Can a change in Preamplifer lead to a 3db boost in spl?

medium jim · « **on:** 5 Jan 2013, 03:54 am »

A few weeks back, I retired my McIntosh MX110 Preamp/Tuner and have been using my Bottlehead Foreplay. What I have noticed is that with the Bottlehead that I have about 3db more of SPL. I know this seems illogical. The McIntosh in the preamp stage uses 12AX7's (I had a trio of Telefunken ECC83 smooth plates in it), and the Bottlehead is set-up for 6FQ7's. The 12AX7/ECC83's are 100MU gain tubes, whereas the 6FQ7/6CG7's are rated at 24 MU Gain.

The Bottlehead that I have has the following mods:

Pseudo dual mono power stage, each signal wire is shielded and has its own ground; The volume controls are in shunt mode; It has RRSF which is a filter to take any nastiness out of the electricity coming from the wall; Vishay-Dale resistors; Modified to use 6FQ7's instead of 12AU7's for a deeper Soundstage.

The spec are:

Maximum output is 10V rms (28V pk-pk) before clipping. Gain is 10 dB. Signal to noise ratio is 90 dB at 2V rms output (a typical CD player max output level), and 114 dB at 10V rms output. Input impedance is 33K ohms-48K ohms (variable shunt attenuator). Output impedance is 700 ohms - recommended output load is 10K ohms or greater. Frequency response is +/- 0.5 dB from 2Hz to 50kHz. Phase is inverted.

The specs on the McIntosh:

Response 20-20kHz (+0.5 -0.5dB) , Distortion 0.2%. Input sensitivity and impedance: aux: 0.3V at 200k, phono: 3mV at 47k, tape head: 3mV at 220k, tape monitor: 0.3V at 100k. Hum and noise: high level -80dB, low level less than 3uV.

Note: I was using the Aux in.

Is is merely a case of System Synergy or is there something in the math that is giving me that extra usable 3db of SPL? Do Magnepan's perform better with low gain preamp's and that is why this is? Or am I crazy and this cannot be possible?

Jim

richidoo · « **Reply #1 on:** 5 Jan 2013, 04:12 am »

Nominal voltage gain for the Mac aux input is 20dB, typical of vintage preamps. http://www.roger-russell.com/tunpre.htm

Were the Mac's input gain controls maxed? They are hidden under the wood.

Foreplay is nominal 19dB gain, but mods can make it anything.

medium jim · « **Reply #2 on:** 5 Jan 2013, 04:17 am »

The gains on the McIntosh are on the sides, early model, and I do/did have them maxed. The Bottlehead has 10db of gain the way it was built if that helps.

Rich:

I think I understand what you are driving at, I am was using too much gain instead of letting the tubes do what they are designed to do. I should have known this as I bias the output tubes a little on the cool side.

Funny thing, I really like the Bottlehead in the main system.

Thanks,

Jim

medium jim · « **Reply #3 on:** 5 Jan 2013, 04:46 am »

I guess I still am learning about this stuff and hope I do until the day I start pushing up daisies!

So I'm not crazy and I did gain 3db's...

Jim

Ericus Rex · « **Reply #4 on:** 5 Jan 2013, 01:04 pm »

Quote from: medium jim on 5 Jan 2013, 04:46 am

So I'm not crazy and I did gain 3db's...

Jim

How did you measure that? The only way I can think of measuring the two accurately would be to max both volume controls and take readings. Fill me in, please.

medium jim · « **Reply #5 on:** 5 Jan 2013, 04:07 pm »

Quote from: Ericus Rex on 5 Jan 2013, 01:04 pm

How did you measure that? The only way I can think of measuring the two accurately would be to max both volume controls and take readings. Fill me in, please.

Not so scientific as that, what I've been doing is listening to it as loud as it would play without hints of compression and sibilance in female vocals and used my trusty Digital RS SPL meter on the fast C weighted calibration at the listening position. I already knew what the McIntosh/Marantz soft clipping point was and know I know what it is with the Bottlehead/Marantz setup is. With the McIntosh it was at 96db average SPL and with the Bottlehead is 99db. This is not scientific, but it is conclusive to me.

It all happened by accident as I was listening and I thought, wow this is louder than I normally listen to and started measuring. I had already done the same with the McIntosh as I wanted to know the upper limits of my system at that time.

My thinking is that less gain results in allowing the tubes to not overdrive too early and provides more clean headroom. I need to note, that I have to use a lot more of the volume knobs and that sort of freaked me out at first, but as long as there is no distortion or stuff, it really doesn't matter.

Jim