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Old 12th April 2020, 11:19 PM   #1
H713 is offline H713  United States
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Greetings,

This is my first solid-state power amp project (though I've built a few tube amps). I'm not going to pretend like this is a "groundbreaking" design- it's heavily based off the works of Douglas Self and (especially) Bob Cordell. I figured it would be best to start off with something "normal". After simulations, I committed the huge sin of going straight to a PCB layout, with the expectation that there would be a second revision. Images of my layout as well as a schematic are attached. You can see that there are some extra footprints on the PCB- I wanted to make sure that I had places for caps that might be needed.

To make a long story short, I have so far been unable to stabilize this thing. It's particularly bad into capacitive loads, as expected. The attached image shows an oscillation that occurs once the amplifier output exceeds something like 22 volts peak. This occurs into resistive, capacitive and open loads. Currently it is running off of +/- 34 V rails from my bench supply, which is current limited to 4 amps. In this case, the IPS, VAS and output stage are all running off the same rails (that's the orange wires in the photo). Note that the emitter resistors on the outputs are actually 0.47 ohms, not 0.22, simply because of what was on hand.

The Zobel network and output inductor aren't shown on the schematic. The Zobel network consists of a 10 ohm resistor and a .1 uF capacitor. The output inductor is of unknown value and is in parallel with a 3 ohm resistor.

C19 and C20 (which are in place of D1 and D2 on the board) made things a lot better, and C16 seemed to help some as well.

Here's a list of things that I tried that didn't help:
-470pF miller caps on the drivers and outputs
-33pF cap in parallel with R10 (unsurprisingly, it made things worse)
-Eliminated the pre-driver transistors and connected the bases of the drivers to the bias spreader with 4.7 ohm resistors (obviously the bias spreader had to change for that). Surprisingly, this didn't seem to help much, if at all.
-Second Zobel network after the output inductor.

Increasing the size of the miller cap C4 does eventually help, but at that point the slew rate is noticeably below 10V/uS. I'd like to see it better than that.

At this point, I'm starting to think that it must be a bad layout or major schematic error, since none of the things I've tried have solved the problem. I'd greatly appreciate some input from those with more experience dealing with this kind of thing.
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File Type: jpg 20200412_164505.jpg (751.7 KB, 438 views)
File Type: jpg 20200412_164520.jpg (551.4 KB, 409 views)
File Type: jpg 20200412_164229.jpg (1.02 MB, 414 views)
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Old 13th April 2020, 12:55 AM   #2
AKSA is offline AKSA  Australia
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You will need to reveal your design if you want advice, this is the kicker.......

HD
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Old 13th April 2020, 01:01 AM   #3
H713 is offline H713  United States
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Quote:
Originally Posted by AKSA View Post
You will need to reveal your design if you want advice, this is the kicker.......

HD
Helps to hit the upload button!

Edit: Just noticed that the drivers are still labeled as being KSC1381 and KSC3503. That is a mistake, they are BD139 and BD140 like the VAS transistors.
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Last edited by H713; 13th April 2020 at 01:16 AM.
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Old 13th April 2020, 06:37 AM   #4
BesPav is offline BesPav  Russian Federation
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Check this one:
The YAP Output Stage
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Old 13th April 2020, 06:48 AM   #5
Michael Bean is offline Michael Bean  United States
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Where's the Boucherot cell?

Mike
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Old 13th April 2020, 07:01 AM   #6
basi is offline basi  Croatia
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Your output Ground goes to Input.Separate Out Ground and put it between power capacitors.
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Old 13th April 2020, 07:04 AM   #7
AKSA is offline AKSA  Australia
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I would make a few changes to the VAS.
First, remove the cascode BD139 in this section and replace the 139 with a C3503 with low Cob. I would take the collector from the driver C1845 to ground via a 10k resistor and a 330R resistor from emitter to negative ground. This ties the base of the VAS to negative rail and sets at least 2mA plus a few uA through the C185, rather than running only very low base current which highly drives transconductance and loop gain to high levels.

I am guessing now, but I would then decrease Miller cap fropm 100pF across base to collector of the new C3503 to 68pF, and add a phase lead of 15pF from VAS collector to the base of the fb transistor.

This will bring back loop gain, improve stability of the amp, and increase intrinsic linearity before applying fb.

HD
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Old 13th April 2020, 07:35 AM   #8
H713 is offline H713  United States
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Quote:
Originally Posted by basi View Post
Your output Ground goes to Input.Separate Out Ground and put it between power capacitors.

The photos are a bit deceiving. The ground connection near the output is tied to the PS ground. The other ground connection is just an extra that I'm using as a test equipment ground connection at the moment (blue wire).
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Old 13th April 2020, 07:37 AM   #9
H713 is offline H713  United States
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Quote:
Originally Posted by Michael Bean View Post
Where's the Boucherot cell?

Mike
External, at least during testing. It's the ugly thing you see near the output terminals.
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Old 13th April 2020, 08:01 AM   #10
basi is offline basi  Croatia
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Quote:
Originally Posted by H713 View Post
The photos are a bit deceiving. The ground connection near the output is tied to the PS ground. The other ground connection is just an extra that I'm using as a test equipment ground connection at the moment (blue wire).
Thats the thing.The blue wire is signal ground and u should leave it there and then u take another separeted wire only for speaker return and take it directly to PS ground,dont connect this wire on your amplifer board.Your signal ground on board is clean ground and your speaker return wire is dirty ground if u know what i mean.
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Last edited by basi; 13th April 2020 at 08:08 AM.
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