Rotel Bias ...

[Arfa Daily]

Anyone know off the top of their head, or have the manual to check what the
correct bias figure for a Rotel RB970BX Mk II is ? Working along the lines
of the 'standard' 20mA figure (about 5mV across the TPs which are across a
0.22 ohm emitter resistor) the output stages idle quite cool, and I see no
signs of x-over distortion on the 'scope, but would be nice to know the
correct recommended figure.

Arfa

 

[Eeyore]

Anyone know off the top of their head, or have the manual to check what the
correct bias figure for a Rotel RB970BX Mk II is ? Working along the lines
of the 'standard' 20mA figure (about 5mV across the TPs which are across a
0.22 ohm emitter resistor) the output stages idle quite cool, and I see no
signs of x-over distortion on the 'scope, but would be nice to know the
correct recommended figure.

20mA sounds a bit low to me. I'd go for at least 30mA.

Graham

 

[Trevor Wilson]

Anyone know off the top of their head, or have the manual to check what
the correct bias figure for a Rotel RB970BX Mk II is ? Working along the
lines of the 'standard' 20mA figure (about 5mV across the TPs which are
across a 0.22 ohm emitter resistor) the output stages idle quite cool, and
I see no signs of x-over distortion on the 'scope, but would be nice to
know the correct recommended figure.

**8mV across TP1 & TP3 & Across TP2 & TP4. Those are the figures for the
RB970BX. (Not MkII). I'll try to locate the data pertaining to the model you
have.

Trevor Wilson

 

[William Sommerwerck]

If you have a distortion meter -- especially one that can show the levels of
each harmonic (eg, an audio spectrum analyzer) -- you might try adjusting
for the lowest distortion or the least odd-order distortion.

 

[Arfa Daily]

**8mV across TP1 & TP3 & Across TP2 & TP4. Those are the figures for the
RB970BX. (Not MkII). I'll try to locate the data pertaining to the model
you have.

Trevor Wilson

Thanks very much Trevor. That equates to not far off Graham's figure of
30mA, so that's where it's now set, and going back to the shop this
morning - it's one of those jobs where the owner is panicking the shop for
it on a daily basis. Customers, don't you just love 'em !! d;~}

Arfa

 

[Arfa Daily]

20mA sounds a bit low to me. I'd go for at least 30mA.

Graham

I can live with that, and 30 is pretty close to Trevor's figure (see his
post).

Thanks
Arfa

 

[Arfa Daily]

If you have a distortion meter -- especially one that can show the levels
of
each harmonic (eg, an audio spectrum analyzer) -- you might try adjusting
for the lowest distortion or the least odd-order distortion.

I do have one, but it only shows THD. I was expecting the figure to be
somewhere between 20 and 50mA, and was erring on the side of caution and
cool running with 20mA. It's now set for about 30 per Trevor's useful info,
and heatsinks are just barely off room temp at idle. I doubt that there
would be too much to be measured as a difference in THD between 20 and 30mA,
and the owner probably wouldn't be able to tell the difference ...

Arfa

 

[William Sommerwerck]

I do have one, but it only shows THD. I was expecting the figure to
be somewhere between 20 and 50mA, and was erring on the side
of caution and cool running with 20mA. It's now set for about 30 per
Trevor's useful info, and heatsinks are just barely off room temp at idle.
I doubt that there would be too much to be measured as a difference
in THD between 20 and 30mA, and the owner probably wouldn't be
able to tell the difference...

I'd be foolish to suggest there even _might_ be an audible difference.
However, the harmonic-distortion spectrum of transistor amplifiers can vary
quite weirdly with bias and signal level.

 

[Trevor Wilson]

Thanks very much Trevor. That equates to not far off Graham's figure of
30mA, so that's where it's now set, and going back to the shop this
morning - it's one of those jobs where the owner is panicking the shop for
it on a daily basis. Customers, don't you just love 'em !! d;~}

**Some. I have a few clients who are like gold. Some, OTOH, I just never
want to see again.

Trevor Wilson

 

[Arfa Daily]

**Some. I have a few clients who are like gold. Some, OTOH, I just never
want to see again.

Trevor Wilson

Amen to that, Trevor !!

Arfa