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Old 15th October 2003, 03:28 AM   #1
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Question BOZ problems - PLEASE HELP!!

OK,

I'm having some problems with my BOZ... it's finally together!!!

I have about 58V at the output of my power supply feeding the drain resistor (R104). However, I'm only getting about 5.5V at the drain (as opposed to the 20V that should be there). I am biasing the gate at about 8.5V and getting about 4V at the source. Music plays, but its very distorted, and I'm guessing its because the Vds isn't large enough. I just don't understand why this is... I'm using the specified value of 1k ohm for R104... and as Nelson says, 40mA through 1kohm is about 40V.

I'm stumped... PLEASE HELP!!!
I'm going insane... I want to hear this thing in all its glory!!!

btw, Nelson, you rock Thanks so much for your help in the community and your enthusiasm to teach!

Seriously though, PLEASE HELP !?!?
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Old 15th October 2003, 03:42 AM   #2
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What happen at the Drain when you turn P102
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Old 15th October 2003, 03:55 AM   #3
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you rock my world...

I just biased it down to 6.6V at the gate... and got about 21V at the drain. I couldn't get any closer to 20V at the drain due to poor sensitivity of the P102...

OK... IT WORKS!!! MUCH BETTER!! Can anybody tell me why??? I don't understand...
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Old 16th October 2003, 01:53 PM   #4
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I have a question for you !!! When you put the volume of your BOF at the max, do you have some noise at the output ????
And what is the resistor do you use at the input of the circuit !!!
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Old 16th October 2003, 03:00 PM   #5
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I think what happened is that you saturated the irf610 with so much gate voltage - you were biasing it up to the point where it was almost fully on, an in a very non-linear portion of its transfer curve.

Fig. 3 at http://www.irf.com/product-info/data...ata/irf610.pdf is a typical transfer curve that is specified at 50 Vds. At lower Vds values the curve shifts down and to the right, with a larger curved portion.

For low distortion you want to operate the device so that bias + signal keeps it in a linear portion of its transfer curve. (an ideal gain device would have a perfectly linear transfer function and therefore no distortion) At 8.5 Vgs you were probably on the "knee" of the transfer curve, causing the distortion. Notice that IRF used a 20 uSec pulse, since at 50 Vds and 4+ amps the device is dissipating over 200W.

This explanation may not be 100% accurate, since I my EE "knowledge" is self taught. I hope that it helps give you a macro understanding of distortion in gain devices. Somebody please let me know if I am competely off the wall, since I'd like to understand what I am doing, too.
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Old 16th October 2003, 03:03 PM   #6
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because i'm running it into my Parasound amp, I haven't gotten anywhere near cranking it up to full volume, so I haven't had a problem with noise really. but, i did crank it up once without any material playing through it, and there was some pretty significant noise... but it sounded to me like 60/120Hz from transformer and rectifier diodes. I don't have any shielding from the power supply on the actual audio circuit, and i'm guessing that's 90% of the problem. That, and I'm using a surplus transformer that was only like $4... I expect better results from a torroidal when I can afford to make some upgrades. At the levels I have run at (about 1/3 volume is about the max volume I have run it at), I don't have complaints about the noise. However, like i said, if i crank it up a lot without source playing so as to not blow up my speakers, the noise is very pronounced.

The input resistor?? I am not padding down the input via a voltage divider if that's what you mean. I'm actually running it very close to stock. changes made were to use a 50k pot at the output (rather than the specified 5k), used MUR840 diodes in the rectifier rather than the 1n4004's spec'd out, and I added 1uF bypass film/foil caps to the power supply caps. Other than that, I have the exact schematic circuit in Nelson's documentation.
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Old 16th October 2003, 03:13 PM   #7
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Um btw...

HOLY CRAP this thing sounds amazing! Talk about open soundstage.... <jawdrop>... <drool>

The background detail is just astounding... I can't wait until I get around to building my Aleph I do notice the distortion, but it is definetly "different" than the typical odd order harmonic. It's not nearly as "unpleasant" I guess. The bass detail and umph is also quite impressive! I am running a Sony NC-650V modded up... see the "Digital" forum here for more info... not your average player

Do yourself a favor and build one if you haven't already!

Thanks mr. pass...
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Old 17th October 2003, 07:55 PM   #8
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BobEllis: you write : "At 8.5 Vgs you were probably on the "knee" of the transfer curve, causing the distortion"..... Vgs is not 8,5v but 8v - 4v = 4 v because Vg=8v and Vs =4v

I take the mesure on my BOZ and I have that:
Vg=7.6v
Vs=3.5v
Vgs will be 7.6 - 3.5 =4.1v

If I turn p102, the 20 v of Vd change dramatitly


Tieftoener: Me too I don't have to complain the noise at 1/3 or 1/2 of the max...... it is after that...... maybe this is normal mecause the Zen have less gain than every amplifier on the market, so I have to drive the BOZ more that you....... maybe....
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Old 17th October 2003, 09:39 PM   #9
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Tieftoener,

I have to chuckle here a bit....I went the same route that you did....building the preamp first(Aleph L, old version), then the Aleph 2's, then Jens digitally controled analog attenuator. Just wait till you build one of the digital attenuators such as Jens unit. You think its good now ha....with a digitally controled analog attenuator not only will your jaw drop on the floor(again), but your dog will then pick it up and run off with it!! Enjoy!

Mark
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Old 4th December 2003, 08:03 PM   #10
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Default REVISION 2

OK...

I was having some problems with pretty significant noise at high volumes, but because the gain is so high, I never had the volume high (going into my Parasound amp). This was kind of annoying, but not so serious given my previous setup.

Longer and more serious listening pointed out some distortion problems - sounded like clipping - when I picked some "heavier" music selections. I was really frustrated and angry. It was really minor and only happened in loud passages, so I was really confused at first. A friend mentioned to me that perhaps the input signal was too much and it was overloading the input. It had never dawned on me prior to that because I'm so used to putting the pot/volume control on the INPUT, not the OUTPUT like the BOZ has. At first I thought "No, couldn't be - it's biased up at like 8V... leaving +/- 2V swing before turnoff (approximately)..." But then I remembered that my biasing was pretty messed up (see above posts). Suddenly it made perfect sense. I was really releaved, because at first I had thought that maybe it was just lots of distortion due to it being a single stage without feedback. That really didn't make much sense since it only happened during peaks and loud sequences. But my bias was down near 5.8-6V. This only left about +/- 1V - and that's peak... NOT rms. Some CD players have 1-1.5V rms output, so this really reassured me that I had found the problem.

Before I even got it working, I had messed up the traces coming off the rectifiers and got + and - mixed up. As soon as I plugged it in my TIP29 blew up... literally. It was cool I left it plugged in, not thinking much of it while I was inspecting and gazing at the exploded BJT... then the main (1st) power supply cap begain emitting "magical blue smoke." It only took me a few seconds to figure out the supply was reversed. I cut some traces and soldered some jumpers to fix it. I still had no sound. It turns out that I had mixed up some traces between the source and the gate - the input and bias was going to the source and the gate was grounded. Again, I had to rearrange some components and cut some traces.

In addition, I took the preamp to show some other friends and my buddy had killed a channel. He was helping me load equipment up and knocking it around and grabbing it all over the place - being very rough. I was kinda' ticked, but nothing physically broke, so I didn't say anything. It just bothered me he didn't take better care of it considering the time I had put into it. Anyway... When we got to hooking it up to listen to it, one channel didn't work. I was ticked and spent a good 15minutes going over wire connections to make sure a solder joint hadn't broken off one of the pins on the POT or the RCA connectors or something. After a while I was pretty confident all wires were in the right place. I had a flashback of my buddy grabbing the board to put it back in its open enclosure... it hit me - he killed one of my FETs. (I took the Zeners out due to the previous problems). This was the last straw... time for Rev2...

So after all that, I redid the layout and fixed the power supply problem... fixed the input traces... and I added a voltage divider resistor network at the input and cut the CD signal 6dB that feeds the input. I also put all new components in the board. Plugged it in and everything worked great. I adjusted the bias, and things sounded wonderful right at Nelson's recommended ~8V bias. The drain voltage was spot on with Nelson's approximation of 20V. Nothing much as changed other than rearranging a few things, and now the original noise at loud volumes is totally gone. I can put volume at max, and not hear any noise that was there before.

However, now, I'm getting a CONSTANT noise level, no matter what the volume. It's minor and barely audible at my chair. The transformer is a cheap wholesale piece of garbage that I got for like $4... so I can't complain much. The transformer does mechanically vibrate - I can feel it with my hand, and hear it when I get close to it. That noise is actually louder than the noise coming through the speakers. But it is the exact same noise. No matter how loud the volume is, the noise through the speakers remains the same... this is what is stumping me. In the next few weeks, I'll be purchasing an Avel transformer as a replacement. I will let you all know if anything changes.

Again, I'd like to thank Nelson Pass for sharing his knowledge and time with us!! This thing sounds absolutely AMAZING. I have three buddies that want one now - they want me to do a 6 or 8 channel version for home theater... i just have to figure out how to switch 6 channels at a time for multiple inputs - a multiplexer i suppose, but I don't know of any that are good that would affect the sound as minimal as possible.
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