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Hot bias - unnatural smooth sound

M

Meehael

New Member
Greetings from Slovenia! My first post here.

I just need some clarification regarding biasing effect on a sound of AU-X701.
When I got my amp back from full restoration (Elna Silmic II caps) I raised the bias from recommend value of 17-19mV to about 25-27mV. It worked well with previous speakers - they were playing faster and more dynamic. With new speakers which are more transparent, everything sounded too smooth, too silky, unnatural, very detailed but not too bright. Cymbals didn't have the natural attack that they should, like they were covered with silk cloth, guitars and saxes were strange sounding also. I've spent days busting my head to find the problem and even got buyers remorse. Today I've set the bias to recommended value and it sounds normal.
So, the question is, what happened with the sound? Was this cleanness or smoothness actually a distortion? It felt like transients weren't right, like attack and decay somehow overlapped, hard to describe as the details weren't missing. Usually higher bias in other amp designs produces less distortion, but in this case I think it went the other way. I guess Alphas are different.
 
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the au-x line is the lowest in distortion out there from Sansui so I suspect some dc was getting through with the higher bias. I know my 711 needs a tone inputed to set it (I believe). Maybe that was the issue? Actually I just looked the tone was for setting the vol pots
 
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Maybe, I doubt it. I know I've set it lowest I could with the original trim pots, but it drifts +- around 0mV. New pots are planned for the future. Would it still drift with new pots or is this normal for this design?
 
moefuzz said:
Getting rid of the old pots will probably clear up some of the static in them and could
result in more stable settings but there will always be a bit of 'drift' as you actually
play music thru the amp and this is normal.

Setting the bias to low also has it's disadvantages. Keeping with in manufacturers specs is
always best practice for things like the transition timing of the + and - cycles.
Too much bias creates added heat and distortion as the wave form transitions from positive to
negative.
And setting bias to low can result in problems with matched pair output transistor timing.
As I understand it, a small amount of bias is necessary for critical timing of the npn pnp transistors
and too much creates wave form distortion as well as unnecessary heat.


cheers,
moe
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Yes, it's set to specs.
I was talking about DC offset drifting. Thanks for confirming that too much bias can create some kind of smearing. I always believed settings bias hotter was good for sound and I often set my amps a tad hot.
 
Meehael said:
I always believed settings bias hotter was good for sound and I often set my amps a tad hot.
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Not so, this is a classic audio myth and is nonsense. There are a tiny handful of amplifiers, and I mean tiny, that can benefit by a small increase in bias - the AU-X701 isn't one of them. There is one setting for DC offset (on this series of amplifier), which is not possible to achieve, and this is the 'Hot to Ground' measurement, this setting will wander around endlessly, (+/-200mV often seen), this is normal, you should adjust it to as close to zero as possible and then leave it. The Hot to Cold measurement can be easy to set to zero, and should have a much more predictable slight drift after warm up. Bias will also drift, but should be set after the unit has warmed up for best results. Note that bias may interact with DC offset during adjustment and vice-versa.
tonysrep said:
I know my 711 needs a tone inputted to set it (I believe). Maybe that was the issue?
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You don't need to input a tone for setting the bias or DC offset - the only reason for the Service Manual asking you to do this is to warm the unit up faster. You can achieve the same thing by leaving the amplifier on for ½ hour or more, preferably with music playing at moderate volume. But for the actual adjustments No tone input, and No music.

Finally, all of these settings will drift less if the unit has been properly restored, and be much easier to adjust with replacement, preferably 'multi-turn' trimmers.
 
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Meehael said:
Maybe, I doubt it. I know I've set it lowest I could with the original trim pots, but it drifts +- around 0mV. New pots are planned for the future. Would it still drift with new pots or is this normal for this design?
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If it still has the "original" trim pots, you didn't get a "full" restoration. That makes me think all you got was a "recap job"...
 
Skywatcher said:
If it still has the "original" trim pots, you didn't get a "full" restoration. That makes me think all you got was a "recap job"...
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Yes, 63 caps were changed, all except two mains, pots were cleaned and thermal paste changed. I'll probably get Bursons done myself. Should there be anything else done? Sounds good now as it is.
 
Hyperion said:
In my opinion it isn't worth the wait, :rolleyes: I like to use capacitors that 'burn in', 'condition', 'form' - much quicker than that. :)
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Same here Brother. :beerchug: I only use them if the customer "insists" and I can't persuade them otherwise. And I tell them it will add 2 weeks minimum to their timeline. I don't want to hear them complain :rant: about how they sound, or "don't sound" for weeks being "virgin" and claim I "did something wrong". :blah:
 
I've done a bit of experiments on bias, but using only hearing - the final product anyway. Lowering bias below recommended setting can give a grainy sound. Increasing bias, does very little, except to add heat. On older quasi-complementary amps it's a moot exercise since they are inherently misalligned by design unless a bandaxall mod is applied.

On some models there might exist two biases: An optimal bias and a safe bias. This would be considering that units were exported to all sorts of the climates, let alone stuffed into confined spaces by some users. The engineer department in these cases might have been told by the legal department to make sure no houses would be set on fire, since the account department would not budget for large enough heatsinks....who knows.

Imagine a 400 m relay run. The first guy runs the top half of the course (positive side of sinus curve) - when its time to pass on the baton to the guy who will run the bottom half of the course (negative side of the sinus curve) the question is, should the first guy completely stop right next to the second guy - hand over the baton, and then the second guy sprints from standstill (Zero Bias). Or, should the first guy keep some speed at the end of his course, then being met by the second guy at equal speed, and then at the exact time the second guy begin his part of the course, the baton is passed over (Bias). How fast they run at baton transfer time would the bias. The analogy even works with class A amps - here there is only one runner, no transfer.....so he gets very hot from doing all the work :-)
 
stereofun said:
bandaxall mod
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Sorry for the tangent, but I'd like to say thanks for introducing me to that idea.

I'm also wondering how much it was used in production designs? I don't think I've ever encountered it - or at least, I never noticed it. Did it just not "catch on", or did it only arise at about the same time that full-complementary output stages were becoming practical?

Same Baxandall that designed the near-ubiquitous tone control circuit, I assume? That guy got around!

Thanks,

chazix
 
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@Leestereo and @ConradH are the gentlemen I reckon to have have implemented it's use on a couple of sansui amps.

Here is a really good intro to the workings of it.

Seems like the load equalisation of the drivers is not something Peter Bandaxall specifically developed, but more that principles from earlier work were used. I love the history and theory, so I'll dig further into this.
 
That's a great thread; I don't think I've seen it before. The only thing I'll emphasize is that some of these old amps sound the way they do because of their imperfections. Make your mods easily reversible because you may decide that cleaner doesn't sound better, and that's OK. I happen to like very flat low THD amps, with decent damping factor, where the output looks like the input, but some might call them cold or sterile.
 
Leave the bias as its stated in SM
Pay attention to set offsets close to zero, if you want the best accuracy during hot/gnd setting you must connect two DVM's to the bases of pre-driver transistors, and set kVR4 to get equal reading on both meters. Thats the correct way, the one mentioned in SM is a bit unaccuracy, and setting kVR4 to show 0v between Hot and Gnd will cause little unbalance
And last, Silmics need at least 500 hours of burn in to give you proper sound
 
I traded a mint looking Sansui 5000a that had the boards re-done to an electrical engineer, who set the bias too high, changed the sound and literally smoked the receiver an hour later.
He has not re-offered the receiver, so I doubt he got it's issues fixed.
I was thrilled to get a pair of Infinity Kappa 200's in trade, I now have $250 in the Infinity's.
The EE has about 20 items for sale on CraigsList, so I assume this was a rare mistake for him.
I didn't like the "new" sound before it started smoking, but it was a great looking 5000a.
 
Bumpalump said:
Pay attention to set offsets close to zero, if you want the best accuracy during hot/gnd setting you must connect two DVM's to the bases of pre-driver transistors, and set kVR4 to get equal reading on both meters.
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I think you meant to say kVR2 here (as there is no kVR4), and for pre-drivers I am guessing kQ54 & kQ56 and kQ53 & kQ55 - but I am unclear on which of the four pre-drivers (4 in each channel), you mean for monitoring of base voltage with 2 meters when making this adjustment? - please can you clarify this?

@Bumpalump
 
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Yes its kVr2 in AU-X701/901 - my bad, kVr4 is in alphas afair.
Actually it doesnt matter which pair of predrivers, just much be the same circuit - kQ53 and kQ56 or kQ55 and kQ54
Base voltage should be within SM spec 1.8v and -1.8v
When this adjustment is out of balance you will get for example -2V on kQ56 and +1.6 on kQ53

I have explained meaning of this adjustment in this movie (English subtitles)
From 21:00
 
Bumpalump said:
I have explained meaning of this adjustment in this movie (English subtitles)
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Thanks @Bumpalump :thumbsup:,I watched many of your videos in your youtube channel. I will try this approach on my AU-D707X when I rebuild it because right now I completely dissmantled it and will make new wood side panels for it .
 
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