headphones

[dmannnnn]

I'm wondering if I could power my 300 ohm headphones off of my 300b paramount monoblocks.  I was thinking of making a black-box with a headphone jack that could be attached to the paramounts binding posts.  Is this feasible or silly?  I'm assuming the signal would need to have some attenuation for a listenable level, but what are the other issues?  Would this be very complicated?

[dmannnnn]

If this has been discussed before, maybe someone has a link to a thread.  The search function at the old forum yields many results but no real matches.

[Paul Joppa]

In the SEX amp we found no problems with a lightly loaded output transformer, so a direct connection of headphones should work pretty well.

The only risk is that some headphones are extremely sensitive, so much so that hum and noise could be audible even with the Paramount's DC heater supply. If you find this problem, post again and we can look at the various options to resolve that problem.

As an aside, normally for headphones one would install a 120 ohm resistor in series with the headphone output; this is the IHF standard for headphones and helps keep the loudness similar with headphones that have widely different impedances. This would make little difference for high-impedance headphones such as the 300-ohm ones discussed here, and many low-impedance headphones of recent design do not actually work well with the IHF standard; they prefer a low-impedance output. I am still reviewing that issue.

[dmannnnn]

Thanks Paul,  I will wire up a jack tonight and give it a try.

[dmannnnn]

Ok, I wired up a jack and gave it a try.  It sounds good but the hum and noise are very obtrusive.  There is a deep hum that was mostly remedied using the paramounts hum pots.  One amp almost lost all hum and the other is still audible enough to cause problems.  There is also a large amount of hiss.  I know one of my foreplay tubes has some rush so I swapped them out for my super low noise pair.  This did lower it a little.  After fooling around with it for awhile it seems as though most of the hiss is coming from the preamp tubes, but there is also some hiss from the paramount tubes, though not as much.

I have been trying to downsize for a move and built a S.E.X. amp to replace the paramounts and take double duty as a headphone amp.  I love the S.E.X. amp with headphones.  It is just a joy to listen to.  But, I have decided that I will be buried with the paramounts so they will stay and the S.E.X. will move on.  I have no problems taking this project to the next level. 

I don't mind if it would be a time consuming undertaking.  I enjoy working on them.  I am not versed in electronics so I'm not sure how to get rid of the noise.  Are the paramounts OPT's just not a good match?

[Grainger49]

Did you try the 120 ohm resistors?  This may be the situation like a sensitive amplifier.  You just need a little padding.  The series resistor will kill some of the signal.  The noise will be subdued by wasting some of the output.

Also, have you tried tweaking the hum pot with a meter on the output?  I don't remember if this is in the Paramount manual.  In your situation you want to put the meter on the headphone leads and set your meter to AC Volts.  Adjust for the lowest reading.  I have gotten exceptional results with my Paramours this way.

[Paul Joppa]

I think your headphones are quite sensitive, so they are picking up the noise floor of the amps. Also, you probably have more gain than you need so you hear the Foreplay noise floor as well. The solution to both problems is the same - some attenuation at the Paramount output, and some more at the Paramount input. (See my white paper on signals and noise on the Community page for more depth, if you have the patience... :^)

The first check is to be certain which noise comes from which source. Short the Paramount inputs (make some shorting plugs if you don't have some, they are quite handy if you do DIY electronics at all) and listen to the headphones again. This will eliminate any Foreplay noise.

Assuming you still have some residual noise, you'll want an attenuator on the Paramount output. The noise floor of a Paramount is about 7dB greater than a SEX amp, according to my notes. The maximum output of the 300B Paramount is about 6dB greater than the SEX amp, so the signal to noise ratio is about the same for both, at 83dB. Therefor, in theory at least, all you need is 6-7dB attenuation of the headphone output. I would use a simple pot to make it adjustable; a linear pot would give 6dB attenuation at the midpoint which makes a fairly repeatable reference, and this way you can change the setting if you use different headphones. A pot of 250 ohms would give 125 ohms equivalent series resistance at the midpoint to match the SEX amp's IHF specification, or you could use a lower value (maybe 50 ohms?) to give a low drive impedance to work with a wider variety of headphones and still be large enough to not interfere with loudspeakers.

Now you can re-connect the Foreplay. If it again produces too much noise, an attenuator at the Paramount input will be needed. I have some 12dB inline attenuators in my Paramounts which worked fine until I swapped in an Eros; now my system gain is too high again and I need more attenuation. When I do that, I'll probably put a 100K pot next to the 12AT7 socket, upside down and attached with a dab of silicone goop, so I can adjust it again next time I make a system change.

Kind of a long-winded post; I hope it helps!

[dmannnnn]

This sounds good.  I am making some shorting plugs now.

I can put the potentiometer in the black box with the headphone jack.  This will limit any modifications to the paramounts.  I had previously been running the paramounts with an l-pad at the iinputs. 6db should still work fine with the RF-7's.  I think.

[dmannnnn]

After trying the shorting plugs it seems that I will need most of the attenuation at the paramount input.  Probably 90% of the rush is coming from the Foreplay.  There is some slight hiss from the paramounts but a minimum of attenuation should be able to take care of it.  The main problem with the paramounts is microphonics.  The Sennheisers really are sensitive!  Some tube dampers might be in order.

I'm going to try the 120 ohm resistors at the headphone jack.  That might be all I need on that end.  Then I will put together a 6db L-pad at the paramount input. I think this just might end up working better than I thought it would.

Grainger, if you want to adjust your hum pots without a meter, just try the 300 ohm headphones.  You can get it down to the last click just by ear!

[Grainger49]

I have an upstairs digital meter and a downstairs digital meter.  The results of many, many checkouts.  The four digit Fluke is really best and it reads very low, I forget where, when the Paramours are well tweaked.

[Paul Joppa]

After trying the shorting plugs it seems that I will need most of the attenuation at the paramount input.  Probably 90% of the rush is coming from the Foreplay. ...

Do you have the C4S upgrade in the Foreplay? If so try the cap bypass trick on the shunt reg boards; the rush may be coming from the shunt reg chip more than the tubes.