Hello - I'm restoring a Dual CV40 amp to use with a Shure M95 G-LM cartridge (requires 47k input load).
I have a question regarding the modification of the resistive input load in the phono preamp (schematic attached).
If I understand correctly, in the set up shown, the signal sees the resistive load formed by the series resistor R2 and the parallel network of R1, R3+R8 (1.5k + approx 77k) - is this correct?
If I wanted to bring the load down to approx 47kohm, changing R1 to approx 53k should do the job?
Thanks in advance for your help!
Martin
I have a question regarding the modification of the resistive input load in the phono preamp (schematic attached).
If I understand correctly, in the set up shown, the signal sees the resistive load formed by the series resistor R2 and the parallel network of R1, R3+R8 (1.5k + approx 77k) - is this correct?
If I wanted to bring the load down to approx 47kohm, changing R1 to approx 53k should do the job?
Thanks in advance for your help!
Martin
Do you have an Audiogenerator and a Millivoltmeter / Scope ?
Use a 47KOhm Resistor in series from the generator output to the magnet input.
Feed 6mV from the generator and measure the input voltage of the magnet input.
If that is close to 3mV ... you are already at 47KOhm Input impedance.
If you are above it change R1 until you see 3mV on the Input.
You could also measure the outputlevel of the preamp and bridge the 47KOhm resistor ... if the Outputlevel doubles ... you are at 47KOhm.
If it less than doubles you are above it .....
Do not use much more than the 3 / 6mV mentioned ... you may create distortion / overload on the output and the feedback will corrupt the results.
Best regards
Peter
Use a 47KOhm Resistor in series from the generator output to the magnet input.
Feed 6mV from the generator and measure the input voltage of the magnet input.
If that is close to 3mV ... you are already at 47KOhm Input impedance.
If you are above it change R1 until you see 3mV on the Input.
You could also measure the outputlevel of the preamp and bridge the 47KOhm resistor ... if the Outputlevel doubles ... you are at 47KOhm.
If it less than doubles you are above it .....
Do not use much more than the 3 / 6mV mentioned ... you may create distortion / overload on the output and the feedback will corrupt the results.
Best regards
Peter
Thanks for your comments and tips 🙂 Obliged me to read up on negative feedback and its effect on impedance! So in this case the feedback circuit reduces the input impedance (derived from T2 emitter current and fed in parallel)?
I have a signal generator, scope and millivoltmeter so I'll try Peter's test at the weekend.
Thanks again!
M
I have a signal generator, scope and millivoltmeter so I'll try Peter's test at the weekend.
Thanks again!
M
It's a variant of a circuit I know from an AES article of Hoeffelman and Meys. The input parallel resistance is strongly dependent on R5, but as that is a trimming potmeter, I don't know what resistance it has, so I can't estimate the input parallel resistance.
It's interesting that this Dual amplifier is from 1968 according to https://www.hifiengine.com/manual_library/dual/cv-40.shtml while Hoeffelman and Meys's article is from 1978.
It's interesting that this Dual amplifier is from 1968 according to https://www.hifiengine.com/manual_library/dual/cv-40.shtml while Hoeffelman and Meys's article is from 1978.
According to https://www.hifi-wiki.de/index.php/Dual_CV_40 the input resistance is 47 kohm (and it's from 1969).
When I try to calculate the input resistance range for R5 from 0 to 1 kohm, I end up with 2 kohm to 62 kohm, so 47 kohm would be in the trimming range. It's a very rough calculation with lots of approximations.
When I try to calculate the input resistance range for R5 from 0 to 1 kohm, I end up with 2 kohm to 62 kohm, so 47 kohm would be in the trimming range. It's a very rough calculation with lots of approximations.
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Regarding measuring the parallel resistance, it is bound to be frequency dependent. What matters most is the resistance between 10 kHz and 20 kHz, so I guess you should also measure it there.
You have to ensure that cable capacitance and oscilloscope input capacitance doesn't affect the measurement then. It's probably best to place the 47 kohm resistor close to the amplifier, so there is hardly any wire between the resistor and the amplifier, and to measure the level at the amplifier output.
You have to ensure that cable capacitance and oscilloscope input capacitance doesn't affect the measurement then. It's probably best to place the 47 kohm resistor close to the amplifier, so there is hardly any wire between the resistor and the amplifier, and to measure the level at the amplifier output.