300b lovers

@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.       

@donsachs ​​​​@lynn_olson ,

Your detailed (And well written) explanations are greatly appreciated. It heightens one’s admiration for the decision making, knowledge, skill and simply hard work required to design and build very high quality excellent sounding amplifiers.
 

This thread could legitimately be separately filed/classified as a teaching course. No doubt that many following this thread have learned a lot and expanded their knowledge base.

Charles

What @pindac described is exactly what I heard when I first built the rather primitive initial "silicon assisted" stereo version of this circuit with a CCS on the plate of the driver tubes and a single regulated supply for each channel.  I could hear things like subtle inflections in vocals, or the resonance in the low notes of a piano in a way that I had not encountered in all my years of building and restoring amplifiers.  I had heard the stirrings of such things in single ended triode amps, but not with the drive and authority that this circuit presents.  So then we spent 18 months or so experimenting with every permutation and combination and ended up with mono block amps with dual independent regulated supplies and all custom interstage coupling, and some old school VR tubes as well.   I have not heard anything like it.... and the final version walks all over the one presented in Seattle.

It sounds like it does for the reasons described above....

Lastly, I have said this before in this thread.  It is about your design goals.  We could make a low powered amp that uses 45 tubes in push pull and it would produce maybe 5-10 watts and stay in class A.  I am sure it would sound incredible.  If you had 94+ dB speakers it would probably be plenty of power.  But 45 tubes cost a LOT, and many folks have 88-90 dB speakers and maybe a larger room.  So our amp is 25 watts, well really about 27 watts, and has easily driven 88 dB speakers to screaming levels.  Would the 45 amp sound better on more efficient speakers?  I don't know.  It would certainly sound a bit different, and possibly better.  We tried to make an amp that had a much wider appeal and used modern production tubes that didn't cost a fortune.   So it depends on your design criteria.   The Citation II amp will drive darn near any rational speaker.  It has three nested feedback loops and is very stable.  It is one of the few tube amps you could let idle away for an hour on a bench with no speaker load attached and it would not oscillate.   That said, it doesn't have the clarity and just spaciousness of the zero feedback DHT circuit.   It makes wonderful music and is non-fatiguing to listen to, but it doesn't have "the piano is in the room" sound of the 300b project.   I know this because I rebuilt about 80 of them, and lived with one for a few years.  I could rebuild one in my sleep:) 

Ultimately,  it depends on what you want the amp to do.   I want an amp that will drive a pretty large number of speakers and have the clarity of a flea watt DHT, with the drive and authority of a push pull amp.   So that is what this project is about.  Trying to get a sweet spot that will make a lot of people happy and not cost a fortune to re-tube, and to run the tubes at really sane operating points so they last a long time, while pushing the state of the art sonically.   There is no single design that can make everyone happy and drive all speakers and retain all the sonic characteristics we desire.  There are always compromises.  Tubes vs SS, SE vs PP, etc...  Within SS and tube worlds there are many topologies, devices, and tube types.  So there are many choices.  For me to say our way is the best is ridiculous.  So our comments in this thread just point out why we have made the design choices we have.

I should note my description of feedback circuits is a grossly oversimplified, non-mathematical overview of a complex subject. For the curious, read about how op-amps are stabilized, and the concepts of loop gain, excess gain, dominant-pole compensation, and phase margin. Once you get a reasonably firm grasp of how it works, then read about slewing distortion and settling time. I tend to use settling time as a figure-of-merit when looking at op-amps, or more complex discrete circuits.

It all comes together at the summing node, which is simply an analog comparator between input and output. In an op-amp, which has extremely high forward gain, the high gain of the op-amp forces the differences between the two nodes to zero. This is fine until the op-amps clips or slews, which creates very large error voltages at the comparator input. The large error voltage can force the comparator itself into nonlinearity, and feedback theory relies on a distortionless comparator.

In addition, if the comparator is saturated, or if the power supply sags or is discharged, then recovery time can be quite long (tens or hundreds of milliseconds), much longer than the original clipping or slewing event.

During this settling time, amplifier distortion can be quite high, since feedback is only partially effective. This will not appear in FFT harmonic distortion or multitone IM distortion measurements, which are taken over several seconds and then averaged.

This is the gap in existing measurement techniques. Harmonic and IM distortion are averaged over several seconds, and do not sense events happening in microseconds or milliseconds. High-speed scope measurements are insensitive to distortion unless it is very high, such as 10% or more, where it becomes visible. Transient distortions, in the microsecond to millisecond range, are not seen.

The key principle of non-feedback amplifiers is they are insensitive to transient upsets or interactions with the load. Steady-state distortion is higher, but there are no issues with phase margin or settling time.