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Old 23rd October 2006, 04:23 AM   #81
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Join Date: Apr 2006
Location: Dallas, Texas
Default Re: spice, output_impedance, damping factor, and more

Quote:
Originally posted by audiorob

As I understand it, damping factor is the output impedance / the
nominal impedance of the speaker (or close enough...). In the paper,
Nelson mentions that the damping factor is about 4 for the Zen 5. I
assume that is the output impedance of about 2 ohms divided by a
"normal" speaker's impedance of about 8 ohms. If spice is correct, that
brings the damping factor value to about 11.
Uhh... Oops!

I'm sure it is obvious to everyone that I have the equation inverted. It
should say that damping factor is the speaker's nominal impedance
divided by the amplifier's output impedance.

I hate when I do that...

Robert
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Old 24th October 2006, 09:36 PM   #82
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Join Date: Apr 2006
Location: Dallas, Texas
Default Rail voltage vs output voltage

As mentioned in Babowana's, "Papa, I want a zen 5 amp" thread, I did some
experiments with rail voltage. I posted the results there, but they really should
have been posted here, as this is the "problems wth zen 5" thread. That is to
say, sorry for the cross post...

As weird as this sounds (or, *I* do not understand this), this is what I have
discovered. With the "standard" (ie: per Nelson's paper) Zen 5, and re-biasing
for the different rail voltages:

+/- 20V Rails -> +/- 17V (34Vpp) output before clipping
+/- 25V Rails -> +/- 18V (36Vpp) output before clipping
+/- 30V Rails -> +/- 17.8V (35.6Vpp) output before clipping

I guess +/-20V rails is the sweet spot.

Does anyone have an idea why the output voltage does not rise above +/- 18V,
regardless of rail voltage? Could this be due to a max Vgs of 20V for the
IRFP240 and IRFP9240? I guess I need to trace voltages on the gate to see
if that might be it. Other than that, I'm out of ideas...

thanks,
Robert
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Old 24th October 2006, 10:36 PM   #83
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Join Date: Apr 2006
Location: Dallas, Texas
Default Re: Rail voltage vs output voltage

Quote:
Originally posted by audiorob

As weird as this sounds (or, *I* do not understand this), this is what I have
discovered. With the "standard" (ie: per Nelson's paper) Zen 5, and re-biasing
for the different rail voltages:

+/- 20V Rails -> +/- 17V (34Vpp) output before clipping
+/- 25V Rails -> +/- 18V (36Vpp) output before clipping
+/- 30V Rails -> +/- 17.8V (35.6Vpp) output before clipping

I guess +/-20V rails is the sweet spot.

Does anyone have an idea why the output voltage does not rise above +/- 18V,
regardless of rail voltage? Could this be due to a max Vgs of 20V for the
IRFP240 and IRFP9240? I guess I need to trace voltages on the gate to see
if that might be it. Other than that, I'm out of ideas...

Uhh... never mind. My scope seems to have a 25V, apparenty pp scale, that
I thought was +/- 25V. I discovered this while measuring gate and source voltage
with the scope and got strange results. As the source pins are (almost directly)
tied to the rails, I measured rail voltage with the scope and got bogus numbers. I
found the different scale, and... well... there you have it.

Another example of a gibbon at the controls.

I haven't measured rail voltage vs output voltage yet. I wanted to post this
first, to hopefully stop any waisted effort caused by my stupidity.

My apologies to any following my bad info.

Robert
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Old 7th December 2006, 04:49 AM   #84
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Join Date: Apr 2006
Location: Dallas, Texas
Default Project summarization

Hi all,

As I have the "stock" Zen 5 running and stable, I thought I would summarize some of the things I discovered as I tried this project. Hopefully these notes will help the next person who tries the Zen 5. No, I am not finished experimenting with the Zen 5, but those experiments probably should be in a different thread.

The very first thing I tried to do is to build the zen5 amplifier section only, and use a bench type of power supply to power it. My bench power supply is 30V, 3A. I tried to use what I will call a floating ground (I don't know if that is the right term or not) to get something like this:

-15V-----GND-----+15V

To make a long story short, that did not work. I needed to get another power supply (another thanks to Nelson - what an incredible guy). With the second bench power supply, I was able to get plus and minus voltages, and get the amplifier board working.

While getting that first board biased, it became apparent that I should have put both POTs on the same side of the board, so that I did not have to jump to different sides of the board to set the bias/offset.

Most of my remaining problems (arguably) are related to heat. I discovered very late in the project (uhh, I'm still discovering?) that as the different parts heat, and possibly heat at different rates, the parts vary the voltage and/or current going through them. This variation does not usually do good things for performance (sound quality). So one lesson that I learned is that having certain parts (like the output transistors) synced in temperature will help with bias and DC offset drift.

With regard to the output transistors, I was mostly concerned with dissipating their heat on heatsinks. I thought I would have one big (area wise) heatsink, and put the transistors on opposite ends of it. I made several mistakes in this regard. The output transistors should be mounted close to one another on the heatsink so that their temperature is basically the same. Next, as a rule of thumb, you get the best efficiency from the part of your heatsink that is within 10 times the base thinkness of the heat source (the transistors). My heatsink was very wide and long (like 12"x19" or so), but only .3" think. I should have had a heatsink with a much thicker base.

I made the same mistake with the transistors for voltage regulation. As your power supply rails vary in a similar way due to heat, they should be mounted close to one another on the same heatsink. That heatsink should be sized to handle the heat load of both transistors.

I think it could be argued that the same concept (ie: temperature syncronization) holds true for the source resistors too. This probably does not make nearly the same difference as syncing the output or voltage regulation transistors, but stablizing voltage is incredibly important.

I don't know if others think there is a problem with the amp reproducing square waves or not. In my efforts to reproduce better looking square waves, the length of wires (within reason) made no discernable difference. I cannot say that I fully understand Miller effect, but my guess about issue of the amp rounding 20KHz square waves is due to Miller effect and the high Ciss of the IRFP240 and IRFP9240 transistors. I also think that the "ramped" low frequency (say 100Hz) square waves is due to low input impedance. Increasing the input impedance does improve the look of lower frequency square waves, but it changes a lot of other parameters of the amp too. I am a novice at this. I could be very wrong about both.

As to the issue of sound quality, this is so very subjective that I am hesitant to address the issue. I will try, but please recognize that these are my opinions only. My amp required almost +/-4V (8Vpp) to drive it to full output. I think this makes the music reproduced by the amp to be somewhat uncompressed. To me, this was done so much as to make some quieter passages inaudible. I have added a first stage to my amps, which I think corrects the situation (+/-1.5V in -> full output). However, I think that discussing modifications to the zen 5 is beyond the scope of this thread and I hope to start a new thread on that topic soon. With the changes mentioned above, I think this amp sounds very good - but I'm sure I'm biased. It has very good bass, but I think for my speakers, it could use more damping factor. Simply for comparison, I think the Zen 5 has better bass than my X150. The midrange is also very good. However, *I* have not heard anything with better midrange than an X150, and I still have that opinion. I also think the Zen 5 has very good high end - almost as good as the X150.

So, there it is. I hope this information helpfull to others. In this regard, I would like to thank all of you that helped me with this project. It has been (and still is) a LOT of FUN. I hope to see you in a "zen 5 mods" thread.

Build carefully and be safe,
Robert
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Old 7th December 2006, 05:37 AM   #85
Formerly "jh6you". R.I.P.
 
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Default Re: Project summarization

Quote:
Originally posted by audiorob


I think the Zen 5 has better bass than my X150. The midrange is also very good. However, *I* have not heard anything with better midrange than an X150, and I still have that opinion. I also think the Zen 5 has very good high end - almost as good as the X150.
My experience on J-Low

Bass: very good +
Mid: very good +
Treble: very good +

Warm and sweet
Even at high volume, never bawl at me
Lovely

For your info, today I use input coupling caps of 6.8uF film,
replacing the original 47uF electrolytic

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