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.

Quote from: Den. on August 20, 2011, 09:38:30 AM
Thanks.  Den.

And you don't have to renew it; looks like there is gobs of it there. Just be careful not to wipe it off.

Quote from: JHow on August 08, 2011, 07:29:45 PM
Personally, I was always fascinated by the "rivets".  I wasn't sure if it was supposed to look like fancy upholstery or kind of medieval theme, but distinctive, nonetheless.

Their job was to be ... riveting. As you can see, it worked.

All that's left to say is: bottle it and sell it. 

Quote from: Kaz Kylheku on August 03, 2011, 11:51:40 PM

Here is an idea: replace your op-amp chip with an IC socket, so you can "plug and play" different chips that are pin-compatible. You can do an instant A-B listening comparison of different ones.

It was not without some reservations that I posted that, and I was originally going to put in a little cautionary note about considering the downsides of adding a bunch more electrical contacts to a circuit than it strictly needs.

Teemu's book has some good words about this:

Quote
Some IC sockets are extremely poor quality and develop intermittency or
high resistance after a very short period of time. Many high-speed OpAmps are also
very sensitive to stray capacitance the sockets introduce. Generally, decision whether
to use sockets or not is a compromise between easy serviceability and reliability.
Sockets should be used only with expensive ICs, ICs likely getting damaged (meaning
mainly those interfering with the outside world, i.e. the input and output stages) and
naturally for ICs that need to be upgraded now and then.

Quote from: nichapat on August 04, 2011, 11:30:22 AM
But I think classD amp does not work well for this application. I have  to go back to AB amp.

Class D, and you still need all that heat sinkage?

In class D, the power transistors act as switches, just on and off. They should generate little heat.

Why do you think it does not work well?

Quote
And for speaker it is normal PA type,15" .

What is "PA type"? Is this an instrument speaker? (Make, model, ...) 15"? Is this for guitar or bass?

Why do you think class D won't work?

Evidently there are some bass guitar amplifiers out there using class D.

Quote
I have just reduce the gain of first OP-amp (Reversed circuit) . And leave the last OP-amp of tube simulation as buffer. So it seem OK for me now.

Good job! So too much front end gain was hissing out. No need to try different op-amps.

If you're itching for cabinet simulator schematics, not necessarily these ones, download the ADA Microcab ones from ADA depot: http://www.adadepot.com/adagear/gearpages/cabsims/ADA-Microcab.htm

The Microcab is based on op-amp resonant cicuits (gyrators).  Gyrators simulate inductance, eliminating the need for hefty inductors in the circuit:

http://en.wikipedia.org/wiki/Gyrator

The Microcab has something like five of these circuits. I think the one connected to the "thump" control is intended to represent the basic speaker driver resonance and the others do some cabinet resonances.

Quote from: nichapat on August 03, 2011, 12:00:37 PM
Thank you so much for all of your advice. This is what I call the power of network. We are friends.
This is the original unit with noisy problem . Sound like "SSSS....." .
I have reversed the circuit as your comment. Sound is tighter and less noise.  . But it is not enough for me so I have to do something further.

Great to hear that there is an improvement.

Still not quiet enough, though? Could it be that the op-amp chip you selected for this project is noisy?

Here is an idea: replace your op-amp chip with an IC socket, so you can "plug and play" different chips that are pin-compatible. You can do an instant A-B listening comparison of different ones.

Quote from: J M Fahey on August 02, 2011, 01:32:17 PM
Let's see it step by step
The 47pF capacitor is not "suspicious" but good engineering practice, lowering gain outside the guitar range and preventing instabilities.

I thought so too, but it's value is too aggressive, relative to the resistance in the circuit, to be doing (just) that. To provide more stabilizing feedback at high frequencies, you can use quite a bit smaller cap value so that this feature does its job outside of the audio range.

Since the circuit already has a treble control, why allow the gain to mess with treble?  If you want to control the upper highs with this box, just add another pot and call it "presence", letting gain just be gain.

You're right about the input coupling cap. It may be the case that the 0.022 is sufficient due to the high input impedance there. The 470 nF going out, though, turns into a bass sucker if it faces a poor impedance in the next device. Suppose the next device has only, say, an 8K impedance. Then 470 nF coupling means 3dB loss at 42 Hz.  Maybe okay for guitar, but not bass.

If the guitar amp already cuts bass, that's fine, but the boxes in front of it don't also have to do that. If you want to re-purpose the boxes for something else, you will be glad they were more generally designed.

People design these kind of boxes with regard to their one single deployment scenario. "It works exactly how I like in my chain in between these two other units, for my instrument, and I will have you know that even the presence-cutting gain is a deliberate feature". 

The trick to finding some of these schematics is to do a Google image search, not a regular search.

Here is one source, where someone posted them to some forum:

http://www.audunmelbye.no/wp-content/uploads/2011/04/rocktron-velocity-300-main-schematic.gif
http://www.audunmelbye.no/wp-content/uploads/2011/04/rocktron-velocity-300-power-section.gif

I found another version of the above which is very similar, but the VR2 and VR4 pots are clearly labeled as reactance. Ah, here we go, I ferreted it up. It was way deeper down in the image search. This one labels the reactance potentiometers as such:

http://www.vagus.it/images/Velocity_300_Schematics.jpg

(I already have copies of all this stuff stashed, of course. Get it while it's hot! Some dude 30 years from now will thank you that you're the only one who has it, haha.)

As for the newer schematics, I also found them in some forum. Some guy who was debugging his Velocity requested them from Rocktron and then posted them because the schematic did not match his unit exactly:

http://music-electronics-forum.com/t16420/

See the "Velocity 300 Sch (2).pdf‎" attachment.

It's a damn annoying schematic. Everything is broken into parts, and there are no clear labels about what connects to what. There are some misspellings, and the identifying legend is in Chinese. But you can easily recognize the pre and power sections.  The drive for the main transistors, which are some kind of darlington device, is an op-amp. I.e. there is no discrete differential input section like in the older Velocity; it's just a big triangle symbol.

This is regarding the Tonemender.

When we examine the schematic closely, it is a silly circuit in a number of ways.

The overall topology is that the signal goes through an op-amp buffer (an amplifier with unity gain), then through some passive tone controls, and through another op-amp amplifier.

This topology means that the tone control is not suitable for instrument level signals, but only for line level signals. (Just like, for instance, an equalizer component from your home stereo: plug your axe into that, and you will get noise, by design.)

Surely enough, the buffer creates a high input impedance, isolating the input from the tone controls, ensuring that the tone controls won't load down your guitar pickup. That's very good. But because the buffer has unity gain, it means that if the input is some weak signal like 200 mV, the signal going through the tone control is at the same level. This means that your tone control has to be completely shielded, just like the conductor in your coax cable and your guitar's cavities.

What you want to do is reverse these two elements. The first thing the device should do is boost the signal through an amplifying op-amp to get it from instrument level to line level as early in the chain as possible. This is where you can have your variable gain also. The buffer can be placed after the tone control, to isolate the tone control's impedance from the next device in the chain.

Secondly, a number of things stand out. Firstly, most of the pots used are 500K. Also, on the gain control, there is a suspicious 47 pF capacitor. Basically, this device looks like it was designed by a guitar player. (``500K knobs are nice and bright in my guitar, so they must be great everywhere.  A tone bleed on my strat's volume knob ensures bright highs when I turn down the volume, so I can just transplant into this op-amp circuit and it will do exactly the same thing!'')

I wouldn't touch the pot values in the tone control because they will change the sound. But the 47 pF cap in the gain circuit will cause some loss of high frequencies when you crank up the gain! The capacitive reactance of 47 pF with respect to a 10 Khz signal is about 339K. That is significant in regard to the 500K impedance of the pot. The cap provides somewhat of an easier path for high frequencies around the pot, thereby increasing the amount of negative feedback for those frequencies, thereby quieting them down. Anyway, you could use a 10K pot here (with a compensating for the 4.7K resistor going to ground).

Bigger resistances invite noise because less current flows through them. One source of noise are resistors themselves: they create what is called "thermal noise". It comes from the random movement of electrons in the material itself, and when it is amplified, it sounds like hiss.  The way you fight thermal noise is to have a decent signal current flowing, so that the thermal noise is small in comparison to the current.  Now in an op-amp feedback circuit, you could achieve the same amount of gain using a 1K resistor over a 2K resistor, or using a 100K resistor over a 200K resistor, or  using a 1M resistor over a 2M resistor. It's the voltage divider ratio that matters. However, the absolute resistance also matters: more resistance means lower quality feedback. The op-amp's input does not have infinite impedance; some current flows through the feedback into it.  Bottom line: smaller pot.

Further reading: http://en.wikipedia.org/wiki/Resistor#Electrical_and_thermal_noise

The coupling capacitors have wacky values. The 22nF input cap is very low. The purpose of the big resistors around it is to mitigate the loss of bass frequency response that this causes, but it won't work very well. The 10M resistor is in parallel with the output impedance of the previous device (e.g. guitar pickup and volume knob). Your 500K guitar volume knob provides an easier path for bass than the 22 nF cap, and the 10 M resistor might as well not even be there. You should remove these silly resistors and put in a 10 uF capacitor.

The 470 nF coupling capacitor at the output is equally silly, as is the 470 nF capacitor at the bottom of the feedback voltage divider. These both suck bass response. Make them 10 uF.

The 100K output resistor is pointless.  It basically means that your device's output faces an impedance that is no higher than 100K, which is a waste if the next device in the chain has a nice and high impedance in the megohm range. Furthermore, your op-amp's output impedance should already be pretty low, so the 100K won't do anything to lower it. You want our output impedance to be as low as possible (ideally zero), and the input impedance to be as high as possible (ideally infinite), for the best possible voltage bridging. Any silly resistors going to the ground at the input or output are just sucking away tone. Coupling caps should always be at least 10 microfarads. You want to block DC, and it's okay to block some very low frequency that nobody can hear, like 3 Hz. But 20Hz and up should nicely pass through.

Lastly, I did a google for this Tonemender and found its home page. Whatever you do, do not build it using the plain perfboard method that is illustrated. This is completely silly. Nobody in their right mind uses plain perfboard, because there exist PC perfboards which not only have copper pads, but also traces connecting those pads in a kind of "breadboard-compatible" layout. You can lay out your components on a breadboard and test the circuit, and then transfer the same layout to this type of board, where you neatly solder everything to the board. The existing traces minimize the jumpers that you have to use, and when you do need a jumper, it is nicely installed through the board like a resistor.

On the other hand, I can't believe someone produced a PC board layout for this device, without correcting any of its obvious flaws. :loco

In addition, the following marketing bull is found on various websites, without any attribution as to where it came from. I cannot find it on the Rocktron web site itself:

Quote
The Rocktron Velocity 300 Rack Power Amp was designed in a joint effort between Rocktron and integrated circuitry design engineer Derek Bowers. The Velocity 300 is the first commercially available amplifier to be based on current-feedback technology. This technology provides extremely high-speed, wide-bandwidth amplification. The bandwidth of the power amp section is considerably higher than typical amplifiers based on voltage feedback designs therefore providing much more punch and definition than previous designs have allowed for.

What? First commercially available amplifier with current-feedback? Just yesterday I was looking at some Roland JC-120 schematics from 1979 with CF. (teemo piqued my interest.)  Straight-faced lie.

Okay, never mind the claim that it is first. I'm pretty sure the thing does not have current feedback!!! There are no obvious signs of any current feedback in the old 1990's schematic or the newer one. Unless the schematics are a decoy.

Bandwidth? Lots of audio voltage amps have bandwidth way beyond 20 Khz.

QuoteAnother of the Rocktron Velocity 300 power amp's innovative features lies in the reactance simulation circuitry (controlled by the front panel "Reactance" control. At the minimum setting of the reactance control, the Velocity 300 will provide the added punch and improved transient response of the high-speed current-feedback design. As this control is turned up, the Velocity 300 will begin to simulate the interaction that takes place between the tube amplifier and the guitar speaker cabinet.

If so, the schematics are phony.

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

Interesting stuff!

I started looking for some schematics because of this.

The vintage ones from the 1970's are harder to find. Here is a time saver for ya: google for the file name "Roland JC-120.zip".

http://www.current-drive.info/

xP

Enjoy ...