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John Curl's Blowtorch preamplifier part III
John Curl's Blowtorch preamplifier part III
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Old Today, 01:47 AM   #22481
Max Headroom is online now Max Headroom  Australia
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Join Date: Jan 2013
Location: West Australia, near to the beach, natural ambient sounds mostly.
Originally Posted by john curl View Post
Thanks yet again, Markw4. I need to know the problem areas, if I am to address them. Of course, analog is relatively easy to 'improve' but it is not enough. Power supplies can be dealt with, but other digital based problems are a mystery to me.
You can experiment with decoupling cap types around DAC chip, each cap type sounds different as I am sure you know .
IIRC Joe Rasmussen advocated Supercaps across DAC chip supplies.....they can contain carbon in the internal electrodes so this might be what is different in sound to other caps, I have not tried this.
PM me for other info if you like.

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Old Today, 02:55 AM   #22482
Evenharmonics is offline Evenharmonics  United States
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Originally Posted by syn08 View Post
Thanks for the confirmation, 44 degrees phase margin, a disaster waiting to happen. And that's not even with the output close to the power rails.

For your education, given an N pole compensation schema (you are showing for N=2) there is still not much room to wiggle the ULGF while not affecting the stability margin. This is because audio amplifiers are by their nature minimum phase systems, therefore the magnitude and phase of the loop gain are not independent, you can get the phase(w) by simply applying a Hilbert transform to the gain(w). The only way to pull more loop gain from an audio amplifier, without pushing the ULGF to absurd values, is to use a higher order compensation network, a typical example would be the Cherry NDFL (which was practically used in the PGP amp). Or, in general, to use N poles followed by N-1 zeroes in the loop gain (to bring back the phase) and at the same time taking care of the sensitivity to stay within reasonable limits. I know of a couple of amplifier that are using N=5, one is an amazing small scale commercial class D amp, the other one was designed and build by a former member here (GK) as a proof of concept, but that one had other issues (was going berserk when approaching clipping).

And this is where the free lessons ends. For more, I can send you my hourly rate .

BTW, there's no relationship between the gain/phase plots you are showing, it's like you are pulling rabbits from a hat. I'm sure you have more .
Thanks, syn08 for your latest input. It is just the sort of info that I need.
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