My tube preamplifier project

[thomsva]

My next project will be a four channel tube preamplifier. It will be mostly used for two channel audio, but I want to occasionally enjoy multichannel SACD:s. Power amplification already takes the multichannel approach into account.

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Design goals:

No gain
As my cd player gives out more than enough voltage to drive the power amp to maximum power I can't see any need for gain. A unity gain buffer is what I will be looking for unless someone can point out why I would need gain.

Good volume control
I hate channel balance problems on low volume settings. I may end up with a dact or similar to get even attenuation curves. The rear channels can have their own attenuator. Multichannel recordings can be very different and I think it has to be easy enough to adjust the level of the rear channels. I don't need a remote control.

One 4-channel source, two or three stereo sources
I will need something to switch between sources. A two pole switch is enough as the rear channels can be dimed out trough their volume control. Unless there is a need to switch signal grounds as well. Many commercial preamplifiers leave the grounds of all sources connected but the ultimate solution would be to disconnect the unused sources completely.

RIAA
One of the stereo sources will be a turntable. I might leave space in the chassis to fit a riia stage. But I haven't given this much thought yet.

Pleasant sound
I want a system that highlights the music and provides easy listening. Most of all I want to avoid any excess harshness. I have solid state power amplification and I hope that a good tube-pre would improve my system.

Low noise
I listen a lot at night with low volume. Even if my speakers aren't the most sensitive, I want the noise floor to be as low *** possible.

High input impedance
I believe that my system will sound better if the preamp is a very easy load for the source. With a high input impedance the source won’t have to push mush current to drive the preamplifier.

Low enough output impedance
I would not like my system to be very dependent on cable capacitance etc.

Low enough distortion
I want low enough distortion with the second harmonic dominating the distortion spectrum. The second harmonic will always sound musical in relation with the actual note. The third and other uneven harmonics seldom will. I believe this is one reason why tube amplifiers sound nice.

Not too much heat
All tube amplifiers are inefficient. I want to build a good performer but I don't want it to be too much overkill. I have other ways to heat the house. As a compromise I plan to use solid state rectification instead of very inefficient tube rectification. But I plan on using a tube regulator in the power supply. Audio tubes like ECC99 or 5678 would be very nice, but to save some current on both heater and anode I would prefer smaller tubes.

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Solutions:

Cathode follower stage
Without having tried anything yet, a cathode follower stage would do what I need at least technically. As I’m a fan of tubecad.com I plan on using this circuit by John Broskie, the brains behind the Aikido preamplifier.



In this circuit the cathode resistor is replaced by the other triode half. It’s grid is driven by a portion of the power supply noise in order to maximize ripple rejection, an Aikido trick.

I will bias the triodes for 10-15ma of anode current. Enough current will ensure that the stage can drive difficult loads without problems.

Input impedance
As the volume control will be before the tube stage, and the tube’s grid has a very high impedance, it will be the volume control that determines the input impedance. A high input impedance makes life easier for the previous stage, but if a high value volume pot is used it will be very sensitive to any capacitance between the volume control and the next stage. The Miller capacitance between the tube’s grid and cathode will generally dominate and it can filter out high frequencies. But here is a good thing about the cathode follower. The cathode’s voltage will move along with the grid. This effectively eliminates the influence of the capacitance. I think I could easily use a 250k volume control without losing bandwidth.

The PSU
I will start by using my existing LM317 regulated psu. Later I will build a new tube regulated psu. Separate PSU:s for left and right channels can be considered but my feeling is that it would be a bit overkill…

Tubes
I have almost settled on the ECC88. I have a pair, and they will be the ones I do my testing with.

Capacitors
To be decided….

Chassis
I have plans to disassemble a rack mounted 1U high amplifier and use it’s chassis for this project. For better looks I will replace the 19” faceplate with a piece of wood. This chassis will contain only the audio circuits. Because of the low profile tubes will have to be mounted horizontally.

The plan is to use a similar chassis for the PSU. If I disassemble two of these amps I will have two big low profile toroids to connect back-to back for the psu.

PCB vs point to point
As there isn’t many components, point to point would be easy. Also horizontal tubes work well with point to point. But I don’t rule PCB:s out yet.

Internal cables
Shielded or not? Maybe unshielded is better as the PSU is outside the chassis.

Other parts
Hmm… these teflon tube sockets at Svalander.se are beautiful:


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These were my current thoughts on the subject. I'm still in the process of looking for parts so I can still change just about anything. It will also be easy to try out different kind of preamp circuits around the ECC88.

Comments are appreciated!

[SY]

Although my phono stage article isn't ready yet, I did go through a design for a unity gain line stage in a fair amount of detail. You may or may not want to duplicate my design (it's not a super-simple one), but the discussion of how to do it might help answer a lot of the questions you're posing here.

Part One:

A Heretical Unity gain line stage