I wonder,why you don't modify your amplifier according to as built schematic that was published after prototype testing?😕
Not sure what you mean. When I received the PCBs - I thought I implemented all the mods and recommendations up to that point. These were all the changes to resistor values basically that were then captured in a list by Prasi and I double checked that list with what written in several posts and one compiled, I believe by yourself. So I think I did indeed build mine to the latest as built by a combination of Terry, yourself, and Ranchu32, with suggestions added by Aksa.
After that amp was built it had a big DC offset of -2.5 to -2.8v. It seems the main cause was a combination of my LEDs having too much voltage drop and the resistor values as recommended did not work well together.
I think the offset issue is now solved. What remains is for me to debug what is causing my #1 amp to remain bad after I shorted the base of BD139 to 35v rail.
This is the schematic as built,first time working.
If you want modify yours according to this.
Attachments
There are four factors which influence the offset range. Generally you can move the range around, but differences in the parameters in the four aspects mentioned do not guarantee that the offset will finish up in the middle of the VR1 trimpot at 0mV.
This is one of the most irritating aspects of amp design and production and I live with it. There is no wrong and right; absolutely wasting time laying blame for this. Watch worries me is that output offset from -100mV to +100mV has NO EFFECT on the quality of the sound, let alone any possible damage to the speaker.....
Hugh
This is one of the most irritating aspects of amp design and production and I live with it. There is no wrong and right; absolutely wasting time laying blame for this. Watch worries me is that output offset from -100mV to +100mV has NO EFFECT on the quality of the sound, let alone any possible damage to the speaker.....
Hugh
I may try etching a new board today and set it up per the latest suggestions. To be clear, is R4 to be removed?
As for the sound, I never tried listening until I had the offset to 0v so I can't say if the sound was the same with higher offset.
I listened with -2.5v offset (yikes!) and it sounds pretty good 🙂
When I had R4 in place, my offset would not budge and was positive 2v or thereabouts.
Up to now, I had built mostly amps with twin transistor LTP differential input stages. My job was simply to match Hfe of the pair and bingo - offset was typically +/-1mV to 5mV no offset trimpot needed. So I can see that is a good way to go if you don't like futzing with DC offset. However, I guess that kind of input stage has a different sound than what we are doing here. Why was not an LTP used here?
When I had R4 in place, my offset would not budge and was positive 2v or thereabouts.
Up to now, I had built mostly amps with twin transistor LTP differential input stages. My job was simply to match Hfe of the pair and bingo - offset was typically +/-1mV to 5mV no offset trimpot needed. So I can see that is a good way to go if you don't like futzing with DC offset. However, I guess that kind of input stage has a different sound than what we are doing here. Why was not an LTP used here?
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Yep remove R4 it changes both range and the mid-point of the trimmer; way off 0mV!
A LTP with perfect offset and balance delivers low second harmonic and generally sound cold and sharp due to higher 3rd harmonic. A singleton like this quasi circuit gives high H2 and H4, not much H3. These low orders in the distortion profile have profound influence to the sound quality. But the downside of a singleton is the input bias voltage - look at the problem this has caused here! - but the LTP is foolproof and also removes any startup and turnoff thumps. This is the reason it is popular and avoidance of adjustments at production is vital for economy.
HD
A LTP with perfect offset and balance delivers low second harmonic and generally sound cold and sharp due to higher 3rd harmonic. A singleton like this quasi circuit gives high H2 and H4, not much H3. These low orders in the distortion profile have profound influence to the sound quality. But the downside of a singleton is the input bias voltage - look at the problem this has caused here! - but the LTP is foolproof and also removes any startup and turnoff thumps. This is the reason it is popular and avoidance of adjustments at production is vital for economy.
HD
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@Aksa: I can see why you prefer singleton inputs - recall reading that is what you prefer to use lately. It does subjectively sound different when compared to the VSSA, the sound is 'sweet' - euphonic components I suppose.
@Mosquito: I tried the sharpie pen trick already. Not dark enough at all. Maybe black paint applied with a small brush would be better. It's quite amazing how brilliant that blue LED is - very piercing high intensity light.
@Ashok: thanks for the tip on shrink tube! Perfectly good solution. I already use them to tie LTP's together for thermal stability. Forgot all about it...
First, I'm not sure if my question about removing R4 was answered. Is R4 to be removed from circuit?
Secondly, The circuit calls for a 3.3V zener. I used a blue LED because I didn't have a zener that small. I'm not sure what the fuss is about how bright a blue LED is. It will likely never be seen as the case will hide it but if it offends use the zener. Problem solved.
AFAICS XRK has still not solved his offset issue. Not to mention that I changes drastically as the amp heats up. A few days ago I set the offset to zero after the amp played for about 10 minutes. Then I let it play fairly loud for about an hour and when I checked it again the offset was 40mV. I had to adjust it again. The next day I started it up cold and it was off by 40mV again. Probably not a big deal but I thought I would mention it.
Secondly, The circuit calls for a 3.3V zener. I used a blue LED because I didn't have a zener that small. I'm not sure what the fuss is about how bright a blue LED is. It will likely never be seen as the case will hide it but if it offends use the zener. Problem solved.
AFAICS XRK has still not solved his offset issue. Not to mention that I changes drastically as the amp heats up. A few days ago I set the offset to zero after the amp played for about 10 minutes. Then I let it play fairly loud for about an hour and when I checked it again the offset was 40mV. I had to adjust it again. The next day I started it up cold and it was off by 40mV again. Probably not a big deal but I thought I would mention it.
Yes sir, some people worry very much about even a few mV. 100mV translates to 1.25mW of dissipation in speaker, am I correct (0.1V^2/8 ohm)? There should be absolutely no danger to speakers at this offset. As Ranchu32 mentioned earlier, he doesnt want to complicate the circuit with a servo. I feel it should be kept as it is with components as optimized by you.
reg
Prasi
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I agree it doesn't the matter of blue led visibility
Please let us doing the best for offset stability.I'm afraid that isn't a way.
This topology without servo controller isn't possible to stay at 0v as Q1 raise or fall his temperature.
When i tested this amplifier i put the Q1 into a piece of polyurethane. Much stable offset😉
Good idea about temp stabilization of Q1 Thimios. Some foam core hot glue and shrink tubing should fix this one right up. 🙂
But how far off centre does the speaker driver get pulled with that output offset?
A very sensitive driver could be pulled so far it uses up a significant portion of it's Xmax.
its at zero hz, so tweeters are out of question. (tweeters should not be used w/o a cap). For full rangers, even if we take a 5w sensitive speaker (fostex FF85WK) 0.1V respresents 2% of voltage required to drive it to 5W. so cone displacement will be very less. sensitivity I think does not relate to cone displacement http://www.fostexinternational.com/docs/speaker_components/pdf/ffwk.pdf.
I would not put a 100w monster amp to drive a 5W speaker, sensitive or not.
XRK, I think builds more speakers than me, so he is the right guy to answer.
reg
Prasi
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Same question here
D1 and 2 = 1n4004
D4 =1n4002 why not a 1n4004 too?
Take a look here this thread is becoming veryvery long
http://www.diyaudio.com/forums/soli...imentary-mosfet-amplifier-builders-guide.html
D1 and 2 = 1n4004
D4 =1n4002 why not a 1n4004 too?
Take a look here this thread is becoming veryvery long
http://www.diyaudio.com/forums/soli...imentary-mosfet-amplifier-builders-guide.html
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MM1964 has opened a new thread here for builders. just came to know while browsing solid state index. for info to all. great going MM1964!
http://www.diyaudio.com/forums/soli...fet-amplifier-builders-guide.html#post4791034
http://www.diyaudio.com/forums/soli...fet-amplifier-builders-guide.html#post4791034