Negative Feedback question

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Negative Feedback question

Postby Scott Anderson » Tue Aug 15, 2006 12:41 pm

My next project will be a 120 watt bass amp based on the Sunn 2000S. My question is that the output transformer has 16, 8 and 4 ohm taps. A good replacement transformer is the Hammond 1650T. With the secondary of this transformer wired for 8 and 4 ohms, there is no 16 ohm tap. In the schematic below the feedback resistors and caps are connected to the 16 ohm tap.

My question is, can I use the 8 ohm tap and use the same values or should they be changed?

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Postby EWBrown » Wed Aug 16, 2006 5:14 am

(corrections/updates made from original posting here)

Increase the 750 pF cap to 1000 pF (0.001 uF) and decrease the resistor to 680 - 750 ohms. the "fudge factor" here is square root of 2, or 1.414,
as 8 ohms will have 0.707 of the output voltage as would 16 ohms.

Doesn't have to be exact, +/- 5% is more than good enough, just pick the closest standard value component...

The 680 ohm resistor from 16 ohms to ground doesn't factor into the NFB equation, it's just there as a light "safety load" just in case someone forgets to plug in a speaker. Yellow_Light_Colorz_PDT_09

The 390 pF cap can remain the same value as this is on the primary side of the OPT, this is an old Hafler / Dynao "trick" used with the cathodyne type phase splitter.

If ypu decide to forego the NFB, just disconnect the NFB at the point where it enters the VA stage's cathode. THis way, NFB could be switched in or our as required. As stated in a later post, sometines no NFB is better for guitar amps. I'm stuck in the "hi-fi" mode most of the time, myself Yellow_Light_Colorz_PDT_11

/ed B in NH
Last edited by EWBrown on Wed Aug 16, 2006 7:53 am, edited 3 times in total.
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Postby dhuebert » Wed Aug 16, 2006 7:11 am

3. leave out the NFB and see how you like it. (I have built 2 bass amps without NFB and the customers seem very happy)

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Postby Scott Anderson » Wed Aug 16, 2006 7:27 am

Don and Ed thanks.

Ed, in step 2 you mentioned the 690 ohm. I'm thinking you mean the 680 ohm connected to pin 9 of the 6AN8 tube and not the 680 connected to the 16 ohm tap.

Don, I'll certainly try your suggestion. I have read that GNF is good for tonal response especially in bass amps.

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Postby EWBrown » Wed Aug 16, 2006 7:44 am

I was referring to the 680 ohm (not 690, phat phinger phumble) resistor from the 16 ohms tap, and now, on second thought, that step won't do a doggone thing, so just forget step 2. I'll remove it from my post Yellow_Light_Colorz_PDT_09

It's just there as a "safety load" in case someone forgets to plug in a speaker.

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Postby Scott Anderson » Wed Aug 16, 2006 8:03 am

Thanks ed, I thought that's what you meant.

That bring up another question. A bass player on a guitar forum, http://www.fenderforum.com/forum.html?db=&topic_number=580768&lastpost=2006-08-1404:50:34 wondered what harm would it do to the amp if he turned the amp on without the cabinet connected. You can read the different responses.

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Postby erichayes » Sat Aug 19, 2006 8:25 pm

Hi All,

Bottom line on that one is that Mr. Bolton nailed it from all angles, and the other "experts" have subtituted opinion for knowledge. Sadly, there's a lot of that going around in both the guitar amp and hi fi worlds.
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Postby dhuebert » Mon Aug 21, 2006 8:01 am

I don't like to turn on an amp without a load because sometimes accidents will happen, like if the end of your guitar cord should happen to come into contact with your thumb fer instance. I've heard enough squeals and thumps to know it's the ones you don't hear that do damage.

As for NFB (National Film Board?): For a bass it does make sense to me that Global NFB (multi-stage) would be a good thing but you can read some long tirades against it on the net. I have thought carefully about this and talked to alot of "gurus" and what it really comes down to is: How do you like the sound? It's good to understand what GNFB does but in the end theory don't matter. What I did was to use Local NFB in the form of unbypassed cathode resistors, miller capacitors on the input tubes and ultra-linear (U-L) output with a quartet of output tubes.

The idea, particularly with bass amps, is to raise the damping factor as high as possible. This is one of the things that GNFB will contribute to. As I understand it, damping factor in a tube amp is typically ~5-10 (no units) whereas transistors exceed 100. Using four output tubes with U-L will work to raise damping factor in the absence of GNFB.

GNFB will also extend frequency response and lower gain (the two are related). For a bass amp there is no reason to have bandwidth exceed 10KHz so we don't need GNFB for that and you want to maximize power for those low notes and GNFB will reduce that.

Finally, GNFB will reduce distortion, its original intent. Again:As I understand it the human ear can't actually hear those low bass notes so what you want is distortion. Let me explain: (PRR on Duncanamps.com disagrees with me here) the amplifier makes 2nd order harmonics at all times. These harmonics are musically related, ie. they are octaves, your ear does not hear the fundamental but does hear the harmonics. The signal processor in your brain hears these harmonics and inserts the fundamental into the perceived sound. This allows you to hear something you don't really hear and gives the amp a warm rich sound.

EOT (end of tirade)

This is my thinkling on this matter. I thank you for your kind attention.

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Postby erichayes » Mon Aug 21, 2006 7:59 pm

Hi All,

I agree completely with Don about making sure a load is connected at turn-on. Taking this paranoia one step further, turn all gain and volume controls to zero if possible (difficult to do post-sound check).

(Global) negative feedback--"global" was added to the term in the last few years by those folks who wanted everyone to think they'd invented something new-- seems to be one of the most misunderstood and, therefore, vilified tool available in amplifier design. A well designed, stable amp doesn't "need" NFB, but nevertheless will benefit from judicious application of NFB. On the other hand, a poorly designed amp will require NFB just to keep it from breaking out in glorious ultrasonic song, causing its owner to wonder why he's always replacing speakers. My philosophy with amp design, regardless of application, is to make them as clean and stable as possible to begin with, then go back and apply muck and filth as needed.

The one aspect of NF that seems to be the most misunderstood is that it decreases the maximum power output of an amp. This is simply not true. Power output is determined by the power supply. What NF does affect is gain structure. You might need to add an extra stage of amplification to get your 100 watt amp back up to 100 watts at full volume, but that can be done in the preamp section of the amp.

One interesting thing I did for a guy with a Princeton Reverb several years ago was to add a "Magic" control (his term, not mine, and preceded by "f'ing"; the control was labelled, simply, "FM"). It consisted simply of putting a 50K pot in series with the feedback resistor coming off the output transformer. Thus, the feedback could be reduced (and the gain and distortion correspondingly increased) from the factory value to virtually none with a twist of a knob. The only downside was the strain of adding one more contol for the guy's already overtaxed and maginally functional mind to deal with.

Several bassists I've been consulting with lately, two of whom are classically trained, point out that the newer generation bass amp/speaker systems are significantly wider in frequency response and lower in distortion than older amps. This is primarily due to the style of bass playing that has evolved in the last few years (slap, funk etc.) from the old stodgy thumbless keep-the-beat background mantra. With all the new harmonics being generated, bass players have moved into the forefront and want the audience to hear all the sounds they're creating now. There's a semi-myth that even though bass guitars are tuned the same as upright basses, the short string length of the guitar prevents the fundamental frequencies from being produced in any useful quantity and all you're hearing is the first harmonic. When we plugged them into the 100 watt acoustic instrument amp (running flat into a matched speaker system) prototype, all of their jaws dropped to the floor. The fundamentals have been there all along; the amps and speakers just weren't capable of reproducing them.

Damping factor is an indication of how high or low the overall output circuity internal impedance is. Back in the 1950s, when speaker construction techniques were primitive, low damping factors were actually desirable as the speakers, themselves, were highly damped (cone excursions of less than ¼" were not uncommon). In order to get any meaningful bass response out of a system, the amp makers would deliberately mess with the damping by putting small resistances in series with the output and the speaker and simultaneously changing the cathode resistance on the output tubes. The net result was variable damping from "unity" (actually 1/∞) to around a maximum of 9 or 10. As speaker manufacturing techniques improved and evolved, the need for higher electrical damping increased, causing higher power amps to be created. The highest damping factor I ever saw in a tube amp was 30, which is about 4 or 5 higher than the calculated upper limit needed for present speaker designs. Solid state amps with damping factors of over 100 (one bragged of a DF of >1600 back in the '70s) sound particularly awful on older speaker systems and cabs because they won't let the speaker do anything below 100~ or so.

Determining damping factor of an amp is ridiculously simple, but, for some reason, has a shroud of secrecy surrounding it. Ironically, it involves a step that got this whole subthread started: removing the load with signal present on the input. Although any output voltage will work, I prefer 1.00 VAC as it is easily to compute with and low enough to prevent harm to the amp.

The first step is to hook up a non-inductive load that matches the output impedance of the amp. Then pump in a sine wave signal (I prefer 40~, but anything up to 1kc or so will work if it's a good amp. When in doubt, do them both and compare notes) that gives 1.00 VAC across the load resistor. Disconnect the load resistor and measure the unloaded output of the amp. Reconnect the load or turn off the amp. Subtract the loaded voltage (1.00) from the unloaded (1.XX) and take the inverse of the difference. Thus, if your unloaded voltage went up to 1.18 VAC, the difference would be 0.18; 1/0.18 = 5.55.
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Postby EWBrown » Tue Aug 22, 2006 7:15 am

Just another crazy untested thought:

In the VA section of the 6AN8, change the 680 ohm resistor to 620 ohm, replace the 47 ohm resistor with a 100 ohm pot, and attach the NFB to the pot's wiper. Voila!, instant variable neg feedback (AKA PFM) . as in Pure F????? Magic :o

The Peavey 50/50 PPP (8 EL84s) stereo "instrument" amp used a similar means of varying the NFB, as previously mentioned (the 50K pot), I think they called it "presence". Said amp, after I re-tubed it, is now serving in the sound system at a local area "teens" club, they seem to like the tube sound.

Uh-oh, I just found another "smilies" site:

http://dungpow.com/smilies.html


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Postby dhuebert » Tue Aug 22, 2006 8:19 am

it decreases the maximum power output of an amp


Yes, of course your right, I suppose what I meant was that for a given topology NF will reduce power.

short string length of the guitar prevents the fundamental frequencies from being produced


Not being a bass player I can't answer this. What I was talking about was the reduced sensitivity of the ear at those frequencies.

From what I understand, with the new switcher style power amps there is so much power available to bass players these days that it's changing the roll of the bassist on stage. You can have 1500 watts in a 10 pound package, giving bassists possibilities that just didn't exist before. Hard to keep up without an SVT (of which I'm seeing more and more). I wonder if bassists aren't going thru the same process that electric guitar went thru in the sixties where technology and playing style got into a positive feedback evolution.

Thanks Eric for a good reply.

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