Thread: Valve preamp.
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Old 18th Jun 2019, 11:02 am   #49
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Default Re: Valve preamp.

Quote:
Originally Posted by Diabolical Artificer View Post
I built Fred Nachbaur's phono amp in the interest of science and all that, attached schematic below again, but found it to lively. Just tweaking it to get the DC conditions right took a lot of doing and I still couldn't get it bang on. Also the OP was "boingy", a bit like a wein bridge oscillator with a lamp as the gain control. So I won't be building it.
It's only a phono preamp. No rocket scientists need to be harmed in the creation of one. Trouble just setting the DC conditions does not give any confidence in the quality of a design. And the 'boingy' description, while it is both poetic and liable to attack as undefined, sounds to be a strong indication that the circuit is of only marginal stability. File under a-for Avoid. With a capital 'A'!

We are in the 21st century and it's important to consider things in context. THere are NE5534 phono stages which offer low noise, low cost and just work. They've been on the go long enough to be considered certainly classic if not quite vintage yet.

So given the existence of the NE5534 and things like Hugh Walker's 3-transistor job, why use valves?

1) If you want to learn about valves. Fine! Good! Great! But maybe you want to learn from a good circuit and not something with major problems? However you'd learn a lot from analysing those problems but this would be entirely a learning exercise and would leave you with something you would not want to use.

2) Just for the hell of it. Fine, Good fun! It can make something which can be a conversation piece and mystify visitors.

3) Because careful analysis and measurement show that valves can perform better than semiconductors in this application.... No, sorry, not generally true. Some good valve designs are better than some bad semiconductor designs but that gives no guide to the value of good semiconductor designs. Building a bad valve design is pointless unless you are studying the design errors.

4) Because you believe valves have stange and wonderful properties that science just can't explain and these translate into a wonderful sound which only true aficionados have the skilled hearing to be able to detect. Yes, well let them get on with it. Experience shows that such aficionados can never be convinced otherwise. They are best left to enjoy the specialist websites and magazines which cater to their beliefs.


There are some god semiconductor-based designs that are well-proven and sure-fire. You'll have to work quite hard to do a valve design which can equal them, but you could get to the point where no-one with normal hearing should be able to distinguish between them.

I agree with Joe that when you get to sub-dB differences it's time to stop worrying. THe recording studio probably had a guy setting powerful equalisers to wherever he liked the sound - entirely by ear. The record-cutting department would likely use a precision network to do the cutter equalisation, but the combination with the settings on the desk make the overall shaping less certain.

I agree with Craig that when analysing things, you might as well do the arithmetic with enough precision to be far better than what can be done in reality. If you don't, you add errors which then become significant.

My hearing has limitations. This gives me freedom to engineer within those limitations. But sometimes I like to engineer things to stupid levels of performance, just for the sheer hell of it, just to prove that I can. It's the same freedom that lets people choose what technologies they wish to use.

David
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