300b lovers

@donsachs 

It also has 6 dB of negative feedback.  That isn't necessarily bad, but our amp project has zero feedback and you can tell.  So again, they have made a nice compromise with good tubes, obviously good parts and a bit of feedback

Don I’m convinced of the merits of zero NFB tube circuit amplifiers implemented appropriately. Is this harder to pull off successfully? Does utilizing some degree of NFB confer a bit of a safety net? I inquire because its use is nearly universal with power amplifiers. Zero NFB  amplifiers are a small niche.

Charles

NFB covers a lot of sins.  You have to do everything right to have a stable circuit without it.  It is actually considerably more complicated than that, but that is a good way to think about it.   That allnic amp ad boasts a perfect square wave response.  The feedback helps with that.  It doesn't mean you cannot build a nice amp that uses NFB, but that "air" and sense of "realism" that you treasure is hindered by NFB.  I can see what is inside their amp and have a pretty good idea what it sounds like.   It is a dance.  We have been working on this amp for quite some time and it will go into production late fall.  We have tried many coupling methods and topologies and the fact that there is zero feedback lets you hear major differences between them.  Again, there are many great amps in the world, most of which have some amount of NFB to make them really stable and limit distortion.  That approach works, but has costs in terms of realism and spaciousness.  That said, it doesn't mean those amps sound bad.  Just different.  I know the sound I value and I cannot get it with NFB, but others have different tastes or the need to drive speakers which are not at all tube friendly.  So there are many paths.....and different sounds associated with them.  There is no right way, just what we like.

Negative feedback is the right choice for the vast majority of amps, particularly direct-coupled solid-state, where you can pile on the gain and use that "excess gain" to minimize distortion via feedback. To oversimplify, if you have 20 dB (a 10:1 voltage ratio) of excess gain, you can have 20 dB of feedback, which will reduce the distortion in direct proportion to the feedback ratio ... in this case, ten times. Pretty slick trick.

In practice, as the excess gain goes up, and the feedback ratio increases, problems with stability creep in. Marginal problems with stability result in overshoots on square waves, and as it gets worse, brief periods of near-oscillation, and then full-power oscillation, which usually destroys the speaker. So you have to take account of the total phase shift on both ends of the spectrum, which includes the output transformer if it is included in the feedback loop. The phase shift of an output transformer typically limits tube amp feedback to no more than 20 dB, but this can be evaded by having multiple nested loops, as in the Citation amplifier deigned by Stu Hegeman in the early Sixties.

But now we get into the (much) deeper waters of both slew-rate limiting and settling time, which are interrelated. That’s beyond the scope of this discussion, but they are limiting factors in any feedback amplifier. Multiple feedback designs can achieve impressively low distortion figures, but settling times can be much longer, since each nested feedback network has to leave saturation, return to controlled operation, and return to zero with its own time constant.

These are not trivial design concerns, and made more complex by load dependence ... a reactive loudspeaker load will decrease the phase margin of the amplifier, and that in turn leads to longer settling times. As the phase margin erodes, settling times get longer and longer, until the amplifier breaks into self-oscillation.

The other consequence of loss of phase margin is an increase in distortion, mostly at high frequencies, with the limit case of oscillation, which can be considered 100% distortion, with the output effectively decoupled from the input.

For obvious reasons, great care is taken in the design phase to avoid oscillation, but there are amplifiers where stability is conditional on the load, with transient overshoots visible under some conditions of load and input stimulus. This was a serious problem with first and second-generation transistor amplifiers. (Which were designed with nothing more than slide rules and nomograms, so you can’t really blame the designers back then.) Nowadays, software modeling programs allow designers to avoid the stability problems of the early transistor amplifiers.

If you want to jump down into the rabbit hole, read about "Nyquist Stability Criterion", followed by "Slew Rate Mechanisms" and "Settling Times in Feedback Circuits". For advanced practitioners, read about "Mixed Feedback Designs" and "Combining Feedback and Feedforward".

By way of comparison, neither the Raven nor the Blackbird use any form of feedback, either local (around the tube) or global (around the entire amplifier). The incoming audio signal only flows forward, with no secondary paths around the circuit. Further isolation is imposed by isolated B+ supplies for input+driver and output sections, so there is no secondary path for B+ power supply intermodulation (clipping in the output section has no effect on the preceding circuits).

This means distortion is entirely the result of device linearity in the specified circuit. The gains are scaled so each preceding stage has 3 to 6 dB of headroom compared to the following stage, so in practice clipping only happens in the 300B power section. The B+ regulator for the output section has a peak output of 200 watts, so the only limiting factor is the peak current capability of the output tubes. The performance of the 300B pair sets the performance of the entire amplifier.

By contrast, in a feedback amplifier (of any kind, solid-state or tube), clipping and/or slewing creates large error transients at the feedback summing node. This can saturate the input stage, which means the entire amplifier is now clipping, and can lengthen the recovery time from clipping.

@lynn_olson Another topic of discussion that has been undertaken by myself recently.

Note: As stated previously, I am no EE, I discuss notions with EE's and EE minded people.

The recent discussions taken part in, has been about the signal travelling through the audio system with as little of a interruption or diversion as can be put in place, with the value of such a approach, being that a direct signal is able to be produced as sound with no loss of content, however miniscule, resulting from an earlier manipulation of the Signal Path.

It was strongly suggested, a later arriving signal (it is going to reach the speaker) can be a info that when finally processed to a sonic, can be perceived as having a effect on the content of the produced sound, such as a smearing/masking of a particular dynamic, detail or frequency extension.

A demo' was also carried out to assist with my helping understand this at a very basic level, and I would settle / will be settling for the measures shown to help reduce the effects of what is claimed to be a delayed signal.

In the case of the delayed signal, it was made known both circuit design, topology and component selection can all be a contributor.  

In the case of the demo' it was quite obvious that at certain places within the replay, there was a Vocal that was more comprehensible and certain notes were sensed as being a rendition that had attained a step further to being a more honest presentation. 

When I read your post, it does look likely to myself, through your circuit design and time spent voicing the sonic, that many of the items that were 'above my head' to address at the time of my discussion are addressed in your 300b Amp' Design. 

The description certainly prompts the idea, that it would be a real pleasure to be able to be in a room with this 300b design one day.