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PostPosted: 17 Jul 2010, 20:23 
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I put the 0.1uf caps on the inputs to the LM317HVs in my oddwatt just for kicks.

I hear no difference. No apparent loss of highs. No apparent anything.

Until I put a scope and an RTA on the amp I won't comment other than the above and to say my LM317's input was more than several inches from any other component and I don't see how the cap can hurt. It's too small to attenuate the signal enough to affect the amp but if the chip designers say it needs some capacatance to be stable and have the best transient response I wouldn't question them when such an inexpensive part is all that's required.

It would take hundreds of uf to bypass the audio at that point in the circuit - we're only talking about 0.1 right near the input terminal to stablize the LM317.

I'll also run some PSPICE simulations when I get time but it sounds to me like the 0.1uf on the input of the LM317 has no detrimental effect on the sound.

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PostPosted: 17 Jul 2010, 20:34 
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sampleaccurate wrote:
I don't believe it's intended to bypass audio signal, it appears to be intended to bypass ultra-sonic frequencies that are above the audio spectrum.

Hi.

You're right the I/P cap on the chip data sheet is intended for RF bypass to make the LM chip's DC functioning more stable.

But I look at it at the audio aspect of it whereby the LM chip also serves as the only signal path down to the ground. By adding a bypass cap will allow the signal flow thru smoothly w/o having to pass thru all those not-so-linear P/N junctions inside the chip.

c-J

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PostPosted: 18 Jul 2010, 03:39 
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cheap-Jack wrote:
But I look at it at the audio aspect of it whereby the LM chip also serves as the only signal path down to the ground. By adding a bypass cap will allow the signal flow thru smoothly w/o having to pass thru all those not-so-linear P/N junctions inside the chip.

There should be no signal flow through the CCS.

The CCS (LM317) is the only path to ground for the cathodes of both output tubes. Ideally ALL of the current flow to ground should be through the CCS and the CCS should have an infinite AC impedance meaning it wouldn't bypass any of the signal to ground.

The job of the CCS is to 1) keep current flowing at all times in both tubes for class A operation, 2) respond to the change in flow of current through the top tube (which is regulated by the input voltage to its grid), and force the exact equal and opposite amount of current through the bottom tube by maintaining a constant DC current flow in spite of the AC signal it sees at its input. It must have a bandwidth over which it can react which must include the audio spectrum and it must be flat over the audio spectrum and have low noise.

Ideally the CCS would represent an infinite impedance and zero audio signal current would flow to ground through the CCS. If the need for biasing didn't exist the circuit would work without the CCS. It passes no signal, and shouldn't pass any signal other than a constant DC current.

The top cathode can only drive the bottom cathode if the CCS can respond to the signal current coming from the bottom cathode fast enough in order to keep the constant current "constant", which if operating ideally would force the bottom cathode to see exactly the same signal as the top cathode - the CCS would carry no signal current. It does this by altering its voltage at the input terminal in sync with the current in the top tube so that none of that current flows into the CCS, while the CCS maintains a constant current through itself.

Hope that makes some sense. The concept of infinite impedance is a little strange. Think of a CCS as having a near-perfect inductor in series with it.

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PostPosted: 18 Jul 2010, 14:31 
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Hi Everyone, I'll see if I can clear up the fog. 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. There are, as noted, many SS devices inside the LM317s. This would seem to be a major source of problems. Measurements contradict this. I consistently get measurements below 0.1% distortion and linearity within 0.1db from 15Hz to past 25kHz. So if the LM317 causes problems they must be quite small. There are easier ways to keep from having the amps respond to crud above the audio band. There are grid stoppers in all stages, plus there is a small but carefully designed amount of global NFB. Together they keep the amps stable. With as many as are out there, both diy and kits, I have not heard of any problems in this area with ones built according to the plans. I am sure someone has tried something really unusual and has had difficulties, but that is beyond the scope of the project. Hope this helps.

Good listening
Bruce

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PostPosted: 18 Jul 2010, 16:40 
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Bruce

Did you mention at some time you had looked at or were looking at tube CCSs? Or have I thought about it and are blaming you?

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PostPosted: 18 Jul 2010, 17:01 
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Hi Mark, I have considered it, but the complications in doing it are much greater than the LM317 route. Simple it seems is better (cheaper and more reliable as well). To do a tube CCS, you would need to supply it with sufficient voltage to operate well and then derive a high gain circuit around it to keep it functioning in a true CCS mode. For small values of current this is sort of what takes place in SRPP stages. The upper section acts much like a CCS. For power amps, you would need tubes that could handle the full current of the outputs. A FET based circuit is much more practical and if I design anything larger than the KT88/KT120 circuits it will be a must as I have just about exhausted the capability of the 317s.

Good listening
Bruce

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PostPosted: 19 Jul 2010, 03:41 
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What of a single transistor (e.g. 2N3055) or a darlington pair etc. set up as a constant current source? How much volatge would the the transistor have to handle??

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PostPosted: 19 Jul 2010, 09:49 
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gofar99 wrote:
There are, as noted, many SS devices inside the LM317s. This would seem to be a major source of problems. Measurements contradict this. I consistently get measurements below 0.1% distortion and linearity within 0.1db from 15Hz to past 25kHz. So if the LM317 causes problems they must be quite small.

HI

Disclaimer: I am NOT here to thrash the usefulness of any SS devices. I am speaking purely on my aural experience.

I am glad to note you'v have raised the long standing 'misconcept' that meaurement using conventional static data, e.g. THD will tell all.

If such measurement can tell us anything remotely related to our aural conception, designing the best high defintion audio amp will be a piece of cake. The problem is THD does not tell us how an audio amp would sound.

Otherwise we don't need to build any tube amps given any SS audio amps already measured so much much superior than any tube audio amps since the invention of SS devices. But our ears tell us something else.

What worse is: many get pre-occupied by the immaculate THD data of SS audio amps & tend to believe those amps sound excellent like how they are measured given limited musical listening experiences.

Our ears don't work like the way an audio device is measured.

c-J

PS: Like it or not, audio is ALL about music listening. Anything else is not as important.

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PostPosted: 19 Jul 2010, 11:31 
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The above post is sort of along the same lines I was thinking about today, when you think about hi-fi (high fidelity) do you look for an amplifier which reproduces the music perfectly where you can hear the guitarists pick when it hits the strings or the singers breathing pattern, after all that is as close as possible as being in the recording studio.

OR do you want an amplifier to 'play with' the sound and make it a more enjoyable listening experience i.e. vacuum tubes adding warmth to the music you are listening to or a solid state amp with bass tone all the way up so the music rumbles its way towards you.

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PostPosted: 19 Jul 2010, 12:03 
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kkkittehz wrote:
do you look for an amplifier which reproduces the music perfectly....?
kkkittehz wrote:
OR do you want an amplifier to 'play with' the sound and make it a more enjoyable listening experience i.e. vacuum tubes

Hi.

Maybe I put my ambition too lofty to achieve...

I want to own an audio amp that can deliver music most accurate & most enjoyable. I believe tubes can deliver this task, if not now, .... one day.

I hate sugarly so called 'tube' sound, yet I dislake coooold SS sound. A tube amp if properly design/built can sound very accurate & yet can be extremely musically involving.

c-J

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