2010 News

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2010 News

Dec 25 , 2011

Less than a year later I got another message from Jon that the scratchy pot was back...those electrolytic caps really do need to be polarized to keep leakage currents at bay. The long term solution is the use of low dielectric constant film caps...the downside are the large sizes. In the olden days, the high input impedances of tube amps meants that these film units were used throughout the amplifier...no wonder the units sounded so good! With the advent of transistors and wide bandwidth design (i.e. lower 3dB frequency) to please the marketing people (specmanship), higher value caps were used and this invariably meant electrolytic caps.

C23
(click to enlarge)

From: Ray C. 
Subject: Re: REPLACING CAPS IN SLA-1
To: jon
Date: Sunday, December 25, 2011, 4:26 PM

Hi Jon,
Good to hear from you and I wish you well during these times. 
I suspected that the problem would come back. The cap(s) replaced 
last time was C23 that is in line with the PCB writing 
"UNIKA L.T.D" near the center of the board for channel 2. The 
other cap is also in line with the markings but nearer 
the back and called C24.

I would just replace the existing 10uF with a 5uF as most people 
will not notice the missing lower octave...if you do...just 
add another 5uF in parallel  (capacitance adds when done in parallel).

Try this for a while to make sure things work...the sound should 
break in and the film cap should sound really sweet compared to 
the electrolytic.

good luck and keep me updated,
Ray

July 15 , 2010
After having got my First-Aid training, CAN-BIKE certification, and a cell phone, it was with much regret that I had to pull out of volunteering for the Waterfront Trail ride...not a good day based upon the advice from a Doc.

May 5 , 2010

canbike class
(click to enlarge)

That's my graduation photo you are looking at from my CAN-BIKE course. My years of riding back and forth from high school had served me well and many of the points made in the course seemed logical and well thought out. The thing that made the most impact were people who came into the course so that they were not so fearful of riding within the city. One person got caught on the street car tracks on two successive days...can you spell body slam !!!

Jan 10 , 2010

I got pinged by a someone playing in a small band about noise when operating the volume know on his Art SLA-1 amplifier. The input stages of op-amps typically are AC coupled by large capacitors to remove the DC content. If not, the wiper of the potentiometer would gradually degrade over time. The degradation are resistive hotspots developing between the body of the resistive element and the surface where the wiper taps. One way to make a non-polar electrolytic from a polarized unit is to wire them up back-to-back (i.e. tie the minus together when they are used in series). A variation of this is show in the diagram below from the volume section of the SLA-1:

C23
(click to enlarge)

From: Ray C. 
Subject: Re: Before you start "fixing" SLA-1
To: jon
Date: Tuesday, January 26, 2010, 2:39 PM

While I'm pretty sure that C23 is the cause of the troubles, it damages 
the pot by degrading the wiper contact. If you have junk car speaker, 
you could hook that up to the noisy speaker output to test when things 
are working.  The probing of voltage can sometimes cause noise injection 
that gets amplified to the speaker out and causing tweeter to blow...so 
hook up a speaker that you will not miss dearly.  The very first thing 
to try is actually to see if you can measure a DC potential on the 
output (pin #7) of the AN6554 quad op-amp chip.  This DC can only get 
through if the cap (C23) goes bad and gets conveyed to the other side.

I'm still thinking about whether C35 can go bad and caused any issues. 
In the case of C23 going bad, it can also block out the DC (on pin #7) 
from passing. Part of the sweet sound of the SLA-1 are these back to 
back potentiometers...so if one gets changed out...so does the other.

Last, if the pot is damaged, you can try some "contact sprays" to see 
if things clear up. I'm still trying to think up a "model" of why it 
can clear up the scratchiness but I know it helped in a unit that I 
repaired for a friend when I knew much less about volume pots in amplifiers.

Last, there have been a couple of mods involving these 10uF caps that 
involve replacing them with higher quality film units. These mods are 
very standard in audiophile equipment and can lend themselves to subtle 
differences in tones that musicians and equipment lovers crave. I think 
you may have re-ignited that project for me.