It should be noted that when you hijack one of the primary windings to make high Voltage, you should de-rate the VA of the transformer by half.

You can probably use an effects pedal as a preamp, but not all will be suitable. You don't need a DI box as long as the pedal has enough output to drive the power amp. A wall wart is a good choice for a power supply as long as it has enough current capacity for the power amp. It should be enough for both preamp and power amp. Both circuits can be built into the same box.

A WORD OF WARNING: If the power supply does not have a safety ground, you will probably have hum on the output of your amp. In addition, you may get a mild to moderate shock if you touch someone playing another amp when you both are touching your guitar's strings. BE SAFE, GET A POWER SUPPLY WITH A SAFETY GROUND !

Quote from: shasam on November 29, 2020, 09:50:50 AM
So I should use a circuit like the one in attachment, with no other base resistor?
In a lot of schematics in Douglas Self's book, he use the other circuit (base of the reference transistor connected on the VAS current source emitter). Do you know why, please?

I can't speak for Mr. Self but perhaps he doesn't listen to his amps when they are clipping.  A base resistor on Q90 or Q91 might tweek the temperature coefficient slightly or suppress any tendency of the circuit to oscillate.

Quote from: shasam on November 29, 2020, 09:50:50 AM
Do you have a suggestion for a faster turn on please?

Try one of these:
1) Replace C32 with a zener diode.
2) Replace R156, R157 and C32 with a high impedance current source.
3) Connect the bottom of R157 to the minus rail and change R156 to 22K. Connect the anode of a 1N4007 to the junction of R156, R157 and C32 and the cathode of that diode to ground. This will force C32 to charge fast to the rail Voltage, then slowly charge to about 1.5 times the rail Voltage.

Use the circuit on the left. Problems occur when the VAS clips. The VAS transistor turns off and the VAS current source has no where for the collector current to go, so it sucks a big base current until it's Voltage reference gives up. R7, limits the base current and prevents the input stage current source from going crazy.

Another subtle problem. C1 is intended to filter the reference current for both current sources. But, the way you have it grounded, it doesn't do it's job. It needs to be connected between the plus rail and the junction of R3 and R13 so R3 doesn't see the ripple on the plus rail. Be sure to connect the + side to the plus rail.

This creates another problem, it limits how fast the LTP current source turns on when the amp powers up. That creates a big pulse on the output. If you can make the current source turn on fast, the output will be almost silent when the amp  turns on and off.

To really study this subject, you would need a lab full of equipment. You would need to simultaneously monitor Voltage and current in a transistor, then multiply the two to get power. In a practical sense you would need to look at the instantaneous power over a time scale and compare that to the SOA limits from a transistor data sheet. I say practical because you don't want a 100 W power amp with 20 output transistors.

Attached is the SOA graph from an On Semi power transistor. If you look at the 1 Second line, it stays at 250W out to about 50V. I have extended the 250W line to show the Second Breakdown region in red. The 1 Second line is the one bordering the bottom of the red zone. As the Voltage increases beyond 50V, the allowable power is reduced. The 50V limit for full power is common to many transistors. Some have a lower limit, a few higher up to 80V. Let me know if you find any bypolar transistors that are higher than 80V.

To keep your output transistors out of the red zone, you need to limit their Voltage and that means stacking them in series. Otherwise you need about twice as many transistors to be safe.

MOSFETs don't have Second Breakdown so that is a big incentive to use them.

Edit: Changed pic slightly to make it more clear.

Sorry, I thought I had included the link. I edited the post above but here it is again

Link: https://music-electronics-forum.com/forum/fun-with-computers/36422-testing-zip-files?t=35493

Check this thread over at MEF.

Link: https://music-electronics-forum.com/forum/fun-with-computers/36422-testing-zip-files?t=35493

If you are not a member there, I think you will have to join to see the attachments. What is shown is an X--Y movie clip of current and Voltage in a speaker when connected to a solid state amp banging rail to rail.

When the trace is above mid screen, current is positive flowing through the transistor connected to the + rail. See how some of the time it stays above the mid screen line all the way from the right side of the screen to the left side ? The inductance of the speaker keeps the + side transistor on even when the Voltage goes negative.

Nelson Pass designed a preamp with the NuTube and released it via the diyAudio store. A number of people have reported microphonic issues sometimes cured with foam between the NuTube and the PCB. Seems to be a hit or miss kind of thing.

Keep in mind that there is no second source and Korg could discontinue making them at any time. Wait 'till you can't tell the real ones from the fakes from ... you know where they come from.

So you built the power amp ? How many Watts ? What did you use for a power supply ?

This circuit has a temperature compensation problem, as it warms up, the bias current goes lower and lower. I started a discussion of it here: https://www.diyaudio.com/forums/solid-state/355583-class-ab-hexfet-amp-temperature-compensation.html ... but so far no one has responded.

OK, I looked at the Ap Note and it's hard to follow. On the layout they provided there is no Q1 and two Q3's. The board side view doesn't line up with the component side view. There are also places where you need to add jumper wires that are not shown. So I built a new version and I'll show my layout sketch and the mods I did to the schematic. One additional mod I did was to add a capacitor to the input.

Most things I build are on a proto board with holes on 0.1 inch centers and copper pads on one side. To this I add eyelets from Keystone in three sizes. In some places you need to remove the copper pads because the pads cause shorts between eyelets. There are multiple ground wires that come off the power amp board to a star on the power supply board that I haven't built yet. I built the power amp board and got it running on my lab power supplies. Still might make some changes there. Another mod I allowed for is mixed mode feedback. There is an eyelet on the power amp board labeled IFB. That hooks to a low value resistor that goes between the low side of the speaker and ground. If you just want Voltage feedback, ground the IFB pin.

The attached schematic shows the short circuit protection and zener diodes on the MOSFETs.

I use Vector Flea Clips.

Link: https://www.ebay.com/itm/Vector-Push-In-Flea-Clip-Terminal-Printed-Circuit-Board-/362593814146

Notice how they extend from the hole in only one direction. You use two rows of holes on the board, each with 3 holes. For the center pin (3), orient the extended portion of the clip towards you, away from the heatsink. To the right and left of that pin, orient the extended portion of the clip way from the center pin to the right (pin 5) and left (pin 1). On the row between that first row and the heatsink, you use the holes behind those two outside pins except you orient the extended portion of the clip towards the center so you can solder to pins 2 and 4 of the chip. Did that make any sense ? Read it again.

From the top it looks like this:

HEATSINK
2x4
135

x= hole not used

JFETs with low pinchoff are easier to bias like tubes. If the pinchoff is too high, the bias will drift around when you overdrive the JFET unless you great lengths to control the bias.

This is an old design that was in an IR data book in the 80's. It doesn't have short circuit protection, but that can be added. You also need gate protection zeners for the output MOSFETs. I built one and it sounded good. You can get to 50W at 4 Ohms with +/- 25V rails.

Link: http://www.cieri.net/Documenti/Altri%20marchi/International%20Rectifier%20-%20Linear%20Power%20Amplifier%20Using%20Complementary%20HEXFETs%20(AN948).pdf

Edit: Attached below is the PCB layout from the Ap note article edited to show the components X-ray style over the etch.

Not exactly a cascode circuit because the gates are connected together.

A quick experiment on the curve tracer shows that it knocks Idss down to about 60% of the single JFET value. Gain is also lower. The curves look slightly more linear.

Big filter caps are inductive above a certain frequency. It depends on the internal construction of the cap. The smaller caps (which have lower internal inductance) are added in parallel to insure that the impedance across the big filter caps remains low across the audio band up to some upper limit probably to a couple of hundred Kilohertz.

This is mostly an audiophile thing that may or may not be measurable.