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 4 responses by dhl93449

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.
Al:

I understand what you are pointing out, but higher harmonics of the 60 Hz fundamental will be significantly decreasing in amplidtude as you get to higher orders.

One way to determine this experimentally would be to observe the noise at a given volume setting (of the line amp), then disconnect the interconnects and replace them with (a) shorting plugs or (b) better yet shorting plugs with resistors simulating the source impedance of the phono pre-amp. If the noise levels drop considerably in this test, then perhaps you are right and the increased noise levels are due to ground loops or IC issues. If not, they are simply the thermal noise generated by the various stages of amplification.

Everyone tends to assume that line stage noise is insignificant compared to phono stage noise, but depending on the relative gain (and other issues such as source impedances) this may not always be the case.

Also, one needs to abandon rules of thumb based on the rotational position of gain controls. There is no reference standard here, and the gain vs position is highly product design dependent.

Regarding my spectrum analyzer measurements, I was looking at the spectrum from about 50 Hz- 20k Hz. I did not apply any weighting (A or otherwise) to my measurements as the Tek will not do this (it is a vintage 1980's SA but has a S/N/distortion capability of -130 dB).
Captain Winters:

Re your question of acceptable levels, this is a bit subjective. For me, -60 dB would not be good enough, as I can easily hear this level from my listening position at the highest sound levels I listen to.

You can check out my post "Impressions of the PS Audio GCPH" under the amps/pre-amps section for the actual numbers, but they were on the order of -90 to -100 dB (unweighted), depending on gain, referenced to a 1 volt output. You can add about 6 dB to those numbers since my normal line input levels (to the line amp) is 300-500 mV at max listening SPL. Even in the worst case of GCPH input gain of 66 dB and MAX volume gain control, noise was at -75 dB unweighted. This level is almost inaudible from my listening position.

I emphasize "normal max" listening volumes, which for my JC2 is 9-10 on the volume pot. If I crank up the gain to 12 or full CW rotation (4-5) the -75 dB levels become very audible.
Captain Winters:

Yes, I was thinking about your measurements, and at 55 dB SPL you are nearing ambient levels, depending of course on your house and environment. If you are picking up ambient contributions, your baseline noise floor of your audio system may be lower than what your SPL meter indicates. Food for thought.

That is why I like the direct measurement with a SA. You can also make this type of measurement with a distortion analyzer, but the SA has the advantage of showing you graphically exactly what frequencies are contributing to the noise levels, especially useful if you have 60/120/240 Hz line related contributions.

Line noise is particularly insidius due to the emphasis of the RIAA EQ curve. Remember that when you speak of phono pre-amp gain, it is usually referenced to 1 K Hz, which is right in the middle of the EQ curve. 50-60 Hz signals (noise or otherwise) entering the phono pre-amp inputs are amplified by 20 db (a factor of 10x) over the 1 KHz level, so any low frequency noise in this region is amplified considerably. Likewise, very high frequency noise is attenuated by the same 20 dB in the other direction.