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Messages - rowdy_riemer

#16
Of course, this idea might not be so applicable to solid state power amps and might be better posted on a tube amp forum, but then again, maybe not. It used solid-state components.

Also, even if there's some reason I'm not quite on the right track here, shouldn't there be some way of providing a reactive load that mimics the reactance of the speaker via feedback?

A somewhat related idea I recently had was to use some digital means of providing a reactive load as well as vary the frequency response of a small-signal amplifier for a line out to use as an active-load speaker emulator. Perhaps an arduino board doing some frequency counting could do the job. You could then programatically change the behavior of the active load and frequency response of the line-out as you wish to mimic several different speakers.
#17
Ok, this was just a spur-of-the-moment idea, and I figured I'd run it past y'all to see what the problems with the idea were.

The idea behind this is that R1, RS1, The capacitor I forgot to label (let's call it C1), and Q1 form an active load. R2 allows us to bias Q1, but is high enough to not provide a significant shunt to the signal fed across C1. The signal applied to R1 is also fed to whatever I use to attenuate the signal. The output of that is fed to a power amplifier, perhaps a simple source or emitter follower. The power amplifier feeds the speaker, which is in series with RS2. The voltage between the speaker and RS2 is amplified by a factor that is the reciprocal of the factor by which the attenuator attenuates. The amplified voltage is then compared to the voltage between R1 and RS1. The output from the comparator is tied to the gate of Q1, so that the conductance of Q1 is increased/decreased to reduce/increase the load from R1/RS1/Q1.

The idea behind this is that the load caused by the active load mimics the speaker's load. The impedance of this load should be about the same as if you plugged the speaker directly into the amplifier.

Any ideas?
#18
Ok, let me post a drawing (PLEASE FORGIVE MY CRAPPY DRAWING). Then, I'll explain.
#19
Amplifier Discussion / Re: Light Bulb Limiter
March 25, 2011, 12:14:15 AM
Sorry for the late comment, but this is cool, JM.  :tu:
#20
I added a tone control to this circuit for a treble booster. Here's the new schematic:



The tone control looks a little odd, but seems to work well. Here are bode plots (from simulation) with full bass cut and no bass cut:



With peak cut, this circuit has about 23.4 dB of gain at about 4.8 kHz and -4.5 dB of gain at about 32 Hz (according to simulation). When I put the real thing in front of my Valveking Royal 8, the bass cut makes a big difference in "de-muddifying" the sound. And with a mostly clean output voltage at around 16 Vpp, the Valveking has distortion suitable for Metal.

This is my first time really trying to roll my own tone control. Hopefully, my lack of experience isn't showing.  :) Either way, I like the results so far.
#21
Amplifier Discussion / Re: Help needed!
March 07, 2011, 05:38:13 PM
Well, the zener should be parallel with your circuit. It clamps the supply voltage to it's rated voltage. As your preamp draws more current, the zener draws less as less is needed to clamp the voltage. By pulling the supply voltage down to +- 7.5V, you do not have much headspace, so you get more clipping. What is the supply voltage when you use voltage regulators?
#22
Amplifier Discussion / Re: Help needed!
March 07, 2011, 03:42:00 PM
I don't think the zeners will have any effect on the sound other than to limit the voltage "seen" by the op amps. What voltages do you measure accross the zeners?
#23
Amplifier Discussion / Re: Help needed!
March 07, 2011, 11:44:07 AM
Why not use IC regulators?
#24
If you need to stick to a 12V supply for whatever reason, maybe you should look into using two IC's in a bridged design.
#25
Schematics and Layouts / Re: Digital JFET Test Rig
February 25, 2011, 12:41:38 PM
I sent an email earlier to the email address listed on their main page suggesting that they offer such a service regardless of what testing rig design they come up with. I included a link to this thread. Mine is certainly simple and certainly not complete enough to start feeding reels of JFETs through. But they are certainly welcome to use as much of my design as they want.
#26
Schematics and Layouts / Re: Digital JFET Test Rig
February 25, 2011, 10:37:35 AM
Quote from: J M Fahey on February 25, 2011, 10:28:57 AM
...
*Why* Fet manufacturers don't offer this service is beyond me.
...

This is something vendors like Small Bear could do, since their customers are primarily builders of stomp boxes and amps. They might could sort JFETs into groups by narrow criteria and tack on a few more cents on the price. I think I would pay an extra 20 or 30 cents per JFET if I knew what I was getting rather than order a few hundred and have to measure them to find the few that I want to use.
#27
Schematics and Layouts / Re: Digital JFET Test Rig
February 25, 2011, 10:32:20 AM
Thanks, JM.  And thanks Joe Cool (fogot to thank him earlier for saying this project was neat).
#28
Schematics and Layouts / Re: Digital JFET Test Rig
February 25, 2011, 10:01:16 AM
Well, it's eventually supposed to be a means for measuring them more quickly. It might only eliminate a few steps, but if you have a few hundred JFETs to measure, eliminating a few steps can make a big difference. Also, suppose you are mass producing amplifiers and you want to use JFETs in your design. If you have a test rig with a reel feeder, you can send a whole reel of these through, and quickly find the ones whose characteristics are in the range deemed acceptable for your design.

Obviously, with my bread board setup, it's not very fast. This is mainly because I have to pay close attention to where I place each JFET on the breadboard. Also, I don't have a spare momentary switch button set up yet to trigger the test, so I have to type in 't' in the serial port monitor on my lap top, and click the send button. But I can through this design on some perf board, put that in an enclosure, and have a box with a button and a socket for the JFET. And, of course, a way to connect the arduino board. Then, it is as simple as inserting the JFET into the socket and pressing the button. The lights will indicate when the test is complete. Then pop in the next one. The output from the arduino has all the measurements, so as long as I keep the JFETs in order, I don't even have to write the measurements down. I don't have to worry about screwing up dividing the voltage by 100 for the Idss test. Of course, that's simple math, but when you're measuring a whole bag of JFETs, brain-fart mistakes happen.

I think I might can even eliminate the pressing-the-button-to-start step. I bet, in between tests, I can check for a voltage on the analog input of the arduino, and trigger a test when it reads a voltage. I can adjust the trigger code to only trigger if there is a voltage for a few thousand samples to eliminate false triggers from interference on the analog input.
#29
Schematics and Layouts / Re: Digital JFET Test Rig
February 25, 2011, 01:03:49 AM
I'll try to remember to post the source code later.
#30
Schematics and Layouts / Re: Digital JFET Test Rig
February 25, 2011, 01:01:07 AM
I hooked this rig up to my Arduino tonight. When I did my testing yesterday, I forgot Arduino has 5V digital outputs. Initially, using the Arduino to enable the test rig, cut off voltages were a bit low. I used an LM324 comparator to bump up the 5 volt Arduino outputs to 12V. This seemed to work much better.

My arduino program (sketch in Arduino terminology) enables the test rig, makes sure the mode output is low, measures the cutoff, sets the mode output high, measures Idss, and then sets the enable output low. It writes the results to the serial port (well, ftdi chip connected to the PC via usb connection). The measurements are slightly lower than with the multimeter and standard measurement rig. I don't know which is more accurate. I bought my multimeter from a flea market years ago, so I don't have a lot of faith in it.  :) In any case, I imagine the measurements are close enough.

Here's a demo with the arduino added to the test rig. http://www.youtube.com/watch?v=OVMhmgWExIo