Bias adjustment = better bass if it was low? Allways ?

Jauk

Active Member
As per the title.
Generally, if I adjust the bias and it was bellow spec before the adjustment, is it granted that the bass will improve ? Is it granted that raising it above spec will also improve bass (not advisable, of course) ?

Thanks
 
Adjusting the bias won't help the bass , it will lower the distortion to a point , most noticable at low volume level the bass will be more affected by current drive and damping factor
 
Define "better bass". How much lower bias current?

With bias low, you're likely introducing a lot of distortion (or more than there would be with properly adjusted bias). This can sometimes add "richness" to the sound, which some people appreciate.

Aside from that, there is no correlation between low-adjusted bias and an (objective) improvement in bass quality. In fact, it's typically (pretty much always) the opposite.
 
After testing my newly acquired Sansui AU-X701 with my favourite piece of test music and finding some bass artifacts 'missing' - I adjusted the offset and bias - both of which were slightly 'off'

Re-playing the test music, I find the bass is back - and it sounds just right. The amplifier design is unusual (an internally bridged design), and I couldn't adjust the bias to spec' as it is a complicated adjustment. The bias adjustment requires the use of a signal generator, dummy load and a true RMS voltmeter. All I did was to even-up the bias using 2 of the 4 adjustment pots for this, I will of course re-visit this adjustment after Santa has visited :D

It is wrong to assume that increasing the bias above the manufacturer specified figure will result in better bass or better anything in sound quality/quantity terms. Bias which is set too low, or in an unbalanced way (as in my case above) WILL result in sound reproduction anomalies as mentioned in this post and others in this thread.

So, in short, my bias was set too low and in an unbalanced fashion which when attended to, was probably responsible for the restoration of sound quality in my case. YMMV
 
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Ok, I allways like to know the theoretical but also the practical parts of it, thanks:)
 
As per the title.
Generally, if I adjust the bias and it was bellow spec before the adjustment, is it granted that the bass will improve ? Is it granted that raising it above spec will also improve bass (not advisable, of course) ?

Thanks

No, bias just regulates the DC voltage at the output. You want it to be zero so that your positive and negative voltage rails don't clip as soon as they would if there were DC at the output. that won't affect bass, it will allow you to turn the volume up louder without clipping. I find that clipping is way more audible in the higher frequencies so if anything this will improve the sound of the treble more than of the bass.
 
Doesn't to low on bias bring in some distortion and to high will push making the heat sinks run excessively hot?
 
No, bias just regulates the DC voltage at the output. You want it to be zero so that your positive and negative voltage rails don't clip as soon as they would if there were DC at the output. that won't affect bass, it will allow you to turn the volume up louder without clipping. I find that clipping is way more audible in the higher frequencies so if anything this will improve the sound of the treble more than of the bass.

I think you are getting a bit confused about the effects of Bias and DC offset.

Doesn't to low on bias bring in some distortion and to high will push making the heat sinks run excessively hot?

In a nutshell, yes
 
No, bias just regulates the DC voltage at the output. You want it to be zero so that your positive and negative voltage rails don't clip as soon as they would if there were DC at the output. that won't affect bass, it will allow you to turn the volume up louder without clipping. I find that clipping is way more audible in the higher frequencies so if anything this will improve the sound of the treble more than of the bass.

you are getting confused with dc offset ...
the bias sets the idle current to the outputs
 
I thought about starting a new thread but instead decided to resurrect this one.

The negative and/or positive effects of running an amp at lower than specified bias?

I use four stereo Classe amps for an 8 channel active crossover system. Two CA-100 for mids and highs and two CA-150 for low and subs. Every couple years I go through the SM bias setting procedure. I do this for all four amps at once because it takes several days. Classe recommends turning down the bias to 14mv and run them with load and input for 24 hours. Then remove load and input then set them to 24mv and run them with load and input for 72 hours. Then remove load and input and recheck to 24mv and repeat until stable.

I use the load and inputs from my main system for the run times which gives me a chance to listen to the amps in both the low and high bias settings. The positive aspects with the low bias is much lower temperatures on the heat sinks but I really don’t notice any negative aspects to the sound. Maybe a little less output that requires a bit more gain from the preamplifier.

I know my speakers really don’t need anywhere near the level of output these amps are capable of so I am curious if there is any reason not to run them at low bias. Is distortion, damping factor, clipping level, dynamics or frequency response effected negatively or positively with such a reduction in bias? I have to admit I don’t listen at really high levels when set at the lower bias because I fear perhaps my speakers are in harms way? Is there any reason to be concerned with the speakers when running at reduced bias?

The main benefit to reducing bias is to reduce temperature and possibly long term health of the amps. Are there other benefits or concerns?
 
on my Technics SA600...
This is what got me in trouble after adjusting it the first time in 40 years I noticed the Left Channel didn't have the same excursions on the Woofers so I would readjust I did this about 5 times over the course of 2 months the last time I did it my probe for my Fluke meter weekend and broke the test point and blew my whole unit now I need it repaired ! If you search my history go back until June when I did this I asked for help finding a technician here in Michigan. .. so a word to the wise be careful ! !!!!
And yes in regard to the bass this is what happened with my Technics receiver it would be below specified millivolts and when brought back up to specification the bass and excursions were back to normal on that left Channel...
I should have left it alone it was fine working like new before I had to mess to with the bias adjustment. .
Specifications for 8.0 millivolts and on the very first test it something like 12.7 and 13.1 and everything worked beautifully I'll never make the mistake of messing with something that is not broke. ...
 
on my Technics SA600...
This is what got me in trouble after adjusting it the first time in 40 years I noticed the Left Channel didn't have the same excursions on the Woofers so I would readjust I did this about 5 times over the course of 2 months the last time I did it my probe for my Fluke meter weekend and broke the test point and blew my whole unit now I need it repaired ! If you search my history go back until June when I did this I asked for help finding a technician here in Michigan. .. so a word to the wise be careful ! !!!!
And yes in regard to the bass this is what happened with my Technics receiver it would be below specified millivolts and when brought back up to specification the bass and excursions were back to normal on that left Channel...
I should have left it alone it was fine working like new before I had to mess to with the bias adjustment. .
Specifications for 8.0 millivolts and on the very first test it something like 12.7 and 13.1 and everything worked beautifully I'll never make the mistake of messing with something that is not broke. ...

Always good to learn from your mistakes but my ACD (Audio Compulsive Disorder) forces me to check all of my equipment per the SM every couple of years. Granted I don’t adjust anything unless I really think it is required but I do like to know everything is working at least very close to spec. Yes, you must be careful and that is why manufactures are required to put that sticker on the case that says no serviceable components inside. You can fry both the amp and yourself very easily. The Classe is extra tricky in that it does not have test points but you have to clip on the emitter resistor. I am aware of the risk and am qualifed and experienced to bias most any amp.

What I am not experienced in is using a scope or distortion analyzer etc. Thus I am asking the more experienced members what are the general expectations with amp and sound performance on using a low bias on an amp when the speakers really don’t benefit from the wattage potential of the amp at normal bias?

In either case I would always set the bias on all 8 channels equal.

Also, am curious what the theoretical wattage reduction would be at 14mv vs 24mv?
 
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No response from the electronics wizards out there?

Well, I found this article which is quite imformative but mostly wanting me to ask more questions:

https://www.electronics-tutorials.ws/amplifier/class-ab-amplifier.html

Realize I am a total noob when it comes to amplifier theory and design. I know the Classe is an AB amplifier. Will lowering the bias increase or decrease crossover distortion? Will lowering the bias decrease or increase the level of class A operation?
 
increase ..

decrease

So decreasing bias in an AB amp will increase the amount of B operation?

Does it effect output level?

So it reduces heat because there is less A operation?
 
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If you look at the topology of class A and class AB amplifiers you will find that they are essentially the same. So why does one require huge power supplies and heatsinks and the other doesn't. The answer is in the way the output transistors treat the signal they are carrying.
Let's start at the beginning. An audio waveform (sinewave or music signal) has a negative and positive going wave which oscillates around a zero point. The amplifier output stage consists of a pair (or multiple pairs) of transistors. One transistor amplifies the negative part of the waveform and the other amplifies the positive part. Each transistor takes an input for it's half of the waveform at it's base connection. It then reproduces a larger copy of that by taking energy from it's supply rail through it's collector connection (1 positive and one negative) and produces a proportionate voltage at its emitter connection via a resistor and send that to the loudspeaker. So the loudspeaker gets the whole waveform, half from the negative transistor and half from the positive transistor. With me so far? Good.
Now, the problem arises in that each transistor will not start to conduct right from a zero signal. It will start to conduct when the base signal reaches about 0.7volts (or minus 0.7 volts for the negative side). This would give a small step in the output waveform every time the input waveform crosses the zero region of the waveform. This is how a class B amplifier works and it sounds terrible. To get over this problem the input signal to the transistor is superimposed on a small DC signal which prevents the input to each transistor from dropping into this 'switched off' region. This is called output stage biasing and is usually adjustable.

Power transistors are current amplifier devices and the output biasing is actually measured as a current not a voltage. I mentioned previously that the output from the transistors go to the loudspeaker via a resistor. This is called the emitter resistor and it is there to ensure that the transistor always has some load, otherwise it would become unstable. As all the output passes through these resistors, they have to be quite large. As the output current passes through this resistor a small voltage is developed across it. When the whole amplifier has no input signal, the only current passing through the emitter resistor will be from the DC bias. This current passing will develop a small voltage across the resistor. It is this voltage that you are measuring at the bias test points. The setting is for bias current, but you cannot measure that without disconnecting an emitter resistor and putting an ammeter in series with it. But, using Ohms law and a bit of maths you can work out what the bias current is by measuring the voltage drop across the resistor. What the manufacturer will do is to give the voltage (in millivolts) that they consider gives the best minimum emitter current to ensure that the output transistors are held just outside of that zero switch on point.
If you decide to increase this given value you will effectively be converting your amplifier into a class A unit. The higher the bias current, the higher the minimum current the transistors will pass - remember that current equals heat. A class A amplifier usually biases it's output transistors to half the supply voltage and this means that their current is always running at maximum. This creates a lot of heat which is why class A amps need large PSUs and heatsinks.

O.K. That will do for now.
 
O.K. That will do for now.

Excellent! Explained very well and thank you! So at the minimum 14mv bias setting the amp is running 100% in class B all the time?

At 24mv (the manufacture’s highest recommended bias) the amp is running at maximum class A possible within the amp’s design and heat sink capability etc?
 
Excellent! Explained very well and thank you! So at the minimum 14mv bias setting the amp is running 100% in class B all the time?

At 24mv (the manufacture’s highest recommended bias) the amp is running at maximum class A possible within the amp’s design and heat sink capability etc?
think i missed what amp it is you are working with .
 
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