Home » Audio » Thermionic Emissions » Class A, AB1, B, C Operation/Modes (How Class A, AB1, B, C Modes Work and Their Strengths and Weaknesses)
Re: Class A, AB1, B, C Operation/Modes [message #96455 is a reply to message #96450] Wed, 01 March 2023 19:45 Go to previous messageGo to next message
gofar99 is currently offline  gofar99
Messages: 1926
Registered: May 2010
Location: Southern Arizona
Illuminati (5th Degree)
Hi, Indeed. The way I get around this in all my gear, both commercial and diy is to isolate the signal ground from the chassis. I always use the third wire (AC mains ground) and attach it securely to the chassis. The internal circuitry ground is connected to the chassis via a type X2 capacitor and parallel resistor. (some folks prefer a diode bridge) There can be no other connections between the two grounds. This is important. Beware of chassis mounted jacks as they can defeat the protection. What this does is prevent ground loops through the AC mains but allows the chassis to still protect the users. A side benefit is the chassis still can act as an EMI shield for the internal components. This arrangement complies with electrical standards and makes for a quiet piece of gear no matter what it is connected to. Any faults in the circuitry are either handled by the fuse (an absolute necessity) or at least kept from harming the user. The typical component values are between 0.1 and 0.2uf and 100-150 ohms 1-2 watt size is usually fine. The capacitor needs to be AC mains rated thus the X2 designation. Usually the voltage ratings are 275 or 350 VAC. There are a few less common AC mains rated caps but the X2s are easy to source and not costly. Even though many companies used other common types in the past they are not recommended as they are not self healing and can short through. Good discussion.

Good Listening
Bruce
Re: Class A, AB1, B, C Operation/Modes [message #96471 is a reply to message #96455] Sun, 05 March 2023 00:33 Go to previous messageGo to next message
positron is currently offline  positron
Messages: 106
Registered: May 2020
Viscount
gofar99 wrote on Wed, 01 March 2023 19:45
Hi, Indeed. The way I get around this in all my gear, both commercial and diy is to isolate the signal ground from the chassis. I always use the third wire (AC mains ground) and attach it securely to the chassis. The internal circuitry ground is connected to the chassis via a type X2 capacitor and parallel resistor. (some folks prefer a diode bridge) There can be no other connections between the two grounds. This is important. Beware of chassis mounted jacks as they can defeat the protection. What this does is prevent ground loops through the AC mains but allows the chassis to still protect the users. A side benefit is the chassis still can act as an EMI shield for the internal components. This arrangement complies with electrical standards and makes for a quiet piece of gear no matter what it is connected to. Any faults in the circuitry are either handled by the fuse (an absolute necessity) or at least kept from harming the user. The typical component values are between 0.1 and 0.2uf and 100-150 ohms 1-2 watt size is usually fine. The capacitor needs to be AC mains rated thus the X2 designation. Usually the voltage ratings are 275 or 350 VAC. There are a few less common AC mains rated caps but the X2s are easy to source and not costly. Even though many companies used other common types in the past they are not recommended as they are not self healing and can short through. Good discussion.

The code seems ok, but I am not sure code covers every scenario.
I don't mean to be critical of the code, if I may present an interesting scenario.

The chassis is grounded and as stated above, the fuse size is larger
than the current through the resistor between signal ground and chassis ground.
We also use a capactor across the resistor.

I believe the key is the value of the resistor vs the fuse size.

Now let's suppose the AC wire shorts to signal ground, and 120 vac
occurs between the signal ground and the chassis (worst case scenario).

The resistor will probably overheat and open (depends upon resistor
wattage), so only the capacitor is connected. The impedance of the
cap at 60hz is basically a non factor unless the large ufd value.

The AC voltage between chassis and signal input/output jacks, will be
120 vac. Contacting both the chassis and jack(s) with fingers will
give a nasty shock at minimum.

May I suggest multiple high wattage resistors "X" ohmage in
parallel between chassis and signal ground, creating a high wattage
and very low ohmage resistance. As such, the current through the
resistor is larger than the fuse value, so the fuse blows.

We should be safe.

The musical signal return current through pin 1 will still be very low
due to "X" resistance, yet the risk of shock is virtually zero
as the voltage between signal ground to chassis ground stays very low.

I think the keys are:

1. that the value of the resistor be such that the fuse will blow.

2. The value of the resistor is low enough that one should never be
shocked.

I would think this would cover things well.

Other thoughts are much appreciated.

Cheers

pos
Re: Class A, AB1, B, C Operation/Modes [message #96474 is a reply to message #96471] Sun, 05 March 2023 08:05 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 18723
Registered: January 2001
Illuminati (33rd Degree)

I connect signal ground to chassis ground, but at only one location.

Any device that is remote must be isolated in some way, without having grounds connected. The isolator can be a transformer or something like an opto-isolator.

Where this can become tricky is in the definition of "remote." Some might think of this as a physical distance, and in fact, that is often the case. But in fact, what makes a device "remote" is the resistance and reactance of the ground conductor.

If all devices had a hypothetical perfect zero resistance connection between grounds, there would be no possibility of the condition we call a ground loop. What causes the problem is the difference between the local ground potentials at each device. And this is due to the resistance between them.

So I try to limit resistance between ground connections everywhere that's possible. That's the case inside a chassis as well as outside. But if resistance cannot be reduced to very, very low levels - close to perfect zero ohms - then isolation is necessary.

And that includes reactive effects too, which makes things even trickier. A resistance of zero ohms at DC doesn't matter much if the circuit is operated at 10Mhz and there is reactance in the ground conductor, making it higher than zero ohms in the passband of the circuit. Where there is any resistance or relevant reactance in the ground conductor, we must abandon the approach of connecting the circuits and instead completely isolate them by transformer or opto-isolator.
Re: Class A, AB1, B, C Operation/Modes [message #96475 is a reply to message #96471] Sun, 05 March 2023 08:10 Go to previous messageGo to next message
gofar99 is currently offline  gofar99
Messages: 1926
Registered: May 2010
Location: Southern Arizona
Illuminati (5th Degree)
Hi Pos, That could happen....but to have that occur you have to violate another portion of the codes. The one about all exposed metal parts need to be either double insulated from contact or earth grounded. The failed transformer should be one or the other and then there is no hazard.

The resistor is actually best thought of as a low frequency path for crud between the circuitry and earth. Many designs leave it out. I find that it helps with the S/N a bit. Also the use of a large rectifier (often a bridge) between the two grounds could fail if there is the second grounding violation. I don't care for the rectifiers as they leave the chassis about 0.7 volts different from the earth and 0.7 Volts is a lot of potential noise that is not eliminated.

Just my two cents on how the codes work.


Good Listening
Bruce
Re: Class A, AB1, B, C Operation/Modes [message #96481 is a reply to message #96475] Mon, 06 March 2023 19:01 Go to previous messageGo to next message
positron is currently offline  positron
Messages: 106
Registered: May 2020
Viscount
A I had an experience that moved me to post.

I went to an audiophile friend's house after he mentioned being
shocked when he plugged in an ic to his ST70 amplifier. It turned
out that the pin 1 (ground wire) was not connected to the chassis
and the power transformer internally shorted resulting in some
high AC voltage on the chassis. Of course the ics were grounded,
so when connecting to the ST70s, he was shocked.

ST 70s are known for power transformer failures, so
I would make sure you do not defeat the ground wire to your ST70,
or someone else has not done so. This includes not using a 3 to 2
plug adapter.


If it is disconnected, please reconnect it. If you cannot, please
disconnect all AC power plugs before connecting audio component ics.
Then plug in your components.

It has made me a little sensitive in this area.
The resistor wattages I use are multiple parallel 10 watters with
total resistance around 1.5 - 2 ohms or so. This keeps the chassis
ground to jacks/signal grounds at very low voltage differences, so
no shocks. The resistors also do not open while the fuse blows as
intended.

As a former designer/manufacturer, I plug and unplug ics all the
time and I need personal protection, and to keep all component
plugs connected to outlets when performing listening tests.
Unplugging and plugging in the power cords could possibly taint
the listening tests.

I use my own non shielded 6N copper wire in my ics to the amp
for improved sonic quality. (Also for other source components
to the preamplifier inputs.) As for any hum, I designed a circuit
specific 60hz hum canceling circuit in my amplifiers. Fortunately,
ear on the driver and zero hum.

(I edited for fuller and easier understanding of why I do what I do.)

cheers to all.

pos
Re: Class A, AB1, B, C Operation/Modes [message #96679 is a reply to message #96475] Sat, 06 May 2023 22:32 Go to previous messageGo to next message
positron is currently offline  positron
Messages: 106
Registered: May 2020
Viscount
gofar99 wrote on Sun, 05 March 2023 08:10
Hi Pos, That could happen....but to have that occur you have to violate another portion of the codes. The one about all exposed metal parts need to be either double insulated from contact or earth grounded. The failed transformer should be one or the other and then there is no hazard.

The resistor is actually best thought of as a low frequency path for crud between the circuitry and earth. Many designs leave it out. I find that it helps with the S/N a bit. Also the use of a large rectifier (often a bridge) between the two grounds could fail if there is the second grounding violation. I don't care for the rectifiers as they leave the chassis about 0.7 volts different from the earth and 0.7 Volts is a lot of potential noise that is not eliminated.

Just my two cents on how the codes work.

Hi Bruce,

I reread your post and it appears to me that the only difference is
100-150 ohm resistor between the chassis and signal ground while I am
advocating 1 to 2 ohms at 30 watt resistor rating.

Both our transformers are double grounded for safety so there should
not be any problem meeting code for either of us.

I am a worry wart, if lightning strikes and shorts the double
insulation, or wiring short I just want to be as safe as possible.
I don't perceive any noise, but we probably use different tubes and
designs anyway.

Anyway, all is well, all the best Bruce.

pos


Re: Class A, AB1, B, C Operation/Modes [message #96688 is a reply to message #96679] Tue, 09 May 2023 21:30 Go to previous messageGo to next message
gofar99 is currently offline  gofar99
Messages: 1926
Registered: May 2010
Location: Southern Arizona
Illuminati (5th Degree)
Hi, There are several ways to make the chassis to signal ground connections. Everything from the old school way of using the chassis as the signal ground (usually too noisy for me) to huge bridge rectifiers. Any of them can comply with the various electrical codes as long as the user is protected from accidental contact with a live chassis. How we all lived through the 2 wire AC mains days with tube gear is a miracle. What I find is there is a sort of sweet spot when using an X2 capacitor and a resistor in the 120 to 150 ohm range. It is not designed to protect from faults like lightning etc or really any external faults. That is why the chassis is AC mains earth grounded. It will provide a path to the earth ground if there is an internal fault but that is not the main purpose. It is not really all that good at that as the impedance is fairly high. Its purpose is generally accepted as two fold. One it acts as a ground loop prevention measure when other gear is connected that passes signals to the subject device. The signal ground on the one will not find an alternate path through the AC mains and cause hum. Second it allows the chassis to act as an EMI shield without being in direct connection. IMO your one ohm resistor will comply with the codes....but may not provide as much ground loop hum rejection as is possible. And as nearly everyone knows...I hate hum and noise with a passion and the higher value resistor helps. BTW wattage is not really critical (I use 1 watt ones) as fault protection is not the primary function...that is what the chassis and three wire mains connection is supposed to do. Even if the resistor failed, the user is still protected. Now to be difficult...I could make a case for if the resistor and X2 fail open and an internal circuit fault energized the signal ground and it was connected to either an input or output cable that had a circuit ground conductor and the user was touching the ground conductor and something else that really was grounded there could be a hazard. Such a failure would almost always manifest itself as an anomaly in the gear and require attention. But then that sequence can happen with anything attached to the AC mains like lights, appliances etc. Opinions anyone?

Good Listening
Bruce
Re: Class A, AB1, B, C Operation/Modes [message #96689 is a reply to message #96688] Thu, 11 May 2023 00:26 Go to previous messageGo to next message
positron is currently offline  positron
Messages: 106
Registered: May 2020
Viscount
gofar99 wrote on Tue, 09 May 2023 21:30
Hi, There are several ways to make the chassis to signal ground connections. Everything from the old school way of using the chassis as the signal ground (usually too noisy for me) to huge bridge rectifiers. Any of them can comply with the various electrical codes as long as the user is protected from accidental contact with a live chassis. How we all lived through the 2 wire AC mains days with tube gear is a miracle. What I find is there is a sort of sweet spot when using an X2 capacitor and a resistor in the 120 to 150 ohm range. It is not designed to protect from faults like lightning etc or really any external faults. That is why the chassis is AC mains earth grounded. It will provide a path to the earth ground if there is an internal fault but that is not the main purpose. It is not really all that good at that as the impedance is fairly high. Its purpose is generally accepted as two fold. One it acts as a ground loop prevention measure when other gear is connected that passes signals to the subject device. The signal ground on the one will not find an alternate path through the AC mains and cause hum. Second it allows the chassis to act as an EMI shield without being in direct connection. IMO your one ohm resistor will comply with the codes....but may not provide as much ground loop hum rejection as is possible. And as nearly everyone knows...I hate hum and noise with a passion and the higher value resistor helps. BTW wattage is not really critical (I use 1 watt ones) as fault protection is not the primary function...that is what the chassis and three wire mains connection is supposed to do. Even if the resistor failed, the user is still protected. Now to be difficult...I could make a case for if the resistor and X2 fail open and an internal circuit fault energized the signal ground and it was connected to either an input or output cable that had a circuit ground conductor and the user was touching the ground conductor and something else that really was grounded there could be a hazard. Such a failure would almost always manifest itself as an anomaly in the gear and require attention. But then that sequence can happen with anything attached to the AC mains like lights, appliances etc. Opinions anyone?

I worry about jacks becoming hot, then we would have AC voltage between the jacks and chassis ground. It is a long shot to be sure, I have had plenty of shock therapy when I was a kid. Amazingly, we survived those old AC/DC radios. Laughing

There always seems to be a problem with ground loops; it seems the more components, the more difficult the problem. I did not want the hassle, and wanted my ics using 6N copper wire, so I designed a circuit in each monoblock so that I can completely dial out the hum and garbage, works perfectly.

I did need to shield my ic from TT to phono stage as a slight hum occurred with volume cranked way up. (Even ics with 6N wire need to be properly terminated for accuracy. Not a small feat.)

I think we both have systems to be happy with.

cheers

pos

ps. Sometime I want to dig a little more into "Tube Operating Curves" attached below. Right now, snowed under with responsibilities.

Re: Class A, AB1, B, C Operation/Modes [message #96973 is a reply to message #96689] Tue, 05 September 2023 23:48 Go to previous messageGo to next message
positron is currently offline  positron
Messages: 106
Registered: May 2020
Viscount
I found that B1 graph in my initial post was obviously wrong.
I have attached the correct graph, B, to this post.
I apologize for the inconvenience.

I would like to add a quote from the RCA Radiotron Designers
Handbook, 1960, written by 26 engineers in Collaboration.

This concerns Push Pull operation.

"A Class A amplifier is an amplifier in which the grid bias and
alternating grid voltages are such that the plate current of the output
valve or valves flows at all times. The suffix 1 indicates that
grid current does not flow during any part of the input cycle."

"A very useful operating condition is the borderline case between
Class A and Class AB1, that is when the plate current just reaches
the point of cut-off"

Notice, each tube in the Push Pull output stage operates
Class A until each output tube just reaches the point of
cutoff. That means each output tube conducts the entire
musical waveform.

Class A output wattages can range from small to many watts output,
and with extremely low distortion, especially in the first watt
out.

cheers

steve


Re: Class A, AB1, B, C Operation/Modes [message #96977 is a reply to message #96450] Fri, 08 September 2023 21:12 Go to previous messageGo to previous message
gofar99 is currently offline  gofar99
Messages: 1926
Registered: May 2010
Location: Southern Arizona
Illuminati (5th Degree)
Hi, BTW using an RC or pair of rectifiers between the grounds is not new. I have seen it used as far back as the early 50s. As soon as 3 wire mains showed up it was not far behind apparently. In addition to discovering that old school 2 wire powered radios could bite you I wanted to add to thoughts on separating the grounds and the possibility of getting zinged by an internal fault energizing the input or output jacks if the RC combo I suggest fails. You need two faults one internal and the RC. I considered that. It is a difficult scenario to achieve. Everything I tried resulted in either an amp that would not work or would work so badly that it was clear there was a fault. Anything that resulted in the two grounds being directly connected caused a serious ground based hum. No hazard, but bad operation. I could not energize any input jack or output terminal without it causing an obvious fault in the device. No sound, loud hum or noise. I will grant that in theory if the RC fails open (both parts) and you could cause the signal ground to become highly energized with nothing connected to it that might bleed off the voltage it might be possible to get shocked. It could only happen if nothing else was connected to it that would have its own ground. Anything carrying a signal into the device that had its own ground would act as a path to bleed off the voltage. If the device was already powered up and then the external connection was made there could be a shock hazard between the jack and interconnect. Before that could occur I would expect that the failed device would either blow its fuse or behave so badly that the user would notice and investigate what is wrong. I could wander off into low probability scenarios like lightning strikes that welded parts together and such, but I expect that any user of an electrical device would have at least a minimum of intelligence and accept some responsibility for their own safety.

Good Listening
Bruce
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