From: "Saved by Internet Explorer 11" Subject: Autobios improved Date: Wed, 7 Oct 2015 14:51:37 -0700 MIME-Version: 1.0 Content-Type: multipart/related; type="text/html"; boundary="----=_NextPart_000_0000_01D1010F.AAC1A650" X-MimeOLE: Produced By Microsoft MimeOLE V6.1.7601.17609 This is a multi-part message in MIME format. ------=_NextPart_000_0000_01D1010F.AAC1A650 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Content-Location: http://www.mennovanderveen.nl/eng/autobias-improves.html
WHY AUTO-BIAS SOUNDS SO GOOD?=20 | |
I know I invented my special Vanderveen auto-bias circuit and = Guido=20 Tent made it a beautiful unit. So, one might think that the = following=20 article is nothing else than advertising. So wrong this opinion = is,=20 because the last month I really was amazed about the remarkable=20 improvements in sound quality with this new unit. The base = sounded much=20 cleaner and much more micro details were noticeable. I started a = deep=20 going research in literature, used earlier work, performed new =20 measurements and I got support from students (especially Lukasz = Jazczuk).=20 All this new work created insight in what happened and that is = what I am=20 going to tell in this article. I now can scientifically = (objective)=20 explain why the sound quality (subjective) has improved so = much.
The schematic below shows the UL40-S2 in its former = configuration,=20 without auto-bias or any other modification. The cathodes of both = power=20 valves (B2 and B3) are connected to a resistor Rc =3D 220 Ohm to = ground, in=20 parallel with Cc =3D 1000 uF. Without any music signals, the = quiescent=20 currents of the power valves go through Rc//Cc and create a = voltage over=20 this circuit. Because the control grids of B2 and B3 are at = ground level,=20 the voltage difference over Rc//Cc sets the working point = (quiescent=20 point). Any difference between B2 and B3 can be corrected with P4 = to make=20 both quiescent currents equal. However, when the amplifier is fed with musical signals, the =
mean=20
currents through B2 and B3 get larger (depending on the output =
power). At=20
higher output level the voltage difference over Rc//Cc gets =
larger and=20
consequently the quiescent working point changes and the =
quiescent=20
currents through B2 and B3 get smaller. All this would be no =
problem if=20
both valves are exactly equal. Reality is that this never is the =
case.=20
So, at higher output levels, the quiescent currents through B2 =
and B3=20
become unequal. Then there is a quiescent current difference and =
this=20
will DC-magnetize the core of the output transformer, which is =
very =20
disadvantageous. STABILITY WITH THE NEW AUTO-BIAS UNIT With the new auto-bias unit of Guido and me, the situation = changes=20 totally. See the schematics below. The unit measures the = quiescent=20 current per valve in a very small current window, and does not = react at=20 all on the output power level. Under all levels the quiescent = currents=20 stay rock stable, constant and equal. Now there is no difference = between=20 the quiescent currents through B2 and B3 and the core of the = output=20 transformer is not DC-magnetized (assuming correct AC-balance, = see=20 later). The measurement below shows that the 50 Hz sine = distortion has=20 disappeared. This explains exactly why the base reproduction has improved = so=20 largely with the auto-bias unit. The DC-magnetization distortion = is=20 absent now, the base tones are clean, independent of the output = level.=20 But, more is going on.
Click on figure to enlarge. This measurement shows a remarkable result. It proves that = small=20 signal levels get weakened and it shows that the calculated and = measured=20 starts of weakening lay at different output levels (they differ = almost a=20 factor of 10 which equals 20 dB). What is the cause of the = difference?=20 Closer research showed that the quiescent currents of the power = valves=20 were not exactly equal during this measurement, they differed = about 0.3=20 mA, which is little. This small difference current was able to=20 DC-magnetize the core of the OPT and consequently the magnetic = domains =20 became extra bound to their position. This created the extra = weakening in=20 the measured situation. If we apply the new auto-bias unit, then = the=20 quiescent currents are equal within 0.05 mA (or better) and the = dotted=20 line shifts left towards the red line. In other words: without = any=20 DC-magnetization of the core we can hear almost 20 dB deeper into = the=20 sound before any extra weakening of the micro details occur. Now = it is=20 clear why we hear the micro details much better with the = auto-bias=20 unit.
In this research and article I am able to explain = scientifically why=20 base tones sound cleaner and why micro details are better = noticeable with=20 the new auto-bias unit.
In the UL40-S2 the 6550 power valves operate optimal at a =
quiescent=20
current of 80 mA. The high supply voltage is then 355 V, so the =
anode=20
dissipation equals 355 * 0.08 =3D 28,4 W per valve. This is =
smaller than=20
the maximum dissipation allowed of 40 W and ensures an acceptable =
long=20
lifespan of the valves. Also in this setting the amp operates =
almost=20
completely in class A, which minimizes the dynamic damping factor =
distortion (DDFD) as explained in my books and publications.
I hope that the visitors of my web site have noticed that I = gave my=20 UL40 a lot of attention during the last month. The introduction = of the=20 ST-mode and the new auto-bias unit made it possible to discover = new=20 refinements in the sound quality of this amplifier. For me = personally it=20 became of the utmost importance to reach again for the top and to = understand scientifically why the improvements were so good. I = notice=20 that in my present work objective and subjective are coming closer = and I=20 really do like that. Enjoy listening to my UL40-S2 in ST-mode with = auto-bias! (Menno van der Veen, June 2008) |