Negative feedback mod. question

[ASC67]

So I have a Music Man HD-130 210 with the 12ax7 P.I. that I got a little while back. The previous owner installed a neg. feedback switch on the amp. Stock is negative feedback on and when you throw the switch there is no neg. feed back. My question is what exactly does this do and what exactly is neg. feedback and impact on sound without ?
Thanks

 

[callaway]

Well, if you can't tell the difference in the sound between the two positions of the switch, then your answer is effectively that it does nothing for you

Theoretically, negative feedback provides a higher bandwidth and increases headroom, making the amplifier closer to an ideal pure gain stage. It also tends to tighten up the bass, making it more punchy or snappy. The last thing I can think of is that it increases the odd-order harmonic content and can be a little noisier. (The increased white noise is always a trade off with increased bandwidth.)

In practical terms, no feedback should feel looser in the bass, probably warmer up top, break up earlier, and generally sound a bit fatter. Adding negative feedback should make the amp more punchy and give higher headroom.

 

[celeste]

Negative Feedback NFB is a way to lower distortion, reduce output impedance and lower gain of the stages it is applied to. What it is, is taking some of the signal and feeding it back to an earlier stage where it will be out of phase with the signal there.

What it does is makes the stages it is applied to stay clean longer, but snap to distortion more abruptly, you have less of that soft compression that builds up as you dig in. That change happens quicker and at a higher volume.

 

[Wally]

Her's the schematic for that amp. IT appears to me that swtiching out that NFB loop might not have a great effect. I coudl be wrong. If so, I am hoping that Celeste will give us the benefit of greater understnding, but it looks to me that that NFB has quite a bit of resistance going on, and so removing it won't have as much effect as say increasing by removing or changing the resistance in a TR's NFB loop would have. What say Celeste? I can't find my notes on how to figure the total effect of that loop.

http://www.schematicheaven.com/newamps/musicman_2100-130.pdf

 

[callaway]

Oh, an interesting note is that a presence control is usually implemented as a pot and cap hanging off the feedback node. As the resistance decreases, more high frequencies are shunted to ground. This effectively makes the amplifier have negative feedback on only the low and mid-frequencies, but not the high end. This makes the whole high end sound brighter.

 

[JohnnyCrash]

I wrote an article on the main TDPRI page about this a little while back.

As others have mentioned, negative feedback widens the frequency bandwidth (lower bass, higher treble). It also smooths the frequency response, which means it evens out frequencies across the board (the lows, mids, and treble are all at sorta more equal volumes). Finally, it increases clean headroom. This is not to say your amp will be louder overall. Instead this means overdrive is held at bay for more of the turn on the Volume knobs.

Finally, theoretically, negative feedback is sort of "defeated" or negated when the amp starts clipping (ie distorting). This sort of depends on where in the circuit the NF loop is inserted, the amp, the NF circuit (output stage to preamp or locally like a preamp's cathode bypass), where overdrive occurs (in the preamp for instance), and a few other variables. In almost all cases, in my experience, even when the amp is fully dimed I still hear a difference either with NF or when it's off... mostly in frequency/tone and not in overdrive.

Cathode bypass capacitors are a form of NF and in cases like that it increases preamp gain, therefore overdrive - again, when and where in the circuit overdrive/clipping occurs matters. This might seem counterintuitive to the claims that NF increases headroom. Also, when used in cathode bypass circuits it is usually frequency dependant.

Presence controls are NF with a capacitor to select which frequencies are not fed back, therefore it sound like they are increased (cranking a Presence knob usually sounds more bright and trebley). Resonance controls are the opposite of Presence controls and are NF where bass freqs bypass being fed back and sound more pronounced.

In the end, it matters what you hear. What differences do you notice at different volumes with the NF on or off?

 

[DrewB]

In almost all cases, in my experience, even when the amp is fully dimed I still hear a difference either with NF or when it's off... mostly in frequency/tone and not in overdrive.

It's definitely an audible difference in a lot of the range of the volume pot. In my case, disconnecting the loop in my 5F1 build made all the difference. The amp is dirtier, thicker and, at the same time, brighter and clearer through the full range of the volume pot. It used to be fairly muddy below 1:00 (the cleans were useless); now the cleans have a nice sparkle to them. The whole amp sounds better, period.

 

[ASC67]

Thanks for the responses,
Well, with it turned off the amp is a bit brighter and more open sounding if that makes sense. Since this amp is such a clean sounding amp it doesn't seem to effect breakup , at least at the volumes I've had it at.

One other question I have,
This amp has the high low power switch. When in the low power mode there is a bit of a back ground fizz that seems to be related to the pre- volume. When you turn the volume all the way off , if you pluck the string it makes this fizzy sound. Now in the the high power setting it doesn't do this at all. Any ideas as to what would cause this ?

 

[Wally]

Johnny, I have never thought of bypass caps as a sort of negative feedback control.
These caps are not reinserting signals in order to control through phase cancellation any part of the signal, are they? In my limited understanding, they are more or less a frequency selective constant gain function...for those frequencies selected. Bigger caps pass more lows. Smaller caps pass higher freqs. Whatever the frequency chosen by the size of the cap, the effect is one of gain..... versus phase cancellation through application of the inverted signal from a later stage of the amplifier into an earlier stage which is what a negative feedback loop does, right?
Willing to learn if there is a different explanation.

 

[JohnnyCrash]

Johnny, I have never thought of bypass caps as a sort of negative feedback control.
These caps are not reinserting signals in order to control through phase cancellation any part of the signal, are they? In my limited understanding, they are more or less a frequency selective constant gain function...for those frequencies selected. Bigger caps pass more lows. Smaller caps pass higher freqs. Whatever the frequency chosen by the size of the cap, the effect is one of gain..... versus phase cancellation through application of the inverted signal from a later stage of the amplifier into an earlier stage which is what a negative feedback loop does, right?
Willing to learn if there is a different explanation.

I mispoke. Without bypass caps, negative current feedback occurs... in this way the NF circuit in a preamp's cathode is a little different.

Adding a large bypass cap decouples the cathode. Using a small cap instead only partially bypasses the cathode. That is to say, this only bypasses the frequencies above the cap's shelf and does not bypass the lows - in this way it reacts just like a Presence control by bypassing that upper shelf of frequencies from NF whereas freqs below that cap's shelf are reduced by NF.

 

[celeste]

Cathode bypass capacitors are a form of NF and in cases like that it increases preamp gain, therefore overdrive - again, when and where in the circuit overdrive/clipping occurs matters. This might seem counterintuitive to the claims that NF increases headroom. Also, when used in cathode bypass circuits it is usually frequency dependant.

Johnny, you have it backwards, an unbypassed cathode resister is degenerative feedback and the cap shorts it out. The cap and the cathode circuit impedance form a highpass filter for the feedback, so a small cap will short out high frequency feedback and give more gain there.

The headroom thing make since if you think of it as feedback holds off distortion till the tube runs out of gain, then it collapses and you get a rapid transition to distortion.

 

[celeste]

Her's the schematic for that amp. IT appears to me that swtiching out that NFB loop might not have a great effect. I coudl be wrong. If so, I am hoping that Celeste will give us the benefit of greater understnding, but it looks to me that that NFB has quite a bit of resistance going on, and so removing it won't have as much effect as say increasing by removing or changing the resistance in a TR's NFB loop would have. What say Celeste? I can't find my notes on how to figure the total effect of that loop.

http://www.schematicheaven.com/newamps/musicman_2100-130.pdf

This thread got posted in amp central as well and I lost it, not seeing my reply there, I thought I had clicked on preview rather then submit. Sorry it took so long to reply

Wally, given that the NFB is taken from a 8 ohm tap and not a 16 ohm, and that there is a 5:1 voltage divider there is not much feedback there.

I have to say I do not use global feedback. I much prefer local feedback options when I want what NFB brings to the table, cathode feedback, UL, E-linear, anode to grid cap to name a few. So I would have to look up the math for loop gain and feedback ratios as well.

 

[ASC67]

One other question I have,
This amp has the high low power switch. When in the low power mode there is a bit of a back ground fizz that seems to be related to the pre- volume. When you turn the volume all the way off , if you pluck the string it makes this fizzy sound. Now in the the high power setting it doesn't do this at all. Any ideas as to what would cause this ?

Could it be bias related ?

 

[JohnnyCrash]

Johnny, you have it backwards, an unbypassed cathode resister is degenerative feedback and the cap shorts it out. The cap and the cathode circuit impedance form a highpass filter for the feedback, so a small cap will short out high frequency feedback and give more gain there.

The headroom thing make since if you think of it as feedback holds off distortion till the tube runs out of gain, then it collapses and you get a rapid transition to distortion.

Ah, thank you!

Always learning... and putting my foot in my mouth...

 

[celeste]

Ah, thank you!

Always learning... and putting my foot in my mouth...

Looks like I responded before I read the whole thread, and you caught yourself. Yean, that whole NFB being subtractive, so more equals less confuses things so often. YOu know what is right, and it just comes out wrong.

 

[bigruu]

Great read, lots of useful info even for a newbie. Thanks a bunch, folks.


Rudy

Can't have too many tube amplifiers.

 

[Wally]

Welcome, Rudy.