ACS500 hiss removal help - chip replacements?

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Greetings all!

I have an Altec Lansing ACS500, a self-powered 3-piece speaker set from the mid 90's with a bad case of "quiescent hiss." Turned on with no signal applied, the satellites and the woofer will issue a quiet hiss that is simply annoying when nothing is playing, to the point that I will turn the speakers off until I want to listen to something.

I'd like to tweak/upgrade the guts in the hopes of getting rid of the hiss. I did a little exploratory spelunking and found some possible culprits:

Both the sub and the sats feed the input signal through some simple resistor networks, through an anonymous black-sheathed aluminum electrolytic cap, and into a JRC-made 4558D op-amp. The heavy lifting in the sats is courtesy of three Toshiba TA8238K's and one KIA6210AH in the sub. Power supply filter caps are small and of questionable pedigree. I added some stiffening caps to the sub PS and notice a substantial improvement in transient response and tightness, but that didn't help the hiss any. Power for the sub and the sats is from the same transformer but through separate full-wave bridges and filter caps.

So, I guess my questions are:

1: Is it possible to quiet these things substantially with op-amp or cap replacement strategies? I'd even swap out the main drivers if it would produce a noticeable improvement/quieting of the system, but that's a big job with the sat amp board's configuration.

2: If replacing the op-amps is a good idea, what pin-compatible replacement should I use? I've seen many suggestions across many boards around the 'net, but nothing with reference to noise. I'm willing to spend a few bucks on replacement op-amps if it will get rid of the hiss.

3: Is it worth replacing the aluminum electrolytics in the signal path with something else? If so, with what?

I searched for "hiss" and "4558" but turned up only one inapplicable thread. Other searches didn't seem to answer these questions either. :-(

Thanks much for any advice!

Best,
tribble
 
Hi Tribble,

the JRC4558 op-amp was design for general purpose audio use and is not really state of the art. However it is sufficient and the noise level is tolerable. Many manufacturer use them in their products (i.e. Magnat in their old car-amps have used them extensively).
To my best knowledge the noise/hiss usually comes either from the signal source or is based on bad component selection. In the latter case check if carbon resistors are used instead of metal film resistors. Carbon resistors have in most cases an occer body color while metal film resistors have a (light) blue body color. Metal film resistors are always preferred not only because their noise value is smaller!
According to the datasheet of the TA8238K the output noise voltage is typically 0.2mV and might reach up to 0.7mV. A level that could be noticeable.
By the way - does the hiss level rise/fall if you turn the volume up/down?

Finally the conclusion would be:

If no parts are aged excessively, no bad solder joints could be found, metal film resistors are used and the amps basically works there is not much you can do about it.
To increase the load capacitors at the power supply increases the capability of peak-power performance and will result in the effects you already mentioned in your questioning.

On the other hand, since you mentioned the system is from the '90, there is a slight chance that the electrolytic capacitors have lost some of their capacity. But this wouldn't be a reason for the "hiss".

There are many more things I would like to ask for, but I guess you do not have an o-scope to measure the voltages at certain pins of the amps. If possible figure out the gain of the TA8238K amp. To do so check the components connected to pins 2 and 6. Possibly you'll find a resistor and a capacitor in series to the ground potential (GND). Calculate the gain according to the datasheet. For a maximum output power of round about 4W on 4 ohms you need an output voltage of 4V. In relation to an estimated input voltage of 0.775V the gain should be just 20 x log(4V / 0.775V) which is equal to approx. 14dB. In this case the feed back resistor should be equal to approx. 7134 ohms. Maybe the real value you'll find is 6.8 kiloohms. The more the real feed back resistor value is below this calculated value the greater is the gain and the chance to amplifier all kinds of noise with the same gain factor.

I hope this will help you.

Greatings from Germany and from Corax
 
Great tips, thanks!

The hiss remains constant throughout the rotation of the volume control knob, up until the last 10% or so, at which point it increases. From 0% to 90%, the hiss is constant.

Shorting the inputs does not affect the hiss at all.

The input resistors are all carbon composition; they are tiny. I will have to investigate their replacement.

Thanks for the investigation hints on the TA8238K; I will disassemble the unit and investigate it thoroughly per your tips this coming weekend, regarding the gain calculation. I take it that if I reduce the gain, the noise will drop, and I can simply run the unit with the volume control slightly higher than it has been in the past?


I do have an oscilloscope and could do some signal voltage measurements at various pins. I will have to track down some new probes, though, so that could take some time. I will look in to that as well.

Thank you again!
 
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