Posted by Jim Womble (A) on June 20, 2006 at 17:17:05

In Reply to: Re: Power supply question posted by arend-jan on June 20, 2006 at 12:53:10:

Thanks for your input. I built a 2a3 amity, and it sounds very good - but I didn't pay any particular attention to the power supply - maybe I was lucky. I used 9uf/7hy/200uf. It is quiet. Someday I'll put better caps in to see if I can hear any difference (I am using Solens and JJ electrolytics, however I am using motor runs for the WE caps and as bypass caps on the JJ's on both stages). Next project is the 300B version. Anyhow, I decided I wanted to try the two section filter to see if I could get smoother dc with smaller caps. I have just started playing with the sim program, and I thought I was seeing a ringing on startup. It looked as though 2.2u/8h/47u/8h/94u didn't show the ringing, and had almost no ripple. Am I reading the sim correctly? I was planning based on 200ma current draw, hoping for around 450-475 volts (I am not going to run quite as hot as Lynn's version) using the lundahl 500v trans with a hybrid bridge.

• Re: Power supply question - arend-jan 15:28:11 06/21/06 (5)
  In Reply to: Re: Power supply question posted by Jim Womble on June 20, 2006 at 17:17:05:

  Hi Jim.

  I can't speak for Kevin or Lynn, but there probably is a good reason why they are both using a single LC network in the power supply. Perhaps Kevin could tell us?

  You undoubtedly have noticed that there is a heated debate going on on the tube diy forum on this topic, so please take anything I say as my personal opinion. If you want to be on the safe side, stick to Lynn's schematic (this is what I would do anyway).

  About the capacitors, I think it is benifical to keep the first cap low (as much as output voltage allows) because a higher value will also give higher pulse currents on the rectifiers. Use a good high voltage wideband snubber type here. The last cap should be able to deliver the currents requested by the output stage when it transitions into class AB, so it should not be too small. If you are going to experiment with exotic capacitor types I'd start with this WE oil type that is closest to the output stage. This one is probably the most audible.

  I don't think it's easy to design a good LCLC power supply. In fact, I think that power supply design is often underrated/underestimated. I certainly don't think I'm qualified to design one, so I'd build the simple one LC section as per Lynn's schematic first. If you breadbord it, you can always experiment and see if you like it with two sections later on.

  A nice and easy experiment to do, is to add a RC section. This lowers the ripple quite effectivly and is less likely to mess things up. Another interesting option I still want to try is to add a shunt regulator on the output stage.
  
  

  ---

  • Re: Power supply question - Jim Womble 17:56:27 06/21/06 (4)
    In Reply to: Re: Power supply question posted by arend-jan on June 21, 2006 at 15:28:11:

    Thanks for you input. Seems like we exchanged posts regarding driver stage harshness in the amity design. You are probably right about sticking to the less complicated scheme of a single lc. I was looking for a little less ripple. On my SE 300b I had to use two stages to get the hum down to an accepatble level (less than 1mv) - I am using a pentode driver, which has poor ripple rejection. On my pp2a3 I am using a knockoff of the GE 97fxxx series motor run for the WE cap and also as the cap that essentially bypasses the final ps cap. What type of caps did you use in your PS? Did you use the shunt regulator tube on the driver stage?
    Jim
    
    

    ---

    • Re: Power supply question - arend-jan 15:18:06 06/22/06 (3)
      In Reply to: Re: Power supply question posted by Jim Womble on June 21, 2006 at 17:56:27:

      Well, with a PP stage you get a free 20-30 dB rejection of power supply ripple. Not the case with SE so you typically need more filtering in the power supply.

      About the capacitors, I'm using the exact same caps as you are, JJ electolytics and motor run oil caps. I'm not so fond of parts rolling and generally try to find improvement on the algorithmic front (the circuits) first. But I may try some different brands when there is nothing else left to tune.

      About the driver stage, yes, up until now I've always had the VR tube in place. If you leave out the shunt regulator on the driver (but keep the CCS) and you remove the capacitor between the centertap of the output transformer and the virtual cathode, the operational mode of the driver shifts from PP to differential (forced balance). Lynn once wrote that he prefers to "let the tubes find their own balance", but it's easy enough to try both to see what you like. The shunting cap (WE cap) is IMO one of the most interesting parts of the design, although I have not been able to get it to work as intended (reducing 3th and 5th harmonic) yet.

      So many things to try and so little time :-)
      
      

      ---

      • Question for arend-jan re: Input stage - Ironmonger 01:35:01 06/25/06 (2)
        In Reply to: Re: Power supply question posted by arend-jan on June 22, 2006 at 15:18:06:

        Hi arend-jan,

        May I ask what tubes you are using on your input stage?
        ECC99's/5687's etc or DHT's as in Lynn's original Amity DHT?
        Has anyone tried the EML 20's or 30's or AVVT in the input?
        
        
        

        ---

        • Re: Question for arend-jan re: Input stage - arend-jan 07:58:08 06/25/06 (1)
          In Reply to: Question for arend-jan re: Input stage posted by Ironmonger on June 25, 2006 at 01:35:01:

          Hi,

          I'm using the ecc99 in the input stage. 5687 is very nice too. Also tried the 6N30P but did not like them very much in this position.

          Never tried a DHT but went on to build a Karna instead. I picked the AVVT 8B as a driver but with the increased gain and double the filament voltage compared to a 45, I could not get away with AC on the filaments. I think you will experience the same with a EML 20 + you have to deal with microphonics. But it could be a very nice amp if you manage to get it right!
          
          

          ---

          • Re: Question for arend-jan re: Input stage - Ironmonger 10:40:13 06/25/06 (0)
            In Reply to: Re: Question for arend-jan re: Input stage posted by arend-jan on June 25, 2006 at 07:58:08:

            Hi arend-jan,

            Thanks for sharing this information!

            I've had Lynn's Amity DHT cct. diagram for over 6 years and would love to have a go at building this amp when I get the time and money!
            The cct looks simple enough but I think there would be a lot of small issues to solve including as you point out microphonics, power-supply design and the sensitivity problem which would require a Pre-amp suitable of producing 18.5 vrms output to get full output with AVVT 8B's.

            Thanks again, Russell
            
            

            ---

Post a Followup: