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Messages - Kaz Kylheku

#61
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/
#62
I have a couple of comments.

1.  Op-amp choice.

Do not use a 4558 op-amp, argh!  :duh It's a piece of crap that belongs in low-end audio gear from the 1980's. There is no reason to use it in new designs, even if you were making 10,000 copies of your preamp and having it all cheaply built overseas. Use NE5532 or better.

2. Ref voltage.

If you can come up with one more op-amp, you can improve on the voltage reference circuit by decoupling the generation of the reference voltage from its use. Take the output of your VREF voltage divider and put it through an op-amp buffer, the use the output of the buffer as the reference voltage everywhere else.

3. Too much buffering

Turns out, you don't need another dual op-amp chip to get an op-amp for the ref voltage. You can simply take one out of the existing circuit! See, you have too much buffering.  After your virtual earth mixer, you do not need to buffer the signal one more time. It's pointless. I think you're only doing it to decouple the volume attenuator from whatever circuitry follows.   But you can get active volume control by putting a pot in the feedback of the inverting mixer (configured as a rheostat). That nicely goes from zero to whatever gain you want.



#63
Quote from: Enzo on November 30, 2012, 01:51:51 AM
That sort of approach might work, but the example in the paper only provides 100ma output current.

I didn't even notice that. That is pitiful compared even to linear regulators like 78XX with no heatsink on them. Switching regulators are supposed to provide more current, since the pass transistor is fully on or off.

Maybe the authors were just going after "proof of concept" to show the establishment of voltage, and so they didn't use "beefy" external components.

In any case, going to read that thing in more detail; it's interesting.
#64
Here is a Texas Instrument paper about obtaining a +/- 12 dual polarity supply from a 5V DC input, using the UCC3803.

http://www.ti.com/general/docs/lit/getliterature.tsp?literatureNumber=slua288&fileType=pdf

Building the circuit described in this paper might not exactly be a good beginner project, though.

It should be straightforward to modify it to +/15V rails and a different input voltage to match the available battery.

#65
What you want is a 1/4" mono jack with an isolated make circuit.

http://www.switchcraft.com/Drawings/13_cd.pdf

"Isolated make" means that a connection is made when a jack is plugged in, and that connection has nothing to do with the signal pins of the jack.

"Isolated break" means that a connection is broken when a jack is plugged in.

Here is the page that is linked from: http://www.switchcraft.com/productsummary.aspx?Parent=68

PG's circuits, with the switch that disconnects the battery from only part of the circuit, are completely silly. But +1 for the charge pump.

Are you using a charge pump IC for your on-board pre-amp? Please post about how your project goes!
#66
Quote from: J M Fahey on November 13, 2012, 01:43:18 PM
The Jensen is a bright speaker, your problem does not lie there.
That Heathkit is "not a guitar amp" but a "hi Fi" amp with tremolo and reverb added.
Input impedance, Tone Controls and Equalization are all wrong for a "Rock" amp.
Simple as that.

I think the Baxandall tone control could work, if it was "primed" so that it has a mid-scooped EQ curve with a bit of emphasis on the treble, when the controls are in the middle position, and also playing with the location of the knee frequencies. The skew of the controls could be achieved by splicing some series resistors with the pots. For instance, the middle could be set up so that it cuts extra deep when turned left, and only goes to a flat midband when turned right (which, for guitar, sounds like a honky mid boost).

Another thing that could help this amp might be a bit of current feedback in the power section. It has only a voltage feedback line. That alone will kill a bunch of sparkle and life. Current feedback will add significant high end.

Speaking of feedback, a presence control could be added in the feedback line for extra control over the high end.
#67
Quote from: Roly on November 12, 2012, 11:27:13 AM
Kaz - you seem unaware that the "death cap" and three position mains switch were standard kit on American guitar amps for many years, certainly in the 60's when this was built.  Most of the Fender amps I have worked on in Australia have a three position mains switch, albeit modified on import for our LEN system.

You're right. I know about death caps, but I don't know all their specific configurations like this DPDT switch. In any case, it's not up to modern safety standards. Safety first, then input impedance, and all the rest.

I heard stories about how players used to be electrocuted by their guitar amps in the olden days, which should never happen.

QuoteI don't know where you get the idea this is a line level input amp since the original spec for this and its mate the TA-16 show an input sensitivity of 25 and 35mV on the normal and reverb channel respectively.

Sorry, I should have said line impedance. The level is really neither here nor there; with the volume attenuator we can adjust the output of the first stage all the way down to zero.
#68
I think modeling is fantastic. It lets people get into guitar and make some nice sounds who will never play outside of their bedrooms, and who could never put together a decent tone using a normal way even if they spent their life savings on real non-emulated gear. Emulation is probably very useful to the pro. You can practice with headphones, do some convincing sounding direct recording, or even play live by going into a PA instead of lugging an amp.  Not all music needs the best possible guitar tone. Just like not all music needs the best possible sax or strings, and synthesized sounds are passable. The pro can set his ego aside and realize that sometimes his contribution is just a small part of a larger tapestry.

I think modeling can do a good job of things like speaker cabinets and room reverberation. However, the distortion isn't there. At least not the heavy distortion. The mild crunches and such, perhaps.

The problem is that if your digital sampling process introduces X decibels of digital noise, and then you simulate 80 decibels of gain (or whatever), then you have X + 80 decibels of digital noise. The quantization noise and jitter artifacts are amplified too by the same amount of gain.

These guitar boxes do not have the quality A/D conversion to begin with. Nothing like expensive audio interfaces used in professional studios.

I think it will be a long time before it makes sense to do anything but generate distortion with analog circuits and sample it afterward. At least for situations when music is being made in which the guitar is important, rather than just some embellishment to add a rock and roll flavor to a passage.
#69
Adding  FET buffer to this thing is a waste of effort. Just let it be what it is: a power amplifier with EQ, tremolo, reverb, and a line-level input.

You need something before this anyway if you want distortion or other effects. That something can provide the instrument-level input.

On a different note, that stupid C103 capacitor by the power switch should go, along with the stupid DPDT power switch.  A
three-prong cord should be used.

What the hell is with that? In one position, the cap is connected to hot, in the other to netural. If that cap fails short, you have a problem: one position of the switch will connect your circuit's ground to hot. It is a form of  "death cap" that should never be used in modern designs.
#70
This design is from the 1960's?

How large would that 2500 uF coupling cap to the speaker have been in 1960-something?
#71
Amplifier Discussion / Re: SS amp building...No treble
October 28, 2012, 03:30:03 PM
Quote from: paku5535 on October 27, 2012, 04:08:21 AM
3. yea tried connecting the speaker from my computer power amp. response is more or less same. But when i connect my computer speaker power amp after Gt2 and taking speaker o/p from that power amp, i have got some treble making the sound more beautiful...

So you get more high end if you substitute another power amp?  Is that enough so that the highs are where you think they should be, or is the difference small? Is it beautful or just more beautiful?  :cheesy:

I think that for this kind of reference testing, you should have a better amp than a computer speaker one.  Get some used 100W per channel studio or sound reinforcement amplifier for a hundred bucks. Something that is 20-20Khz into 4 ohm loads. This can be used as a reference benchmark.

You may have made a mistake in the assembly or a poor solder joint.   Lots of gain at low frequencies that rolls off rapidly could point to too much gain (inadequate negative feedback).

Maybe you put an incorrectly valued capacitor somewhere.  (Eg mix up picofarads and nanofarads),

You cannot even rule out that you have a bad amplifier IC. You have only reproduced this issue in a single unit that you have built. By some extremely bad luck, you could have a bad chip and tear your hair out looking for problems elsewhere.
#72
Quote from: Roly on May 26, 2012, 11:28:36 PM
Quote from: J M Fahey
So I stick to my guns about having independently invented and applied mixed feedback technology in 1972.

No argument from me JM, quite a few things have been invented independently in more than one place, Calculus for example.   :cheesy:

Just to note that the RCA scheme I posted above is a bridge that actually allows the current feedback to be varied through zero to either phase.

Variable Damping Factor Control by Charles A. Wilkins, Audio, September 1954
http://www.aikenamps.com/vardamp.pdf

Quote: "A voltage is developed across the 0.47-ohm and the 0.27-ohm resitors that is proportional to the current flowign through the speaker load. This current-proportional voltage is sampled by the slider on the 25-ohm potentiometer and fed back through low-pass filter RC as current feedback  to the cathode of the first stage where it is added with the over-all negative voltage feedback."

"Added with the overall negative voltage feedback" means "mixed feedback"!
#73
Quote from: mojah on November 04, 2011, 09:00:51 PM
I looking at adding a dampening factor adjustment to one of my SS amps. I'm looking at loosening up the feel a bit and before I put it under the soldering iron I thought I would ask around here.. My first impression would be to add more series resistance in the neg feedback loop at R108 any other thoughts?

Looking at the PDF, I see your Bandit is loaded with piece of crap RC4558 op-amps (if we are to believe that the schematic gives the part number that is actually installed).  Nobody in their right mind would use this for a serious piece of audio gear.

The explanation for this is schematic date: 1989!

The first thing I'd do is replace all these with something for audio quality, like LM4562 or NE5532. 

In 1989, an NE5532 was maybe still a bit of an expensive part for an amp like this, and the LM4562 was nearly 20 years away from development.

After putting in better chips, I'd play the amp and re-evaluate if it's worth it to muck around in its current feedback structure.
#74
Amplifier Discussion / Re: Light Bulb Limiter
October 09, 2012, 04:04:30 PM
The FAQ maintainer for the sci.electronics.repair newsgroup also maintains a SMPS repair FAQ.

This repair FAQ has a whole extensive section on lightbulb limiters.

http://www.repairfaq.org/sam/smpsfaq.htm

He claims that "[M]ost of the time you will get away with putting [the light bulb] in series with the AC line".

But it is cautioned that "[SMPSs] can still supply bursts of full (or excessive) current even if there is a light bulb in series with the AC line" and "at some range of line voltage, the output regulation may not work properly and the output(s) may go much higher than expected".
#75
Amplifier Discussion / Re: Grounding techniques
October 09, 2012, 03:31:15 PM
Should we worry about star grounding in some line-level audio equipment that doesn't produce high output currents?  Like some equalizer box or preamp or whatever.  Say it's made up half a dozen dual op-amp chips, all dual-supply, which are mostly at unity gain or very modest gain.  It's convenient to just run a single ground trace reaching all the places on the board which need a ground.