Published in the October 2000 issue of ...

    Analog Maintenance and Upgrade Considerations 

    Part-1: Caps and OpAmps
    ã 2000 by Eddie Ciletti

    Go to Part-2 Opamp Upgrades

    I get plenty of e-mail regarding "modifications and upgrades" to recording consoles and other pieces of audio gear. I chose those words carefully to make a point. More often than not, modifications can be as expensive as a new piece of outboard gear, or worse, a major detour into oscillation-land. Why modify a console mic preamp when there’s already a product who’s designer auditioned each component for its sound, or lack of same? By contrast, the decision to upgrade should be borne from the maintenance history of the console or tape machine. 

    If you want to know a little more about basic maintenance, enough to make educated decisions or possibly go a little further, ya gotta be in the ballpark to see the game and ya gotta practice to play the game. So let’s find our seats and check out the opening line-up. What better way to demonstrate the process of comparative analysis than by testing a recording console or tape machine, good channel next to bad channel?

    Capacitors: Square Biz

    Geeks in training need to start with maintenance fundamentals and this includes familiarity with test equipment and electronic components. (A list of test gear was detailed in the May 2000 issue.)

    Capacitors — the weak link in almost any aged product — are failure prone over time making travel for low frequencies difficult. The end result is "thin" sound. You can test for bad capacitors without pulling a module; the fastest way requires a square-wave oscillator and an oscilloscope. 

    Figure One shows a family of square-waves starting with the 500Hz reference wave (a), bass roll-off simulates the tilt caused by bad capacitors (b), while bass boost tilts the wave in the opposite direction (c). (Any midrange frequency will yield similar results.) This test is particularly useful when checking old Pultec and Neve modules or any equalizer that has switched EQ settings. Not only do you want to know that all frequency options are functional, but also that the center détente is really "flat" and that the output transformer (if applicable) is properly terminated.

    Also included are snapshots of treble boost (d) and cut (e). All "visual effects" were produced with a shelving equalizer where the slope is a gentle 6dB per octave. A low-pass (high-cut) filter has a much steeper slope, at least 12 dB / octave leaving more bandwidth untouched and leaving a recognizable footprint as seen in Figure –1 (f). Notice how the rise time of the square wave is affected. The images will vary depending on the slope of the EQ curve. Boosting the midrange EQ at the same frequency as the oscillator will turn it into a near-sinewave.

    Figure One: A family of Square waves

    Module Etiquette

    Once the bad modules reveal themselves, it’s time to find the bad caps. Be sure to mute the speakers and power down the console when removing and replacing modules. An extender card makes it easy to probe individual modules. If you don’t have that luxury, pull a few modules to clear some real estate. A schematic will help locate input and output caps, most of the time the circuit board is silk-screened with part designations. Using a scope probe, locate a cap at the input or output of an amplifier and check each leg for the "before and after." 

    You might find that many of the same caps in each module have failed. The decision is yours to change only those that typically fail or go the "wholesale replacement" route. Caps can fail in batches, some are more stressed than others — and then there are caps that just plain suck. Once you begin to recognize them, order a quantity to have on hand when some "downtime" opens up. Digi-Key is a good place to buy mass quantities.

    HFS, HFQ and FC

    Note: Both HFS and HFQ series caps by Panasonic were being discontinued at the time this article was published in October 2000. The "replacement"  is the "FC" series.  One reader also recommended the Sanyo Os-Con and Elna Starget.  Anyone with recommendations is encouraged to drop an e-mail and share their knowledge.

    There is also an issue of component style - Radial and Axial are shown above.  Going vertical obviously saves space and it seems the Panasonic FC series, at least, is only available in this package.

    I like to replace electrolytic inter-stage (coupling) caps with Panasonic’s HFS series, designed to withstand the abuse of switching power supplies and rated for 105° C. The difficulty with selecting replacement caps is that many were originally chosen for their small footprint, which is mostly determined by selecting (some would say compromising) voltage and capacitance values. Construction materials also affect size. The ambient temperature inside the product must also be considered. Heat shortens life, so the 105° C rating is a safer bet than the more typical 85° C. 

    As a rule of thumb, the power supply rail(s) should be within 70% of cap’s DC voltage rating. Quite often caps ride the rails much closer than that (not good). For example, a 24-volt rail wants a 35-volt cap. Using one rail of a bipolar supply as reference, a 16-volt rail wants a 22.8-volt cap, which you’re not going to find. Capacitor voltage ratings are stepped — 10v, 25v, 35v — so the next highest value is a good place to start. For additional comfort margin, add the bipolar rails together, but remember that larger capacitance values and voltage ratings make bigger caps. Good compact caps cost even more.

    Yes, you can argue that different capacitor materials have a sound, Dave Hill of Crane Song once told me a story about replacing the Tantalum caps in an old Neve module. Apparently the non-tantalum replacements changed the sound of the module. Dave’s research determined that Tantalum caps contributed more even-order harmonic distortion, not as clean, but pleasing.

Op Amps

Before considering an opamp upgrade — those little ICs that contain dozens of transistors — other items are worth considering. Just as capacitor ratings are often just a few volts above the rails, many of the "affordable" consoles have power supplies running close to capacity. (Adding a fan, if none exists, is the best life-extending preventive maintenance.) "Speed" is important to sonic transparency — specified as Slew Rate, in Volts per microsecond or V/m S — but the most critical parameter is actually current consumption. 

If, for example, the board is filled with hundreds of TL072 dual op amps — at 3 milliAmps (mA) per chip — there is no way you can "upgrade" to an OP275, at 5mA per chip, without putting the power supply into shock. If, on the other hand, the board is stocked with NE5532s (8mA per chip), then the OP275 is an option. See Tables One and Two for dual and quad opamp comparisons, respectively. (The two parameters in Tables One and Two only scratch the surface. Any opamp will behave differently when operating in a unity-gain circuit or in a high gain application. )


Op amps

Slew rate
3 mA

1.4 ~ 2.5 per amp

13 V/m S

at unity gain

Texas Instruments
22V/m S
Analog Devices
8~ 10mA
9V/m S
Signetics / Philips
25V/m S
Table One: Dual opamp comparisons

Op amps

Slew rate

1.25~2.8 per amp

1.7V/m S
Texas Instruments
6 mA

1.4 ~ 2.5 per amp

13 V/m S

at unity gain

Texas Instruments
25V/m S comp

50V/m S decomp

Table Two: Quad opamp comparisons

A customer once delivered a mixer to my shop after carefully desoldering the chips, adding sockets and experimenting by ear with different opamps. His techniques and intentions were good, but the end result was intermittent oscillations. Modifications have a way of revealing a product’s soft underbelly — idiosyncrasies pushed to the edge, in this case, by the opamp’s increased speed and current requirements.

There is no perfect capacitor or opamp, these are just two of many other variables that can all contribute to subtle variations of sonic character. Since this article is about "global restoration" and not specific to any one product, I welcome your questions via e-mail.  I encourage you to collect schematics, pop the cover to peek, probe and better know your gear. How else will you know how to ask the "right" questions? 

Click Here for an example of a console grounding upgrade

In addition to Dave Hill at Crane Song, my thanks to: Greg Gualtieri at Pendulum Audio and Dan Kennedy at Great River Electronics for their input. Next class: op-amp upgrades to the UREI LA-4 limiter, plus power and ground distribution improvements to the Trident series 65 console. See you online at

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