Posted by Sheldone (A) on January 17, 2007 at 21:04:53

I have a resistor loaded 801 amp based on a driver design by Steve Bench. It's cap coupled to the OPT, which is probably larger than I need, as the amp puts out a thundering 0.5W of power. I also use it for a TAD compression tweeter on a multiamped system. I was wondering if there would be a benefit to using a transformer closer to the power requirement. The closest I've seen are some EXO 100 cobalt OPT's on the Magnequest site. Are they still available? Would they be a potential upgrade?

Sheldon

• Re: EXO 100 - SLM 17:35:22 01/18/07 (2)
  In Reply to: EXO 100 posted by sheldone on January 17, 2007 at 21:04:53:

  Hi there

  Sorry to weight in here - I use a single stage parafeed 801a amp on some compression drivers. Mikeys Exo 45 transformers - following gordon rankins advice I use 5k transformers with the 801a choke loaded running at 400v/40 ma/-20v. Sounds super clear with a fantastic midrange. "flipped" B7 15 k at the input.

  At your operating points of 325 volts and -27 bias it looks like the 801 a is running at about 15 ma from the curves . The combination of a 27 k resistor and a 10K transformer gives an effective load of around 7.3 k - I suspect the 801 might like a little more current at this load - you will probably need more B+ but it would be worth exploring if you can

  800v should give you 20 ma?

  801a parafeed is the best of maybe 20 amps I have built

  Simon
  
  

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  • Re: EXO 100 - sheldone 18:45:06 05/29/07 (0)
    In Reply to: Re: EXO 100 posted by slm on January 18, 2007 at 17:35:22

    That's a pretty strong recommendation. I think I'm going to try this. With my present power transformer, it looks like I can rework it to give me about 30mA at 400V on the plate. With the 10K EXO 100 and the Dowdy? Mike, what size is the Dowdy compared to the Exo? Got a couple of those?

    Sheldon
    
    

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    Edits: 05/30/07
  • Re: EXO 100 - Sidewinder 11:24:48 01/19/07 (0)
    In Reply to: Re: EXO 100 posted by SLM on January 18, 2007 at 17:35:22:

    Hello Simon,
    Your circuit sounds interesting. Is it possible to post your schematic or a link to the circuit. thanks.
    David
    
    

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• Re: EXO 100 - mqracing 22:28:25 01/17/07 (9)
  In Reply to: EXO 100 posted by sheldone on January 17, 2007 at 21:04:53:

  Hi Sheldon:

  I'd love to see a schematic for your amp. Small amps can be absolutely amongst the best sounding ones. Years ago I remember talking to Mr. Atarashi on the phone and he told me that one of the most beautiful amps he had ever built was a 71A PP amp for like 1.5 watts output.

  the exo-100's cobalts are long ago sold out. But we can make the EXO-100's with other cool mats like high or low nickel, M4, M3, pinstriped, etc. At small power level of 500mw a lot of lams would do quite well.

  best to contact me by phone (evenings best) at 215 288 4816 or by email at acrosound at aol dot com

  Mike

  
  
  

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  • Re: EXO 100 - sheldone 15:40:14 01/18/07 (8)
    In Reply to: Re: EXO 100 posted by mqracing on January 17, 2007 at 22:28:25:

    Thanks Mike,

    I figured, given the date on the site, that they were gone. But I'm still interested in trying smaller transformers. I'm not sure how to post a schematic, so here's a link: http://www.diyaudio.com/forums/showthread.php?postid=883558#post883558

    By the way, I got the original transformers from Jack at Electra-print. They are 37:1, I believe, and are small transformers intended for maybe 5 or so PP watts. They sound nice, but wondering if smaller wouldn't be better, especially on the top end.

    One other project I'm planning, is to use this amp as a driver for this clever little FET follower amp, for when I want more power: http://www.susan-parker.co.uk/zeus.htm

    But instead of using the regular amp transformer output and a step-up interstage, I'd want to take the output directly from the output cap, and put that into a step-down interstage to give me a voltage swing of about +- 10V to the FET's (about 10:1). I've got some Lovaltec JFETS that I want to try here (won't need the bias supply). Any thoughts on that one. There looks to be a unit from Lundahl which might work, but custom would be cool too.

    Sheldon

    
    
    

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    • Re: EXO 100 - mqracing 16:43:31 01/18/07 (7)
      In Reply to: Re: EXO 100 posted by sheldone on January 18, 2007 at 15:40:14:

      Hi Sheldon:

      thanks for the urls to the schematics. The 801 schem looks interesting... would love to hear all these different incarnations.

      we still have some exo-100's... and would be happy to build a pair for you. I think they would do quite well with your 801.

      MSL
      
      

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      • Re: EXO 100 - sheldone 16:54:45 01/18/07 (6)
        In Reply to: Re: EXO 100 posted by mqracing on January 18, 2007 at 16:43:31:

        Sounds good. However, I only picked the EXO 100, as it appeared the closest to what I'm looking for (not that I know much about what to look for). If that's the best choice, I'd go for it. What lamination configuration would you suggest for this application, remembering that I don't need the low end.

        Sheldon
        
        

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        • Re: EXO 100 - mqracing 17:05:48 01/18/07 (2)
          In Reply to: Re: EXO 100 posted by sheldone on January 18, 2007 at 16:54:45:

          Hi Sheldon:

          You could really pick from the full range of core mats that we have (at the power levels your talking). Then it is a question of voicing... each sounds a bit different. The nickel adds a bit of warmth... but has good low level resolution... the M4 tends towards very good clarity and see-through... some of our customers really prefer the admixture of M6 and nickel (as a pinstripe) and feel it sounds best. We offer the range for you to choose from (knowing that it can be a difficult choice).

          The nickel would be the most expensive... the pinstripe in btwn all nickel and all M6.

          these tranneys are relatively small guys... so they come in channel frames with solder lugs. They have a single impedance secondary of 8 ohms.

          Mike
          
          

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          • Re: EXO 100 - sheldone 17:19:47 01/18/07 (1)
            In Reply to: Re: EXO 100 posted by mqracing on January 18, 2007 at 17:05:48:

            Thanks Mike,

            I have no knowledge base on which to make an informed decision. The reason I want to try this, though, is to test the idea that proper sizing of the transformer and proper choice of materials should give the lowest zero crossing distortion, and maybe better low level quality. A bit more delicate, if you will. I'd go with your recommendation here. If you see the speakers, and given the time spent already, you'll guess that moderate cost differences are not a major issue (excluding platinum laminations and gold wire with rare silk insulation - type of thing).

            Sheldon
            
            

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            • Re: EXO 100 - mqracing 17:25:34 01/18/07 (0)
              In Reply to: Re: EXO 100 posted by sheldone on January 18, 2007 at 17:19:47:

              After looking at your system.... and reading your comments above... the nickel might actually be my recommendation for you. Nickel seems to come alive early and at low, low induction levels. And it would give you, yet, another new experience.

              msl

              
              
              

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        • Re: EXO 100 - here's the output end - sheldone 16:58:51 01/18/07 (2)
          In Reply to: Re: EXO 100 posted by sheldone on January 18, 2007 at 16:54:45:

          BTW, this a brief description of the entire system: http://www.diyaudio.com/forums/showthread.php?s=&threadid=69906

          Last couple of posts describe the amp side.

          Sheldon
          
          

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          • and with the russian teflons you are using.... - mqracing 17:19:57 01/18/07 (0)
            In Reply to: Re: EXO 100 - here's the output end posted by sheldone on January 18, 2007 at 16:58:51:

            forgot to mention that the exo-100 is made with teflon insulation--- I've always liked the sound of the teflon. I think we are one of the few (actually I don't know of anyone else) using teflon in audio tranneys.

            MSL
            
            

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          • Re: EXO 100 - here's the output end - mqracing 17:16:12 01/18/07 (0)
            In Reply to: Re: EXO 100 - here's the output end posted by sheldone on January 18, 2007 at 16:58:51:

            Hey... greeat pics. I spotted the russian teflons as well as the russian bathtubs in the power supply. If you have a stash of the 20uf at 400v... let me know :=)

            the pic of the speaker with the top cover off... couldn't tell if my eyes were playing tricks on me or not... looked like there were multiple compression drivers in that stash...

            and... I really liked your choice of wood for the chassis and what appears to be a brass top plate. Even our competitor's tranneys looked good on your amp.

            man... them russian teflons do eat up some space though, heh?

            the exo-100's are going to look lost on your chassis... perhaps you need some of the dowdy chokes to fill the empty top spaces and plate choke load your 801... only other thought is that you may want to increase your blocking cap from 2.2 mfd to something larger in say the 4 to 6 mfd range. You could experiment and scope out the differences.

            msl
            
            

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