As promised a while back, I simulated the NAD 3020's line stage. Total output noise comes out to ~3.7 µV A-wtd. Including the 23.7 dB power amp gain of this 25 wpc amplifier that brings this up to ~56 µV, this means a total (best-case) SNR of about 108 dB(A), slightly short of the claimed 110 but still very respectable for a little old amplifier.
With the (20k) volume pot turned down for a 2.83 V output, I'm getting ~111 µV(A), so about 88 dB re: 1 W / 8 ohms. Not too shabby at all.
Turned down further for 50 mW / 4 ohms, it's 68 µV(A), so about a 76 dB score on that.
There used to be amplifiers with even lower noise despite substantially higher output power, but those used a two-stage volume control (4-gang volume pots).
LTspice schematic attached.
With the (20k) volume pot turned down for a 2.83 V output, I'm getting ~111 µV(A), so about 88 dB re: 1 W / 8 ohms. Not too shabby at all.
Turned down further for 50 mW / 4 ohms, it's 68 µV(A), so about a 76 dB score on that.
There used to be amplifiers with even lower noise despite substantially higher output power, but those used a two-stage volume control (4-gang volume pots).
LTspice schematic attached.
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You only need Bob Cordell's models (which I figured most would already have) for the BC550, the universal opamp model comes with LTspice.
When I try to run the asc file, a window pops up that says Couldn't find symbols: voltage3 and poti. The schematic references lib potentiometer.sub which I seem to not have since the schematic is missing the bass and treble pots. voltage3 must be the V+ and V- voltage sources, both also missing when I open the schematic. I have the BC550 on the schematic as expected. I am using LTspice IV version 4.19a, not the new release if that matters.
Not sure if this is the right thread, but could someone explain what is the strange volume potentiometer? It has four contacts in one row and two contacts on a second row. Is there some kind of extra loudness tap?
Or have I been shown a wrong picture? Is it four plus two or two rows of three?
I simulated the preamp section, based on what I could understand from the service manual. I assumed an ordinary dual log pot there. What I noticed was that with loudness off, there was a wide +2dB boost centered around 400 Hz, +0dB at 100 Hz and -2.7 dB at 20 Hz. Is there something I'm doing wrong? The treble response was straight. Both tone controls were centered in my simulation.
Edit: Looking further at the picture, it seems the pot has four plus four contacts. The pot in the picture is cracked and is missing two contacts in one row. So there is a loudness tap.
Or have I been shown a wrong picture? Is it four plus two or two rows of three?
I simulated the preamp section, based on what I could understand from the service manual. I assumed an ordinary dual log pot there. What I noticed was that with loudness off, there was a wide +2dB boost centered around 400 Hz, +0dB at 100 Hz and -2.7 dB at 20 Hz. Is there something I'm doing wrong? The treble response was straight. Both tone controls were centered in my simulation.
Edit: Looking further at the picture, it seems the pot has four plus four contacts. The pot in the picture is cracked and is missing two contacts in one row. So there is a loudness tap.
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Edit: I was doing something wrong. The tone controls are centered when at 15 % of their value, i.e. at 1k5 position.
Edit: Which ones of the traces on the circuit board come from the loudness switch? Is it possible to replace the original potentiometer with a normal log potentiometer without jumper leads? In case one wanted to forfeit the loudness function entirely.
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