Simple way to measure preamp output voltage/amp input V...

[Selmerdave]

In that case I'd do it aurally with a known mono recording. Set both volume knobs to the exact same place and then adjust level pots at the back until image is centered.

I think it's unlikely the motorboating issue has anything to do with output voltage. Others will be able to help far more than me, but perhaps the other preamp is putting out a frequency out of the audio band that is causing the oscillation?

 

[knockbill]

I could do it with a tone recording also,,, for some reason I thought measuring output was the was to go... probably wrong tho!
As for the motorboating, may be the amp has too much gain in the 1st stage (paralleled 6SL7 triodes) for the SS pre... I may go back into the amp at some point and rebuild the front end, but this SS phono pre is the only thing it had a problem with in a couple years...
Thanks for the input,,, just trying to learn a little more on each project...

 

[knockbill]

I set up a variac and a small PT using the 6V secondary to put out 1VAC into each preamp separately, and with level pot and VC pot on full ,,, #1 reads 4.02VAC Out ,,, and #2 reads 4.00VAC Out... Not sure if this proves anything, other than approx the same amount of gain is going thru each preamp line stage,,, still have to recap the phono signal sections...
Thanks for the input and conversation... Without knowing the input requirement of the amp they are pushing, seems just using level pots and VC should balance the output (music) fine...

 

[PakProtector]

4:1 gain is not so high, and they are effectively matched quite well. The component values could easily leave it much larger.

On an amp otherwise happy, oscillation can be triggered by capacitive loading. The amp might just be amplifying an oscillation created further upstream...some further investigation will likely find something silly.
cheers,
Douglas

 

[knockbill]

Thanks for the help,,,, I matched all the resistors I put in as close as possible,,, figured comparing in to out V would be worthwhile,,, I'm surprised how nice these sound,, so far... Now I need the phono section to come out as well!

 

[JRM]

Hi All. I found this old thread and I'm hoping someone will see this and offer some advice. I think my preamp output might be too much for my amplifier input as my amplifier VU meters sit nearly maxed out nearly all the time when I am at the volume I actually enjoy.

I have a VTL 2.5 tube preamp and a Phase Linear 400 power amplifier. Is my assumption correct and how can I address this to correct the situation?

 

[gadget73]

If you're talking about the meters on the PL 400, those measure output power, not what comes off the preamp. If they are pegged in the corner either you listen at ear-bleed levels or there is something wrong. 0db should be 200 watts per channel output. I'd guess there is a problem with the meter circuit. Haven't studied up the schematic on that one but its probably just a pair of resistors across the output wired in series so it knocks down the voltage to a proper level to run the meter. If the resistor on the ground side is high value or bad the meter will read much too high.

 

[whoaru99]

my amplifier VU meters sit nearly maxed out nearly all the time when I am at the volume I actually enjoy.

You'll need to better define "the volume I actually enjoy" and the approximate listening distance from the speakers to make heads or tails if there might be a problem, or if it's simply that you need more rig for the gig.

 

[JRM]

You'll need to better define "the volume I actually enjoy" and the approximate listening distance from the speakers to make heads or tails if there might be a problem, or if it's simply that you need more rig for the gig.

Well the volume on the preamp sits at around 11'O clock because that is where it actually gives of the kick that i enjoy. I would definitely not define that as being at max output of the pl400 because picking the volume higher would reflect some change in output but seeing what the needles are doing, I'd dare not even go there.

The needle on the pl400 sits in the red zone with slight jumps and reducing the volume makes the needle move like it should.