deepak Posted February 7, 2012 Report Share Posted February 7, 2012 I am in need of a preamp with some gain and I was thinking about a DIY tube preamp. Doesn't need to have crazy high gain as the power amps will have voltage gain as well. Right now what looks attractive is the K&K Audio active output stage. The level 2 kit comes with most parts including the OPTs. Looks like a fairly good design to me? If not any other suggestions? Preferably something that is well documented. Quote Link to comment Share on other sites More sharing options...
guzziguy Posted February 7, 2012 Report Share Posted February 7, 2012 There's always the bottlehead.com Foreplay III linestage kit. I haven't heard the III, but the earlier versions sounded nice. Quote Link to comment Share on other sites More sharing options...
dsavitsk Posted February 7, 2012 Report Share Posted February 7, 2012 K&K Linestage : Foreplay 3 :: Acura NSX : Honda Accord I think the K&K is probably hard to beat. The only thing I might suggest is that for output duty, I prefer the LL1930's in this topology to the LL1674's. Kevin disagrees with me on this, but while I think the 1674's are unequivocally the best input transformers available, for output duty they sound a little cold to me. The 1930's are slightly warmer, and are slightly more forgiving. And they cost quite a bit less (due to using mu-metal cores instead of amorphous, and not being shielded, which is not necessary in this application). Not sure if Kevin's board will accommodate them, but if not, it is pretty easy to mount them off board. Quote Link to comment Share on other sites More sharing options...
guzziguy Posted February 7, 2012 Report Share Posted February 7, 2012 K&K Linestage : Foreplay 3 :: Acura NSX : Honda Accord You mean that the K&K is faster and holds the road better, but has limited interior space, no luggage space and is much costlier to insure as compared to the Foreplay? Quote Link to comment Share on other sites More sharing options...
deepak Posted February 8, 2012 Author Report Share Posted February 8, 2012 K&K Linestage : Foreplay 3 :: Acura NSX : Honda Accord I think the K&K is probably hard to beat. The only thing I might suggest is that for output duty, I prefer the LL1930's in this topology to the LL1674's. Kevin disagrees with me on this, but while I think the 1674's are unequivocally the best input transformers available, for output duty they sound a little cold to me. The 1930's are slightly warmer, and are slightly more forgiving. And they cost quite a bit less (due to using mu-metal cores instead of amorphous, and not being shielded, which is not necessary in this application). Not sure if Kevin's board will accommodate them, but if not, it is pretty easy to mount them off board. Cool I will ask Kevin about the 1930s. Any suggestions for an attenuator, I'd like to get something nice so maybe a TKD pot, Sowter (9335) or Intact TVCs? Quote Link to comment Share on other sites More sharing options...
cetoole Posted February 8, 2012 Report Share Posted February 8, 2012 Cool I will ask Kevin about the 1930s. Any suggestions for an attenuator, I'd like to get something nice so maybe a TKD pot, Sowter or Intact TVCs? Quote Link to comment Share on other sites More sharing options...
deepak Posted February 8, 2012 Author Report Share Posted February 8, 2012 The preamp is single ended. Seems like an awful waste to throw money for those other 2 channels I won't need Besides one RK50 is enough insanity Quote Link to comment Share on other sites More sharing options...
ujamerstand Posted February 8, 2012 Report Share Posted February 8, 2012 You could always get the 2 channel version. Quote Link to comment Share on other sites More sharing options...
Craig Sawyers Posted February 8, 2012 Report Share Posted February 8, 2012 K&K Linestage : Foreplay 3 :: Acura NSX : Honda Accord I think the K&K is probably hard to beat. The only thing I might suggest is that for output duty, I prefer the LL1930's in this topology to the LL1674's. Kevin disagrees with me on this, but while I think the 1674's are unequivocally the best input transformers available, for output duty they sound a little cold to me. The 1930's are slightly warmer, and are slightly more forgiving. And they cost quite a bit less (due to using mu-metal cores instead of amorphous, and not being shielded, which is not necessary in this application). Not sure if Kevin's board will accommodate them, but if not, it is pretty easy to mount them off board. I have to say that I had not come across this beauty before - but it certainly looks the part. Quote Link to comment Share on other sites More sharing options...
deepak Posted February 8, 2012 Author Report Share Posted February 8, 2012 You guys are brutal see: Besides one RK50 is enough insanity And the German seller that has the 2-ch RK50 wants more than what I paid for the quad gang. Quote Link to comment Share on other sites More sharing options...
grawk Posted February 8, 2012 Report Share Posted February 8, 2012 Penny&Giles Quote Link to comment Share on other sites More sharing options...
MASantos Posted February 8, 2012 Report Share Posted February 8, 2012 The K&K looks like a nice preamp! I didn't know them. I'll be looking for a tube preamp in the near future to use with Nelson Pass' F5 turbo which was released yesterday. This might be a good option. Quote Link to comment Share on other sites More sharing options...
spritzer Posted February 8, 2012 Report Share Posted February 8, 2012 I'm sure you can get a two gang RK50 for a sane price in the US. I know Woo has have been using them so you could drop them a like. The TKD stepped attenuators are also very, very nice. Quote Link to comment Share on other sites More sharing options...
dsavitsk Posted February 8, 2012 Report Share Posted February 8, 2012 (edited) The preamp is single ended. Seems like an awful waste to throw money for those other 2 channels I won't need The K&K is differential. For use as a preamp, you need to use a pair of input transformers (Kevin suggests the LL1690's -- same core as the 1674's, but 1+1:1+1 instead of 1+1:4+4). Due to the input transformers, it will accept balanced or single ended input. If you leave out the IPTs, then you need to feed it with a balanced source. Here is the simplified schematic (no cascodes). However, for a pot, you can use a 2 gang Alps in a differential shunting configuration ("B"). This effectively removes the pot from the signal path making the quality not much of an issue. (The TKDs don't work in this configuration, b/t/w/) Here's some background on the linestage: http://www.raleighau...m/chapter_4.htm Edited February 8, 2012 by dsavitsk Quote Link to comment Share on other sites More sharing options...
deepak Posted February 8, 2012 Author Report Share Posted February 8, 2012 Thanks for the schematic Doug. Sorry I should have been more clear that my intended use is purely single ended only. Quote Link to comment Share on other sites More sharing options...
deepak Posted February 8, 2012 Author Report Share Posted February 8, 2012 The K&K looks like a nice preamp! I didn't know them. I'll be looking for a tube preamp in the near future to use with Nelson Pass' F5 turbo which was released yesterday. This might be a good option. Are you building the v3? Quote Link to comment Share on other sites More sharing options...
Pars Posted February 8, 2012 Report Share Posted February 8, 2012 However, for a pot, you can use a 2 gang Alps in a differential shunting configuration ("B"). This effectively removes the pot from the signal path making the quality not much of an issue. (The TKDs don't work in this configuration, b/t/w/) You mean like the 2P2500/2511 pots? Interesting, I wonder why? Electrically they should be indentical to an Alps RK27. Quote Link to comment Share on other sites More sharing options...
spritzer Posted February 8, 2012 Report Share Posted February 8, 2012 No, the TKD's are a bit different. For instance the curve is different and so is the treatment of resistive track. Quote Link to comment Share on other sites More sharing options...
nikongod Posted February 8, 2012 Report Share Posted February 8, 2012 However, for a pot, you can use a 2 gang Alps in a differential shunting configuration ("B"). This effectively removes the pot from the signal path making the quality not much of an issue. (The TKDs don't work in this configuration, b/t/w/) I also like this technique/circuit quite a bit, but Im also curious why it would not work with a TKD pot. No, the TKD's are a bit different. For instance the curve is different and so is the treatment of resistive track. That doesn't seem right to me. Thanks for the schematic Doug. Sorry I should have been more clear that my intended use is purely single ended only. The neat thing with a transformer coupled preamp is that the active components work exactly the same way no mater what inputs/outputs you use. There is never a situation where half of the amp is sitting around doing nothing like there may be with DC or cap coupled gear. When you add the advantage of ground-loop resistance transformers are really quite nice. Quote Link to comment Share on other sites More sharing options...
deepak Posted February 8, 2012 Author Report Share Posted February 8, 2012 I also like this technique/circuit quite a bit, but Im also curious why it would not work with a TKD pot. That doesn't seem right to me. The neat thing with a transformer coupled preamp is that the active components work exactly the same way no mater what inputs/outputs you use. There is never a situation where half of the amp is sitting around doing nothing like there may be with DC or cap coupled gear. When you add the advantage of ground-loop resistance transformers are really quite nice. Definitely, but close to $300 in input transformers does hurt the wallet Quote Link to comment Share on other sites More sharing options...
Beefy Posted February 8, 2012 Report Share Posted February 8, 2012 (edited) That doesn't seem right to me. The TKD are indeed funky. I asked the question here: Datasheet and explanation of their attenuation curve. Edited February 8, 2012 by Beefy Quote Link to comment Share on other sites More sharing options...
Pars Posted February 8, 2012 Report Share Posted February 8, 2012 Hmm, I'll have to ohm my 50k out tonight per your post at the linked thread. I don't recall seeing anything unusual in mine, and the values you showed don't seem to make sense. I actually liked their curve vs. an Alps. Quote Link to comment Share on other sites More sharing options...
Beefy Posted February 8, 2012 Report Share Posted February 8, 2012 Hmm, I'll have to ohm my 50k out tonight per your post at the linked thread. I don't recall seeing anything unusual in mine, and the values you showed don't seem to make sense. I actually liked their curve vs. an Alps. I just checked a 10kohm 2CP-2511 that I have in the cupboard, and it does the exact same thing Quote Link to comment Share on other sites More sharing options...
Pars Posted February 9, 2012 Report Share Posted February 9, 2012 (edited) Interesting. Yes, mine does behave rather strangely. From 2 to c (wiper to ground), it jumps up to 1K or so with very little rotation, then to 8k and then to 15-19k for much of the rotation, sometimes going down. Once past 3/4 rotation or so, it increases fairly linearly to max of 44k (50k pot). On the input (1) to wiper, it does actually go above the max pot value (to 55k was the highest I saw). I remember Morsel on hf said these things work somewhat like stepped attenuators... They sure work and sound good for a pot though... think of the beautiful beautiful music... <<ducks and runs like hell>> Edited February 9, 2012 by Pars Quote Link to comment Share on other sites More sharing options...
deepak Posted February 9, 2012 Author Report Share Posted February 9, 2012 I've been talking to John Chapman from Bent Audio and for about $1000 would get a full remote AVC (TAP-X modules I believe) with remote volume control, source selection and volume indicator on the front (includes custom paneling for front and rear). I've never really needed a remote with my preamps, but the thought is tempting now! The KG attenuator is another option for far less and what I'm leaning towards since I have the boards. Quote Link to comment Share on other sites More sharing options...
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