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Messages - Loudthud

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Amplifier Discussion / Re: Peak dissipation in class B output stage
« on: September 09, 2020, 10:08:46 PM »
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.

Amplifier Discussion / Re: Peak dissipation in class B output stage
« on: September 08, 2020, 12:01:25 PM »
Sorry, I thought I had included the link. I edited the post above but here it is again


Amplifier Discussion / Re: Peak dissipation in class B output stage
« on: September 07, 2020, 09:27:25 AM »
Check this thread over at MEF.


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.

Amplifier Discussion / Re: Pedalboard amp
« on: July 11, 2020, 01:27:34 AM »
So you built the power amp ? How many Watts ? What did you use for a power supply ?

Schematics and Layouts / Re: Who has Layout for Mosfet P. Amp?
« on: June 06, 2020, 07:15:36 PM »
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: ... but so far no one has responded.

Schematics and Layouts / Re: Who has Layout for Mosfet P. Amp?
« on: May 29, 2020, 02:04:18 AM »
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.

The Newcomer's Forum / Re: Tda ICs Pentawatt PCB
« on: May 28, 2020, 11:45:40 PM »
I use Vector Flea Clips.


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:


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.

Schematics and Layouts / Re: Who has Layout for Mosfet P. Amp?
« on: May 22, 2020, 01:37:18 PM »
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.


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.

Amplifier Discussion / Re: PS Audio hifi amp capacitors question
« on: April 15, 2020, 02:23:19 AM »
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.

If you ask 50 people your question, you will likely get 50 different answers. Now if you set up a test where you ask 50 people to play an amp and ask them if it's tube or solid state, it's one of those situations when you can fool some of the people most of the time, but you can't fool all of the people even once.

In my experience, you can come pretty close, but you can fool more people if you think outside the box (non-conventionally).  A tube power amp has a high output impedance (low damping factor), it has some gain compression and produces a non-symmetrical square wave when you overdrive it. If you can reproduce those characteristics,  you've come a long way.

Guitar News / Re: Coronavirus hitting the music industry
« on: March 14, 2020, 10:47:28 PM »
The Elephant in the room; :o
It took ~~ 40,000 Yrs to reach 200 Million people on this planet
At the time of Christ it's estimated that it was a shade under 200Million.
By the 1830's we cracked the 1 Billion mark.
Less that 200 Years later we have hit the 7.8 Billion.
Now if that does not tell you the real reason for the worlds problem I don't know what will. 8|
Unlike the FAKE hockey stick curve presented by Al Gore this one is REAL.
Scroll down a bit for the graph,, scary :'(

There are people that suggest that the human population on Earth be limited to 500 million. The first place I ran across this was here:
It's interesting that they can't seem to be able to track down the people that paid to put these things up.

Guitar News / Re: Coronavirus hitting the music industry
« on: March 13, 2020, 03:08:25 AM »
Justin Bieber had a big tour booked in the US but ticket sales were slow so they had to change venues to smaller places. This was before the virus really got serious press. Now they may have to cancel the whole thing or postpone it.

The question I have is: If you get it once, are you immune or can you get it again ? Or is it like the flu and it mutates every year ?

CMOS circuits can fizz, especially when you lower the Voltage (this increases the gain) and you won't see any ringing on the waveform. Increase the Voltage and the gain goes down and they start clipping softer. Fizz is mostly from too much feedback. Solid state circuits have hundreds of times more feedback of tube circuits. A tube power amp may have 20dB of feedback. A typical opamp has 40 to 60dB or more feedback. Same goes for chip power amps.

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