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Old 3rd July 2015, 01:37 AM   #1
bwaslo is offline bwaslo  United States
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Default So... should I build this?

I did a thread earlier about a zero global feedback SE MOSFET amp I did. The thread didn't generate much enthusiasm here, but the amp does sound awfully good to me.

I forgot to turn off my train of thought about such things, though, and it has since occurred to me that a variation could be made so that the design is DC coupled (well, sort of, there will be an inductor across the speaker!), and capable more power (>16W, in class A, able to do more at lower impedances by dipping into AB). Generating 36VP-P from a 13.8VDC supply, and 28% class A efficiency via the magic of inductor current. The inductor for this one could be smaller since push-pull operation would cancel the flux in the core from DC (current from each side regulated to match), and an output capacitor wouldn't be needed. It wouldn't be as simple as the last one (which was only 4 transistors per side, this one would be 12 transistors), though the same basic topology.

The amp posted about previously plays my system as loud as I need, and its sound doesn't leave me wanting something else, really. I also have several sets of nice mono amp pairs and have some (also un-needed but...) class D modules coming. So it would be money and effort spent only 'because'.

Here's the SPICE schematic of the basic idea. SPICE says it will do 1W level at under 0.004% THD (but I strongly doubt it will... probaby 3x to 5x that would be more likely).
Click the image to open in full size.

So, I thought I'd put it up here to see if it looks interesting to anyone else, and to ask whether you all think I should just go ahead and build it for the glory of DIY and to help curb the worldwide power amplifier shortage.

Votes?
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Old 4th July 2015, 09:08 PM   #2
bwaslo is offline bwaslo  United States
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Well, after a rousing response totalling. .. none at all (!), I guess I can conclude that this scheme doesn't stir any interest. So, I'll go back to making speakers (that I also don't need) for a while.
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Old 4th July 2015, 10:27 PM   #3
Zero D is offline Zero D  United Kingdom
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Don't give up too soon

Could you explain further how it works
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Old Yesterday, 08:26 PM   #4
bwaslo is offline bwaslo  United States
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Thanks Zero D for the interest (attention?)! Be glad to explain a little better, get it off my mind at least

Here's a stripped-down diagram (with the supporting circuitry and servos removed to make it less tedious to follow).
Click the image to open in full size.

M2 and M5 are a differential pair of depletion mode mosfets, with the 'long tail' part at the bottom done with a pair of current sources. That arrangement lets the input be DC ground referenced while still needing only a single supply for the entire amplifier. M3 and M4 are cascode tops, to avoid the signal voltage effects on the D-G capacitances in M5. M3 and M4 could also be BJTs (might be better, actually). This stage provides all the voltage gain in the circuit, and swings a lot of voltage with good linearity, due to being bootstrapped by the amplifier's output voltage swing (the input stage and the output followers all get their current through and hang off of the output inductor L1/L2.

The signals at the drains of M3 and M4, in reverse polarity to each other, feed "super followers" made up of Q1/U1 and Q5/U2. These two-transistor circuits provide output current drive and have rather amazingly low output impedance. Their outputs get bias current from and drive into coupled inductors L1 and L2, which are actually just one inductor, center-tapped. DC going in opposite directions in the windings prevents any standing flux that could saturate the inductor (the single-ended version of the circuit, on the other hand, needs an inductor that can handle all the class A DC bias current).

Using inductors to feed current to the 'super followers' lets the circuit output larger voltage swings with a low voltage power supply. It can also allow the circuit to transition to class AB if wanted, much like the output transformer in a class A push-pull tube amp. The inductor characteristics don't effect the performance as much as do transformers, since the signal doesn't actually get transferred to the load through the inductor.

If you were of the persuasion that capacitors in the signal path are evil, then this circuit has none, along with no global NFB either. I'm actually ok with output caps, even electrolytic ones, myself, but hey, it's for the challenge, right?

But after all that, I looked at some numbers and parts prices yesterday, and I think my next build will be just another SE type of higher power. The efficiency of this type topology is actually higher in SE than in PP, since about 5V of the supply is used up in drop across the inductor, the bias voltage of the large FETs, and the input cascodes, and those voltage losses are pretty much the same set amount no matter the overall supply voltage. By using a higher overall supply voltage, rather than the P-P arrangement with lower supply voltages, less power is lost. And more efficiency means smaller heatsinks and enclosure and smaller power transformers, even if the inductor type needs to be more expensive (and an output capacitor is needed). THD will be higher, but the difference is mostly in 2nd harmonic, which isn't a very big problem (some think it is actually a good thing).

Thanks for putting up with my geeky babbling
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Last edited by bwaslo; Yesterday at 08:31 PM.
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Old Yesterday, 10:22 PM   #5
sreten is offline sreten  United Kingdom
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Hi,

I'm not convinced at all that a single centre tapped inductor
with flux cancellation will do the job of two separate inductors.

rgds, sreten.
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Old Yesterday, 10:28 PM   #6
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One inductor will work, and will be smaller since doesn't need to be gapped like a single one would.
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Old Yesterday, 11:47 PM   #7
bwaslo is offline bwaslo  United States
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Quote:
Originally Posted by sreten View Post
Hi,

I'm not convinced at all that a single centre tapped inductor
with flux cancellation will do the job of two separate inductors.

rgds, sreten.
Try thinking of it being like a single-ended amp transformer (needs a gap in magnetic path to avoid saturation) vs. push-pull transformer (no gap required). Other than the lack of secondary windings, it's the exact same thing. Two inductors wouldn't work as well as the one.

But, ignore the 20mH winding inductance values shown (which would be a 80mH CT inductor). That should ideally be higher. I was considering trying a small 28VCT power transformer for this purpose. It should work as long as the windings or core don't do anything bad enough to output, which with the 50mOhm output impedances of the followers, would take a big issue to do.

But, as mentioned above, efficiency favors a SE version.
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Last edited by bwaslo; Yesterday at 11:51 PM.
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Old Today, 03:42 AM   #8
Zero D is offline Zero D  United Kingdom
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I don't pretend to understand it all, but i get the general concept. It seems clever to me !

What are the simmed specs like ?
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