Maybe the RM has considerably more self-noise than the phono stage?
Phono Section, Preamp, gain matching for noise
I set out to optimize my phono section and preamp for the lowest noise. First my equipment, VPI Classic 3 Dynavector XX-2 MKII, Avid Pulsus phono stage, Acurus RL-11 Preamp, Music Reference RM-9 MKII amp (Genalex Gold Lion KT77 and Russian 6922) and Sophia II speakers.
Having owned the RM-9 MKII amp for 18 years, I settled long ago on the “high gain” setting for 36 db, this being the lowest feedback setting sounds the best, especially with the Sophias. So that is one variable out of the way. With the RM-9 set to “High” gain I had plenty of gain at any setting since the Acurus has 17db of gain (BTW, I have also had the Acurus RL-11 for 18 years). So I had three options from the phono section 48db, 60db or 70db. At 48db the volume control was the traditional 10:00 – 2:00 range. At 60db 9:00 – 12:00 and at 70db and even smaller range.
I set the Avid Pulsus to 48db of gain, 100pf, 100R and started the test. I set up a sound pressure meter pointed 1" from the tweeter (since white noise from the phono mainly comes from the tweeter). Using a test record I set the 1Khz tone to 114 db (wearing ear plugs). At that same setting the noise from the tweeter when I picked up the tonearm was 60db. At this setting I get perfect 9:00 – 2:00 volume control on the Acurus preamp.
I then tried the 60db setting on the Pulsus, with the same 114db sound pressure, volume control reduced of course, because of the higher gain from the phono section, the noise form the tweeter went down to 52db when I picked up the tonearm. The even higher setting on the phono section produced similar results.
Therefore for my system, the 10:00 – 2:00 rule for preamps produced 8db more noise for the same sound pressure of 114 db. I was able to reduce that noise by 8db at 114db SPL by setting up my preamp for the 9:00 – 12:00 range, by increasing the gain on my phone section from 48db to the medium gain of 60db.
RM9 MK II Tube Amp
low gain 2 db damping 0.7db feedback 1 db Input for 125watts 1.2V
Med gain 32db Damping 2.5db feedback 14db Input for 125 watts 0.8v
Hi gain 36 db Damping 1.5 db feedback 10 db Input for 125 watts 0.4v
Avid Pulsus Phono Section (Solid State)
GAIN
48dB – 60dB – 70dB
INPUT RESISTANCE
100R – 300R - 500R – 1K – 5K – 10K – 47K
INPUT CAPACITANCE
100pF – 200pF – 500pF
DISTORTION
Less than 0.001%
RIAA ACCURACY
+ 0.5dB 5Hz – 70KHz (Neumann HF correction)
NOISE (A weighted)
MM –81dB MC –67dB (high setting)
MAXIMUM OUTPUT
18V RMS
MAXIMUM INPUT @1KHz MM
120mV
MC
4.8mV (MC high)
CHANNEL SEPARATION
< -85dB 5Hz – 20KHz
Dynavector XX-2 MkII output 0.28mv
Acurus RL-11 Pre-Amp (Solid State)
Frequency Response: 20Hz-20kHz
Total Harmonic Distortion: 0.06%
Signal to Noise Ratio: 95dB A Weighted
High Level Sensitivity: 60mV
Maximum Output Voltage: 8 Volts RMS
Voltage Gain as Full Volume: 17.7 dB
Input Impedance: 10K ohms
Output Impedance: 47 ohms
Having owned the RM-9 MKII amp for 18 years, I settled long ago on the “high gain” setting for 36 db, this being the lowest feedback setting sounds the best, especially with the Sophias. So that is one variable out of the way. With the RM-9 set to “High” gain I had plenty of gain at any setting since the Acurus has 17db of gain (BTW, I have also had the Acurus RL-11 for 18 years). So I had three options from the phono section 48db, 60db or 70db. At 48db the volume control was the traditional 10:00 – 2:00 range. At 60db 9:00 – 12:00 and at 70db and even smaller range.
I set the Avid Pulsus to 48db of gain, 100pf, 100R and started the test. I set up a sound pressure meter pointed 1" from the tweeter (since white noise from the phono mainly comes from the tweeter). Using a test record I set the 1Khz tone to 114 db (wearing ear plugs). At that same setting the noise from the tweeter when I picked up the tonearm was 60db. At this setting I get perfect 9:00 – 2:00 volume control on the Acurus preamp.
I then tried the 60db setting on the Pulsus, with the same 114db sound pressure, volume control reduced of course, because of the higher gain from the phono section, the noise form the tweeter went down to 52db when I picked up the tonearm. The even higher setting on the phono section produced similar results.
Therefore for my system, the 10:00 – 2:00 rule for preamps produced 8db more noise for the same sound pressure of 114 db. I was able to reduce that noise by 8db at 114db SPL by setting up my preamp for the 9:00 – 12:00 range, by increasing the gain on my phone section from 48db to the medium gain of 60db.
RM9 MK II Tube Amp
low gain 2 db damping 0.7db feedback 1 db Input for 125watts 1.2V
Med gain 32db Damping 2.5db feedback 14db Input for 125 watts 0.8v
Hi gain 36 db Damping 1.5 db feedback 10 db Input for 125 watts 0.4v
Avid Pulsus Phono Section (Solid State)
GAIN
48dB – 60dB – 70dB
INPUT RESISTANCE
100R – 300R - 500R – 1K – 5K – 10K – 47K
INPUT CAPACITANCE
100pF – 200pF – 500pF
DISTORTION
Less than 0.001%
RIAA ACCURACY
+ 0.5dB 5Hz – 70KHz (Neumann HF correction)
NOISE (A weighted)
MM –81dB MC –67dB (high setting)
MAXIMUM OUTPUT
18V RMS
MAXIMUM INPUT @1KHz MM
120mV
MC
4.8mV (MC high)
CHANNEL SEPARATION
< -85dB 5Hz – 20KHz
Dynavector XX-2 MkII output 0.28mv
Acurus RL-11 Pre-Amp (Solid State)
Frequency Response: 20Hz-20kHz
Total Harmonic Distortion: 0.06%
Signal to Noise Ratio: 95dB A Weighted
High Level Sensitivity: 60mV
Maximum Output Voltage: 8 Volts RMS
Voltage Gain as Full Volume: 17.7 dB
Input Impedance: 10K ohms
Output Impedance: 47 ohms
- ...
- 13 posts total
Al: I think he is measuring mainly white thermal noise as opposed to 60/120/240 Hz hum (via the tweeter). I found something similar with the PS Audio GCPH. That pre-amp has a low noise input stage and a gain cell buffer/output stage. The gain on the input stage is determined by a set of fixed gain settings (48/54/60/66 dB) and the gain cells have a volume control. So the input stage is analogous to your the pre-amp and the gain cell similar to the line amp. I found that the lowest noise (as measured by my Tektronix 7L5 spectrum analyzer) was found at the higher input gain settings, minimizing the output stage gain. In other words, the output buffer gain stages were noisier at higher gains than the input stage was. In fact, with gain increases of 6 dB of the output gain cells, I gained 8-12 dB in noise. But this trend was very non-linear, and this effect occured only over the last 6 dB of gain in the gain cells. Below that, the noise was dominated by input stage gain. |
I'm not sure exactly what the question is myself, but I do use an Avid Pulsus. You need to use at least the 60db setting (or even the 70db setting) on the phono stage for a cartridge with that low an output. I would forget about the "10:00 - 2:00" rule and use what's best for your cartridge. It also sounds like your amp produces more noise than the Pulsus. |
05-22-12: Dhl93449Hi DHL, I realized that he is not measuring hum. Note that I said "even though no hum occurs." Ground loop effects can cause noise at high frequencies, as well as low frequency hum. And it is conceivable to me that the former could happen in the absence of the latter. If high frequency distortion components and/or noise that are on the AC line were to couple through stray capacitances onto the chassis and/or the circuit grounds of each component, to a degree that is unequal in the two components, and if the components have their chassis and circuit grounds connected together (or connected together through a low impedance), and if 60 Hz and its lower order harmonics are not able to so couple (to a significant degree), that is what would happen. See this reference, and keep in mind that the interface between the particular components that are involved is unbalanced: http://www.jensen-transformers.com/an/an004.pdf Note this statement: The noises originating with the power line are generally described as EITHER [emphasis added] "hum", which is predominantly 60 Hz, or "buzz", which consists of a mixture of high-order harmonics of 60 Hz. These harmonics are the result of power line waveform distortion, which commonly reaches 5% THD and is caused by many types of non-linear power line loads. Because the human ear is much more sensitive to frequencies in the 2 kHz to 5 kHz range at these very low levels, buzz is usually more audible than hum, even though the hum level may be electrically larger.RFI/EMI pickup in the cables is also a possibility, as I indicated. Best regards, -- Al |
- 13 posts total