Acoustic B20 combo: op-amp replacement for reduced hiss?

[PhiDeck]

When I practice through my Acoustic B20 combo using headphones I hear some hiss. The amp uses four 4558 dual op-amps. Three of those probably operate at or near unity gain, but the first one in the signal chain does not.

Has anyone replaced the pre-amp stage 4558 and thereby noticeably reduced hiss? If so, with which op-amp?

Philip

[dune2k]

Originally Posted by PhiDeck

When I practice through my Acoustic B20 combo using headphones I hear some hiss. The amp uses four 4558 dual op-amps. Three of those probably operate at or near unity gain, but the first one in the signal chain does not.

Has anyone replaced the pre-amp stage 4558 and thereby noticeably reduced hiss? If so, with which op-amp?

Philip

Hi,
not only the preamp can produce hiss, but also the power amp. So changing opamps might not bring that much relief.
If you decide to replace the 4558 opamps I'd recommend NE5532 opamps. They are relatively cheap (0.40-0.70€ around here), at least a lot cheaper than the OPA ones, and low noise.

[gerryjazzman]

I've done this as part of modding a Carvin PB15 preamp. It also used 4558's (these are the absolute cheapest and practially oldest opamps around). I used TL072's in several of the front end and EQ sections and it made a noticeable difference in hiss level. TL072's are pretty old too but considerably better than 4558's and are designed for low noise. They have been used for years in the Eden WT series amps. There are others that are even better now, but I haven't kept up with all the latest op-amp designs.

The NE5532 as mentioned is also a good low-noise audio opamp and might work as well, but if I recall are not unity gain stable, meaning they may or may not oscillate in your application depending on the circuit design.

[chucko58]

TL072s are not particularly "low noise". If you want to go that route, try some TLE2072s. They're TI's improved low-cost FET op amps.

5532s are unity gain stable. They also pull a bunch more power supply current than the 4558. They're not the best choice for the input stage of a bass amp (with a passive instrument, anyway) as they have higher current noise. They're a bad fit for the Carvin PB-15 in particular due to the high resistor values in that circuit.

You might want to try some TI OPA2134 chips in the input stage. They're truly low noise FET input op amps. They also draw more power supply current.

The LM4562 is another possibility for the tone control stages etc. It's an improvement over the 5532, but it draws about the same power supply current.

[PhiDeck]

Originally Posted by dune2k

Hi,
not only the preamp can produce hiss, but also the power amp. So changing opamps might not bring that much relief.

Agreed, although my experience is that the pre-amp, where voltage gain is taken, is more often the culprit. Moreover, the pre-amp is often easier and less expensive to modify (eg. swap an op-amp) that is the output stage, where (primarily) current gain occurs. If the problem turns out to be in the output stage, I'll cross that bridge next, or learn to live with the hiss.