wrote:
Wow, thanks for the quick responses!!
Brett - I have only a schematic and a parts list. The board is not
marked with component positions.
I'm in the UK so a call to ARC would be a little pricey. Their
website says there is no public email access at the moment either!
ARC are too old fashioned to maintain friendly email facilities.
They also like to keep as much secret about the devices they use to
ensure ppl
only buy spares from them and deal with them only at a price per hour
far in excess of what ordinary
ppl like me earn.
They also make everything far more complex than it needs to be.
Its engineering for the sake of engineering; and each amp is an ode to
somebody's skills
at putting so many bits together, usually surrounded with loops of lots
of NFB,
most of which isn't needed at all.......but anyway, we can find our way
with enough
applied zeal....
Patrick - On the schematic, resistor 28 and 29 are bottom right - to
the left of the manual mute switch (28 is above 29 - both rated at
39.2K 1%) and it says 'see note 2'. The notes are directly below
the symbol for R29. Note 2 says 'Add R28 39.2k to cut gain 6 dB'
I don't have the same schematic as you have because that comment does
not appear
in the notes list on my schematic from
http://www.analogstereo.com/audio_re...ce_manuals.htm
It does say to remove R27 for less gain. This R is soldered between two
points meant to allow gain adjustment resistors to be added/removed
The right hand side of these resistors leads up to a group of 4
capacitors and the left hand side runs up to the line that goes
between a 1.28k resistor and tube no. 4 .
I'm afraid that the thing I just spotted flying over my head must have
been your explanation! (as I said, I'm an electronics virgin!!
NFB - Negative feedback?
NFB = negative feedback.
NFB + electronics virgin = man in quandary = confused be-fuddled person.
Without any NFB network between output and input, the line preamp would
have a gain
of about 150, way too much. This gain without any NFB is called the
"open loop gain".
When a sdample of output voltage is fed back to one of the two inputs
of the amp, ie, the cathode terminal of V4, there is the input signal
applied to the
V4 grid from the gain control, and the NFB signal applied to the
cathode.
Both are of the same phase, and the difference between them is
amplified.
So if you have 0.2Vrms signal from volume control applied to the grid,
then there may be 0.18Vrms of NFB signal applied to the cathode, and the
difference = 0.02Vrms.
The open loop gain amplifies 0.02V by 150, giving an output = 3Vrms.
The gain with NFB connected is called the closed loop gain, and in the
example I just mentioned
the closed loop gain with NFB = output voltage divided by grid input
signal = 3V / 0.2V = 15.
The NFB signal applied to V4 cathode is a sample fraction of the signal
at the V6 output cathode terminal.
The size or amplitude of the V4 cathode signal may be varied by altering
the
resistance network between V6 and V4 cathodes.
But first we must know we are looking at the same schematic,
and then be able to allow you to identify the R you need to remove or
add to alter the gain.
Its a very easy simple thing you wish to do once you know what you are
doing.
Techs would do all this without a board layout diagram which is
usually in the workshop manual for the amp.
But by deduction we find out where we are on a board from just the
schematic
R values, other things adjacently connected and and voltage
measurements.
Patrick Turner.
On my diagram R27 that you mention is a 33k value.
Cheers
Chris