Re: Where is Graham's mixer design? (was Re: RAT Tube Mixer update?)

Ian Iveson wrote:
> Ian said:
> 
>> Well, I've made my position clear a while back and I have
>> been busy lately doing part 2 of NFB 101 in the process of
>> which an idea struck me. Has anyone thought of using a
>> grounded grid amplifier as a mix amp?
> 
> Yes. It seemed the obvious choice for the bus. An
> alternative would be an ordinary gain stage with a low value
> grid resistor.
> 
> I couldn't maintain interest in mixer design beyond getting
> the ideas straight. It seemed like a fool's errand, and
> still does. I can't see why a valve mixer is a good idea. A
> box of mic amps, yes, with a state of the art SS mixer amp
> and chip earphone amp, maybe.
>

I guess it is the same reason some people prefer valve amps to SS ones - 
oersonal preference for the 'sound'. There are certainly a large number 
of pro and semi-pro mic pres that are aither all valve or use a valve or 
two somewhere in the chain to obtain that elusive sound.

For myself, I have been a keen sound recordist for over 40 years and 
even worked at Neve in the 70s designing pro mixers. Now that I am 
retired my main hobbies are music recording and audio design. Since I 
was 'brought up' on valves I am having great fun revisiting them and my 
main current project is to build myself a couple of all valve mic pres 
to supplement  my existing recording kit


> And I could only copy a respected valve mic amp, considering
> I've never done any recording and have no idea what they are
> supposed to sound like.
> 
> And I noticed that no-one else has actually posted very
> specific details. Coyness, in addition to the usual
> animosities, was clearly going to frustrate any attempt at
> serious discourse.
> 

That surprised me too. Maybe it was the thought of making money from the 
project that caused people's reticence. For me it is an interesting 
exercise in design. However, the design is just the start of the job. 
The physical implementation is at least as difficult.

> And I don't know enough about noise to adopt a rational
> strategy, and this particular mixer idea isn't a good
> learning vehicle because I don't care much for the money or
> its man, for whom I suggest Eeyore is a good match, which
> is what I said, and still think. The man doesn't want what
> he's asked for, and Eeyore won't provide it.
> 

Noise is obviously a crucial issue in mixer design but the principles 
are applicable to any audio design. The first stage is the crucial ones 
and for valves the most straightforward way of obtaining a respectable 
noise performance is to use a 10:1 mic transformer which will have a NF 
of a dB or so and nearly 20dB of gain which makes the noise performance 
of the first valve almost irrelevant. Thereafter it is mostly a matter 
of ensuring the signal remains adequately above the noise floor.

> John seems to be taking it seriously. Never seen him so
> keen, bright-eyed and bushy-tailed, deserves a prize of some
> sort.
> 

Yes, I like John. He is pretty knowledgeable even though mic pres seem 
outside is experience and you can at least have a sensible conversation 
with him without it descending into the sort of vicious diatribe that 
has become all too common on this group.


Cheers

Ian