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Old 12th June 2011, 02:29 PM   #1001
GregH2 is offline GregH2  Australia
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All,

I have just made yet another incarnation of this amplifier that I think looks very promising. You may recall that I wasn't impressed with the JFET front end and BJT VAS. Well that was when I thought I needed a lot more drive for the fets so I was using the BD140. This one uses the BC560, which I think the JFET will have much less trouble driving.

Note that it needs no compensation, just a very small output snubber to keep it clean with no load attached.

Check out that 20kHz square wave into a 8R load at nearly full power...perfect!

I need to get some more current through the jfet, but my current dual led bias arangement won't let me as I can't turn on the JFET hard enough to get enough current through a smaller drain resistor. Will add another LED tomorrow so I can get enough voltage drop across a smaller resistor (like 150R or so).

Anyway, will post listening impressions tomorrow. I think it could be quite good!
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File Type: gif 20ksquare.gif (7.7 KB, 591 views)

Last edited by GregH2; 12th June 2011 at 02:45 PM.
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Old 12th June 2011, 02:45 PM   #1002
GregH2 is offline GregH2  Australia
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Mikelm,

To answer your questions.

No, I have never tried parallel ZVN3310s. Might be worth a go, but I really have a thing about paralleled devices, similar to your capacitor phobia. It is probably completely unwarranted though.

Actually, I have just realised over the last few days the audible affect of global feedback on the sound, and this perhaps makes some of my comparisons not completely equal. Now I understand this was a mistake and I will try to stick to a universal value from now on.

I have to say I think the amplifier sounds better with less feedback. I was trying hard to get the ideal amount of gain to suit my needs. I wanted to just be able to make the amplifier clip with a 2VRMS input. If I could not get it to stabilise I would then reduce the feedbeck until it did.

Now I can see this was a bad idea, as the feedback can really change the sound.

The 200kHz monster I posted a day or so ago sounded pretty clinical at first. But when I reduced the feedback (can't remember what to exactly, can check if you like), everything just came to life. It was like the amplifier could suddenly breathe. With the original circuit I was sitting there very analytically thinking "yes this sounds very good". When I dropped the feedback it induced foot tapping. The difference in perception was very apparent.

I am now going to stick to a 1:15 ratio for the feedback network for all future designs. Does that sound like a good compromise?

I'm sorry for my non apples with apples comparisons. I can now see that I wasn't rating everything on an equal footing.

In any case you're bound to decide on a different final design to me, I just hope the baseline I have provided you with is a good start for your project.

How is it coming along? Have you put iron to solder yet?
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Old 12th June 2011, 04:02 PM   #1003
GregH2 is offline GregH2  Australia
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All,

Please forgive me for all the continual scope shots. I'm a scientist by trade and I love playing with scientific instruments. I hope you guys get as much out of this a I do.

The schematic I posted in post #1001 is looking very good indeed...on paper at least. I have dropped the jfet drainn resistor to 200R and added another LED to the bias stack. Oh, the input cap is 2.2uF too.

I did not think 7mA would be enough to drive the laterals that well, but check out the 100kHz square wave into 8R! Not bad at all!

Also attached is the 30V p-p 1kHz fft. Is this quite good for a lashed up amplifier sitting on the floor of my room with computers and other gear lying around?
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File Type: gif 100kbc.gif (7.6 KB, 535 views)
File Type: gif 100kbc2.gif (10.6 KB, 511 views)

Last edited by GregH2; 12th June 2011 at 04:14 PM. Reason: wrong fft image
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Old 12th June 2011, 04:24 PM   #1004
Sheldon is offline Sheldon  United States
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This is a fascinating thread. Much more civil interplay between the engineering approach of "wire with gain" and some deliberate seasoning. I think it's assumed that the issue that triggers conflict, is that it's considered bad engineering to stop short of lowest distortion and perfect stability. But a big part of the hackles rising, is just that scientific methodology may be ignored. This needn't be the case. Because, although it's often not, seasoning, and conclusions arising from experimentation can be done from an engineering perspective. After all, the objective is not to make an instrument (as in measuring instrument) amplifier, but to create a stable, long term illusion that makes us let go and enjoy "they are here", or "we are there" - without sacrificing accurate tonality.

What is required is an attempt to systematically study different modifications and gather data on how their interactions can be predicted to a degree. The present project is ideal for that, because of its bare bones simplicity. It's going around in circles a little bit, but that's OK. It would be terrific if what has been learned (even tentatively) can be summarized down the road a little. Wahab's amp might represent the maximum measured fidelity end. And, the various changes and incarnations given a subjective analysis of the specific ways that loosening up the amp modifies the illusion (remembering that we never listen to an amp, but to a system in a room, or headphones). This won't be completely definable, but it's reasonable to try, with appropriate qualifications. It should be remembered that the stereo illusion is, for the most part, an artifact. When I go to live venues, and close my eyes, it harder to pinpoint the sound source that it is at home. For amplified music it's darn near impossible. But, we don't need it in the live venue, because our eyes coordinate with our ears and fill in the missing or garbled information. We need the image illusion less for video, for the same reason. So even though stereo imaging is largely artifact, it is a very useful one, and makes the reproduction more accurate, in the sense that it fools us into believing we are witnessing a live performance - which is the point after all. Same probably applies to a little added timbre.

BTW, my observations from afar, and through watching various wrestling of this issue, is that distortion and distortion profiles don't seem to be the only seasoning. More than one person has suggested that perfect square waves and perfect stability seem to weaken the illusion (as mentioned above). This is not just with SS: Is it worth using anything other than DHTs for preamps? I don't think that distortion is the issue in this citation, because the profiles can be made virtually identical for various tubes. I suspect that some physical parameter, such a microphony in the right amount and right frequencies adds a little touch of realism. Might be the same with a little ringing in the right place and right time.

Carry On

Last edited by Sheldon; 12th June 2011 at 04:27 PM.
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Old 12th June 2011, 04:51 PM   #1005
GregH2 is offline GregH2  Australia
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Sheldon,

Thank you for your insightful comments.

Yes, you are right, we are going around in circles a bit and this is mostly due to the fact that I am currently the only active builder (as far as I'm aware). Unfortunately I am perhaps not the best person to take things forward in a logical and systematic fashion. I have only been into this hobby for a short time and am still full of excess enthusiasm.

I try something and then think "I wonder how that would go with a bjt/MOSFET/ccs/bootstrap/bigger cap/smaller cap/etc/etc." Next thing I know I have a completely different amplifier!

However convoluted the road may be, I assure you we'll get there in the end!

I also think there have been some good examples here where simulations have supported listening impressions.

Anyway, I hope you continue to enjoy this thread. It has been a very interesting and educational exercise. Soon mikelm will have an operating amplifier and we can move forward in a more logical fashion. Can't wait!
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Old 12th June 2011, 04:58 PM   #1006
mikelm is offline mikelm  England
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Quote:
Originally Posted by swordfishy View Post
I have to say I think the amplifier sounds better with less feedback.
One point about terminology for clarity of discussion:

If OLG = 100dB ( x 100,000 )

and CLG = 24dB ( x 16 )

then there is 76 dB of "feedback"

So if OLG stays constant and CLG is decreased then the feedback has increased.

or to look at it another way if CLG stays constant then the "feedback" increases as the OLG increases or decreases as the OLG decreases.

So when I said you seem to like high feedback I was not talking about CLG.

You may know all this already, but I wasn't sure

cheers

mike

p.s. I don't like caps coz I tried amps with & without them and heard the difference

Last edited by mikelm; 12th June 2011 at 05:03 PM.
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Old 12th June 2011, 05:30 PM   #1007
GregH2 is offline GregH2  Australia
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Yes mikelm,

I understand the feedback terms. I prefer the sound with less feedback (more CLG) and no compensation, over less CLG ( and more feedback) with compensation. Once you hit a CLG of 10 or so it seems almost impossible to get stable without significant compensation. I now prefer to have more CLG (less feedback) with no compensation, though I didn't make the connection until a few days ago.

Btw, my capacitor comment was not intended as a stab at you so I hope you weren't offended. I was just saying that I have a weird thing about paralled devices that is probably completely unjustified, and I feel as strongly about it as you do about capacitors. It just doesn't feel right for some reason. I'd rather use a larger device. Weird huh?
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Old 12th June 2011, 05:40 PM   #1008
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I'm not sure you want to drive output MOSFETs directly from VAS & Spreader.
Cause Miller capacitance grows crazy near the rail, when the drain loses bias.
And the bootstrap implies your intent "to go there". I think you may need EF
stage between spreader and output gates... Else you don't know what goofy
unsimulatable currents are flowing across spreader resistor due variable CGD.

Then again, you got big feedback loop. And this problem doesn't much affect
the behavior at crossover, where CGD is better behaved...

Last edited by kenpeter; 12th June 2011 at 05:46 PM.
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Old 12th June 2011, 05:43 PM   #1009
mikelm is offline mikelm  England
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I think it's best to choose the CLG you need and engineer the OLG to give the amount of FB such that compensation is hardly needed.

Of course, manufacturers are duty bound to make amps that are stable with any load, which might mean they sound less than optimum with any particular speaker.

As DIYers we can engineer our amps to sound most ideal with our own speakers which I think gives us a distinct advantage.

Why should our amps have to be totally stable with 100nF when the load will never be more than 1nF ?

No offense what so ever - just thought it might be worth a try with doubling up
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Old 12th June 2011, 05:44 PM   #1010
GregH2 is offline GregH2  Australia
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Mikelm,

I can see how I confused you. It's only the last few days that I have made the connection between feedback and sound quality. Prior to that I was striving for a clg that could just clip with a standard line level signal, or , failing that goal, the most feedback I could get and remain stable. I didn't like the idea of having to attenuate my source.

As I am only using 25v rails, this meant quite a lot of feedback.

Now however, i have realized, especially with the 200khz monster, that feedback has a profound effect on the sound, and I actually prefer to have more CLG (less feedback), especially if it means I can do away with compensation. I think you will find that the miller cap especially has a detrimental effect. I would recommend trying every trick in the book to avoid using one.

Anyway, hope I have cleared that up.
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