TREBLE BOOST for PPA, Pimeta, MMM!

[amb]

zxc, yeah, that alpha dual pot should work fine. I think it's going to be very hard to find those special center-detent versions that I had.

 

[morsel]

amb: nice tone control!

zxc: Much great information has already been posted in this thread. Judging from your reaction, you desire full tone controls, not treble boost, which would only increase, not decrease treble.

With this level of complexity, you might consider a separate circuit that connects to the amp input and provides initial gain, tone control, crossfeed, and bypass for tone control and crossfeed. This would perform signal altering functions on the highest possible signal level prior to volume control, thus reducing noise. The amp itself could then be configured with unity gain.

 

[stadams]

So, I suppose the challenge is to develop the mathematical equations for this circuit also?

Where's Snoop? This might be fun considering... If I can find the time.

Later,

 

[zxc]

I have one question (again)?

I have no idea what the difference is in design; but some tone controls introduce noise (like the ones in my old Sansui amp) and some don't (Arcam).

Is amb's tone control design likely to introduce any noticable noise or distortion? Or will a design like you said (from which I understood was to design a 'pre-headphone-amp' with tone-controls and no volume control) needed for it? Are there any ready schematics for such design, or is it easy to put together?

 

[morsel]

I merely point out that gain applied before tone control, crossfeed, and volume control minimizes noise. Whether you or anyone else will notice the difference is unclear. Some adventurous soul would have to experiment with it. Using a 10K volume pot also helps reduce noise. Using low impedance high efficiency headphones and boosting the treble is more likely to reveal noise issues.

I would draw something up but I'm really busy with PPA v2 and M³ issues at the moment. If there is enough interest, perhaps this would be a good next project.

 

[zxc]

Unless anyone has any suggestions I will build it according to AMB's schematic and post the results here. If this doesn't turn up to be a good solution or if there are any different ideas (like just a bass and treble boost circuit instead of a whole adjustable design, or something like a "crossfeed and tone control" pre-processing board) I will bug you again! Thanks to all; especially AMB, and Morsel.

 

[stadams]

Has anyone tried to simlulate this thing yet? I've noticed their is some peaky midrange response depending on the setting of the tone potentiometers. With the bass pot at 1/4 and the treble pot at 1/2 I am seeing a peak at 2 kHz and as the treble boost is increased, the peak backs further down in to the midrange until it reaches 1.6 kHz where the boost is much greater in the midrange region than what I would want for a treble boost. For lower settings of the treble pot it does produce a nice shelving response but still to early in the frequency spectrum. Finally, as the pot reaches the end of its range, the boost frequency is nice but the circuit gain is extreme.

Caveat: Or my simulation could be wrong.

amb,

Can you adjust this network to move the treble control into the upper octaves?

Later,

 

[amb]

Hi stadams, care to share what you're simulating with?
To increase the treble turnover frequency, simply decrease both of the capacitor values on each side of the treble pot, but keep their 1:10 ratio (e.g., try 0.001uF and 0.01uF, etc.). Also, with simulation you deal with ideal pots, but in reality the pot may not be centered electrically when it is mechanically, so you may or may not need to make adjustments to the 499 ohm resistor value. Ideally it ought to be 1K ohms, but I used 499 ohms for the particular pot I had.

 

[zxc]

I also noticed the DIY PASSIVE CROSSFEED FILTER at http://home.t-online.de/home/meier-audio/index.htm.

I have doubts about how easy it is to build it from scratch but it seems a good combination for bass-boost and crossfeed combined, something like Morsel was talking about. What do you think? Has anybody built, owned, or listened to it? Does it have noticable treble boost; and is it quiet (that is, no noise)? What are your opinions?

 

[SnoopyRocks]

Quote:

Originally Posted by stadams So, I suppose the challenge is to develop the mathematical equations for this circuit also? Where's Snoop? This might be fun considering... If I can find the time. Later,

Fun?

Fun?

I'm around...This one will require a bit more effort than the bass boost circuit since this is a 4th order network instead of a 1st. The analysis is not difficult, just tedious. If there is enough interest I could be persuaded to work it out.


I would not put a tone control on an amp personally. Why make such a big deal about DC coupling the amplifier (as is done in this forum) only to intentionally add circuitry that introduces the same issues that DC coupling strives to eliminate. That is more of a rhetorical question than not. I realize that some might want to add circuitry like this to make up for load (speaker, headphone,...) or source shortcomings. I thought that this was an audiophile forum...

 

[amb]

Quote:

Originally Posted by SnoopyRocks I would not put a tone control on an amp personally... I thought that this was an audiophile forum...

I'm definitely with you there

. For my personal amp I'd ditch the tone control too, but hey, to each his (or her) own.

 

[zxc]

The reason that I wanted the tone controls integrated in the amp was to minimize noise/distortion. But I get the impression that this is not true. Please let me know if a passive section between the amp and the source (something like the "crossfeed+tone controls" of meier audio) is a better idea... Let me have my descent tone controls; I don't want to sound dramatic but am I alone?

 

[SnoopyRocks]

Quote:

Originally Posted by zxc The reason that I wanted the tone controls integrated in the amp was to minimize noise/distortion. But I get the impression that this is not true. Please let me know if a passive section between the amp and the source (something like the "crossfeed+tone controls" of meier audio) is a better idea... Let me have my descent tone controls; I don't want to sound dramatic but am I alone?

Distortion:
Tone networks inherently introduce linear distortion. This is ostensibly what you want though - to color the input with more bass and treble or leak the stereo input from side to side (crossfeed). Nonlinear distortion (single input tone yielding more than one tone at the output -> SFDR, THD) is another matter. The resistors will not be a problem here. The capacitors might be. I doubt that it would be significant however.

Noise:
The capacitors are not a problem here. The resistors will introduce thermal noise. The noise power is proportional to the resistance. Yes, the circuit will be noisier. I doubt you will be able to tell/hear the difference. For a cascaded system ( Amp1 -> Amp2 -> Amp3 ...) the input referred rms noise is reduced by the gain of the stages that precede it. Consequently the noise of Amp1 is more important that that of Amp2, Amp3, etc. Introducing noise sources within the feedback loop of an amplifier is not the same thing.

Bottom line:
These issues are over your head but should not concern you. Build the circuit. If you like the way it sounds, great. If not, you got your hands dirty and learned something in the process. Analytical treatments of both of these issues (linearity and noise) are not simple and typically require advanced degrees to delve into with more than superficial treatments.

My advice is to build the circuit and enjoy the music.

That's why we're all here in the first place. Right?

 

[stadams]

Quote:

Originally Posted by amb Hi stadams, care to share what you're simulating with?

Unfortunately, the only tihng that I had at hand, an evaluation version of PSpice using the ubiquitous uA741--the only op-amp available for simulation.
Quote:

To increase the treble turnover frequency, simply decrease both of the capacitor values on each side of the treble pot, but keep their 1:10 ratio (e.g., try 0.001uF and 0.01uF, etc.). Also, with simulation you deal with ideal pots, but in reality the pot may not be centered electrically when it is mechanically, so you may or may not need to make adjustments to the 499 ohm resistor value. Ideally it ought to be 1K ohms, but I used 499 ohms for the particular pot I had.

I tried this, scaling the resistor values up and down to see how it effected the response and did not recieve what was expected. Maybe I did not change the values enough to see a large enough change although they were changed by a factor of ten.

Later,