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Neutralizing An Amplifier
Neutralizing An Amplifier
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Old 21st November 2012, 05:40 PM   #1
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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Default Neutralizing An Amplifier

Hi all guy´s. Some years ago I saw a circuit in where a push pull driver around a 12AX7, and neutralizing them crossing 1.5pF between their grid and plate of different output circuits, like it is done in RF circuits. Did any of yours try this method in audio? What is your opinion or taste about it?

Osvaldo.
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Old 21st November 2012, 05:52 PM   #2
SY is offline SY  United States
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Neutralizing An Amplifier
Yes, tried it. It works, but you have to be VERY careful about strays, which can easily throw things off balance. Norman Crowhurst had a nice description and schematics in "Understanding Hifi Circuits."
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Old 21st November 2012, 06:10 PM   #3
counter culture is offline counter culture  United States
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RF is normally narrowband, high frequency. Solutions that work for RF are not always directly transferrable to audio.
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Old 21st November 2012, 06:30 PM   #4
SY is offline SY  United States
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Neutralizing An Amplifier
The neutralization trick is not narrowband, it specifically compensates Miller capacitance.
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Old 21st November 2012, 08:50 PM   #5
counter culture is offline counter culture  United States
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'most neutralization circuits tend to neutralize the amplifier at the operating frequency only and may cause problems (instability) at other frequencies.' - Chris Bowick, RF Circuit Design - P126
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Old 21st November 2012, 08:53 PM   #6
SY is offline SY  United States
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Neutralizing An Amplifier
The ones being discussed here do not fall in that category. If it's unclear, check out the Crowhurst reference, or I could draw you a picture.
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Old 21st November 2012, 09:54 PM   #7
Sch3mat1c is offline Sch3mat1c  United States
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Some methods do, some don't. It's noteworthy that Miller effect is itself frequency-dependent (it's a capacitor), so you need a frequency-dependent method (in the same direction!) to cancel it for all frequencies.

Three common methods are:
1. Resonating Cmiller with a neutralizing inductor
2. Inductive loop from input to output coupling circuits (negative feedback)
3. Cross-connected capacitors

Of these, 1. is inverse frequency dependent, so it truely works only in narrowband applications. I would guess, in a circuit which requires heavy neutralization, you wouldn't get more than 20% bandwidth or so (i.e., +/-5MHz out of a 50MHz VHF amp). 2. is frequency independent, so it works over a wide bandwidth (with good coupling, it'll work over the full bandwidth of the input/output coupling coils/transformers). Unfortunately, miller is not, so it only needs a certain amount of neutralization over part of that range, and more elsewhere. So the bandwidth would be higher, but not wideband.
3. has the potential for wideband operation, as long as the phase shift between phases is maintained well. This requires tight control over parasitics and very good transformers.

Now, for audio purposes, capacitance is capacitance, and adding more won't magically fix anything. You can get some PP enhancement with a small miller-canceling cap, but you can't reduce the effective capacitance below, I would guess, about double the interelectrode capacitance. Attempting to do more will load it down more, and make it more prone to weird things like oscillation.

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Old 21st November 2012, 10:55 PM   #8
counter culture is offline counter culture  United States
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Quote:
Originally Posted by SY View Post
The ones being discussed here do not fall in that category. If it's unclear, check out the Crowhurst reference, or I could draw you a picture.
I thought this was a open discussion.
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Old 21st November 2012, 11:02 PM   #9
SY is offline SY  United States
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Neutralizing An Amplifier
I don't get your point. Did you read what the OP was describing? Or do you need a picture drawn?
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Old 21st November 2012, 11:14 PM   #10
gingertube is offline gingertube  Australia
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I did try netralisation caps (grid to opposite anode) on a differential splitter using 12AX7.
I could'nt get it stable, always ended up with an oscillator. I think I was using a cap of around 10pF which was probably too big.

Try putting a short piece of wire on each of the anode and opposite grid and then twist the wires around each other, more twists => more capacitance, there was a "rule of thumb" of pF per twist for a particular type of wire but I can't remember it. For 1.5pF you would probably only need 2 or 3 twists.

Cheers,
Ian
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