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Doug Self Preamp from Linear Audio #5
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Old 31st December 2017, 07:21 PM   #941
DouglasSelf is offline DouglasSelf  United Kingdom
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Quote:
Originally Posted by diyralf View Post
There is already problem number 1 for me: 4-way potentiometers are very expensive or difficult to obtain. I have been playing for quite some time with an active Baxandal volume control. What worries me is the characteristic. At 25% pot position you have just ~ -12dB. More would be better (e.g. -25 ..- 30dB).
I quite agree. You get a much better law by using the active-passive system.
The downside is what I call 'the overlap penalty' whereby headroom is reduced if the active part is amplifying while the passive part is attenuating.

The only complete solution to this is switched resistor ladders, which solves the pot problem but requires something like a 60-way 4-pole switch.
Such things exist but they are NOT cheap.
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Old 31st December 2017, 11:57 PM   #942
diyralf is offline diyralf
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Happy new year!
May I ask you something else? I simulated circuit Fig. 4.19 from Audio Power Amp Design book with LTSpice.
According to Fig. 4.10 THD would have to be 10 times less. But the simulation is the other way around. There it is about 10 times lower, if R2 and R3 are zero. As the current increases to 9.7mA, the values get even better. What's wrong here?

Fig 4.19
Freq., current, R2/R3, R6, Total Harmonic Distortion
1khz (9.7mA, 0, 68): 0.000043%
1khz (6.1mA, 0, 68): 0.000053%
1khz (9.7mA, 100, 68): 0.000502%
1khz (6.1mA, 100, 68): 0.000507%

20khz (9.7mA, 0, 68): 0.000443%
20khz (6.1mA, 0, 68): 0.000573%
20khz (6.1mA, 100, 68): 0.009438%
20khz (9.7mA, 100, 68): 0.009438%


Last edited by diyralf; 1st January 2018 at 12:26 AM.
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Old 1st January 2018, 05:44 PM   #943
DouglasSelf is offline DouglasSelf  United Kingdom
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Originally Posted by diyralf View Post
What's wrong here?
]
In general, the fact that SPICE distortion results are not dependable because of the approximations in component modelling.

SPICE is never, never, a substitute for measurement.
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Old 1st January 2018, 09:01 PM   #944
diyralf is offline diyralf
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Understood. Then I need an AP ;-)
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Old 3rd January 2018, 05:26 PM   #945
scottjoplin is offline scottjoplin  Wales
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Why is an 18V transformer specified in the BOM, wouldn't a 15V mean less heating of the regulators?
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Old 3rd January 2018, 09:05 PM   #946
diyralf is offline diyralf
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15V + 3V headroom for reserve?
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Old 3rd January 2018, 09:30 PM   #947
Carl_Huff is offline Carl_Huff  United States
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Originally Posted by scottjoplin View Post
Why is an 18V transformer specified in the BOM, wouldn't a 15V mean less heating of the regulators?
The short answer is, "Yes". The power supply as specified is 'universal' as it can be adjusted to provide higher voltages to power JFET and legacy 2520 (old pro audio format) circuits.
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Old 3rd January 2018, 09:46 PM   #948
scottjoplin is offline scottjoplin  Wales
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That explains it thanks
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Old 4th January 2018, 08:34 AM   #949
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by scottjoplin View Post
Why is an 18V transformer specified in the BOM, wouldn't a 15V mean less heating of the regulators?
Quote:
Originally Posted by diyralf View Post
15V + 3V headroom for reserve?
Quote:
Originally Posted by Carl_Huff View Post
The short answer is, "Yes". The power supply as specified is 'universal' as it can be adjusted to provide higher voltages to power JFET and legacy 2520 (old pro audio format) circuits.
An 18Vac transformer will give 24Vdc to 28Vdc after rectifying and smoothing.

Allowing for low mains and high load current let's start with 24Vdc as our worst case.
Allow 2Vdrop through the regulator, allow 0.5V for the half ripple (1Vpp ripple)
That leaves a guaranteed 24-(2+0.5) = 21.5Vdc as the highest regulated voltage that is free from drop out during worst case operation.

For 15Vdc operation the extra 21.5-15 = 6.5 volts will be dropped across the regulator in addition to the 2Vdrop already taken account of.
Some regulators perform a bit better when there is excess voltage above their specified drop out. Do we have any evidence that 8.5Vdrop gives better performance than 4Vdrop ?

Now look at normal operation where the available DC voltage is at 26Vdc.
Required output is 15Vdc. Allow a normal regulator Vdrop for the 1.5V drop out voltage and 0.3V for the half ripple. The regulator now has an excess of 25-(15 +1.5+0.3) = 9.2V
The result is an excess of dissipation in the regulator. Say the circuit is drawing 150mA and the reg Vdrop is 11V, then the dissipation 1.65W
That is extra ventilation inside the equipment.

An 18Vac transformer can easily drive a 18Vdc regulator and in most circumstances would drive a 20Vdc regulator.

For 15Vdc, there is no need to go above a 15Vac transformer.
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Last edited by AndrewT; 4th January 2018 at 08:37 AM.
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Old 4th January 2018, 08:41 AM   #950
J.M.K is offline J.M.K  France
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Hi everybody,

Never took the time to go into this here,
but I moved my PSU (original design) out of the preamp box
which resulted in a very significant reduction of the floor noise (on my oscilloscope)
on the output.
For what it's worth ...........
All the best

J-M
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