedance=zero current).

A 300VA will have 5% to 7% regulation.

1.04 * 44 * sqroot(2) =64.7Vdc This could be +-1V dependant on winding accuracy.
Take off one diode drop when running quiescent current and you are left with 64.0Vdc. When on heavy load the diode drop will be nearer 1V and the resistive losses will bring the voltage down even further.

Yes the rails sag and they have very significant ripple on them when driving hard and there are semiconductor losses and then the resistive losses in all the cabling and the emitter resistors.

For a GOOD bass amp, all these losses should be minimised (within economic reason) to allow sustained current to get from the PSU to the Voice Coil.

BTW. if you allow for mains supply coming in at maximum tolerance (+6% in the UK) then 44Vac usually require 75Vdc capacitors. Whereas 40Vac can get away with 63Vdc smoothing caps.
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Old 9th November 2006, 06:22 PM   #10
Alcaid is offline Alcaid  Norway
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I will use 80V caps, no problem there. Have a bunch of Cornell Dubilier 10.000uF snap-ins.

If 8ohm driver is called for then I guess the GB300D would be a pretty good high-end choice.

The P101 would give a lower power output due to the 12 Vds of the output mosfets... Power is always nice for a subamp
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