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Old 6th July 2002, 05:15 AM   #11
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The Doz has a bipolar input, which is much lower impedance than a FET input, and a poor choice. It has capacitively coupled output instead of direct coupled output, a total nightmare for quality audio. The output impedance is too high for low impedance headphones. Real world headphones have impedances ranging from 30 to 600 Ohms. A quiescent current of 330mA, gah. I could go on and on, but basically, it is not a great headphone circuit. If you are insistent on discrete design, at least look at Kevin Gilmore's class a amp.
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Old 6th July 2002, 11:06 PM   #12
paulb is offline paulb  Canada
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Hi morsel,
Most of the disadvantages you refer to are common to other DIY designs, like the Zen series. And the DoZ is very similar to the classic JLH, still going strong after 25 years or so (which also has headphone amp variants). If it were just a matter of specifications, then I'm sure the opamp/buffer would win hands down. But listening tests are the only true story. I'm going to have to build both and listen before I'm truly convinced that one sounds better or worse than the other. I'm not sure, for instance, that an output coupling cap is the instrument of evil some think it is.
By the way, don't get me wrong, I think you and tangent have done a great job with making this board available to DIYers. On behalf of all of us, thank you. I just placed my order.
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Old 7th July 2002, 05:30 AM   #13
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Default Regarding PassAmp or Szkeres' Derivatives

What Paul said is very true. MOSFET designs like Szkeres or even Pass amp have a little higher distortion rating. Also, Mosfets aren't necessarily so linear. On top of that, it has gate capacitance which screams for attention.

However, many Pass amp derivative fans stubbornly insistant. But they DO have valid reasoning. Somehow, I find MOSFETs to sound very warm and soothing when it deals large quescent current. I was told that this comes from high low-order distortions. Some Mosfet designs deliverately promote that, making it "warm sounding" or more typically "laid back" sounding.

I built Szkeres amp nearly ... 2 years ago. I had good time with it. I wasn't so satisfied with linearity so I added CCS. It got a little better. (I recommend this upgrade. Easy, cheap and effective. It will improve slewrate also.)

Please note, HOWEVER, that this issue is pushing the boundaries of technical suggestions to personal tastes. So I will not say which is "better." Sorry for vagueness. You just have to taste both and pick which ones you prefer. (or you can have them all!)

Tomo
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Old 7th July 2002, 05:53 AM   #14
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Default Regarding Capacitive Coupling

I think I have had fair share of dealing with capacitive coupling. I have tried few different methods of capacitive coupling and several capacitor models.

Morsel is true that capacitors can be simply considered as a source of distortions. Ideally, not having capacitor is most preferable. Though it is not completely the whole story.

I, for one, prefer no capacitors at all. However, some people use them to flavor the sound. I am not a specialist and I don't know but some people can really craft sound well. That is why not everyone go for DC coupled.

Lastly, capacitive coupling leaves very large freedom to designers. I found this rather fun and cool. I don't have to recalibrate everything to set zero DC offset all the time which tends to restrict what I can do. With cap coupling, I can try out very weird combinations without going deeply into circuit theories. (Of course, I will do so when I am building "the" amp. But sometimes, I just like to build one for giggles. That is when cap coupling is useful.)

See? It isn't much of nightmare. However, it is a matter of compromise, I have to admit. But I think you shouldn't put it entirely out of your mind.
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Old 7th July 2002, 06:57 AM   #15
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Default About Solid State Devices

(I hope you don't find me writing separate reply for each diff subject ... I have gotten wearly of miscommunication due to my poor English.)

Chip combinations or chip alone versus discrete designs have been heated issues for quite sometime. But I found it just a matter of compromise. (again!)

Opamps must have strong feedback. This is because opamp by definition have massive gain. You will have to tone down using negative feedback. This is considered to be poor among some circles. However, recently the performance of opamps have improved durastically. And opamps should be reconsidered.

Descrete design is generally considered to be better than opamp using circuits. This isn't necessarily true. Unlike opamps, interconnects that connect transistors and other components have more inductive and capacitive properties which tend to be unpredictable. So poorly built descrete design can be pretty poor, not to mention hard to fix.

So on and on. You can go on forever on this topic. I just wanted you to know both require some compromises. So keep in mind to check both types of designs.

I currently use both types of amps. I like both of them in different ways. I don't find either to be significantly favorable than the other. I listen to one one day and the other other day.

Lastly, input impedance. Generally, like morsel mentions, higher input impedance is better. But again it isn't the whole story. If so, it would be common to strap input directly to fet gate. This isn't good idea. Unfortunately, where low impedance line is connected to significanly high impedance load, you will get RF pick-ups. You have to be careful about this especially when you deal with high speed and high bandwidth drivers. i.e. My opamp based amp has input impedance of 10Kohm.
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