I'd make a proper shunt regulator out of that zener. The TL072 only draws about 0.9 mA per channel and that's not quite enough to keep the zener happy. I'd regulate to something like 27-33 V.
Tom
Tom
The Zener isn't very critical. Its intent is to drop the 37V supply enough to keep the opamp below its maximum allowable operating voltage. That turns out to be 40V! But I'd still drop it some for better safety margin.
One item I did neglect is that my suggested change of the resistors at the first stage emitter does raise the open loop gain by the ratio of about 220/91. So the compensation cap at the 2SC1318 might want to be revised to 82pF.
One item I did neglect is that my suggested change of the resistors at the first stage emitter does raise the open loop gain by the ratio of about 220/91. So the compensation cap at the 2SC1318 might want to be revised to 82pF.
The OP asked for a small improvement, and I suggested one. He is not going to get more power without a significant redo.
Ed
From what I can tell, this Kenwood receiver's power transformer doesn't 'sag' 7 volts down to 30V.
More like maybe 2 to 3 volts.
So 'sag' is not an issue here, since it's got to power a whole chassis including the tuner and preamp areas.
Actually, at idle, the B+ sits around 40V.
More like maybe 2 to 3 volts.
So 'sag' is not an issue here, since it's got to power a whole chassis including the tuner and preamp areas.
Actually, at idle, the B+ sits around 40V.
There are 3(+) good reasons OP-amp IPS are not more common.
1. Op-amps are internally compensated for unity gain from their own output so if you add gain or even an output buffer, feedback stability is difficult. A good design wraps as few stages as possible in the global feedback loop, and the op-amp already has several stages.
2. Chips like LM4562 have an output saturation that limits their output to about 75% of the rail voltages, which is a loss of about 50% of the potential power outputs.
3. Above 40V total supply voltage, the op-amp requires its own reduced supply voltages, and the output must have the gain to take the +/-13V up to the rail voltage, which is aggravates stability problems with a second dominant pole.
4. Today we have many exceptional op-amps, but simple and cheap also means no proprietary parts, nothing over $2. This is not an audiophile design. This entire amp module should cost about $3 in parts.
Oh? Why are multi-stage amps not good designs? Does this mean that my amps are bad? How about Benchmark or THX AAA?
I argue that it's the end-to-end performance that matters. What the amp looks like inside is not particularly relevant. Using an opamp is like using another transistor. It's just another part. An opamp like the NE5534 or NE5532 would cost less than a typical transistor these days.
Really? According to the data sheet the output of the LM4562 will go to ±14.1 V with a ±15 V supply. That's 94 % of the rail voltage. The THD does start to rise before then, but it's still below -120 dBc at 8-9 V RMS output (±15 V supply, 2 kΩ load). That's 12.7 V peak.
OP's circuit won't go anywhere near -120 dBc at any operating point, so I'm not sure what you think is lost by using an LM4562. Except for the $0.50 it costs to pull one off a reel of 2500 and some fun in the lab getting it to work.
There are some good ±22 V opamps available now, but, yes. If you want more output power than you can get with ±22 V you'll need an output stage with a bit of gain if you're using an opamp.
And if that's OP's top priority - great. All I did was to suggest that you could make this quite a bit better and increase the output power in accordance with OP's wishes if you added an opamp. I'm not forcing anybody to follow along here. I'm just pointing out that such an option exists.
And, yes. Amplifiers with high loop gain need some care and feeding to get the stability right. Just as the original circuit did.
Tom