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Messages - Kaz Kylheku

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Amplifier Discussion / Re: Serious Blocking Distortion
« on: August 13, 2013, 11:38:05 PM »
Here is the amp as it stands. I have added the two zener diodes to protect the JFET and increased the voltage from the PSU to 30volts to increase the volume. I may change the JFET source resistor to 5K6 to lower the gain slightly, which may also give me a fuller tone. Do you think I will benefit at all from changing the 10K Pot to a 100K Pot? I should see an increase in volume shouldn't I? Actually I think the preamp will just be stupidly overdriven.

I would say that the thing to do is to revise the impedances so as not to have to use these ridiculous 22 uF capacitors in the signal path.

They are actually already too large.  You have a 22 kOhm impedance into the TDA stage input. If the coupling capacitor is 22 uF, the -3dB frequency is 0.33 Hz, which means you have nearly flat frequency response down to 3.3 Hz (10X the -3dB frequency: this is a rule of thumb).

That is ridiculous even for hi-fi, let alone guitar.

Amplifier Discussion / Re: G-K RB400 On w/no sound
« on: August 13, 2013, 10:57:54 PM »
Do not use lubricant on electrical contacts!
A lubricant is a film of some hydrocarbon, silicone or fluorocarbon.

What do these materials have in common? They don't conduct!!!
That's *exactly* the point, we don't want randomly conducting stuff there.

When you put a lubricant film between metal contacts, you're creating a capacitor.

If and only if they do not touch each other .... which is not the case.   ::)

Okay, J.M. has set my head straight about this.

After some weeks of experimenting, I'm a convert: lubricant is good, at least in sliding contacts.

Hypothesis: the coating provides an oxygen barrier, inhibiting oxidation, and when you insert the plug into the jack, it wipes off, allowing for the metal surfaces to cut through and make contact.

I have used this product to clean all the jacks and plugs in my rack. Guitar to rack, between the rack units, and power amp to speaker:

I use this light machine oil on bicycle chains and all small lubrication tasks that don't call for something specific.

The audio results are fantastic. Smooth, silky, well-articulated sound. I just did a re-application because I was hearing something "off", some harsh brittleness in the tone. It was instantly gone.  The bass response is excellent, which dispels the hypothesis that the film creates poor contact combined with capacitance that lets through higher frequencies.  Obviously there is good DC contact.

So, to heck with boutique contact cleaner: a $6, half-liter bottle of oil does the job.

This particular stuff is great: according to the MSDS it has hydro-treated as well as napthenic oil, a petroleum solvent to help it penetrate (smells like fuel or naptha), as well as zinc alkyldithiophosphates: anti-wear, anti-corrosion additives.

Someone has studied the latter in connection with electrical connections. I'd love to get my hands on the full text of this paper: "Electrical contact resistance studies on zinc dithiophosphates".

Now I wouldn't go as far as using oil on non-sliding contacts, because of the lack of a wiping action to cut through the film. Oil inside a relay? Forget it! The best thing for non-sliding contacts is to be oxidation-free, enclosed in an air-tight container that is filled with an inert gas. E.g. magnetic reed switches are that way.


Check out the way the resistors run under the trimmer pot, with a screw between them, and everything clears.
By placing the trimmer footprint over the resistors, I got it all to fit into a 1.1 x 0.9 inch outline.

This is not just idle curiosity; armed with this, I'm going to design a better current feedback circuit that doesn't cause a DC offset like the one I'm using now.

The above was no idle threat. I mulled over alternatives and then designed and implemented a solution (in one channel so far) several weeks ago.  It's been working beautifully.

The DC offset problem, previously 2.7V (!) (with a 4 ohm speaker load!) is completely cured.  I can dial in any mixture of voltage and current feedback without shifting the offset, and yet everything is DC coupled.  I even removed the CX13 capacitor: the one in the feedback loop that reduces gain to unity at DC. The amp now has gain all the way down to DC.

How did I do this?  I figured, since bias cancellation techniques are used in op-amps to null offsets, why not try it in a discrete power amp. So I designed a temperature-compensated input bias current cancellation circuit, consisting of a small (0.9"x1.1") circuit board with four transistors, seven resistors, trimmer pot and cap.  This circuit taps into the main board via four hookup wires: ground, -48V, and connections to the two bases of the input pair.

The rig is now producing some great tones! All I want to do is play guitar because the sound is just so good every time I pick up the guitar.  I've taken to sitting on the floor right in front of the 4x12, so I can just bask in the pure tone in my face.  And it's been consistently good. Turn it on in the morning or evening, hot day, cool day, doesn't matter: same great tone.

As far as the DC offset goes, when you now turn on the amp cold, it starts at around -30mV, and immediately starts ramping positive, until in a couple of minutes it reaches around +30 mV or so. It stabilizes there and then reverses, falling into the neighborhood of +12 mV and thereafter fluctuates as the amp is left on all day. I've seen it as high as around 20 mV, and as low as only 6 mV.  Given that I was prepared to be happy with anything below +/- 200 mV, I am very pleased.

I will post pics and schematics of the project later.

i powered up the amp and ran a chord from the Polytone send to a traynor 20 watt amps return and the volume is low.

I'm afraid that no hard conclusion can be drawn from this type of thing. We would need some assurance that the line levels in the effect loops of the two amps are nominally supposed to be the same.

Jumping volume levels can be caused by worn or dirty potentiometers, due to the wiper making an intermittent contact with the resistive element.

The first thing to do is give the unit a thorough cleaning: pots, jacks, and any switch contacts that carry audio that you're able to reach with spray or swab.

sometimes i can hold a key down and the volume will jump up to full volume for a split second if i pick the right key to hold.

That sounds like it could be a problem with the keyboard (in addition to the amp problems). Do you have another amp with which the keyboard can be shown to behave itself?

Amplifier Discussion / Re: how build a small amp for practice
« on: July 25, 2013, 03:40:53 PM »
By the time you get your act together and build something, your guitar will sit in its case for another five years.

You should just get some kind of amp and start playing!  Get those fingers back into the game. If you ever build something, you need to be able to play in order to evaluate how it sounds.

Here is another idea: if you really have an urge to tinker, start with existing equipment and mod it. Modding takes vision and know-how, but is less time consuming than building something from scratch. You can get a lot of satisfaction from making good mods.

A related way to save time and money if you build something of your own is to re-use the chassis from an existing piece of gear, along with some existing furnishings like power supply components. This saves you from the trouble of the mundane stuff like making a box out of sheet metal, and installing power entry sockets, fuse holders, and a transformer.

Preamps and Effects / Re: Pre-amp / compressor for bass guitar
« on: July 22, 2013, 05:20:03 PM »
9V is a problem if you're using audio workhorses like the NE5532 which don't go close to the rails.

A problem like that can be overcome by using a charge pump, which can generate a 2V and -V rail from a V rail.

The power consumption of the 5532 is also an issue for batteries.  If you can implement this circuit with two amplifiers instead of four, you can cut the IC-related power use in half. Then if you can use chips that have half the consumption of the 5532, your overall improvement is 4X.

One nice bipolar IC that can replace the 5532 if lowering the current is important is the MC33078. At ordinary temperatures, it consumes about 2 to 2.5 mA per amplifier, so about 4-5 mA per package. Compare that to the typical 8 mA and maximum 16 mA of the NE5532: the MC33078 eats about 3X less.

The MC33078 has good figures, distortion-wise (distortion versus frequency, and such are in the datasheet).  The output stages on the MC33078 are non-complementary and supposedly are free from crossover distortion.

It needs dual rails though: minimum of -5 to +5.  Running it on a 9V supply therefore violates the data sheet, but a charge pump to create -9 to +9 would work.

There are lots of devices out there; and many ideas are possible.

P.S. If you're designing around a JFET, it's also worthwhile to look at a very similar device from the MOSFET world: the depletion-mode MOSFET. This is like a JFET in that it is normally on, and a voltage below the source (for an N-channel device) is required to shut it off. Depletion MOSFETs use the same biasing and can be "dropped in" for JFETs. However, being a MOSFET, the depletion mode MOSFET has an insulated gate: there isn't a diode there, and so unlike a JFET, it has a next to nonexistent (DC) gate leakage current.   The existence of these transistors increases your options if you're designing a JFET type circuit.

Preamps and Effects / Re: Pre-amp / compressor for bass guitar
« on: July 18, 2013, 09:36:00 PM »
I gots me some comments too.

If it were me, I'd try to somehow roll the vactrol into the feedback of an inverting stage, to control gain rather than just attenuation. That could actually eliminate a buffer stage: one amp does everything. And since it is inverting, you preserve phase through the box (as I can see you are diligently doing).

Another musing that occurred to me. You have this rectifier and peak detector circuit, whose output is then buffered and used to drive current into the vactrol.

Perhaps the buffer could be eliminated. If you can reduce the compressor signal path to one inverter with vactrol-controlled gain, as above, and also eliminate the buffer from the rectifier/peak-detector, then you can make this device with just one dual-op amp IC, which is cool engineering. You trim the idle current consumption, save space, etc.

When choosing the capacitor and resistors for the peak detector, just take into account the impedance of the transistor base, rather than making it very high with a buffer; I suspect it can still be made to work fine.

The QN2 circuit could put all the loads on the collector side so that it's a current driver. What you have there now is an emitter-follower: as it stands now, you're driving the LED (the display one and the one in the vactrol) with voltage rather than current, which is basically wrong. If the loads are put to the collector side, you can reduce R12 and put some resistance on the emitter for a little negative feedback to control the gain and improve the impedance of the base.

On an opposite note, if you keep the op-amp buffer in this circuit, it can be configured as an active low-pass filter rather than as just a "dumb" buffer.  Why would you do that? Because thanks to the Miller effect, you can achieve the same frequency roll off as the peak detector has now, but using a much smaller capacitor due to the Miller effect of the cap being in the feedback. Cool engineering, again. Whenever you catch yourself building an RC filter immediately followed by a unity gain buffer, it's useful to consider whether they might not be combined.

Amplifier Discussion / Re: G-K RB400 On w/no sound
« on: July 13, 2013, 01:55:35 AM »
Quote from: Kaz Kylheku
The brand name DeOxit is priced like it's some of elixir of immortality made from the extract of the flower of a rare herb that only thrives in one single tiny forest somewhere in Nepal, and blooms only for one week, once every 23 years.

...gathered by comely virgins on the full moon (who make $1000/m speaker cable on their days off).

Thanks for the clue about oleic acid.

But, wouldn't you know it, oleic acid is also priced wildly, like the milk squeezed from the teats of Buddha.

Some medical grade of it ready for use with bacterial cultures and can set you back a hundred bucks for a measly 25 ml!

Some other grades are more reasonably priced, like 25-30 bucks a liter.

Check the lab chemical suppliers "Sigma-Aldrich" and  "VWR".  I contacted the chemistry department at my old alma mater, UBC, and they tipped me off to these.

Amplifier Discussion / Re: G-K RB400 On w/no sound
« on: July 10, 2013, 02:07:09 PM »
See here:



Amplifier Discussion / Re: G-K RB400 On w/no sound
« on: July 10, 2013, 01:58:05 PM »
The "medicinal ingredient" in Deoxit is oleic acid: a fatty acid found in vegetable oils such as olive oil.

The same stuff is also used in some traditional metal polishes. I think Brasso contains it, or may at one point have.

It cuts through oxidation, perhaps not so differently from the action of solder flux.

I've read some claims by hobbyists that you can make your own homebrew Deoxit equivalent by cutting some oleic acid in a petroleum distillate: camp stove fuel like Coleman or lighter fluid. I haven't tried it myself, yet, but it sounds promising.

The brand name DeOxit is priced like it's some of elixir of immortality made from the extract of the flower of a rare herb that only thrives in one single tiny forest somewhere in Nepal, and blooms only for one week, once every 23 years.

Amplifier Discussion / Re: G-K RB400 On w/no sound
« on: July 09, 2013, 08:00:22 PM »
Do not use lubricant on electrical contacts!

A lubricant is a film of some hydrocarbon, silicone or fluorocarbon.

What do these materials have in common? They don't conduct!!!

When you put a lubricant film between metal contacts, you're creating a capacitor.  If the contact was so dirty that it didn't work, the lubricant may improve it, but the result won't be ideal.

The best contact is made by smoothly polished metal surfaces that are free of any dirt, film or oxidation. The smooth polishing maximizes the contact patch area, and freedom from nonconductive material and oxidation ensures that the contact points have near zero resistance, and are ohmic (distortion free).

Use metal polishing paste on a cotton swab to polish the contacts inside the jack: only a small quantity worked into the swab, so as not to leave a build-up in there. Follow up with a clean cotton swab loaded with rubbing alcohol to remove the paste, and give it a final rub with a dry swab.   

(I use toothpaste, with great results.  When I run my tongue over the jack contacts, they have that fresh-from-the-dentist feeling, and none of my gear has ever had a cavity!)

The Newcomer's Forum / Re: Analog + digital ground separation?
« on: July 08, 2013, 07:53:58 PM »
3) it's fed +/- 7.5V @ less than 1 mA , *easy* to get from the +/-15V you already heve for the Op Amps

4) it costs *less* than 1U$S [/color]

What's not to like about it?  <3)

That it does not run on the +/- 15V you already have for the Op Amps.

Amplifier Discussion / Re: Transformer on Line6 Spider111 75 watt
« on: July 08, 2013, 03:55:40 PM »
If the amp hums, the transformer is still alive; it just forgot the words to the song.

On a non-electronics topic completely; it sounds like the club owner did something stupid (and possibly illegal, in contravention of electrical code).

He should compensate you for repairs and replacements.

I bet his insurance company (assuming he even has one) would not be too thrilled to hear about this, not to mention whatever authority that gives out club licenses (city hall, ...).

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