Fixing a JBL 6290 amp. Very critical that the low voltage supply 15 volts plus and 15 volts minus. Last two attempts gave me 15.1 volts plus and 14.9 volts minus. Using 7815 positive regulator and 7915 negative regulator purchased from Mouser. All filter caps and diodes check good. Suggestions?
Hi, In my opinion and in this particular ‘JBL 6290 Amplifier’ case those voltages should be fine as they are and also your current readings of +15.1V and -14.9V are perfectly safe and well within normal operating specifications. Standard 7815 and 7915 linear regulators have a manufacturer output voltage tolerance of ±4% to ±5%, meaning any output between 14.25V and 15.75V is technically “in spec.”
These rails power the front-end op-amps and protection circuitry. Modern and vintage op-amps feature a high Power Supply Rejection Ratio (PSRR), meaning a minor 0.2V imbalance between rails will have zero measurable impact on audio performance, distortion, or DC offset etc. etc.
Now, on the other hand there are a number of question you need answer to:
On another note: if you really aiming at an absolute precision and/or symmetry then there are some options available: 1. The so called: “Component binning” in which you will have to purchase a batch of let’s say 5 to 10 pieces of both the 7815 and 7915 and test them (individually) on a breadboard under a minor load (let’s say a 1kΩ resistor at the output) and keep doing it until you find a pair that natively hits closer to 15.0V or matches each other perfectly.
Another method would be to trim the ground reference: fixed regulators (usually) determine their output relative to their ground pin so, because of it you could “trick” the negative regulator to bring it closer to the positive rail (in your case 15.1V). So to raise the output of the 7915 (moving it from -14.9V toward -15.1V): place a low-value resistor (10 to 47Ohms) or a forward-biased signal diode such as an ordinary 1N4148 in series with the GND pin of the 7915 regulator before it connects to the chassis ground chassis.
This trick should raise the ground reference slightly, pushing the output voltage further negative. Job done !. but, please beware of the fact that you cannot easily lower the +15.1V on the 7815 using this method, as inserting resistance into its ground pin will only increase the output further. !
And finally the ultimate solution is to Upgrade to a proper set of Adjustable Regulators: such as LM317 and LM337 which are way better and far less noisy that the 7815/7915
1: De-solder the old Lm7815 and 7915 Regulators, 2: Drop in an LM317 (positive) and LM337 (negative) adjustable regulator pair. 3: Add a pair of 1% metal film resistors and a multi-turn precision potentiometer to manually calibrate the rails to exactly 15.00V. On how to do things exactly read the DataSheet of LM317/Lm337. It should all be there.
Good luck.
Hello, I agree that the problem here doesn’t seem to be the voltages, but, I wanted to comment that if some time you need symetrical supply voltages you could use a tracking power supply configuration. Tracking power supplies track one voltage to generate the output, in this case, the negative supply would be a copy of the positive supply, negated, of course.
CH
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