In Reply to: Re: Lundahl Transformer Attenuator?? posted by KevinC on October 14, 2002 at 20:06:04:

How is the transformer attenuator compare to resistive attenuator when use in an active (tube) preamplifier?

Also, have you tried to put an input transformer (1:1) before the resistive attenuator? My friend tried that and he said the result is excellent, and he claimed the transformer attenuator seems to have problems in tube preamplifier circuitry due to the impedance issue. What's your thought?

• Re: Lundahl Transformer Attenuator?? - gdahl 23:06:12 10/15/02 (7)
  In Reply to: Re: Lundahl Transformer Attenuator?? posted by SuperB on October 15, 2002 at 03:11:44:

  Hi Super B,

  I agree that in many cases adding a 1:1 input transformer will improve results with a resistive attenuator, but you still have to watch the impedance relationships. If you want to use a transformer attenuator, it is important to plan the surrounding stages so that it will perform at its best.

  The TX-102 likes to be driven by a fairly low impedance. This might make it difficult to incorporate in certain tube circuits, but in my opinion is worth planning around...most things like being driven by low impedances anyway!

  In my system, I use the TX-102 at the input of the Aurora integrated amps, which have their line stage built in. Using an integral line stage allowed a favorable gain structure for the amp...the line stage has a healthy amount of gain, which makes the driver stage's job easier, but the voltage level between the line and driver stage is higher than a typical "line level". With the tranny attenuator at the input, level control takes place at a voltage level that is quite appropriate for its optimum use.

  As I see it, one of the main advantages of the tranny attenuator is the fact that as you turn down the volume, you step down the source's impedance. In contrast, turning down the volume with a resistive attenuator increases the source impedance, at first...as you continue to turn it down, source impedance begins to drop as the wiper approaches ground. But wait! Even though the source impedance is dropping, the source has less control over its load. Turning down the volume with a tranny attenuator is like shifting into a lower gear--the engine has greater control over the car's motion. Using a resistive attenuator is like keeping your foot on the accelerator and controlling your speed with the brake!

  If you add a 1:1 input transformer to a resistive attenuator, you still have the disadvantages of a resistive attenuator, except its lack of isolation.

  Another thing Lynn and I will be doing soon with the Aurora: a fully-balanced version, where the TX-102 is used as a phase splitter at the input. Since phase splitting and level control are accomplished at once, the circuit is elegantly simple.

  I have had no problems so far driving the TX-102 with digital sources. As for phono stages, it gets a bit trickier. Phono preamps usually come in two varieties: RIAA only, and RIAA plus line stage. The ones with RIAA only often have a high output impedance, which won't drive the TX-102 well. The ones with an integral line stage end up having too much gain.

  Several solutions have come to mind, and I'm planning to try more than one of them. What sort of preamp are you thinking of using? Maybe one of my ideas will be of interest to you.

  Best regards,
  Gary Dahl
  

  • See my Aurora amps here...

  
  

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  • Re: Lundahl Transformer Attenuator?? - SuperB 03:08:11 10/21/02 (0)
    In Reply to: Re: Lundahl Transformer Attenuator?? posted by gdahl on October 15, 2002 at 23:06:12:

    Hi Gary!
    I have a question for you. I just looked at the Lynn Olson's website, and found that Lynn used the Pro-Cap in all his design. However, Pro-Cap is not exist anymore. What sort of capacitors you guys currently using?
    
    

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  • Re: Lundahl Transformer Attenuator?? - SuperB 00:11:36 10/17/02 (4)
    In Reply to: Re: Lundahl Transformer Attenuator?? posted by gdahl on October 15, 2002 at 23:06:12:

    Gary,
    So which version of S&B TX102 that Bent Audio selling? MK I or MK II?

    ***The TX-102 likes to be driven by a fairly low impedance. This might make it difficult to incorporate in certain tube circuits, but in my opinion is worth planning around...most things like being driven by low impedances anyway!***

    Therefore, do you think the TX-102 drive the K&K Audio Single-Ended Triode Parallel Feed Line Stage Kit properly?

    Thank you!
    SuperB
    
    

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    • Re: Lundahl Transformer Attenuator?? - gdahl 08:05:32 10/17/02 (2)
      In Reply to: Re: Lundahl Transformer Attenuator?? posted by SuperB on October 17, 2002 at 00:11:36:

      You won't have much to worry about with what you put on the output side of the TX-102, because it steps down the source impedance of whatever you feed its input (except at full volume). The area of concern is what you connect to its input. A tube RIAA with a plate-loaded output, such as the RCA circuit, will very likely have bandwidth problems when driving a TX-102. OTOH, a phono stage with a cathode follower output or a step-down transformer will have no trouble.

      Bent Audio only sells the MK II versions now, as far as I know. There are no differences between the MK I and Mk II that would involve usable impedances.

      For a thorough discussion of these units, you might find Thorsten Loesch's application note useful. This is posted at the Bent Audio forum on Harmonic Discord--the link appears below.

      Gary Dahl
      

      • Thorsten's TX-102 application note

      
      

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      • Re: Lundahl Transformer Attenuator?? - SuperB 01:11:35 10/19/02 (1)
        In Reply to: Re: Lundahl Transformer Attenuator?? posted by gdahl on October 17, 2002 at 08:05:32:

        Hi Gary!
        Thank you for the advice! I'll order the S&B TX-102 transformer attenuator and try into the K&K Audio parallel feed line amplifier.

        I have a preamplifier which has 2 stages: one gain stage direct couple to the cathode follower without feedback, and the cathode follower has a cathode resistor. Some people suggested I try the C4S from Bottlehead, and the other suggest I put some type of iron (I gues they meant choke or sth.) to replace the resistor on the cathode follower.

        Do you suggest any iron from Lundahl or other manufacturer I can replace the cathode resistor of the cathode follower in my preamplifier?

        Thanks again!
        
        

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        • Re: Lundahl Transformer Attenuator?? - KevinC 06:07:08 10/19/02 (0)
          In Reply to: Re: Lundahl Transformer Attenuator?? posted by SuperB on October 19, 2002 at 01:11:35:

          There are a couple of things that I would suggest. You can use a Lundahl LL1667/15mA or LL1668/25mA audio choke depending on the amount of current passing through your cathode follower. Or you could use the K&K Audio CCS, which is an easy-to-apply 2 terminal current source. It is used as the plate load in the SE line stage circuit (see the schematic on my webpage). Either choice will work, but I have no sonic experience with this application, because I tend to steer clear of RC coupled cathode followers. They tend to "dry" the sound out, in my experience. I think that the use of either of these devices will help relieve some of the dry character of the cathode follower.

          Kevin
          
          

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    • Re: Lundahl Transformer Attenuator?? - KevinC 03:28:40 10/17/02 (0)
      In Reply to: Re: Lundahl Transformer Attenuator?? posted by SuperB on October 17, 2002 at 00:11:36:

      I have tried this and it works perfectly fine.

      Kevin
      
      

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  • Re: Lundahl Transformer Attenuator?? and Phono Stages - KevinC 04:51:51 10/16/02 (0)
    In Reply to: Re: Lundahl Transformer Attenuator?? posted by gdahl on October 15, 2002 at 23:06:12:

    Gary Dahl writes:

    < >

    I will make a plug for the new SE phono stage kit here. It has an output impedance of around 200 ohms due to the parallel feed transformer output coupling and drives my TX102 most admirably. ;=)) The gain without an MC step-up input transformer is about 45dB. It wll be available for viewing on the website after the weekend.

    Kevin
    
    

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