Preamp - 18 dB Gain too low?

Coming into this late but the first thing you need to do is differentiate between gain and attenuation. Gain is fixed at 18dB and attenuation is variable.

The 800 pound gorilla in the corner is lumping gain and attenuation into an arbitrary setting of a dial without reference.

A decade ago I measured some flavor of the day 10K pot to relate the gain to rotation in both dB and voltage ratio. By no means is this a standard, it is just a statistical sample of one but hopefully it illustrates the issue. (remember these values are for attenuation only)

(Apologies the formatting doesn't hold it is a three column table)

Rotation Db attenuation voltage ratio
7.00.00 AM 0ff NA
8.00.00 AM -50.798242 400
9.00.00 AM -37.669889 78.78787879
10.00.00 AM -28.995433 28.10810811
11.00.00 AM -21.98006 12.14953271
12.00.00 PM -13.492213 4.727272727
1.00.00 PM -8.299467 2.524271845
2.00.00 PM -5.2231697 1.818181818
3.00.00 PM -2.7932399 1.382978723
4.00.00 PM -0.7861947 1.06122449
5.00.00 PM -0 1

This is where it gets fuzzy without the actual numbers of "your" attenuator wrt "clock rotation" If we make the grand leap and assume all attenuators have this particular taper, it becomes easy.

With 18dB of gain, you would have overall unity gain between 11 and 12. (again assuming you have the attenuator I measured installed) This means if you listen at 1-2 the concept of a passive is really not an option since you would actually need gain. (again look back at the assumptions in this story)

Unfortunately there is no bureau of standards for the audio industry so you have either have to roll with it or put in the time to learn some of the basics to make an informed decision.

My gut feel is adding more gain to simply add more attenuation is not the way to go.

It is also important to realize I have only addresses the gain aspect of this situation. There are two other key points that play a more important role in system synergy which are impedance matching and headroom. Unfortunately neither of them can be directly read by the dial rotation value.

dave

Hi Dave,
Thank for sharing the detail of your measurement. I swapped the active preamp CJ 18 dB gain with a passive Lightspeed Attenuator, and I found that most of CDs, I have to push the volume knob to 3 - 5 PM (MAX == 5 PM) to obtain the desirable volume. The low sensitivity amp, 4 V, seems to be an issue. Bought this amp without understanding the input sensitivity spec...

What is attenuation voltage ratio and what does it tell us?

Some amps have high input gain because they are designed to be used with passive preamps which have no gain at all. I can tell you that CJ amps need a preamp with relatively high gain to properly drive them. 18db of gain is higher than average.

I have owned some CJ preamps and they do invert phase. I corrected the phase by reversing the wiring to the speaker binding posts. I couldn't tell any difference myself. It could be because some recordings are already recorded with inverted phase.

Hey,

'between 3-5pm' are just arbitrary numbers and without seeing an actual attenuation plot vs. knob rotation of the LDR, you really don't know that much. Also seeing an input and output impedance plot of the device WRT attenuation would be needed to see if any red flags go up.

dave

Nguyen hi, just looking at the factory specs for your 845 amps they are very sensitive at There is something not right with your system as Red Book standard for cdp is 2v out and most are higher than this so your Lightspeed should be below half way. What is your cd or source (has it been modded?) please give model no's

Cheers George