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

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.

>
>> 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