Read several posts of experts, including papa, advising to Bias the amps, in some cases, higher than the manufacturer's service manual.
One example is the 40mv for the Nakamichi 'stasis based' PA-7, some folks advises even 110mv! Papa says to wait 30 minutes and bias until the heatsinks hits 50ºC.
So I would like to know what are the benefits of high bias (higher than manufacturer's indications) of class A and AB amps, also on Marantz amps that I think are 99% B class.
1-Is it really matters in regards of sound quality? In class A, AB and B
2-What are the problems concerning high bias in lifetime of an amp?
3-How much is a 'safe limit' percentage of high bias over a standard one?
4-What can go wrong in high biasing?
5-Other than crossover distortion, any improvements in soundstage, bass extension, treble top end, etc, these audiophile subjective 'hard to measure' topics?
6-any tips to get the correct high bias using a meter, a scope or a thermometer?
One example is the 40mv for the Nakamichi 'stasis based' PA-7, some folks advises even 110mv! Papa says to wait 30 minutes and bias until the heatsinks hits 50ºC.
So I would like to know what are the benefits of high bias (higher than manufacturer's indications) of class A and AB amps, also on Marantz amps that I think are 99% B class.
1-Is it really matters in regards of sound quality? In class A, AB and B
2-What are the problems concerning high bias in lifetime of an amp?
3-How much is a 'safe limit' percentage of high bias over a standard one?
4-What can go wrong in high biasing?
5-Other than crossover distortion, any improvements in soundstage, bass extension, treble top end, etc, these audiophile subjective 'hard to measure' topics?
6-any tips to get the correct high bias using a meter, a scope or a thermometer?
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1-Is it really matters in regards of sound quality? In class A, AB and B
Yes
2-What are the problems concerning high bias in lifetime of an amp?
If output devices run too hot, they don't last as long.
3-How much is a 'safe limit' percentage of high bias over a standard one?
It's more of a matter of how hot you can run them.
4-What can go wrong in high biasing?
If you mistakenly turn the variable resistor too quickly and surpass the rating of the transistor, it could burn up. Work slowly.
5-Other than crossover distortion, any improvements in soundstage, bass extension, treble top end, etc, these audiophile subjective 'hard to measure' topics?
Probably. You will be the ultimate judge of the quality when you change the bias.
6-any tips to get the correct high bias using a meter, a scope or a thermometer?
Having the schematic helps. Knowing where to monitor the bias adjustment is important. If you know the test point for bias, measure it before changing it so you have a reference. Use a multi-meter and temp probe to give yourself an idea of where the manufacturer set the bias and heat sink dissipation. Be careful where you touch the probe. Avoid toughing component leads. Let the amp run for an hour at least to get it up to temp.
Then do your measurements.
Make small changes and let it sit for a while to warm up, then measure again. Remember, if you measure on a cold day and run it hot, then when it's a hot day, the amp could overheat and fail.
Yes
2-What are the problems concerning high bias in lifetime of an amp?
If output devices run too hot, they don't last as long.
3-How much is a 'safe limit' percentage of high bias over a standard one?
It's more of a matter of how hot you can run them.
4-What can go wrong in high biasing?
If you mistakenly turn the variable resistor too quickly and surpass the rating of the transistor, it could burn up. Work slowly.
5-Other than crossover distortion, any improvements in soundstage, bass extension, treble top end, etc, these audiophile subjective 'hard to measure' topics?
Probably. You will be the ultimate judge of the quality when you change the bias.
6-any tips to get the correct high bias using a meter, a scope or a thermometer?
Having the schematic helps. Knowing where to monitor the bias adjustment is important. If you know the test point for bias, measure it before changing it so you have a reference. Use a multi-meter and temp probe to give yourself an idea of where the manufacturer set the bias and heat sink dissipation. Be careful where you touch the probe. Avoid toughing component leads. Let the amp run for an hour at least to get it up to temp.
Then do your measurements.
Make small changes and let it sit for a while to warm up, then measure again. Remember, if you measure on a cold day and run it hot, then when it's a hot day, the amp could overheat and fail.
I've been running a lot of SPICE simulations on class A amps, and the distortion figures go down considerably with more voltage, more current, and more heat.
Another quote from Nelson goes something along the lines of: MOSFETs sound better and better the hotter they get right up until they catch on fire.
Another quote from Nelson goes something along the lines of: MOSFETs sound better and better the hotter they get right up until they catch on fire.
Even more so, I should think, as you'd be expanding their class A envelope. On the other hand, they're less likely to have enough heat-sinking to do anything meaningful.
I think Bob Cordell showed that there is an optimum bias point of minimum THD for a class AB BJT follower output stage, increasing bias higher than optimum yields increasingly higher distortion.
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