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

Discussion in 'Amps and Cabs [BG]' started by PhiDeck, Jan 1, 2012.

  1. 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?

  2. dune2k


    Sep 26, 2009
    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.
  3. gerryjazzman


    Dec 31, 2006
    New Jersey
    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.
  4. chucko58


    Jan 17, 2002
    Silicon Valley, CA, USA
    I paid for all my gear myself. Well, me and MasterCard.
    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.
  5. 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.
  6. mrpict


    Jun 7, 2012
    I just ordered an Acoustic B20 and would like to replace the 4558(s) in the preamp. I presume it is the first in the chain only that provides gain and might benefit from replacement? Is that correct or should all 4 be replaced? Did anyone actually find a suitable replacement and fit it to find a reduction in hiss? - or is it the power amp that fizzes? I'm willing to experiment and try a few different opamps, but can someone please tell me what package the preamp opamps are in please? 8 pin DIP, SO-8?? Thanks, John

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