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Preamp to use with Dynaco ST-70?

nerdorama said:
I like a 6SN7 Aikido line stage. You can build one fairly simply. The other options mentioned above would be good too.
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+1 on the Aikido's. I've built a 12AU7 and 12AX7 version and then bought a 6SN7 version that I'm using now. They're very nice sounding preamps if you don't want or need tone controls. I've rarely felt the need for tone controls with this preamp. I built the Universal preamp just for fun and it's very simple, cheap, and easy to build. It uses just one dual triode. I've tried it with all my tube amps and it works well.
(Edit) so far not having a cathode follower hasn't really been an issue, although it would probably be better the have one.
 

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gadget73 said:
Personally I'm not over the moon with preamps that just have a lot of gain, so I'd probably be going for something thats on the low side.
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I've been thinking about this a lot recently in terms of preamp transparency. Does a minimalist design actually need any gain at all? A choke-loaded cathode follower has huge headroom with virtually zero distortion, and bandwidth can easily extend beyond 100kHz. This might be a future project for me.

Jack
 
John Ess said:
Or a simple buffer like this... https://wtfamps.com/muchedumbre-3/
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Yes, it doesn't have to be choke-loaded, only correctly designed. I see he also addresses the characteristic of the extremely high input Z of the follower. The output stage in my current preamp project takes advantage of that by using an ultra-transparent polystyrene at the grid. It's only .0047uF. :)

Jack
 
TriodeLuvr said:
Yes, it doesn't have to be choke-loaded, only correctly designed. I wonder why he used such a large input coupling cap. The input impedance at the grid is in the multiple-megohm range because the stage is bootstrapped. The output stage in my current preamp project takes advantage of that by using an ultra-transparent polystyrene at the grid. It's only .0047uF. :)

Jack
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Dunno...but had given this circuit some thought to replace the output ic in my TU-660...I've got 2 so one try with. It would have to be outboard of course.
 
Magnolia Tom said:
Will this setup work well?
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This should work fine, assuming the st70 is ready to go!
Vintage tube equipment may need maintenance prior to use. It does look like your amp has some newer capacitors, but I can't tell much from photos. I hope the seller offered you some details as to whether your amp is 'plug & play'!
 
John Ess said:
Dunno...but had given this circuit some thought to replace the output ic in my TU-660...I've got 2 so one try with. It would have to be outboard of course.
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I also have a TU-660, but it doesn't get much use. The audio quality of most FM broadcasts is terrible now, so I'm not tempted to spend much time or trouble trying to improve the tuner itself. I listen to CDs frequently, but LPs are still my gold standard.

Jack
 
TriodeLuvr said:
I also have a TU-660, but it doesn't get much use. The audio quality of most FM broadcasts is terrible now, so I'm not tempted to spend much time or trouble trying to improve the tuner itself. I listen to CDs frequently, but LPs are still my gold standard.

Jack
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Yeah..I get it. Curious though...that tuner works well and has decent audio quality. I get just enough good stations to make me think about improving the antenna and the audio output.
 
TriodeLuvr said:
I've been thinking about this a lot recently in terms of preamp transparency. Does a minimalist design actually need any gain at all? A choke-loaded cathode follower has huge headroom with virtually zero distortion, and bandwidth can easily extend beyond 100kHz. This might be a future project for me.

Jack
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depends on your source and the amp I suppose. One of my sources is the computer which is good for about a quarter volt output. My usual amps need a bit over 1 volt for full output so I need some gain there. I actually really need less on the phono stage, it was designed to run an LOMC without a head amp. Even with the gain set as low as it goes its louder than the CD player at the same volume control position. Luckily its passive EQ so I should be able to sub a 5751 and knock that back without screwing up the frequency response. I have a set somewhere, just need to find them and confirm it works as I expect.


my other two preamps are both 1/2 12au7 for gain and 1/2 12au7 for cathode follower. Neither have bypass caps so its not a high gain setup. The Bottlehead I added feedback to so it has even less gain than they normally have. Not enough power transformer in either to swap it to a 6cg7 unfortunately.
 
I recently tested a couple small toroid transformers for possible use matching a headphone amplifier to somewhat higher Z phones. The bandwidth was really excellent. I'm wondering now if this might also be a useful technique at the output of a line stage to increase overall voltage gain. I regret now that I didn't test the response of the transformers with the lighter loads typical of line level circuitry. I'm knee-deep in a project at the moment, but this is something I want to revisit soon.

Jack
 
If you know how to solder take a look at the Bottlehead Moreplay preamp, it uses 6V6 tubes or any others in that family. Very easy to follow assembly instructions and best of all it sounds fantastic.
 
I use various Sansui receivers and integrated amp as preamps for my ST-70. I like the sound and the features of all the inputs.

Sansui Seven, 5000x, AU-717 all work great as pre's. FYI those Mullard power tubes are worth more than the amp, get some new production el34's while you sort everything out as those tubes are irreplaceable.
 
mrwood200 said:
This should work fine, assuming the st70 is ready to go!
Vintage tube equipment may need maintenance prior to use. It does look like your amp has some newer capacitors, but I can't tell much from photos. I hope the seller offered you some details as to whether your amp is 'plug & play'!
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He stated it was looked at by a technician and passed with flying colors
 
Honestly you could do waaaaay worse than an ST70 as an intro to tubes. A metric buttload of these were sold for a reason. I should own one but frankly I don't need one. I worked on one purely for the purpose of having one to listen to vs my Pilot Sa260 though. It's a nice amp. It did absolutely nothing wrong, but sounded very different from the 260. I made a post about it on here somewhere but I'm on my phone and like 4 beers deep right now so I'm not going to look for it.

Anyway it's a good amp. Enjoy it.
 
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