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PostPosted: 19 Jul 2010, 13:50 
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gofar99 wrote:
Hi Everyone...The LM317 used as a CCS in the cathode circuit maintains a constant current, but allows the voltage to fluctuate in step with signal applied to the tube connected to the previous stage. This in turn provides the drive for the second tube. Recall that the it is the difference between the grid and cathode that control the current flow in a tube. With the second tube's grid at essentially ground potential it gets the difference when the cathode voltage changes. So.... anything that upsets the ability of the CCS to be linear and respond rapidly will cause non-linear action in the outputs.

Very well put IMHO. I have to admit it's surprising that the LM317 does such a good job, but my ears (and my test instruments) tell me it does.

To me the easiest way to think about the CCS is that there are 3 parts attached to the CCS input node:

-top cathode
-bottom cathode
-CCS

There is a simple law of physics that says the sum of the currents at any node is zero, that is to say, if current flows into a node (a node being a point where components connect togehter) the same amount of current must flow out of the node. Current can't magically appear and dissappear. Pretty simple concept.

So if the top tube's cathode reacts to a rising grid input voltage by increasing its current flow, and the CCS won't let the extra current pass through itself since by definition it's a "constant" current source and will resist any change in current, then all of the current from the top cathode must reach the bottom cathode and none of the change in the current through the top cathode passes through the CCS, which is what I meant by "no signal is passed by the CCS". The voltage fluctuates but the current through the CCS does not, and the fluctuation in voltage is just enough to cause the exact same amount of current flowing out of the top cathode to flow into the bottom cathode.. Essentially if the CCS were perfect there would be zero signal current flowing through it, ONLY DC. ALL of the signal current from the top cathode will reach the bottom cathode.

I DO however have a strange problem with my amp that might be the regulator. I'm getting "ripples" in the frequency response when the amp is driven hard. They look like resonance points across the audio spectrum. Up until now I haven't been able to track them down. Perhaps it's the long wires to the regulator, perhaps it's some other parasitic interaction of components or wiring, who knows? All I know is I'll keep looking until I find it and NO COMPONENT is above suspicion at this point. I'll run some new RTA tests this week once I install my new LM317 boards with provisions for any or all caps recommended by the datasheet.

If the ripples in my response disappear when I add the caps to the LM317 then problem solved. If not I'll keep looking until I find it. I'll find a pic of what I'm talking about. I think it's somewhere in another post. I also have a dual n-channel depletion mode FET CCS I'll try if the "ripples" are still there after playing around with the LM317.

Here's the trace of the response "ripples". Any ideas?

Image

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PostPosted: 19 Jul 2010, 14:42 
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Hi, in answer to a question a few posts back. The pass transistor would have to sustain up to 50 volts at what ever current you selected. For something like the KT120s it could be as high as 250 ma. (If pushed in class A even up to 300 ma a pair). I would not use a junction device if I could avoid it. Just a personal choice, I think the high power FETs are more linear and better suited for this use.

On the later comments and a bit OT. I agree that music is what we are tying to hear. Most likely it is impossible to recreate an authentic performance. It is possible to create something we find satisfying though. The method and hardware we use to do this is really quite a personal choice. I have no intention of trying to convince folks with SS gear that valves are better, nor would I suspect any real diyer would do the opposite. They are just two ways to accomplish what we all seek .... a satisfying listenen experience. 'nuf of my soap box and 2 cents.

Good listening
Bruce

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PostPosted: 19 Jul 2010, 16:05 
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I would never argue (like I used to) that valves are inferior or better than SS. I agree, the goal is a satisfying listening experience. I was skeptical about tubes until I actually built a tube amp.

The only thing I WOULD argue to most engineers who were educated in the 70s and later is that tubes are AT LEAST as capable as SS devices at audio amplification. They may be more expensive, heavier, bulkier, hotter - the list of disadvantages goes on and on, but INHERENTLY INFERIOR SOUND QUALITY is NOT among them. If you want to call that an opinion that's fine by me. I know what my amps sound like to my ears.

For those who have a "problem" with the CCS, you can always use a tube phase splitter and use fixed bias on the grids or self bias with cathode resistors and forego the SIPP configuration. My money says the amp will sound no better, perhaps worse, and it will be harder to build.

Any ideas on what could cause the ripple shown above in my response curve?

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PostPosted: 19 Jul 2010, 16:53 
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Cheap-jack said something like - The problem is THD does not tell us how an audio amp would sound. This is what I have been saying for a long, long time. We only measure what we can and it is simple and crude. Our hearing and emotions aren't.

Distortion as a measurement on or under 5% means nothing depending on the type of distortion and what makes it up. In fact I believe we, as multi-cell, carbon based, bipeds enjoy music more when there is a healthy serve of some types of distortion.

I only listen to tube amps these days (apart from my surround sound system and that may change soon) and I’m constantly moved emotionally by what I hear. And I’ll lay $100 down that if my systems was to be subjected to the best of the crude measurements we can perform the distortion levels would be very high compared a reasonable sand amp.

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PostPosted: 19 Jul 2010, 20:52 
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Hi All, Good comments. At the risk of being a little OT, but still on the subject of the "oddwatts" I was looking through my Mullard Tube design book today and found a few interesting tidbits. All the OWs from the Poddwatt to the KT120 version of the OddBlocks have rather low distortion compared to the "historic" designs. Not to take away from them, but things have moved on since the 60-70s. The best of the Mullard designs in the book had distortion levels of 2-3% before NFB and with 20-27 db of NFB got down to about 0.5%. This is with a bandwidth of 30HZ to 15K (some went to 20K). All the OWs will do under 1% without NFB and with only 3-5 db of NFB will go as low as 0.2% through mid power. At 1 watt (a common level for measurements in the early days) they are usually at the 0.1% or better. All with a power bandwidth (at full power ) of below 20 (usually 10-12HZ) to past 25KHZ Historic designs were typically +/- 1 db at 1 watt and worse at high power. The OWs are usually +/- 0.2 db at full power. So times have changed. New amp designs can be a lot better than folks think and in many cases remember. I am sure we have all heard horrible tube amps from the past :eek: . Now being fair, new SS designs are excellent. The bandwidth and distortion levels are far superior to any tube amps I know of. I have several that go from DC to several hundred kilohertz. All with distortions of below 0.05%. Good stuff, but they don't sound musical to me. This is where the personal preference comes in. I like the sound the tube amps deliver. What I wish to do with my designs is provide alternatives that are relatively easy to build and will deliver high performance to the diy community. This is probably preaching to the choir as we say as since you are reading stuff in the tube forum, you probably have already built and like the sound such projects can deliver. I apologize for getting wound up, but like many others in this forum, I'm passionate about quality music reproduction.

Good listening
Bruce

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PostPosted: 19 Jul 2010, 22:14 
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I'm still trying to figure out what's causing those ripples in my response curve. The amp sounds great, but the RTA trace bothers me.

Adding NFB extends the high frequency response but doesn't change the ripples. Increasing the amplitude of the input signal make the ripples worse. Same frequency, greater amplitude.

:confused:

What's also strange is that the peaks seem to be integer multiples of 60Hz. This tends to point away from the CCS, but it's still a mystery. The 60Hz spacing could be a coincidence.

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PostPosted: 20 Jul 2010, 00:04 
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Try some different leads. Is it lead capacitance of resinance?

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PostPosted: 20 Jul 2010, 00:08 
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What do you mean 70's design. Its only 1975 isn't it. That may explain a few of my designs??!!?? Thought I was feel'n old.

Next there will be an Iliac in every home and we will be joined by some mesh of cyber connections. HA!!!!

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PostPosted: 20 Jul 2010, 10:48 
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Looking closely at the response plot reveals (assuming the scale is accurate) that the ripples are less than 60Hz apart. Not much but enough to conclude it's a coincidence and nothing to do with the power supply. The first three lie almost exactly at 60, 120, and 180 Hz but then they start to deviate from integer multiples of 60Hz, although not by much.

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PostPosted: 20 Jul 2010, 11:03 
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gofar99 wrote:
The best of the Mullard designs in the book had distortion levels of 2-3% before NFB [/b]and with 20-27 db of NFB got down to about 0.5%.

gofar99 wrote:
All the OWs will do under 1% without NFB and with only 3-5 db of NFB will go as low as 0.2% through mid power

Hi.

As I posted here before, so many many judge an amp on its measured THD rather than by listening due to disliking or limited experience in, music listening, maybe. But does THD mean anything to what we listen, I doubt very much.

Let me quote an evidence published in Cheever's Theses on Total Aural Disconsonance Daniel Cheever auditioned a SET power amp using 45 with 5% THD without NFB vs a SS power amp with 0.5% THD. He did hear any distortion with the 45 SET but he heard distortion with the SS amp.

Why? We got to know more about the behavior of our ears. Our ears is NOT linear at all. In simple language, the non-linear behavior of the outer hair cells of the inner ear cochlea cause some masking effect to filter out certain patterns of distortions. Apparently the 45 triode generates a bunch of distortions that fit into the masking behavior of our ears.

So we just can't hear any distortion at all.

I don't mind go into more details on this human ear distortion behavior with our readers upon request.
gofar99 wrote:
All with distortions of below 0.05%. Good stuff, but they don't sound musical to me. This is where the personal preference comes in.

With the above scientific backup, I don't think this is purely "personal preference".
gofar99 wrote:
Now being fair, new SS designs are excellent.

We can't dispute this fact. But, but it still can't beat a triode which does NOT get the knee-shaped not-so-linear transfer characteristics of a P/N junction (and a pentode :worried: )

c-J

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