John Curl's Blowtorch preamplifier

[Bonsai]

 

[tomtt]

Bob Cordell attacked MY sincerity and honesty, this week, on this thread.

such attacks go with this territory -

When true genius appears,

you can know him by this sign:

that all the dunces are in a confederacy against him.

jonathan swift

 

[ostripper]

Blowtorch thread = 1/5 useful info/BS .. Exponential BS/ sqrt
of post 500.. I really don't mean to be harsh, but for those
of us "student's" this is bad for the teachers..

Everybody who "happen's" upon DIYaudio checks out this
thread and then they see the interpersonal BS instead of the
thoughtful insight they expected. Bad for the site... very freakin'
irritating as well. We who do do not know as much as YOU
would appreciate more "meat" to latch on to.


OS

 

[andy_c]

I'm not sure if anybody's posted this yet, but there's a brief section of Doug Self's site related to transformer imbalance and its possible effects here.

 

[john curl]

I would take 'Damnitall' but it is illegal.

 

[maxpou]

Hi,

Chris has apologised to me both publicly and privately, which is very gracious of him, and I accept in full his kind and thoughtful words.

I honestly did not seek any such apology, but I did sincerely hope that perhaps drawing attention to some unfortunate points (which I believed were relevant to make) might just help to dissuade some others from apparently and regularly criticising 'out of hand' those posters who are *not only concerned with measurements*, which any other sensible individual in this field attempting to improve their audio systems must surely appreciate *are also as vitally important* as any listening trials/experiments, as of course do I.
For the vast majority of those posting here this comment doesn't apply, so please don't take exception to what I say here.

I guessed that there had to be some misunderstandings here, partly (and entirely unwittingly in my opinion) due to a very minor slip (one letter added unfortunately, that is all) in Charles' earlier posting which when read quickly altered what I am sure he intended to say. However, I still believe that this would have been less likely to have occurred if all Members would simply give others a fair chance in this thread whether they understand or agree with what is posted or not, and not immediately (over?)react with such enthusiasm.

Personally, I am also extremely sorry that such a daft situation arose at all, and that at least in part I was responsible for any time-wasting off-topic unpleasantness, but I can assure everyone that I had no intentions that this should happen.

Maybe everyone involved, including myself, has learned something worthwhile from this, and I do also sincerely hope that this will not arise again. It makes life unbearable in my view, and for no sane reason. In my earlier posting I willingly acknowledged the almost impossible tasks faced by Moderators on such a large forum as this, I meant what I said then, and it is to my regret that my intentions might have been misinterpreted.

Please, just let us all give each other a chance to have their say and not automatically dismiss their comments, as I for one am still learning from what I see here on occasions, and with life being so short (especially at my age!) sharing experiences is extremely valuable to many of us, even though many others don't perhaps hold similar views.

Regards,

Well spoken M. Bobken,
I don't like this quarrel I have caused with my question from beginner. Thank's again for your answers. Maxpou

 

[Johnloudb]

When we were testing the switches we were using a D/A converter made by Jocko Homo that had a variable output with an output impedance of around 50 ohms, I would guess. This fed a power amp with an input impedance of 10 kohms. So not a heavy load, but not too light. (Our current designs have a 1 Mohm input impedance!)

As far as the "psychological" part, I have found that to be silly and a waste of time. I've tried very controlled tests, I've tried tests where I've deliberately tried to fool people, I've tried all kinds of things. In the end, it's not hard to tell what's going on if you have a careful listener that is intimately familiar with the system under test and the specific source material being played. I've found these last two items to be the most important items for conducting listening tests.

As a recent example, we were comparing the sonic difference of taking the 64-bit output of a digital filter and reducing it down to 24 bits for input to a DAC chip. We tried three methods:

a) Rounding.
b) +/- 1 LSB TPD dither.
c) +/- 1/2 LSB TPD dither.

The engineer who programmed the digital filter knew which DIP switches on the rear of the unit would create what type of bit-depth reduction. I had no idea. I told him to see if I could guess. So I listened and got all three correct.

The odd thing was that the audible differences were so obvious. I would have thought that the noise in our circuitry (not to mention the limitations of the DAC chip!) would have made it so that *anything* you did to the 24th bit wouldn't matter. But there you have it. Another case of easily audible and difficult to explain.

Cheers,
Charlie Hansen

Charles,

It's really great to here about your experience on this subject. I fell behind on this thread but I'm catching up.

As I learned from an experience with my hearing (see "Hearing Beliefs and Blind Testing" in my signature), experience and familiarity with sound is very important. The ear filters out sounds that we hear on a regular basis that have no particular importance. It's called habituation. So, that might explain why the noise didn't obscure what you heard. I think it's also why blind testing should be done on system you're familiar with.

I don't know how you knew how those LSB changes would sound, but I don't understand why I occasionally have dreams about future events either. Just your experience I guess.

The control was the "bypass" connector. Remember, we made up a batch of connectors. Each was a pair of rhodium-plated RCA connctors, one male and one female, with the grounds directly soldered together to make one large back-to-back connector.

Then we soldered short lengths (approximately 2") of Cardas hook-up wire to the center terminal of each connector. For the "bypass" reference, we simply soldered the two wires together. For each DUT, we would solder the DUT (relay or switch to be tested) to the wires. In this way the reference "bypass" had the exact same number of connections, solder joints, and wires as the DUT's. The only difference was whether there was a DUT in the signal path or not.

The arrangement was to plug the reference "bypass" directly into the input connectors of the power amp. Then the cable from the preamp would plug into the reference "bypass". For the testing protocol we would listen to about a half dozen tracks of various types until we felt we had a good fix on the sound of the system. The volume control of the preamp is a stepped attenuator, so it is trivial to ensure that the volume is always the same if we changed it for different tracks.

Then we would plug the DUT in place of the reference "bypass" and listen to the same songs at the same volume. If there was any question about what we were hearing, we would then simply replace the DUT with the reference "bypass". It would only take about 15 seconds to change between the DUT and the reference "bypass".

The person changing the DUT's obviously knew which one he was inserting. The other people listening (typically one and two others) sometimes were told what the DUT was and sometimes not. We tried it both ways many times and it never made any difference.

Essentially the experiment would be like putting 10 different slices of cheese on a plate and comparing their taste to a "reference" cheese. You would taste one cheese and see if it tasted the same as the "reference" or not. At any time, you could take a bite of the "reference" to get a baseline.

Listening to the different DUT's was as easy as tasting different types of cheeses. They all had marked "flavors", EXCEPT for the Shallco switch. With that one we had to go back and forth several times. There was a *very* small difference compared to the reference "bypass", but it was so small that it was hard to describe and I highly doubt that I could identify it if somebody inserted it into the system without my knowledge. In contrast, all of the other switches and relays we tried made an easily identifiable change in the sound of the system.

This is great! You did a controlled blind test that works! I keep saying you got to do a blind test that takes into account of how we really hear. I didn't think that a relay or switch would be audible in the signal path. But I've never had a problem hearing differences in cables though. Because of the experiences I've had with my hearing I wasn't sure I could tell a difference in a blind test.

The mind is very open to suggestion, as I explain in my paper "Hearing, Beliefs and Blind Testing." But, if a sound is audible, at all, and you're familiar with the sound you can hear it. The brain turns up the gain for familiar sounds with importance. Even the quietest sounds can be very distressing. Anyway, you're welcome to check it out.

I still say people need tone controls!! But, I know some people enjoy everything on the recording, the good and the bad. I prefer to have some control over my system.

Thanks for sharing,

John

 

[john curl]

Over the years, this thread has been fairly popular. It is not necessarily just about the Blowtorch and its general design, but what it takes to make a successful audio product.
Of course, this creates controversy, as many, for some reason, think that quality audio design is easy and obvious.
When designers like Charles Hansen and I talk, we usually speak to each other, much like we address each other here. A little gossip, a little exchange of useful information, and as light hearted as possible. This is because we are competitors and colleagues engaged in the same goal, to make the best audio possible. It is an obsession for us.
Today, I spoke to Walt Jung (at anatech's suggestion) and our conversation was more formal, yet we have known each other for more than 35 years. We discussed circuit details that most here would not understand, especially beginners, as well as a little social exchange.
For the record, I am not going to break down what we discussed, because it is more than I am willing to contribute, just like I will not put up my latest schematics, in general. I am not here to teach you how to copy me.
Oh sure, we get complaints that we don't give everything that we know away. It must be remembered that others pay us for the same general info that you want. It would be unjust for them to pay, and for the rest of you to get it all for free. Also, they might stop paying and just ask questions on this thread, and I need the income, so please don't complain and expect to get my favor in some way. Thanks in advance.

 

[anatech]

Hi John,
I can understand what you are saying here. I will simply point out that most people do not know you and Charles Hansen. So when you talk casually, it may appear completely differently to an outside observer (the rest of us). To avoid any misunderstandings, I can only ask that you keep a respectful tone. That really applies to however is addressed. Of course this applies to everyone, I don't mean to single you out.

Hi Bobken,
I'm glad there are no lasting hard feelings. Thank you for that, and your kind words. You have given us some wise advice, something I'm sure that we can all agree with.

-Chris

 

[vuki]

I'm not sure if anybody's posted this yet, but there's a brief section of Doug Self's site related to transformer imbalance and its possible effects here.

Thanks for the link, but I have quite different experience with balanced mains than D. Self. In fact, he doesn't have experience but that doesn't stop him to make a comment about it's effects on audio. Recently I had have such isolation transformer made for an audiophile friend. I really didn't believe it would have any effect on sound, but to my surprise it really did notably improve sound in three systems we had tried it in. Yes, the soundstage was larger, but also the bass was firmer and everything seemed cleaner and more natural.
I don't know the exact mechanism of why this thing works that way - could be mains filtering effect or ground currents cancelling effect or both. The transformer is made on 2kVA UI core with primary on one and bifilary wound secondaries on the other side. It is "soft" transformer- meaning that prim/sec coupling is loose (the voltage sags ca. 5V for 500W load) but that also helps (with some additional capacitors) for a good filtering effect. I was afraid that I would notice somewhat reduced dynamics but that was not the case - it brought just benefits to the sound even dynamics seemed to be better. So maybe mr. Self should first try something before calling it rubbish?

 

[analog_sa]

Hi Vuki

Thanks for posting this. Did your components use safety earth? Is it difficult to repeat the test with the safety lifted?

 

[vuki]

Hi Vuki

Thanks for posting this. Did your components use safety earth? Is it difficult to repeat the test with the safety lifted?

Right now it is impossible to do any test because the transformer is already in my friends system, but as I liked what it does I ordered one such transformer for me too and I guess I'll have it in a couple of weeks.

 

[Bob Cordell]

Well Scott, in attempting to reduce the residual harmonic series in my Sound Tech this morning, I added an output cap across the gen out, and reduced the harmonic series considerably. I still have some nagging 7th harmonic that seems to come from the input stage of the analyzer. I might, in future, force the IC's on the front end to be class A. What do you think? PS I might do that to the oscillator too!

Hi John,

Let's move on to something technical that people can perhaps profit from.

You have mentioned several times that you can measure distortion products down to about -120 dB. You have also stated that you care a lot about the 7th being way down. These are measurement-based things that we can both agree on.

Measuring distortion products down to 120 dB is not easy. Can you tell us what your setup is and how you do it? For example, are you able to measure the 7th harmonic of 20 kHz down to -120, or are you referring to a lower fundamental frequency? What analyzer bandwidth do you have available on the SoundTech? I don't think the SoundTech performance specification comes even close to being able to see -120 (0.0001% - I think the SoundTech can do maybe 0.001% on a good day), so I'm guessing there is more to your setup than that.

Thanks,
Bob

 

[Bob Cordell]

Transformer polarity has been understood for as long as there have been transformers. The primary winding goes on first with the secondary over it. When the primary in connected one way, the neutral is wound closest to the core. The other way, the hot is nearest the core. The leakage is different because the potential is different. This only applies to one hot and one neutral mains connections. US type 220VAC is different.

This can easily be measured with a digital meter. Connect one lead to ground and the other to the transformer frame. If the transistor has been varnished you will need to cut through the varnish to make an electrical connection. Check the AC voltage with the transformer primary wired each way. One way will be high and one low.

For toroidal transformers (no metal exposed), simply place a sheet of metal under the transformer and measure between that and ground.

It is not necessary to load the transformer for this test. Use the polarity that shows the lowest voltage reading.

Hi Steve,

Thanks for this explanation; it seems to make sense. I guess you are mainly referring to the capacitive coupling from the hot-side part of the primary windings to the core.

This got me thinking, though. Although neutral on a mains circuit can be quite dirty, we usually do tend to think that hot is worse. If the issue is capacitive coupling from the hot parts of the winding, we have a choice of whether to allow that coupling to go to the core, which will be typically at earth ground, or to the secondary, which will typically be related to the circuit's star ground.

Perhaps I read you wrong, but I kind of got the impression that you were saying that it is better to have the hot part of the primary windings away from the core, resulting in less coupling of garbage into the core. But this then means more coupling into the secondary. Neither choice seems good, but if the issue is capacitive coupling as you have described, which type of coupling do we think is less bad?

Thanks,
Bob

 

[PMA]

Just measure leakage currents, I have dozens of such measurements.

I am often being surprised how little evidence is shown here. Too much talking.

 

[PMA]

Potential difference between two class II audio equipment.

 

[john curl]

Actually, for working audio engineers, Bob, I have been very helpful.
The situation is this:
My ST analyzer, has been modified by Scott Wurcer and me to perform better than its stated specifications. This has been due to using Scott's AD797 IC's as the input to the analyzer, and Scott's 744 IC in the oscillator section. I could probably replace a few more IC's, but so far, I can get what I want, once I analyze the residual with a HP 3563 FFT based analyzer with a 256K clock, so that I can signal average, and screen out the distortion products
Of course there is a little 2'nd and 3'rd residual, but they are at .001% and below, so I ignore them and concentrate on the 5th, 7th, and 9th harmonics. Ideally, I can have fairly little higher order harmonics, and I can test this with a bypass test using the best measuring cables in my possession and noting the residual. I can always subtract this residual from the actual measurement, if I chose to do so, but I have not found it that important to do up to this time.
Now, what I am having trouble is a nagging residual 7th harmonic that I would like to reduce or remove, so that I don't have to do differential subtraction, either in the test equipment or in my head. Then when I spot some 7th harmonic on the screen, I will be able to generally attribute it to the DUT, rather than the test equipment. It is more a matter of convenience to reduce the 7th harmonic, but it might be a useful improvement.
One approach that I have NOT tried at this point, is to force the output stages to run Class A, by adding a fet current source on the output to the - supply, and hoping for an improvement. Of course the 2'd harmonic might rise slightly, but MAYBE the 7th will drop.
As Scott Wurcer has, at least, past experience with his own IC output stages, I thought that he might give me some useful input in doing this.
Repeatedly, I have asked Scott for the current of peak beta of the npn output device, but so far no luck. I would make a difference to me, if it is .5 ma , 5 ma, or 50 ma. From my perspective, all 3 are possible from my experience with discrete devices.
Now, have I made myself more clear to both you and Scott?

 

[GRollins]

What are you talking about? What do you think I meant anyway?

What I want to know is what set you off. I didn't pick a fight with you.

Chris, you're passive-aggressive. It's damned tiresome.
If you and others of like mind are so sure John, Charles, I, and others who want to talk about these things are so full of (ahem...feces) then don't post in it. Don't read the thread. Don't participate in any way. But, no, you have to come 'witness' (those who don't live in the Bible Belt here in the US will be mystified by this term--you can probably Google it to catch my drift) on those who do find things of interest here.
I quit posting in threads like the simulation thread...might I suggest that you and those who believe as you do abandon this thread? No? Then I have to question your motives.
Which leads me back to "Science as religion," and other such motivating factors. Syn08, for example, is at least 90% troll (a clear motive), yet you never seem to do anything about him. Why? "Fish don't notice water," perhaps? Or just plain hypocrisy? John does get a bit, well, abrasive at times, and you jump on him for doing so...yet I cannot recall a single post wherein you call syn08 (just one of many) to task for the things he says and the attitude with which he says them. John is both wrong (in that he gets frustrated and hostile) and right (in that you employ a double standard in your moderation). But the knife's edge he has to tread online is similar to the one I have to live here in reality in SC, so I can sympathize.
If you and those who feel it so necessary to witness on everyone about the glories of THD as lord & master of all audio would simply not participate in this thread, then John (and Charles) would not be goaded into intemperate posts and everyone would be happier. Okay, maybe you'd sneak a peak into the thread and have an apoplectic fit whilst restraining yourself from posting, but most everybody would be happier.
I will stay out of John's relationship with SY (the only other moderator I can think of who posts in this thread regularly). They clearly know each other personally and appear to have achieved some balance that suits them.
How about it, got the guts to get out...and enforce the same on syn08, et. al.?
Or would you like me to get into the simulation thread for a couple of weeks so you can see how it feels when the shoe's on the other foot?

Grey

 

[anatech]

Hi John,
I am curious why you don't want to build your CCS with a BJT and LED + 2 resistors. I find them more stable with temperature and finding the zero tempco point for J Fets is another task I would rather avoid doing. I know with BJTs, I can grab the parts and simply get the current I want. I also believe the BJT version has a higher impedance than the J Fet types. This would show up as a better PSRR when you are measuring "way down there".

Just a thought.

Another thought (I know, I'm over my quota ). If you are using J Fets for current sources, and making measurements to -120 dB and beyond, might the higher impedance from a BJT version clean up the noise in your circuits? That's assuming the impedance on the BJT is in fact higher than the J Fet. It would provide more isolation from supply noise. Most regulated DC supplies either pass or pick up far more noise than an audio circuit is capable of.

I have been playing with using a J Fet for CCS duty since Walt was kind enough to tell me how he does it. I really have to say that he has far more experience than I do, so he probably knows early on from other measurements he makes which parts will give him the current he wants. I have no such luck, so I'll have to continue in order to gain experience with this.

Your quote from SY :

Originally posted by SY I think the worry is a legal one, not necessarily a true safety concern.

I think he was referring to the removal of the safety ground, not the lead content issue.

-Chris

 

[john curl]

Grey, I have a feeling that you feel under siege here. Actually, many of these comments were not directed personally at you.
For example, I think that SY was worrying about 3rd wire grounding when he made his statement. I was mostly concerned with Alansawyer's attack on Bobken, as if Bobken was doing something immoral by donating his electronic parts to a 'school'.
I think that lead problems are HIGHLY overrated, and while there may be a sincere need to remove it from the environment, lead solder just isn't that much of a big deal.
In any case, we are generally in your camp.