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Vacuum State RTP3C

Hello all,
I've been in the mood to change up my analog setup a bit lately. I have been using an Aikido phono stage with lundahl step up transformers. I have thought about building a new phono pre, and have decided on the Vacuum State RTP3C.

I just have a couple of questions about it. Has anyone built/heard this unit? I know it's extremely subjective, but if anyone has heard it, could they describe the sound of it? Also is there anything I need to know about building this particular circuit? It seems pretty straight forward, but I just wanted to seek the expertise of someone who has possibly gone down this particular path before. I appreciate any help, thanks. Hope to hear from you soon.
 
I strongly advise to read Allen's "tube preamp cookbook", which describes the development of the circuitry over the years. You'll have a lot of fun reading it and and all the information you need.
BTW: I do have an RTP3 at home ATM (borrowed from a friend). It is one of the most neutral, detailed and fastest preamps I have ever listened to. It's not that warm and sweet tube thing, it is... well, straight in ya face :D
 
I haven't heard this preamp in a familiar system, but the design is VERY similar to one I've been using since the '80s. Allen has engineered it superbly well and I think you'll be quite happy. I would strongly suggest the use of his boards, if available- high bandwidth circuits can be tricky to stabilize. And do not stint on the power supply; the PSR of the cascodes is 67% of bupkis.
 
Hi,

I have not build the rtp3c but the fvp5. I've been living with the fvp for years and wouldn't part with it whatever. The rtp should be better. In fact since i'm going balanced after building the alephX and pumpking, my next try is going to be the rtp5.

Indeed read Allen's preamp cookbook and build a proto, it's fun.

regards,
Joris
 
I cannot comment on the Phono aspect of the RTP3C, but have built two versions of the FVP5 (without phono). Absolutely brilliant detailed, punchy and clean sound. Both versions have been mostly point to point builds and have shown no instability. Allen Wright only recommends his Super Reg to supply it, but I built a simple three stage CLCRC which is fine.
I listened to it up against an Aikido and can say that they sounded almost the same. If you use a different volume control then this is likely to swamp any of the sonic differences between the line stage circuits. This is not totally surprising since they are very similar circuits. If you include the Super Regulator then you will probably hear a more pronounced difference

Shoog
 
Shoog, I was hoping to hear from someone who has heard both. I wasn't planning on using the super reg since I've already built Broskie's Janus regulator. I was planning on using this with the RTP. Does anyone have any experience with the differences between these 2 power supplies. Does one sound significantly different from the other?
 
If you are going to build the older iteration, the RTP3C, it will be relatively straightforward if you avail yourself of Allen's documentation.

If you are going to build the latest iteration, the RTP3D, you will have to put your thinking cap on, because the documentation is out of date.

If you construct to a (self-imposed) high standard, you will likely find building one of these a trial by fire, but you will be amply rewarded with, as HBarske above says, a neutral and detailed preamp that can hold its own against much more expensive gear. And it looks the part, too.
 
Another reason to read The Tube Preamp Cookbook is that it contains a neat little circuit for matching J-FETs. Whether you will be using (increasingly scarce) 2SK369s, as in the earlier versions, or (readily available) BF245s, as used in the latest incarnation, they will have to be matched, obviously. Allen still sells matched pairs of 2SK369s.
 
Another reason to read The Tube Preamp Cookbook is that it contains a neat little circuit for matching J-FETs. Whether you will be using (increasingly scarce) 2SK369s, as in the earlier versions, or (readily available) BF245s, as used in the latest incarnation, they will have to be matched, obviously. Allen still sells matched pairs of 2SK369s.

I've seen MAT02's mentioned elsewhere as well, though they're expensive and seem hard to get. Digikey ran out a week or two ago. So given all these choices what's considered best these days for this circuit? I have LSK389, LSK170 and real 2SK170s in my parts bin.

Hopefully Allen has my book in the mail.
 
Allen doesn't use MAT02s in his phono stage any more. He uses a precision dual transistor chip, the SSM2210. J-FETs (BF245Cs) have been retained, but they serve as CCSs to bootstrap the SSM2210 cascoded to a 6922. The original circuit, as SY suggests, is a celebrated design that has been taken up by various enthusiasts. K&K Audio's phono stage is similar, as was Thorsten Loesch's Toccata MKII pre-amp, itself based on a design by Arthur Loesch. Allen's use of an SSM2210 is a refinement to get around the difficulty of obtaining obsolete low-noise J-FETs and then having to match them.
 
One builder of the RTP3C, Alex Megann, website here, says with regard to the suitability or otherwise of 2SK170s: "Allen has suggested to another RTP3 builder that the specified 2SK170bl JFETS, which are expensive and quite hard to find, can be replaced by the RF device BF245C. These have a large Idss, quoted in the datasheet as in the range 12-25 mA, which makes selection for the required currents (particularly in the output stage, with its standing current of about 12mA per leg) easier and more economical than with the 2SK170bl parts, which have an Idss of 6-12mA. I […] found that they worked well in all the current sink circuits, with a variation of only about 5% in the current across the whole batch."
 
I have built the RTP3C, FVP5 and a couple of variations on each. The best part of them is the first stage - the RTP line I find unnecessarily complicated and can be equalled with other topologies.

Mine are in storage until the TT's are back on line, but my only sonic comment would be 'clean'. I'm not one to use flowery words to describe sonics.
 
One builder of the RTP3C, Alex Megann, website here, says with regard to the suitability or otherwise of 2SK170s: "Allen has suggested to another RTP3 builder that the specified 2SK170bl JFETS, which are expensive and quite hard to find, can be replaced by the RF device BF245C. These have a large Idss, quoted in the datasheet as in the range 12-25 mA, which makes selection for the required currents (particularly in the output stage, with its standing current of about 12mA per leg) easier and more economical than with the 2SK170bl parts, which have an Idss of 6-12mA. I […] found that they worked well in all the current sink circuits, with a variation of only about 5% in the current across the whole batch."

Indeed.

To clear up any misunderstanding, my comments only apply to the JFETS in the constant current circuits: I used SSM2210 matched bipolars in the input of the phono stage, as recommended by Allen.

You can buy a kit from Allen or you can build your own from scratch. The latter works out a lot cheaper, but you need to know what you are doing and have plenty of patience. As I mention in my article, there is information out there (particularly photos) to help you on your way. My own build was slow - the RTP3 is a very complex circuit - but actually I had very few problems and the preamp worked fine first time. This is in contrast to the active crossover I built subsequently, based on the RTP5 hybrid SLCF buffer stage, which I had all sorts of problems with, though many of these were to do with the shunt regulator (not a SuperReg) which I used it with.

Oh, by the way, the RTP3 sounds excellent! The comparisons I have made are against an Audio Research SP-11 and a Croft Super Micro A, and also compared the phono stage with an Audio Synthesis ADEQ, and the RTP3 comfortably beat all of them. I don't know if I can describe the sonic character of the RTP3, since my impression is that it doesn't get in the way of the music much at all, but my overall impression is that it sounds very natural, particularly in its dynamics.

Alex
 
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I have a detailed schematic for the RTP3c line amp (without phono; all tube with 2sk369-V as ccs and e88c) with all voltages in the schematic. You can ask me by PM.
I do not see why building from this schematic would be any more difficult than building any other tubeamp. Of course you should not build this as a first experience.
With some experience this design could be altered to accept the 6H30 or 6N6P.