Speaker bias?

stretchl

Q: I recently had someone talk with me about tweaking an amplifier’s bias to match that of my Lsim705 speakers. I never knew that speakers had bias. Any insight into this? Thanks.

mhardy6647

In an amplifier, a DC bias voltage (and current, per Ohm's law) is used to adjust the operating point of a Class A (single-ended or push-pull) or a Class AB push-pull amplifier (output) stage to optimize its efficiency, lifespan , power output and distortion.

Normal passive loudspeakers don't require any bias. Indeed, a fixed (DC) bias - I would think! - would be a bad thing to the extent that the bias voltage/current would perpetually energize the voice coil (electromagnet) of the woofer -- just generating heat and lessening its life.

The bias (i.e.,operating points) of a Class AB amplifier stage might perhaps benefit from a little tweaking for any given load -- but in general I'd say stick with the mfgr. spec. for bias and make sure the amp's output section components are up to snuff to hold that bias point as the amplifier warms up and operates in the real world.

But (short answer) I have no idea what that someone you talked to was on about, without further info (context), sorry!

tonyb

Like Mark said, speakers don't have a bias, amplifiers do. Normally you don't screw with an amps bias, it's designed to operate within a set standard with a tad bit of leeway. Going outside those parameters can do damage.

motorstereo

If you have an autoformer based Mac amp you could change things up a bit by swapping impedance taps

stretchl

Thanks, all. I’ll see if I can get more clarification and report back.

daddyjt

The purpose of bias in output transistors is to curb or eliminate commutation distortion. When a transistor goes from zero input at its base to receiving a signal, this is commutation - think of it as a “hard start”. This can happen literally thousands of times a second during normal music playback. It’s not generally perceived, as the intervals are in the micro second range - however, these “hard starts” create distortion. Bias is a tiny (usually 0.5-10mv or so) current that is always being applied to the transistor to keep it from ever having zero input and creating a “hard start”. Think of it as driving a large truck in town - as you approach a red light, you want it to turn green before you have to reach a complete stop, making the impending acceleration easier if you are already rolling, even a little bit.

The byproduct of bias is heat, as that’s the only way for the unused current to shed.

Speakers (unless self powered) do not have bias. Further, I cannot see a situation where you would ever modify an amplifier’s bias to “match” a given speaker (presumably taking the amps bias out of spec).

mhardy6647

The bias described above is the essence of "Class AB" operation, to avoid crossover distortion in push-pull circuits. Pure Class B operation would operate at lower bias (and also higher electrical efficiency), at the cost of more or less significant crossover distortion. In class B, one "side" of the push pull pair of output decices is effectively "turned off" when the other side is active -- every time the signal crosses the zero (voltage) point, the other "side" turns on. As implied above, when the amp is biased "on" a little bit (Class AB), the distortion associated with the turn-on is minimized or eliminated.

Class A bias is much "higher" ("hotter", so to speak). An amplifier stage can operate in Class A whether the stage is single-ended or push-pull (although a single-ended analog amplifier must operate in Class A by definition). In Class A, the output devices (transistors) are fully 'on' all of the time. Very inefficient electrically, but (potentially) very low distortion as well.

daddyjt

Correct - I should have specified I was discussing AB operation. I omitted class A and class B in the explanation, as the percentage of them comprises a vanishingly small percentage of SS amplifiers.

mhardy6647

Some of us are pretty pro-Class A, albeit not necessarily all that pro-solid state

There are - and have been - some pretty fine Class A soiled solid state amplifiers, though.

In the early days of solid state hifi, there were components running in Class B, but that was a pretty short-lived "trend" - for exactly the reason you mentioned.

WLDock

Are you running a tube or transistor amp?

Amp bias has to do with the steady DC operating point to drive a circuit. It also has to do with the class (Class A, Class A/B) the amp is operating in.
The amp manufacture tech support or a bench tech familiar with the amp can talk more in depth about a particular amps capabilities.

https://www.youtube.com/watch?v=c6cmkm3UPUI

What's common in the audiophile world is to bias an amp towards class A (Steady output current during the whole cycle) at lower watts then class A/B as the power rises. This of course will increase heat.

Look at Parasound's new A21+ for example, that list: High bias Class A/AB operation. These amp are class A for about the first 6-8 watts, etc.

daddyjt

I once had an extended listening session with a pair of Levinson ML2 amps pushing Wilson MAXX speakers. Intoxicating, to say the least....

mhardy6647

The early Class A Mark Levinson amplifiers (from the real Mark Levinson, when Mark Levinson was involved with the company) were mighty nice.

The big-boy amplifier here is Class A but low power. The output devices (direct heated 2A3 triodes) are biased "on" all the time -- most of the power being generated is dissipated as heat from the plates; some goes to the loudspeakers. The amp draws about 130 watts from the AC mains to produce ca. 7 watts (3.5 wpc) of audio output power. The rest of that power is dissipated as heat (thermodynamics being what it is).

Actually - come to think of it - there's one Class A solid state amplifier here (Nelson Pass' "Amp Camp Amp") and one with switchable bias (Yamaha CA-800, which is a class AB amp @ 45 wpc that can be switched to operate in Class A @ 10 wpc).

None of which is probably helping the OP much at all, though!

stretchl

It’s all good @mhardy6647

I tend to keep reading, whether I understand what someone is saying or not, hoping I’ll pick something useful up by osmosis.

I want to clarify what the individual I’m referring to, who knows amps backwards & forwards, was referring to. I may have mixed things up badly between that conversation and my post above.

Will clarify when possible -