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

In article <tugts3p9n831vqc87guusnnbr94ngahk7m@[EMAIL PROTECTED]
>,
 The Phantom <phantom@[EMAIL PROTECTED]
> wrote:

> On Tue, 04 Mar 2008 19:41:35 GMT, John Byrns <byrnsj@[EMAIL PROTECTED]
>
wrote:
> 
> >In article <fqf4uh$tam$1@[EMAIL PROTECTED]
>,
> > Ian Thompson-Bell <ruffrecords@[EMAIL PROTECTED]
> wrote:
> >
> >> Eeyore wrote:
> >> > 
> >> > Ian Thompson-Bell wrote:
> >> > 
> >> >> Has anyone thought of using a grounded grid amplifier as a mix
amp?
> >> > 
> >> > No point as far as I can see.
> >> > 
> >> > Graham
> >> > 
> >> 
> >> I can think of several.
> >> 
> >> 1. Only needs one triode
> >> 2. Input/output transfer characteristic is very linear when used with

> >> current source inputs
> >> 3. No Miller effect.
> >> 4. Input impedance approx 1/gm
> >
> >Are you sure about that?  By my calculation that approximation only 
> >holds as long as Ra is small compared with the load resistor.  Perhaps
I 
> >made a mistake in my hand done calculations and should try to look the 
> >result up in the RDH4th?
> 
> I calculate input impedance as:
> 
>       RL + ra
>      ---------
>      1 + gm*ra
> 
> which would indicate that ra should be large compared with the load
> resistor for the approximation 1/gm to be good.

Yes, that is the same formula I derived.  The Ra vs. load resistance 
thing in my commentary was a typo, thanks for the correction.  I am 
always afraid that I am going to flip the two components of a comparison 
around like that when I put it into verbs, and sometimes I actualy do.  
It sort of made nonsense of my comment a couple of paragraphs down.


Regards,

John Byrns




> 
> >
> >> 5. Gain approx anode load resistor over input resistor
> >
> >Doesn't that gain approximation assume that Ra is low compared with the

> >value of the load resistor?
> 
> I calculate gain as:
> 
>           (1 + gm*ra)*RL
>     -------------------------
>     (1 + gm*ra)*Rin + ra + RL
> 
> which means that for the given approximation to be good, ra + RL must be
> small compared with (1 + gm*ra)*Rin.
> 
> >
> >There seems to be a serious compromise here with respect to achieving a

> >low input resistance while maintaining the gain close to unity!
> 
> As one might expect, to get this result gm*ra should be large.
> 
> >
> >> Disadvantages are:
> >> 
> >> 1.Heater cathode leakage can inject hum/noise into input path.
> >
> >Is this a serious problem assuming a DC heater supply is used, and the 
> >heater bus is well filtered/isolated to prevent the entry of other
noise 
> >sources?
> >
> >
> >Regards,
> >
> >John Byrns

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