Hoffman Amplifiers Tube Amplifier Forum
Other Stuff => Other Topics => Topic started by: shooter on August 25, 2019, 06:20:07 pm
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Trying to wade in the kiddy pool n my 1st stops got me tangled!
Here’s the snippet I’m looking at;
“A broadly useful input sensitivity is 200mV. It will cope with most sources. A lower value of 400mV suits silver disc players and modern tuners that typically give 1V output. It is too low for many external phono stages however. An input sensitivity of 90mV such as that of Naim amplifiers is very high, meaning volume will have to be kept low from CD.”
Why is 200mV a higher value than 400mV?
Going for the 2nd half, if my input impedance is off does the signal get dragged down toward 200mV or 400mV?
(for reference, I’ve always worked with the notion input sensitivity is the MAX pp signal that gets MAX clean out)
Thanks
dave
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Because they're talking about sensitivity. 200mV (for full output) is more sensitive than 400mV.
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for full output
Thanks
ok, I'm term deficient :laugh: logic is still correct
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It's 5' up from the bottom. :icon_biggrin:
Re input impedance. Because we're dealing with voltage transfer (as opposed to current transfer) from one device to another, we want impedance "bridging" -- generally the input impedance should be 10X the output impedance of the source device. This is just another example of the Rule of Tens. The input impedance of the typical tube input stage is often the grid leak resistor of the input stage, with a typical value of 220K - 1M (for a 12ax7).
If the input impedance is too small, it places a heavy load on the incoming AC signal. I.e., signal voltage will drop too much. IOW, there is nothing here special, or that you don't already know.
Said voltage drop varies by frequency. A resistor is a low pass filter. If the impedance of the grid leak resistor is too small, it will shunt mids & lows to ground, leaving mostly high frequencies to reach the input tube's signal grid. The result will be a thin, weak, nasally tone.
Note that the input impedance may be in parallel with the output impedance of the source device. A SS source device, or modern mic, may have an output impedance as low as 20 - 600 Ohms. That, in parallel with a 220K - 1M input impedance, yields still only about 20 - 600 Ohms net input impedance for the tube amp. There may be no way to directly bridge the output impedance of the source device to a tube amp's input impedance. One solution is a buffer.
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:laugh:
As I was typing the post I was chuckling just knowing this was a ( - apple )thing :icon_biggrin:
the last 24yrs fixing MRI, Receiver cal's were a monthly thing X 5 systems X 24yrs is ALOT of sensitivity but it was Never sensitivity for me It was 12' bnc, male adapter to SMT, one end to J1 other to Sig out, start cal software - good, next
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Please note my revised post above re impedance.