To adjust Bias:
You will want to place a volt meter across the output transistor
while the unit is off.
Turn on the unit. (with cover on if possible.
Wait a while: 30 minutes. . .you really want it at thermal stability.
Take note of output resistor value.
If the output resistor is 0.22 ohms and there is 22 mV across the resistor, then you have 100 mA flowing through it.
That might be too high. . .
I would adjust until it is around 10 mV. No reason to burn too
many electrons while the amp is doing nothing. The whole idea
is to reduce distortion, not increase it. Since pots are not perfect,
I kept adjusting until both channels were the same, which ended
up being somthing less than 10 mV, but higher than the original
around 6 mV.
Chris
You will want to place a volt meter across the output transistor
while the unit is off.
Turn on the unit. (with cover on if possible.
Wait a while: 30 minutes. . .you really want it at thermal stability.
Take note of output resistor value.
If the output resistor is 0.22 ohms and there is 22 mV across the resistor, then you have 100 mA flowing through it.
That might be too high. . .
I would adjust until it is around 10 mV. No reason to burn too
many electrons while the amp is doing nothing. The whole idea
is to reduce distortion, not increase it. Since pots are not perfect,
I kept adjusting until both channels were the same, which ended
up being somthing less than 10 mV, but higher than the original
around 6 mV.
Chris
Redrawn Schematic Rotel RB850
Here is a redrawn schematic of the Rotel RB850 and part list:
Resistors
R602 1k input +
R604 27k input -
R606 470 CSS LTP
R608 6.8k CSS LTP 1/2 W
R610 1.5k LTP / VAS
R612 470
R614 10k CSS BIAS1/2 W
R616 100 CSS BIAS
R618 8.2k BIAS
R620 2.7k BIAS
R622 47 VAS
R624 33k
R626 220 1 W
R628 220 1 W
R630 0.22 2 W
R632 0.22 2 W
R634 0.22 2 W
R636 0.22 2 W
R638 10k NFB
VR602 2.2k BIAS
VR604 100 LTP / VAS
POWER SUPPLY
R902 5.6k 1/2 W
R904 5.6k 1/2 W
Capacitors
C602 100V 1 elect
C604 100P
C606 1000P
C608 6.3V 100 elect
C610 150P
C612 330P
C614 50V 1 elect
C616 5 pF
C002 Disc sig to chasis
POWER SUPPLY
C910 50V 10000 elec
C912 50V 10000 elec
C914 ceramic
C916 ceramic
Transistors
Q602 2SA1016 pnp LTP
Q604 2SA1016 pnp CSS
Q606 2SA1016 pnp LTP
Q608 2SA1016 pnp CSS
Q610 2SB631 pnp CSS
Q612 2SC1941 npn VAS
Q614 2SD600 npn BIAS
Q616 2SD600 npn UNITY
Q618 2SB631 pnp UNITY
Q620 2SD1047 npn OUT
Q622 2SB817 pnp OUT
Q624 2SD1047 npn OUT
Q626 2SB817 pnp OUT
Diodes
D602 1N4148 CSS LTP
D604 1N4148 CSS LTP
Here is a redrawn schematic of the Rotel RB850 and part list:
Resistors
R602 1k input +
R604 27k input -
R606 470 CSS LTP
R608 6.8k CSS LTP 1/2 W
R610 1.5k LTP / VAS
R612 470
R614 10k CSS BIAS1/2 W
R616 100 CSS BIAS
R618 8.2k BIAS
R620 2.7k BIAS
R622 47 VAS
R624 33k
R626 220 1 W
R628 220 1 W
R630 0.22 2 W
R632 0.22 2 W
R634 0.22 2 W
R636 0.22 2 W
R638 10k NFB
VR602 2.2k BIAS
VR604 100 LTP / VAS
POWER SUPPLY
R902 5.6k 1/2 W
R904 5.6k 1/2 W
Capacitors
C602 100V 1 elect
C604 100P
C606 1000P
C608 6.3V 100 elect
C610 150P
C612 330P
C614 50V 1 elect
C616 5 pF
C002 Disc sig to chasis
POWER SUPPLY
C910 50V 10000 elec
C912 50V 10000 elec
C914 ceramic
C916 ceramic
Transistors
Q602 2SA1016 pnp LTP
Q604 2SA1016 pnp CSS
Q606 2SA1016 pnp LTP
Q608 2SA1016 pnp CSS
Q610 2SB631 pnp CSS
Q612 2SC1941 npn VAS
Q614 2SD600 npn BIAS
Q616 2SD600 npn UNITY
Q618 2SB631 pnp UNITY
Q620 2SD1047 npn OUT
Q622 2SB817 pnp OUT
Q624 2SD1047 npn OUT
Q626 2SB817 pnp OUT
Diodes
D602 1N4148 CSS LTP
D604 1N4148 CSS LTP
Attachments
Part List taken from Schematics:
SCHEMATIC [COMPUTER]
LOCATION [NUMBER] PART DESCRIPTION
Q601, Q602 [TR10000023] 2SA1016 (G,H) DIFFERENTIAL AMP
Q603, Q604 [TR10000023] 2SA1016 (G,H) CONSTANT CURRENT
Q605, Q606 [TR10000023] 2SA1016 (G,H) DIFFERENTIAL AMP
Q607, Q608 [TR10000023] 2SA1016 (G,H) CONSTANT CURRENT
Q609, Q610 [TR10000047] 2SB631K (E,F) CONSTANT CURRENT
Q611, Q612 [TR30000169] 2SC1941 (K,L) PRE-DRIVER
Q613, Q614 [TR40000069] 2SD600K (E,F) IDLING DRIVER
Q615, Q616 [TR40000069] 2SD600K (E,F) POWER DRIVER
Q617, Q618 [TR10000047] 2SB631K (E,F) POWER DRIVER
Q619, Q620 [TR40000045] 2SD1047 (D,E) POWER AMPLIFIER
Q621, Q622 [TR20000040] 2SB817 (D,E) POWER AMPLIFIER
Q623, Q624 [TR40000045] 2SD1047 (D,E) POWER AMPLIFIER
Q625, Q626 [TR20000040] 2SB817 (D,E) POWER AMPLIFIER
D601 - D604 [DD10000100] IN4148 BIAS COMPENSATOR
D901, D902 [DD10000147] KBPC802/PBPC803 RECTIFIER
D903 [DD20000028] UZ-20B (M,H) ZENER REGULATOR
D001 [DD40000095] LN224R POWER LED
D002 [DD40000095] LN224R BRIDGED LED
VR601, VR602 [RV40000022] RM6CKPV-1S-B 2.2K BIAS ADJUST
T001, T002 [PT23308103] TT-233H POWER TRANSFORMER
F901, F902 [FU11000074] FUSE, LONG 250V 4A
F901, F902 [FU12000045] FUSE, UL 250V 4A
F901, F902 [FU21000071] FUSE, MINI 4A
F901, F902 [FU23000049] FUSE, S 250V 4A
S001 [SH40000155] C-4176A01 POWER SWITCH
SCHEMATIC [COMPUTER]
LOCATION [NUMBER] PART DESCRIPTION
Q601, Q602 [TR10000023] 2SA1016 (G,H) DIFFERENTIAL AMP
Q603, Q604 [TR10000023] 2SA1016 (G,H) CONSTANT CURRENT
Q605, Q606 [TR10000023] 2SA1016 (G,H) DIFFERENTIAL AMP
Q607, Q608 [TR10000023] 2SA1016 (G,H) CONSTANT CURRENT
Q609, Q610 [TR10000047] 2SB631K (E,F) CONSTANT CURRENT
Q611, Q612 [TR30000169] 2SC1941 (K,L) PRE-DRIVER
Q613, Q614 [TR40000069] 2SD600K (E,F) IDLING DRIVER
Q615, Q616 [TR40000069] 2SD600K (E,F) POWER DRIVER
Q617, Q618 [TR10000047] 2SB631K (E,F) POWER DRIVER
Q619, Q620 [TR40000045] 2SD1047 (D,E) POWER AMPLIFIER
Q621, Q622 [TR20000040] 2SB817 (D,E) POWER AMPLIFIER
Q623, Q624 [TR40000045] 2SD1047 (D,E) POWER AMPLIFIER
Q625, Q626 [TR20000040] 2SB817 (D,E) POWER AMPLIFIER
D601 - D604 [DD10000100] IN4148 BIAS COMPENSATOR
D901, D902 [DD10000147] KBPC802/PBPC803 RECTIFIER
D903 [DD20000028] UZ-20B (M,H) ZENER REGULATOR
D001 [DD40000095] LN224R POWER LED
D002 [DD40000095] LN224R BRIDGED LED
VR601, VR602 [RV40000022] RM6CKPV-1S-B 2.2K BIAS ADJUST
T001, T002 [PT23308103] TT-233H POWER TRANSFORMER
F901, F902 [FU11000074] FUSE, LONG 250V 4A
F901, F902 [FU12000045] FUSE, UL 250V 4A
F901, F902 [FU21000071] FUSE, MINI 4A
F901, F902 [FU23000049] FUSE, S 250V 4A
S001 [SH40000155] C-4176A01 POWER SWITCH
New Bias Resistor and trimpot
Against anatech's advice (sorry) I did install the multi-turn bias pot.
For the 'reason' that the amount of DC current in the circuit through
R619+VR601 is very small. The measurement across these two
devices is the same points as the voltage of (be).
Left Channel:R619+VR601
Right Channel:R620+VR602
Left DC offset: +31.8mV
Right DC offset: +24.4mV
The old circuit path was R619 = 2.7kohms.
The new cirucit path is R619 = 3.58kohms.
The old VR601 was a 2.2kohm single turn.
The new VR601 is a 500 ohm multi-turn.
Spec is at 4mV across the 0.22ohm output resistor for 18.2mA.
Left
R619+VR601=3989ohms with 154.2microamps (615mV)
VR601=408.6ohms
Right
R620+VR602=3872ohms with 157.8microamps (611mV)
VR602=291.5ohms
Spec+8mV = 12mV across the 0.22ohm output resistor for 54.5mA
Left
R619+VR601=3752ohms with 162.6microamps (610mV)
VR601=172ohms
Right
R620+VR602=3670ohms with 165.9microamps (604mV)
VR602=90.4ohms
Against anatech's advice (sorry) I did install the multi-turn bias pot.
For the 'reason' that the amount of DC current in the circuit through
R619+VR601 is very small. The measurement across these two
devices is the same points as the voltage of (be).
Left Channel:R619+VR601
Right Channel:R620+VR602
Left DC offset: +31.8mV
Right DC offset: +24.4mV
The old circuit path was R619 = 2.7kohms.
The new cirucit path is R619 = 3.58kohms.
The old VR601 was a 2.2kohm single turn.
The new VR601 is a 500 ohm multi-turn.
Spec is at 4mV across the 0.22ohm output resistor for 18.2mA.
Left
R619+VR601=3989ohms with 154.2microamps (615mV)
VR601=408.6ohms
Right
R620+VR602=3872ohms with 157.8microamps (611mV)
VR602=291.5ohms
Spec+8mV = 12mV across the 0.22ohm output resistor for 54.5mA
Left
R619+VR601=3752ohms with 162.6microamps (610mV)
VR601=172ohms
Right
R620+VR602=3670ohms with 165.9microamps (604mV)
VR602=90.4ohms
Look in your PMs
hi
i did the 100uf bypass caps on the diodes and i got 6db rms improvement on noise floor.
and yes higher thump noise on startup but not big deal.
few years ago i did recap, ufast diodes ,non magnetic resistor ,bourns bias pots.
now this mod and little higher bias gives big improvement in sound stage.
thanks all.
e.g the mods is on rotel rb960bx the amp almost identical to rb850.
i did the 100uf bypass caps on the diodes and i got 6db rms improvement on noise floor.
and yes higher thump noise on startup but not big deal.
few years ago i did recap, ufast diodes ,non magnetic resistor ,bourns bias pots.
now this mod and little higher bias gives big improvement in sound stage.
thanks all.
e.g the mods is on rotel rb960bx the amp almost identical to rb850.
Last edited:
Great to hear you had good results modifying this amp series -- I never touched the PS diodes or bypass caps. I just finished re-capping back in January. I didn't replace the PS caps as I'm not sure how I will be mounting and soldering in points for the new caps -- just gonna give it some time to roll around in the old cranium.
I have been running the amp with fixed resistors in the bias adjust circuit for many years now; I might still add a switch to have two bias points (high and low) -- it just depends. I found a good value for each channel that drifts within a good 'zone'. Life isn't perfect. I enjoy listening more now than earlier in my life; modding is still a fun past-time.
Wow, just noticed my last post on this thread was 12 years ago. . .how time flies!
Another thread with pictures of the recapping:
Rotel RB850 Rebuild 2019
I have been running the amp with fixed resistors in the bias adjust circuit for many years now; I might still add a switch to have two bias points (high and low) -- it just depends. I found a good value for each channel that drifts within a good 'zone'. Life isn't perfect. I enjoy listening more now than earlier in my life; modding is still a fun past-time.
Wow, just noticed my last post on this thread was 12 years ago. . .how time flies!
Another thread with pictures of the recapping:
Rotel RB850 Rebuild 2019
Last edited:
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