After fighting my battles with the transformer hummmmm on my DoZ, I thought I had the bugs worked out. However, there was one problem that really bugged me...on power-up, the bias current is very very high...my meter, which is slow, shows about 5A while there is still 35+ volts on Q3 (see schematic). I suspect that, if I could see instantaneous values, I'd see 6 amps or more current through the output stage while nearly the whole supply voltage is still on Q3. It falls 'fairly' quickly, but I was worried about the instantaneous values that I could not really see and suspected that the SOA was being violated. I wrote Rod and he replied:
After about 3 weeks, one channel blew. It was not overheated, as this amp is fan cooled. All it could be is SOA violation at power up hammering away on the output transistors, eventually killing 'em.
I have a buddy, ilimzn, who frequents this forum, as well as Audiokarma.org. I asked him to look into this huge current spike at power up, and perhaps suggest a way (besides using more than one pair of transistors) to make this a relaible amp. Here's his take:
I replaced C4, the 100µf 63V cap with a 2200µf 10V cap, and I reduced C6 from 470µf 25V to 330µf 25V. C4 does not need to be rated at anywhere near 63V...at power up, the voltage on it rises to a max of about 5.2V, and then falls to about 4.1V as the bias stablizes. I also got a .05 ohm 3W resistor designed for current monitoring and stuck it on the emitter of Q3.
NOW when I fire up the amp, the current rises slowly to a maximum of about 2.5A instead of 5+ amps, and then falls to about 1.5A where it slowly waits to rise as the transistors begin to heat up. Also, because the current rise at power up is slow, C6 has a chance to charge, and instead of a large portion of the supply voltage being on Q3 when the current peaks as it used to, the voltage is almost centered when the current peaks and therefore is well within the SOA of the output devices. I've also noticed that the addition of the .05 ohm resistor has made the bias current much less dependant on the temperature of the output devices...
I would add that if parallel output devices are used and current sharing is working, the amp may very well deal with the turn-on current spike just fine. My amp and others, using only one pair of devices per channel, just can't handle that huge current.
I just wanted to share the info for others, as the DoZ seems to be a popular project. I'd bet Rod would have tackled these problems had he actually built the amp and used it for some length of time, but he admits that he has not. But I have read numerous posts about the DoZ here that go something like "It sounded pretty good until it blew up for the 5th time". Perhaps with these modifications the DoZ fans here might be able to enjoy a bit of reliability to their amps.
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I liked the silent start-up, so I left C6 alone.
After about 3 weeks, one channel blew. It was not overheated, as this amp is fan cooled. All it could be is SOA violation at power up hammering away on the output transistors, eventually killing 'em.
I have a buddy, ilimzn, who frequents this forum, as well as Audiokarma.org. I asked him to look into this huge current spike at power up, and perhaps suggest a way (besides using more than one pair of transistors) to make this a relaible amp. Here's his take:
Well done!!
I replaced C4, the 100µf 63V cap with a 2200µf 10V cap, and I reduced C6 from 470µf 25V to 330µf 25V. C4 does not need to be rated at anywhere near 63V...at power up, the voltage on it rises to a max of about 5.2V, and then falls to about 4.1V as the bias stablizes. I also got a .05 ohm 3W resistor designed for current monitoring and stuck it on the emitter of Q3.
NOW when I fire up the amp, the current rises slowly to a maximum of about 2.5A instead of 5+ amps, and then falls to about 1.5A where it slowly waits to rise as the transistors begin to heat up. Also, because the current rise at power up is slow, C6 has a chance to charge, and instead of a large portion of the supply voltage being on Q3 when the current peaks as it used to, the voltage is almost centered when the current peaks and therefore is well within the SOA of the output devices. I've also noticed that the addition of the .05 ohm resistor has made the bias current much less dependant on the temperature of the output devices...
I would add that if parallel output devices are used and current sharing is working, the amp may very well deal with the turn-on current spike just fine. My amp and others, using only one pair of devices per channel, just can't handle that huge current.
I just wanted to share the info for others, as the DoZ seems to be a popular project. I'd bet Rod would have tackled these problems had he actually built the amp and used it for some length of time, but he admits that he has not. But I have read numerous posts about the DoZ here that go something like "It sounded pretty good until it blew up for the 5th time". Perhaps with these modifications the DoZ fans here might be able to enjoy a bit of reliability to their amps.
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An externally hosted image should be here but it was not working when we last tested it.
Well, I could comment that I am ashamed of my own typos in that quote above 
There was a thread on the DOZ, maybe this should have been posted there, or at least referenced from there. Or maybe people are more interested in gold leaf papyrus in virgin olive oil capacitors wound by peruvian virgins during a fool moon (now movin into my flame proof bunker )
There was a thread on the DOZ, maybe this should have been posted there, or at least referenced from there. Or maybe people are more interested in gold leaf papyrus in virgin olive oil capacitors wound by peruvian virgins during a fool moon
(now movin into my flame proof bunker )
Lol!! I fixed a few, but the concepts are there, so we don't care about stinkin' typos...
I can't thank you enough for your help...I consider myself quite fortunate to know you.
sith, the changes have made a definate improvement in reliability. I would recommend them to anyone.
Here's something Rod doesn't give you:
An externally hosted image should be here but it was not working when we last tested it.
Note that I show the 1K resistor for the current limit for the zener as well as the cut trace, and the zener is on the lower right. I also do not trust the pot at the input to be the only DC return, so I added the R702, the 464K resistor. I also show the trace I cut to add the .05 ohm resistor...I drilled holes in the board and put the resistor on the foil side to keep it all neat. Lastly, no filtering on the zener bugged me, and I added the 47µf 50V cap.
The capacitor, C6, could probably be reduced even further to 220µf and that would probably reduce the current spike at start up even more, but the values shown work fine, and I'm not worried about 2.5A @20V on the output devices for as short a time as it lasts.
Hi EchoWars,
Thanks for posting your findings on the DoZ. I hadn't replied as my DoZ test amp has been out of action for last week waiting for the temporary wiring to fixed up. Your post has prompted me to complete the wiring. I'm using the pre-zener circuit and so far, nothing has failed after a month. I hadn't noticed the surge current issue you mention until I turned it on tonight. Initially my multimeter showed a overload condition and by the time I changed scales the current was at 2+ amps. It soon dropped to normal. I haven't had a chance to let everything cool to try again.
I realise you have been concerned with reliablility but have any of your mods had an effect on the sonics of the amp.
BTW your link worked the first time I looked at it.
Thanks for posting your findings on the DoZ. I hadn't replied as my DoZ test amp has been out of action for last week waiting for the temporary wiring to fixed up. Your post has prompted me to complete the wiring. I'm using the pre-zener circuit and so far, nothing has failed after a month. I hadn't noticed the surge current issue you mention until I turned it on tonight. Initially my multimeter showed a overload condition and by the time I changed scales the current was at 2+ amps. It soon dropped to normal. I haven't had a chance to let everything cool to try again.
I realise you have been concerned with reliablility but have any of your mods had an effect on the sonics of the amp.
BTW your link worked the first time I looked at it.
Hi guys,
I have been seeing a lot of posts about the DoZ.
May I ask what the reason is that so many folks build this amp? I have never heard one. Is there something special about the way they sound as say opposed to Rod Elliot’s P101 or P3A?
I am very new to this hobby so this may seem like a dumb question to some of you, but I have read many posts about this amp and have yet to really understand the attraction for this design.
Thanks, Terry
I have been seeing a lot of posts about the DoZ.
May I ask what the reason is that so many folks build this amp? I have never heard one. Is there something special about the way they sound as say opposed to Rod Elliot’s P101 or P3A?
I am very new to this hobby so this may seem like a dumb question to some of you, but I have read many posts about this amp and have yet to really understand the attraction for this design.
Thanks, Terry
The DoZ, JLH, Zen variants are all class-A amps. P3A and P101 are class-AB.
I think the idea is to eliminate crossover distortion and produce "happy" distortion somewhat like tube amps. Check out Rod's Class A Amplifiers - A Brief Explanation. Last updated 5/02/2005.
Also look start looking at Nelson Pass' Zen articles at http://passdiy.com
As to why DoZ v. JLH v. Zen? I chose the DoZ a couple of years ago as my first DIY amp. I knew nothing at the time (I know next to nothing now). With a whole forum dedicated to Pass and his throngs of admirers, I chose to root for the underdog. Largely an emotional decision. Also, the DoZ seemed more economical to build at the time, but I cannot tell you if that is actually true.
I'd like to go through the Zen series soon and start to really understand how all this stuff works.
I think the idea is to eliminate crossover distortion and produce "happy" distortion somewhat like tube amps. Check out Rod's Class A Amplifiers - A Brief Explanation. Last updated 5/02/2005.
Also look start looking at Nelson Pass' Zen articles at http://passdiy.com
As to why DoZ v. JLH v. Zen? I chose the DoZ a couple of years ago as my first DIY amp. I knew nothing at the time (I know next to nothing now). With a whole forum dedicated to Pass and his throngs of admirers, I chose to root for the underdog. Largely an emotional decision. Also, the DoZ seemed more economical to build at the time, but I cannot tell you if that is actually true.
I'd like to go through the Zen series soon and start to really understand how all this stuff works.
Hi ultrachrome,
Thanks for the reply. So, in your estimation, does the DoZ actually sound "happier" than a P101? I have built the P101 and it sounds really good.
In almost every post I see about the DoZ, the guys are asking what they can do to make it sound good. They also are asking what they can do to get rid of some this heat.
In my mind, that heat = wasted electricity. Do these class A amps use more energy than AB amps of the same wattage?
Thanks, Terry
Thanks for the reply. So, in your estimation, does the DoZ actually sound "happier" than a P101? I have built the P101 and it sounds really good.
In almost every post I see about the DoZ, the guys are asking what they can do to make it sound good. They also are asking what they can do to get rid of some this heat.
In my mind, that heat = wasted electricity. Do these class A amps use more energy than AB amps of the same wattage?
Thanks, Terry
Based on what I have read and a little of what I have heard, class-A amps are generally considered to sound more tubey/warmer/smoother than your average class-AB amp. But as always YMMV.
I thought mine sounded pretty good out of the box but I still have more work to do on it.
Class-A runs hot. It's the nature of the beast. They also like regulated power supplies which add even more heat. That's why they are usually lower powered designs so at least the heat is manageable.
Read Rod's article I linked earlier. It will answer a lot of your questions.
I thought mine sounded pretty good out of the box but I still have more work to do on it.
Class-A runs hot. It's the nature of the beast. They also like regulated power supplies which add even more heat. That's why they are usually lower powered designs so at least the heat is manageable.
Read Rod's article I linked earlier. It will answer a lot of your questions.
It is this large current that is banging away at the output devices, and is what I wanted to get rid of. The solution is simple, and will increase reliability tremendously.grege said:
The amp both measures the same on the bench, and sounds the same. Nothing I have done would or should have any effect on the sound of the amp.
I found an error after I posted it the first time. I'm using ImageShack for pics, and I have no control over image names. Since it was past the time DIYAudio allows for editing, all I could do was post another link to the corrected drawing.BTW your link worked the first time I looked at it.
Sorry Rod...
I thought it quite nice to have a component view from the foil side...sure made t-shooting easier. Did not intend to step on anyone's toes.
I don't suppose I could get Mr. Elliot to chime in with his thoughts? Unlikely now, I supposed, as I seem to have stirred up a hornets nest.
I thought it quite nice to have a component view from the foil side...sure made t-shooting easier. Did not intend to step on anyone's toes.
I don't suppose I could get Mr. Elliot to chime in with his thoughts? Unlikely now, I supposed, as I seem to have stirred up a hornets nest.
I hope not...we've corresponded briefly, and I must say I'm an admirer of his efforts.paulb said:
Certainly I would not recommend this without the boards from Rod. They're cheap and save a whole bunch of trouble. I'd like to see more about the amp here at DIY...I'm definately a fan of minimalistic designs.
hi still4given,
I built a DoZ because I wanted to try a simple class A amp and learn what hot is. The DoZ was very simple and relative cheap to build and I already had the output transistors from an old power supply.
The DoZ compares favourably to my other amps (a P3A for example). The only potential issues are its 20 watts, its inefficient and its hot.
BTW: 20 watts seems more than adequate for me.
I built a DoZ because I wanted to try a simple class A amp and learn what hot is.
The DoZ was very simple and relative cheap to build and I already had the output transistors from an old power supply.The DoZ compares favourably to my other amps (a P3A for example). The only potential issues are its 20 watts, its inefficient and its hot.
BTW: 20 watts seems more than adequate for me.
What voltage are you running the amp at? I have ordered my boards and cant wait to get going. I'm going to use BrianGT's snubberised power supply per channel to start with and work from there.
Is stability at start up an issue straight from standard or is it from any mods/differences you have made?
Is stability at start up an issue straight from standard or is it from any mods/differences you have made?
Read the first post...stability is not really the issue...relaibility is, and it is due to the original design...the mods I have made have only been to reduce this large current at power up. The large value of C6 along with the small value of C4 causes the large start up current. Make C4 as large as possible (I of course used a 2200µf 10V cap), and make C6 330µf as I did, or even 220µf (if you go to 220µf or smaller, you will almost certainly need the zener stabilization added). I also strongly recommend the .05 ohm 3W resistor on the emitter of Q3.JRKO said:
Also, Brian's power supply is a split supply...+/- voltages available. You plan on using split voltages for the DoZ?
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