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In Reply to: RE: LCR Phono stage posted by dave slagle on April 28, 2021 at 16:42:09
What about a simple MosFet emitter follower?Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 04/28/21Follow Ups:
Hey,
This enters the "slippery slope" area that often derails audio. A mofset follower absolutely will work and the measured results will be nearly "ideal". Since we all know "perfect" devices do not exist it brings up the old adage that just because you can do something doesn't mean you should. Way to many people in audio base their adver-speak on the engineering/technical size since that is unassailable. I tend to gravitate to and make note of those who have ears on experience that does not 100% align with the engineering status quo. Audio is one of those odd slivers of reality that is akin to the wild west and the guy with the best aim will win not the guy with the most expensive or most accurate gun. Audio systems should be curated and not engineered.
As a footnote, this is a Vacuum tube based audio forum so right there we are already against 80% of the audio world and 99% of the engineering world. Suddenly reverting back to the ideals of the 99% seems hypocritical to our fellow 1%er brethren.
dave
OK. So, in your opinion after building one and massaging it to bring out the most musicality, what is wrong with the sound of a MosFet emitter follower sitting between a tube gain stage and a LCR phono EQ followed by a tube gain stage?
Or maybe I should ask, where does a MosFet emitter follower rank on you list of ways to drive a 600ohm load but "I [don't] want to listen to for more than 15 minutes."?
First? Last? Doesn't even make your list?
Thanks
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I haven't tried the a mosfet follower to specifically drive a 600Ω LCR but have heard several variations at the input of an autoformer volume control in an attempt to buffer it. The results were similar to a cathode follower in that the cure was worse than the disease. With a source that has a 3KΩ output Z I preferred the sound without the buffer... moving up to a 7KΩ source it was closer to a coin toss and to me the proper solution is to chose a suitable "low Z" source rather than buffering a high Z one. Maybe there is a magical symmetry out there between one specific Mosfet / 600Ω LCR pair and I haven't explored all the options but I do discount general statements in audio suggesting a generic "perfect" solution to a problem. Ultimately it becomes a balancing act between choices of compromise and the choices made are what leads to a "House Sound"
dave
"With a source that has a 3KΩ output Z I preferred the sound without the buffer"
This part is interesting to me. What is the reactance, @20Hz, of the inductance of the autoformer volume control?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
figure 200hy's so 25KΩ. I have had several people use them with tubes like the 26, 27, 56 or 76 and love the results but it is a bit to close to the "edge" for the left side of my brain to comprehend.dave
Edits: 04/30/21
This is the silk volumcontrole.
The measurement is made with a load of 10k. With the more traditional load of 100k the peak is +15dB.
Full review on https://audio-creative.nl/projecten/diy-passieve-voorversterker-met-tvc-deel-1/4/
I agree with you about this type of volume control.
Walter
That isn't necessarily representative of all magnetic volume controls.
This one isn't much better. And 5k output impedance is a very high output impedance.p.s where is the 10Hz-1000Hz part?
Edits: 04/30/21 04/30/21
5K output impedance? where did you get that number from? Did you read the source impedance of 500Ω in the plot title as 5000?
The full details including full bandwidth response against source impedance were in the referenced link titled "The details"
the LF plot from that link is below and aside from the settings with gain, the LF response doesn't change for a given source Z so repeatedly showing the 10-1000hz trace doesn't add any new info. The LF behavior is based on the inductance and once the full winding is used that number does not change. When a part of the winding is used to achieve gain then the sub 1kHz info can then be related to the source impedance.
dave
So I need to select at the best the source otherwise I will get a peak or a low pass?
This is a limit for me.
Walter
Oeps, sorry. Yes, the picture low resolution did make me read 5000 instead of 500
I wonder how it would measure as an AVC?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
The problem with ALL transformer based volume controle is bad coupling.
Excessive ringing. The ringing will be less if you have an outputimpedance which is "high". But a high output impedance dosn't help for a good frequency response either. Tribute for instance use a fluxring at the inside of the (toroid)core but that helps just a little. A snubber wont's work either.
Edits: 04/30/21
"The problem with ALL transformer based volume controle is bad coupling.
Excessive ringing"
I don't see that at all.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Just measure* it and you'll see. Any standard resistance** control does a better job.* measure with a low output impedance ofc and with a system that is not limited to 200kHz or less
**use a 10k to 50k value.
p.s the most measurements i see from manufactures are tuned for optimum results for just 1 setting
Edits: 04/30/21 04/30/21 04/30/21 04/30/21 04/30/21 04/30/21
Can you spell that out? I'm a little dumb.
10K to 50K source impedance?
low output impedance ofc?
Thanks
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I did mean any standard volume controle, a potentiometer, 10k-50k
But a 50k ohm pot will have an output impedance of 12.5k ohms at the -6db point of rotation and that will not drive cable capacitance very well.
The output impedance of a AVC at the -6db position will have an output impedance 1/4 of the source.
I've looked at my AVC using square waves and sine waves and I don't see anything to cause alarm. I've even adjusted the source impedance, up and down, and I still don't see any issue that should cause alarm.
Maybe you can show us the ringing that you think is problematic?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Why would you use a potentiometer to drive a cable directly? That is really silly.
But what about de capacity load for the tvc? Did you measured that too.
Edits: 04/30/21 04/30/21
Don't blame the cathode follower, blame the autoformer volume controle which is far more imperfect then the cathode follower. Secondly blame the guy who thought it was a good idea to use a cathode follower because it enlarge the faults of an autoformer volume controle
Edits: 04/30/21
the subject at hand was a mosfet follower driving a low impedance load and I listed my only experience with them. The load was not difficult and yet they still left me wanting and sounded better without the buffer.
It seems that all of the praise with the followers stems around the need for a near infinite load in the cathode and that infinite load becomes superfluous when you parallel it with a low impedance. The only place I see a CF as a viable option is driving an A2 grid.
dave
I can't undestand why there are a posts around trafo volume control.
Then the use of CF to drive in A2 is a bad solution due the non linear current in a grid of power tube because the impedance falls quickly with the swing (a lot of volts) in the region of positive Vg; just to look on the data sheet.
And it is not a case that on curves of output tubes for Audio the positive region of Vg aren't specified. They stop at 0 volt
In same case with big triodes but there is also the figure of the grid current that is always not linear
In the LCR phono the swing is limited and the correct set point for this job allow a very goodo performances.
Walter
Hi
excuse me but I can't understand what are you sayng
Walter
Just measured a PEC potentiometer 50K with a output source of the amplifier of 500 Ohm. Capacitive load 100pF.
Correction: the 100 pF is wrong, i measured with my 1:10 probe which is 15-17pF
This is the worse setting at 3 o'clock
Measured at 12 o clock setting
And this is measured at the 9 o'clock setting
Looks "bad" but look at the scale, flat within 0,3dB.
Edits: 05/01/21 05/01/21 05/01/21 05/01/21 05/01/21 05/01/21 05/01/21 05/01/21
The problem with measurements and sims for that matter is they only show you what the presenter wants you to see. You were trying to show how perfect a 50K pot was but fed it into an unrealistic 15pf load. Using the 1X probe and its ~100-150pf capacitance is much more realistic of what may happen in the real world. In the plot below a 50Ω source is driving a 50kΩ pot set to -6dB. The green trace is 15pF, blue 30, red 60pf cyan 125pf Violet 250pF and gold 500pF.
Granted this represents the absolute worst case setting scenario (-6dB) and explains why many opt for a 10K pot for attenuation which makes things appear much better and 1K would be nearly perfect but then we are back to the question of how to properly drive a 1K volume control and the answer is obviously a cathode follower loaded with a CCS and we are back on topic. :-)
10K pot set to -6dB into varying capacitances
The approach the the tests are done just to show what the presentr want is out of the reality.
When we test the equipmente on Audioreview magazine these tests follows ( in part) the intl indication then there are other test but are well documented and replicable.
Otherewise the reputation will be lost.
Also when I present on some forum the circuit and measurement.
The REAL test are replicable and there aren't nothing to manipulate.
And, in my opinion, the simulation are the main problem that we can met in forums; there are thousand of "designer" that presents lot of circuit tested only with simulations.
And is is almost impossible to see the proto of that strange things.
Some of them are a strange mix between tubes and sand.
After that I will return on topic
Walter
Nice plots Dave, and you are right about the 15pF for a normal tube circuit although if we use a ecc88 the total capacity is less then 60pF and if it is a cathode follower even a lot less.
but what about the cable capacitance that is added to the Miller of the tube?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 05/03/21
If you know how to design the cable capacity is not a big issue.
If you are using a pot for a passive preamp (and some people do) and you have a cable between it and a power amp, that cable will have at least 20pf per foot. That can be a big issue.
That what I was referring to.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I already posted that that is a stupid thing to do and the plots from Dave shows what actually happens when you add to much capacity to a volumcontrole.
Also transformer coupled volume controle have issues with to much capacity.And if people insists on a direct connection potentiometer to the next amplifier, keep de value of the pot low, LOW + the length of the cable short.
You can calculate (or measure) the hf roll off
Edits: 05/04/21
"Also transformer coupled volume controle have issues with to much capacity."Mine don't.
Edit, I should have said anything circuit or device has issues with too much capacitance.
With the normal capacitance of a tube input plus some cable capacitance my AVC doesn't have issues.Let me ask you a question. Does a signal level step down output transformer have trouble with a normal amount of capacitance? What is a TVC if not a signal level step down output transformer?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 05/04/21 05/04/21 05/04/21
Well, a normal transformer has not the high L value as a inductive volume controle and also the winding capacity is totally different, actually the whole design is different. So why you compare them that way?
The small signal output transformers that I am familiar with in vintage tubed pro studio gear are not gapped like an output transformer for a SE power amplifier. They are used in what one might call a "parafeed" arrangement (no DC in the primary) and I see no reason that they wouldn't have relatively high inductance. Also I see no reason that the winding capacitance would necessarily be high.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Correct and the opposite is the normal line output transformer with low induction and high capacity.
Totally different designs.
But i would be glad to see the measurements so we can see how the transformer in real life behaves with a capacitive load.
Edits: 05/05/21
Please send the test at - 6dB from the max position of volume.
Which brand is the pot?
Walter
Edits: 05/01/21
Btw, the test was not done with 100pF load but with my 1:10 probe which is about 15- 18pF. So the 3 o'clock setting is about what you asked for (-4dB, almost -6dB )Walge, the capacitance load of the potentiometer largely determines the frequency response. So the capacitance load of the next stage is very important.
The great advantage of a normal potentiometer is the absence of all kinds of resonances at higher frequencies. At the same time, the impedance of a normal potentiometer is still high, that's is why professional equipment with tubes usually use lower values and also compensation circuits.
Edits: 05/01/21 05/01/21 05/02/21
Hi
I know well the problem and, when possible I use a low value of pot.
In the last numbero of Audioreview magazine there is a presentaion of a potential kit of pot with 6 relè and a (almost) costant impedance ( done by a collaborator).
Will be the possibility to change the value desired.
Walter
What is there not to understand? It has been my contention that benevolently driving a 600Ω load is not an easy thing to do independent of it being resistive or reactive. Since this is about 600Ω LCR units, Tre asked about mosfet followers. Since I hadn't tried the exact combo he asked about but have experience with a similar situation of using a buffer to drive a difficult load I offered that example. As for the grid of an A2 tube, that is also a difficult load but unlike the benign flat impedance of the 600Ω LCR, it varies substantially. The key point here is with an A2 tube, you are stuck with the load the grid provides and if you choose A2 you must find a way to deal with it. With a 600Ω LCR, within limits you have the option of changing that value to something that is easier to drive without compromise. Thats my opinion and I'm sticking to it :-)
dave
I still think a lot people use not so good designs. It dosn't have to be sofisticated but it should be good designed, designed for the whole audio system, not just one stage.
Maybe some people here should read a little bit more in the books about cathodefollowers.
Here is a start.
http://valveradio.net/audio/low-distortion-cathode-follower.html.And this is for the Brosky fans
https://www.tubecad.com/2011/08/blog0212.htm
Edits: 04/30/21
Hi
personally, as said, I don't like to mix sand with tubes.
Walter
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