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Helix 21st July 2002 02:47 PM

300watt in to 8ohm amps
i have a 8 ohm sub and i am trying to find some schematics and plans for amps that will produce 350 watt in to this 8ohm speaker.

What amps are there to build?

Super leech ? i have seen leachs site, it seems quite complex amp but people say that it is very good.

something of holdens? these are genally classic AB type designs that are very proven

how many output transistors will i need on these 'cos it will be about 700 watts in to 4 ohms and almost a 1.2kw at lower impedence (but i don't plan to drive less then 4 ohms)

I guess i need rails at around 75volt or more

what else is there ?

Seeeing as at the moment i olny have 1 sub it might be worth making a over-engineered monoblock, rather then trying to squeeze a dual channel in to a case (and my budget :))

any comments are most welcome!

JonMarsh 21st July 2002 03:01 PM


Another tack you might take is to build a balanced bridge amp. If you're only driving a single or dual sub drivers, you can take a circuit with supply voltages that's more appropriate for 150-200 watts at 8 ohms, beef up the output stage and power supply, and in balanced bridge you'll see very high output power with more manageable stress on the output transistors. Remember, bipolar output transistors handle current pretty well, but because of second breakdown, don't like high voltage (over 50V or so) all that much. ;)

As an example, consider the Aragon Palladium, which is basically an 8008BB with the inputs wired to a single XLR (for the standard version) or to a bridging differential amp (for the single ended version) (For a simple very high grade bridging op amp, see the TI Differential Op Amps, like the THS4131, which is very low noise and distortion with true differential outputs.)

The Palladium, while biased to operate class A to 125 watts, clips at about 600 watts at 8 ohms and nearly 1000 watts at four. Your requirements are less demanding, but even with 60-65 volts on the rails, fairly high power can be managed with a balanced bridge circuit. There's lots of designs to borrow and build on in that voltage range.



Helix 21st July 2002 03:26 PM

i had concidered this but i thought, once i made two of whatever amp i choose, i will then bridge those

Helix 22nd July 2002 10:27 PM

OK i have decided what to do.

I think i will build the AV-800 watt am of athony holden


I think i will run the rails at around 80volts ish (depends on transformer) and use a 1Kva tranny and twin rects on each amp.
With twin power supplys do they ALL share on star ground? i guess so

a few qestions i am currently thinking is....

two 0.5k/w heatsinks bolted together make a 0.25 k/w heatsink, correct or not? this should be enough i think at 80 volt rails especially once the heatsinks are attached to a big case.

The diagram has two diodes 1N4936 on the output and says they are optional. I guess the are there to catch back EMFs? and what are there pros and cons? whats wrong with the diodes built in to the FET's?

Hopefully i should fit two including there trannies in a 3U case, with about 40,000uf per amp (i shoud really put on more i guess).

I could bridge them (8 ohm load only) for 700 watts in to 8 ohm.

It says the bandwidth is 10hz upwards, is this OK for big subwoofer use. I think so

The inputs are balanced, to connected unbalanced input i just connect the signal ('hot') to the non-inverting and the ground of the RCA to the inverting input correct or not?

Is this thing stable? i don't want to see the biggest most expensive flash of light (second only to edinburgh newyear fireworks :)).

Protection - I will fit a couple of relays on the output. It should have DC protection, Temp protection and soa protection (for soa, is it ok to detect the voltage across one of the 0.22 source resistors?)

I don't like the idea of rail fuses, cos if one goes for no good reason, the thing will latch to a rail then the output stage blows up :rolleyes: . What about inrush limit the mains then put a smallish mains fuse, so at least the house won't burn down.

I have seen thePCB design on the website and i don't like it, but i appreciate is very difficalt to do complex output stagesonly only using one plane (single sided). My not liking is about the FET outputs/speaker connection/feedback track. Actually i can't see the output for the speaker but the feedback is at the bottom, i always thought, from D Self's advice that it is important to get the feedback from where the N and P channels join and go off to the speaker because of resistive losses in the tracks and hence not fully balanced feedback.

any other high power amp circuits i should think of?

Any help would be really cool, oh and don't worry i will not concider the 'transformer-less' idea:p

i think i need many ppl's 2cents;)

Nelson Pass 22nd July 2002 11:16 PM

Go out and buy a DVD copy of "Attack of the Giant Leaches",
a Corman classic and an absolute must when plannng the
size of your amplifier.

The Saint 22nd July 2002 11:20 PM

The Design of the AV800 PCB takes into account all of Douglas Self's Suggestions on PCB design.
I have read his book as well :)

This amplifier is very stable and delivers a very clean and detailed sound stage.
The design also lends it self well to bridging. With the right PSU your can expect up to 4 Times the power O/P at 8 Ohms.

Quite a few people now from all over the world have built this amplifier with great success and they are very happy with it...

Anthony Holton :)

NU_NRG 23rd July 2002 01:29 AM

The Saint- What would you call a decent PSU for the AV-800 ??


The Saint 23rd July 2002 01:45 AM

It Really depends on what power o/p you require.

But for 800 watts into 4 Ohms, Single channel. 1.2 KVA transformer min is required. for 2 channels 2KVA works well.
With the 2KVA Over 1600 Watts RMS into 4 Ohms can be achieved in bridge mode.

The DC filtering, I would recommend as much as you can afford.
However as little as 10,000uf per voltage rail has been used in some tests.
But as a starting point I would say 40,000uf per rail.

Higher power levels could be achieved by increasing the size of the power transformer....

I hope this helps...


Anthony Holton

NU_NRG 23rd July 2002 07:42 PM

Sure does mr.Holton
thank you for the info.
holy...cow 1600W... now if only I had drivers that could handle this much power :)

Samuel Jayaraj 24th July 2002 08:15 AM

So this amp can deliver into 2.6 ohms (3 x 8 ohm in parallel)! Can it be used in this mode to deliver continuous high power in PA applications?

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