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Hi Kevin.
Can you follow up at all in regards to the 4P1L line stage you alluded to in a Facebook post in March?
Best regards.
Follow Ups:
I agree, this is looking to be yet another cool Kevin Carter design: Coleman filament regulators, Shunt regulators for B+, choke input in the B+, the provision to use DHT's, Lundahl output transformers, the provision for unbalanced/balanced outputs and affordability! What's not to like? With an unbalanced circuit costs can indeed be less...just need to tweak that operating point.
Best,
Anand.
Here is a photo of the second prototype. Across the front you can see the filament supply components. The large heat sink accommodates the Coleman current regulators for the filaments. The raw power supply sits just to the right, one channel's supply stacked on the other, and a pair of rigged PC mount filament transformers in the right front corner. The high voltage supply is fed by an R-core transformer and Triad choke (choke loading), raw supply and time delay, then shunt regulator. The 4P1Ls are each on a PC board with integral socket (using Mill-Max pin sockets) and the 13R cathode resistor (with filament current passing through it) on the heat sink. This board is not mechanically decoupled, but could easily be arranged that way with EAR-type stand-offs to minimize chassis-borne microphonics. Finally, the plate load transformers, which are Lundahl LL1692A/18mA.
This layout is probably not optimum, but it does illustrate the real estate necessary to get the job done at this level of performance. In the near future I will be be redesigning the raw filament supply and getting some beta boards for the circuitry together for a few of you to try.
Kevin Carter
K&K Audio
www.kandkaudio.com
This looks wonderful.
I am curious how you intend to incorporate the Rod Coleman filament supplies into this offering. Are you considering importing a small batch for resale?
Also, what are the dimensions of the chassis shown?
I am happy to still have already ordered a few of the standalone modules for this kit. Despite having a pair of your regulator boards, I really want to try out VR tubes. I would like to know if you have managed to coax satisfactory performance out of any of the family of VR tubes.
I have approached Rod about reselling them and he is OK with it.
The pictured chassis is 17 inches wide, 11 inches deep, and 3.15 inches internal height.
I have had no recent experience with VR tubes, so I can't say what they will do in this design. If you observe the restrictions regarding capacitive bypassing, they certainly work well enough, but how they would affect the sound would require trying them.
Kevin Carter
K&K Audio
www.kandkaudio.com
I can follow-up...
I finished the second prototype build about 2 weeks ago and have been listening to it since then. I am very, very pleased with what I hear. A Rod Coleman filament regulator provided a better solution to the DC filament challenge than the others I'd tried, which was the major issue that needed to be resolved. In my "spare time" I will be working out the remaining details to facilitate making the parts for such a preamp available.
At the beginning at least, I am strongly considering offering an "a la carte" selection of modules for DHT line stage use, with the 4P1L and then, perhaps, the 26, plus tested schematics to guide the assembly of one of these preamps.
Kevin Carter
K&K Audio
www.kandkaudio.com
Not going to use LUNDAHL for the output transformers but would like to use them for the power transformers.
Would you tell us something about the power transformers and choke on the right side. None of that stuff looks like LUNDAHL to me.
The choke looks like HAMMOND and what I assume to be the filament transformers looks like an unusual amalgamation of HAMMOND split bobbin transformers. The B+ transformer I have never seen before.
The 4P1L is a great hobbyist tube, glad to see you, too, have taken an interest in it.
From what I have read there are those who think 18 mA is sufficient but then there are others who think one should go higher - I tend to go with the latter based on nothing more than instinct (and Moglia).
Looking forward to seeing what you come up with. Obviously I am more interested in parts ...
The prototype I've shown was built trying to put the money where it counted, sonically speaking, namely the Lundahl output transformers and the Coleman filament regulators. I can't say whether it would sound better with Lundahl PS magnetics and won't know until I try it. In any case, the LL1683 will work fine in place of the R-core power transformer for the high voltage supply and the "auxiliary functions", like time delay. The LL1685/100mA will work well in place of the Triad choke. Lundahl sells no transformer that is suitable for the filament supply. There was someone recently selling Lundahl power transformers on eBay that were destined for ss power amp use, but the amp didn't make it commercially so they were surplus to the company's requirements. I don't see them up there now, though.
As far as operating points go I don't operate by intuition, rather I try a range of voltages and currents to see what sounds the best. I've not yet done this with the 2nd prototype as I've initially focused on getting the preamp quiet. Operating point optimization is a lot of work if unless you're lucky.
Kevin Carter
K&K Audio
www.kandkaudio.com
It might be useful to state that the 4P1L preamp I've built has a gain of 8dB and an output impedance of about 300 ohms with an LL1692A (3.75:1) as a line output transformer at the operating point I am currently using (150VDC plate voltage and 18mA). That amount of gain is good for me under the current circumstances, but it can be modified using different operating points or a different line output transformer turns ratios.
Kevin Carter
K&K Audio
www.kandkaudio.com
Thanks for the additional details. I'll be checking back frequently for updates.
I'll be checking back frequently for updates...........+1
Cheers
FM
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