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Old 10th February 2012, 05:44 PM   #1911
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Quote:
Originally Posted by dadod View Post
Maybe that was not clear enough. Barny Oliver optimum point is 26mV across one emittor resistor(plus re of the transistor) to set quiescent current. If set to high you get more A class power but after that there is more distortion. 300mA is way to much.
With 0.1Ohm, 300ma makes 30mv. Not so far away.
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Old 11th February 2012, 09:48 AM   #1912
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Quote:
Originally Posted by Esperado View Post
With 0.1Ohm, 300ma makes 30mv. Not so far away.
Yes that is not so far away, but in most of the schematics I have seen in this thread it was used 0.22ohm and that gives 110mA or close. It will be good when talking about BJT output stage quiescent current to connect it to the emitter resistor used in a case.
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Old 11th February 2012, 04:39 PM   #1913
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Hello, Jay
Your amp building experience is wide and rises definite respect. At the same time, I can not agree with attributing resulting sound to a kind of active parts - tubes, mosfets, etc.
jFETs can result in amplification stages sounding better than tubes - one shoud choose those of them with few pF Ciss, rather than 100pF like for 2SJ74.
Hi Vladimir, actually we have no disagreement here.

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With tube designs, choke input PS and high quality caps usually is a must
For me it is always a must. Even for SS design. For example, find two vanilla amp designs where amp A is preferred by 60% audience and amp B preferred by 40% audience. Then give a good regulated power supply for the front end of amp B (amp A has no dedicated front end supply). I will not surprised if there would be a massive preference switch to amp B.

I was unable to integrate a tube line amp into my system, until I found out that I have to pay critical attention to the heater power supply. Yes, I use chokes for this.

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With my gratest respect to Lazy Cat and excellent measured performance of the SSA designs, I am not active in this thread because of impossibility to implement my empirically found design principles.
If one uses standard PS and Push-Pull output stage at SS design, irrespective of the rest of schematics and measured parameters, one must forget about competition with good SE Tube amps at mids and highs. With other design principles one can repeatedly win against best tube designs.
Of course, for cost-no-object design you wont consider the SSA. Neither do I. I pick the SSA for my class-B high power (at least 75W) amp. For smaller wattage amp, who wants an amp with global feedback???
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Old 11th February 2012, 10:51 PM   #1914
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Default Bias Levels

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Nico Ras - I read some years ago that John Curl also recommends 25mV across Re.
If you bias up 26mV across the emitter resistor, then re = Re, since the Zout of an emitter follower is 26/Ic.

This halves the transconductance of the output device, and it may be minimising distortion at crossover because it brings the gm doubling issue into line at the crossover event where this problem is greatest.

Your thoughts, Nico?

Hugh
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Old 12th February 2012, 10:05 AM   #1915
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According to upper claims, meaning all output transistors in A-class are way out of their optimal operating conditions, producing unavoidable self distortions ...

Are A-class crossover distortions so more beneficial and prevail over transistor's excessive bias current conditions? Where's optimum? Using 30 pairs of outputs at 100 mA each for proper A-class?
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Old 12th February 2012, 10:43 AM   #1916
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Quote:
Originally Posted by Lazy Cat View Post
According to upper claims, meaning all output transistors in A-class are way out of their optimal operating conditions, producing unavoidable self distortions ...

Are A-class crossover distortions so more beneficial and prevail over transistor's excessive bias current conditions? Where's optimum? Using 30 pairs of outputs at 100 mA each for proper A-class?
It looks like you do not understand upper claims.
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Old 12th February 2012, 10:52 AM   #1917
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Enlighten me
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Old 12th February 2012, 11:42 AM   #1918
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Me too.....

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Old 12th February 2012, 11:43 AM   #1919
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I guess u just have to decide what kind of distortion / noise you prefer.

Some low order HD does not offend me, switching noise - that not sound nice.

But I suggest forget theory - just try different bias currents and listen.
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Old 12th February 2012, 11:47 AM   #1920
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Quote:
Originally Posted by AKSA View Post
If you bias up 26mV across the emitter resistor, then re = Re, since the Zout of an emitter follower is 26/Ic.

This halves the transconductance of the output device, and it may be minimising distortion at crossover because it brings the gm doubling issue into line at the crossover event where this problem is greatest.

Your thoughts, Nico?

Hugh
You put it so simply, it must contain some truth. Using mostly lateral MOSFETS I find that the lowest harmonic distortion resulted between 160-190 mA bias current (using 2SK1058/2SJ162 combination). Since reading John Curl's advice I adjusted Rs for the required 25 mV with acceptable and consistent audible results (proportional local negative feedback).

Whether this number is as Hugh says, or from John's experience or just hocus pocus is hard to tell but it seems to have the desired effects.
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Last edited by Nico Ras; 12th February 2012 at 11:50 AM.
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