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Old 15th May 2003, 01:27 PM   #11
panos29 is offline panos29  Greece
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Lightbulb Some light at the end of the tunnel!

O.k
After alot of search on the amp and many hours , I found out that the current source should be at exactly 50% setting otherwise distorts all signals above 10Khz very much. Moreover I found that feedback cup should be more than 10p(to my amplifer) towards 15-18p to remove the square wave ringing and distortion. Additionaly the 1n cap between the gate and and drain of negative side mosfets is very much needed as installed in the aleph3 and aleph5. Biasing the circuit to 3 amp without the active current source in the game(R21 removed) gave the best square and sine wave responce by far. Obviously the active current source is more of a marketing trick to get some more power in 4 ohms(flames welcomed ), or on the other hand maybe there is something I do not understand at all, maybe there is not a direct relation between sound quality and square wave responce. O.k I know this is something overtalked in here but I wish somebody to remind the exact procedure to measure the current source with and without R21(aleph30)? Measurements were taken with a tektronix 475 and a Fluke 8840A (true rms option installed). I will post the exact measurements hopefully tonight. thanks to everybody for the interest!
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Old 15th May 2003, 03:00 PM   #12
Raka is offline Raka  Europe
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Panos,

What about the sound now? Do you still think that your gainclone equal aleph sound?
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Old 15th May 2003, 07:41 PM   #13
panos29 is offline panos29  Greece
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Some new measurements were made right now. Vcc=+-28, bias 1,6A, at 11Vrms on 7,5Ohm square excellent at 1khz to 5khz, good to 40Khz, sine wave perfect to 60Khz. At 15vrms square excelent at 1khz good to 5khz acceptable to 20khz, sine excellent to 50khz. At 4ohm, excellent square 1-5khz good to 40khz, sine perfect to 60khz. Frequency respose at 7,5ohm from 100hz 15Vrms to 30khz 14.7Vrms. Notes: in all measurements distortion is evident in increasing frequency or power output. Current source behaviour is : increasing the gain corrects the sine wave but distorts the square wave so I choose a middle solution. Can someone guide me to measure the current gain please?
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Old 15th May 2003, 08:10 PM   #14
panos29 is offline panos29  Greece
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Default Current gain measurements

Forgot to mention the AC current measurements on the 0,47ohm power resistors measured with 9Vrms ouput power on 8Ohm load. So, on the 6 paralleled output resistors I got 83,3mV AC, on the negative side resistors 87,3mV AC on each one and on the current source resistors 89.5mV AC on each one. R21 is a 2k trimmer. What is the current gain and how can I calculate it?
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Old 16th May 2003, 06:35 AM   #15
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Hi Panos,


I think the current gain is defined as the percentage of the total output current that is delivered by the active current source.

So at 1A bias and 2A peak output current the ac-current-gain is 50%. (1A extra)

At 1A bias and 3A peak output current ac-current gain would be 66.6%

The simplest way to measure it is to measure the bias and the peak output current (not directly but with the voltages over source and output resistors/or load resistor)

If total bias is b and peak output current is c the ac-current-gain would be ( 1-b/c) x 100%

You can also look at the ac-currents through the source resistors. At 50% these should be the same for the positive and negative side (source and output) ((with a load on the amp of course))

So when you have 89.5mv and 87.3mV your ac-current gain is a tad over 50% (50.6%).


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Old 16th May 2003, 06:44 AM   #16
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Post Re: Current gain measurements

Quote:
Originally posted by panos29
Forgot to mention the AC current measurements on the 0,47ohm power resistors measured with 9Vrms ouput power on 8Ohm load. So, on the 6 paralleled output resistors I got 83,3mV AC, on the negative side resistors 87,3mV AC on each one and on the current source resistors 89.5mV AC on each one. R21 is a 2k trimmer. What is the current gain and how can I calculate it?

According to your measurements you have: 1,06 A output current, 557 mA on the negative side and 571 mA on the current source. The output current should be the sum of the other currents (well, minus the current flowing in the feedback resistor..), but taking in account wirewound resistors value spread (often 10%) your measurements are OK.

Therefore your AC current gain is slightly higher than 50% (calculated values can be between 50.6% and 54%, depending on which currents you assume as "good"), so I think everything's OK.

To make a more precise measurement there is another thread where such a method is described.

Cheers

Andrea
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Old 20th May 2003, 07:22 PM   #17
panos29 is offline panos29  Greece
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Default Measurements

I finally found some time to take some pics of the measurements I described earlier in this thread. After 1 hour warming at 60 degrees celsius, +-28v, biased at 1,6A. At top of scope is an RMS AC voltmeter and coresponding readings. No1 and 2 pics are 1Khz sine and then 1khz square just before clipping using the same setting on my signal generator, just pushing the square button after sine.
No3 and 4 the same at 10khz.
No5 and 6 the same at 20khz
No7 clipping sine at 1khz
No8 testbed

And my final question is: are these measurements typical of aleph topology or something is wrong? Soundwise the amp got lots of power and good control to my 15" Tannoys, but compared to my gainclone is a bit tiring to the ear although a bit more "alive" but less gentle and analytical compared to the simple gainclone using the same powersupply.
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Old 20th May 2003, 07:23 PM   #18
panos29 is offline panos29  Greece
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Old 20th May 2003, 07:25 PM   #19
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Old 20th May 2003, 07:28 PM   #20
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