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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

Drop in replacement for NE5532?
Drop in replacement for NE5532?
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Old 14th August 2019, 02:25 AM   #241
sgrossklass is offline sgrossklass  Germany
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Quote:
Originally Posted by dreamth View Post
I built some tapeheads preamps last summer and winter and decoupling near the playback op-amp is a big problem.The only decoupling used in the industry is just one capacitor(either low esr, either no polarized between +- rails).That's all.You don't need any noisy ground around a tapehead preamp.They sometimes use a decoupling 1n...10nf cap between the input shielding ground and the ground but far from the op-amp.
Seems realistic. That's what you get with shared signal and power ground returns. Two ways of addressing this issue:

a) Use dedicated, separate signal and power ground returns (Proper Star Grounding[tm]), thus keeping bypass capacitor current away from signal ground entirely. Designers in the 1980s would shy away from this as they had enough complexity to deal with on their single-layer boards as-is. It's much less of an issue if you can afford dual layer boards (or *gasp* a dedicated ground plane) or some flying leads. People have had good luck bringing noise down in vintage equipment by "reinforcing" grounds with external wires, thus bringing down shared return resistance... clearly a sign of injected noise.
b) Insert some series resistance in the power rails, preferably for each chip individually. Maybe 100-220 ohms or even more, depending on what you can afford. The bypass capacitor nets you a voltage divider between power supply source impedance and shared ground return resistance at high frequencies, potentially allowing a substantial amount of HF noise to drop over shared ground return resistance if power supply source impedance is low, and it can be as low as a fraction of an ohm after all. That extra resistance could easily make 40+ dB of difference.

Both approaches could also be combined, of course.
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Old 14th August 2019, 03:10 AM   #242
anatech is offline anatech  Canada
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Drop in replacement for NE5532?
Hi dreamth,
You could use local regulation like Nakamichi did. They used transistors to send the ground current of the regulator to the opposite rail before the other regulator. This reduced injected noise by the beta of the transistor used.

Pretty darned smart if you ask me.

-Chris
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Old 14th August 2019, 04:58 AM   #243
Rick PA Stadel is offline Rick PA Stadel  United States
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Well worth it IMHO if you're powering a discrete low-level amp. But some of these applications are power rails for op-amps that already have 60 - 80+ dB PSRR at audio frequencies. I'm sorry, but I don't think a few microVolts of rail noise are worth bothering about in that case.

Best Regards,
Rick

Last edited by Rick PA Stadel; 14th August 2019 at 05:02 AM.
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Old 14th August 2019, 08:23 AM   #244
dreamth is offline dreamth  Romania
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Quote:
Originally Posted by anatech View Post
Hi dreamth,
You could use local regulation like Nakamichi did. They used transistors to send the ground current of the regulator to the opposite rail before the other regulator. This reduced injected noise by the beta of the transistor used.

Pretty darned smart if you ask me.

-Chris
I had 2 CR4...but bx300 was the smartest of them all .
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Old 14th August 2019, 03:31 PM   #245
anatech is offline anatech  Canada
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Drop in replacement for NE5532?
Hi Rick,
Sometimes it does matter, even when you think it may not. That suggestion certainly isn't expensive or too complicated to do yourself. That and power lines tend to have currents and voltages from other stages that only show up when the system is processing a signal. So it isn't just the static mV worth of supply noise you're worrying about.

-Chris
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