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

[Kaz Kylheku]

Please don't multiply feedback values by 100, it does not work that way.

This is a good discussion topic. Can you explain why, for sampling the voltage to be fed back, a more resistive voltage divider can't used which has the same ratio between the resistors? Broadly speaking about amplifiers in general, what are the issues to watch out for with doing that. I'd still like to try it. If it doesn't smoke, it's good!  I should be able to tell if the amp becomes obviously more gainy. If it oscillates, I will hopefully be able to kill it before it kills the speakers.

(One obvious issue might be that the feedback input may have a lower impedance than expected, which could makes the more resistive voltage divider less tall, so to speak. I.e. the same issue as with the potentiometer being too resistive is just relocated somewhere else, but now causing a real problem: less negative feedback, destabilizing the amp.)

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.

Yes, no FET input there. You cannot count on megohms of input impedance.

[phatt]

Quite obviously some of this is over my head ,,,but my guess is also quite simple.
This amp is going to go poof fairly soon if he keeps fiddling with it.

Hint,,, don't mess with DC coupled power Amps unless you know *EXACTLY* what you are doing.
I took out at least a dozen Power TR's before I got wise.

From memory a $5 power transistors will blow about 100 times faster than  a 5 cent fuse.
Cheers Phil.

[Kaz Kylheku]

Experiment settles it: too much overall resistance in the feedback path is very harmful.

Firstly, the diminished current in it causes the feedback path to pick up hum (bad S/N ratio).

Secondly, frequency response is impacted quite severely: highs are rolled off.

Third, although there doesn't appear to be a noticeable rise in gain, the amp's biasing appears to be destabilized. It normally runs cold, but with this change, its output transistors warm the heat sink. I was watching for that and shut it off immediately.

[J M Fahey]

Well, it sort of answered itself.
The Lin structure power amp (99.9% of all modern amps) is built out of 3 basic blocks:
* an input differential stage, which compares (substracts) the input signal with a sample from the actual output signal which is driving the speaker (negative feedback), generating an error correcting signal (very intelligent chap, this Lin)
* a very high gain stage, basically a single transistor, which provides most of the amp open loop gain, plus the full rail to rail voltage swing
* since you already have all the peak to peak or rail to rail your power supply can give you, you only need to amplify the *current*, from a few mA into a couple to tens of Amperes, as needed to drive the speakers fully.
* the input differential stage has 2 inputs ; one of them gets the input signal, the other gets the feedback signal, which is the output signal attenuated by a couple resistors, plus some capacitors if needed.
* the input impedance of said feedback input is usually not very high, say 10 or 20KOhms.
A classic feedback net has usually the input to ground resistor between 5K and 100 ohms, so it's affected very little by the NFB input impedance, but if you multiply everything by 100x .... you now know what happens.
Also any stray capacitance, say in the order of 100pF which could previously be ignored, now becomes a highs killing monster or the contrary, an unstable nightmare.
In a FET input TL071 (as an example), the input resistance on either input is around 1000000000000 ohms. No, it's not a typo, one million megohms, there's 12 zeros there.
http://www.datasheetcatalog.org/datasheet/stmicroelectronics/2296.pdf
Even so, if you use a feedback loop of, say, 20Meg/1Meg , I'm *sure* you'll lose highs because even the tiniest real world stray capacitance you'll get there, will kill all your sparkle and then some.
But now you know that, of course. 

[Kaz Kylheku]

Hi all!

Sorry about the necropost. I just wanted to drop a note in here that I revisited the mod recently and reworked it. I got rid of all the complexity (including the bias current cancellation jig I built (for the left channel only)).

I now just have a simple switch (per channel) to enable current feedback.

This just propagates the signal from the top of the 0.39 Ω current-sensing resistor through a 0.47 uF capacitor and 1 kΩ resistor to the feedback point in the amp.

I also restored the 39 kΩ feedback resistors in the amp boards; i.e. the feedback resistors are no longer relocated to my circuit board. That's a bad thing; you want to keep the core feedback structure in one tight location.

I kept the circuit boards in this rework, though, honestly, if I were to do it from scratch, honestly, I would probably go point-to-point.

Since I built the original mod, it wasn't long before I stopped playing with the variable knob. I ended up using it as a switch: in one extreme position or the other. Then, I stopped fiddling with it at all and just had it in the maximum voltage mode for years. I got a good tone with external EQ.

Because I made improvements elsewhere in the signal chain, plus gained a decade of experience tweaking a 31 band EQ, I didn't need the current drive as a band-aid for tone problems. But I wanted to revisit that again, and have a proper and simple implementation. I'm now using it again, and liking it. I still find voltage mode useful; it can be used as a kind of mid boost that brings the guitar into the foreground, but with less presence, for smooth leads.

[J M Fahey]

Thanks for posting.
We may _guess_ all day long but at the end "the proof lies in the eating"

Or, as we say in Argentina: "en la cancha se ven los pingos" literally meaning "racing horses are proven in the racetrack" 😉

[Kaz Kylheku]

So I've been using the revamped mod for a couple of weeks, in current drive mode.

For a long time, I had the rest of the signal chain set up for the best possible sound in voltage mode, but that turned out to have way too much scoop under current drive.

I took the mid control of the ADA MP-1 from -12 (max cut) to only -8.

I also knocked down the presence from +4 to 0.

Now the overall tonal profile is similar to before, only way better sounding.

When I switch to voltage mode now, it's a joke. Blatty midrange with poorly defined highs, overly tight bass.

Current drive is cool: it shapes things so you need less EQ in the front, and does things that are hard or impossible to get with EQ in the front.

In The MP1, the EQ is  +6 -8 0 0 (bass mid treble presence).   Bass is 180 Hz and below, mid is centered on 600.

My 31 band EQ looks like this; plus the SMF-1 unit below has a two-pole low-pass filter (leftmost knob) set to around 7 kHz. That knob is the first thing to reach for when adjusting presence.