[Freo-1]

Roger,

I would like to hear more about the testing procedure you mentioned for stability, overload recovery, etc.

I responded to claims about the HK Citation II in another forum and referred to your findings posted above. Well, I found out very quickly that the Cit II is a sacred cow to some people; a lively discussion ensued 

Having used various Dynaco tube amps for most of the last 30+ years and listened to a host of others, I have yet to find the distortion-free amplifier. Therefore I am not inclined to question your findings. Your writing here and on your website is very interesting and informative, so I would like to know more about the subject of these tests if you are willing to take the time.
Thank you.


Best regards,
Mike Hazel

We Citation owners are very passionate about our amplifiers, and do not take kindly to smack talk

Seriously, the Citation II has some of the best output iron ever made, so the amp is capable of great sound. I had my amp re-done by a local expert up here in the Northeast. The pentode front end was replaced by using two 5687 tubes per channel. The 5687 front end is a tried and true setup used by a lot of DIYers, and is capable of providing some of the best sound regardless of price.

Perhaps, one day, I'll get my hands on a RM-200. I've talked to Roger in the past about tubes, and found him a very well informed and schooled master of his craft. Talking to Roger about tube audio is always enjoyable.

[Roger A. Modjeski]

To all in this lively discussion: At the risk of smacking the Citation I propose the following. Anyone who has a scope and generator should take the CITATION power amp and clip it, reduce the signal level 20 db and watch the thing oscillate at about 1 Hz.

I have measured the "iron" in the Citations and found it to be nothing unusual.

I was happy when I sent mine to its next owner.

[Freo-1]

To all in this lively discussion: At the risk of smacking the Citation I propose the following. Anyone who has a scope and generator should take the CITATION power amp and clip it, reduce the signal level 20 db and watch the thing oscillate at about 1 Hz.

I have measured the "iron" in the Citations and found it to be nothing unusual.

I was happy when I sent mine to its next owner.

Hello, Roger.  Good to hear from you.

I assume you are referring to a original Citation II, as opposed to one where the front end has been re-done to work with a different driver tube. 

By the way, was the next owner Bruce Moore?  I remember that he gutted many a Citation II for the "iron" for use in his big ticket MFA amplifiers.  There are obviously a lot of people who think otherwise about the quality of the iron.


Here is a blurb on the Citation II: "  Hegeman's approach to designing the Citation line was from a professional recording engineer's perspective. When you listen to recorded music for a living, listener fatigue becomes a major concern. Hegeman felt that distortion and frequency response were main factors in amplifier design and superior performance. He believed lower distortion, wide bandwidth and multiple feedback loops were essential for realism and to reduce listener fatigue. The amplifier must have minimal distortion to reduce the overall distortion generated from the cutting head, cartridge and speakers. "Distortion", he said, "is a deviation from the original. It includes harmonic, transient and inter modulation distortion components as well as phase response, restricted dynamic range and restricted distribution patterns from microphones and musical instruments".

Frequency response of the amplification system was another major design consideration. The musical response bandwidth must extend considerably beyond the hearing characteristics of the human ear in order to provide satisfactory reproduction. Thus, the concept of "wide band" amplification further developed. Hegeman felt that amplifier performance below the 20 cycle range is very important to a tight and clearly defined low end. Conversely, an amplifier which has a frequency response beyond 100,000 cycles without evidence of ringing or instability when hooked to a reactive load can offer clean, transparent tone in the higher frequencies with outstanding instrument separation. "

Here is a link some additional background information

http://www.bassboy.com.au/getreel/site/classicamps/files/amps/harmkard/ctation2/amp.htm:

[jon_010101]

To all in this lively discussion: At the risk of smacking the Citation I propose the following. Anyone who has a scope and generator should take the CITATION power amp and clip it, reduce the signal level 20 db and watch the thing oscillate at about 1 Hz.

Hi Roger,

I'd be curious if the low frequency oscillation was due to the circuit itself, or simply due to exhausting the charge in the high-ESR vintage electrolytic caps.  Did the tested example have the original power supply caps intact, and were you able to monitor B+ voltage during the test?  With stock value parts, I did find mine to be a bit "sludgy" and "bloopy" at high levels, which might indicate stability issues.  With Jim McShane's power supply kit installed, I have not had any complaints (unless running a impedance mismatch), with zero sign of instability.  But, I also haven't been able to clip it in use (and have not tried on the bench)!  I've run it at levels which should have been more than halfway to clipping into my old AR 302s, and did not see any oscillation.  Unfortunately, it is 2000 miles away right now, so I don't have the ability to test/abuse it further.  Perhaps someone else can give it a try on a restored example ...

[Roger A. Modjeski]

Although I did these tests many years ago, I distincely remember the cause as being coupling capacitor values causing the problem. Some Williamson amplifiers have the same problem and some don't. One would think the Williamson circuits are very similar to one another but they are not. After I noticed this flaw I did a survey of circuits and found many that had low frequency poles stacked upon one another. This flaw demonstrates a lack of understanding of stability which is still found in some amplifiers today and is responsible for giving feedback a bad name. I first tackled this low frequency problem in the RM-1 preamp. I was looking at the step response at frequencies below a few Hz. I found several well known preamps that would oscillate at a Hz or less and take many seconds to settle down. I made sure the RM-1 was free of this problem by separating the LF poles by 20 to 1. Stacking implies 1 to 1 and you can see that is quite a difference. The power supplies in that unit were low impedance to down to DC so there was no way for their response time to influence the behavior.

This is difficult stuff to understand so I will put it another way. If there are two couplings in a feedback circuit they had better be of different frequency by at least the amount of feedback applied. Otherwise there will be low frequency oscillations. The same applies for high frequencies.

I have purposely made this explanation short and encourage readers to mull over it. Sadly there are many amplifier designs past and present that do not take account of this problem going all the way back to the Williamson era.

[Flyquail56]

We Citation owners are very passionate about our amplifiers, and do not take kindly to smack talk

Seriously, the Citation II has some of the best output iron ever made, so the amp is capable of great sound. I had my amp re-done by a local expert up here in the Northeast. The pentode front end was replaced by using two 5687 tubes per channel. The 5687 front end is a tried and true setup used by a lot of DIYers, and is capable of providing some of the best sound regardless of price.

Perhaps, one day, I'll get my hands on a RM-200. I've talked to Roger in the past about tubes, and found him a very well informed and schooled master of his craft. Talking to Roger about tube audio is always enjoyable.

Hi Freo,

You sound like a more reasonable person than the H-K people that I encountered in the other forum. I wasn’t trying to give anyone a bad time; I simply asked how it was that the Citation II could be as unconditionally stable as they claimed when Roger had evidence to the contrary. One offered as evidence that it can be turned on without speakers hooked up, and nothing bad happens (!?). A few people offered opinions as to why they did not like the Citation II. And finally, Jim McShane went to some length to discredit every dissenting opinion, including Roger’s, by stating that they were either ignorant (haven’t heard one in proper operating condition), biased (do not understand the unusual circuit design and therefore do not like it), or from persons belonging to the DHT/SET crowd and don’t like it because it is a push-pull amp. I asked him again to either explain to me why Roger’s findings are wrong or provide his own testing that contradicts Roger’s, which he said he would.  It’s been six weeks and I am still waiting… Unfortunately, he seems to prefer to attack the person rather than address the original questions.

And hey, you may want to keep it down about having changed the front-end circuitry on your amp, even if it does sound better. They may not let you into the inner circle.

Thanks for your comments though. It's refreshing to hear from a Cit II owner without a chip on his shoulder!

Best regards,
Mike

[Freo-1]

We Citation owners are very passionate about our amplifiers, and do not take kindly to smack talk

Seriously, the Citation II has some of the best output iron ever made, so the amp is capable of great sound. I had my amp re-done by a local expert up here in the Northeast. The pentode front end was replaced by using two 5687 tubes per channel. The 5687 front end is a tried and true setup used by a lot of DIYers, and is capable of providing some of the best sound regardless of price.

Perhaps, one day, I'll get my hands on a RM-200. I've talked to Roger in the past about tubes, and found him a very well informed and schooled master of his craft. Talking to Roger about tube audio is always enjoyable.

Hi Freo,

You sound like a more reasonable person than the H-K people that I encountered in the other forum. I wasn’t trying to give anyone a bad time; I simply asked how it was that the Citation II could be as unconditionally stable as they claimed when Roger had evidence to the contrary. One offered as evidence that it can be turned on without speakers hooked up, and nothing bad happens (!?). A few people offered opinions as to why they did not like the Citation II. And finally, Jim McShane went to some length to discredit every dissenting opinion, including Roger’s, by stating that they were either ignorant (haven’t heard one in proper operating condition), biased (do not understand the unusual circuit design and therefore do not like it), or from persons belonging to the DHT/SET crowd and don’t like it because it is a push-pull amp. I asked him again to either explain to me why Roger’s findings are wrong or provide his own testing that contradicts Roger’s, which he said he would.  It’s been six weeks and I am still waiting… Unfortunately, he seems to prefer to attack the person rather than address the original questions.

And hey, you may want to keep it down about having changed the front-end circuitry on your amp, even if it does sound better. They may not let you into the inner circle.

Thanks for your comments though. It's refreshing to hear from a Cit II owner without a chip on his shoulder!

Best regards,
Mike

Thank you, Mike. Appreciate the feedback.

This hobby we pursue can easily get out of the enjoyment realm if one lets it. There are a whole host of engineering solutions out there for audio, and while solid engineering practices should always be the order of the day, no one set of solutions are absolute. Amplifier and speaker dessign are truly "horses for courses".

The original Citation II sounds good to me, and to a lot of people. In my case, I think changing out the front end to 5687 opened up the sound stage and clarity notably (along with re-doing the power supply and bias setup).  Other people may demur (and that's fine as well).

Some people think the Citation II is hard sounding, and prefer a more traditional tube sound (bass not tight, slightly depressed treble, and midrange emphasis).  The reality is that it is almost impossible to obtain " you are there" sound from any playback system. They all deviate in one matter or another. 

I've always thought sound reproduction was 90 per cent engineering, and the remaining 10 percent art. So, what do we argue about most of the time? The 10 per cent that is art, that's what. As long as lively discussions are respectful, have fun with them, and you may learn something along the way to boot.  We are never too old to learn new ideas.

Cheers

[Roger A. Modjeski]

If you ask an amplifier maker if his amp is unconditionally stable or if his tubes are matched or if the noise is low I imagine you will get a "yes". Why would he say otherwise. He also knows that you will have a hard time finding out if his isn't as he claims. So we have the problem of "empty claims". Earlier this evening I started a new topic: preamp/driver triodes: Gm vs Mu have a look at that example. The same goes for stability. Is the maker going to tell you that .1 uF of cable capacitance will send his amp into supersonic oscillations? Did he check to see if the back EMF of a large cone woofer creates birdies under certain conditions?

"Unconditionally Stable" has little to do with an amplifier behaving itself with no input and no output. Mine don't mind either or any variation of inputs/outputs floating. If an amp cannot behave itself under those conditions I wouldn't even consider it. These days "unconditionally stable" is claimed even if it isn't. As I have mentioned before, I can present an amp with all sorts of reasonable loads and watch all sorts of instability in most.

As to the defenders of the Citation II, I find that those who adamantly stick to their position in the face of solid logical evidence have nothing useful to contribute to knowledge. All they have is an opinion they cannot support which will be further supported by those who do not know enough to judge the truth for themselves. I know nothing of Jim McShane beyond what I have read here. I have shared my findings and cited specific flaws of the Citation II amplifier that will not go away without reworking the time constants in the driver and feedback loop.

I did measure the transformers and although better than the standard Dyanco they offered nothing that I found exceptional. Those of you who have followed my research,which I am happy to share with you, know that I have a different approach to transformer design. Bigger is not better. More iron is more work to drive. More iron presents a longer path to encircle and more winding resistance. Bigger transformers have diminishing returns.

Here's a tip: I clearly recall that I found the Citation V to be the better of them and a bargain since everyone is chasing the II. The wattage difference is not important. I had both at the same time. I kept the V and sent the II to Hawaii to rust in peace.

Sadly, the paragraphs cited by Stu Hegeman are early examples of the "fluff" we see all to often today. If nothing else, let's award HK a prize for writing so much that means so little.

In my opinion, Sid Smith designer of the Marantz amps was heads above Hegeman, may they both rest in peace.