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Old 6th April 2005, 06:49 AM   #1
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Default Newbie Amp Design Question (Please Respond)

I've done a fairly substantial amount of reading in Self's Audio Power Amplifier Design Handbook , and I believe I have a quite good grasp on how each section works, etc. But looking through the book and every resource I have been able to find online, I can't find anything that tells me how to even START selecting specific component values and such. I would really like to design and build my own "blameless amplifier," but this seems like an insurmountable problem. Please help.

Thanks in advance,
Scott
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Old 6th April 2005, 07:24 AM   #2
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scott,

I can probably help you as I was designing commercial PA amplifiers very similar to Douglas Self's designs - back in 1974 using MJ15003/4.

The one thing he glosses over and is a prime concern of mine is PSRR or Power Supply Rejection Ratio. Miller compensated amps are particularly prone and even his own example benefitted greatly, from memory, with THD dropping from 0.04% to 0.008% simply be adding a regulated power supply. If that isn't an indication of supply sensitivity I don't know what is.

The same occurs in many integrated OP amps where chip space is at a premium and capacitors consume real estate. The usually opt for the smallest on chip comp capacitor which is a miller cap and the PSRR to that supply side goes down the toilet. Rarely do they bring out a comp to ground pin.

The basic Self amp topology has great potential particularly with FET inputs, bipolar or CFP MOSFET outputs and comp from the Vas output to NFB return point.

Hope that helps.
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Old 6th April 2005, 07:37 AM   #3
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There was some great information in there, but I still don't know where to start deciding on values for each resister and capacitor and transistor, etc.
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Old 6th April 2005, 07:45 AM   #4
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You need to identify the goals your amplifier is to achieve. Power, THD , crosstalk, budget.

Identify Type of output BJT or MOSFET?

work out supply rails Regulated or unregulated?

how many channels 2 to 6+ . If you want you could run them all off one power supply in one big box?

What quality source are you to use?
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Old 6th April 2005, 08:52 AM   #5
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hmm... looking at more schematics, most of the resistors and caps and such look predetermined. If this just leaves the transistors and rails and such, do I figure out how much power I want, and what rail voltages, and then pick transistors based on these values?
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Old 7th April 2005, 12:37 AM   #6
AndrewT is online now AndrewT  Scotland
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Hi,
Self is good reading.
I second the removal of the Miller comp cap and choose a Leach or Cherry feedback topology or as amplifierguru suggests for HF feedback.
Read the tech notes in the ESP site along with the schematics etc.
Start with a lower voltage PSU (+-40v?) and save blowing up your transistors until after you have understood temperatures and why they happen, protection and how to implement and peak currents/voltages and their effects on components.
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regards Andrew T.
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