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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
captain_winters

Showing 2 responses by captain_winters

captain_winters OP
133 posts
 
Thanks for the responses, of interest, and an indirect question, was based on the SNR published for the Acurus RL-11 of 95db A Weighted. The Avid Pulsus on the other hand has a noise spec from -81db to -67db. I have plenty of gain so operating the Pulsus at 48db of gain, even though the XX-2 is 0.28mv, still gives me the “typical” preamp range of 10:00 – 2:00. If strictly going by that rule, I would have set up the Pulsus gain that way. If you look at the SNR in that configuration it is SNRdb = Psignaldb – Pnoisedb. SNRdb = 114db – 60db = 54db. In the Pulsus 60db gain configuration. SNRdb = 114db-52db = 62db. Looking at the specs, Pulsus being noise floor at gain, and Acurus being SNR, it is not apparent to me how you would surmise this result analytically, without actually measuring.
It looks like Dhl93449 hit it on the head when he said
“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.”

A related question, has anyone actually measured their SNR using this method? I believe I have a quiet system and I also believe 62db is a good result, but what is a reasonable expectation?
captain_winters OP
133 posts
 
Al, DHL93449
Good points, at 0.00028v with 60db of gain with the Pulsus, gets me to 0.28 volts. My RM-9 MKII Amp at the high gain setting, lowest feedback (which I prefer) puts out max watts (125w/channel) at 0.4 volts (spec in original post). Assuming its proportional, then 87.5 watts = (125*0.28)/0.4. So from 0 – 87.5 watts the Acurus RM-11 preamp is attenuating the signal. From 87.5 watts to 125 watts only an additional 3.1 db is needed. So out of the 17.7db of gain available from the Acurus RM-11 the most that will be used is 3.1db and that is at very high listening levels. So with this configuration 99.999% of the time the signal will be attenuated by the Acurus preamp. There is a lot of discussion on this forum about passive preamps and “possibly” the loss of dynamics with those devices. However, many point out that it is due to the input and output impedance of the passive devices. In my case, I presume, even though the signal is attenuated, the input impedance of the Preamp is still very high (10Kohms) and the output impedance is still low (47ohms). If I recall the RM-9 MK II input impedance is also 10Kohms. So there should be no signal frequency interactions due to impedance mismatching. Therefore even though the signal is attenuated almost all the time I am getting the benefit of an active preamp, would you agree?
If I make the preamp more active, set the pulsus to 48db of gain. Then the 0.00028v cartridge signal exits the Pulsus phono stage at 0.071. Assuming proportionality, then 22 watts = (125*0.071)/0.4. Therefore the Acurus RM-11 preamp is attenuating from 0-22 watts and from 22 watts – 125 watts it is providing 0db – 15.1 db. The preamp range is from 0 – 17.7 db, so on paper these seems the way to go, however, in my Sound Pressure Level measurement, it produced more white noise.
For reference, when cranking up the volume the noise I hear is pink in nature. I won’t call it pink, since it covers a limited frequency spectrum, It seems like all frequencies are being crossed over to the tweeter. I have no buzz or hum, which I am thankful for, since I have spent a lot of time in the past trying to track down those problems in a different system configuration. This noise is inaudible when I play a record or lift up the tonearm at almost all levels from normal listening positions.
DHL93449
Thanks for the reference, I will look up the GCPH post. Yes as far as my Sound Pressure Level test, it was very limited. First the lowest measurement on my SPL meter is 55db, hence I had to crank up the volume to 114db to even get a noise measurement. And there was only one sample 114db. A multi-meter test with multiple samples at the speaker output terminals would be a much better test. Maybe I will do that today, for the heck of it. It would be interesting to calculate the noise level using voltages at normal listening levels and see what the resulting SNR is. That would compute total system performance without the limitations of my SPL meter and the air gap variable.
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