I understand that it's optional, but i'm wondering what those options accomplish? More power, less noise, etc...
You keep focusing on power (or current), but that is all determined by the basic circuit and power supply of an amplifier design. You can't buy or build an amplifier and drop a bigger engine into it. The "engine" is the amplifier design itself. Carrying the analogy to its logical extreme, if you had an amplifier enclosure, you could drop another amplifier assembly into that enclosure and get more power, but it would be a completely different amplifier in the same skin.
As for the schematic, the answer to your question is less noise, less distortion, better frequency response, and more convenient usability. It all depends on which parts you're talking about. Keep in mind that Pete Millett's principle in designing this amplifier was for a
Starving Student. As Pete put it in terms of
Starving Student costs, "about two delivery pizzas and drinks."
When Dsavitsk (of ECP Audio) designed the PCB version of the Starving Student Millett Hybrid, he chose to add some additional parts that made improvements (some significant) with a minimum of cost. Keep in mind that once you design a PCB, you're already increasing the cost significantly from the original. Likewise, design that PCB to fit into a custom-made case, you increase the cost even more. The comparison of the added parts becomes insignificant. To understand the perspective, however, realize that Pete's version was absolutely the lowest cost possible - similar in concept to a Tube CMoy. There was no case - you had to come up with it on your own. This led to many creative concepts in the early days of DIY-er's constructing the Starving Student. Nevertheless,
casework often represents fully half the cost of building an amplifier. It also involves a completely different skill set than populating a PCB by soldering - often compared to a paint-by-number painting you might purchase in an arts and crafts store.
So, keep in mind that the goals between Pete's original design and the SSMH PCB are slightly different. The goal in the SSMH PCB was to reach more of the masses by greatly simplifying the construction in providing a PCB and a custom-made case. This makes assembly a piece of cake by comparison to Pete's original design, but it necessarily increases the cost. The SSMH PCB version is currently $150 at Beezar.com, whereas Pete's original Bill Of Materials (BOM) totaled about $36.
One complicating factor to this is that due to the overwhelming popularity that Pete's design caused, the tube prices of the 19J6 increased radically. Today, you will find 19J6 tested and matched pairs for as much as $49.95 on ebay. When Pete first made the design available, these tubes cost about $2 each. So just the tube costs alone have far exceeded Pete's
total Bill Of Materials for the design.
Anyway, with all of that background, here are the reasons for the parts we added in the schematic you referenced from
www.diyforums.org/SSMH/SSMHschematic.php :
Strike-Through Parts
- C1, C6 - These are the classic "power" caps in most amplifier circuit designs. While replacing these do not increase power, they lower the impedance of the power supply ahead of the amplification circuit and provide more ripple removal. In other words, upsizing these caps will have a good result in increasing the availability of current in the circuit and decreasing noise. They were up-sized to 680uf to make use of the Dsavitsk-PCB and Beezar-custom-case space. The 63V rating was considered the safest (with an ample safety factor) for the 48V power supply (Cisco VoIP power adapter).
- C2, C4 - These are the interstage capacitors that AC-couple the tube output to the MOSFET buffer. Simply put, if these caps are not sized large enough, you may lose some bass response. This is also related to the amplifier's output coupling caps. More on that next.
- C3, C5 - These are the primary output coupling caps for the Starving Student. Any output coupling capacitor on a headphone amplifier will combine with the impedance of the connected headphones to form an RC circuit. This type of circuit forms a bandpass filter, meaning some frequencies are filtered out. In this case, if the value of RC is not high enough, audible bass will be filtered out. Pete originally specified 150uf capacitors to keep costs down. However, this results in -3dB at 33Hz for a 32 ohm headphone. Actual bass begins dropping at much higher frequencies than that. Up-sizing these capacitors to 470uf means the cutoff frequency is down to 10.5 Hz. That still has some audible drop above 20Hz, but not enough for most headphones. Once these capacitors were up-sized, it made sense to also up-size C2 and C4 above to ensure that these did not cut out any bass frequencies, too.
Yellow-Highlighted Parts
- C3a, C5a - Common practice, especially with many of Pete's designs (such as the original Millett Hybrid in AudioXpress magazine) is to use bypass film capacitors with electrolytics, particularly in the signal path. The reason for this is that electrolytic capacitors do not sound well with high-frequencies and can add distortion. Bypassing with a small film cap allows the film cap to actually pass those frequencies, instead. Keep in mind that this part is mostly empirical and listening preference, but it will make things sound better than just the electrolytic itself. This can be mitigated further by use of true, audiophile-quality grade electrolytics, too (Black Gates, Muse ES, FG, KZ, Elna Cerfine, Silmics, etc.).
- C7, C8 - without going into the specifics of the design with cathode bypass tube circuits, use of this capacitor can also increase the bass frequencies around the tube. Again, a small tweak that may or may not cause much of an audible difference, but on paper - very little cost for some gain.
- R14, R15 - these are normally referred to as grid stop resistors. In some cases, they can lower distortion in tubes and ensure that the tube does not oscillate. They can also cut down on outside interference that tubes can sometimes pick up. It can be argued that none of that is a concern in this amp, but in this case - very little downside to adding these resistors for only pennies.
- R16, R17 - These resistors were specifically suggested by Dsavitsk and can vary in 50K increments - 50K, 100K, 150K, etc. The reason for these resistors is again - "Starving Student." One of the most expensive components of an amplifier can be the volume control. In this case, an inexpensive Alpha stereo volume pot (RV1) is specified in the BOM. Unfortunately, cheaper volume control pots are sometimes not well matched at lower volume knob travel (9 o'clock and below). Because the 19J6 tubes already have a pretty high gain compared to most headphones, it means that without correction, most of your listening may be in the 9 o'clock positions and below. These resistors (must be coordinated with your particular headphone's sensitivity) "push" the volume range further into the mid-travel of the pot's rotation. The idea is to use the best increment of 50K that results in comfortable listening levels around the 12 o'clock position of the volume pot.
This is the reason you didn't get much response the first few times because you were not asking a simple question with a simple answer. Hopefully, this covers it. If not, there's not a whole lot more that I've got.
