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Cheebatone
01-10-2007, 05:00 AM
Sorry for re-hashing a discussion I've had before, but I can't find the thread where it was held and I need the info...

I was always under the impression that Presence was just a super-high Treble control, but a forum member (John Phillips, I think) set me straight. Unfortunately, I can't fully remember what he told me! :o

I think he said that Presence was governed by controlling something called Negative Feeback? Is this right? He also said, whatever it is, could be used to vary the other end of the sonic spectrum as well, and that that is precisely what the 'Resonance' control on the Peavey JSX amps is.

So, anyway, could someone varify or correct me on this, with a brief summary of what I need to know? Please (and this isn't the first time I've asked this) help a dumb-ass...

Cheers!

Charly

John Phillips
01-10-2007, 05:23 AM
Your memory isn't quite as bad as you think it is :).

The negative feedback loop in an amp is a sort of 'damping' (that isn't quite the right electronics term, but it will do) and makes the power section run more linearly, or 'smoothly' - by taking some of the output signal, running it 'backwards' (that's the negative bit) and re-applying it to the input, as a sort of self-regulation.

If you put what are essentially tone controls into the negative feedback loop, you can affect the degree of smoothing at those frequencies. Presence affects the top end, resonance the bottom end. Basically turning either of these 'up' actually turns those frequencies down in the loop, and allows those ranges to be less restricted. They don't sound the same as simple bass and treble controls because they affect the dynamics of those frequencies more than the 'amount' of them - which is why you tend not to hear them doing very much at low volume, but once the power section is really cranked they can become more effective than the normal tone controls.

Cheebatone
01-10-2007, 05:41 AM
Thanks John! :)

I seemed to remember you said something about them only really working well when the amp was on full blast, so my thinking is to have Negative Feedback Loop controls, one each for the top and the bottom (and a regular Mid cut/boost tone control in the middle), in a single valve amp, designed to produce power-amp overdrive/saturation etc, for use in recording and pratice situations.

Will this still work at low volumes, or does it have to be flat-out and very loud?

Jon Silberman
01-10-2007, 06:02 AM
Only thing I would add to John's post is that he's discussing true presence controls. In fact, there are amps out there where what's labeled "presence" is actually a trad high-end treble control.

brad347
01-10-2007, 10:52 AM
to put it another way (in more layman's terms from john's great description):

So you know that two audio waves that are "out of phase" wil cancel each other out, right?

Negative feedback exploits this. Tube amps are hardly "linear" or "flat" when it comes to frequency response. Specifically, there is usually a 'bump' in the midrange. In many hi-fi circuits (and guitar amps starting with the fender brownface amps, since Leo was always looking for a more 'hi fi' sound) there will be a small amount of the output signal "fed back" into the audio path at an earlier point with polarity inverted so that it is out of phase with the main signal.

Since we know that two signals out-of-phase will cancel, this means that a little bit of the audio gets cancelled out. This works to "flatten" the frequency response, since the frequencies that are loudest in the fed-back signal get attenuated most in the overall output. The loudest bits of the fed-back signal are doing "the most cancelling," to put it another way.

Now, if we add tone controls (like a simple capacitor treble-bleed off tone control as on your guitar) to the negative feedback circuit, we can control how much treble gets fed back and "cancelled out." By turning the presence "up" we are actually 'darkening' the signal in the negative feedback loop, making it so that the treble frequencies are "less cancelled." This gives not just a brightness, but an overall liveliness in the top end that cannot be accomplished with a simple treble boost.

You could also experiment with removing the negative feedback in your amp entirely. It only involves disconnecting one wire. For some things, the sound of the "naked" power tubes is just the thing. The speaker will be more responsive, the transition into overdrive from clean smoother, and the amp a touch louder, midrangier, rawer, and 'tweedier' with the NFB dis-engaged. On my BF deluxe, I removed the 'death cap' and routed the NFB through the now-defunct ground switch so I can switch it in or out. I think of it as a "blues/jazz" switch.

Hope this helps!

waveman
01-10-2007, 01:00 PM
So is the a book with some schematics that discuss how to add a presence control to an amp?

brad347
01-10-2007, 01:03 PM
Not all amps have negative feedback. If your amp doesn't utilize negative feedback in its design, then it's impossible to add a true presence control.

axepilot
01-10-2007, 01:05 PM
Great thread and great responses! :AOK

I've noticed that the presence control on my Germino C40 doesn't really come alive until the amp gets up to a good stride.

brad347
01-10-2007, 01:06 PM
also, as far as adding a presence control, if your amp has negative feedback in its design, I would guess it's simply a matter of inserting a regular passive-type tone control in series with the loop. Experts?

marshallnoise
09-06-2008, 04:48 PM
Ok, so when I turn up my presence control above 8 on my Laney, it sounds great and clears up on the distortion channel, but on the clean it introduces a very loud high pitched squeel and so does the resonance switch.

What is up with my negative feed back loop?

brad347
09-06-2008, 05:01 PM
Ok, so when I turn up my presence control above 8 on my Laney, it sounds great and clears up on the distortion channel, but on the clean it introduces a very loud high pitched squeel and so does the resonance switch.

What is up with my negative feed back loop?

You are getting a parasitic oscillation.

See if you can make the wires connecting to the grids of the power tubes just a little bit shorter-- sometimes even an inch shorter is enough to fix it.

If your amp has PCB mount tube sockets, then it's a flaw in the design and you're stuck with it.

909one
09-07-2008, 08:52 PM
I put a switch in the negative feedback loop of my 5E7 Bandmaster Clone. It changes the tone significantly when there is no feedback loop. It defintitely sounds more raw, midrange present and has a little more distortion. It sounds more like an AC-30, which I believe have no feedback loops... But I might be really wrong about that.

abro163
09-08-2008, 05:30 AM
The 5e7 has two feedback loops, one is the 10m resistor the other the more typical 56k at the speaker jack, which one are you talking about? (speaker one I presume)

marshallnoise
09-12-2008, 06:20 PM
You are getting a parasitic oscillation.

See if you can make the wires connecting to the grids of the power tubes just a little bit shorter-- sometimes even an inch shorter is enough to fix it.

If your amp has PCB mount tube sockets, then it's a flaw in the design and you're stuck with it.

Those are the ones going from the output transformer, right? Mine are not PCB sockets.

brad347
09-12-2008, 06:51 PM
No. Those are the plate wires. Those can be long.

The grid wires are the ones connecting to pin 5.

But speaking of those wires connecting the OT to the plates... you can also try reversing those (they might be brown and blue). This may cure your oscillation or it may make it worse.

marshallnoise
09-12-2008, 09:55 PM
No. Those are the plate wires. Those can be long.

The grid wires are the ones connecting to pin 5.

But speaking of those wires connecting the OT to the plates... you can also try reversing those (they might be brown and blue). This may cure your oscillation or it may make it worse.

Ok, the daisy chaning wires that go from tube to tube in the power section. Those look good, but they could be looked at again. What about the preamp tubes? Or were those what you were referring to in the first place?

I recently went through the whole amp (Laney VH100R) and replaced all 6 preamp sockets and the power amp sockets too. It does have a new OT and I left those long in length for a "just in case" scenario. Although, that makes very little sense in the long run. I'll shorten those up appropriately.

brad347
09-12-2008, 09:58 PM
If it didn't do the squealing before you put the new sockets and OT in, then I'd be almost certain that you need to reverse the brown/blue (if that is their color) wires on the OT that connect to the power tube sockets.

marshallnoise
09-13-2008, 12:17 AM
http://www.thegearpage.net/board/showthread.php?t=183750

In that thread, I did as you described with a different OT. While I am not having the severe issues I had before when I had the wires crossed up, I think you might be on to something. This is a different OT with different specs so perhaps the differences in the actual symptoms would be similar but different.

I'll give that a try soon and see what happens.

marshallnoise
09-15-2008, 12:22 PM
Hey Brad, if you see this, you were right about the grid wires. I shortened them by about 2 inches each side and it worked great. I tried swapping the OT primaries to the tubes...NOT GOOD! Darned near went deaf. But once I got it back, it was awesome. I still think I have some tube issues (quality that is). But other than that, the presence works great now on both channels.

brad347
09-15-2008, 12:49 PM
great! Glad to hear it.

Sorry, I should've warned you about the potential violent oscillation that can occur if the OT leads are hooked up backwards! :o

marshallnoise
09-15-2008, 03:58 PM
great! Glad to hear it.

Sorry, I should've warned you about the potential violent oscillation that can occur if the OT leads are hooked up backwards! :o

Scared me poopless! :rotflmao

But, it does not make those noises anymore and I am happy with that.