[[email protected]]

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

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?

Thanks for any help you can offer.

[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.

[Patrick Turner]

"François Yves Le Gal" wrote:

On Mon, 23 Jul 2007 17:05:50 GMT, Patrick Turner
wrote:

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

Have you tried the ARCDB site? They've got quite readable schematics of most
versions of the SP-8 as well as of many vintage ARC products.

http://www.arcdb.ws/SP8/SP8.html

Thanks Francois,

The schematic here is exactly the same one as at
http://www.analogstereo.com/audio_re...ce_manuals.htm

But your source is much clearer, because ARC scanned it at higher dpi.

I was right. Alter gain by varying R27 between points A&B.

Do NOT change the value by adding or subtracting another R to what is
shown as R83, 33k,
which is a dc carrying load R for V6.

Minimum gain is when R27 is omitted entirely.

Benn should try omitting R27 immediately, and if ordinary levels are
enjoyed with gain pot set to the 12 o'clock position then all is well
and nothing
more needs to be done.

However, a good tech would perform a stability check.
Its unlikely that loooong cables with high capacitance effects are being
used,
and unlikely HF oscillations will occur with the increase in NFB by
removing
R27.
The reason is that the removal of R27 also increases the local current
NFB
of V4, thus reducing the open loop gain of this triode and thus the open
loop gain of the
two triodes, V4, V5, so although the global NFB is increased, open loop
gain is decreased,
and so the amount of reduction of open loop is kept more constant this
way.

Patrick Turner.

[[email protected]]

It seems that you have do have different schematics to the one that is
in my SP8 manual. I looked at the one on the ARCDB and it is from a
later version. From the photos on that website it seems I have a Mark
I preamp. I don't know whether it's revision 2 or 3, but the schematic
does mention changes made in 1981, so I would assume it's not the
original version - called revision 1 on the ARCDB (although how the
first version of something can be a revision, I'm not sure!).

Anyway, my manual does not show this resistor no. 27 in this
position. Rather, resistor 28 is in a similar place to resistor 83
and there is shown a line parallel to it that has a gap in it. The
lines end in circles with crosses in them similar to the ones at
either end of your r27 on the ARCDB site. This is the place where it
says that r29 should go if needed. I also note that the gain
reduction quoted here is 10 dB's rather than the 6dB's mentioned for
my 39.2k resistor.

If you look at the photo of the Mark 1 SP8 internals on ARCDB you can
see the bottom of 2 of the posts I was referring to where they
protrude from the other side of the double sided circuit board.
Frustratingly, there is no photo of the underside of the board & mine
is very different from the underside photo of the later amp that is
shown.

Here's how you find them:
If you locate the yellow Wondercap to the far left, below the mains
transformer, you can see the shiny edge of the heat shield I referred
to. Now, between the Wondercap and the heatshield there are 4 solder
tracks that go under the cap. The left hand one goes under the left
hand end of the cap, leaving 3 more going under the upper edge of
it. The track to the left and the one to the right show the bottoms
of these posts as shiny discs. I think r29 should be soldered across
these posts because I measured the resistance between them and found
39.2, which I assume to be the value of the paralled resistor, r28.
If you then look to the right of these posts, there is a black
resistor with its left hand wire going into a spot of track with no
begining or end - I think this is r28.
If you want to see what the posts look like from above there are two
more over near the top of the amp between the topmost psu capacitor
(one of the upright ones) and the transistor mounted in the heatsink.
There is a black device soldered between these posts - not sure if
it's a resistor or not. The lower of the two posts is easier to make
out than the other one.

I wonder if anyone looking at this has a similar vintage SP8 with a
resistor across these posts on their amp?

Cheers
Chris

[Patrick Turner]

wrote:

It seems that you have do have different schematics to the one that is
in my SP8 manual. I looked at the one on the ARCDB and it is from a
later version. From the photos on that website it seems I have a Mark
I preamp. I don't know whether it's revision 2 or 3, but the schematic
does mention changes made in 1981, so I would assume it's not the
original version - called revision 1 on the ARCDB (although how the
first version of something can be a revision, I'm not sure!).

Anyway, my manual does not show this resistor no. 27 in this
position. Rather, resistor 28 is in a similar place to resistor 83
and there is shown a line parallel to it that has a gap in it. The
lines end in circles with crosses in them similar to the ones at
either end of your r27 on the ARCDB site. This is the place where it
says that r29 should go if needed. I also note that the gain
reduction quoted here is 10 dB's rather than the 6dB's mentioned for
my 39.2k resistor.

If you look at the photo of the Mark 1 SP8 internals on ARCDB you can
see the bottom of 2 of the posts I was referring to where they
protrude from the other side of the double sided circuit board.
Frustratingly, there is no photo of the underside of the board & mine
is very different from the underside photo of the later amp that is
shown.

Here's how you find them:
If you locate the yellow Wondercap to the far left, below the mains
transformer, you can see the shiny edge of the heat shield I referred
to. Now, between the Wondercap and the heatshield there are 4 solder
tracks that go under the cap. The left hand one goes under the left
hand end of the cap, leaving 3 more going under the upper edge of
it. The track to the left and the one to the right show the bottoms
of these posts as shiny discs. I think r29 should be soldered across
these posts because I measured the resistance between them and found
39.2, which I assume to be the value of the paralled resistor, r28.
If you then look to the right of these posts, there is a black
resistor with its left hand wire going into a spot of track with no
begining or end - I think this is r28.
If you want to see what the posts look like from above there are two
more over near the top of the amp between the topmost psu capacitor
(one of the upright ones) and the transistor mounted in the heatsink.
There is a black device soldered between these posts - not sure if
it's a resistor or not. The lower of the two posts is easier to make
out than the other one.

I wonder if anyone looking at this has a similar vintage SP8 with a
resistor across these posts on their amp?

Cheers
Chris

I can't help you any more than I have.

Perhaps you could trace out the schematic.

It does appear the R27 or "black device" between two posts
is probably a resistor; unsolder one end and measure it.
some R are plain black bodied things with small lettering of ohm value
on them.

When removed one end you can see what that does to gain without risk of
blowing anything up.

Patrick Turner.

[[email protected]]

Patrick, Thanks for your help.

I called ARC and a guy called Leonard who does servicing confirmed
that on my model I had indeed found the right spot for the
resistors.

Later versions require a resistor removal just as you described. I
shall do some experiments to see whether there is a loss of sound
quality, if there is then I'll work with attenuating the signal of
line sources further up the chain.

Cheers

[Arny Krueger]

wrote in message
oups.com
It seems that you have do have different schematics to
the one that is in my SP8 manual. I looked at the one on
the ARCDB and it is from a later version. From the
photos on that website it seems I have a Mark I preamp. I
don't know whether it's revision 2 or 3, but the
schematic does mention changes made in 1981, so I would
assume it's not the original version - called revision 1
on the ARCDB (although how the first version of something
can be a revision, I'm not sure!).

Anyway, my manual does not show this resistor no. 27 in
this position. Rather, resistor 28 is in a similar place
to resistor 83 and there is shown a line parallel to it
that has a gap in it. The lines end in circles with
crosses in them similar to the ones at either end of your
r27 on the ARCDB site. This is the place where it says
that r29 should go if needed. I also note that the gain
reduction quoted here is 10 dB's rather than the 6dB's
mentioned for my 39.2k resistor.

If you look at the photo of the Mark 1 SP8 internals on
ARCDB you can see the bottom of 2 of the posts I was
referring to where they protrude from the other side of
the double sided circuit board. Frustratingly, there is
no photo of the underside of the board & mine is very
different from the underside photo of the later amp that
is shown.

Here's how you find them:
If you locate the yellow Wondercap to the far left, below
the mains transformer, you can see the shiny edge of the
heat shield I referred to. Now, between the Wondercap
and the heatshield there are 4 solder tracks that go
under the cap. The left hand one goes under the left
hand end of the cap, leaving 3 more going under the upper
edge of
it. The track to the left and the one to the right show
the bottoms of these posts as shiny discs. I think r29
should be soldered across these posts because I measured
the resistance between them and found
39.2, which I assume to be the value of the paralled
resistor, r28. If you then look to the right of these
posts, there is a black resistor with its left hand wire
going into a spot of track with no begining or end - I
think this is r28.
If you want to see what the posts look like from above
there are two more over near the top of the amp between
the topmost psu capacitor (one of the upright ones) and
the transistor mounted in the heatsink. There is a black
device soldered between these posts - not sure if it's a
resistor or not. The lower of the two posts is easier to
make out than the other one.

I wonder if anyone looking at this has a similar vintage
SP8 with a resistor across these posts on their amp?

Probably not anybody who posts here regularly.

It appears to be that you are either underequipped, undertrained, or too
timid. If the shoe fits there are probably skilled professionals who can
serve you.

If you've got some traces on a circuit board that go who-knows-where, you
can always power the unit up, maybe send a tone through it, and probe around
with a scope and/or a meter.

[[email protected]]

Arny......

It appears to be that you are either underequipped, undertrained, or too
timid. If the shoe fits there are probably skilled professionals who can
serve you.

Thanks for your vote of confidence.
With the greatest respect, did you inherit your audio electronics
skills from a long line of engineers?
Unfortunately I'm not so lucky and find I have to learn gradually
through experimentation.

[[email protected]]

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!

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'

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?

On my diagram R27 that you mention is a 33k value.

Cheers
Chris

[Patrick Turner]

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

[Mike Mueller]

I have an SP9
It's a solder connection on the PC board that neeeds to be made open.
You will need a schematic for the board. The original instruction manual
show's you specifically where it is. Call ARC. tech help will tell you
what too do.
I needed because of too much gain from my CD