less powerful, but better accuracy?

[wwmhf]

Since I am using my SEX amp with a Sennheiser HD650 headphone exclusively, I do not think I need any power around 2W. Is there a simple way to reduce the amp's maximum output power a little for a better accuracy?

[Paul Birkeland]

That is the function of the 120 ohm resistors soldered to the headphone jacks.  Instead of an 8 ohm speaker load across the autoformer output, you now have well over 400 ohms across said output.  This provides a voltage divider (reduces output level) and a higher reflected load for the tube (less distortion, or more accuracy as you put it). 

I know Paul Joppa is working on a short paper in regards to the various values of resistors that can be used there, but I would say that the greatest improvement would be to install the iron upgrade.

[wwmhf]

Changing that resistor to a larger value does reduce the power, but it also reduces the dynamics if my feeling is right. I have tried several values: 100, 120, 150, and 180 ohms. I am using the 150 ohms in the amp now. The amp is still very loud. Most of the time, I put the pot dial somewhere between 8:00 and 9:00 positions, rarely above 9:00. I would like to tune the amp such that the pot dial is at around 11:00 position for my usual listening. Of course I would like to do this without scarifying the sound quality.

As for the iron upgrade, I think prefer to use a better iron starting from scratch. The amp in my hand now was bought used, mainly for revitalizing my tube experience after many years. The amp came with many problems, especially with numerous bad solder joint; however, I consider these are just part of the fun. I have done the following since I got it:

1. Changed the pot.
2. Replaced the C_heater and C_bias with better capacitors of much large values.
3. Bypassed the Cpsu.
4. Re-soldered many joints.
5. Got rid off the speaker outputs. They can be easily put back if desired.
6. Bypassed the Cpf capacitor.

Now, it sounds much much better than when it came. I am waiting for parts to replace the CC (Russian or Obbligato), Cpf (Russian or Obbligato), Cpsu (oil), and Cd_psu (oil) capacitors. I am sure that these will make it sounds even better. I conclude that I have known the S.E.X amp's potential. I would like to keep this amp as an experiment bed and I have decided to build another one starting from scratch with better parts including the iron upgrade.

I either have or know where to buy almost every part for a Bottlehead S.E.X amp except for the power transformer. Do you know if I can buy one from Bottlehead directly? Any suggestions for alternatives?





     

[JC]

Here is where I would caution against confusing Voltage Gain with Power Output.  Since the input pot is really controlling how much signal voltage gets into the amp to begin with, I am guessing that your loudness issue is more related to the former.  IOW, you put less signal Voltage in, you get less volume out, of course.  This issue has been kicked around a lot here, and I understand that Paul Joppa has a white paper on the community pages that specifically addresses gain in great detail.

Still, if you want to try reducing the Power Output a bit further, and you have the original Speco output transformers, you may want to try switching to the 4 Ohm output.  I don't have the paperwork in front of me, but I know it amounts to switching one wire on each side and has been addressed here before.  This should reduce the Power Output a little more, and theoretically at least, improve distortion a bit.

I have my doubts, though, whether it will change the volume control position all that much.  I suspect you will need to address the Voltage Gain issue to adjust that to your liking.

[Paul Birkeland]

I would strongly caution against replacing capacitors in the power supply with other caps unless they are the same capacitance or greater.  There is the decoupling resistor/cap for the driver stage that can be easily swapped, but the power supply caps in the voltage doubler need to be pretty big.


In terms of using more of the volume control, that is more easily accomplished by adding some series resistance between the input jacks and the volume pot.  Adjusting the resistors on the headphone jack does a little more than just shed output voltage. 

[Dr. Toobz]

What about changing the power triode halves of the circuit to accommodate "the rest" of a 6SN7, and leaving the input stages as-is? I would imagine only about a watt of ouput power that way, but I do recall that many people feel the 6SN7 is one of the best triodes out there. Perhaps using both halves of a 6SN7 would be a little more linear than the stock S.E.X. configuration, where IIRC, the voltage gain triode of the 6DN7 (smaller one) is basically a 6SN7 itself, whereas the bigger half has twice the anode dissipation (10W vs 5W for the 6SN7, correct?) and is completely different.

[wwmhf]

JC: adjusting the voltage gain seems to be one interesting way to go, but it is a little technical for me at this moment. I will definitely pursue this later.

Caucasian: I totally agree with you about the capacitance values for the power supply capacitors. I always try to use larger values. The other challenging for using an oil capacitor to replace an electrolytic capacitor with an equal or larger capacitance is the physical size of the oil capacitor. One often has to be creative to install. Your suggestion of using a resistor in serial of the pot makes sense. I will try it later.

[Paul Joppa]

At this point, the PT-3 is not available for experimenters. (It's used in the Crack headphone amp as well, and I posted on this subject in that thread a day or two ago.) Eventually I hope to have a new version that is more robust.

In my humble opinion, the iron upgrade is the most effective upgrade sonically, and the C4S driver plate load is very nearly as significant. The signal path capacitors are next, parafeed cap first, coupling cap second. Power supply caps are third because with parallel feed very little signal current actually runs though the power supply - unlike series feed, where the power supply is more important to the sound. Incidentally, especially for low-level listening, I prefer the nickel-core output transformers, which have higher inductance at small signal levels.

Just for everyone's information, in stock form the SEX amp makes about 50mW into most headphones - it is not a 2-watt amp unless it is loaded with an 8 ohm speaker. You could of course generate 2 watts into a 300 ohm load if you had a suitable output transformer matched to that load, but neither we nor Magnequest offer such a transformer.

Seems to me that I posted once, a long time ago, about using a 6SN7. I did the analysis back then but I can't find my notes (if I made any!) and I don't have time to re-do the analysis. The pinout is the same; the heater voltage will be high since it draws less current, and the power section may or may not run about 5 watts. Only the -GTA and -GTB versions are rated for 5 watts or for more than 300 volts. I don't promise you won't damage anything until I find or repeat the analysis though!

[wwmhf]

Thanks you all for sharing your opinions/experiences.

I would like to clarify that my desire for a "better accuracy" does not mean I am implying this amp is not accurate enough. In fact, this is the most enjoyable headphone amp I have up to now (I have/had too many!). I just would like to know if we can make it sound even better. I am really surprised by how well the amp responded to my tweaks. Of course, re-soldering joints and replacing the pot should be considered as repair, not really tweak. When I got amp, I immediately found many problems, but I was also attracted to its nice structure for easy remodeling/repairing. So far, I am glad I did not chose to return the amp. I have got a lot of fun and music from this amp.       

[wwmhf]

Paul's order of modding effects is very valuable. I am benefiting from it and will benefit from it more.

I also would like to mention that Caucasian's suggestion of bypassing 47uf power capacitor does have an obvious positive impact on the sound. It makes the sound more transparent.

[Grainger49]

One thing that PJ didn't mention.  Changing a Parafeed cap, which makes a small but meaningful difference costs ~$50.  Upgrading the iron costs hundreds and rewards you in proportion.  That is probably why we do the caps first.  But... always a but, you need to change the parafeed cap in most Bottlehead equipment when upgrading the iron.