V1.1 Input Sensitivty

[galyons]

I am getting close to building my V1.1/45 Paramounts.  I plan to use my current 300B Paramounts to drive the bass bins on my speakers and the V1.1/45's for the mid/treble.  The speaker's sensitivity is about 102dB/1W/1M.  The P V1.1 manual states that the sensitivity for full power is at 0.625v (300B); about 0.4v (2A3).  Linestage is Smash/Up.

I calculated the sensitivity difference to be about 4dB, (300B v. 2a3)? Does that seem right? What impact will using 45's , (EXO45ni + BCP15), have on the 2A3 sensitivity spec?  Would using 7N7/6SN7's  in both configurations, rather than the 5670's change the ratio?  (I assume not, but thought better to confirm!)

I plan to put a 10 position stepped attenuator in place of the 250K load resistor covering about 12dB total to provide balance between the bass and mid/treble. .  I would prefer to put it in the 300B base amp.  I can also adjust the sensitivity reducing resistor value on the speaker crossover to reduce the mid/treble.  Trying to find a happy balance without giving up too much sensitivity at the speakers.

Any thoughts, comments and/or suggestions will be appreciated.

Cheers,
Geary

[Paul Birkeland]

Are the speaker impedances in question equal between drivers?  Are the driver sensitivities equal?  If you have a passive crossover that is padding one driver way down (say, perhaps, a horn tweeter?), then removing the pad on the driver will allow you to be more confident that you are padding the correct amplifier. 

[galyons]

Thanks PB!

Are the speaker impedances in question equal between drivers?  Are the driver sensitivities equal?

All of the drivers are 8 ohm.  The bass bins are 2 4ohm 12" speaker in series. The bass bin is 101dB/1W/1M and the tweeters are about 104dB/1W/1M.  The mid is about 104dB/1W/1M. ( But I have never been able to find a sensitivity rating into the horn on the Sentry IV-A, I am estimating based on the spec sheet with a horn that has the closest low freq cut off.)

If you have a passive crossover that is padding one driver way down (say, perhaps, a horn tweeter?), then removing the pad on the driver will allow you to be more confident that you are padding the correct amplifier.

Passive crossover.  The padding resistor is only on the mid. Voltage divider, R1 = 2.5 ohm, R2 =13 ohm,  so not a huge drop. The tweeter has a passive EQ circuit that drops the sensitivity to 102dB/1W/1M.  The balance is good between the bass, mid & treble.  The bass bins drop off pretty quickly after 50Hz.  I am looking to boost the bass a bit, after I get the 45 Paramount done, but first want to balance what I have with only variable being the amps.

Does that make sense?

Cheers,
Geary

[Paul Birkeland]

That makes sense. 

I would get the amps built, then send 0.5V of 60Hz signal through them and check the gain.  The 45 will have a little more mu than a 300B, but the output transformer you're using has a greater step-down ratio. 

-PB

[Paul Joppa]

The sensitivity has not been updated for the driver change in Paramount v1.1. My numbers are 1.56v for full power into 8 ohms with a 300B, 0.94v with a 2A3. The EXO-45 reduces the sensitivity, to about 1.25vRMS for full power. With a 6SN7 you will need about 2.1v with the EXO-45 and a 45.

[galyons]

Thanks Paul,
I assume you are quoting the revised gain based on the 5670 driver.  Sounds like I should convert the 300B amps back to 5670 for more sensitivity and do the 7N7/6SN7 for the 45.  Hopefully the voicing difference wouldn't be particularly noticable.  That should give me more gain, (2.58dB?), from the 300B, versus the 45/EXO45 , for the same input?

Would it make sense to forgo the stepped attenuator on the 300B and put a single value voltage divider for the input load?  I could dial it in with a 250k pot first. Then I could make changes to the crossover, and/or a voltage divider in the 45 amps,  if more gain differential is desired.

Thanks for your input!

Cheers,
Geary