Tube preamp suggestions?

[deepak]

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.

[guzziguy]

There's always the bottlehead.com Foreplay III linestage kit. I haven't heard the III, but the earlier versions sounded nice.

[dsavitsk]

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.

[guzziguy]

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?

[deepak]

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?

[cetoole]

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?

[deepak]

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

[ujamerstand]

You could always get the 2 channel version.

[Craig Sawyers]

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.

[deepak]

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.

[grawk]

Penny&Giles

[MASantos]

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.

[spritzer]

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.

[dsavitsk]

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

[deepak]

Thanks for the schematic Doug. Sorry I should have been more clear that my intended use is purely single ended only.

[deepak]

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?

[Pars]

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.

[spritzer]

No, the TKD's are a bit different. For instance the curve is different and so is the treatment of resistive track.

[nikongod]

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.

[deepak]

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

[Beefy]

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

[Pars]

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.

[Beefy]

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

[Pars]

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

[deepak]

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.