Headphones for a SET Amp

sweet & juicy SE amp for 1626 Darlings and the 6L6 family

Headphones for a SET Amp

Postby Shannon Parks » Mon Mar 05, 2012 7:00 am

Today I experimented with adding a headphone jack to my Darling, Clementine ULtra (now a 6BG6 amp, but I'll share that later) and also my get*set*go. I looked at three methods: direct connect, 100 ohm resistor in series and a 20-20 circuit (20 ohms in series and 20 ohms in parallel). I used my Sony MDR-V6 headphones, which have a nominal impedance of 63 ohms and 106dB sensitivity at 1mW.

Image

Looking at the chart, I have the frequency response of all three methods in comparison. Don't worry about the relative gain too much, as ultimately you want to balance this with your source input and whatever volume balance control you'll be using, either the source's volume control like on an iPod, a volume pot adding to your tube power amp, or a combination of both.

DIRECT CONNECT
The Darling SET isn't a frequency bandwidth champ as we are about -1dB down at 20Hz and 20kHz with the direct connect output. But considering $37 output transformers, this is pretty good. Damping also isn't half bad due to the lower plate resistance of the 1626, in characteristic triode fashion. This damping factor, or low amplifier impedance, is a key in keeping the output voltage constant with a varying speaker impedance, as all speakers have a variable impedance over frequency. The nice thing about the Darling is that the output will never be large enough to damage a lot of headphones. Consider that a 2.5Vrms output is what it delivers into 8 ohm load for its 3/4W. This 2.5V with a 63 ohm load is a perfect 100mW max. My Sony's can supposedly handle 1W max. For lower impedance headphones, like 32 ohm Grados, this max output doubles to 200mW.

100 OHM IN SERIES
This is a classic method for adding a headphone jack to a power amp, with a resistor in series with the secondary '+' and the headphones. The upside: you can adjust the size of this resistor (even use a stereo pot) to fine tune your headphone level. I think for headphones under 100 ohms impedance I would use a 200 ohm stereo volume pot, and probably 500 ohms or 1K for headphones 100 ohms and greater. The downside can possibly be a major problem though: this resistance is now your amplifier impedance and your damping is horrible. This means your headphones will exhibit their full sonic signature, which could be flat or might be as curvy as a roller coaster. Look at the chart. Now we see a hump of about an extra 1dB at 80Hz to 100Hz. This doesn't seem very bad to me, and this method would probably be sufficient for these Sonys.

20/20
I used a voltage divider of 20 ohms and 20 ohms for this test. I used a couple 1/4W resistors for this test, but you'd probably want to use 1W rated parts for these SET amps. In addition to giving control over the output level with the divider, I have a 20 ohm resistor in parallel with the headphones. This greatly lessens the dynamic impedance characteristic of the headphones, as the 20 ohms is the dominant resistor in a paralleled resistor configuration. See how the output level looks almost like the direct connect level. For lower impedance headphones lie the Grados, you would probably want to lower this paralleled resistor to 10 ohms. To adjust the base headphone level, or to limit the max output level, you would adjust the series resistor.

EPILOGUE
The 20/20 method seems to address all the issues the best. You can adjust the output level for your headphones, and you can still get a nice flat output to your headphones (to the limitations of the amplifier). One more major plus for the 20/20 method is that the effects of residual hum and noise is lessened. You can adjust the series resistor higher to help lower this to a reasonable level. Note that your headphone sensitivity plays a large role in that, too.

Shannon
Last edited by Shannon Parks on Sat Mar 10, 2012 5:59 am, edited 2 times in total.
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Postby TomMcNally » Mon Mar 05, 2012 9:26 am

The Lafayette $ 11.88 headphones I bought when I was 13, came
with a blue metal can with a phone jack on one end, and 100 ohm
resistors inside, and 4 leads for connection coming out a grommet
on the other end. So they knew about the 100 ohm trick back
when dinosaurs still roamed the earth.

Image

The can didn't supply a load, so it was best used with solid state
amps, that didn't care if they had a load or not.

In your 20-20, does the first resistor go directly across the OPT,
with the second resistor in series with the phones? Or the other
way around? Or does it matter?

I had one bad experience with a guy who wanted to use a Darling
with Grados and bitched about the residual hum. I've stayed away
from headphones users since.

I'll try the 20-20 method.
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Postby Shannon Parks » Mon Mar 05, 2012 9:56 am

I drew a nice little picture right now, but can't upload it. I'll add that tonight. The first resistor is in series between the output transformer secondary and the TRS jack L (or R) input. The second resistor goes from the TRS jack L (or R) input to its Ground lug.

In the original Darling, the hum will be greater than the Darling Clementine, as the Clementine as one extra filtering stage (it's a CLCRC). But the 20 ohm series resistor can be increased to probably 40 ohms in the standard Darlings to help knock the hummmm down. My SWAG is this will work. Unfortunately, any larger might cause the Darling to run out of headroom.

I did experiment with adding a second filter choke where the 100 ohm 3W resistor is at in my ULtra's power supply, just to see what it could do. Definitely helped some, but the 20/20 did the most improvement and seems like the best solution.

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Postby Shannon Parks » Sat Mar 10, 2012 6:05 am

Sorry for the delay in posting these. I couldn't quickly find my stereo jack symbol, so I just used a speaker, fyi. The first shot shows the traditional 100 ohm resistor in series mod. The second shows the 20/20 mod.

Shannon

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Postby EWBrown » Sat Mar 10, 2012 3:34 pm

The approach used with the Bottlehead 6DN7 SET "SEX" amp was to connect an 8 to 10 ohm, 2W resistor across the (8 ohm) speaker terminals (to act as a dummy load) and 120 ohms in series with the "hot side" of headphone jack, for each channel. 100 ohms is also a good value, BH used 120 ohms as they considered it to be an industry "standard" value.

Using the 8-10 ohm "dummy load" wasn't absolutely necesary, but it was recommended, as it kept the power triode's plate loading in the correct operational impedance range..

This approach was mostly intended for use with 300-600 ohm headphones, though with most 32 ohms headsets, it worked well, too.

But what REALLY counts, is to use whatever approach sounds best, to you [:)

/ed B
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Re: Headphones for a SET Amp

Postby ekubota » Sat Feb 07, 2015 8:47 am

Shannon and Ed, any thoughts on the necessity of using a dummy load with a Clementine 6L6 Ultra? Would 8-10 ohm/2W still be the recommended value? Also, what sort of arrangement have people used to switch from the speaker outputs to the headphone out/dummy load?

For context, I'm planning to use a 6L6 Ultra with AKG K701s (200 mW max power, 62 ohm impedance, 105 dB SPL/V). However, I am also using Stax electrostatic headphones and speakers; the Stax phones run from a step-up adaptor box connected between the amp and speakers that incorporates a switch between speakers and phones. Thus, even with the speakers connected, I could switch the output to the Stax phones (effectively muting the speakers) and use the AKGs instead. But I'm not sure what sort of load the Stax adapter presents by itself.

In any event, if it's acceptable to run headphones from the Clementine without a dummy load, I would prefer this approach to limit the number of additional switches. Hopefully this makes sense.
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Re: Headphones for a SET Amp

Postby ekubota » Sun Feb 08, 2015 9:28 am

FWIW, I finished the Clementine 6L6 (non-Ultra for now), and it seems to work well using the AKG headphones without a dummy load and with the Stax adaptor set to the Stax phones. I'm satisfied, although the level through the AKGs is a bit high with the 20 ohm resistor, so I may try 30 ohms in series.

The Clementine also has plenty of power with my 89 dB/W speakers, even before the Ultra conversion; it's uncomfortably loud with the Budgie line stage set around half volume. Any thoughts on whether it still makes sense to do the Ultra conversion?
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Re: Headphones for a SET Amp

Postby Shannon Parks » Mon Feb 09, 2015 7:34 am

ekubota wrote:FWIW, I finished the Clementine 6L6 (non-Ultra for now), and it seems to work well using the AKG headphones without a dummy load and with the Stax adaptor set to the Stax phones. I'm satisfied, although the level through the AKGs is a bit high with the 20 ohm resistor, so I may try 30 ohms in series.


Your AKGs are similar to my Sony MDR specs. With the 20 ohm in series and parallel load resistor, you are only attenuating 2.5dB - not enough. Changing to a 30 ohm in series will just lower it another 1dB. Leave the 20 ohm resistor and then I would use a 10 ohm resistor in parallel with the headphones. You'll have -9.5dB reduction - probably right where you want it. This is where I use my Sonys and A-T ATH-M50x headphones. You'll have flatter frequency response, too.

ekubota wrote:The Clementine also has plenty of power with my 89 dB/W speakers, even before the Ultra conversion; it's uncomfortably loud with the Budgie line stage set around half volume. Any thoughts on whether it still makes sense to do the Ultra conversion?


Sounds like you don't need it.

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Re: Headphones for a SET Amp

Postby ekubota » Wed Feb 11, 2015 10:21 pm

Thanks. I tried the 20 ohm in series/10 ohm in parallel. It is better (although the level is still a bit high compared to my speakers and Stax electrostatic headphones). I also converted the amp to Ultra mode; it's much cleaner at high volume, perhaps because in triode mode the Budgie line stage had been overloading the more sensitive input?
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Re: Headphones for a SET Amp

Postby Shannon Parks » Fri Feb 13, 2015 10:04 am

ekubota wrote: I also converted the amp to Ultra mode; it's much cleaner at high volume, perhaps because in triode mode the Budgie line stage had been overloading the more sensitive input?


It's because it has more gain and output power in ULtra mode.

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