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Old 15th March 2012, 12:05 PM   #1101
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Hi guys I haven't post anything for a long time but I still working on this project I just want to say thank you guys for all the tips helps on my building specially mister Destroyer X, I have plans to order a set of board for myself in case a have a mayor burn out lol, well guys have a wonderful day.

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Old 1st April 2012, 05:18 PM   #1102
bonfis is offline bonfis  United States
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Default Changing the Feedback Ratio

Quote:
Originally Posted by BMW850 View Post
Andrew, please tell me how I can do it otherwise, so I change the input sensitivity 390R to 2.2K to the desired result with my preamp?
I am open for everything even for other suggestions, but please tell me what to do.

You tell me the test and reported conclusion are completely flawed.
That may be, but this amplifier sounds now to a level where I want to go maybe further, even though it is true no more how this amp is designed.

You says, I listen to two different amplifiers, which I can confirm because it sounds different , but as I said this is the level where I want to go.

I have listened a number of years ago to a UCD400 classd amp with a tube entrance, that sounded a lot better than the default UCD400.
I know this has nothing to do with this amp, but the design was also completely different.

Hopefully I become understood correctly, English is not my mother tongue.

Regards,
Rudy
I would also like to reduce the amps gain to something closer to 20X. My reading suggests that increasing the global feedback will also require a change in Cdom. Although I've located the formula for calculating the correct value for Cdom it requires knowlege of the amps closed loop gain bandwidth and the transconductance of the input LTP. I didn't find these parameters available anywhere within the forum but perhaps someone knows what they are or how to derive them from the schematic. I'm concerned that simply increasing the negative feedback without adjusting Cdom might lead to instability.
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Old 2nd April 2012, 09:01 AM   #1103
AndrewT is offline AndrewT  Scotland
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Do you have a scope?
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Old 2nd April 2012, 02:38 PM   #1104
bonfis is offline bonfis  United States
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Yes
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Old 2nd April 2012, 03:02 PM   #1105
AndrewT is offline AndrewT  Scotland
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Build it exactly as designed.
Test it.
Change the NFB and test again. Are the results acceptable?

If not, then come back with some pics and/or test results.
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Old 2nd April 2012, 04:33 PM   #1106
bonfis is offline bonfis  United States
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Thanks. First channel is under construction. Will report back.
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Old 3rd April 2012, 02:04 AM   #1107
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Interresting point bonfis. You seem right saying that by changing NFB we should reconsider the miller cap value. Because I barely start understanding its role in the schematic, I decided to compare DX amps to identify similarities. Here are my findings

Here are the gain vs Cdom values of the amps mr. DX has developped:
ES = 26.5x (28dB) = 82pF
ST = 26.5x (28dB) = 100pF
MKII = 31x (30dB) = 100pF
MKII-SC = 58x (35dB) = 100pF
HX = 121x (41dB) = 180pF

The "default" value Dr. Self uses for his amps is of 100pF.

I decided to simulate my schematic and compare THD at near-full power at 20Khz.

Simulation details:
Gain= 32x (30dB)
Frequency = 20kHz
Output power = 200W RMS / 8ohm resistive load
Input = 1.244V RMS.

with 180pF Cdom = 0.063% THD
With 100pF Cdom = 0.039% THD
With 82pF Cdom = 0.033% THD

Interresting... but is it stable with lower values?... I don't have a scope to test the real thing Only my ears, and they tell me that the amp sounds good with 180pF, but can it be good to lower that value to 100pF?.

share your findings!


Mart.
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Old 4th April 2012, 12:49 AM   #1108
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About the miller capacitor, Destroyer X said in his video Monologue detailing the MKIII-Hx power audio amp:

"...I have used in some schematics 82pF, yes you can use that value if you want but it is dangerous, better for you to keep it 180pF because it is tricky to check if it's oscillating you need a scope to see ocillations trigerred by the signal..."

Then later on in the same video:

"...do not change it, keep this damn thing 180pF, it's gonna be safer this way..."

So to my understanding, lowering the value can elevate the risk of oscillations.

But what is the relationship between the gain and the Cdom value?
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Old 4th April 2012, 01:09 AM   #1109
bonfis is offline bonfis  United States
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Hi Mart-
Thanks very much for the simulation results. It does seem that for the Blame MK III the distortion results benefit from a reduction in Cdom when feedback is increased. The formula for calculating Cdom (from Bob Cordell's book) indicates that the correct value for Cdom is inversely proportional to closed loop gain if the input stage transconductance and closed loop bandwidth remain constant. So a reduction in the closed loop gain should require an increase in the value of Cdom (everything else being equal). Perhaps the only consequence to be considered when calculating Cdom is stability and not distortion. However, since global feedback is being increased in the process of reducing overall gain I would expect distortion results might improve also.
Steve
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Old 4th April 2012, 01:52 AM   #1110
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Has anyone here heard from Evette .....?
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