'Ultra Class A Superdrive Power Amp' :: Questions

[tca]

If I'm seeing this right the MPSA12 can dissipate about 612mW, in a class a configuration you can only get at most half of it, 306mW. For 9V you will need a transformer of 135Ohm for the primary. This is, somewhat, the value (100Ohm) that appears in some class b push-pull late 50-60's transistors amplifiers. There is no way around it.

If you look at Escobedo's circuit he says that the current draw is about 4-6mA, and so, for 9V this gives the amazing 22.5mW! Unless your speakers are really, really, sensitive >=100dB it is better to build another amp (that is what I've did).

P.S.
Although it is always fun to play with this circuit.

[PRR]

Time to re-draw.

http://i.imgur.com/0Wa9R.gif

> B 3.5v
> E 2.8v

I'd expected B to be more than 1V above E; but the meter may be loading-down the B point a whole lot more than the E so this may be OK.

The E number should be trustworthy (no meter loading) and is in the OK ballpark.

It may be good-as-it-gets. However it is odd that two respected gurus have drawn and posted this.

[chptunes]

Yea, I may be "barking up the wrong tree". ..just thought it'd be cool to get a simple low powered transformer amp working.

I brought the 680k up to 890k and now I'm reading 3.5v on the Emitter.. still, the output cuts as signal weakens.

Enough is enough, I guess.  Thanks for all of your help.

-Corey

[PRR]

> the output cuts as signal weakens.

It may be enlightening to meter that point while you play.

We may have run into another problem. The emitter cap can charge-up a whole lot better than it will bleed-down. So bias is probably changing all the time.

I can think of so many ways to modify this, each opening new problems, and no good reason to complicate.

There's lots of other transistor+transformer power amps. Some REAL power, such as 1956 Cadillac car radio. Delco transistor on heatsink the size of two packs of Parlaments (not the funkadelic kind). But it's hard to expect to be convincing without three stages. (IIRC, that's about the minimum I've seen in commercial practice amps.)

[PRR]

Mispost.

[PRR]

> get a simple low powered transformer amp working.

I have cooked-up a scheme for a low-power transistor transformer amp.

It should perform excellently, within its roughly 0.3 Watt ability.

But it isn't particularly simple. Two small transistors, a TIP120 (or similar TO220 Darlington). Chip-amps are much simpler.

And the transformer. A *good* class A SE OT is not common. I thought of a hack using a part which used to be common, and it seems to have gone out of style. (25V 5W speaker transformer.) I'm finding like just 22 or 8 in stock at prices like $8 and $16, plus $5-$10 shipping.



So the bare parts cost nearly $100 per Watt! ($1/watt has been the mark for decades, at least in larger utility amps.)

And it is NOT a battery amp. Scaled for 12V 120mA. OK, it will run all week on a car battery, all day on eight D-size NiCads, but small batteries will fade fast.

[chptunes]

Wow!  Sweet design PRR.. I'm tempted to search out parts and get started.  Do you think this amp will remain clean with a boosted guitar signal driving it?  Which component would overdrive first, if overdriven by a boosted guitar signal?

.. ...

-Corey

[PRR]

Notes:

Stuff in dotted boxes is simulator fluff. Far left is just a signal source. In the middle, R9 R10 is a 100K Volume control.

Transformer must be rated for _25V_ line at 5 (or 4) Watts. The typical speaker line transformer is covered with taps. Be sure you know which ones are 25V at 5 Watts.

As a check: feed 6VAC 60Hz (heater transformer) to the suspected "25V 5W" taps. At the 8 ohm winding you should get 1.5VAC. (1.0VAC at a 4 ohm winding.)

Supply should be 11V-12V DC. Don't go higher, it'll melt. Lower, it will suck.

The TIP120 in TO220 package can run without a heatsink, but don't lean a finger on it.

Target bias is about 1V at the 10 ohm power transistor emitter (about 100mA). The first stage should show 4V to 6V at collector.

[PRR]

> Which component would overdrive first

Power is precious. You paid for your mighty 0.4 Watts, you want to get ALL that you paid for. Power amps are normally designed so the power stage overloads first.

The first stage is designed clean up to 500mV signal, a hot guitar.

The power output stage will or will not overload depending on the Volume control.

Classic guitar amp. Somewhat scaled from the better Fender Champs. Hot guitar wall barely strain the first stage, then a volume control lets you select the level of strain in the output stage.

If you truly find it "too loud", we can re-tap the transformer and increase the emitter resistor for lower power output.

[tca]

...$1/watt has been the mark for decades, at least in larger utility amps...

Nice benchmark... in my case that would be 0.75€/watt.

[tca]

I remember seeing a half a watt TL431 (adjustable shunt regulator) in the datasheet made by Motorola (http://pdf1.alldatasheet.com/datasheet-pdf/view/5774/MOTOROLA/TL431.html)
Check page 9 Figure 29.

But never built it.

Cheers.

[chptunes]

Thanks for the link tca.

To be sure.. I am enjoying my 1w Punch Amp very much.  Cool clean little power amp.

> Which component would overdrive first

Power is precious. You paid for your mighty 0.4 Watts, you want to get ALL that you paid for. Power amps are normally designed so the power stage overloads first.

The first stage is designed clean up to 500mV signal, a hot guitar.

The power output stage will or will not overload depending on the Volume control.

Classic guitar amp. Somewhat scaled from the better Fender Champs. Hot guitar wall barely strain the first stage, then a volume control lets you select the level of strain in the output stage. 

Yeah!  I'm grateful that you shared this cool design PRR.  In order to show my gratitude (and for fun), I'm gonna build it.  I'll lend you my impressions and thoughts.

I have everything in stock except for the 10 Ohm Resistor and the Transformer.  I'll order the Bogen T725..  Data Sheet

Several stores sell this Bogen device on-line, including Amazon and Best Buy:  http://www.bestbuy.com/site/Bogen---T725-Step-Down-Transformer/4472885.p?id=1218477791887&skuId=4472885

-Corey

[PRR]

Questions:

> -What is the arrow symbol near the 8 Ohm speaker load?

Simulated "voltmeter probe" so I can see what comes out.

> A single TIP120 Darlington can be used instead of the "Q2N6059" pair, right?

Yes. My sim doesn't have TIP120. The '6059 lacks the '120's base resistors, but I did simulate with more '120-like values and no real difference (because the driver stage current is ample-enough).

[chptunes]

Cool.. thanks..

One more.. The Negative side of the output is connected to Common Ground.  This is not the case with other small amp circuits I've been seeing.  Is there a clear explanation for this?

-Corey

[Perrow]

Cool.. thanks..

One more.. The Negative side of the output is connected to Common Ground.  This is not the case with other small amp circuits I've been seeing.  Is there a clear explanation for this?

-Corey

I think it's to tie the lower end of the output to "something" stable, so you can "measure" the output on the upper end. You can connect any side (in simulation, or in the real amp) to ground with no difference in function.

[PRR]

> The Negative side of the output is connected to Common Ground.

Why not?

(One exception: BTL transformerless outputs have neither end grounded.)

Yes, it will work if the secondary is left floating.

Historically, we did NOT, because the grunt came from a 105 Volt - 300 Volt vacuum tube. If the OT developed an internal short, there could be high voltage on the speaker wires. Bad for humans. If the speaker line is grounded, something else will blow, humans are safe.

Yes, this is only 12V.

Another thing: for "best fidelity" we would run negative feedback from the speaker side of the OT back to an earlier stage. Since that earlier stage is ground-referenced, we need one end of that winding referenced to ground.

However I don't know the phase of your transformer. (They are not marked.) If you phase it wrong, it howls full power. (Less ear-splitting here than on a Fender Twin.) So I took NFB from the primary. This was common before the hi-fi damping-wars of the 1950s. The polarity is unambiguous. The distortion is reduced (>26% toward 10%). There's some speaker damping. We don't want hi-fi infinite damping in a stage amp (dresser amp) because the speaker's liveliness is part of the flavor.

And the exact reason is that my sim complains about nodes with *no* path to zero volts reference.