Hey all,
I just love my Music Reference RM5 - awesome preamp that replaced quite a few other preamps...but:
How the hell does Roger (God rest his soul) equate a 800 ohm output impedance with a single 6922, no cathode follower?
I am getting over 2k...22k plate resistor, 475 ohm cathode resistor, 1000uf // .39uf bypass...80v on plates
Drivin me nuts!
I just love my Music Reference RM5 - awesome preamp that replaced quite a few other preamps...but:
How the hell does Roger (God rest his soul) equate a 800 ohm output impedance with a single 6922, no cathode follower?
I am getting over 2k...22k plate resistor, 475 ohm cathode resistor, 1000uf // .39uf bypass...80v on plates
Drivin me nuts!
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Could get there with the (150k resistor) loop feedback.
Reduces the gain and lowers the output impedance.
Reduces the gain and lowers the output impedance.
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Didn't even see that feedback loop. I must have seen this schematic 50 times and never noticed it....that could do it?
Yes, the RM5 gain appears to be configurable. The output impedance of the open loop circuit will be reduced by the
amount of negative feedback. The lower the closed loop gain is set, the lower the small signal output impedance.
This is an inverting circuit, sometimes called an anode or plate follower (although it has voltage gain).
Say the open loop voltage gain is about x50, and the closed loop voltage gain is set at about x3.
Then the factor by which the gain is reduced by feedback is 50/3 = 17. If the open loop output
impedance is say 15k, then the closed loop output impedance will be 15k/17 = 880R.
This is not to say this circuit can drive 1k loads well. The 880R is only the small signal output impedance.
The load impedance should be at least several times 1k, perhaps 5k. Even then, the distortion will rise in
comparison to lighter loads (50k and up), due to the reduction of open loop gain from the heavy loading.
amount of negative feedback. The lower the closed loop gain is set, the lower the small signal output impedance.
This is an inverting circuit, sometimes called an anode or plate follower (although it has voltage gain).
Say the open loop voltage gain is about x50, and the closed loop voltage gain is set at about x3.
Then the factor by which the gain is reduced by feedback is 50/3 = 17. If the open loop output
impedance is say 15k, then the closed loop output impedance will be 15k/17 = 880R.
This is not to say this circuit can drive 1k loads well. The 880R is only the small signal output impedance.
The load impedance should be at least several times 1k, perhaps 5k. Even then, the distortion will rise in
comparison to lighter loads (50k and up), due to the reduction of open loop gain from the heavy loading.
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