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RE: Question on VTL 2.5

Posted by Palustris (A) on January 19, 2017 at 09:26:17

In Reply to: Question on VTL 2.5 posted by dyl71 on September 9, 2016 at 14:16:01:

Most systems with a separate preamplifier have too much gain. The input sensitivity of your amplifier with no attenuation at the input is 1.4Vrms. This means that it takes only 1.4Vrms to drive the amplifier to clipping. Since we know that the Red Book industry standard output voltage for a CD player is 2 Vrms, it stands to reason an amplifier with 1.4Vrms input sensitivity does not need any further amplification in the signal chain. Indeed, any further amplification adds noise and distortion.

I know this is an unpopular message: we guys are gear junkies and like to have two boxes. The truth is, though, that each gain stage adds both noise and distortion. In fact, you could reasonably argue that each component the signal passes through changes the signal, even if ever so slightly. Add RCA jacks and cable into the mix and the potential for corruption of the signal increases.

So the noise you are hearing is the first gain stage triode "tube rush" being amplified throughout the entire rest of the signal chain. The gain of your preamp is 2.5 (on the 8dB setting) and the amp has a gain of 28. So if we assume that there us 15mV (.015Vrms) of noise in the first stage amplify that by 70 and you get a full volt (1Vrms) of noise on the output of the amp with no signal!

Well, if you are not concerned with having the best sound possible, you can add -6dB attenuators between the preamp and the amp. That will knock the gain of the preamp down to 2dB and it will attenuate the noise -6dB. So you will take the 2Vrms from the CD player amplify it by 8dB, then attenuate by -6dB and use the volume control on the preamp to ensure the amp never sees more than 1.4Vrms and you have just taken a pristine signal from the CD player and added noise and distortion in an attempt to fix too much gain in your system.

So, what's the "take away?" Reduce the number of gain stages to the minimum; use only one attenuator within the signal chain; know the input sensitivity of your power amplifier; if you have too much gain in the system reduce the number of gain stages. It that was my amp I would punch a hole in the front panel and install a 50k volume control, but naturally, that would affect the resale value.

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