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Created by
 Western Graphics
Last modified
 14 April, 2006

Analog Metropolis Guide to Quality Components & Upgrade Tips

Overview The AM Modules are replicas of old circuits dating back to the early 1970's, many of which include long since obsolete parts. This often means substituting a modern equivalent to get the circuit working at all or an upgrade is needed to make some worthwhile performance improvements. Here is my guide to getting the right replacement to work! I follow these tips when building my analog modules, and they also apply when I upgrade an old analog synth.

1) Upgrade the CV Op Amps  The basic 741, 748 and 301 Op Amps of the early 1970's are Jurrasic designs, which should almost always be upgraded to improve drift, tracking and stability. Use a high quality low offset Op Amp, such as:

• Linear Technology LT1013 (dual)
• Linear Technology LT1012 (single)
• Burr Brown OP07 or OP177 (single), F are better than G
• Burr Brown OP2277 (dual)

These chips will give you more accurate control over control voltages, especially frequency. Low temperature drift and low offsets mean the control voltage is accurate and does not create unexpected changes. They have slow slew rates, typcially a lot less than 1V/μs, and therefore must not be used in the audio path. Control volatges typically vary up to 100Hz, which is well within the speed on these low drift op amps. Audio paths need 20kHz and a much faster slew rate to avoid distorting the waveform.

2) Upgrade the Audio Op Amps Dave Rossum used the 556 Op Amp in many of his designs in the mid-70's, as it was one of the best parts about. It was slightly better quality than the 301 and 741 Op Amps, and therfeore it was used before the arrival of JFET/Bi-FET Op Amps (like the LF353), where a good frequency response was critical. The RC1456 and RC1556 Op-Amps were manufactured by Raytheon. They are pretty much impossible to locate now, but it is safe to substitute with a Bi-FET (TL08x or TL07x) or the inimitable bi-polar NE5532 (dual) and NE5534 (single).

Audio op amps need to have fast slew rates (10 - 25V/μs) , low noise, low distortion and a stable design. Diferent designs may "sound" marginally different, but thats rather a subjective area.... However all the old bi-polar Op Amps from the early 1970's, like the dual 1458 and 4558, are best upgraded when you see them in the audio path. Even the early Bi-FET amps such as the LFxxx and TLxxx, can be significantly improved upon. When replacing old designs start by using the TL072 or even safer still the NE5532, then move up to higher quality Op Amps, such as:

• Analog Devices OPA134
• Analog Devices OPA132 (good stablility)
• Burr Brown OPA275
• Burr Brown OPA604A

These are only going to cost you $3.00 each and offer a worthwhile upgrade. I make use of the OPA134/132 in my AM modules for customers. However you can go for esoteric Audiophile quality at over $15.00, for example:

• Burr Brown OP627
• Analog Devices AD843

However there are diminishing returns in this price bracket and I only use super-Op Amps when I need the speed for clean waveform generation or for high bandwidth applications as in the core of very high Q state variable filters. Burr Browns tend to be smoother, whilst the Analog Devices are more aggressive. But beware of inducing Op Amp instability and (sometimes fatal) HF oscillation with the replacement. It's not as easy as it looks! Check the output of the replacement Op Amp with a 'scope to see if there is any HF. If there is you'll need to think again.

3) Don't upgrade all the Op Amps! There are some notable exceptions. Some analog designs rely on older, slower Op Amps for waveform shaping or power supply regulation. In these cases don't try replacing Op Amps. They are usually 741/748's.

4) Power Supply capacitors When repairing an analog synth, the power supply capacitors are always up for replacement and upgrade. Over time, they age, and the ESR (internal resistance) goes up, they can also heat up, with the result of output ripple into the audio circuits and sound. High temperature versions (105°C) and low ESR types should be used, and once again the Audiophile industry has some proven solutions. The same rules apply when building from scratch, low ESR, high ripple capacity, high temp, such as the Panasonic HC series we use in AM modules. These capacitors are only slightly more expensiver than standard 85°C items, and they provide a better design and longer life.

5) Voltage Controlled Filter Core Capacitors Generally it is wise to use 1% polystyrene capacitors in filter cores, as they provide tighter matching and a much better sound. However this doesn't always work out and you will need to use multi-layer polyester capacitors in some designs. I carefully test out a range of capacitors when prototyping AM modules, this ensures that the customer shipped production modules and PCB's use the best designs.

6) Audio Path Capacitors Jurassic capacitors can be found in most analog synth designs from the 1970's, typically those horrible big ceramic disc capacitors and nasty Tantulum bead capacitors. These have to go. I use high quality low-K ceramic capacitors for Op Amp bypassing, high quality Panasonic audiophile electrolytic capacitors and WIMA polyester capacitors for audio paths. The end result is a much cleaner audio path, these simple upgrades can make a big difference.

4) Power supply bypass capacitors It's best practice to have power supply bypass capacitors on every IC, I tend to compromise a bit on this rule and bypass every 2 or 3. But I always put more serious isolation in for LED drives and I keep digital sections of the circuit on separate power feeds, often on separate PCB's. The bypass capacitors smooth out the power to the chip by removing the alternating currents caused by ripple voltages, they also provide localised power for fast responses. A good  default is an 0.1uF 50V ceramic axial capacitor.