Amp design attempt number 2 (simpler) - Page 24 - diyAudio
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Old 16th August 2013, 11:34 AM   #231
mcd99uk is offline mcd99uk  United Kingdom
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Andrew,

I am always willing to try ideas out. The loop areas are already very small around this area. The layout was done with loop areas in mind.

Will implement your suggestion over the weekend. How would you physically place the PSU star ground and the signal star ground in relation to each other?

Thank you

Paul
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Old 16th August 2013, 11:40 AM   #232
AndrewT is offline AndrewT  Scotland
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The PSU output should never be used as a Star Ground (Main Audio Ground).

The Main Audio Ground serves the Speaker Return and PCB Power Ground with links running to the Signal Ground and PSU Zero Volts.
Because the MAG serves primarily Speaker and PCB, it should be placed close to these two locations. If the PCB output end were very close to the speaker terminals, then in my view the MAG can be on the PCB or on the Speaker Return terminal or in the short wire connecting the two. But NEVER at the PSU.
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Old 16th August 2013, 11:56 AM   #233
mcd99uk is offline mcd99uk  United Kingdom
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I thought I had a basic understanding of grounding until I tried it. Then I found out otherwise.

Shall follow your instructions to the letter and report back my findings.

Thank you again.
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Old 18th August 2013, 02:57 PM   #234
gootee is offline gootee  United States
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Quote:
Originally Posted by AndrewT View Post
The PSU output should never be used as a Star Ground (Main Audio Ground).

The Main Audio Ground serves the Speaker Return and PCB Power Ground with links running to the Signal Ground and PSU Zero Volts.
Because the MAG serves primarily Speaker and PCB, it should be placed close to these two locations. If the PCB output end were very close to the speaker terminals, then in my view the MAG can be on the PCB or on the Speaker Return terminal or in the short wire connecting the two. But NEVER at the PSU.
I still don't understand this very well. If everything runs there separately, then as long as it's not upstream of any reservoir cap, why could it not be near the PSU output?
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Old 18th August 2013, 03:08 PM   #235
mcd99uk is offline mcd99uk  United Kingdom
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Well the results are in...

AndrewT's scheme works. The amp was still conditionally stable though. Probing the output with the DMM sent it into oscillation. So I went back to the sims. Found a peaking that I had somehow missed in the closed loop response. So there was a control loop problem in there. Increased the shunt compensation at the output of the VAS/TIS. This has solved it. Appears stable now. No oscillation on the scope. Need to double check things still, like feeding a square wave back to the rails via a 1K resistor as keantoken suggested.

Its been a productive weekend so far.

I have a theory (could be wrong) about the MAG not being near the PSU ground and that would be voltage gradients caused by the potentially high currents in the PSU. Apparently you can get these in the star point itself. This would be the same reason why the speaker out and feedback take off point need to be on their own spur.

I have got to say making mistakes is the best way to learn.
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Old 25th August 2013, 07:22 PM   #236
mcd99uk is offline mcd99uk  United Kingdom
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Default Stereophonic sound...

Now in stereo and stable. No output inductor yet but happy driving the Coda 7's. They have never sounded so good. (Maybe some bias But pleased that I haven't designed a dud).

No measurements yet but they will come later in the project.

Now I turn to thinking about the power supply requirements. Was thinking of running a second toroid for the front end at a higher voltage to allow some PSU filtering to be using. Some sort of "ripple eater"

In the mean time its time to do metal work and rework the case. Have bigger heat sinks to fit (330mm x 125mm x 40mm) One per channel.

Here's the latest schematic.

Cheers

Paul
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File Type: jpg Prototype.jpg (384.4 KB, 190 views)
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Old 26th August 2013, 01:33 AM   #237
gootee is offline gootee  United States
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The main "ripple" will be caused by playing music. The reservoir and decoupling caps' current IS THE MUSIC, directly. So the louder the music the bigger the ripple, since the cap VOLTAGE is proportional to the integral of the music current, between charging pulses. The best thing you can do, to prevent the music current from affecting the voltage of anything else that uses the same supply is to put lots of decoupling capacitance right AT the power rail connections to the active output devices. Use smaller caps REALLY close to the devices, and larger ones slightly farther away. A larger number of them in parallel will do slightly better than one (or a few) larger ones, for the same total capacitance. And separate parallel paths will do better than a buss topology, for the caps, especially the multiple smaller one that should be really, really close to the device, that handle the transient currents at up to a few hundred kHz. You want roughly at least 10 uF per watt, for those, but should also have a closer core of 1uF to 10 uF that are film or C0G ceramic type.

See the rather-enlightening image of the LT-Spice simulation plots of cap currents and output voltage, in the post at:

Resevoir capacitors for Chip Amps

This post might also be helpful:

Resevoir capacitors for Chip Amps

Cheers,

Tom

Last edited by gootee; 26th August 2013 at 01:48 AM.
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Old 1st September 2013, 08:19 PM   #238
Hlaford is offline Hlaford  Croatia
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Quote:
Originally Posted by mcd99uk View Post
Here's the latest schematic.
Could you please share the spice file?

Thanks.
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Old 1st September 2013, 10:28 PM   #239
mcd99uk is offline mcd99uk  United Kingdom
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Quote:
Originally Posted by Hlaford View Post
Could you please share the spice file?

Thanks.
No problem. Hopefully you'll get some value from it

It uses Bob Cordell's models for the smaller transistors and Exicon models for MOSFETs

To get it to run properly need to set:

Gmin = 1e-009
Abstol = 1e-006

Use GEAR solving method.

Paul
Attached Files
File Type: zip exicon mosfet.zip (9.3 KB, 11 views)
File Type: asc compdiff Prototype1.asc (35.9 KB, 10 views)
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Old 1st September 2013, 10:50 PM   #240
mcd99uk is offline mcd99uk  United Kingdom
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Gootee,

Thank you for that info and the links.

Some of your suggestions can be implemented. The only part I don't think I can do is the inner core of film caps. I only have 330n per device.

Your spreadsheet looks interesting! The only part I haven't figured out is the Vclip parameter. What does this relate to? My limited mind can't process this part.

Paul
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