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In Reply to: RE: Garg0yle 2A3 DC posted by drlowmu on January 19, 2015 at 12:27:25
DrlowmuChokes not only filter residual ripple well, they have directional momentum.
In this application the momentum helps to stabilize the B+ against transient demands.When the voltage tries to drop, the magnetic field in the choke starts to collapse, releasing energy that opposes the voltage drop.
So while you think of my 110 ohm choke as a resistance, when called upon in action it is not that way at all.
Edits: 01/19/15Follow Ups:
"When the voltage tries to drop, the magnetic field in the choke starts to collapse, releasing energy that opposes the voltage drop."
That's actually the opposite of what happens. If the amplifier creates a sudden demand for current, the choke "bucks" it and it's output voltage goes down. This is why it's critical that the last cap have sufficient storage. Note that SE amps make different demands, as their average current draw doesn't change significantly.
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Buy Chinese. Bury freedom.
Triode_Kingdom, here is how I understand it.What I am describing is the similar to what generates a spark in an automotive ignition coil. A steady current passes through the coil, when the flow is interrupted an large surge of energy is released.
Power absorbed by an inductor is the product of the voltage across it as well as the current flowing through it.
By changing the voltage dropped across the inductor it should be altering the current through it.Here is an exaggerated view using an inadequate 1H choke in spice.
You can see red choke voltage drop while it lifts the green B+.
Edits: 01/19/15
"A steady current passes through the coil, when the flow is interrupted an large surge of energy is released."Yes, that's the same mechanism, but you're describing what happens when the load suddenly draws *less* current. Under those conditions, the field collapses, and voltage at the output of the choke spikes. The opposite occurs if the load draws more current (an action that can possibly drain the last capacitor). The magnetic field then grows larger, and the choke's output voltage momentarily dips. Both these actions are the inverse of what we want.
To expand on this concept, there is a camp on this forum that eschews large filter caps. Regardless of the reason for that preference, reducing the values of filter caps (particularly the last cap) requires that the choke's constant current mechanism be similarly reduced. Otherwise, output voltage will swing wildly in response to the amplifier's changing current demands. The downside to these Low-C, Low-L designs includes reduced energy storage and less effective filtering, and they can be significantly more susceptible to line noise, ripple, audio-band resonances and intermodulation effects. The latter is sometimes mistaken for increased bass response and "speed."
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Buy Chinese. Bury freedom.
Edits: 01/19/15
"Yes, that's the same mechanism, but you're describing what happens when the load suddenly draws less current. Under those conditions, the field collapses, and voltage at the output of the choke spikes. The opposite occurs if the load draws more current (an action that can possibly drain the last capacitor). The magnetic field then grows larger, and the choke's output voltage momentarily dips. Both these actions are the inverse of what we want."
I will have to digest this some more. However I am still seeing the output side of the choke swinging both positive and negative with respect to B+, one of those swings would have to drop the current flowing through the choke a bit releasing stabilizing emf similar to what we agree happens in an ignition coil?
I won't get to hung up on it, I will ponder and reconsider it in the future.
"To expand on this concept, there is a camp on this forum that eschews large filter caps. Regardless of the reason for that preference, reducing the values of filter caps (particularly the last cap) requires that the choke's constant current mechanism be similarly reduced. Otherwise, output voltage will swing wildly in response to the amplifier's changing current demands. The downside to these Low-C, Low-L designs includes reduced energy storage and less effective filtering, and they can be significantly more susceptible to line noise, ripple, audio-band resonances and.."
Actually I am working on some sims at the moment that correlate with this, I will my findings shortly.
"..intermodulation effects. The latter is sometimes mistaken for increased bass response and "speed."
I could see how little random high energy blips of bass could be heard. Inter-modulation with the source material would cause peaks when a transients hits on one of the wiggling B+ peaks.
Similar to what Triode_Kingdom was talking about, I was able to find a bit of a sweet spot of capacitance for my choke.The spice model is somewhat arbitrary (Has SS diodes, generic PT) but it gives me some insight as to what I can expect.
The test is at 30Hz.I started of increasing the bottom capacitor below the ultrapath in 50uF increments up to the 20% ultrapath ratio that thermonic addictions recommended. (100uF/400uF)
These C increases lowered the wiggle in the lower capacitor almost proportionately from 2.8V to 0.8V. It did not however help the B+ which stayed around 7.5V.Next I doubled the ultrapath which cut the B+ wiggle almost in half to around 4V
Making the ultrapath 300uF over a 400uF is the point of diminishing returns as 400/400 did not get any better.
So the 300uF ultrapath over 400uF yeilds wiggle of around 2V and 0V respectively.
While 700uF total seems like a lot of capacitance, due to the series connection it is much less.
**EDIT:The 400uF in series with the 300uF ultrapath would yeild around 171.5 uF as a "second" cap.Paul Joppa was right on with the ~40% ultrapath ratio he recommended earlier.
Edits: 01/19/15 01/19/15 01/19/15 01/21/15 01/21/15
I was able to find some bigger caps locally today.
I got a 270uF/400V ultrapath and a 330uF/250V lower series capacitor.
They are a little bit smaller then I would have liked, I wanted 300uF/400uF, but the big stuff was shy on the voltage ratings.
I am going to start the swap tonight.
As I mentioned earlier, the 6J5 plate choke seems to not really be needed.
I may or may not take mine out, depending if I need the room or not.
OK got the new capacitors in. They worked out really well.
-They gave the low end more definition, as it was a little smooth and whispy before. This really helps out at lower listening levels.
-They also give the transients the kick in the pants I was after. Percussion instruments have better attack now, they sound nice and jumpy.
Those where the two main objectives I was after, so it was a definite success.
As an added bonus surprisingly the capacitors did a lot to warm up the mid-range. I was not expecting this. I was able to raise the L-pads back up on my speakers. I have some small horns in my system, they sound sandier now, not as glassy as before. It's nice.
It's too bad it's so late, I will have to wait until tomorrow to really put the pedal down!
Can anyone identify this anomaly at the start and end of this waveform?
As I start to lower the source frequency below ~500Hz I am starting to notice a voltage offset. The swing seems to be fine at first, but as I go lower the relative short rise of the first swing, seems to coincide with a bit of an extra swing at the end of the waveform. It does start to compress the first swing a bit at lower frequencies.
I think I have isolated it to tye 6J5 area. My 6J5 model is not be great, I guess I could figure out how to use a 6SN7 model to try in place, thought I would ask first.
Does this have something to do with the influence from the plate choke on the 6J5? (Paul Joppa specified a 150H hammond (159H would be 20Hz -3db)) I tried playing around with the values a bit, it doesn't seem to correct much so I am not sure.
I am not going to get too excited about it, just wondering if I am seeing this correctly or if it's due to an obvious simulation error.
Below is my current schematic and a screen shot of the 6J5 plate connection.
OK well I have figured out that once the plate choke for the 6J5 gets up over 10H (series resistance being the same), causes the phase shifting I see at lower frequencies.This of course assumes my model is accurate enough.
It makes me wonder if I would be just as well to use a resistor instead of the Hammond 156C.
With just the resistor I am seeing 3mV of ripple at the 6J5 plate vs the 1mV ripple of the choke. This seems like fairly minor difference.
Audio Illuminati
Edits: 01/31/15
I decided to try the 6J5 circuit with a 10K plate resistor instead of the choke, 1.5K cathode resistor, 150u cathode bypass cap. (These values were found by trial and error if they seem odd.)On the surface it seems I have smoothed out the response and phase issue.
The 30Hz transient looks good now. It also has the same swing here as at 1Khz.
The choke setup seems to be a less linear, having higher swing at 1K.You may have noticed in the previous sims that the 6J5 plate was not sitting at 135V. I can't seem to balance the 180V, 135V and 3.4V bias readings at the same time.
Hopefully it's just a glitch in my modeling, I will have to measure the amps and see what is happening.I will have to digest this for a bit, to see how I can get this result combined with the parameters of Paul Joppa's original BBProof.
Audio Illuminati
Edits: 01/31/15
I made an EDIT to the post I am quoting.
It should have read 171.5uF as the equivalent series capacitance.
OK so now I modeled the plate capacitor.
When 100uF plate capacitor is used with the 100uF ultrapath and 100uF cap below it, (as per BB Proof Rev 1.2 drawing), the wiggle on the cathode of the 2A3 is 2.62V
Without the 100uF plate capacitor the wiggle is 2.75V (As mentioned in the previous simulation.)
Unless someone can point out additional attributes that justify the use of the 100uF plate capacitor on the 6J5, I would say it is not worth it.
The 100uF is better combined into the other positions. (Ultrapath and lower series capacitor.)
I ponder the idea of paralleling 100uF caps for lower ESR to offset the series ESR, or to just get some bigger electrolytics.
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