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Messages - Koreth

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1
Amplifier Discussion / Re: Introduction, and question about power supplies.
« on: February 15, 2010, 05:09:14 PM »
Maybe part of the problem is that you're looking at toroidal transformers? An EI core transformer, while bulkier, is easier to make, and thus costs less. Yes, a toroidal transformer is smaller and leaks less EMF noise, but they also have higher inrush current, and thus all other things equal, are more likely to blow fuses on power up than an EI core tansformer.

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Amplifier Discussion / Re: Introduction, and question about power supplies.
« on: February 14, 2010, 05:42:03 PM »
It sounds to me like that would work for a quick and dirty but functional power supply. Connecting the -12V from one transformer to the +12V from another sounds like it should work for a virtual center tap/common/neutral/whatever the appropriate term is.  However since I'm still in the learning the theory stage of power supply design, I'd sooner take the advice or opinion of someone who has actually designed and built a power supply over my own.

3
When speaking of this 3/2 law or 1.5 law, are you referring to the spacing, or the slope/curvature of the grid curves in a triode, or both?

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Amplifier Discussion / Re: Introduction, and question about power supplies.
« on: February 14, 2010, 03:16:29 PM »
Do the wall warts put out 12VDC or 12VAC? 12VAC, after being put through a full wave rectifier and a couple smoothing caps is closer to 16 or 17VDC. 12VDC has already been rectified and is just that, 12VDC. You an run a TDA2030 on 12VDC, but you won't have as much headroom or power output than if you gave it a higher supply voltage.

5
If it is saturation effects in the output transformer that plays such a large role in the 'valve sound', than I can't help but wonder if one could reproduce some of these effects or similar effect by employing a transformer in an SS design. I know Teemu's book talks about some early SS designs that employed transformers, but that is largely unnecessary know, since SS devices have such an incredibly low output impedance. The impedance matching function an transformer serves in a valve design isn't needed to drive the speakers in a SS design. I wonder if one could work some transformer magic with an appropriately built 1:1 transformer between the output device(s) and the speaker. I'm pretty sure I'd want to be careful with impedance mismatches. Unless I'm misunderstanding data sheets for various SS devices, under-loading them is a great way to destroy them in a hurry.

Another thing to add to the (growing ever longer) list of things to research and experiment with, I guess.

6
Thank you. I have read Teemu's book and attempted to read it. It's a hefty read in some parts, and as such, I've mostly only skimmed it thus far, but I agree, it is a good book.

So I promised to make a point about the nonlinearities of a triode. Such non-linearities are present in all tubes used in guitar amps, really, and I think it is safe to assume, all tubes. The above graph of output characteristics given for a pentode was for a 6L6GC.

Here's a KT88, used in output stage of the Ampeg SVT, and a few other amps.


Here's an EL34, used in many popular Marshall designs.


Here's the EL84/6BQ5, used in the renowned VOX AC30, and many other designs.


And I couldn't quickly find a graph for the 6V6GT, used in many lower output Fenders, but I can assure you it follows the same pattern. I do realize that the 6L6GC and 6V6GT are considered beam tetrodes and only the EL84 and EL34 are "true" pentodes. Fair enough. The operation characteristics of a pentode and beam tetrode are slightly different and this may account for part of the difference in sound between a 6L6 amp and an EL34 amp, but functionally, they serve the same purpose and will be considered functionally the same for now.

One thing I want to point out is the spacing of the input voltage curves. Towards the bottom of the graph, the curves bunch up and get closer together. Towards the top of the graph they spread apart. FETs do this too, but it is not as pronounced, IMO. Neither a pentode or a FET are the perfect, ideal component that is easily described mathematically, but tubes are far less so.

The curves on a triode do the same thing, as seen in this plate characteristics graph for a 12AX7 triode, one of, if not the most commonly used tube in guitar preamps.


Well that's nice, but what does this mean? This means, that the gain of the tube, pentode or triode, changes during operation, depending on what point it is operating at at any one instant. When the signal inside a tube swings positive, gain goes up until the tube maxes out and clips, and when the signal swings negative, gain is reduced until the cuts off  and clips. Notwithstanding negative feedback or any other tricks applied to a tube gain stage to make it more linear, any signal fed into the tube, no matter how perfectly symmetrical and balanced will not come out as perfect. It's like we have a compressor on one side of the signal and an expander on the other, playing tug-of-war with the signal. Furthermore, this change in gain as the signal swings from one end of the graph to another in itself isn't linear. The closer the valve comes to clipping on the positive swing, the mroe quickly the gain ramps up. The more quickly the valve comes to clipping on the negative swing, the more quickly the gain goes down.

To demonstrate this, I took the plate characteristics graph from a 12AX7 datsheet and plotted a load line for a 100k plate resistor, a B+ of 300V, and "center biased" with the grid sitting idle at -1.5V -- an arrangement similar to that found in many Fender amps. I took note of what the plate voltage would be at each grid curve, figured the size of the out voltage swing for each .5V step of the grid and the resultant gain. The numbers are below.

Vg is voltage at the grid. Va is voltage at the plate. DeltaV is the change in Va from it's previous value.
       Vg     Va       DeltaV   Gain
Clip   0.00 092.0036.0072.00
       0.50 128.00   33.2566.5
       1.00 161.2531.2562.5
Center1.50192.5000.00--.-
       2.00222.0029.5059.0
       2.50250.0028.0056.0
       3.00272.2522.2544.5
       3.50292.0019.7539.5
Cutoff3.75?300.0008.0032.?

AFAIK, nobody has attempted to replicate this nonlinear variable gain characteristic of a triode in a SS preamp circuit. For all I know, this characteristic may be unimportant when it comes to trying to get SS components to mimic the distortion. If nothing else, I think it would make for an interesting experiment to make an SS design with a preamp which mimics this specific characteristic of a tube, and compare it to an otherwise identical SS design that did not, and see if there was an audible difference. Perhaps an actual valve preamp that the SS design was mimicking would be a good comparison also.

7
I figured this was the best section for this post. If it's not, I apologize and can repost it where it is appropriate.

This is something that's been occupying my thoughts for a while. Off and on, I've been looking at datasheets, jotting down notes, doing bits of math and generally mulling things over. I figured it would help to put my thoughts down in writing where people smarter and more experienced than myself can off their input and comment if they want. The point of this isn't try to prove how the valve sound or solid state sound is inherently superior or more desirable, but focusing more on why they are different and assuming one wants to accurately replicate a valve sound with solid state parts, how that might be achieved. I know this has been discussed before, if not here, then plenty times elsewhere. It probably sounds like I'm about to start beating a dead horse and parroting the same out-of-context half-truths that abound on the Internets. For that, I apologize. I'm only posting this because I've made a few observations I believe haven't been discussed to death. Though I warn I may still ramble a bit.

So there's two tube types commonly used in guitar amps, triodes and pentodes. Pentodes are commonly used in the output stage for their greater capabilities. Triodes are commonly used in the preamp. There's exceptions, but that's the norm. Both are voltage-controlled -  no appreciable amount of current in the control grid during normal operation. In fact, current on the control grid is a bad thing unless you like a hard, sudden clipping on the positive swing of your signal (Makes for a great crunchy distortion!  :)). You probably knew that already.

There's two main types of transistors, BJTs and FETs. FETs are like tubes in that they're voltage-controlled devices. I don't know that any appreciable current flows in the gate in a JFET during normal operation, and I can't see how current can flow at all in a MOSFT, what with that insulated gate and all. Bipolar transistors however work with current. It's the changes in the current from the emitter to base that causes changes in the current from emitter to collector. Again you knew probably already knew that.

Transistors and pentodes have similar output characteristics. Look at the graphs in the datasheet for your favorite BJT, FET and pentodes. The curves have a very similar shape. They all sprout up from 0V/0I, shoot up sharply, then bend sharply or softly after a small portion of the max voltage and swing almost horizontal.

Here's a FET...


...here's a BJT...


...and here's a pentode


So we have a SS equivilent of the pentode -- the FET. Cool. Of note is how the curves are mostly horizontal after a small fraction of the output voltage. After the knee of each curve, a change in input voltage is going to affect more change in output current than it will in output voltage -- a low internal output resitance. That's why a pentode makes for as such a great output tube, poor damping characteristics of a low output resistance notwithstanding.  A huge voltage swing is nice, but without current capability to back up that voltage swing, getting some decent output wattage and driving a loudspeaker is gonna be harder.

So high current capability is desirable on the output of the amp, but what about on the input of the amp, in the preamp? Here we want voltage gain and lots of it. The output signal of a guitar pickup is of so little current and voltage that it doesn't take much attenuation to kill it entirely. give a .1Vp guitar signal a gain of 100, and now we a have a voltage swing that's much easier to work with. We can play with its frequency content to make it sound prettier, compress it so it's levels are more even and easier to work with, clip it for some crunchy sounds, and bring the voltage swing up high enough that with some current behind it a la the power amp/output section, can drive a loudspeaker.

Yes, that's nice Koreth, congrats on finally getting guitar amps 101. Now go put your newfound understanding to use and go build a Noisy Cricket already. Do you have a point, or are you just rambling?

Yes there's a point. We need voltage gain in our guitar preamp. Enter the triode. The curves for the output characteristics of a triode stretch more vertical than they do horizontal. A change in input voltage gives more voltage swing than current swing. Yes, a pentode can be made with with far more gain than a triode (the upper limit for a triode is about 100µ, pentodes can get over 1000µ.) But a pentode also costs more, is more prone to noise and microphonics (more active elements to go wrong). and requires a more complex circuit. We don't need a gain of 1000+µ in our preamp, plus noise and microphonics are Bad Things™ in a preamp. So if we could use a tube that requires less external parts, gives us the gain we need, isn't as prone to noise and microphonics, and on top of all that, costs less to boot, why wouldn't we? I suspect this is why the triode is the common preamp tube. It could also be that everyone is simply copying Leo Fender, and Leo was simply copying the RCA handbook, but that's another discussion.

The point being is that unlike the pentode and his SS brother, the FET, there is no SS analogue of the triode. Maybe the Trioderizer counts. (http://radiomuseum.org/forum/the_trioderizer_a_solid_state_triode.html). Put some local negative feedback on a MOSFET and it starts to behave much more like a triode than a pentode (the SS verson of Ultralinear mode maybe?) But with only 10 hits on Google, either nobody cares, or there must be some fundamental problem with it that keeps it from seeing more widespread use in SS designs trying to cop a valve sound. So without a SS equivalent to the triode, where do we get our voltage gain for our preamps? I see opamps in a lot of schematics. Op Amps are great for the application. They have downright stupid amounts of gain available, so much we have to use local negative feedback to get the gain down to a useful level.

Problem being is that especially with all that negative feedback improving it's operating characteristic, an Op Amp is really linear across the range of audio frequencies, AFAIK. But linear is good! We want linear! Harmonic Distortion BAD! Yes, and all this linearity is probably why a not insignificant portion of the guitar playing populace prefer tube amps to SS amps. An SS amp can be too accurate. The mysticism and some sort of placebo effect surrounding tubes aside, their imperfections apparently can color and distort the sound of a guitar in a pleasing way for some people.

Great, it's two in the morning I'm tired and just realized I've just spent over a thousand words running in a giant circle without coming to a point. There's one, I promise. I'll post it later when I'm rested. It involves the nonlinearities of a triode.

People smarter than me are welcome to point and laugh at the rambling newb now.

8
Amplifier Discussion / Re: Modifying a First Act MA104
« on: February 08, 2010, 01:01:47 AM »
Let the record show that I am blind. My multimeter was sitting on the shelf where I last placed it all along. I just hadn't stared at the exact spot it was sitting hard enough.

I took a measurment of the gain pot. Let the record also show that I am an idiot. The taper in the simulation was way wrong.

An oscilloscope would still be useful to have.

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Amplifier Discussion / Re: Modifying a First Act MA104
« on: February 07, 2010, 11:20:11 PM »
So I figured out a few more things with LTSpice, and even found enough models of parts used in the MA104 for a simulation of the whole amp to run. Yay!  :) Thus, I have spent most of my weekend making tweaks to circuit in the simulator and staring in fascination at pretty colored graphs of the resultant waveforms and voltages in various parts of the amp's circuit. Good times. I also redrew the old schematic, because it was a pain to try to follow. The new one is attached. I didn't draw out the output stage because I can't find a model for the TDA2003 and the arcane art of making models from scratch is still beyond my ken.

I made a few discoveries. Unless the 4558 model made by Texas Instruments is innaccurate, the 4558 can't swing all the way to it's voltage rails, it clips hard about 2V before them. I was tempted to blame the model first, but when I tried other opamp models that came with LTSPice, they did the same thing, clipping hard a volt or two before hitting the power rails. So I guess it is a behavior of op amps. I didn't know op amps did that, but it is a good thing to know.

However there's been some differences between the simulation results and the results of plugging a guitar in and playing the amp that are puzzling me. According to the simulation, with the gain, volume and tone knobs cranked, the highest voltage swing that could happen at the preamp's output is about 350mVpeak. The TDA2003's gain is presently set to 10, and it is getting about 13V from the power supply. So, unless I'm Doing It Wrong, a 700mV peak to peak times a gain of 10, means the TDA2003 should have a signal of about 7V peak to peak on it's output, well below it's power rails, and shouldn't be clipping, ever. However, when I plug in my guitar and strum as hard as I can, I can hear a bit of breakup on the attack with the gain knob set at 3.

Now, my big 100W tube amp has it's input stage biased at 1.5V. So the hottest signal my guitar could possibly put out without overdriving the input stage and giving me dirt on the clean channel (which it doesn't) is 1.5Vp. I doubt even the bridge pickup puts out that hot a signal. Now in my simulation, a 1.5v signal with the gain knob set at 3 doesn't even cause the opamp to clip. The clipper diodes have been lifted from the circuit for now, the 2nd opamp is set at unity gain, and the TDA2003 is set to a gain of 10. So where the heck is this distortion coming from?  >:(

Some thoughts. My simulation is wrong? (likely). If that's so, I'm not sure where to start looking. Maybe I have the taper of the gain pot set wrong? (i.e. What LTSPice thinks is 70% rotation is actually 30% on the pot itself). The TDA2003's gain is set too low and the chip is unstable? (maybe?) The chip gets warm enough to feel that it's warm when the amp is running, but it doesn't get HOT like I've read about in these forums when a chip oscillates. The datasheet specs a minimum closed loop gain of 92.3, but stock, gain was 32.1, I don't think it had any problems being set that low.

I wish I could find my multimeter and take some measurements, instead of making blind guesses like this. An oscilloscope would be nice too.

I suspect those of you who know what you're doing and know how to use SPICE are probably rolling your eyes about now. Fair enough. But your input would be appreciated.

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The Newcomer's Forum / Re: More datasheet questions
« on: February 04, 2010, 03:29:09 AM »
I know it's not a voltage question, but it is a datsheet question and I figured better here than to start a new thread.

What about the closed loop gain rating seen on the datasheets for various chip amps? For the TDA20xx series, it's specified with a min, typical and max in dB. Is the minimum gain specified on the sheet mean that's as low as you can set the closed loop gain and any attempt to set it lower will result in some internal configuration in the chip taking over and keeping it at the max closed loop gain? Or is it more like the max voltage ratings, IOW, go too low and you risk damaging the chip, having stability problems, or other badness? Or is this a thing which can vary from manufacturer/chip family/lunar cycle to the next?

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Amplifier Discussion / Re: Modifying a First Act MA104
« on: January 11, 2010, 01:57:16 AM »
Success.  :)

C8 is now .047µF, and C10 is .0022µF. According to the simulation, this produces a notch -14dB down at about 330hz and spreading two octaves from center with the tone knob on 10. Perhaps not ideal, but it definitely gives the tone knob a usable sweep. I ran some more simulations and found that with C8 at .1µF, C10 at .001µF, and R7 at 6.8K (which I think is a standard value) I could make the tone knob approximate a Fender tone stack with the mid knob set at 2. I'm not sure how much more I want to play with the tonestack without changing to a better speaker.

Either way, the amp sounds much better and useful now.

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Amplifier Discussion / Re: Modifying a First Act MA104
« on: January 10, 2010, 04:31:12 PM »
I've done some more tweaking of the circuit. I first tried replacing C8 with a .01, thinking it would get me more highs. It did, slightly, but it didn't cure the muffled/muddy sound I'm still getting from the amp, and worse reduced the audible effect of sweeping the tone knob to almost nothing.

So I did some more reading and finally figured out how to use LTSpice to do more than just draw circuit diagrams. I made a schematic of just the tone stack starting at C7 and ending at the wiper of VR3, then did an AC analysis with sweeps for a few different positions of the wiper of VR3 to see just what the heck the tone stack is doing. I'm actually amazed by its simplicity. At one end of VR3's sweep the tone stack is a simple high cut filter. At the other end, it is a notch filter with the notch gradually shifting forward flattening out to smoothly transition into the high-cut filter. Thinking about how I run my EQ's on the other amp, it actually makes sense. Clean sounds tend to have a mid-range scoop to counter the mid-range emphasis that magnetic pickups supposedly have for a more natural, balanced sound. But when running heavy distortion, I tend to pull my highs back a bit to keep the distortion from sounding too harsh. So yeah, once again, the First Act designers knew exactly what they were doing.

By changing C8 to the same value as C10, I pretty much flattened out the mid-range notch. All lows and mids with rolled off highs sounds pretty muffled. Playing with a few values in the simulation I found out I actually want C8 to be a larger value than C10. Yes, a larger C8 rolls off more highs, but the greater the difference, the notch gets narrower and shifts it down in the frequency range. There are other places in the circuit I can get my highs back or reduce lows to keep the amp from sounding muffled. Also, changing C7 to a smaller value has a negligable (<-3dB) on the bass until the cap gets down into the .1µF range and lower. If I change C5 and C7 to 1µ, I get the bass down about -1.5 dB. I think I'd have to change C15 and C24 to 1µF to as well to see a -3dB reduction in bass. It'd be easier to change the input caps at each gain stage from .022µF to .01µF, methinks. So I'm not going to bother with C7 just yet. I'll play around with the values of C8, C10 and R7 in the simulation and see if I can't tweak the tone stack to my liking in the simulation, then I'll try it and report my findings.

Yes, I know I'm wasting my time. This is a fun learning experience.

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The Newcomer's Forum / Re: Voltage supply rating on datasheets
« on: January 02, 2010, 02:30:47 PM »
So then an opamp, output chip or whatever else with a supply voltage rating of 18V used as a voltage amplifier could only swing a max of 18Vpp assuming it received an 18V supply, but one with a +-18V supply rating could swing 36Vpp, again assuming it received its maximum rated supply voltage.

In the various schematics I've looked at, I don't know that I've ever seen any op amp or chip amp run at its maximum rated voltage supply. This makes sense to me as running things at their max all the time leaves no room for error. If ever there was a momentary surge from the power supply for whatever reason, you'd risk blowing the device. I'd like to think that the manufacturer's ratings are somewhat conservative, but better safe than sorry. So, that said, is there a general rule that's considered good design or best practices when deciding what voltages to run devices at, (e.g. so many volts under the max, a fixed percentage of the max, etc.)?

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Amplifier Discussion / Re: Modifying a First Act MA104
« on: January 02, 2010, 01:14:33 PM »
While I would generally agree with that, until their forward voltage the diodes should do nothing. So the 1N4148s should have no effect on the signal until the signal coming out of the op amp reaches 1.3-to 1.4Vpp. The bridge humbucker on my guitar isn't that hot as is evidenced by there being no clipping with the gain knob is turned down until the volume knob (located after the clipping diodes) is turned up. As such I want to look at the gain of the output stage or the opamp before it. But I'll try lifting D2 and D3 to see if that makes a difference.

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The Newcomer's Forum / Voltage supply rating on datasheets
« on: January 02, 2010, 01:12:51 AM »
I've been looking at datasheets for things like op amps and chip amps the past few days, and some of the specs in them confuse me a bit. Some say something like 18V, others say +-18V. Are these meaning the same thing or something different. I assume that 18V means it can have a total of 18V across it's power supply terminals before taking damage, and that +-18V would mean that the V+ terminal can be +18V, and the V- terminal can -18V, for  total of 36V across its power supply terminals, but we all know what happens when you assume. Am I on the right track here or am I off in la la land?

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