Menu

Show posts

This section allows you to view all posts made by this member. Note that you can only see posts made in areas you currently have access to.

Show posts Menu

Messages - Scott Novak

#1
Zener diodes are NOISY!  I suspect that they are also much faster reacting than any op amp based regulator, so in that regard they may have some advantages.

Another approach is to use a zener diode as a reference for a series pass transistor.  You filter the zener diode well so that it's noise is reduced, then the transistor handles most of the load.  Select a fast transistor and you may well have a better sounding regulator than many, if not most op amp based designs.

But if you are using an op amp based design, keep in mind that at higher frequencies it's the decoupling capacitors on the output of the regulator that are going to be supplying the transient currents to the amplifiers, so pay special attention to power supply decoupling.

Don't forget that capacitors have series inductance and a natural resonance.  You might use a 10µF electrolytic with a low ESR capacitor bypassed with a 1uF polypropylene film cap, bypassed with a 0.1µF polypropylene film cap.  If your amp runs cool enough you may get by using polystyrene capacitors which are even better sounding, but more expensive and physically larger.

Also pay close attention to avoid ground loops within your layout.  Pay attention to where the decoupling capacitors are grounded.  You don't want the transient currents flowing through the capacitor ground connections to modulate another part of the circuit.

Scott Novak
#2
Schematics and Layouts / Peavey IP1 Preamp Schematic?
September 01, 2019, 06:36:19 AM
I'm seeking a Peavey IP-1 preamp schematic.  It's about 1979 vintage. The preamp seems to be a good candidate for upgrades as the PWB already has sockets for the Op=Amp ICs.

It's using 4558s and some other op amps that don't appear to have any markings on the top or bottom.  Based on their positions on the PWB I suspect that the unmarked Op Amps are a lower noise type.

After cleaning the pots with Deoxit D5, channel 1 seems to be operating normally.  However, on channel 2 when I rotate the gain control I hear noise and when the control is rotated near maximum gain I hear what sounds similar to a high frequency oscillation.

The block diagram on the top of the preamp shows the "gain" control being part of the negative feedback loop, which may explain the behaviour of the control.  I haven't hooked up an oscilloscope to look at it yet.

Swapping out the 4558s in favor of OP-249s is a no brainer.  But please let me know if you know of any better dual Op Amps.  I've noted that the DC offsets on the OP-249s are so low that I've been able to remove electrolytic capacitors from the feedback circuits.  The electrolytic capacitors cause so much noise and DC drift that the DC offset varies less WITHOUT the capacitors.

Does anyone have any idea what type the unmarked Op-Amps might be?

Scott Novak