Archived from groups: rec.audio.tech (
More info?)
"Dave Platt" <dplatt@radagast.org> wrote in message
news:10quvfek9d94a34@corp.supernews.com...
> >If memory serves me, the B board on the C-4000 Sonic Holography
> >Autocorrelation PreAmp uses Texas Instruments RC4136 quad op-amps. These
are
> >rather slow slew rate op-amps, and I was considering replacing them with
> >OP275 op amps or equivalent.
>
> If you replace 'em with faster op amps of any sort, take care to check
> and perhaps upgrade the op amp power-supply bypassing arrangment, and
> do some form of stability check on the outputs. Modern high-speed op
> amps can be prone to oscillate when used in some circuits designed for
> older, lower-speed op amps, due either to inadequate power-supply
> decoupling, stray capacitance in the feedback path, excessive
> capacitive loading, etc.
Good post. I'm going to print this out for reference, as I tackle the
restoration of the preamp.
There is a thread on the internet about upgrading the Urei LA4 compressor,
which uses the same op amps. They link to a site that sells the circuit
boards to convert the OP275 to the pin configuration needed by the 4136.
That might be an option.
I will probably improve the decoupling circuitry while I'm at it. It seems
there is a lot to be desired in terms of PCB layout (with respect to
decoupling and grounding) and perhaps something to be gained by adding
electrolytics in parallel with ceramic or poly caps to provide a low ESR
across the band.
> This may be an issue of additional concern since you're going to need
> some sort of klugeboard arrangment to replace a single quad op amp
> with a pair of duals. If you build such a board, I'd encourage you to
> include a PC-board pattern which will let you stick .1 uF
> monolithic-ceramic bypass caps right at the op amp V+ and V- pins, to
> as short a ground trace as can reasonably be managed.
Good point about integrating the bypass caps. I'll check out that offering I
found on the Urei upgrade page to see if they include bypass caps on the
converters.
> >My question is this:
> >
> >Does the Hologram Generator work it's magic BECAUSE of the slew rate
> >limiting of these op-amps, or would I stand to improve the hologram image
at
> >higher audio frequencies by upgrading them?
>
> I don't recall the details for certain, but my recollection is that
> the hologram circuit is some form of cross-channel inverse-phase
> mixer. My guess (and it's only a guess) is that any necessary
> frequency shaping and equalization is performed through discrete RC
> circuitry rather than through the op amps' limitations.
I may order a copy of the service manual on Ebay, so I can study this
further. Hopefully you are correct.
> Replacing the op amps with better ones might reduce distortion or
> noise to some extent, or it might have no audible effect at all, or
> (if the replacement is done poorly) it might leave you with an
> unstable circuit which oscillates in the ultrasonic or low RF
> frequencies.
Good points. I will not do this upgrade without improving the decoupling
drastically.
> >My TI chips have degraded somewhat, causing audible distortion when the
> >hologram is switched in, until a 5-minute warmup has passed, after which
it
> >sounds fine. So the question is whether to replace with the same type, or
> >attempt an upgrade with the possibility of improving high frequency
imaging,
> >or ruining the whole effect altogether.
>
> I suggest what I think is a much more likely suspect for the
> distortion you are hearing: dried-out electrolytic capacitors. The
> caps in your preamp are a quarter of a century old, and it's very
> likely that some of them have gotten "leaky", changed capacitance,
> developed a high equivalent series resistance, or some combination of
> these. It's a common fault in older electronic equipment of all
> sorts. I have a Singer FM radio service monitor whose 1 kHz
> Wien-bridge oscillator wouldn't start up, for this very reason, and
> it's of basically the same age as your Carver.
Well, originally, this preamp had the A board. I heard about the B revision
hologram, so I had it upgraded at a shop. It was about a year or two later
that the distortion problem happened to appear. The preamp was less than
three years old at that time, being 1984 when I had it done.
I had the preamp apart last night, as part of a process of modifying the
chassis so it can be rack-mounted properly. While I was in there, I went
through and cleaned contacts and switches and I noticed a problem that had
plagued many of a product I used to service in the late 1970s. The techs
where I used to work referred to it as "TI crud", the silver plating that
tarnishes to black, forming dendrites between pins on the IC, thus causing
intermittent and varying resistance and making the op amps unstable. All of
the chips on the hologram board were black with tarnish! Drawing from my
experience with servicing thousands of FDM boards as a service tech in the
70s, I tried cleaning the tarnish off the pins (it cured the problem in 50%
of cases I serviced so long ago), but in this case, the preamp didn't fall
into the 50% that were successfully-remedied by this cleaning process. Often
the chips themselves are damaged, or the corrosion is underneath the pins
and inaccessible to cleaning.
Another thing I was hoping to achieve with this upgrade was the reduction of
hiss. Since the B board seems to move the hologram to a spot downstream of
the volume/balance controls, there is an audible hiss with the volume turned
down. Granted, it wouldn't be noticed on home hi-fi speakers, but this
particular C4000 is connected to a monster sound system with speakers that
have a 106dB @1W A@ 4' sensitivity, driven by a multi-amped setup of many
large power amps. If I can achieve any reduction in hiss in this circuit, it
would help greatly.
> Dirty/oxidized switch contacts are another option. My Phase Linear
> 1000 autocorrelator (same basic circuit as is used in part of the
> Carver) needed switch cleaning every couple of years, or it'd start
> sounding distorted.
Yes, the Carver has simply awful switches. The AC power switch became so
intermittent that I had to jumper it out. Other switches are intermittent. I
had to do a lot of cleaning and injecting of lubricants to get them working
well again.
As an aside, I have a Phase Linear 1100 Parametric EQ that developed a very
annoying problem with the output relay. It would lose the right channel
intermittently. Other times, the right channel would be there, but would
sound 'dark' and would also be about 10dB down from the left channel. I
finally opened that relay up and ran a thin strip of rough paper between the
contacts, then a strip of paper impregnated with Calilube R5 power booser
lubricant. It seems fine since then.
As for the dried out cap story, I have a Phase Linear D500 power amp in my
rack that saw some early failures in the control circuits. The low voltage
(approx 150VDC) power supply smoothing cap decided to open one day, causing
relay chatter. I have another cap failing somewhere, because since July of
2000, the D500 started powering itself on, even when the power switch is
off!
While I'm busy making the rack modification (involves countersinking the
holes for the handles and replacing the 1/4-20 pan head screws with 1/4-20
countersinking heads, so I can get this thing to mount flush against the
rack rails, I'm going to investigate another annoyance: the hum in the left
channel. Since the power transformer is on the left side of the amp (all
78lbs or so of it), I figure the low level audio circuits are getting
hammered with mucho magnetic fields. So I'm going to experiment with
different shielding methods to try and clean that up. (I'm in the midst of a
major overhaul of the two amplifier racks, to solve the hum problems caused
by the D500 magnetizing the rack and injecting hum into all other rack
devices. I'm planning to move all the low level stuff into a separate rack
and put the power stuff in the other rack.) The right channel is silent, but
the left channel has what sounds like a capacitively-coupled AC hum
(harmonics only). Big project, doing it a little bit at a time.
> The cure, fortunately, is an easy one - re-cap the circuit and clean
> the switches. From the fact that you mention distortion, but not hum,
> I'd guess that it's the inter-stage AC-coupling caps which have
> deteriorated, but it'd probably make sense to replace 'em all. Buy
> yourself an assortment of high-quality 'lytics (the Panasonic
> 105-degree-C long-life types are a good choice), unsolder the old and
> solder in the new, use a squirt of a good contact/control cleaner on
> each of the rotary and pushbutton switches for good measure, and I
> think you'll find that the preamp's performance has been restored.
Yes, I think I'm eventually going to re-cap most of this gear. The Phase
Linear stuff has been notorious for dried out caps, mostly in control
circuits, rather than audio paths, but I think it's time to recap all of it.
I'm thinking it might be possible to improve the accuracy of the hologram
generator by improving the slew rate of the chips, such that the hologram
would be effective above 5khz. A difficult thing to measure, and largely a
subjective listening issue.
--
Take care,
Mark & Mary Ann Weiss
VIDEO PRODUCTION • FILM SCANNING • DVD MASTERING • AUDIO RESTORATION
Hear my Kurzweil Creations at:
http/www.dv-clips.com/theater.htm
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www.adventuresinanimemusic.com
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