A preamp for montesquieu

[Radford Revival]

I am in the process of building a prototype bespoke preamp for montesquieu, I thought people might be interested in seeing it unfold.

The preamp will be a hybrid valve / solid state design, using valves and MOSFETs. The FETs will be acting as voltage followers, but at the same time performing a bootstrapping function, allowing the valve itself to operate with lower distortion than it otherwise would (not that it would be particularly high in the first place).

Tom / montesquieu wanted to have a preamp that used 6SN7s, which as well as being aesthetically nice valves, are well known to be nicely linear and tend to produce remarkably little distortion when implemented correctly.

Here are some of the bits gathered so far. The mains transformer is bespoke for the project, the pot is a simple ALPS affair. The angled brackets you see are so that the valves can be mounted horizontally, 6SN7s would otherwise be too tall for the case.

Tom wanted a tape loop, so to avoid impedance issues I may implement a buffer using the venerable NE5532 opamp, possibly a somewhat contentious issue for a purist! In all honestly the distortion introduced by an opamp (especially the NE5532 which still remains a good performer even today), especially connected as a voltage follower, is so minuscule, it's simply not worth worrying about.

This of course brings about the argument that one could just as well use an opamp for the main gain section of the preamp, and not bother with the valve at all. This is entirely true, and from an engineering perspective makes absolute sense. From my perspective, it can be just a matter of enjoying the fact that the amplification is done by a valve, and as the colouration by either technology can be so low as to be irrelevant, it really can just be down to that alone. Tom's views may differ from that of course so hopefully he'll like the way it sounds

[montesquieu]

My prejudices say all-valve, but I trust Will's instincts as well as his technical expertise, so not going to presume to tell him how he should go about designing it (even if I had the language to, which I dont). It will be fascinating to hear how this works out. My suspicions are it will sound ... Like a Radford!

[RobsterD]

What no foo parts,such a cheapskate Tom

[Radford Revival]

My prejudices say all-valve, but I trust Will's instincts as well as his technical expertise, so not going to presume to tell him how he should go about designing it (even if I had the language to, which I dont). It will be fascinating to hear how this works out. My suspicions are it will sound ... Like a Radford!

Normally (without the tape loop) there will be one solid state component in the signal path, and MOSFETs behave in a way very reminiscent of high gain pentodes so you can pretend it's a plastic valve

[awkwardbydesign]

Love the brackets. Every expense spared!

[Ali Tait]

Not much wrong with a NE5532 TBH. Not something I'd have wanted until recently myself, but since getting hold of a Tandberg R to R, it would be a necessity for me too now.

[Firebottle]

Liking the approach Will, I will follow with interest.

How do you tell which has the left electrons and which has the right electrons with all white phono sockets

Alan

[Andrei]

The angled brackets you see are so that the valves can be mounted horizontally, 6SN7s would otherwise be too tall for the case.

But still visible?

[awkwardbydesign]

But still visible?

That's the only purpose of valves, isn't it?

[CageyH]

Not in my pre-amp.
It has to be "little finger" safe. Anything that glows is a magnet.

I get the sonic properties of valves without the worry about burnt fingers.