LM3875 Poweramp- reflections on components...

[ThorstenL]

Hi,

What's "lawfaked" mean?

Lawfaked refers to making a linear LAW pot behave more or less like a log LAW one.

What the LAW means is that at halve rotation you have around 20db attenuation (log) vs. 6db attenuation (lin) and at further rotation the attenuation becomes progressively more.

A "usual" methode of lawfaking to take a resistor appx. halve the value of the "nominal" potentiometer value if a log law pot was used and use a linear pot with around 10 - 20 times the value of the resistor. The resistor goes from wiper to ground, the pot is connected normally.

If the resistor is 10k and the pot is 100k the attenuation at halve rotation is around 17db, close to the normal log curve, the minimum input impedance (volume all the way up) is around 9kOhm, with a few db attenuation it is above 10k.

If we made the resistor 22k and the pot 470k we would have 22db attenuation at halve rotation and a minimum impedance of 21K.

Clearer?

Sayonara

[pinkmouse]

Originally posted by Kuei Yang Wang
Falls Dich der hintergrund dieser seite interressiert, e-mehl mir doch mal....

Is it scandalous then, Kuei?

[Pedja]

Kuei wrote:
“Transformer based Attenuation”

What is the price?

Kuei wrote:
“Good quality Cermet/Carbon track Linear Pot lawfaked”

The problem with it is that lawfaking resistor should have small enough value (to be effective). That means at higher levels you’ll get relatively low input impedance. Further, that means tubes can not drive it.

Kuei wrote:
“Single Series Resistor Single Shunt Resistor Attenuator”

Is this volume control or what?

Kuei wrote:
“True "Ladder" Attenuator”

It needs one whole day of soldering to sort it and cost is not negligible. It may look nice you have only one resistor at signal path (and only one shunt resistor), but truth is also you have two switches at signal path. I’ve never compared it directly with something else (just listened with and without it, difference was very small), but some reports I can not ignore say it is not any better than Black APLS.

Kuei wrote:
“Series Resistor Chain Attenuator”

Cheaper and much easier to sort than ladder attenuator. Not bad idea, if you have small enough number of resistors and if you can live without continuous control.

Kuei wrote:
“Conductive Plastic Log Pot (linear w. Lawfaking not applicable due to distortion)”

???

Bob, where is that thread?

Pedja

[Bobken]

Hi Pedja,

I don't agree with the 'order' of choices mentioned by KYW, but as I have covered the subject almost to death in the other post, I didn't wish to waste any more time on these points or any further unnecessary arguing here.

The thread I posted in a lot was "passive preamp" and within that post, there was a reference by Dave the Moderator, to an earlier Post of some ten pages which also covered all of the relevant points about passives too.

Regards,

[Peter Daniel]

Passive Preamp

[Philo]

Got it. Then the 10K 4th lead on my new ALPS pot goes to ground for the LAW attenuation. Correct?

[Bobken]

Originally posted by Philo
Got it. Then the 10K 4th lead on my new ALPS pot goes to ground for the LAW attenuation. Correct?

I very much doubt it. I don't know which Alps pot you are referring to, but I cannot imagine any possibility of a manufacturer making any (mechanical, of course, as opposed to electronic) pot which you can choose to use in a linear or log fashion. As I said earlier, the conductive tracks are made differently.(post #35)

So you will most likely have either a linear, *or* a log pot, of which one lead will need to be grounded (at least) for it to function as an attenuator.


Regards,

[Pedja]

Thanks Bob and Peter. I admit, I rarely go to the Analogue.

Pedja

[Philo]

The 100k log ALPS I have are NOS and have 4th leg with 10k resistance between both ends of the "windings" and is open to the wiper. I wasn't sure if this was related to "law-faking" but yours and Kuei's discussions on this hit a note that reminded me of my measurements of the pots.

[Bobken]

Originally posted by Philo
The 100k log ALPS I have are NOS and have 4th leg with 10k resistance between both ends of the "windings" and is open to the wiper. I wasn't sure if this was related to "law-faking" but yours and Kuei's discussions on this hit a note that reminded me of my measurements of the pots.

Hi,

Not absolutely certain what you have here from your description, but I would put money on the fact that it is an 'older' pot, with a tapping off the track for a 'loudness' control.

In case you are unfamiliar with this, it was usually a (switched) method of boosting the lower frequencies at low volumes to accord with the Fletcher Munsen (not 100% certain of spelling) curves which relate to the ears' ability, or rather lack of ability, to hear bass frequencies very well at lower levels.

Regards,