OTL Asylum

Posted by Frihed89 (A) on April 01, 2007 at 01:37:14

I have a Single Power MPX3 headamp/preamp (SLAM SE Model). It is OTL. Is there any way to reduce the gain in the circuit, other than to attenuate? It has excessive gain as a preamp for my power amp.

Would reducing the gain of the input tube have any effect? What about the gain of the output tubes? Or reducing the output voltage? I can do these things very easily. It takes lots of different tubes and has a low voltage (for 6SN7GTs, and 5687s) and high voltage (for 6SN7GTBs and 6BL7GTAs).

I suspect these things will not really help. I need to kill at least 5dB! I have tried attenuation - Yuk!

• Re: Excessive gain in OTL preamp - Lew 09:53:19 04/03/07 (3)
  In Reply to: Excessive gain in OTL preamp posted by Frihed89 on April 1, 2007 at 01:37:14:

  Can you tell us more about the circuit? It's hard to imagine a commercial product where you can swap a 6SN7 for a 5687 and vice-versa (the two tubes are so physically different). Also, 6SN7GTA/GTB and 6BL7 will all operate at the same voltages required by a 6SN7GT, AFAIK. Granted that the GTA/B has a higher max plate dissipation and max plate voltage compared to a GT, that does not mean that one must run them at higher voltages or current. Also, how does it achieve OTL-ness? Is it capacitor-coupled? Most preamps are, and this is not unconventional at all.
  
  

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  • Re: Excessive gain in OTL preamp - Frihed89 11:06:32 04/03/07 (2)
    In Reply to: Re: Excessive gain in OTL preamp posted by Lew on April 3, 2007 at 09:53:19:

    I have no idea. Contact mikhail Rottenberg at info@siglepower.com. Or look at www.singlepower.com.

    The simple part of the tube swapping is that he makes tube adaptors with the octal base for 3-4 different pin sets. It only takes double triodes as inputs and outputs, but I assure you it will take low and high voltage 6SN7s, 5687s, and 6BL7GTAs and others as outputs. For inputs: 6Au,Av,At7, 5965, 6414, 7n7 E180cc, 5751 (with some outputs), and on and on. I think it is a cathode follower, but i don't know how he avoids the output transformer.

    My MPX3 Slam SE is mainly a headamp, but can work as a preamp.

    Anyway, I tried a 7N7 driving VT-231s. That is as low gain as you can get and it still has excessive gain
    
    
    

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    • Re: Excessive gain in OTL preamp - Lew 12:36:32 04/03/07 (1)
      In Reply to: Re: Excessive gain in OTL preamp posted by Frihed89 on April 3, 2007 at 11:06:32:

      I went to the website. There is very little information to help decide what the circuit is like, except to say that it looks like there may be two tubes used for a gain stage, one per channel most likely, and a third tube used as a cathode follower. It's likely that there is a CF output because you would need that to get the stated output impedance as low as it is. To say that this design is "OTL" is not helpful at all. The great majority of tubr preamps are OTL and instead use a capacitor at the output to block DC voltage. I suppose this is applicable to a headphone amp as well, depending upon the nature of the headphones it is intended to drive. So if you really have excessive gain, perhaps you can bypass the gain stage entirely, and just use the unit as a cathode-follower/buffer. Ask the maker whether you can jumper the sockets for the input tubes and just run straight into the CF. If you are a DIYer, you might try modifying the input to accept a low mu triode, instead of medium mu triodes like the ones you named. (However, off the top of my head I cannot think of any dual section low mu triodes, except as a far out choice the 6AS7. Type 27s are great sounding but are single section tubes; you may not have enough sockets to try 27s [mu = 9], and you would have to change the filament voltage to boot).

      By the way, I assume you are referring above to 12AU, AV, and AT7, instead of "6au", etc. FYI, 12AU7, 6CG7, 6SN7GT/GTA/GTB, VT231, and 7N7 are all electrically identical and differ only in socket type and pin-outs, tho in my book the 6SN7 types sound best. I personally would not sub some of those other types you list into a circuit designed for the tubes I listed, without making some changes to the current and voltage supplied.
      
      

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      • Thank you. I will - Frihed89 21:34:32 04/03/07 (0)
        In Reply to: Re: Excessive gain in OTL preamp posted by Lew on April 3, 2007 at 12:36:32:

        Yes 12A_7 is right. I do prefer the VT-231 for the output tubes and for inputs my favourite with the VT-231s is an Amperex pinched waist E180CC. But i have tried all the 12A_7 varieties up to and including the 12AT7, 5965, 6414, 7N7, 5687, 6829. There are a host of others that will work. These are very nice headphone amps and preamps, too.

        I will call the mfr. I imported it from the US, so I have to solve the problem over here.
        
        

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