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Old 30th May 2005, 03:28 PM   #11
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R45 = 470 ohm
R46 = 220 ohm
cap between PWD FET gate and drain = 0.01u
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Old 30th May 2005, 07:36 PM   #12
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Hi CK!

I am wondering about different R45 and R46.
I cannot exactly read the names in the schematic, but I guess
R45 is one of the gate drive resistors of of the upper MosFets and
R46 one of the lower sider gate drive resistors.
From my perception the large capacitor of 10nF between gate and
drain should easily dominate vs. the nonlinear (and different N-chanel vs. P-chanel) gate charges of the Fets.
In order to improve the phase shift and rectangular response, you could try two steps:
1. Decrease R45 in to 220Ohms and check if the amp is still stable,
I would guess yes.
2. Decrease the gate-drain capacitors to 6.8nF

R51 is influencing the bias of the output stage and your step to increase its value should help to reduce the crossover distorsions, but will generate more heat, due to the increased idle current.
As long as you are not facing any thermal runaway, it should be fine.
On the other hand I WOULD expect thermal runaway, because
there is no thermal compensation and no degeneration resitors.
Are you using exotic MosFets, which offer a negative temp. coefficient already at low currents?

Bye
Markus
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Old 30th May 2005, 08:08 PM   #13
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Dear Folk,

All you guessed are right. I have seen a few circuits like this, they would like to use different value of R45 and R46. Perhaps all the N/P Channel MOSFET with a certain kind of behaviour need to match.

PWD FET is Hitachi J56 , K176 model. I have bought it many years ago.

I will follow your instructions. See how the best of sound will blowing out.

Thanks
CK
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Old 31st May 2005, 08:04 AM   #14
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Hi ChocoHolic,

Just complete some meausrement FYR.

Constant Input : AC 0.42rms / 1.2Vp-p

1) 14.8vrms / 44vp-p / @1k input

2) 14.14vrms / 41.5vp-p / @10k input

3) 13.1vrms / 38.5vp-p / @20k input

4) 9.16vrms / 28vp-p / @50k input

5) 4.62vrms / 17vp-p / @100k input

About phase shift 40 degree at 20kHz. I found such problem also exist on the 4 output copuling caps. Therefore the problem is in the voltage gain area.
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Old 31st May 2005, 10:11 AM   #15
sajti is offline sajti  Hungary
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What is the value of R26?

sajti
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Old 31st May 2005, 03:30 PM   #16
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R26 = 1.2k
R25 = 100k
R27-R30 = 1.2k
R31-R34 = 160k
c42-c45 = 20p
Two tubes = 5751
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Old 31st May 2005, 08:27 PM   #17
sajti is offline sajti  Hungary
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Quote:
Originally posted by ckwong99
R26 = 1.2k
R25 = 100k
R27-R30 = 1.2k
R31-R34 = 160k
c42-c45 = 20p
Two tubes = 5751
Try to remove C42-45. I think that capacitors are not necessary, and do nothing in Your amplifier. So try to remove them, and check again the phaseshift

sajti
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Old 1st June 2005, 06:29 PM   #18
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Another idea could be the low pass formed by the output impedance of the tubes and the input capacitance of the output stage.
You could measure if the tube outputs still have the phase shift, when the output stage is not coupled to the gain stage.
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Old 2nd June 2005, 05:38 AM   #19
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You give me best idea. Once, I get a result will report to you.

It have already replied from Tube Column .......

> 40 degree shift when input signal set to 25k Hz. ... square wave its output waveform change to wave shaped.

Of course. The tubes are a fairly high impedance. The MOSFET inputs are big capacitors. It is going to have a high frequency droop, and detectable phase shift before obvious roll-off. While some folks will disagree, I think 40 degrees at 25KHz is "nothing". (Put a mike right against a tweeter and measure 20KHz phase.....)

THanks to all your valuable inputs.

CK
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Old 6th June 2005, 06:26 AM   #20
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Status report :

Still have 45 degree shift on tube output when it disconnected to the middle stage. Even the 4 small value CAPs had been dismounted.

A new question discovered...

When I attempt to do a maximum power output test. Seems the the clipping single was introduced into the speaker out. When peak voltage was raised in 53VAC at 500 Hz. There is also same as the value 53VDC to the Drain of the output stage.

At that time very bad and 'zzz' sound to be heard.

Should it be increase the DC supply at output stage ? I find this FET output similar design they'd like to use 75-90vDC supply.

Will it bring more benefit when using higher DC ?
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