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Old 5th August 2001, 04:13 AM   #121
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Default How is the performance?

Now that it's finished Petter, what do you think of the performance?

Cheers,

Pete
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Old 5th August 2001, 10:05 AM   #122
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Default Performance?

Pete,

I am suffering from home redecorat-itis so finding time to complete the project is hard. What I could do fairly quickly is get some distortion measurements for the input stage by itself, and I will attempt to get that info to you.

Still, the basic principle and design goal holds. Floating input --> floating output (tied to input, really). The temporary power supply I was using had significant noise which totally disappeared when I ran the unit differentially (according to plan)

Petter
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Old 24th August 2001, 03:41 AM   #123
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Lightbulb Alternative to Active Current Sources

Petter,

Warning! Long post.

I just came across this thread the other day. For the last few years, I have also been playing around with this circuit (along with some of the Zen designs), and I have to congratulate you on your circuit. It is pretty much what I pictured the X topology to look like, so I would agree that you are on the right track.

I have long been a fan of Nelson Pass' amplifiers, and over the years, their sound has continued to improve. One of the things that stands out in his recent designs is their simplicity. Given the choice between two different implementations of a circuit, he seems to choose the simplest. I light of that, I have a suggestion for a simpler current source implementation; it is quite possible that Mr. Pass used simpler, passive current sources (i.e. resistors) in some of the circuit locations.

Referring for a moment to your original schematic, current sources Is3 and Is5 should remain active to provide adequate drive for the followers in the second stage. Is2 and Is6 would be the easiest to change to passive, since the relatively large (and constant) voltage across a resistor would provide a decent current source. Is1 and Is4 could potentially be changed to resistors, as long as you plan on using the output of the amp in its intended differential mode (more on this later). Using resistors for current sources Is1, Is2, Is4, and Is6, in combination with careful matching between transistor pairs (including the transistors for current sources Is3 and Is5), it is possible to use a single potentiometer to null the DC offset. Here is the proposed design process:

1. Knowing the Vgs of the matched pair of transistors to be used for Is3 and Is5, select a fixed voltage reference for the gates (connected together) and a fixed resistor value for the two source resistors, which results in the desired current through each transistor (50mA in your case); no adjustment necessary.

2. Select a resistor value for Is2 and Is6 that will result in approximately the same current as Is3 and Is5. Referring to your original schematic, the resistors will have approximately 46V across them, so 910 Ohm resistors would provide slightly more than 50mA through each of the two input transistors (Q1 and Q20). Note that it is not necessary for Is2 and Is6 to equal Is3 and Is5, respectively. As long as Is1= Is2+Is3, and Is4=Is5+Is6, the DC offset will be nulled.

3. Knowing the Vgs of the matched pair of transistors to be used for Q3 and Q19, create an adjustable voltage reference (with a single trim pot) for the gates of Q3 and Q19 (connected together). Next select a resistor value for Is1 (and Is4) that will result in a current equal to Is2+Is3 (or Is5+Is6) when the gates of Q3 and Q19 are at the nominal value of the voltage reference.

4. Power it up, and adjust the trim pot to null the DC offset.

Now the disclaimer. I have not actually tried this out (and probably won't in the near future, the way things are going at work). However, I did do some simulations, and as much as I hate using simulations without validating the results on actual hardware, the results were very encouraging. Starting with active current sources in all locations, I first converted Is2 and Is6 to resistors. Measuring the distortion of a 1KHz sine wave, the single-ended distortion (either output to ground) dropped by a factor of five (from ~0.5% to ~0.1%), and the differential distortion (across the two outputs) dropped about 1/10 (from ~0.02% to ~0.018%); so far so good. I next converted Is1 and Is4 to resisters; the single-ended distortion went up (from ~0.1% to ~0.8%), but the differential distortion actually decreased slightly (~0.018% to ~0.017%). So it looks like the distortion increase in each half was canceled out by an equal and opposite distortion in the other half (just what a balanced circuit is supposed to do). Not bad performance for a substantial simplification of the circuit. My only other comment is that I haven't done a sensitivity analysis of variations in resistance values, but it would be prudent to match each pair of resistors anyway.

If you decide to try any of the above modifications, please let me know how it comes out.

Dale
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Old 24th August 2001, 10:32 AM   #124
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Default How about posting/mailing your schematic?

I have used FET current sources which are harder to make than BJT. The reason for this design choice was to ensure that there was no "base current", i.e no gate current to interfere with the setup.

What did you use for simualation?

P
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Old 24th August 2001, 07:09 PM   #125
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Default Spice simulators

I have used both PSpice and Electronic Workbench on this circuit in the past. I don't trust the Electronic Workbench models as much, but that is what I used for this most recent experiment because I didn't have much time, and I find that tool faster to use for simple "what if" experiments.

As a personal preference, I generally use simulation only for investigating the behavior of subcircuits rather than entire designs, since the more complex the circuit, the less likely the simulation results will match reality. In this case I only simulated the first stage (everything except the followers).

Dale
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Old 1st September 2001, 04:21 AM   #126
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Talking Re X-100

Hi petter i don't know anythings you have complete circuit this amp and pic plaese send to me .can you help me?
thank
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Old 1st September 2001, 11:32 PM   #127
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I have chosen not to post full schematics etc. on this page out of respect to NP. Nevertheless, there is enough information on these pages to do the required reverse engineering. The conceptual schematic on page 1 is what you need to emulate -- of that I am quite confident.

Another reason for not posting is that I have been to busy to complete but the input stage -- and therefore not proven even to myself that the sound is good and the circuit stable. Now, this will be changing sooner rather than later -- which probably means Xmas

Petter
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Old 2nd September 2001, 04:26 AM   #128
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Oh sorry yanamps@yahoo.com
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Old 4th September 2001, 09:24 PM   #129
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Feel free to post the schematics, I would like
to seem them also.
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Old 5th September 2001, 05:59 AM   #130
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Petter's schematic may not be identical to the original; however, hat's off to you Mr.Pass for being a sport. And thanks a million.

Come on Petter, don't put this off till X'mas, the schematics I mean. Thanks.

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