NFB net gain or loss

[PRR]

Modern speakers and pentodes want NFB.

Speakers are tuned for high damping. Nowadays, nominally zero source impedance, or DF>>40. If they are tuned flat with high DF, and you give zero DF, the bass resonance will rise 10dB or 20dB.

Even guitar amps, where the speaker can be selected for best spectral balance on a hi-Z amplifier, often use about 6db NFB for DF near 1, instead of DF near 0.2 as a naked pentode gives.

Triodes give DF like 2 to 5 without any extra NFB. Rise at bass resonance is only like 3dB. While not "perfect", it usually vanishes in real-room bass nodes.

Ultralinear splits the difference.

Yes, you can enjoy a speaker without good damping, especially if you design the speaker that way. Some of the great consoles of the past ran very low damping with speakers and cabinets adjusted for pleasant bass response.

The problem with NFB and tubes is that: in many practical situations you don't have much excess gain, and you have so many coupling caps and the output transformer that you can't stablize more than 10dB-20dB NFB without crippling the amp or your wallet. That's about enough to damp a speaker, but small amounts of NFB increase IM distortion and high-order distortion.

> Solid state amps generally have a very high open loop gain, thus a lot of excess gain that can be turned into feedback, and the measured non-linearities are often very low in SS compared to tube amps.

The nonlinearity of any simple BJT stage without NFB is huge compared to any tube. But so is the gain. And you can often reduce coupling rolloffs enough to run NFB with stability. They may hide it, but it is very unusual to find a truly no-NFB BJT amp. (FETs are a lot like tubes for gain and linearity, but can often use BJT-like NFB.)

> I applied 19dB of feedback to a PP 2A3 amplifier and at the onset of output stage grid current it reduced distortion from 0.9% to 0.4%.

NFB will actually increase THD above the clipping point.

What was the change in THD at say 3dB below clipping? Just guessing: 0.5% no-NFB, <0.1% with 19dB NFB.

Of course this raises a different question: do we want THD to stay low up to a point, and then rise like a brick wall? Or a more gentle rise through and past the gross-distortion point? And of course that depends on our needs: if we never need to touch clipping, NFB will give lowest distortion in our operating zone; if we like to crank it up until it comes out bent, the gentle rise and low slope of no-NFB may be less obnoxious. (In college, with low efficiency speakers, I cranked the heck out of a 25 watt amp to get 50 watt sound. Decades later at home, I built a similar low-NF amp of 40 watts but with a high efficiency speaker, and just never felt inspired to crank it past the limit.)

[rick57]

PRR

Thank you - you have a great depth of experience !!

[Sheldon]

Originally posted by ray_moth
I find triode stages produce rather boomy bass with the speakers I have, so I apply a modest amount of NFB (say 6 to 10dB).

Originally posted by EC8010
As other posters have said, the main need for feedback is to lower the output resistance of the amplifier sufficiently to allow the loudspeaker to achieve the bass response that the designer intended (and which assumes zero output resistance).

Originally posted by PRR
Modern speakers and pentodes want NFB.

Speakers are tuned for high damping. Nowadays, nominally zero source impedance, or DF>>40. If they are tuned flat with high DF, and you give zero DF, the bass resonance will rise 10dB or 20dB.

Yes, you can enjoy a speaker without good damping, especially if you design the speaker that way. Some of the great consoles of the past ran very low damping with speakers and cabinets adjusted for pleasant bass response.

It's nice to be reminded that a music system must be considered as a whole. If you make your own amps and speakers, you have more degrees of freedom. You can adjust system Q with the amp or speakers. My main amps have an output impedence of 6ohms, which would be considered poor by most current standards. But on my speakers the bass is tight and solid, not underdamped in the least. If you bi or tri amp, you have even more ways to tune.

Sheldon

[EC8010]

Originally posted by PRR
> I applied 19dB of feedback to a PP 2A3 amplifier and at the onset of output stage grid current it reduced distortion from 0.9% to 0.4%.

NFB will actually increase THD above the clipping point.

What was the change in THD at say 3dB below clipping? Just guessing: 0.5% no-NFB, <0.1% with 19dB NFB.

I'll have to fire it up again to check, but I expect your guess about dramatically reduced distortion at lower levels is correct. Sadly, the onset of grid current was 3dB below clipping, so I felt quite aggrieved about the effect of global feedback.

[rdf]

No discussion of damping factor should be without this analysis by Dick Pierce:

http://www.diyspeakers.net/Articles/...G%20FACTOR.pdf

For those unfamiliar with Mr. Pierce he's the antithesis and often the Usenet bane of audio tweakdom and spent many years as an industry consultant. He considers DF above 10 as gravy. I believe that's still out of reach of most non-NFB tube designs, at least SE.

[amperex]

I built a very high quality PP 45 amp. According to others & myself 4dB & 8dB of switchable feedback did not sound as good as zero feedback.

[ray_moth]

No discussion of damping factor should be without this analysis by Dick Pierce . . . he's the antithesis and often the Usenet bane of audio tweakdom

An iconoclast after my own heart! I'm sure the only reason there is a market for expensive speaker cables and connectors is to preserve the very low output impedance of some amplifiers. After all, there's no point in having an amp with OP impedance < 0.1 ohm if your cables & connectors add a much higher impedance, is there?

[gingertube]

The Impedance of your speaker typically rises at low frequencies with a peak at the resonant frequency. It also rises at high frequencies as the inductance of the speaker (tweeter) voice coil becomes more significant.

With a very low damping factor amp the power delivered to the speaker tends to follow the impedance curve of the speaker and the lower end and upper end of the frequency response tends to be exagerated as more power is delivered into the higher impedances.

I have found a damping factor of 4 is more than adequate to balance the power vs frequency issues. In fact I have several amps with damping factors as low as 2 which sound quite glorious. This is speaker dependent. If this damping factor can be achieved without recourse to global NFB then I always choose to do it that way (eg local FB loops, triode connection, ultralinear connection, cathode feedback etc). If I need some global feedback to achieve that balanced frequency response then I have no problems with adding it. I try to keep global NFB to around 6dB maximum BUT have run as high as 15dB with some loss of stereo imaging.

Cheers,
Ian

[rick57]

Thanks again - will factor in to the amp design

Cheers