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Topics - Kaz Kylheku

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I've been driven crazy with bad done over the past week or so.  There has been a bout of wet weather so I blamed the humidity. I ran hot air against the speakers in the 4x12 cab, and put a bunch of silica gel dessicant inside the thing.  I cleaned jacks. I cleaned tube sockets in the ADA MP-1. I repositioned the cab in the room. But  I did the obvious last: try switching to the other channel on the power amp. Aha!

What is the perceived problem? Something harsh and objectionable in the tone in distorted programs, and a stiff lack of touch sensitivity on cleans, which are much more fluid and silky through the other channel.

This morning I cracked open the power amp, and checked the quiescent current in that channel based on the voltage on one of the 0R22 emitter resistors (with the amp completely unloaded: no speaker or resistor). It was around 27 mA. What? I had set it to 5 mA only weeks ago. How can it drift so far, and in such a short time? This is a sign of some trouble. I also noticed some quite marked instability. On power up, the voltage was 6.1 mV on the resistor, soon dropping to around 5.5 mV in less than a minute.  Change with temperature is expected, but I don't remember it jumping by such a percentage in a short time.

I haven't had time to do any more investigating than that, so far.

Maybe a driver transistor is going south?

There is no glaring problem with the amp, like pronounced distortion. It reproduces sound and plays loud.

I'm not going to have time in the next few days, but I will run some signal through it into a load resistor and scope it.

It's nice to have a second channel for backup and comparison.

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There is a compression technique for lower bitrate AAC whereby the high end is basically stripped out, and all that is retained about it is the shape of its frequency profile.  (That and some info about transients and whatnot.)

When the audio is decoded, the lower order harmonics are somehow extrapolated into the high end, which is shaped by the frequency profile. Voila, the high end is back, faked out!

Turns out we don't hear the high end in detail; but we just need it there so that the lower end has definition and "air".

I'm wondering whether this might not actually end up sounding better than the original high end, since it could conceivably have the side effect of cutting away some objectionable harshness, and replacing it with a smooth, calculated high end. If so, it could be a useful post-distortion filter for a guitar signal chain, to smoothen distortion.

It would be after the EQ, so that the high end would follow the contour.

Hmm ...

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Preamps and Effects / Nice: op-amp swap on Yamaha FX-500 processor.
« on: June 23, 2013, 11:13:59 PM »
I have an FX-500 that I bought new in 1989! 24 years ago, man.  8|

I never took much of an interest in the internals.

A few days ago, I ripped out the six RC4558's that make up most of its analog audio path, and put in NE5532's.

Quite an improvement; it doesn't sound so cheap and boxy any more. I mean there are limitations on the digital side, but at least now they are laid bare.

The reverb is smoother, less irritating and less grainy sounding. Now when I heap on lots of reverb (externally, via analog mixing), it does not seem to occlude the dry signal as much as it did before. 

Yet the basic character of the unit is retained; it still sounds like itself, just better.  Clearly, its "personality" is from the digital side; and the analog side is just another op-amp selection cluster-funk.

It's a nice board to work with. The component side pads are solder-masked and so the solder did not capillarize through to the other side to make a double joint, as can happen with plated through-holes.  The chips gave way quite easily after just pumping solder away; they just needed a little twist.  No component side solder manipulation (pumping, wicking) was needed at all.  The holes came out mostly clean. A .028" drill bit went through most of them easily. And so all six were done in under an hour.

Now it's obvious why the headphone output was better at driving a power amp (less tone suck): it's an NJM4556 which can drive 150 ohm loads. I kept that one in there.

I didn't consider if the power supply has the margin for the bigger current draw times six op-amps. I didn't check the temperature of the 7809 regulator, or take any measurements, like ripple on the input side, taking instead the empirical route of: close it and stick it back into the rack and if it works it works.  xP

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Preamps and Effects / SMF-1: analog dry for digital boxes.
« on: December 03, 2012, 06:26:50 PM »
Here is a little project of mine to build a "last piece of the puzzle" piece of rack gear that I badly needed.

It went very well.

http://www.kylheku.com/~kaz/smf-1/

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The Newcomer's Forum / Brand new ADA MP-1 mailing list.
« on: October 08, 2012, 12:02:34 AM »
Hi all,

To fill in the vacuum left by the defunct ADA Depot website, I've decided
to start my own mailing list. (Today!)

No BBS crap with animated smileys, no bullshit. Not even a subscription needed
to start a topic. Just send an e-mail to <ada-mp1@kylheku.com> with your ADA related ideas or questions.

If you want to subscribe, go to the GNU Mailman interface at
http://www.kylheku.com/mp1. Archives will be accessible through that page also.

If interest builds, I will add a file download area (for which no registration will be required).

This is completely non-commercial. You will not see any ad in any web page or
list message footer, unless some third-party spam sneaks through.

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Amplifier Discussion / Roctron Velocity rant ...
« on: July 30, 2011, 09:44:06 PM »
Recently I became curious about what exactly the "reactance" control of a Rocktron Velocity 300 does.  (I don't have one of these units, thank goodness).  According to the marketing spin, it has something to do with speaker interaction. Quote from website:

Quote
The Velocity 300 has a unique “Reactance” circuit that actually replicates the output impedance of tube amplifiers—so you can get the same great sound that a tube amplifier delivers in a reliable solid state design. And, because it is a variable control, you can customize your Velocity 300 to sound like any of your favorite tube amps. Best of all, this feature is available in the mono bridged mode too!

If you Google for what users say about the amp, you can see that people are falling for this bullshit.

The old 1990's schematic shows this to be purely a tone control which mixes between two paths through different op-amp filters. There is no possible way it has any effect on output impedance. I'm guessing that it produces various amounts of a "frown curve" EQ.

In the newer amp, there are two tone controls. The schematic reveals these to be an obvious variation on the Baxandall tone control topology, again, purely in the preamp. Bass must be the reactance, and treble is called this:

Quote
In addition, the Velocity 300 has “Definition” controls to give you that little bit of edge you need to bring your playing out in the mix.

Good grief!

I suppose that if you hear tube sound when you tweak a pre-amp Baxandall bass knob, you deserve Rocktron equipment and the lies that sell it.  xP

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Amplifier Discussion / I just found THE web page for phatt.
« on: July 28, 2011, 03:50:09 PM »

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In the schematic of this amp (easily Google-able), at the very end of the output chain, the output is shunted to ground through a 0.33 ohm resistor R59 in series with a 0.22 uF capacitor.

Is this really just a low pass filter or is something else going on? That cap value seems quite aggressive. Wouldn't this kill the sparkle from the cleans?

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Hi all,

I'm trying to add a damping control pot to my Alesis 100 amp (one channel for now; if it works, then the other channel too).

I have reverse engineered enough of the amp to find the feedback path. The feedback goes through a 39K:1K voltage divider to feed 1/40th of the output stage voltage.

I'm going to rip out the 39K resistor and patch into there. The 39K resistance will be replaced with 100K to reduce the voltage feedback a little bit, so the current feedback is more significant (without having to use a larger current sense resistor that makes more heat!)

I plan to use a 0.22 ohm, 5W current sense resistor, and use a 50K pot.

The schematic is attached.  The terminals on the left are where we patch in place of the 39K resistor. The terminals on the right represent where we patch into the speaker return circuit and obtain ground.

Any advice is kindly appreciated.


10
Hey everyone,

can anyone rationalize why in the Alesis RA-100, two parallel 18 gauge hookup wires go from each power amp board to the speaker out terminal, but from the return terminal to ground, there is just one 18 gauge wire (per channel?)

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