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Old 25th March 2012, 11:31 PM   #41
CBS240 is offline CBS240  United States
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Is the schematic accurate for the driving and output stages? The author says it uses a current feedback pair topology. Personally I have always had a dodgy view of CFP for large power amplifiers. But if the drawing is correct, it is strange that they used the mosfet drivers as source follower. The maximum peak that Vout could be is Vcc-Vgs-VR21-VR19. IRF610/9610 are vertical type fets so count on Vgsth at least 3.5V. The bias on these if the intent is class AB operation (IMHO drivers should be class A) should be at least a couple hundred mA, to mitigate the Gm droop of this type of mosfet at low currents, <100mA.

The SMD input cap is not of the right type. Since this has turned into an evaluation thread, electrolytic input cap is a poor choice as well for full range audio. I prefer polypropylene film but a 10uf PPM cap will be physically large. 1uf is a more viable size but the input Z may not be high enough for 1uf and will result in LF rolloff. Perhaps it could be modified but not without knowing what is inside the IPS block in the diagram.
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Old 26th March 2012, 12:38 AM   #42
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Default Zobel

The 10R + 100nF cap. is the standard and necessary Zobel network or Boucherot cell found in virtually all conventional amp. outputs. Doug Self gives a fair exolanation and examination of the values used. HF oscillation can kill it but not without a fight.

Perhaps the coil shown in your pic. is some more insurance?

I guess the CFP/B design refers to just the shade areas of the schematic as the output stage is EF.
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Last edited by Ian Finch; 26th March 2012 at 12:42 AM.
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Old 26th March 2012, 02:31 AM   #43
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Oops! Apologies again - it is , of course. Source Follower not EF output configuration.
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Old 27th March 2012, 11:26 PM   #44
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Default L20 V9

OK, I've measured the voltage across some of the 0.22 Ohm resistors and it 5 - 6 mV's. Thanks for the tip. I should have done it in the first place but everything is so close together I didnt want to risk it. In future I won't rely on the clamp meter at such low currents.
I can't remember exactly what I did but after restoring the circuit to standard (took out the pots), one channel expired. So it's all going on the back burner for a few days whilst I recover .
I am now using the 70Vac toroids and because the mains was up around 251Vac, the DC was about 74V and there's nothing I can do about it. I ran it at 50W's both channels and the waveforms looked good except there was a little crossover distortion visible on one channel and nothing I could see on the other.
I am now using my HP N&D set as a source (should have done it in the first place) as the kit built oscillator I was using had it's own waveform problems. The amp. waveforms now look much better. To be continued.
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Old 12th May 2012, 03:51 PM   #45
ljm_ljm is offline ljm_ljm  China
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Change a lot. To join the pre-amplifier.
Standard three Darlington structure. A coupling capacitor.
NJW power to change the four pairs tube using ONSEMI to it can achieve 250W 8R.
I named it for the L25.

The preamplifier gain is 4.7 times.
Level after about 25 times.

The above is the simultaneous measurement of the input signal sine wave output signal

2012-5-12
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Old 12th May 2012, 03:53 PM   #46
ljm_ljm is offline ljm_ljm  China
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L25 does not include the version of the pre-amp for L20V9.2.

It is output as a 200W version of the +-63V.

Comparison of previous changes slightly.
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Old 17th July 2012, 11:43 AM   #47
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Default L20 V9

I purchased two new V9 boards and rebuilt the amp, now using the 500VA toroids (+/-73VDC with 251VAC input). I had trouble again with one channel but it turned out to be an intermittent earth on one RCA input. Tested both channels to 30WRMS into 6 ohms with less than 0.01% distortion measured on my HP N&D set. I have been listening for about three hours and the amp sounds very good and getting better, almost valve like. Not as good as my 845 SET monoblocks which clean it up on clarity and detail but great impact which the SET's couldn't do in a fit. Looks like I will be heading for the metal bashers soon to make the casework.
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Old 3rd October 2012, 10:34 AM   #48
xslavic is offline xslavic  Moldova
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Default preamp input

Hi
i have same l20 v9 almost complete,running at 54.5V dc .
I have this question- whats the maximum input V rms input of the preamp ,before clipping ?
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Old 3rd October 2012, 10:57 AM   #49
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Default L20 V9

Sorry, I don't know. I pulled the breadboarded amps apart and have almost finished the final versions. If they were finished I could tell you. Anyway, if you put about 10mV RMS AC in at say 50Hz and measure the RMS volts at the output into an 8 ohm load, you can calculate the output power and then the gain. You should be able to extrapolate from there. If you had a CRO it would be easy to measure the signal across the 8 ohm load at clipping and then measure the input with the CRO. Of course you would need a resistive load rated for the expected power. Don't even think about using a speaker!
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Old 8th October 2012, 05:42 PM   #50
xslavic is offline xslavic  Moldova
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Default Found the problem and saw same on a forum

Low pass audio filter placed after amplifier - All About Circuits Forum

by removing the input capacitor(10uf) the amplifiers started to sound
and quite nice,i should remove the 10k resistor as well
- and of course Replace the 2k2 resistor with a COIL (inductor) and C2 to ,with the calculated value for the 24,5 - 25khz LowPass filter

and bring the input to aprox 6-8V preamplifaction,i dont know the upper limit
that will be normal for high end preamp

the 100k should stay in place because it brings preamp to required input impedance

sO HIGH PASS filter only after amplifier and !! Not Before
- also replaced 1n4007 with ultrafast 35ns diodes ,because 1n4007 limited speed will act as a filter for transistor switching

Last edited by xslavic; 8th October 2012 at 05:46 PM.
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