Hi experts,

How much DC offset would you consider acceptable in a power amp?
What can you do on a rainy Sunday afternoon? I was poking around inside
an old Rotel RA-970BX amp. It works OK except that the DC offset is
about 100mv and 50mv on the two channels, which seems rather high to me.
There is no adjustment, except for bias current, so any tweaking would
probably have to be adjusting the current in one of the long-tailed pairs
by swapping resistors.

--
Eiron.

"Eiron" wrote in message
...
Hi experts,

How much DC offset would you consider acceptable in a power amp?
What can you do on a rainy Sunday afternoon? I was poking around
inside
an old Rotel RA-970BX amp. It works OK except that the DC offset is
about 100mv and 50mv on the two channels, which seems rather high to
me.
There is no adjustment, except for bias current, so any tweaking
would
probably have to be adjusting the current in one of the long-tailed
pairs
by swapping resistors.

--

Have you tried adjusting the bias to see if you makes any difference?

If you Google 'schematic RA970BX' you will get copious information
including the service manual which includes how to set the bias. I
suspect it will correct it.

You should ideally be aiming for 10mV or less offset.



--
Woody

harrogate3 at ntlworld dot com

Eiron wrote:


How much DC offset would you consider acceptable in a power amp?


** An offset of +/- 100mV is Ok for regular 8 ohm speakers - but not for an ESL57 of the like which has a resistance of 0.3ohms.


What can you do on a rainy Sunday afternoon? I was poking around inside
an old Rotel RA-970BX amp. It works OK except that the DC offset is
about 100mv and 50mv on the two channels, which seems rather high to me.
There is no adjustment, except for bias current, so any tweaking would
probably have to be adjusting the current in one of the long-tailed pairs
by swapping resistors.

** That amp is a dog's breakfast.

It uses complementary differential pairs at the input so ought to be almost input current and hence DC offset free - if they merely matched the Hfes and got the resistor values right.

The input has 47kohms to ground while the overall feedback resistor is 12kohms.

Good luck fixing it.


.... Phil





--
Eiron.

On Sun, 22 Feb 2015 19:35:14 -0000, "Woody"
wrote:


"Eiron" wrote in message
...
Hi experts,

How much DC offset would you consider acceptable in a power amp?
What can you do on a rainy Sunday afternoon? I was poking around
inside
an old Rotel RA-970BX amp. It works OK except that the DC offset is
about 100mv and 50mv on the two channels, which seems rather high to
me.
There is no adjustment, except for bias current, so any tweaking
would
probably have to be adjusting the current in one of the long-tailed
pairs
by swapping resistors.

--

Have you tried adjusting the bias to see if you makes any difference?

If you Google 'schematic RA970BX' you will get copious information
including the service manual which includes how to set the bias. I
suspect it will correct it.

You should ideally be aiming for 10mV or less offset.

That's a pretty fair ideal to aim for. However, in the case of a 60W
RMS per channel amplifier, even the 100mV offset isn't of any great
concern. It represents a mere 80mW dissipation in the bass driver coil
and an assymetric clipping level reduction by a tenth of a volt from
the +/- 31 voltage swing available to support driving a 60W RMS
sinewave into an 8 ohm speaker load.

Since this is an old model, the less than ideal DC offset might
simply be a symptom of elderly electrolytics suffering unusually high
leakage current due to lack of use. The DC offset might well improve
after a few hours use (or it just might get progressively worse).

I suggest the OP runs it for an hour or two and recheck the DC offset
for any signs of further drift over the first few hours and then
recheck over the next few days and weeks if nothing untoward develops.
--
J B Good

Woody wrote:


Have you tried adjusting the bias to see if you makes any difference?

** It won't.


If you Google 'schematic RA970BX' you will get copious information
including the service manual which includes how to set the bias. I
suspect it will correct it.


** Not a chance - DC offset is an input problem, usually due to a combination of input transistor bias current and diff pair device matching.


You should ideally be aiming for 10mV or less offset.


** Not necessary at all - rare exceptions already noted.


.... Phil

Probably OK on most speakers, as long as it does not alter as the devices
heat up, and hopefully there is some kind of protection against excessive
currents at DC. I mention this as I had a Memorex brandreceiver, obtained
from Tandy which sounded great, but also had the ofset you mention, and a
few years back one channel died and put some large volts and amps through a
speaker and welded the cone to the pole pieces.

Brian

--
From the Sofa of Brian Gaff Reply address is active
"Eiron" wrote in message
...
Hi experts,

How much DC offset would you consider acceptable in a power amp?
What can you do on a rainy Sunday afternoon? I was poking around inside
an old Rotel RA-970BX amp. It works OK except that the DC offset is
about 100mv and 50mv on the two channels, which seems rather high to me.
There is no adjustment, except for bias current, so any tweaking would
probably have to be adjusting the current in one of the long-tailed pairs
by swapping resistors.

--
Eiron.

In article , Eiron
wrote:
Hi experts,

How much DC offset would you consider acceptable in a power amp? What
can you do on a rainy Sunday afternoon? I was poking around inside an
old Rotel RA-970BX amp. It works OK except that the DC offset is about
100mv and 50mv on the two channels, which seems rather high to me. There
is no adjustment, except for bias current, so any tweaking would
probably have to be adjusting the current in one of the long-tailed
pairs by swapping resistors.

FWIW I'd tend to go along with aiming at around 10 - 20mV or less. And if
its a long-tail-pair (ltp) started design you'd probably need to examine
that end if you want to improve it.

If it starts with an ltp then does it use two devices in one pack? If not,
then you may be able to tweak by selecting a pair. If it is a packed pair
you could find a better matched example.

I don't know the design, but if the ltp is bipolars, consider the
resistances though which each device gets its bias current. If they don't
present similar resistancesfor the bias paths then that may be offsetting
the result. That kind of problem made me prefer matched pairs of fets in a
single pack for this kind of job.

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

On 23/02/2015 10:09, Jim Lesurf wrote:
In article , Eiron
wrote:
Hi experts,

How much DC offset would you consider acceptable in a power amp? What
can you do on a rainy Sunday afternoon? I was poking around inside an
old Rotel RA-970BX amp. It works OK except that the DC offset is about
100mv and 50mv on the two channels, which seems rather high to me. There
is no adjustment, except for bias current, so any tweaking would
probably have to be adjusting the current in one of the long-tailed
pairs by swapping resistors.

FWIW I'd tend to go along with aiming at around 10 - 20mV or less. And if
its a long-tail-pair (ltp) started design you'd probably need to examine
that end if you want to improve it.

If it starts with an ltp then does it use two devices in one pack? If not,
then you may be able to tweak by selecting a pair. If it is a packed pair
you could find a better matched example.

I don't know the design, but if the ltp is bipolars, consider the
resistances though which each device gets its bias current. If they don't
present similar resistancesfor the bias paths then that may be offsetting
the result. That kind of problem made me prefer matched pairs of fets in a
single pack for this kind of job.

Two complementary long-tailed pairs feeding the + and - sides of the
output stage. And the transistors are bonded together after installation
presumably to equalize the temperature.
It's the same basic design as all Rotel amps from the 90s.

I guess the components are matched before installation but as there is
no offset adjustment it either passes or fails on testing.
I wonder what the factory limit is?

An interesting point is that the feedback is taken from the speaker side
of the fuse so if you remove the fuse there is no feedback
and the output voltage drops to -38v.

--
Eiron.

Jim Lesurf wrote:


I don't know the design, but if the ltp is bipolars, consider the
resistances though which each device gets its bias current.

**There are four BJTs, 2xNPN and 2xPNP in complimentary differential mode.

In theory, Hfe matching of all four results in no input bias current at all.

In reality, 1uA or 10mV across 10kohm is easy to achieve and so eliminate the offset trim pot. But Rotel have not matched the resistor values nor matched the four device's Hfes.



.... Phil