300b lovers

@antigrunge2 Yes, IT is known as an excellent way of coupling, but it is expensive for very good ITs, and also they take far more physical space than your typical coupling cap.  So most vintage tube amps use RC coupling due to cost and size considerations.   Bad ITs don't sound very good, they can oscillate, or have very poor frequency response.  Done correctly with top shelf ITs and with a plan for the layout, then IT coupling walks all over RC coupling.   Medium ground is LC coupling with a high quality anode choke and good capacitor.  Also, there are circuits and tubes that are very difficult loads for ITs, so it is process of using the right method for the right circuit.  It is not one size fits all....  But in the right spot, I prefer IT coupling....

IT coupling dates back to the 1920’s Atwater Kent and radios of that vintage. Like field coil loudspeakers, it is the oldest form of coupling of all. But transformers have always been labor-intensive and expensive (going right to the beginning of electronic amplification), so when coupling caps became practical in the late Twenties, ITs mostly went away, although Western Electric and high-end radio builders used them as late as the end of the Thirties.

What pretty much ended them in all applications was the universal use of global loop feedback, with the landmark Williamson of 1948. You can wrap feedback around one set of coupling caps and an output transformer, but two transformers are out of the question. So IT coupling was moribund until zero-feedback amps saw a comeback in the early Nineties.

In the mid-Twenties, the only signal source was AM radio. Electrical phono pickups were just coming on the market, and movie sound was brand-new and experimental. By the mid-Thirties, movie sound was universal, with a bandwidth topping out at 8 kHz. Electrical phonographs could reach 8 kHz, but users often used "scratch" filters to soften the sound of noisy shellac 78’s. AM radio reception could go higher, but people often used the narrowband setting to get rid of interfering whistles from adjacent stations. The only truly wideband source was the Armstrong "Yankee Network" of FM stations in the 42~50 MHz band, which was limited to a few stations in the Northeast. These were the only FM stations in the world, and could be received by the high-end radios of the day.

It was only in the postwar years that wideband (30 Hz ~ 15 kHz) sources became widespread, with LP records in 1948, pre-recorded tapes in the mid-Fifties, FM broadcasting in the postwar 88~108 MHz band, and 70mm movies with magnetic soundtracks in surround sound. By then, all amplifiers were medium to high feedback designs, and used RC coupling throughout.

Modern wideband transformers were in studio use from the early Fifties, and the triode designs of the early Nineties opened the market for more unusual products, such as interstage transformers. True, it’s a 1920’s technology, but they didn’t have modern bandwidths back then ... the recording technology was unforeseen and decades in the future. What we hear now, with our ultra-wide band, ultra-low distortion sources, is a brand new sound, running through new-tech devices.

I hope you don't mind me going a tangent, but delving into modern and old technologies and taking advantage of such knowledgeable folks sharing with an open mind I wanted to humbly ask what function does a preamp fulfill in the context of a single digital source (DAC) driving a good SET? 

Here's why I'm asking: time ago I got into DIY speakers and adopted software-based FIR crossovers and 4-way active stereo, driving amps directly from a multiway DAC. AMT tweeters are driven from a 45 Yamamoto SET (hopefully not considered "junk" here 😊, combo sounds great to me), mids by KT88, midbass and subs by class D. My system sounds better now than it did before with branded speakers with passive xo, but I do wonder if I'm missing something driving amps from a DAC...I did love my Lamm LL2 preamp but I decided to optimize for digital sound reproduction with multiway so a stereo preamp didn't fit anymore.

In the context of not needing more than a digital source, the DAC producing enough gain such that using a digital volume control works well, and gain matching among amps being already solved, what would the advantage of using an interface between DAC and amps? 

Thank you in advance!

@lewinskih01 I have experimented for years with tube preamps.   When I built the Raven preamp it was after I had sent the 300b monos down to Spatial Audio for the Seattle show.  Then I built the matching preamp.  They were able to run the mono amps directly from the tube output stage of the Lampizator Pacific DAC, and they had one with a volume control.  I wanted to test it so I took used my Pacific DAC (no volume control) and implemented digital volume control in ROON.  It worked fine and sounded good.  I built the Raven preamp and inserted it in the system between the DAC and my 300b amps and the improvement was obvious.   I have done this experiment before with other preamps and DACs and every time I have preferred the sound of my good custom tube preamps to a direct connection from a DAC to the amp.  I have built passive preamps with the Khozmo volume controls.  I always preferred the active tube preamp and it was never close.  That is just my experience and others will undoubtedly have different opinions, but I have experimented off and on for years and always preferred a very high quality active tube preamp in the system.  That said, I have never owned active crossovers, nor will I, nor have I ever bi or tri amped a system, nor will I.  So my experience is not really applicable to your case.  I prefer the simplest system with passive crossover and one amp per speaker.   My 2 cents and of course your mileage may vary:)  This is why I build high quality active tube preamps for people with fairly simple systems.  Just my preference.   

@alexberger 

I don't use tube rectifiers in power amps, but I do use them in preamps.  In your amp, if you wish to keep the 5u4g, I would consider using it for the input and driver section and using good quality diodes to drive a separate power supply for the 300b.   I use only regulated supplies, and I have my way of doing it that I don't wish to get into here.  But, yes, it is a good idea to have separate supplies for the input/driver section and the power tube sections.  What sort of supplies is up to  you, but a DHT will echo your power supply AND your filament supply very clearly so make sure they are very good.  

Good luck!

Don