Recent blogs by Paul advocated for valve input and transistor output stages in a power amp as an ideal combination.
Can the same advantage be realized by using a valve preamp and a transistor power amp?
Yes, I imagine so. I added a tube preamp and before the BHK Signature arrived, it was solid state and sounded better because of the tube.
alcarp said Recent blogs by Paul advocated for valve input and transistor output stages in a power amp as an ideal combination.Can the same advantage be realized by using a valve preamp and a transistor power amp?
Paul beat me to it. I’ve been doing this ever since I started my system upgrade 5 years ago (when I replaced my worn-out PS-IVH preamp with a tube AudioResearch LS-26)
But there’s something else. Remember that an amplifier architecture is based on two amplification stages: The first to raise voltage to the point where the second can raise the current (that goes to the speakers) to the proper level. With the tube in the preamp, there is still a transistor Vgain stage (the first) and no matter what Paul tried, even with FETs running at 100V bias levels, nothing sounded as good as the tube, according to Paul. So if anything, the transistor Vgain amp stage will be hurting the sound in this scenario.
Hence, I’m saving my pennies , actually for the last 5 years now . . .
–SSW
I’m not very technically minded.
SSW: can you please elaborate.
OK, I’ll do my best, but don’t even think I can do a good a job as Paul.
You will note that in Paul’s video on the BHK ( http://www.psaudio.com/forum/new-power-and-pre-amp-forum/bhk-amplifier-and-the-cas-first-impressions/page-2/ ) the amplifier had a very difficult job in accurately translating the small signal from the preamp into a high current signal that is sent to the speakers. Paul and most contemporary amplifier makers split this task into two: The first is a Voltage gain stage that only amplifies the magnitude of the incoming signal, but is too weak to drive the speakers directly. The second stage is the Current section that takes that high voltage and is able to control the output transistors (or tubes) to modulate the huge power supply and send the desired current to the speakers.
The voltage gain stage (Paul calls it Vgain) is where the tube is in the BHK. A tube has tremendous voltage amplification vs. a transistor, it think the tube can do at least a 100:1 whereas a bipolar transistor is on the order of 20:1 or so. So a single tube in the Vgain stage can do the same job as multiple transistors attempting to do the same. The more transistors you have, the more distortion you get, so ideally you want to reduce the number of transistors to the bare minimum.
An all-transistor amp will have some sort of transistorized Vgain stage. Hopefully the distortion will have been minimized, but in Paul’s testing of all types of solid state devices, from bipolars to FETs and every possible combination, nothing sounded as good as the tube in that role.
So what I was trying to point out was that, yes, you could put a tube in your preamp and get some benefits of the tube, but that now-beautiful signal will go right to your solid state amp where the transistorized Vgain will, unfortunately, mess it up (albeit only a tiny bit).
In my case of the Streets 950 the Vgain stage is an op-amp. Very common at the time (30 years ago) but not the ideal as there are many transistors in it that can distort the sound. What was unique about my amp, however, was that the power supply used power regulators that kept the supply rails locked in and gave the amp the sweet sound it was famous for. But now it’s time to replace the Streets with something better.
The BHK hybrid concept is intended to eliminate that distortion source int he Vgain stage. Paul’s testing indicates this has, in fact, been done so and, to him, the sound is cleaner than an all-transistor equivalent. But merely putting a tube preamp in front of an all-transistor amp will not do this. It won’t sound bad, but it won’t sound quite as good, either.
Hope this helps.
–SSW
Paul McGowan said Yes, I imagine so. I added a tube preamp and before the BHK Signature arrived, it was solid state and sounded better because of the tube.Paul, how much performance improvement do you get from inserting your tube preamp between the DirectStream DAC and the BHK poweramp? Does the DAC direct route works well? Have you tried anything else besides DAC direct and using your Aesthetix pre? What did Arnie use?
If I end up swapping out my Spectral 360SS II for BHK, the preamp/no-preamp field is wide open again and I’ll try some different options (keep spectral preamp, remove preamp, insert ARC tube preamp, wait for PS Audio tube preamp), but just curious what to expect based on your observations.
SSW … thanks for that explanation … very helpful.
I use a pure Class A 100 wpc power amp. Will this make a difference to the distortion introduced in the Vgain stage?
edorr saidFirst, thanks to SSW for that great explanation. Well done lad.Paul McGowan said Yes, I imagine so. I added a tube preamp and before the BHK Signature arrived, it was solid state and sounded better because of the tube.Paul, how much performance improvement do you get from inserting your tube preamp between the DirectStream DAC and the BHK poweramp? Does the DAC direct route works well? Have you tried anything else besides DAC direct and using your Aesthetix pre? What did Arnie use?
Edorr’s question is a good one and I haven’t fully formed my opinion although I think I know. I am in Montreal at the moment, the show progresses and our room sound marvelous, one of the best we’ve had, frankly, and we’re using some small Neat UK speakers. I haven’t any experience with these but I am impressed. DirectStream feeds the BHK directly through balanced Kimber’s and the entire setup is simple and direct. I have attached a picture of the room. What’s interesting to me is just how good this room sounds and no, there isn’t a preamp.
So DirectStream as its name implies, direct into the BHK. Sounds great. At home, I still use the preamp which is, itself, a tube based preamp and prefer the sound to that of going direct. We’re just now finishing work on the first BHK preamp, again a tube/MOSFET hybrid, and I’ll let you know what I find when I listen. So I suspect yes, the preamp, if it’s the right one, will be better than none at all - but DirectStream direct into the BHK might well be better than a solid state preamp, that remains to be seen.
Strong work on the room, Gordon! He’ll never even know that the resonators are there.
alcarp said SSW .. thanks for that explanation ... very helpful.I use a pure Class A 100 wpc power amp. Will this make a difference to the distortion introduced in the Vgain stage?
Not knowing the exact architecture of your amp, I can’t say for sure, but at first guess I don’t think so, and here’s why: Your amp is 100 wpc class A, but that only refers to the current amplification section, or what Paul calls “Igain.” There is still, most likely, a Vgain stage in front of the class A output system (if the design architecture is indeed split into Vgain and Igain systems as is the custom nowadays).
Adding a tube in front of this will not affect any distortion caused by the actual Vgain stage you have already.
But, saying that, I will confirm that when I put my LS26 tube preamp in front of the Streets 950 amp (replacing the PS IVH I bought 33 years ago) the system did sound a lot better.
So I suspect that whatever distortion you have in your Vgain stage, it is most likely smaller than what is coming out of your preamp. Get a tube preamp and audition it and see what happens. If it sounds better: Keep it! 
I have to admit my system sounded a LOT better when I put in the LS 26 but it took me 6 months to acclimate to it to appreciate all the new things being presented to me.
Hope this helps.
–SSW
edorr saidIf I end up swapping out my Spectral 360SS II for BHK, the preamp/no-preamp field is wide open again and I’ll try some different options (keep spectral preamp, remove preamp, insert ARC tube preamp, wait for PS Audio tube preamp)
Spectrals love Spectrals (also MIT cables and Avalon speakers ) A pre-amp is a delicate piece of equipment, that should match the power amp. For example, I tried Pass Labs x1 preamp with other power amps and I did not like the sound. But x1 sings with PassLabs amps. Spectrals are even more picky.
Speaking of pre- of no pre- - the results are very mixed. Paul likes PWD (I and II) direct to amp and I found it awful. Paul likes DS with a tube preamp (but he’s got long cables), and I prefer DS connected directly. And your mileage may very vary
The combination of tube pre and SS amp (spectral DMA 50 for now, though sweetened it) is the best combo I’ve heard on my speakers, and which is why I’m going with a BHK hopefully this month.
I’ve always found that going through a fantastic tube pre amp (like my Atma Mp1) always seems to enhance the signal from the PWD1/2 and Strain gauge phono stage, both with highly regarded variable outputs that can go direct.
But of course I’ll have to hear the new preamp once it comes out and see if it can outshine my two box tube unit with handbuilt discrete balanced volume control.