K&K Audio / Lundahl Transformers

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K&K Audio / Lundahl Transformers

RE: Power Amplifier link

72.150.130.64

In Reply to: RE: Power Amplifier link posted by dth31 on July 19, 2008 at 09:24:26

It will be awhile I'm afraid. The first entry will be a mod kit for the Dyna Stereo 70 that uses a fully differential Class A 2 stage version of the topology that is used in the Art Audio Vivo. For $250-$350 (in addition to the price of an ST-70), you can build an exceptional amplifier out of a used ST-70. See the picture for a visual preview. You take the ST-70 practically down to the chassis metal, reusing only the output tubes and sockets, the GZ-34 rectifier tube, and the transformers. It really is a very nice amplifier, even without considering the price.

The delay comes as a result of the input transformers being backordered with Lundahl until early October.

Kevin Carter
K&K Audio
www.kandkaudio.com

Follow Ups:

RE: Power Amplifier link - AnandR 11:12:45 07/22/08 (17)
Nice work. In fact, you could easily build a decent EL34 amplifier using Lundahl output transformers, a power transformer, EL34 tubes for just a small increase in price versus purchasing a used ST-70 which can be expensive depending on the quality. Should sound better too!

What input and driver tubes? I am assuming its a differential class A design that is cap coupled.

That PCB is a work of art. Point to point is cool, but when you see PCB's like this I wonder why I go through the hassle of ptp.

I'm moving to Wilkesboro, NC this Friday. We should hook up soon, before I have to start working in mid to late August. That way, I can come over with a cheque to purchase a RAKK dac II which IS LONG OVERDUE!

Anand.
- RE: Power Amplifier link - KevinC 11:48:14 07/22/08 (16)
  Thanks! The input board could be used as a front end for many output stages.
  
  The driver/amplifier tubes are 6N23P/6922/7308. I will use 6N23P in the kit, because I think they sound the best.
  
  We're here most every Sunday soldering and BS-ing. I suspect you'll enjoy the chaos.
  
  Kevin Carter
  K&K Audio
  www.kandkaudio.com
  - RE: Power Amplifier link - AnandR 19:24:35 07/22/08 (15)
    With an experimenter's platform and mind, I do have a pair of 1.6K output transformers (single ended) that was made with the mind of using parallel EL34's. I am wondering if these input/driver stage could be used with this. What is that octal socket on the board? Is it for a GZ34?
    
    Anand.
    - RE: Power Amplifier link - KevinC 19:55:41 07/22/08 (14)
      I suppose it's more than obvious to say that this kit was designed with driving PP output stages in mind, hence the differential configuration, so interface with a parallel SE output stage would be possible, certainly, but awkward. I suspect that your outputs don't have center taps, so couldn't be "forced" into PP service.
      
      The octal socket does indeed harbor a GZ34, in this case used as a delayed turn-on device where the diodes are paralleled to make operation not so near the edge from a current-carrying standpoint. FREDs do the actual rectification.
      
      Kevin Carter
      K&K Audio
      www.kandkaudio.com
      - RE: Power Amplifier link - papagenohyun 23:19:37 07/22/08 (13)
        
        In Reply to: RE: Power Amplifier link posted by KevinC on July 22, 2008 at 19:55:41
        
        how much gain does this board have?
        does it have enough gain for 300b pushpull?
        
        RE: Power Amplifier link - KevinC 03:42:57 07/23/08 (12)
        
        In Reply to: RE: Power Amplifier link posted by papagenohyun on July 22, 2008 at 23:19:37
        
        The board has essentially the gain of a 6N23P plus the possibility of 6dB more from the input transformer, if connected for such. With EL34s, the amp overall has either 12db or 18dB of gain into a 4 ohm load. There will be about 7dB less with 300Bs. The on-board heat sinks for the output stage cathode current sources are also sized for 50mA of current at about a 40-50v bias point, so they will not be adequate for 300Bs.
        
        Kevin Carter
        K&K Audio
        www.kandkaudio.com
        
        RE: Power Amplifier link - papagenohyun 05:15:05 07/23/08 (11)
        
        In Reply to: RE: Power Amplifier link posted by KevinC on July 23, 2008 at 03:42:57
        
        if output cathode ccs isnot applied, do you think it can be used for 300b pp?
        i mean 300b output stage is common cathode resister biased.
        
        and if 6n23p is biased deeply so can accept high level signal from active preamp, i think voltage swing for driving 300b grid can be acquired .
        
        am i incorrect?
        
        i think this brilliant board can be good choice for diy builders who wants some amity style 300b poweramp ( yes, differential except output stage )
        
        thanks.
        
        RE: Power Amplifier link - KevinC 06:23:25 07/23/08 (10)
        
        In Reply to: RE: Power Amplifier link posted by papagenohyun on July 23, 2008 at 05:15:05
        
        The circuit on the PC board is simply not optimized for driving 300Bs. The input transformer is really too small and the input stage is not adequately biased for the large voltage swing that would be necessary to get 15-25watts out of a pair of 300Bs.
        
        Kevin Carter
        K&K Audio
        www.kandkaudio.com
        
        RE: Power Amplifier link - AnandR 07:51:56 07/23/08 (9)
        
        In Reply to: RE: Power Amplifier link posted by KevinC on July 23, 2008 at 06:23:25
        
        Okay, okay, you've twisted my arm enough, I will spring for some push pull trannies. What other output tubes are possible? 6550's or KT88's? You're right about optimzing the driver stage for the output tubes of choosing. Lynn and you are both experts in that regard.
        I've got a quad of EL34's so that's what I will plan on first. This will all have to wait unfortunately since the 1544a's are on back order. Love the way you implemented the GZ34 for a slow turn on rectification. Although my primary choice (as you know) are damper diodes which combine both great rectification properties, and slow turn on in one unit (actually a pair for full wave rectification); a pair of Freds then a GZ34 in series would be the next best choice. And of course, you don't have to spill a bunch of beans on an NOS Mullard.
        What Lundahl output trannies would be your recommendation (no amorphous core for now, just too $$$)? 1623?
        Thanks again, great board.
        Anand.
        
        Edits: 07/23/08
        
        RE: Power Amplifier link - KevinC 09:01:46 07/23/08 (8)
        
        In Reply to: RE: Power Amplifier link posted by AnandR on July 23, 2008 at 07:51:56
        
        You could use 6550s, KT-88s, 6L6GCs, etc. So long as the tube doens't require a bias voltage of much more than -50v or bias current of more than 60mA, the ST-70 mod kit will handle it. Although, you could go off-board with the output stage current sinks, if you wanted to use a "heftier" output tube.
        
        I've been happily using the Valve Art GZ34 with no problems.
        
        For 8 ohm speakers I would recommend the LL1663/PP and for 4 ohm speakers, the LL1682/PP, as the "entry level" transformers. If you want more load flexibility, I would choose the LL1679/PP.
        
        Kevin Carter
        K&K Audio
        www.kandkaudio.com
        
        RE: Power Amplifier link - AnandR 11:04:09 07/23/08 (7)
        
        In Reply to: RE: Power Amplifier link posted by KevinC on July 23, 2008 at 09:01:46
        
        Good. Then when I come over I'll get a Valve Art GZ34 from you, a pair of matched 6N23P's as well. I think this board is going to be quite versatile. Even more than just popping it in an ST70.
        
        Power output in triode I take it is about 12-15 watts/channel with EL34's? Should be plenty for high efficiency speakers that are 98 dB/1w/1meter. I also like the overall gain structure is configurable and plenty low at 12-18 dB.
        
        As usual, good work. Your designs always make sense from an engineering, technical, and financial standpoint.
        
        Too bad I have to pay NC sales tax now.
        
        Anand.
        
        RE: Power Amplifier link - KevinC 11:41:31 07/23/08 (6)
        
        In Reply to: RE: Power Amplifier link posted by AnandR on July 23, 2008 at 11:04:09
        
        The ST-70 modded with the kit makes about 11-12w/ch, the consequences of operating in Class A instead of Class AB1. It's also easy to switch to class AB1 by simply bypassing the current sources with a capacitor. This results in more power, but worse sound.
        
        I actually went to the trouble of building the prototype on a different chassis and made it switchable between the differential current sink biasing, this bypassed by a capacitor, and fixed bias. The differences were readily apparent and left no doubt in my mind about the superiority of differential current sink biasing, at least in this two stage PP topology with these tubes and their corresponding operating point.
        
        Kevin Carter
        K&K Audio
        www.kandkaudio.com
        
        RE: Power Amplifier link - AnandR 12:34:19 07/23/08 (5)
        
        In Reply to: RE: Power Amplifier link posted by KevinC on July 23, 2008 at 11:41:31
        
        I love it when Kevin uses words like 'readily apparent' which he uses sparingly and only in situations where the sound difference is night and day.
        
        Makes it easy to be the diy'er/consumer when you have somebody who takes objective criteria seriously enough to not inflate one's subjective impressions.
        
        What HV and filament secondaries to you recommend for the power transformer? Are you using AC for the 6N23P and EL34 or is it DC rectification?
        
        I've got to think up of a sanskrit or latin name for this amplifier, that simply means 'workhorse.' I've always wanted an EL34 PP amplifier which I can compare to all other amplifiers.
        
        Gotta warm up the Front Panel Express Program. If you have time, shoot me a schematic (you don't have to send me the parts values to protect the privacy of your work) so I have a general idea.
        
        Thanks again,
        
        Anand.
        
        RE: Power Amplifier link - KevinC 13:26:13 07/23/08 (4)
        
        In Reply to: RE: Power Amplifier link posted by AnandR on July 23, 2008 at 12:34:19
        
        The kit is designed to work with the original ST-70 power transformer and uses all of the secondaries that it provides. Filaments are all AC. You will need something like 375-0-375 VAC at 400mA, 2 x 6.3vAC at 3.5A (center tap a convenience, but not necessary), and 25-50VAC at 50mA for the front end negative supply.
        
        Kevin Carter
        K&K Audio
        www.kandkaudio.com
        
        RE: Power Amplifier link - papagenohyun 18:42:56 07/23/08 (3)
        
        In Reply to: RE: Power Amplifier link posted by KevinC on July 23, 2008 at 13:26:13
        
        then, how about 2a3 pp ? i think 2a3pp can be used with that board.
        so marginal spec for output ccs capability?
        
        RE: Power Amplifier link - KevinC 19:07:22 07/23/08 (2)
        
        In Reply to: RE: Power Amplifier link posted by papagenohyun on July 23, 2008 at 18:42:56
        
        Yes, it would work with PP 2A3s just fine from a CCS dissipation standpoint. The issue then is can you live with about an overall 4dB or 10dB of gain into a 4 ohm load?
        
        Kevin Carter
        K&K Audio
        www.kandkaudio.com
        
        So . . . - Ian L 20:08:52 07/23/08 (1)
        
        In Reply to: RE: Power Amplifier link posted by KevinC on July 23, 2008 at 19:07:22
        
        . . .maybe the PX25 would be a good choice here for papagenohyun ?
        
        RE: So . . . PX-25? - KevinC 12:50:43 07/26/08 (0)
        
        In Reply to: RE: So . . . posted by Ian L on July 23, 2008 at 20:08:52
        
        The PX-25 is an excellent choice for such an amplifier. However, unless you want to hunt down original British manufacture tubes, you are left with the excellent, but rather expensive KR PX-25s. The Chinese tubes that I bought two years ago did not have anything resembling PX-25 characteristics, so they aren't really PX-25 replicas. They appeared to be 300Bs with altered filaments (to me). My note along these lines to the seller was greeted with polite dismissal.
        
        Kevin Carter
        K&K Audio
        www.kandkaudio.com

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