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Old 3rd October 2009, 11:19 AM   #11
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There are no boards in an RTP apart from the Superreg. How much of the unit is DIY and how much is already assembled depends on what you arrange with Allen.
If his policy hasn't changed recently, you can have a kit as well as a completed unit.
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Old 3rd October 2009, 05:02 PM   #12
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Quote:
Originally Posted by M Knight View Post
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.
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Old 3rd October 2009, 05:56 PM   #13
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It was at one time offered as a kit....

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Old 3rd October 2009, 09:06 PM   #14
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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.
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Old 3rd October 2009, 10:13 PM   #15
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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."
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Old 10th December 2009, 07:45 AM   #16
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Default 2sk369-v

Anybody that build an Allen Wright Clone (amp) can use some tight matched 2SK369-V. See here: matched 2SK369-V for sale
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Old 10th December 2009, 08:11 AM   #17
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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.
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Old 10th December 2009, 11:21 AM   #18
Alex M is offline Alex M  United Kingdom
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Quote:
Originally Posted by M Knight View Post
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

Last edited by Alex M; 10th December 2009 at 11:29 AM.
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Old 10th December 2009, 12:02 PM   #19
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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.
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Old 10th December 2009, 08:41 PM   #20
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I have built two versions of the FTP5. All point to point and I never had issues with noise or hum. I found them one of the easiest things I have built.

Shoog
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