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High end OP's - LM6171 vs. OPA604 vs. OPA627 vs. ... - Page 2

post #16 of 26
Quote:
Originally Posted by Sugano-san
That's where you're wrong again -- the Opamps in the RKV do much more than correct bias current. As I said, I am not an audio engineer, but Helmut is. If his English were a little better, I am sure he could express with more clarity than I why changing the Opamps influences the RKV's sound so much. But then again, I don't think I did such an awful job in the first place.

In any event, it was my RKV on which the Opamps were changed, and it was the designer and manufacturer of the RKV who performed this modification.

Before I sent my RKV back to Helmut, we discussed the project and its different aspects in length over the phone and by mail.

It was Helmut who said that from his point of view changing the Opamps was the single most promising modification out of the ones that were performed. And it was Helmut again who was right. The dfference is huge. I hardly recognized my RKV when I got it back. And it was Helmut and me (i.e., not you) who did the before/after listening. So we should know.

This just goes to show that one ounce of practice is worth more than one ton of theory.

So my remarks stands: I am using the OPA 637 in my Supermacro and in my RKV, in both cases with excellent results.

I'm gonna ask you a simple question: to change OPAmps in the RKV make the same difference as changing an OPAmp in the PPA or any other amp, in which they are inside the signal path, for example like going from the OPA637 to AD8610...??? If yes, I stand corrected.....and please end of the discussion, the OPAms in the RKV are not used for an audio application, the difference could be audible of course, any change in one amp affects the sopund we all know that, but not as big as in the case of an OPAmp based amp...period....
post #17 of 26
If Helmut tells me that the Opamp "makes" the RKV's sound -- and that's what he told me -- that means to me that the Opamp "makes" the RKV's sound -- regardless of your (esteemed) views on the subject.

I trust this is sufficient to end the discussion.
post #18 of 26
In the RKV Mk II product section of the AudioValve web site, there is a reproduction (very poor quality!) of Helmut Becker's RKV patent.

The schematic diagram shows an op-amp driving the grid of the first amplifier stage. The four tubes are DC coupled and there is DC feedback from the amplifier output (before the capacitor that couples the output signal to the load) to the op-amp input to stabilize the bias conditions and to set the overall gain of the amplifier. The signal input is AC coupled to the op-amp input.

It thus appears that the op-amp is used both to set the operating point of the tubes and to inject the audio signal onto the grid of the first tube stage. This being the case, the op-amp is very much in the signal path and could have a profound effect on the sound of the amplifier.

The photos of the PCB reveal that the op-amp used in the RKV is a generic "741" type, a variety dating back to the 1970s when good audio performance was far from paramount in the mind of the designer! That it provides acceptable results is probably due to circuit conditions that force it into class A operation. It therefore comes as no surprise to me that the sound of the RKV could be improved markedly by substituting an op-amp with better audio characteristics. In fact, I wonder why a better op-amp is not fitted by default. (Cost, I suppose...)

The op-amps are mounted in sockets with the same pinout as that used in the PIMETA, PPA, Emmeline, etc. Could the RKV turn out to be the ultimate tweaker's platform, with the opportunity to roll both op-amps and tubes? Better start a new thread if this catches on!

Disclaimer: I strongly discourage anyone from attempting op-amp rolling in an RKV without a more thorough investigation of the circuit conditions than can be accomplished by inspection of a schematic diagram with no legible component values. Being a tube amplifier, there are voltages present that could potentially (pun intended) fry an op-amp, with possibly catastrophic consequences in a DC coupled amplifier. I'm putting forth these ideas for consideration when more is known about the amplifier -- they are not to be construed as a recommendation for immediate experimentation. YOU HAVE BEEN WARNED.
post #19 of 26
I agree. Helmut told me that the Opamps control not only the tube bias, but also "any and all AC and DC components" in the RKV. The actual juice (tension and current) is generated by the tubes, but the way in which the tubes amplify the sound is controlled by the Opamps.

It has never been a secret that the Opamps are placed in the signal path; this fact is even published on audiovalve's website:

"Mit Hilfe eines Op-Amp im Signalweg konnte ein Servokreis geschaffen werden, der alle o.a. neg. Eigenschaften der Röhren ausregelt."

Rough English translation:

"With the help of an opamp in the signal path a servo circuit could be created that compensates for all above-mentioned negative characteristics of the tubes."

It follows that anyone alleging that the RKV's Opamps are not in the signals path is wholly wrong and that such statements are based on assumptions (at best) but not on hard facts or reliable information.

I also agree that (i) the RKV could become the ultimate tweaker's platform, and (ii) you should not try to roll opamps on your own.

I installed OPA 637s and 627s myself out of curiosity, and I found that they do not work stable in an unmodified RKV (clicking noise). The required modifications were thought out and performed by Helmut, not by myself.

During my experiment, however, and through the noise floor, I also heard a much better sound emanating from the RKV, and this is why I convinced Helmut to modify my RKV.

As I said before, the difference is huge; I hardly recognized my RKV when I got it back. I must mention, though, thatI also had the capacitors replaced with much much better types and that instead of the Alps pot a ladder-style stepped attenuator has been installed. But Helmut told me that changing the Opamps is the single most promising modification you can perform on an RKV.

Opamp-rolling the RKV is the way to squeeze out of this beast a completely unexpected and superior sound quality, and its huge power reserves (for a headphone amp) just add to the truly impressive final result.

As I said, Helmut tells me my RKV measures much better than the stock model, and we both find it sounds much better too! And what else would you expect from an OPA 637 implant.

So much for now. Best regards.
post #20 of 26
Ok, Sorry to stir on the same issue anyway that is not the topic of the thread, but if this is the case, I stand corrected, and sorry for the hassle, but just for the records, there is no information (or at least is confusing) about that particular use in the user manual or website, also they stated there in the specs section that the bias is full automatic and IC controlled and thisi s the only reference ot the ICs in the whole manual (that was why my wrong assumption, but you prove me wrong so no more discussion on the subject)

If the IC's are in the signal path (as it seems to be the case) then the amp is not a pure tube OTL amp (as they stated) it could even be considered as an hybrid OTL amp, nothing wrong on that, but why not stating that as such clearly....Sorry but they are not being accurate in the description IMO...as they stated that it is a tube amp, and that the audio signal is processed by the 4 tubes, that was why my wrong assumption, sorry for the troubles, now from the website "in English", as they have also one in english:


".....The root of the famous AudioValve amplifiers is the RKV - the very first invention by Helmut Becker in 1982 and is protected by patent No. DE 3200 517 in germany.
The basic concept is founded on a conventional OTL principle circut. To find a circut and a harmonizing tube for high output current and very low resistance wars one challenge Mr.Becker has to master.
The second success he had was to realize high output power for nearly all different headphones and their impedances - 32 to 2000 ohms.
Mr.Becker found a tube used in the TV production called PCL 805 (18GV8). With this tube and a full - automatically bias regulator Mr.Becker developd a unique closed DC coupled servo regulator circut whitch guarantees a life long exellent rendering of music.

All tube tolenrances was eliminated by using old or new tubes. All in all this amplifier is so perfect in technical details that until today the circut is the same, now for nearly 20 years.
Only one improvement was made: a output protected circut, who eliminates the little "Blobb" when the amp is switched ON and OFF. This circut is integrated on a small pc board in the new Mark II model but also can be additionally built into the older versions.

tech. description:

OTL - transformer outless tube amplifier (not very accurate IMO if the IC's are in the signal path...it should say outless "hybrid" amplifier)
double mono construction
Alps volume control
1 pair 6,3 mm stereo jacket for headphones
tubes: 4 * PCL 805 (PCL 85 or 18 GV 8 - german old TV "vertikal" ampl. tube )
power output: each channel 3 watt SINUS, ( between 100 to 400 ohm loads )
bandwide: 15 - 100 000 hz
damping 3600 !
distortion 0,002 % at 1 Watt-200 Ohm load
max. output voltage 80V
sens. 0dBm
IC - fullautom. - controlled bias (the only reference to the ICs inside the manual....)

class - a
power consumption 40 watt
size 360*200*120 mm (d.w.h.)
weidht 10kg......"

Anyway, even being confusing, I stand corrected in any case...sorry....
post #21 of 26
OPA627BPs are my favourite by far. I've got 4 of them in my amp and 6 in my cd player. (OK, so 4 of them were samples but I paid for the other 6!) OPA2227s also sounded pretty good to me. Did not like the OPA2134 and OPA2604.
post #22 of 26
Today, I discussed the RKV schematics (which I have) with someone highly qualified in tube amps. This schematic is awfully complicated compared to a pimeta or other similar amps and I never quite caught the use of the opamp.

From what I got: the inverting input opamp creates a feedback loop, in which the tube buffer is included. This global loop has a very high gain, to control the instability of the tubes. Very roughly, the opamp is as much in the signal path as a the opamp in the PPA for example.
post #23 of 26
Helmut told me the Opamp works with "infinite gain", whatever that means, but so much I am able and happy to confirm.

I think this Helmut guy is really something special, and so is the RKV. Even more so after it's been modded...


Quote:
Originally Posted by 00940
Today, I discussed the RKV schematics (which I have) with someone highly qualified in tube amps. This schematic is awfully complicated compared to a pimeta or other similar amps and I never quite caught the use of the opamp.

From what I got: the inverting input opamp creates a feedback loop, in which the tube buffer is included. This global loop has a very high gain, to control the instability of the tubes. Very roughly, the opamp is as much in the signal path as a the opamp in the PPA for example.
post #24 of 26
Quote:
Originally Posted by machead
The schematic diagram shows an op-amp driving the grid of the first amplifier stage. The four tubes are DC coupled and there is DC feedback from the amplifier output (before the capacitor that couples the output signal to the load) to the op-amp input to stabilize the bias conditions and to set the overall gain of the amplifier. The signal input is AC coupled to the op-amp input.

It thus appears that the op-amp is used both to set the operating point of the tubes and to inject the audio signal onto the grid of the first tube stage. This being the case, the op-amp is very much in the signal path and could have a profound effect on the sound of the amplifier.

The photos of the PCB reveal that the op-amp used in the RKV is a generic "741" type, a variety dating back to the 1970s when good audio performance was far from paramount in the mind of the designer! That it provides acceptable results is probably due to circuit conditions that force it into class A operation. It therefore comes as no surprise to me that the sound of the RKV could be improved markedly by substituting an op-amp with better audio characteristics. In fact, I wonder why a better op-amp is not fitted by default. (Cost, I suppose...)

The op-amps are mounted in sockets with the same pinout as that used in the PIMETA, PPA, Emmeline, etc. Could the RKV turn out to be the ultimate tweaker's platform, with the opportunity to roll both op-amps and tubes? Better start a new thread if this catches on!

Disclaimer: I strongly discourage anyone from attempting op-amp rolling in an RKV without a more thorough investigation of the circuit conditions than can be accomplished by inspection of a schematic diagram with no legible component values. Being a tube amplifier, there are voltages present that could potentially (pun intended) fry an op-amp, with possibly catastrophic consequences in a DC coupled amplifier. I'm putting forth these ideas for consideration when more is known about the amplifier -- they are not to be construed as a recommendation for immediate experimentation. YOU HAVE BEEN WARNED.
I can be a bit more accurate and confirm what you got from the very hard to read patent drawing.

The first opamp is inverting, with infinite local gain, and create an overall feedback loop, with a gain of 40. Its power supply is at +22/-6V, to bias the input tube.

Regarding the opamp : the design has been drawn in the early 80's. My version of the schematic is from a kit that was once sold by Elrad. The commercial amp is derivated from it. From what I gathered, the main modifications were some decoupling caps and a dual mono structure. The kit asked for an LF351 opamp. I don't think they tried to cut costs, they probably just continued to use what was once available. The lf351 is a j-fet input opamp. I was warned about oscillations too.

Helmut Becker kindly provided me the old schematic to repair the kit I bought used. I will have to ask him if I can make it public or not.
post #25 of 26
LF351 -- that's it! That's precisely the Opamp which I removed from my RKV.
post #26 of 26
Well, that's good news! The LF351 is quite a bit faster than the HA17741 shown in the RKV photos, so I guess I can include a few more op-amps in my target list for no-modification substitution. The OPA637 is faster still, so it is not surprising that it is unstable without the extra compensation capacitor. I was surprised that 68 pF would make the difference, but it now makes sense knowing that an LF351 is stable.

My long list of hopefully stable op-amps now includes:
OPA27
OPA227
AD743
AD797
LT1028
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