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Different headphone amps
Different headphone amps
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Old 27th September 2020, 10:06 PM   #21
duncan2 is offline duncan2  United Kingdom
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Join Date: Mar 2013
Location: Scotland.
Yes the LM4562 has got impressive figures but those types didn't impress D.Self .
I see what the rejection values are for it but that wouldn't stop me fitting PSU suppression components to it, I am a "belts & braces" man , it comes down to -- does it sound good to ME -- and that is the ultimate test not a THD meter reading of over 100db.
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Old 28th September 2020, 08:39 PM   #22
Antoinel is offline Antoinel  United States
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Location: Pennsylvania
Different headphone amps
Thanks duncan2 for your post. I fully agree with you.

The application understudy is a test method for PSRR. The attached schematic shows:

1. LM308 [U2] is the application understudy. It generate two out of phase output signals; namely Vout [VSA] and Vout [CSA].

2. The signal Vout [CSA] is a "disturbance" on both rails of +/-15 V PSU. It has magnitude and phase.

3. This disturbance causes a change in [U2's] Vout [VSA]; per the definition of PSRR; but I can't measure it at this location.

4. This same disturbance is seperately imposed on the rails of an independent LM308 [U1]. It is idle. The LTSpice model shows an emerging output voltage [Vout PSRR]. It has amplitude and phase. This method is fully applicable to a dual [or true twin] OpAmp like NE5532.

5. 20 Log {Vout [VSA]} divided by {Vout [PSRR]} is a calculated PSRR at 10 KHz. The resultant value is 60 dB.

6. The two signals in point 5 are in phase; but with a phase shift per LTSPice.

7. But suppose the two signals in point 6 are in-phase. This creates or is an internally-induced positive feedback. A possible cause of oscillation if excessive; but maybe beneficial in its abscence if low or moderate.

Vout [VSA] and Vout [CSA] are correlated and out of phase. They are a balanced pair of outputs which are suitable to drive bridge amps.

Attached Images
File Type: png Mono uAmp PSRR.png (36.7 KB, 90 views)
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Old 4th October 2020, 05:03 PM   #23
Antoinel is offline Antoinel  United States
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Location: Pennsylvania
Different headphone amps
Default A Simply-Balanced Stereo Headphone Amp

The attached image shows the prototype's schematic for one channel and its Spice model; which runs smoothly.

Please note the following:

1. This HA has independent +/- 12 to 15 V PSUs. Their regulation in my application is not needed. This PSUs' independence favourably affects channel separation.

2. The stereo phones are independent of each other. They do not share a common/ground. Please recall I had cut the plug on my Grado Labs [SR80] and separated their leads. This favourably affects one's preferred perception of H2; do you like +H2 or -H2? Reversing the leads of each phone will give the opportunity to sample H2.

3. The stereo proto HA uses two NE5532s. One per channel. Sounds superb.

4. I have another application for the idle twin OpAmp in each channel .

5. Spice model says one can increase the value of the balancing resistors [330 Ohms shown] to 10 K each. More output power goes to the phone.

6. The OpAmp inverts phase. Gives one the opportunity to balance volume level between L and R phones. This is done by slight changes in the unity gain of one [say L ch] relative to that of [R ch].

7. The OpAmps can also be operated non-inverting without problem.

Attached Images
File Type: png OpAmp Balanced HA.png (37.3 KB, 77 views)
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Old 14th October 2020, 03:19 PM   #24
Antoinel is offline Antoinel  United States
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Location: Pennsylvania
Different headphone amps
The OpAmp power rails did not have decoupling caps [e.g. 0.1uF] in my last posted schematic. They suppress high frequency oscillations [>100 KHz.]

The attached schematic is an LTSpice model which shows the effect of one decoupling cap [0.1uF] at Vo [current source]. The sampling frequency is 10KHz instead of 1 KHz. The resultant behaviour is a low pass filter for/at the power rails

1. Vout [current source] is reduced in amplitude relative to that at Vo [voltage source].

2. Vo [current source] is phase-shifted relative to Vo [voltage source].

One may get away with lower-valued decouple caps.

I used the learnings todate on a 75 W/ch [8 Ohm] discrete power amp module by Radio Shack [STA-2000D]; vintage 1979. I have its schematic. Here's the experiment and subjective outcome.

1. It retained the decouple caps [0.11 uF] at its power rails [+/-45 V] to ground. Without them, the amp oscillates. May remind of the residual oscillation at the output of Class D; high enough to be irrelevant.

2. Cut the PCB around electrolytic caps [100 uF] connecting Vo[current source] to ground. They'll bypass power to ground!.

3. The amp is stable, no trace of oscilation, and it idles a +/- 70 mA.

An 8 loudspeaker [Karlson] was connected as shown in the attached schematic. It sounded great, loud and clear.

The take-away messages are:

1. Adaptable to low power discrete OpAmps.

2. Scalable for high power integrated amps e.g. LM3886.

3. Audio signals on the power rails constitute a possible added global feedback [- or +] to the stages preceding the power output stage. Granted their circuits ingest this feedback.

Attached Images
File Type: png OpAmp Balanced HA plus.png (41.1 KB, 44 views)
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