Tube buffer stage

[Taito]

Sounds like fun project eh?

Ok, here's the background: I'm most of the way through an electrical engineering degree, love design work and am getting to love the smell of hot solder. I have what I consider to be a very nice stereo (CD player/Pre/Power : Rotel RCD-1072/RC-1070/RB-1080, JM Lab Cobalt 826S speakers). I really enjoy my late night listening sessions, BUT, like everyone here, I am looking for that little bit more.

I am too young for valves (lucky me ), but have been interested in them for a long time. I figure that throwing a tube buffer stage into the mix could be a fun exercise.

At the moment I am in research mode -devouring web sites, have some books on the way from amazon etc.

I'd like to know what you guys think and any suggestions / ideas that you have.

Some questions:
What is your favourite small signal valve?
None of my gear is balanced (although the power amp does have XLR as well as RCA inputs). Should I go balanced or single ended?
Parafeed?
Use an output transformer?
What sort of topologies do people out there like?
All valve, or just valve output stage?

I want to gather as many possibilities as possible so that once I have kulled them back I am heading in a really good direction. All input is wanted and appreciated. Cheers, Ben

[cjd]

While I know this will draw scorn from certain members here...

check out the chip-amp forums (also, others) at diyaudio.com - also, check out Decibel Dungeon - VERY good write-ups for non-ee types like me.

A tube buffered chip-amp is semi-common.

I'm working on a few chip-amp projects, but no tubes expected to be used at the moment.

C

[Saurav]

Try the tube forum on DIY Audio and the Tube DIY forum on Audio Asylum. You'll probably get more advice there.

With my next-to-nothing knowledge of tubes, I would recommend looking into the ECC99. This is a fairly new tube, and is much more linear and has lower distortion than most of the popular small signal tubes. For a buffer you'll want low output impedance, so you'll probably need some kind of totem pole/SRPP topology, or a cathode follower. Try Googling for Pete Millett, he wrote up something about an ECC99 preamp with measurements. You could also look into solid state active loads - those bring down distortion by another order of magnitude, and some of them have a low impedance output tap that you could use, I think Gary Pimm's ones do.

[Dennis H]

JonMarsh aka Evil Twin is our resident professional EE poohbah but he's traveling on business. Wait a few days and he should have some good advice for you. (Just guessing but be ready for a dissertation on low-negative-feedback solid-state designs that will outperform tubes with all of the advantages and none of the drawbacks. It's one of his hobbies and his ideas have made it into some ultra-high-end audio gear from Ayre.)

[Saurav]

Wait... Evil Twin and JonMarsh are the same person? OK, now I feel stupid. Why did I never realize that before? Man, he does a good job.

OK, I guess all the regulars will get a good laugh out of this

[Kevin P]

Hey Saurav, didn't know you hung around here! Or maybe I did but I haven't seen you post in a long time, or I just don't tend to lurk in the DIY section much. I figure I have to contribute something to this thread considering what I use as my avatar.

Taito, it sounds like you're looking to design your own circuit, or are you looking to assemble a unit in kit form? You might check out the Bottlehead kits if you're considering going that route. Their Foreplay pre-amp is nice from what I've read.

If you're looking to design from scratch, well, I have no clue how to help. :duh: Jonmarsh probably will though.

[JoshK]

Wait... Evil Twin and JonMarsh are the same person? OK, now I feel stupid. Why did I never realize that before? Man, he does a good job.

OK, I guess all the regulars will get a good laugh out of this

Your not alone....I was starting to piece it together recently but wasn't sure till just a little bit ago.

[JoshK]

Re: tube buffer stage. While it isn't strictly a tube buffer, I have built the simple 5687 preamp that diyparadise.com documents. I agree that the ECC99 is probably the best tube for such a circuit but as I understand it the ECC99 should slip right in in place of the 5687 with some minor tweaking.

The simple 5687 provides 15x gain which I don't care for (too much) but Thorsten derates the gain in his circuit and others do as well so it can be tailored to be more of a buffer I'd imagine. The 5687 tube sounds great, doesn't add too much tube color but does add a touch of the 3D type of sound tubes are known for. Call it colored if you like, I do, but I still like it once in a while.

[Taito]

Thanks for the input so far everyone! To clear things up, yes, I am intending to design the component myself (from scratch). All of these ideas are very helpful though, as I want to consider as many permutations (at least at the pre-design phase) as possible. If I do see something really clever in somebody else's schematic, I will quite happily include it -coming up with a design which does not use any previous ideas is almost unheard of.

One other thing: while I don't feel that I need another preamp, if this buffer does turn out well, I may alter it and add such things as a TVC (transformer volume control) to make a really special preamp.

Cheers, Ben

[Saurav]

I use a TVC by itself as my passive linestage. Unless you need the gain, I think it works great. We once compared it to a few active linestages up to $4K, SS and tube, and everyone felt the TVC was best for transparency and clarity. The good active linestages were better for bass heft and had a more solid feel to the sound, kinda hard to describe. For about $300, that's not bad

[Spearmint]

Hi Taito

Firstly let me commend you on wanting to design and build your own valve buffer stage, obviously you have done some homework with their attributes. I am using the Musical Fidelity X10v3 unit with my Rotel setup and they do make a nice improvement in audio quality.

Maybe you can get to trial one of these MF X10v3 if you haven’t already done so to realise the benefits which they can provide, this may assist you in your design work as well.

BTW if you are interested here is an inside view of the MF unit taken by another owner.

[Doug Lockwood]

If you want to learn about tube design, check out http://www.tubecad.com/

The programs he sells are also very useful and are a bargain.

For a buffer, I would use a Cathode follower using a bi-polar power supply.
The 6DJ8 and 6NP1 are good choices, among others.

Do not skimp on the voltage in the power supply. While +-35 volts will "work", higher voltages make the device more linear.
If you aren't maximizing linearity, what’s the point?

Also, most of the topologies mentioned are useful for gain stages, but overkill for a buffer. KISS.

JMHO

Doug

[Taito]

Spearmint, actually the X10v3 was more or less my inspiration for this project (That and it seems like a better (for the cost involved) area to get into higher end design that power amps).

Doug, too right about power supply. I would always rather go for overkill when it comes to power supplies. What I am seriously considering at the moment is using a high voltage supply that constant current source(s) draw from.

I must admit, I do find it more enjoyable to go all out when designing, but I'll try to keep it simple.

-Ben

[hired goon]

G'day,

So did you get yer tube buffer stage up and running?

--Geoff