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Let's say 0.3R output impedance and 1.5A bias ?
You can just do a normal Feucht source follower, using 300x 2SK209GR in parallel.
Add another one as current source, you can use split rails and no coupling caps.
Noise will be ridiculously low.
And you can get away with 25V rails with some simple heat sinking.
You can even do push-pull with either a White follower, or a Taylor current source.
Then Zout will even be lower.

Crss is 1nF, so you need a low impedance source.
You you will need 1200 of them (Beast of a Thoussand FETs).
Plenty in stock at Mouser.
2SK209-GR(TE85L,F) Toshiba | Mouser

:)


Patrick
 

PRR

Member
Joined 2003
Paid Member
...the preamp needs to put close to 3V out to have 1W in 8 ohm......

Most analog sources will make over 3V without much strain. PAS-3 will make 3V (in 100k!) at <0.1% IM distortion. PAT-4 made 10V, 5V into 600r.

If you have a lame 2V DAC and must have gain, try another topology.
 

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Most analog sources will make over 3V without much strain. PAS-3 will make 3V (in 100k!) at <0.1% IM distortion. PAT-4 made 10V, 5V into 600r.

If you have a lame 2V DAC and must have gain, try another topology.

Ok.....I will build the MoFo and also a preamp with some voltage gain. The question was just a part of my own education as I original had the impression that with 2V in the amp would deliver 1W in 8 ohm...….then I concluded that the amp had a tiny bit of voltage gain (x1.4) or so. But that was not the case :)
Now this is called a 1W amp......is there a true definition? …..what makes this a 1W amp and not a 0.5W amp or 2W amp? …...is that the distortion at a specific Watts out at a specific load? ...or is it what it can deliver just before it clips at a specific rail voltage in a specific load? or is it more "fuzzy" ….that you say that this kind of design only makes sense at about 1W out in 8 ohm?
 

PRR

Member
Joined 2003
Paid Member
…..what makes this a 1W amp and not a 0.5W amp or 2W amp? …...

"Whatever the Marketing Dept wants to print".

And maybe some design effort to back-up the published numbers.

In this case the "customer" asked for "a simple 1 watt amplifier". Would the designer get shot for delivering a 1/2W? Well, if the mis-fire became public he might get less future work. Would he be tarred-and-feathered for delivering 1.5W? Only in a highly cost-competitive market where pennies matter (which is most audio, but not hobby DIY audio). And over-design is its own punishment: if the "1W" making 1.5W is $1 more than the competitor's "1W" then sales fall off, the company goes broke, the designer is out of a job.

OK, this job is not heavy commercial. What did Mr Pass propose (GIVE us)? In my sim it clips at +6.3V/-5.9V. This is actually 2 Watts! What an over-powered monster! However it is showing 2% THD (and visible flattening). At 4.9V peak 1.5W I sim a fine Sine and 1%THD with falling spectrum. At 4.1V peak, 1W, my sim says 0.8% THD. (My simmed FET is old-old, Pass probably has better parts in sim and in drawers). At 1.64V peak, 0.16W, I sim 0.3%THD, so THD is not falling fast. (Seen the other way, there is no sharp rise of THD, another important aspect.)

6Vpk 2W 2% THD
4.9Vpk 1.5W 1%THD
4.1Vpk 1W 0.8%THD
1.64Vpk 0.16W 0.3%THD

The %THD is not really as important as the distortion shape/spectrum. 0.1% of 13th harmonic will drive you nuts, while 1%-2% of 2nd harmonic will pass un-noticed, or add a slight touch of sugar. But in this simple amp with simple distortions, you can almost pick your "output rating" anywhere from part-Watt to couple-Watt. Since the design goal was "1 watt", the designer merely claims to "meet spec".

It seems reasonable to suspect that, just foolin'-around (with deep experience), Mr Pass hit >1W at <1% and called it "soup".

Now if the customer had *also* specified "0.01%THD", then we would have to reduce drive to the flea-Watt level to make this spec, so the design is probably "failed". (I sim 0.4mW at 0.06%, tho I may be into the sim's numeric noise.)

The fact that there is 2W near-clean and 4W gross-distortion would matter if over-power would twist a drive-shaft or start a fire. But many machines "can" deliver more than rated power, with some sacrifice of other specs. A "100HP" Diesel can be tuned for 120HP, but with dense black smoke. A "1HP" saw will deliver 1.5HP to a knot for a few seconds, though not all-day without burning windings or fuse. A "100W @ 0.1%THD" power amp will usually make 140+W at higher THD.

However audio needs are NOT so precise. It isn't like a need to pump 10GPH of water up 100 feet, a clear and easily calculable power-goal with not much user input. Soundhappy posted a link to a plot showing that, for a certain listening situation and event, the electrical needs were a bit shy of ONE WATT. However I know that if I feel funky I will turn the knob 3dB, 6dB, even 10dB. Now the "1 Watt" must be 2W, 4W, even 10W. OTOH I know that in many situations "1 Watt" is really very ample. I have several old 1W kitchen-radios and each will get "as loud as I can stand". Partly because their light speakers are pretty loud in midrange. (Partly because the THD goes up fast and then faster.) I know if I put the 1W Zenith to an A-7, it would be loud over at the neighbor's house also. Much less than 1W is often ample in efficient speakers.
 
The problem is that even if there was a standardized way to specify Watts for an amplifier the quality of the Watts would be difficult to specify. Even if the distortion numbers for the amps was the same. Maybe no. of Watts is simply a stupid way of specify an amp because it says not much about the amp. No speaker is 8 ohm but is a complex impedance which changes over the frequency band.
I noticed my 15" woofer I use for my OB speakers are rated 1000W (2000W max). But here they refer to a standard (AES) which says:
80 L box, pink noise input signal, 12 dB/oct bandpass filter 20- 20 kHz, 2 hours test duration. So this is a relative objective way to describe the power handling of a speaker. But we don't know the quality of the sound. So for me not a useful parameter as I use small SE amplifiers. But for people who use it in PA setups it will be useful. A similar objective test method could probably be applied to an amp but would not say anything about the quality of the sound.
Your measurements shows that calling it a 1W amp is not far out :) ....my interesting was also only to be a little more cleaver on definition of Watts.......