[Patrick Turner]

wrote:

Hi,
I need some help with my ARC SP8. The manual talks about an option
to add a resistor - R28 (value - 39.2k) to cut gain by 6dB's in the
case of high efficiency speakers or high gain power amps. The usable
area of the Gain control is cramped right down into the first 3 or 4
clicks of the knob so it's either too loud or too quiet and fine
control is impossible. I guess I need to add resistor no. 28!!

Has anyone done this, and can they help me identify the correct
location for these resistors?

I'm a bit of an electronics virgin but I do see the position on the
circuit diagram where the resistor goes. However, I can't be 100%
sure that I've found the position for it on the unit. A double sided
circuit board doesn't help too much either! I am pretty confident at
soldering though. My assessment is that the position for the resistor
is on the underside of the board, on the right hand side. There are
two sets of metal posts that look like they could have been meant to
attach a resistor to and the components upstream & downstream of the
circuit tracks look like they are what the diagram shows. .

I'm fairly sure I'm right but I need some reassurance before I go
ahead & do it.
The first pair of these posts (one channel) are located just in front
of what I take to be a heat shield (a sheet of aluminium rising out of
the circuit board at 90 degrees. The second pair (the second channel)
are about 4 inches in front of these, right at the front of the unit.
The best way I had of confirming that these could indeed be the
positions was that they both showed 39.2k, which is the also value of
resistor 29, that the new resistors need to be wired in parallel
with.

Am I right in my assumption, or am I about to blow my preamp up??

I just downloaded a copy of the SP8 schematic but its such a poor scan
I cannot read the R numbers.

And I could not see any 39.2k resistors.

It seems that what you are intending is to simply increase the NFB from
the line
level amp output by a factor of 6dB, and thus you will reduce gain by
6dB.

This is recommended for where gain is too high, but we are left to
assume the preamp won't become unstable
with the extra NFB.



I'd also like to know what would be the best type of resistor to get
to do the job with?

If its a FB resistor, 1 watt metal film would be OK.



Would it be possible to add a higher value resistor to give me more
than 6 dB's if I need it - without compromising sound quality?

If you are increasing NFB by placing an aditional R across an existing
FB R
then to cut gain more the R would need to be lower value, not higher
value.

But it appears the cathode R from V6 back to cathode of V4 forms the
global NFB path,
and also carries DC for the V6 tube, so this R cannot be reduced too
much lest
Ik becomes too high, since V6 cathode follower grid is direct coupled
off V5.
My schematic has this R = 33k and marked what looks like R83, but
probably its R23,
and it could be simply paralleled with another R, 33k.
There is R27 of some value I can't read to the left of the R23,
and this R27 is between two terminals for gain change.
Removing R27 entirely would maximise NFB without paralleling any other R
or drastically altering the Ia of V6.

There should be a dc voltage reading across this R27.

It partially shunts the NFB applied to V4 cathode.

V4&V5 are cascaded 12AU7 with 1/2 6DJ8 as the cathode follower,
so total open loop gain is around 150.

The NFB probably reduces this to say about 15, or 23dB,
and really all that's ever needed for a line stage for CD source is a
gain of about 4,
or 12dB for most power amps needing a volt for clipping.

So perhaps you need to reduce the gain by 12dB or by 1/4, not 6dB or by
1/2.


Reducing gain is like a giving the amp a vasectomy.
Its OK, usually no damage can occur, and its reversible later.

And your'e right about double sided boards being a PITA.

Its difficult to know where you are, and many amp makers don't
have the R&C numbers printed on their boards,
leaving techs to find their way around in the dark.
Single sided are much better and can be traced out by reading what is
connected to what.
Double sided boards make it all much harder.

So to fix gear from ARC one has to try a lot harder.



Patrick Turner.



Thanks for any help you can offer.