[Captain Humble]

Apparently there is a "bias setting" associated amplifiers.
I'd appreicate an explanation of the term, how the setting is changed and why.

Thanks,
Jeff

[JohnR]

A great many amps operate in what is known as "Class AB." This means that the two output devices have a certain amount of current passing through them when there is no signal. As the signal increases, one or the other device turns off, depending on whether the signal is going positive or negative.

Many tube amps have this as user-adjustable, as it varies with the specific tube and has to be set within a certain range for the amp to operate well. I'm not sure that any solid state amps make it user-adjustable, I expect it's set at the factory.

BTW when I talk about "devices" I mean the tubes or transistors in the output stage, which deliver the power to the speaker. There are two (per channel), because one "pushes" and one "pulls" -- hence the term push-pull to describe that type of circuit. (The other major type is single-ended, which has just one output device.)

HTH

[neilr11]

This is one of many "word games" played in the world of electronics. "Fixed-bias" means that the tube has a preset negative voltage on the controller grid. This negative voltage inhibits flow of electrons (electrons being negatively charges----and "like" hcarges repel)---and sets the idle flow, or bias of electrons from the cathode to the anode.

Many (if not most) fixed-biased amp HAVE a control potentiometer that allows you to vary the negative voltage applied to the grid---and therefore, control the idle bias level of the tube. They are adjustable.

Cathode-biased amps usually use a cathode bound resistor to ground to set the idle level of the power tube. Since the resistor if set---the idle is set at certain resistance. Many Class "A" type amps use this method to set their idle bias. In this since, they a FIXED in a smaller range of bias.

There is a range is which the tube can be cathode-biased. So, if the various brands of tubes have similar electronics---you can interchange them without having to change the cathode resistor (and idle bias). I think this is where the idea of "self-biasing" come from.

For instance, nearly all front end tubes are cathode biased. That is why you don't need to fiddle with any bias settings when swapping tubes. The variance in required idle settings is not great enough to warrant re-biasing these tubes.

But, larger power tubes draw more current---and a possible change in cathode resistor may be necessary if you switch from say, a new Sovtek EL34 to an older NOS Mullard EL34---to set the correct idle.

This idle change can make a world of difference in the tone of the amp. A "cold" sounding amp, can suddenly become very "warm"---once the bias is set correctly.

Then, there are those amps with auto-biasing circuitry. These usually use a duo-triode tube to set the range of idle bias.

To find which type of amp biasing you have---best to ask the manufacturer and check out the schematic.

Regarding SS amps. I spoke with legendary designer J. Curl regarding this and SS design. His thoughts are once the SS amp has left the factory, you don't have to worry about the bias or making any adjustments. Any manufacturing chirping about adjusting/checking the bias in a SS amp once it left the factory is doing it as a marketing ploy to sell more amps or their services.

[randytsuch]

Hi guys,

Here's an interesting link on bias in SS amps
http://www.tnt-audio.com/clinica/bias_e.html

Not that I don't respect John Curl, but I am not sure you can make blanket statements about anythink like that.  Depends on the components used in the amp for one thing, and unless John designed it, he has no control over that.

Anyway, this guy at TNT rebiased his amps, after replacing the bias pot, and found an improvement.

Randy

[neilr11]

Randy,
Thanks for the article. If you read the whole article, it's about biasing a Class A/B amp more into class A, not adjusting the bias after the factory settings for the sake of adjustment. As the author found out, biasing the amp more into class A results in a better, sweeter sound.  This is typical.

Quote: "In closing, I stress once again - the actual benefits and limits will vary depending on the design, but there is no doubt that you will hear a difference just by changing the trim pots if your unit uses poor quality ones, and you will be able to increase the quiescent current somewhat, which will again offer some benefits. But I would strongly suggest you make sure your unit is well ventilated, as it will be producing more heat."

[DVV]

Hi guys,

Here's an interesting link on bias in SS amps
http://www.tnt-audio.com/clinica/bias_e.html

Not that I don't respect John Curl, but I am not sure you can make blanket statements about anythink like that.  Depends on the components used in the amp for one thing, and unless John designed it, he has no control over that.

Anyway, this guy at TNT rebiased his amps, after replacing the bias pot, and found an improvement.

Randy

Yeah - that was me.

And guess what? I'm still at it. To this day, I'm answering mails on that topic, as well as the power supply texts on TNT and my own site, http://www.zero-distortion.com .

Cheers,
DVV
aka Dejan V. Veselinovic

[randytsuch]

Randy,
Thanks for the article. If you read the whole article, it's about biasing a Class A/B amp more into class A, not adjusting the bias after the factory settings for the sake of adjustment. As the author found out, biasing the amp more into class A results in a better, sweeter sound.  This is typical.

Neil,
The author (DVV) rebiased his amp AND readjusted the DC offset of his amp.  I agree that rebiasing is really changing the AMP, but readjusting the DC offset is just that, an adjustment.  If I understand what DVV wrote, he found some rather large DC offsets, which he adjusted back to almost nothing.  These DC offsets can have a detrimentel effect on the sound.

Hi DVV,
Sorry I did not notice who the author I referred to was.  
I actually read that article a while ago, and I forgot you wrote some stuff for TNT.
Please correct me if I have said anything wrong above, which I think you would do anyway, since you are always helpful.
Since I have your attention, I have a couple of unrelated questions (sorry for hijacking the thread  ).  I was just reading your website on power supply design, but this is for a amp PS.  I am kind of working on a preamp supply, and will need regulated +/- 15VDC.  Wondering if you have any suggestions for a good regulated supply (something better than the standard 7815/7915)?  
Second question is if you have ever played with the National power opamp chips (LM3886 or LM3875)?  I am making a gain clone with a 3886 now.
If you have any answers, and would not mind answering, just say yes to one or two, and I will start an appropriate new thread for you to answer in, instead of further hijacking this thread.

Thanks,
Randy

[DVV]

Neil,
The author (DVV) rebiased his amp AND readjusted the DC offset of his amp. I agree that rebiasing is really changing the AMP, but readjusting the DC offset is just that, an adjustment. If I understand what DVV wrote, he found some rather large DC offsets, which he adjusted back to almost nothing. These DC offsets can have a detrimentel effect on the sound.

Yes they can, which is why I felt it necessary to redo them as well. In addition to which, you never know what they were like when you bought the amp, and nobody knows what they are like after say ywo years of service. Of course, in case your amp is equipped with a DC servo circuit, theoretically DC offsets should be around 1 mV or less, which is of no practical consequence.

Hi DVV,
Sorry I did not notice who the author I referred to was.  
I actually read that article a while ago, and I forgot you wrote some stuff for TNT.  

That's OK Randy, little things like this make life a little more fun to live. I have published a lot in life, over 300 texts in paper mags, over 100 texts in electronic media, one book (on text processing with a PC - actually sold out first and second edition, but that was way back in 1990), authored 240 weekly TV shows, over 300 weekly radio shows (now, THAT was all about audio!), so you can imagine I have little vanity left regarding publishing in general.

Please correct me if I have said anything wrong above, which I think you would do anyway, since you are always helpful.

I probably would, but there's no need.

Since I have your attention, I have a couple of unrelated questions (sorry for hijacking the thread ). I was just reading your website on power supply design, but this is for a amp PS. I am kind of working on a preamp supply, and will need regulated +/- 15VDC. Wondering if you have any suggestions for a good regulated supply (something better than the standard 7815/7915)?

Oh lots. I am a certified power supply freak-out, like really gone, man. But like you said, let's not strangle everybody here, you can reach me at dvv@bitsyu.net and we can talk about it.

Second question is if you have ever played with the National power opamp chips (LM3886 or LM3875)? I am making a gain clone with a 3886 now.

No, sorry, I know about them, but I have never actually used one.

If you have any answers, and would not mind answering, just say yes to one or two, and I will start an appropriate new thread for you to answer in, instead of further hijacking this thread.

Thanks,
Randy

Randy, I believe in discrete topology, using classic bipolar devices, with maybe a FET or two in the input stage. If this is of any interest, let me know which thread you started and I'll hop on board.

Cheers,
DVV

[randytsuch]

Hi DVV,
Thanks for the answers.
There is no other threads to point to at this time.
I will email you with some PS questions.

Randy