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pre65
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#31

Post by pre65 »

With reference to the above post and diagram, I'm getting confused. :?

If the 9v battery + goes to the pot wiper, and neg goes to grid (-9v bias) then does the signal go through the battery ?

If not does the input signal bypass the battery ?

Or am I just thick ? :wink: :lol:

This is because I'd rather not use an input capacitor.
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Nick
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#32

Post by Nick »

If the 9v battery + goes to the pot wiper, and neg goes to grid (-9v bias) then does the signal go through the battery ?
The problem with that statement is the word "through". Just what do you think you mean by using it?
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Paul Barker
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#33

Post by Paul Barker »

The AC signal goes through the battery in the sense it goes through a capacitor. Perhaps batteries sound better than capacitors.

It's a little bit of a unnecesary worry when you consider what the battery is doing for you. It is eliminating a cathode resistor and bypass capacitor through which the signal passes also.

If you put the battery on the cathode as the bias the signal also goes through it, but people have found that using a battery in the fixed bias position the sound is better. You may say what is the difference? Shirley a battery on the cathode is fixed bias from another angle? The difference is the battery on the grid sees no current and it holds the grid dc potential absolutely "FIXED" (for the time constants relevant to audio, obviously it discharges over a very long time) whereas on the cathode the circuit is a battery charger, the battery is charged by the electrons passing through it and if those electron flows alter the battery voltage will alter ever so slightly. So it is not as fixed as a pure voltage reference battery as used in the schematic. Perhaps that explains the preferred (by proponents) sound of fixed bias?

It is less a matter of tying oneself in knots trying to find the theoretic minimal component count in the coveted "signal path" these days. The above schematic is simply how some people have enjoyed the results. I am using it at the moment.
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#34

Post by Neal »

It would be interesting to know if using a battery in the config Paul sketched out altered the frequency response of the stage vs battery in the cathode. Batteries are horribly non linear with frequency, impedance at DC can be very low but rises sharply there after, I suspect there would be some roll off depending on the battery and Millar capacitance... Maybe thats why some like the sound of this config.
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#35

Post by Nick »

The AC signal goes through the battery in the sense it goes through a capacitor. Perhaps batteries sound better than capacitors.
But in this case (what I wanted Phil to think about), next to nothing will go "through" the battery, I agree its like a cap, but in this case the only current that will actually flow (that which through can only mean) is charging and discharging the miller cap of the valve. In the case of a cap instead of the battery which will be followed with at least a grid resistor, then the signal will pass current as its loaded by the resistor(s) following it,
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#36

Post by Neal »

Neal wrote:It would be interesting to know if using a battery in the config Paul sketched out altered the frequency response of the stage vs battery in the cathode. Batteries are horribly non linear with frequency, impedance at DC can be very low but rises sharply there after, I suspect there would be some roll off depending on the battery and Millar capacitance... Maybe thats why some like the sound of this config.
Scratch that. Seems impedance only starts to become significant when in the MHz range...
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#37

Post by Dave the bass »

Neal wrote: Scratch that. Seems impedance only starts to become significant when in the MHz range...
Aaah! That interesting, so would it be fair to say then that a coupling capacitor of a known value is more predicable in its behaviour than say a battery in that position Phil is planning to use it in then? From reading the answers about the battery its seems a cap of an unknown quantity (and quality!) is being introduced into the signal path.

FWIW, I tried Batt-coupling/grid biasing a while ago and it sounded OK but nothing to go potty about in the project I used it in.

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#38

Post by pre65 »

So, I've done a diagram.

A) is how I was going to do things, and B) is how (I think) Paul suggested.

Image
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#39

Post by Neal »

Dave the bass wrote:
Neal wrote: Scratch that. Seems impedance only starts to become significant when in the MHz range...
Aaah! That interesting, so would it be fair to say then that a coupling capacitor of a known value is more predicable in its behaviour than say a battery in that position Phil is planning to use it in then? From reading the answers about the battery its seems a cap of an unknown quantity (and quality!) is being introduced into the signal path.

FWIW, I tried Batt-coupling/grid biasing a while ago and it sounded OK but nothing to go potty about in the project I used it in.

DTB
I would lean towards yes Dave. A battery has capacitance and its performance wont be as good as a capacitor. It would also have a lower impedance...the battery testing example I dug up on google suggested 3~4 ohms impedance at 10Khz for a lithium battery dipping down then a very sharp rise in the MHz range...a good polyprop capacitor should show a smooth falling impedance with rising frequency. But hey, in practice the battery may be better sounding!
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#40

Post by Mike H »

Nick wrote:
The AC signal goes through the battery in the sense it goes through a capacitor. Perhaps batteries sound better than capacitors.
But in this case (what I wanted Phil to think about), next to nothing will go "through" the battery, I agree its like a cap, but in this case the only current that will actually flow (that which through can only mean) is charging and discharging the miller cap of the valve. In the case of a cap instead of the battery which will be followed with at least a grid resistor, then the signal will pass current as its loaded by the resistor(s) following it,
The way I see it, it would be an AC signal with a fixed DC offset added to it, plus some aspect of capacitor like behaviour. Which will depend on the behaviour of the battery in response to a small AC current being applied.


:?:
 
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#41

Post by Mike H »

Or if you prefer, like a capacitor with a DC charge across it.
 
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pre65
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#42

Post by pre65 »

Perhaps I need to try both ways. :wink:

Paul's way is more simplerist. :)
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#43

Post by Dave the bass »

pre65 wrote:Perhaps I need to try both ways. :wink:

Paul's way is more simplerist. :)
I would, just to see.

(B) I think has a higher i/p Z . (A) effectively has that 220K in parallel with the pot lowering the i/p Z, I think.

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#44

Post by Paul Barker »

I think we may be guilty reading too much into it.

I would say Phil's diagram one is inferior because it has both a coupling cap and the battery.

The only application I would consider that would beat the fixed battery bias is the unbypassed cathode resistor were that appropriate in the design.
Last edited by Paul Barker on Wed Mar 20, 2013 8:17 pm, edited 1 time in total.
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#45

Post by IslandPink »

Definitely try it both ways Phil - you might be surprised.
Please note that Stephen has been using battery in the grid circuit of his pre-amp for the last 5 years :
http://www.izzy-wizzy.com/audio/preampnew.html

As Nick was saying, the audibility of a battery is strongly linked to how much current modulation there is through the battery - in the case of the grid loop , very little .
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