1. Win a Broadcaster or one of 3 Teles! The annual Supporting Member Giveaway is on. To enter Click Here. To see all the prizes and full details Click Here. To view the thread about the giveaway Click Here.

Mid-fi mini-stereo amplifier idea

Discussion in 'Shock Brother's DIY Amps' started by tvas22, Apr 16, 2021.

  1. tvas22

    tvas22 Tele-Meister

    Age:
    31
    Posts:
    315
    Joined:
    Aug 28, 2016
    Location:
    UK
    TLDR is I'm looking to re-use some parts for a low powered mid/hi-fi stereo amp, asking for feedback on design ideas.

    I have been using my Bassman Micro build as a mid-fi mono amp for listening to music on and off, but my Princeton Reverb build has taken over as my main amplifier.

    I've been considering gutting my Bassman Micro (which I'd already butchered by turning it in to a Trainwreck Rocket style mini-amp) and using the parts to build something actually designed more for listening to music to.

    It doesn't have to be particularly loud, and the build is more going to be about the best I can do cheaply. The 370AX power supply limits my output options somewhat too, and I'd rather use that than buy something.

    Essentially, I'm thinking of mashing together the 4S Universal Preamp, a cathode follower, and the 12AU7 power amp from the Deluxe Micro build, and pretty much keep the Bassman Micro power supply/filtering section. I read quite a few people suggesting adding a cathode follower to the 4S preamp, which is why I've added that in, and I'll probably add in a pre-amp out and switched dummy load so that I could use a more powerful power amp with it in the future.

    I'd seen that someone has already had a go at doing something similar with the Deluxe Micro too.

    This means that I'd need to buy one extra 12AU7, and two output transformers (I'm going to go with the Hammond 1750A), and I should have everything else I would need on hand.

    I have had a go at putting together a schematic for this idea, just showing one channel (obv. it'd be the same for left and right). Does anyone have any thoughts/suggestions? This is only my 5th build so I'm still very much learning.

    Thanks!
     

    Attached Files:

    • HiFi.png
      HiFi.png
      File size:
      211.6 KB
      Views:
      17
  2. Tom Kamphuys

    Tom Kamphuys Tele-Holic

    Age:
    41
    Posts:
    731
    Joined:
    Sep 18, 2018
    Location:
    The Netherlands
    The dotted line needs to be present and the output of the follower needs to be taken from the cathode.

    The 470k grid stopper is not what I would expect. Doesn't that limit the higher frequencies?

    I _think_ it is an unusual hifi design, but it's your amp!
     
    tvas22 likes this.
  3. tvas22

    tvas22 Tele-Meister

    Age:
    31
    Posts:
    315
    Joined:
    Aug 28, 2016
    Location:
    UK
    I wondered about the 470k grid stopper, that came from the Deluxe Micro so I just copied it over - on Robrob's page for the Deluxe Micro, it states that "The large value 470K grid stopper helps prevent blocking distortion in the power amp". The overall pre-amp gain should be lower, I think, as there's one less gain stage (although there's also no tone pot or preamp volume pot) in my design, so perhaps a smaller value would be fine. I can experiment with it and see what happens anyway.

    The cathode follower's output should indeed be coming from the cathode! :( I have changed it.

    The dotted lines are 'present', I just put them as dotted so I knew they weren't connecting to the solid lines they pass through.
     

    Attached Files:

  4. tvas22

    tvas22 Tele-Meister

    Age:
    31
    Posts:
    315
    Joined:
    Aug 28, 2016
    Location:
    UK
    I also realised I forgot to link the 12AU7 grids together.
     

    Attached Files:

  5. Mongo Park

    Mongo Park Tele-Afflicted

    Posts:
    1,112
    Joined:
    Nov 28, 2010
    Location:
    Winnipeg
    Looks like you are stoking out on your own for a personal journey of discovery
    Always a fun and educational. Repurposed parts is always a good thing
    You might find something useful in this amp, many build threads around it. It is designed to be se, inexpensive( it can be built with repurposed iron), and sounds rather good. You can find threads using different tubes. It runs about 4-5 watts. It might give you some insight into what is the essence of a stereo tube amp.
    Good luck on your journey.

    http://rh-amps.blogspot.com/2013/02/rh84-amplifier-revision-2_26.html?m=1
     
    Nickfl and tvas22 like this.
  6. tvas22

    tvas22 Tele-Meister

    Age:
    31
    Posts:
    315
    Joined:
    Aug 28, 2016
    Location:
    UK
    Thanks! That looks great, and sort of similar to the Mullard 3-3 which was a contender too. The problem is that the power supply I want to use has only 58ma available on the secondary, and anything with a ‘proper’ power stage seems to need around double that.
     
  7. printer2

    printer2 Poster Extraordinaire

    Posts:
    7,270
    Joined:
    May 24, 2010
    Location:
    Canada
    You got 23 VA available (and Hammond is usually conservative with their numbers), so take half that and then a third, three-four watts. I would be more concerned with the output transformer putting out bass. Either way this power level will easily handle polite listening levels. I would not bother with the cathode follower though, won't be doing much before you start overdriving the output. Save the VA's for the output tube. You just need a 12A*7 as a driver, not a demanding application. If the price is not too bad I would try for a hifi OT. The 125B or 125C? They all seem to sound the same and the C would give you the most versatility in the future.
     
    Last edited: Apr 16, 2021
    tvas22 likes this.
  8. Mongo Park

    Mongo Park Tele-Afflicted

    Posts:
    1,112
    Joined:
    Nov 28, 2010
    Location:
    Winnipeg
    I can see that is definitely a limiting factor. Which you seem to be able to figure out, beyond my tube skills. watching to see how it comes out.
     
  9. tvas22

    tvas22 Tele-Meister

    Age:
    31
    Posts:
    315
    Joined:
    Aug 28, 2016
    Location:
    UK
    It would have to be one of the SE range, which would bump the price a little, but I also can’t find one of the 125SEs with a compatible primary impedance. I am open to alternates, though.

    Would the cathode follower still help it be a little more versatile if I tapped the preamp out to use with another power amp in the future?
     
  10. tvas22

    tvas22 Tele-Meister

    Age:
    31
    Posts:
    315
    Joined:
    Aug 28, 2016
    Location:
    UK
    Hmm, actually the Deluxe Micro page suggests using a 125ASE as an upgrade, so maybe it’s more flexible than I was thinking. That is 2-10k primary, so I might look at that.

    Would it be beneficial to get a higher wattage version, such as the BSE or the CSE? Both are only a few £ more.
     
  11. NTC

    NTC Tele-Holic

    Posts:
    586
    Joined:
    Mar 17, 2003
    Bigger usually means more low end. That would be a very good thing in an audio amp.
     
    Nickfl and tvas22 like this.
  12. printer2

    printer2 Poster Extraordinaire

    Posts:
    7,270
    Joined:
    May 24, 2010
    Location:
    Canada
    But you also have to see what the induction is. I look at it as a cute project but what if you want to do something in the future. All said and done, tubes are cheap. If you decided to use your iron in a different amp you are not limited because you used a 12AU7. On the cathode follower, everything is a tradeoff. I had a tendency to throw everything and the kitchen sink in, then pared things down to what I really wanted. I do not think it will add anything in a hifi context. If you decide to use it in other ways then it depends on where you go from there.
     
    NTC likes this.
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.