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Re: non-parafeed transformer preamp?

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KevinC

(M)

In Reply to: Re: non-parafeed transformer preamp? posted by bigtube on August 25, 2003 at 20:39:21:

You have it right; the low frequency cut-off is a matter of some concern in these circuits. A rough calculation shows me that, assuming an Rp of 1300 ohms for the 437A and an LL1660 gapped for 36mA, the low frequency cut-off (-1dB) will be about 15Hz, which I think is fine.
The high frequency performance in the audio band will be fine, but there will be high frequency resonances (>20kHz) that result in peaks in the amplitude response. These can be reduced by resistive loading, but don't seem to affect the sound significantly, in my experience. Some people are bothered by the imperfect square wave performance that they engender, but I don't know what square waves have to do with listening pleasure.

Kevin Carter
K&K Audio
www.kandkaudio.com

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Re: non-parafeed transformer preamp? - bigtube 15:35:00 08/27/03 (1)
After a closer look, wouldn't the LL1671 be more appropriate than the LL1660 because of it's higher current capability?
- Re: non-parafeed transformer preamp? - KevinC 16:26:30 08/27/03 (0)
  The LL1671 is not as appropriate as the LL1660, because the primary inductance is subtantially lower, resulting in a much higher low frequency cut-off. Stick to the LL1660.
  
  Kevin Carter
  K&K Audio
  www.kandkaudio.com
Re: non-parafeed transformer preamp? - bigtube 15:21:12 08/27/03 (9)
Thanks again Kevin. The LL1660 should work it appears. Would be interested to hear from anyone who has used the LL1660 in such a fashion. Can you point me to anyone?
Any advantages the LL1660's might have over the Sowter 9560? That would also seem to be a suitable tranny for the straight-ahead topology.
- Re: non-parafeed transformer preamp? - KevinC 16:38:13 08/27/03 (8)
  Other than the difference in ratio, which is an objective thing, one can't predict without hearing.
  
  Kevin Carter
  K&K Audio
  www.kandkaudio.com
  - Re: non-parafeed transformer preamp? - bigtube 19:01:25 08/27/03 (7)
    True, true. Just wondering if I was missing any obvious differrences. LL1660's are less expensive, so that's a nice advantage for them.;o)
    - Re: non-parafeed transformer preamp? - akltam 05:56:54 08/29/03 (6)
      I use 6C45 with the LL1660/25mA in the circuit you are thinking about. It is indeed very nice. You may also want to consider LL1654. (which I will try later) Before I go for the LL1660, I have also considered the Sowter. Maybe one day, one day ... But you can't go wrong with Lundahl. Excellent quality
      Alan
      - LL1660 or LL1654? - bigtube 12:27:21 08/29/03 (5)
        
        In Reply to: Re: non-parafeed transformer preamp? posted by akltam on August 29, 2003 at 05:56:54:
        
        Interesting Alan! Thanks for the advice! The number one concern I(those that have advised me)have is the OPT being able to handle the current of the 437A. I think I'm supposed to run it up around 35mA.
        It appears the LL1654 is designed to handle more current and the application note shows the exact circuit type I'm building. Still, I suppose you might start to have tradeoffs as the tranny gets larger like the LL1654 is?
        Kevin, what do you think about the LL1654 vs. LL1660 in this application? Alan? Anyone else? I respect and appreciate all of your opinions. Thanks in advance.
        
        I've used the LL1654 gapped for 30mA. - JLH 16:24:06 08/29/03 (4)
        
        In Reply to: LL1660 or LL1654? posted by bigtube on August 29, 2003 at 12:27:21:
        
        Guys,
        I've used it with a 12B4 preamp. It sounded very good, and could drive a signal down a pair of soggy strings if needed. All those big dollar cables didn't make a difference anymore. The driving capability and low output impedance allowed it to overcome the cable capacitances and any other factors that normally allow us to "hear" the differences in cables. The only thing that I thought it could do a little better at was the high frequencies. It does roll off sooner than the LL1660. My pair of LL1654's were down 1dB at 25KHz, where the LL1660's I used to have went to about 60KHz @ -1dB. Not that this makes any difference in the listening, but you should be aware of this.
        Rgs, JLH
        
        Comments on the LL1660 and the LL1654 - KevinC 15:28:10 08/30/03 (2)
        
        In Reply to: I've used the LL1654 gapped for 30mA. posted by JLH on August 29, 2003 at 16:24:06:
        
        John's post reveals one of the essential differences in practice between these two transformers. The LL1654 is a five section transformer, while the LL1660 is a three section transformer. The LL1654 is larger and has greater primary inductance, when gapped for a given current, than does the LL1660, but it also has higher capacitance as a result of the greater number of sections. This means that the driving impedance must be lower than it would need to be to get the same high frequency cut-off that you could get with the LL1660. This is reflected nicely in the data that John presented.
        Use of the LL1654 with a tube that has an Rp of greater than, say, 1500-2000 ohms will probably result in roll-off in the audio band. It's a perfectly good transformer that can yield great results when applied properly (see John's comments), but comes with the warning just given.
        
        Kevin Carter
        K&K Audio
        www.kandkaudio.com
        
        Can you clarify .... ? - akltam 00:56:13 08/31/03 (1)
        
        In Reply to: Comments on the LL1660 and the LL1654 posted by KevinC on August 30, 2003 at 15:28:10:
        
        'The LL1654 is larger and has greater primary inductance'
        Do you mean LL1654 is larger and because it is larger, it has greater primary inductance? Or do you mean LL1654 is inherited being build having a inductance that is > than LL1660 with the same gap size ?
        I have got LL1660 25mA and LL1654 40mA. Since they are not the same gap size, I cannot compare them. But the reading of my LCR meter told me that the LL1660 25mA has more than double the inductance of the LL1654 40mA in the primary.
        If they were gaped all to let say 40mA, I will not be sure if LL1654 will have a primary inductance > than 1660. (even though Ll1660/40mA is expected to have much smaller primary inductance)
        Any thoughts ?
        I don't think it matters to me whether it starts to roll off at 10Hz or whether it starts to roll at 20Hz. (as long as they don't roll off within the audio band) Since they are made differently, I would be interested in how they sound. I will post my findings later about 1654. (but 1660 sound very good to me already)
        
        IMHO
        Alan
        
        Re: Can you clarify .... ? - KevinC 04:08:16 08/31/03 (0)
        
        In Reply to: Can you clarify .... ? posted by akltam on August 31, 2003 at 00:56:13:
        
        I shouldn't have speculated on which has greater primary inductance without looking at the datasheets, but the LL1654 is larger and size matters. Simply put, more primary inductance can result from more iron or more turns of wire, all other things being equal. My rough calculations show that they would be about the same inductance when both gapped for 40mA.
        It will be interesting to hear a report of which you prefer sonically.
        
        Kevin Carter
        K&K Audio
        www.kandkaudio.com
        
        I will post when I get the 1654 working ! - akltam 08:13:37 08/30/03 (0)
        
        In Reply to: I've used the LL1654 gapped for 30mA. posted by JLH on August 29, 2003 at 16:24:06:
        
        I have indeed asked for Mr Per Lundahl's advice. He also believes that the LL1654 may overkill when I use 6C45. You never know until you try, so I will post later when I get it working.
        Alan

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