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Old 5th April 2004, 12:47 AM   #1
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Default best topo for 25W class A into 1 ohm resistive load?

What is the best sounding circuit topology to drive a 1 ohm pure resistive load with 25-35 watts of class A power?

The "first watt" is the most important.
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Old 5th April 2004, 01:39 AM   #2
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35w into 1ohm?

just think about the heat on the output devices,
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Old 5th April 2004, 02:16 AM   #3
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Default Re: best topo for 25W class A into 1 ohm resistive load?

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Originally posted by LineSource
What is the best sounding circuit topology to drive a 1 ohm pure resistive load with 25-35 watts of class A power?

The "first watt" is the most important.
This shouldn't be too bad of a problem, and just the sort of thing for a DIY project. (There aren't really any commercial products that will do what you want.)

Let's start with 25 watts, and assume a complementary output stage. This will require 5 A rms, or 7.1 amps peak. Therefore the output stage will need to idle at 3.55 amps in order to maintain class A to full output.

Now this is a fair amount of current, but on the other hand you will only need to swing +/- 7.1 volts peak-to-peak. So let's be generous and use +/-10 volt rails in the output stage. That means the total dissipation in the output stage will only be 71 watts. This is easily handled with a moderately big heatsink.

The latest edition of Self's book will have a lot of useful information for you. I would suggest at least 4 pairs of what he calls "sustained beta" devices such as the MJL1302/MJL3281. You'll want to use very low values of emitter resistors when driving this low impedance load. I would suggest 0.1 ohms if the devices are unmatched but come from the same date code, or else 0.05 ohms if you can match them carefully. They are pretty cheap to purchase ($2 or $3 each), so it's not too painful to buy extras for matching.

Good luck!
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Old 5th April 2004, 09:36 PM   #4
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Default MOSFET OUTPUTs

Are MOSFETs unsuitable or less desirable than bipolars for outputs driving a 1 ohm load?

JFETs are typically used for input stages to get the lowest noise. What JFET input (and voltage driver) topologies are best? Which can drive bipolar outputs?
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Old 5th April 2004, 10:09 PM   #5
sreten is offline sreten  United Kingdom
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Mosfets are poor devices for driving low impedance
loads, unless you have loads of them in parallel.

Gate voltage requirements in this case would change
the needed voltage rails from 10-0-10V to 14-0-14V
just for the same output voltage swing into a normal
load, adding the fact you need more gate voltage for
high current your heading towards 18-0-18v or more.

BJT's are the only sensible option here, a Mosfet output
stage would have at least 3 x the static dissapation,
i.e. > 200w for 25W output.

sreten.
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Old 5th April 2004, 10:18 PM   #6
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Default Re: best topo for 25W class A into 1 ohm resistive load?

Quote:
Originally posted by LineSource
What is the best sounding circuit topology to drive a 1 ohm pure resistive load with 25-35 watts of class A power?
may i ask why do u wanna do that?
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Old 5th April 2004, 10:27 PM   #7
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Hi Linesource,

A 1 ohm load ? and class-A ?

First thought is a matching transformer.

Second thought, I agree with Millwood, the heat in the output devices will be high, even with paralleled devices.

Bipolar transistors need a headroom voltage to operate correctly at high audio frequencies, generally 5V minimum across each half, so you are immediately thinking about Vp-p plus 10V at whatever quiescent current is necessary.

The quiescent current also needs to be more than half of the peak current for proper high audio frequency linearity, this is because of unavoidable gain droop at the high class-A operating temperature with low Vce at maximum output amplitude.
I-q often needs to equal I-p !

So for 25W I am suggesting you'll need 25V @ 7A = 175W, which means a double parallel push-pull output stage, using the devices that Charles suggests and massive heatsinks. The voltage is low so SOAR limitations would not be a problem.

Topology, take your pick.

If your design outputs more than 25W then you won't grumble, and 40W might be possible at low frequencies.

What about the never-off class-B biasing suggested elsewhere, in diy.audio. Maybe by John. Basically this is class-B operation but no crossover switching distortion because neither device actually turns fully off.

Maybe Mosfets would do better, I do not have experience here.

Cheers ........ Graham.
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Old 5th April 2004, 10:38 PM   #8
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by Graham Maynard
Bipolar transistors need a headroom voltage to operate correctly at high audio frequencies, generally 5V minimum across each half, so you are immediately thinking about Vp-p plus 10V at whatever quiescent current is necessary.


So for 25W I am suggesting you'll need 25V @ 7A = 175W, which means a double parallel push-pull output stage, using the devices that Charles suggests and massive heatsinks. The voltage is low so SOAR limitations would not be a problem.


Maybe Mosfets would do better, I do not have experience here.

Cheers ........ Graham.
Well I'd suggest your 5V requirement per BJT is garbage,
and contrary to the laws of semiconductor physics,

sreten.
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Old 6th April 2004, 12:55 AM   #9
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Default INPUT and DRIVE STAGEs

Any better output solution than this?
Output Power Supply +/- 12V - 15V transformers that can deliver at least 15 amps on each leg, 750-1,000VA.
Outputs 6 pairs of the MJL1302/MJL3281 with about 0.2 ohm emitter resistors...Buy from ON semi...(Motorola)
Large 0.35 C/watt heatsink for output transistors and thermal compensation components.

Now...any suggestions or pointers to schematics for a very low noise input and voltage drive stage? Inputs JFETs? Low noise bipolar? The driver power supply will be regulated, but I think current sources sound better than resistor loads. I'll add an opamp to null the DC offset.


Graham...I have found that step-up transformers shift the phase and move the soundstage around. I currently have a big Krell, but I can hear noise in the first 1-2 watts, the most important region. I can hear the ocean waves breaking..........unless I pad it up to 2 ohms.
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Old 6th April 2004, 04:58 AM   #10
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I see no reason to go past +/- 10 volt for the output stage rails. Of course, this means that the transformer only needs to be around 14 VCT. This is kind of an odd value, it will be much easier to find 12.6 VCT, which will give rails around +/- 8 volts. This will still give the 25 watts you are looking for.

As far as current rating on the transformer, you only need 7 amps per channel as a minimum. Remember the circuit idles at 3.5 amps per channel, the only reason you need 7 amp transformers is because a capacitor input power supply is only about 50% efficient in terms of transformer utilization. But ths means that for a 12.6 volt center-tap transformer you only need 12.6 x 7 = 88 VA, which is 10x less than you were speculating! Going with more current won't hurt, but it will be pretty hard to find something much beyond 10 - 15 amps. The part you were suggesting would have a secondary rating somewhere between 50 - 100 amps. You might find something like that in an arc-welder!

Six pairs of outputs should be enough. I would use a smaller emitter resistor than 0.2, but that's a relatively small detail.

As far as schematics, I would suggest grafting the output stage of the Leach amp (triple emitter follower):

http://users.ece.gatech.edu/~mleach/lowtim/

on to the front end that Jam has successfully built:

Amplifier Topologies

In fact for that application, you won't even need that much gain. Instead of the normal 20x used for power amps, you probably only need 3x or so. That way it will clip with around 1.5 Vrms input.

Good luck!
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