Millett "Starving Student" hybrid amp - Page 100

I'm really interested in building this amp and I've checked out the schematics, but there are some things about grounding that I'm not so sure about. Where do you connect the grounding wires/caps/resistors? Do you connect them all to the same point/same chunk of metal or do you connect them to separate points?

I'm a newb when it comes to DIY electrical stuff...

Been wondering if I should even bother posting this, but what the heck.

FWIW, I think this goes against pmillett's design philosophy as I don't think any of his amps utilize this. If you're a pmillett purist or other type of purist, then no need to read this at all, and just skip over it.

I would do something different. I would not do the signal attenuation at all...i.e., resistor in series wired to a switch. That's attenuating the source signal, and with a 15x gain stage, it's not ideal and rather silly, IMO. The ideal would be to reduce the gain by changing tubes. Since that's not really an option (I don't want to scrounge the internet looking for a tube that'll fit and work), I would use negative feedback.

Remove all that signal attenuation stuff, and build in adjustable NFB to reduce the gain, and should lower the distortion as well. Some people hate that, but I think some NFB is good. Again, pick your poison.

I haven't measured or done this, but it's a starting point.

I'm too lazy to draw a diagram for the entire SSMH so I'll describe with words, first, bypass the cathode resistor (R5 and R11) with a 1000uF/16V cap I think even a 10V cap should work, not sure entirely, but 16V is fine. Without R5 and R11 bypassed you will get signal degeneration, so the gain drops some in the default circuit. Bypass it, to avoid some feedback, and the circuit gain should increase. That's right, increase it.

Put in a 10K grid stopper, that is a 10K resistor between the pot and the tube, but put it right on the tube pins (5 & 6). Also, put another 10K resistor right on pins 5 & 6. Connect this to a pot, and then connect that to the output of the circuit, after the output cap and the 2K resistor to ground.

The higher the value of the resistance, the less NFB. So, wire the pot in reverse, basically. Shunt the wiper to the input pin and wire the amp output to the input pin. Take the "ground" pin as the output, wire so it connects to the second 10K resistor.

You can eliminate the adjustable NFB pot, and use a single resistor, once you figure out what you need. I would do some measurements and look for something that gives you about 2:1 for IEMs, or 2x gain. IEMs, ideally, should just use a buffer. Even an IPOD has enough voltage swing for an IEM that can put out 120dB/V. high sensitivity low-Z headphones, I would go for 2x to 4x gain. Higher end Senn dynamics, I can live with 6x to 8x gain. Measure with a 0dbFS file on your computer to a DAC that you know gives 2Vrms output. Feed into the amp, and measure the RMS voltage at the amp output if you really want. Another way is to put a scope on it, but most people don't have one. Measure the pot, add up the resistances and put in the resistor value you want, or leave the NFB adjustable as it'll make the amp more usable for various headphones.

Again, pick your poison. Some people hate NFB, but I think the extreme amount of series resistance coupled with a 15x gain to utilize an IEM or a high sensitivity low-Z headphone is silly.

YMMV.

I hope you find this useful, IMO, it makes the amp more useful for low-Z. With the 150mA Class-A bias, I think this is a good mod for the circuit. You don't need 150mA Class-A bias for high-Z.

Again, I haven't measured this, but that's what I would do. In theory it should work, but the resistor values may need tweaking. That would require sims, and testing, and measurement, something I'm not going to bother doing.

You can build in a switch to drop the NFB completely. Break the circuit and change the grid stopper to a lower value, and remove the bypass cap. Since this is right on the tube socket, you can put the switch nearby.

Google "star ground". This amp is small and simple enough that you don't need local nodes to connect to the star ground. Run them all to the ground strip. You can also use a ground plane. If using the plane, just drop the ground onto the plane from each component, short runs are best. Look at the article on pmillett's web page. 2 implementations. pmillett, ground plane. n_maher, star ground.

That's awesome Holland, since I'm still learning this stuff what would be the downside to implementing a NFB. What are the downsides of using it, i.e. upsetting the purists?

I shouldn't say purists, but there definitely are camps. Some really hate NFB. Just go around and read various forums. But then again, what isn't argued, resistors, capacitors, NFB, etc.

Part of the argument is that you get phase distortions (but you can also get that from output caps). The other argument is that you can get poor transient behavior, less bandwidth, slew rate impacted, etc.

But this amp is hardly in opamp territory for bandwidth or gain.

The mu on this tube, IIRC, is 38 (?) That's not huge for an amp. You should be able to use enough feedback to get what you want. Use your ears to decide.

Though, for you, I would still reconsider. For IEMs, all you really need is a buffer. A JISBOS with a pot, for example, as long as your source can put out 2Vrms. However, adjustable NFB allows you to use your IEMs with the SSMH, till you get some full-sized cans, then you can dial down the NFB and increase the gain for other headphones.

Audio Power Amplifier Design Handbook - Google Book Search

High-Power Audio Amplifier ... - Google Book Search

search for negative feedback. I think it gives a good overview.

First off, it's been said many times now that you can use a 10k pot with a 40k resistor in series and have the same input resistance as a standard pot, with effectively 1/5 the gain.
The type of negative feedback you're suggesting will in most cases cause all sorts of degradations.
If you're a signals expert you might be able to work something out that is close to distortion free in the audio range... maybe. What you're suggesting is a completely different amp either way.
What's so silly about using a series resistance as described above?

Yes, worded improperly. It's not the input resistance, but the ratio. The need to necessitate cutting the input signal by 5 or more. The higher resistance will have more noise if you start with the BOM pot of 50K or a 100K pot, but that's not that big an issue. Nor is the input resistance, in most sources unless it's a portable, which is unlikely in this amp's usage scenario.

Disagree, it's a standard feedback loop. There are some that don't do a feedback loop, the SSMH uses local feedback in the VAS. As I had stated in my prior post, the values can be tweaked, but I'm not going to do it.

What cases do you refer to?

Noted, it's why I said purists need not read it, and why I debated posting it. I wouldn't call it completely different though. Completely different would be a different buffer, for example, at least to me. *shrug*

High signal attenuation followed by high gain vs "normal" attenuation with a more "reasonable" gain for high sensitivity headphones. Using what's appropriate, has been debated for a long time. I won't argue the pros or cons of it here.

If you want to take an extreme example, an amp with a gain of 150 (10x the SSMH). Is there any detriment in cutting the input signal by 50 (1/5 * 1/10) to then amplify by 150? Same point, different numbers. If that doesn't resonate with you, then it doesn't. Again, I won't argue it here, but think about it. If it doesn't mean anything to you then it doesn't. If it does, it does.

Hmmm, wonder if head-fi is going to crash, keep getting failures trying to post this.

I'm basing this on my limited knowledge of signals and control systems rather than instances of actual audio amplifiers.
Of course feedback is generally used to linearize non linear systems.
A few problems with this is that feedback increases the system's sensitivity to noise and decreases its stability.
What you end up with is a control function (which is unknown really) which affects how feedback influences the output. Finding the right balance between transient response, steady state performance, and sensitivity becomes very complicated, and will involve a lot more than just adding a feedback loop.
Beyond that you are effectively removing the "tube sound" by trying to linearize the amplifier's performance.
I think a lot of audio people rightly want to minimize the use of feedback in tube amps. It is much better to use stages that are close to linear rather than trying to linearize with feedback, imo.

Perhaps my memory is lagging, as it's been a long time since I studied linear systems, but I don't really agree with your summation of feedback. IIRC, that doesn't apply to reasonable negative feedback where the feedback signal is an error control and is used to reduce error/distortion. In a positive feedback system, that would be true, but this scheme would use a negative feedback. For there to be issue in a negative feedback system, the amount of feedback will be large I could be wrong though, I admit that freely, it's been a long time. I think you're making it much more complicated than it really is.

The linearizing, I understand could be an issue for some, and not for others. But, if the argument is that the system is already almost linear, then the feedback shouldn't do much but reduce gain. The end goal is not the linearizing, it's the reduction of gain by the negative voltage feedback factor.

The open loop gain of the amp isn't enormous, and the amount of feedback will be small, and realistically is only used for high sensitivity low Z headphones (20-30dB drop).

All that is primarily "different strokes for different folks", and really that's all it is. I'm sure you will find plenty of tube amps with local negative feedback and more than a few with global, be it power amps, or guitar amps. One can also argue that this isn't a tube amp, it's a hybrid.

Basically, if you want to try it, try it, if you don't, you don't. You're in the purist camp, but till you get rid of all your SS amps and SS source, I won't really believe it. We can go back and forth, but basically it's going to be the same no matter how you slice it. Your view is yours and mine is mine. There's nothing wrong with that.

Hey guys, any idea how the SS would compare to the Bijou in terms of power output?

The reason I ask is because I am trying to build a makeshift tube radio for my gramps on a strict budget ($100). I showed him a pic of my Bijou and he started reminiscing about listening to ball games on his old radio. So anyway, I'm probably going to rip up an old alarm radio or buy a radio kit and amplify it with the SS through some 107 or 117dB/W speakers. With my Bijou cranked via hd650's(105dB/W) it is plenty loud to be audible a room away. So I just wanted to see if the SS would be able to supply ample power.
PS: I looked all over for a small speaker tube amp, but all the kits seem to be at least 100$ and I'm not nearly savvy enough to find an unproven schematic and whip one up from scratch :\

just a little update...I was experiencing the coldstart hiccup problems....just swapped out nichicon pw series 180uF (C1) and 680uf (C6).....and replaced them both with Panasonic FC series 1000uF.

Hiccup cured!

Tube Update

I've managed to acquire some 19J6's, potentially as many as 20 tubes. They should be arriving mid next week and once I have them I'll sort them into pairs (construction matched, brand might not be possible) and I'll try to list them for sale at the end of the week. I'm posting this hear so that I don't have to reply to any more PM's about this. From here forward anyone who PM's me asking to be put on a list will be removed from eligibility. I'm sorry to be like that, but I'm trying to be as fair as possible with this and I'm not going to try and keep track of "pre-orders". I'll post a link to the FS thread when I have them listed.

What would anyone guess to be the approximate number of tubes my son needs for a lifetime of listening with his Nascar 24 MSSH? Assuming he turns it on when he gets home from school or work for 6 hours a day before bedtime, and plays music during at least 2 of those hours? Or, if it is easier, how many years would each pair of tubes last with that kind of use - 5, 7, 10?

Id guess he'd get about 8000 hours out of a set. that works out to 4 years per set at 6 hours use a day.