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Grounded Grid?? Really?



 
 
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  #1  
Old November 6th 14, 07:54 AM posted to rec.audio.tubes
Donut
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Default Grounded Grid?? Really?

Hello!
My friend is working on his preamp and he shows me the circuit lately. The preamp named Grounded Grid and claimed that the circuit is GG as well.

However, some people called it a mu follower.....SO what is this?? GG or mu follower?

http://i2.photobucket.com/albums/y48...e/ggpreamp.jpg

I'm really curious!!

thx!

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  #2  
Old November 6th 14, 12:34 PM posted to rec.audio.tubes
Peter Wieck
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Posts: 2,418
Default Grounded Grid?? Really?

On Thursday, November 6, 2014 2:54:05 AM UTC-5, Donut wrote:
> Hello!
> My friend is working on his preamp and he shows me the circuit lately. The preamp named Grounded Grid and claimed that the circuit is GG as well.
>
> However, some people called it a mu follower.....SO what is this?? GG or mu follower?
>
> http://i2.photobucket.com/albums/y48...e/ggpreamp.jpg
>
> I'm really curious!!



I think you are getting hung up on nomenclature, as pretty much any amp/pre-amp is a "mu-follower" at some level, and the schematic you linked is often called many other things than "Grounded Grid" as well. Note that the grid grounds are either via the volume control (shorting to ground on the input side when at lowest volume) and via high-ohm resistors on the output side. Not directly as with (some) power-amp designs. So, the meaning of "ground" is shaded. Note also that one way to test pre-amp noise is to measure with shorted inputs - to ground.

http://analogmetric.com/ebay/GG%20or...0schematic.png

Remarkably similar, isn't it? Another design described as 'grounded grid'.

So, as a general statement, (very nearly) all grounded-grid systems are also mu-followers, whereas not every mu-follower system is a grounded grid.

I am not an EE, I am rendering an amateur's informal interpretation based on limited formal knowledge but a long history of tinkering and experimentation with tube-based equipment.

Peter Wieck
Melrose Park, PA
  #3  
Old November 27th 14, 10:16 AM posted to rec.audio.tubes
Patrick Turner
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Posts: 3,964
Default Grounded Grid?? Really?

On Thursday, 6 November 2014 18:54:05 UTC+11, Donut wrote:
> Hello!
> My friend is working on his preamp and he shows me the circuit lately. The preamp named Grounded Grid and claimed that the circuit is GG as well.
>
> However, some people called it a mu follower.....SO what is this?? GG or mu follower?
>
> http://i2.photobucket.com/albums/y48...e/ggpreamp.jpg
>
> I'm really curious!!
>
> thx!


The circuit you refer us to is not a simple grounded grid gain stage.
On each channel there are 3 triodes. The single left triode is a cathode follower with high Z grid input. Its cathode is low Z output which drives the cathode input to bottom triode cathode of 2 series triodes which are arranged as SRPP pair. The output of the circuit is from top right triode cathode and after coupling cap there is an R divider from output to bottom triode grid of SRPP pair.
This divider is a NFB circuit where = 0.167. The advantage, if any, of this circuit is that phase of input is same as phase of output.
Unfortunately Transcendant Sound omit all the important info such as all the signal voltages and phases which leave inexperienced experimenters floundering because they cannot do basic diagnosis of such a very simple circuit.

I've tried this type of circuit and found no benefits other than phase not being inverted, but after realizing there is nobody in this world who can hear the difference of phase reversals, the ordinary -follower with 2 triodes aka bootstrapped follower is superior to the similar SRPP, and examples of what I mean are at http://www.turneraudio.com.au/preamplifiers.htm

  #4  
Old December 1st 14, 06:03 PM
John L Stewart John L Stewart is offline
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Location: Toronto
Posts: 301
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Quote:
Originally Posted by Donut View Post
Hello!
My friend is working on his preamp and he shows me the circuit lately. The preamp named Grounded Grid and claimed that the circuit is GG as well.

However, some people called it a mu follower.....SO what is this?? GG or mu follower?

http://i2.photobucket.com/albums/y48...e/ggpreamp.jpg

I'm really curious!!

thx!
None of the grids in the subject circuit are at AC ground, so no grounded grid circuits here. Two that could be are at V1B & V2B, but they have AC NFB from their upper triodes. Hope I got that right, the schematic is a bit tough to read. The stacked triodes are connected as mu-followers.

Grounded grid circuits using triodes are common as RF amplifiers for VHF & UHF circuits. The benefit is isolation of the output circuit from the input, thus reducing the risk of positive FB resulting in instability & oscillation. The grounded grid provides the necessary shielding of the output from the input within the tube. The rest is up to the designer to provide a good physical layout.

The other benefit is the good match to a low impedance source. Matching to an antenna at 300 ohms (or 75) with low standing wave ratio (SWR) is important to maximize the signal transfer from the source (antenna) to the RF amplifier.

For audio, the circuit can be used to match a magnetic pickup on a phonograph or microphone to the high impedance of a normal tube circuit. But there would then be a DC biasing of the pickup or matching transformer, not the best idea.

Another use that was fairly common with amateur radio operators was to put a carbon microphone (telephone type) into the cathode of its amplifier. Something like one triode of a 12AU7 works well. Ive done that one for a power line amplifier running at about 600 KHz. It worked very well where I was working at the time (1960).

I am a P.Eng (Electrical), hope that does not disqualify me! Got my tube design experience, 10 years in a research lab as a random designer of random circuits. My experience since then is in power & communication.

Cheers to all, John L Stewart
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  #5  
Old December 13th 14, 01:30 AM
John L Stewart John L Stewart is offline
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First recorded activity by AudioBanter: Jan 2011
Location: Toronto
Posts: 301
Smile

Quote:
Originally Posted by Patrick Turner View Post
On Thursday, 6 November 2014 18:54:05 UTC+11, Donut wrote:
> Hello!
> My friend is working on his preamp and he shows me the circuit lately. The preamp named Grounded Grid and claimed that the circuit is GG as well.
>
> However, some people called it a mu follower.....SO what is this?? GG or mu follower?
>
> http://i2.photobucket.com/albums/y48...e/ggpreamp.jpg
>
> I'm really curious!!
>
> thx!


The circuit you refer us to is not a simple grounded grid gain stage.
On each channel there are 3 triodes. The single left triode is a cathode follower with high Z grid input. Its cathode is low Z output which drives the cathode input to bottom triode cathode of 2 series triodes which are arranged as SRPP pair. The output of the circuit is from top right triode cathode and after coupling cap there is an R divider from output to bottom triode grid of SRPP pair.
This divider is a NFB circuit where = 0.167. The advantage, if any, of this circuit is that phase of input is same as phase of output.
Unfortunately Transcendant Sound omit all the important info such as all the signal voltages and phases which leave inexperienced experimenters floundering because they cannot do basic diagnosis of such a very simple circuit.

I've tried this type of circuit and found no benefits other than phase not being inverted, but after realizing there is nobody in this world who can hear the difference of phase reversals, the ordinary -follower with 2 triodes aka bootstrapped follower is superior to the similar SRPP, and examples of what I mean are at http://www.turneraudio.com.au/preamplifiers.htm
Here is confirmation of Patrick T's comment on our ability to hear phase. Copied from Fred E Terman's Electronic & Radio Engineering, 4th Edition.
A very good reference text for anyone involved in vacuum tube electronics. I got mine about 15 years ago used at a very good price.

Refer to www.alibris.com for many books, both new & used.

Cheers to al, John L Stewart
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  #6  
Old December 13th 14, 02:20 PM posted to rec.audio.tubes
Peter Wieck
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Posts: 2,418
Default Grounded Grid?? Really?

On Saturday, December 13, 2014 1:37:25 AM UTC-5, John L Stewart wrote:
>
> Here is confirmation of Patrick T's comment on our ability to hear
> phase. Copied from Fred E Terman's Electronic & Radio Engineering, 4th
> Edition.


True, & true.

HOWEVER: a couple of times I have made myself look godlike (and that is a vast stretch even to the best of imaginations) to a certain class of audiophile by the simple expedient of connecting their speakers correctly (in phase). On one very special occasion, I had to practically tie one gentleman down as he INSISTED that the right speaker should be connected one way, the left the other way. Until he heard them connected properly.

So, relative phase angles may be inaudible. But 180 degrees out of phase is most definitely audible.

Peter Wieck
Melrose Park, PA
  #7  
Old December 16th 14, 09:28 AM posted to rec.audio.tubes
Patrick Turner
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Posts: 3,964
Default Grounded Grid?? Really?

True, & true.

HOWEVER: a couple of times I have made myself look godlike (and that is a vast stretch even to the best of imaginations) to a certain class of audiophile by the simple expedient of connecting their speakers correctly (in phase). On one very special occasion, I had to practically tie one gentleman down as he INSISTED that the right speaker should be connected one way, the left the other way. Until he heard them connected properly.

So, relative phase angles may be inaudible. But 180 degrees out of phase is most definitely audible.
- hide quoted text -

Peter Wieck
Melrose Park, PA

Phase and audiophiles are not happy marriage material. Phase is a bad word, because ppl talk in terms of being un-phased as a negative experience, and of course NFB is also evil, because its damned negative, and saying it ought to be called "inverse feedback" simply makes audiophiles feel queasy about listening to music standing upside down, or mounting their amps upside down, ie, something must be inverted to counter the inverted NFB.

I don't know how many times I corrected phase for sub-woofers which wen connected by the audiophile made overall bass worse, while putting in an unwanted boom below 30Hz. Measurements at the listening chair were usually appalling, only able to be improved marginally.

It is standard practice by hundreds of speaker manufacturers to have phase of bass and tweeter with "normal" phase, then with midrange deliberately "reverse phase" connected. I've done it myself in nearly all 3 way speakers I made.
One will find it is THE WAY to get a smooth transition of response between bass-mid, and between mid-tweeter, when correctly damped C&L second order crossover filters are used.

Its stupid to worry that the midrange is 180d to bass and tweeter. The essential thing is a flat response, and SAME phase shift of L and R speakers in order to get the best stereo imaging. The only time I have heard any one tell me they didn't like the phase behavior of a speaker was because L and R had different measurements, or someone has replaced a driver and one speaker driver was wrongly phased, and then they couldn't understand why a singer seems to be to the left of a LEFT speaker, when we all know she was standing centre stage when recorded.

Music produced at a live venue has instruments placed at many different distances to our ears, and yet it makes SFA difference if the musos change distance by plus or minus maybe several or dozens of wavelengths, so we never ever can hear music at our ear without the phase analomies occurring because of varied path length. Much sound indoors is reflected, there is a huge amount if phase cancellations and additions, but they sum to give an average level, and its no use thinking about it all because it'd lead to maths equations that are too complex to have meaning, even if worked on with super computer. So what you hear is what you get, and live music needs a nice venue not too reverberent,
and we also need a good room to reproduce that overall "room atmosphere" - and remarkably, if some extra phase jiggery-pokery is added in speaker making, but speakers are good quality, we really do hear what we heard live at the venue.

Scientific American did tests back in 1950s or 60s to find out if ppl could hear phase change. Nobody could. You play music with speaker leads reversed at amp or normal, and nobody could pick it.

But audiophiles are so often not at all scientifically minded, and go through life never ever understanding any formula or technical concept. They will often claim to hear or not hear some peculiar thing in their systems, without their being the slightest real evidence to cause the perceived phenomena, and gently I have had to just demonstrate by doing and measuring and comparing to put them at ease - until next time they find a reason to change amps or speakers. I have surprised myself when I improved total quality of what I made over a number of years which made it easier to sell my gear. If the audiophiles liked what I made, and if they changed houses and it all still sounded well, then that WAS really something.

Oh, and BTW, just about all recorded music has huge post recording processing done on the music, often digitally, and STILL we get some excellent recordings that are so good the word PHASE just never enters our mind.

Patrick Turner.

 




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