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nhuwar 3rd July 2007 12:34 AM

Input impedance
I'm curious what's a normal line level input z for the average tube amp? I was thinking around 15k is this a good number or to low?



kevinkr 3rd July 2007 01:04 AM

Hi Nick,
Depends on what you want to use to drive it, 15K is a pretty tough load for most tube pre's excluding transformer coupled designs. Fine with solid state. I typically use 70K - 100K on power amplifier inputs, and 100K on pre-amplifier inputs. (Essentially the dc resistance of my stepped attenuators.)

Higher impedance inputs reduce the need for very large output coupling capacitors in the preceding component, both a cost and performance benefit.

mcs 3rd July 2007 01:08 AM

I would go with 50-100k. Old amps often had very high input impedances (250k to 1M), but with modern source equipment and preamps there's no reason to go that high.

Best regards,

Mikkel C. Simonsen

nhuwar 3rd July 2007 11:14 AM

It is a transformer and I think it's 15k resistance not z now that I really look at it.


valveluver 20th July 2007 09:41 PM

Assuming the input stage is class A, the grid resistor determines the input.

nhuwar 20th July 2007 09:56 PM

Nope not class a it's a driver stage in push pull. But I dont think I am going to use a transformer on the input.

Just for clarification doesn't the interstage transformer act as a phase shifter?


valveluver 20th July 2007 11:07 PM

Yes many amps use transformers for phase inverters.

I think you would be better off with a cathode follower driver. The transformer would degrade the low frequency performance.

nhuwar 20th July 2007 11:25 PM

Oh I didn't know that. I was under the impression that transformers only effected the high frequency response.


jnb 21st July 2007 12:02 AM

If the impedance of a winding is too low, it attenuates the lows. This is because it forms a voltage divider with the source impedance, and as the winding is an inductance its impedance falls with frequency.

nhuwar 21st July 2007 12:40 AM

So if the inductance of the primaray is to low I will get roll off.

Is this dependent on the inductance in relation to the driver tube anode impedence or just the inductance in relation to frequency?


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