[McRat]

What's a better way to celebrate my 200th head-fi posting than a first post in the DIY section?

I was just doing some research on improving my MisterX-built Millett Hybrid when I ran into this comment:
Quote:

Originally Posted by amb In fact I might change the Millett's 22 ohm output series resistors to a much lower value (or jumper across it altogether) to allow the buffers to really deliver the goods. The BUF634 has internal output overcurrent protection (so does the OPA551), thus the resistor isn't needed. DIY forums thread

So if I'm able to jumper the resistor, I really don't need to worry anymore about the big output coupling cap, right? So I can leave the better quality bypass cap in there to block out the DC from the output. Removing the high-pass filter and the big capacitor from the output should make the Millett a better preamp as well.

Please tell me if there's something seriously wrong with my reasoning?

 

[steinchen]

what high pass filter do you want to remove ? the output coupling cap(s) still form a high pass filter with the impedance of the headphones. If you remove big the electrolytic coupling cap the corner frequency will rise significantly due to the small capacitance of the film cap and will result in significant bass roll off.

nevertheless, you can safely jumper that resistor (as I did with my MHs)

 

[rickcr42]

not all output series resistance is there for current limiting and when absent may cause more trouble in the long run than the minor output current loss

 

[dsavitsk]

Quote:

Originally Posted by rickcr42 not all output series resistance is there for current limiting and when absent may cause more trouble in the long run than the minor output current loss

You may be correct in the grand scheme of things, but you can safely jumper the resistor on this design. From the original article (http://pmillett.com/file%20downloadss/ax_hybrid.pdf)
Quote:

The output of the BUF634 is connected through a 22Ω resistor, which is needed only to help protect the BUF634 in case of a short circuit of the output, but it also affects how different headphones sound. I usually recommend a series resistor of between 10% and 50% the impedance of your headphones

It is unnecessary.

However, do not take out the big cap. The cap is not there due to the resistor, but due to the need to block the DC and set a reasonable 3dB corner. If you use your Millett as a premp only (i.e., into a power amp, not into headphones) you could replace the coupling cap with something smaller (in the 2 - 4uF range.) Into headphones, you need a big cap (100uF for Senns, 470uF into Grados.)

If you want to improve the Millett, the diamond buffers are a decent option. If you want to do it through caps, replace both the cathode bypass caps and the output caps with something to your liking.

 

[McRat]

Okay, thank's for the great answers. I knew there was some aspect I didn't think of and forgot the headphone impedance from the equation.

I'd really like to use this as a headamp/preamp, so I guess I have to wire the preamp output straight from the opamp outputs through a separate small cap.

 

[dsavitsk]

Quote:

Originally Posted by McRat Okay, thank's for the great answers. I knew there was some aspect I didn't think of and forgot the headphone impedance from the equation. I'd really like to use this as a headamp/preamp, so I guess I have to wire the preamp output straight from the opamp outputs through a separate small cap.

A couple of things if you do this. The cap bleeding resistor (the one from out to ground after the cap) should be increased from 1K to more like 50K or 100K if you are using this as a preamp and if you use a smaller output cap. This resistor is in parallel with the load seen by the preamp, and if left at 1K will require use of a larger coupling cap. In fact, you can safely change this for use with headphones, so it is not a big deal.

Also, with the big cap there, and to a lesser extent with the small cap, this will produce an enormous turn on and turn off thump. It is enough to damage both your power amp and your speakers. Make sure the power amp is off whenever you turn the preamp on or off. (This is always the case, but it is particularly important wih the large coupling cap on this amp.)

Last, while you can remove the ouput series resistor, there is almost no benefit to doing so and you do need it there for the diamond buffers, so i would leave it alone.

 

[McRat]

Whoah! Thanks dsavitsk, that was a very thorough answer. A bit more than I bargained for, but I think I'll be able to sort it out.

It does seem that there's no easy way for this amp to act simultaneously as a preamp and a headamp. Would a switch help? Maybe I could make use of the headphone socket bypass and mute functionality, whatever that is called.

Hmm, on second though the headphone socket idea isn't that good afterall, as it wouldn't solve the big cap problem.

The original thought was just to remove the unnecessary resistance from the output, especially for using the amp with Grados. The idea of losing the big electrolytic from the signal path was intriguing, but this amp clearly isn't designed for that. I guess there would be quite a lot of DC going to my phones if the output was altogether without a cap.

 

[dsavitsk]

Quote:

Originally Posted by McRat It does seem that there's no easy way for this amp to act simultaneously as a preamp and a headamp. Would a switch help?

Well, you could switch a coupling cap in and out of the output path, but here's the thing. While big electrolytics in the signal path are a bad thing, this is not a really high fidelity amp. The tube probably does more damage to the sound than than a good quality cap. I would use the best quality electrolytic you can find (which is arguably a blackgate N or NX, though the Std will do fine as will a cerafine, a Muse (I like the ES series a lot), or a couple of other options), take out the wima bypass cap, make sure you have a decent power supply, and leave it at that. The amp won't get a lot better. If you want a better preamp and you want it to involve tubes, you will need more volts.

Quote:

Originally Posted by McRat I guess there would be quite a lot of DC going to my phones if the output was without altogether without a cap.

Around 12 to 15V

 

[rickcr42]

Quote:

You may be correct in the grand scheme of things, but you can safely jumper the resistor on this design. From the original article

Probably there because Pete KNOWS so called "audiophiles" would moan about the resistor anyway so offered the option.

Quote:

It is unnecessary.

seems you folks never heard of feedback loop contamination from signals reflected back in from the output jack.The resitor is not there to protect the buffer but to protect the signal,the buffer is already protected by design.

ANY audio device that uses a feedback loop is sensitive to contamination because it too should be considered a signal but the difference is when you add negative versions to postive versions of the same thing in a differential amp (the opamp) you can cancel any common mode signals depending on how good the CMMR is the better the cancellation but if the signal enters from the OUTPUT there can be no cancelation and that signal will be "mixed" instead of eliminated.

With a SE front end there is not even "CMMR" that so everything at the output is fed back to the input and if that "fed back" signal has RFI contamination that entered through the jack and from the headphone cord it will be treated as just another signal and amplified.

if you folks decide to protect against this or not is personal choice but sound design practice say you should and the impact on the sound far less than a contaminated signal path that you have no idea you even have.

 

[McRat]

dsavitsk:
So I guess I'll just use this as a happy headamp and wait for my real preamp.

I know this isn't such a hi-end amp, but it doesn't hurt to consider these things. Afterall, I'm still just learning basic circuit design.

I do have a monster Mascot PSU coming up on friday and there are already Nichicon Muse caps on the outputs, so I'm pretty much covered. I bit of tube rolling and playing with the bias and probably trying the opa551 as the buffer should keep me happy on the tweaking section while considering some major upgrade.

Rick:
Thank you for the wise words. I'll be sure to consider this if I ever decide to jumper the resistors.

 

[rickcr42]

I like to use these guys on the output of all my "feedback" type amps

http://www.jensen-transformers.com/datashts/oli3.pdf

$5 per channel may seem like a lot but for me knowing it is there doing the job is worth any amount in peace of mind.Again,a personal preference thing but if you read the serious opamp (feedback is what makes it an opamp and not just amp) design manuals alll mention the possibility of feedback loop contamination and why you pretty much see a low value decoupling resistor outside the loop in every single manufacturer data sheet bsed design.

rick

 

[Voodoochile]

Quote:

Originally Posted by rickcr42 The resitor is not there to protect the buffer but to protect the signal,the buffer is already protected by design.

I can see where people would think that, though, because that is what Pete says in his article- that it's there to prevent damage to the buffer in the event of a short on the output.

Strikes me as odd that he would say that, knowing the Buf634 is internally protected. Maybe he originally used a different buffer, like a 2008 or something? I've always wanted to ask him, but seeing that the article is in the archive and out in print for a few years now, it also seems a bit moot.
Fourth paragraph on page five. Odd, for sure.

 

[rickcr42]

Quote:

I can see where people would think that, though, because that is what Pete says in his article- that it's there to prevent damage to the buffer in the event of a short on the output.

could just be a "typo",oversight or just old habit.Back in the "good old days" a short to the output of any monolithic would wipe it out and even the front ends needed to be diode protected from ESD many times so it became habit to add thse in without even a thought.

If we wanted more output current we just added a discrete current pump stage to the output of the opamp and kept the resistor between the two and if the output was for line levels the resistor was typically in the 150-300 ohm range.
The combination of low impedance and easy to drive headphones meant there was now the ability to "direct drive" headphones from an opamp output or opam/buffer combo but being still marginal in that department something had to go and it was the series resistor on the output.since this coincided with beefier output stages and internal protection (full shut down or foldback current limiting) that momentary dead short situation when you plug in your headphones is hadled and the resistor considered no longer needed.

Everyone happy until some noticed this was now a path for all kinds of crap to enter the output and from there the feedback loop which is then fed to the inverting input in a "closed loop" system from whatever cordage is on the output.that cordage headphones or interconnects for audio or test probe leads for test gear and while notr a blatant thing for audio easily seen as contamination in high speed testing so it IS there.

once I had the "doh" moment I began to take my buffers "out of the loop' and run them as open loop voltage follwers which totally cures the problem but since so many like a closed loop I think it worth mentioning this potential for messing up the sonics and especially so when the group is one that will spend $$$ on just the right cap or resistor then use it in a design (not saying this one so easy dammit) already comprimised out of the gate.

Pete may have said "current limit" but I bet he knew exactly what he put the resistor there for,he just didn't tell anyone

 

[00940]

Rick : what the hell are those reflections you are speaking about ?

- Capacitive loading (the reason why the datasheet have resistors outside the feedback loop in all textbooks discussing line drivers) or
- Back emf (never discussed in opamp text books since opamps aren't designed to drive moving coils) or
- Transmission line reflections (only applies to digital transmission with proper terminations, obviously not the case) ?

 

[rickcr42]

Quote:

Originally Posted by 00940 Rick : what the hell are those reflections you are speaking about ? - Capacitive loading (the reason why the datasheet have resistors outside the feedback loop in all textbooks discussing line drivers) or - Back emf (never discussed in opamp text books since opamps aren't designed to drive moving coils) or - Transmission line reflections (only applies to digital transmission with proper terminations, obviously not the case) ?

-every electromagnetic device is capacitive/resistive/reactive
-headphones ARE moving coils
-read up on your transmission theory and line termination theory

finally every single cable has reflections determined by the cable and its length plus being looped are a natural antenna for RFI pickup.This IS mentioned in quite a few typical high speed data sheets and is a very well known fact in high speed testing circles.You don't have to take my word for it look it up