This type of NFB does not have some of the advantages of global NFB. In particular it doesn't improve the noise performance that global NFB brings by cancelling the noise of the subsequent stage(s). Also, this type of NFB increases the output impedance of the stage, which may be detrimental to the HF and transient response. It does not significantly reduce distortion either. It does not decrease the global output impedance, with all the consequences it may have on response.squarewave said:Why don't tube pres use the old fuzz pedal trick where the emitter / cathode resistor is actually a potentiometer with a large cap to ground on the wiper.
It is actually possible to modify a common-cathode stage for a larger range of gain control, but that would be at the expense of increased noise and reduced headroom.Tubetec said:I suppose you could try that ,but my guess is that the amount of gain range you would get wouldnt be massive.
just found this ,
7db difference with 12ax7 bypassed vs unbypassed
Heikki said:Who here measures the performance of the equipment they build anyway.
Tubetec said:In saying what I said above ,of course I dont mean to underplay the importance of proper scientific measurements and testing .However with simple tube and transformer coupled circuits analising about THD numbers doesnt give the full story, the balance between lower and higher order harmonics is more important.
slight derailment, but Ian I'm wondering about REW - when I use it, the minimum output level seems to be -40dB. What do you do to measure higher gain units? Just put a simple pad in front of it? or is there a way to adjust this in the software?ruffrecords said:PC based audio spectrum analysers are cheap nowadays so there is no excuse for not seeing the harmonic structure. REW (free) married to a decent USB powered audio interface (like the Scarlett range for example) is all you need.
Matt C said:slight derailment, but Ian I'm wondering about REW - when I use it, the minimum output level seems to be -40dB. What do you do to measure higher gain units? Just put a simple pad in front of it? or is there a way to adjust this in the software?
No doubt.Tubetec said:Thanks for the reply Abbey,
So maybe a sectionalised,PI wound ,or Bifilar ,
Maybe CJ knows
It's almost impossible to answer this for you, because we don't know what you may or may not like. Distortion in the first stage is generally very different than that of the final stage, since the latter reacts with the output xfmr. Think guitar amps, where most players like the distortion of the output stage being cranked up whilst other are content with input distortion à la Mesa Boogie.Matt C said:Interesting information, thanks. Let me alter my question a bit: when you are getting a tube preamp like this to distort, are you better off creating the majority of that distortion in just one of the stages, or creating multiple points that are all distorting at once? Tonally, what are the implications of each?
You may be on a wild goose chase here. Euphonic distortion in pro audio products is serendipitious; designers design for cleanest, and users abuse gear. I don't believe you can design a jack of all trades that you could dial for clean, crunchy or dirty at the flick of a switch. Just look at how there are so many different overdrive pedals and all have their use and idiosyncracies. In fact the closest thing to what you're asking is a DSP-based modelling preamp. I don't think that's the route you want to take.A bit of background, a while ago I built a two-channel preamp based on the Collins 6Q-1, I love the distortion characteristics when it is cranked but I find it's a bit of a one trick pony. I'm aiming to build something that can give me similar (or even nicer?) distortion, but can also perform well as a relatively clean, high gain preamp.
You will have to experiment with all sorts of variations and topologies in order to find what you like or not, and live with the fact they're all different. That'll give you a good excuse for owning more than one mic pre.The 6Q-1 is just two triode stages, 2nd stage is R-C coupled to the output transformer, B+ is quite low around 150Vdc. So maybe all I need is a similar topology with another tube and some clever gain staging options. All these other arrangements just have me wondering what I'm missing.
Heikki said:Who here measures the performance of the equipment they build anyway.
A few reasons:PRR said:I hate to say it, but I used to do my dirt in the DAW. I really can't accept the idea of distorting a live performance-- what if you over-do it? There is no un-do! Capture clean, then beat it in the CPU, with un-do.
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