Please sanity-check my current feedback mode for Alesis RA-100.

[Kaz Kylheku]

Hi all,

I'm trying to add a damping control pot to my Alesis 100 amp (one channel for now; if it works, then the other channel too).

I have reverse engineered enough of the amp to find the feedback path. The feedback goes through a 39K:1K voltage divider to feed 1/40th of the output stage voltage.

I'm going to rip out the 39K resistor and patch into there. The 39K resistance will be replaced with 100K to reduce the voltage feedback a little bit, so the current feedback is more significant (without having to use a larger current sense resistor that makes more heat!)

I plan to use a 0.22 ohm, 5W current sense resistor, and use a 50K pot.

The schematic is attached.  The terminals on the left are where we patch in place of the 39K resistor. The terminals on the right represent where we patch into the speaker return circuit and obtain ground.

Any advice is kindly appreciated.

[Kaz Kylheku]

By the way, I should be able to use electrolytics for C1 and C2.  I was just being general when I put those in.

The original voltage divider (whose top resistor we are splicing out) has one at the bottom, below the 1K resistor, indicating that the feedback termination point provides DC.  I.e. even if the output stage of the amp swings deeply negative, that does not cause the much smaller sampled feedback voltage to go sufficiently negative to offset the DC and reverse-polarize the cap.

When I get home, I will be sure to measure what that DC level is.

[Kaz Kylheku]

No negative feedback from anyone!

Here is what the partially constructed project looks like. The hookup wires go to a connector (not shown). The 5 pin connector on the board, left to right, is FB-OUT, FB-IN, POT-GND, POT-WIPER, POT-HOT.

I reversed the order of the coupling cap and resistor, because it was convenient for assembly. An electrolytic 10 uF 50V is used.

I dropped the pointless capacitor in series with the 50K pot.

Apologies for the large dimensioned schematic image in my earlier post, by the way.

[Kaz Kylheku]

Well, here is a pic of the kit, ready to go into the amp for a smoke test.

In the chassis, the matching connector is already spliced between the left speaker return and ground.

Cross your fingers! xP

[Kaz Kylheku]

Passed smoke test!

Furthermore, hooked it up to pre-amp, guitar and speaker cab. @#$%-ing works!

Now I will be playing with the pot to see if I can hear speaker damping being affected.

Anyway, I haven't played guitar in like three days, damn!

I told myself: no guitar until this is finished.

[Kaz Kylheku]

Now I will be playing with the pot to see if I can hear speaker damping being affected

It is. Quite terrific!

Firstly, the amp is more alive even in the maximum damping position. I expected this: I changed the voltage divider to feed back less voltage.

When you crank the damping control, there is noticeable drop in gain.

The speaker resonance on various notes is obvious.

My speakers resonate particularly well around C and D. (Think A-string rooted power chords at 3rd and 5th fret). The C and D roots are almost indistinguishable due to the speaker going ape-*s!!t*, and the obvious "oomph" is there long after you mute the note.

Way different beast from the stock unit.

Now I have to drill the hole to mount the damping control.  Also, put some feet on the new circuit board and fasten it in place.  Oh, and do the other channel.   That damping pot is double-ganged for a reason! Heh ...

For time being I will keep the right channel stock to do some A/B comparisons.

Now I will be playing with the pot to see if I can hear speaker damping being affected

It is. Quite terrific!

Firstly, the amp is more alive even in the maximum damping position. I expected this: I changed the voltage divider to feed back less voltage.

When you crank the damping control, there is noticeable drop in gain.

The speaker resonance on various notes is obvious.

My speakers resonate particularly well around C and D. (Think A-string rooted power chords at 3rd and 5th fret). The C and D roots are almost indistinguishable due to the speaker going ape-*s!!t*, and the obvious "oomph" is there long after you mute the note.

Way different beast from the stock unit.

Now I have to drill the hole to mount the damping control.  Also, put some feet on the new circuit board and fasten it in place.  Oh, and do the other channel.   That damping pot is double-ganged for a reason! Heh ...

For time being I will keep the right channel stock to do some A/B comparisons.

So, everything worked, from design, to building, to working the first time on power-up with no debugging. :tu:

[phatt]

The best I can make out of your posted schematic is you have simply added a big fat resistor across the speaker which will just over heat the output devices,,to say the least.
You have no ground point that is clearly marked.
As such it's very hard to make a valued comment.
Phil.

[Kaz Kylheku]

The best I can make out of your posted schematic is you have simply added a big fat resistor across the speaker which will just over heat the output devices,,to say the least.
You have no ground point that is clearly marked.
As such it's very hard to make a valued comment.
Phil.

Sorry, it's not a clear schematic, because it's basically just an excerpt of the whole system covering just the device being built and spliced in.

The terminals on the left represent the replacement of a 39K resistor, which is connected directly to point between the output devices: i.e. takes the output voltage.  This is the top resistor of the amp's voltage-feedback-sampling divider; so our "feedback return" terminal connects to the 1K bottom resistor. This has no DC ground, but an AC ground via an electrolytic cap. You can see that I've replaced the 39K resistor with a 100K one.

On the right side, the terminals are not speaker terminals, but the speaker return terminal and a ground. Normally the speaker returns to the ground. Here, we are spliced in between the speaker and ground. So the high wattage, small valued resistor is in series with the speaker: it is sensing the current and converting it to a small voltage, which is further sampled by the potmeter. This is the basis of the current feedback.  The output devices are not being destroyed. The GND terminal is a real ground to the power supply; I just didn't use the ground symbol anywhere.

[phatt]

Sorry chum but without a clearly defined ground your circuit means little so it is pointless for me to make any comment.
Phil.

[J M Fahey]

It will work, sort of, but your 50K damping pot is *way* too high, in practice it will work like a switch in both ends (max/min damping) .
You should use a 500r or 1K pot there (no more than half the R2 value).
As-is, you *will* hear a sound change in intermediate positions, but it's not what you think.
Good luck.
PS: try to keep schematics to screen-friendly dimensions, say, 640x480 or less for simple ones, maybe twice that for those which should be scrolled.
Good luck.

[Kaz Kylheku]

It will work, sort of, but your 50K damping pot is *way* too high, in practice it will work like a switch in both ends (max/min damping) .

Yes! I had exactly the same thought when I woke up this morning. At 50% position, we have 25K of the pot on top, but the bottom 25K is in parallel with 3K. Completely wacky.

I just used this because I had it on hand (bad reason, I know).

Gee, I'm as dim-witted as that guy in the other thread who wants to but a big ohm volume pot at the power end of his amp.

Thanks.

By the way, wow, how on earth (no pun intended) were you able to make a useful comment without a well-defined ground? 

[teemuk]

I agree, use smaller potentiometer resistance. Additionally, instead of using the typical volume control -style circuit to control the amount of CFB, you could just try wiring the pot as a rheostat in series with the 2K resistor. Most of those caps look redundant too, more than that, at the extremes of the potentiometer's dial they likely introduce some wacky RC filters that could hinder the intented operation. I'm quite sure you don't need at least the one that couples the 50K potentiometer to ground.

[Kaz Kylheku]

I agree, use smaller potentiometer resistance. Additionally, instead of using the typical volume control -style circuit to control the amount of CFB, you could just try wiring the pot as a rheostat in series with the 2K resistor. Most of those caps look redundant too, more than that, at the extremes of the potentiometer's dial they likely introduce some wacky RC filters that could hinder the intented operation. I'm quite sure you don't need at least the one that couples the 50K potentiometer to ground.

I made a note of that ground cap being useless in an earlier posting. The actual circuit I built does not have that capacitor, but does retain the coupler.

I did see the rheostat-like current feedback control in that high-level diagram in your book ,  but I wanted to be able to turn the current feedback completely to zero, so the Alesis can be returned to something similar to its original personality (modulo the diddled voltage feedback amount).

It would be fun to add presence and resonance to the circuit board, what the heck. I would probably want a Baxandall style tone control that is flat when the pots are centered (again, preserving the personality).

Thanks for the comments! And the book.

[Kaz Kylheku]

It's too troublesome to get a 1K pot which is dual-ganged, and I don't want separate controls for the left and right channel.

I think what I will try is to increase the impedance in the voltage divider into which the pot is mixing current feedback.

Now it's 100K:1K, but what if I just change it to 1M:100K, and make the coupling resistor 200K. Basically multiply everything by 100 except for the pot. So, change one resistor on the power amp board, two in my circuit, and I should be able to keep the 50K pot.

The one little uncertainty in this idea is the impedance of the transistor circuit where the feedback ends up, but I suspect it's decently high.

[J M Fahey]

Please don't multiply feedback values by 100, it does not work that way.
Although the power amp is *some* kind of "big Op Amp", it's not a TL071 or similar Fet input one by a long way.
It's been optimized for *Power* (duh), it's not an instrumentation type device or similar.
If you can't get a dual 1K pot, consider a 5 or 10 pole dual rotary switch, with some convenient resistor values.
Anyway there will not be *that* much variation in sound, from one position to the next.