[Blorton]

- All overused cliche's aside - this mod is as subtle as a punch in the gut. -

Those of you familar with the RKV will know that it's one of the reference amps for driving the K1000's.

However, in it's stock configuration, it can be a bit reserved and warm, but it delivers lots of current while doing so. Small wonder it pairs well with the K1000.

Well listen up kiddies - yank the cheap opamps it comes with for a pair of OPA637's and you'll think it's the second coming! You'll need to solder a non-polarized 68pf cap across pins 2 and 6 first. The stock opamps are already socketed, so it's otherwise a very easy swap.

In exchange, you'll get such a gain in dynamics and overall responsiveness you'll bust a gut laughing at what a difference a couple of cheap little chips can make. The entire frequency range takes on amazing clarity and the low end will smack you around with such authority you'll be looking to file a police report.

Kid you not - this configuration easily bests a Singlepower SDS I've listened to recently and any ss amp I've tried. Fwiw, the Singlepower had the best bass control of anything I'd heard up to then.

And if you like it loud - this mod gives the amp so much clarity and punch at high volumes you'll cry uncle long before it runs out of juice - and it will maintain the same incredible presentation the whole time. Simpy amazing.

Staging? Oh Yea. Before the mod, some pieces had a distant sound almost like you happen to be walking through the Quarter and have stopped briefly outside a bar with a live band playing inside. After the mod - the bouncer has grabbed you by the short and curlies and deposited you in the front row.

Softer passages have a wonderful delicacy to them, and changes in dynamics are rendered very well regardless of the overall program level. Truly a revelatory experience.

The mod gives so much clarity and detail to the upper end that certain percussive events like cymbals and brushes on a snare drum float up out of the layers exactly as they should.

And regarding the very low end - this thing clamps on the low end of a pair of cans like nobody's business. I'm currently using a pair of loaner 650's and while I think they are a very good can, they have a tendency to get flatulent down low with lesser amps. Not happening with this thing - nosireebob! Listening to anything from an upright bass to a synth dance groove, you will be blown away. Those of you who are bassheads - or even the ones who haven't come out of the closet about that yet, seriously need to give this setup a listen.

Absolutely recommended.

More later, got to get to bed!

Dan

P.S. Huge thanks to Sugano-san for the information on this mod and for sharing his impressions on having this swap done on his.

 

[purk]

glad to know that you are enjoying the amp man.

 

[Jazper]

considering the cap is a bandwidth restrictor.. I wonder if you were to use a smaller cap (33pf?) whether you'd get even more improvement, you could wire 2x 68pf in series to get about 34ish pf...

And yeah the 637 sounds awesome, pity it's so expensive. For those trying this mod who don't have a whole heap of money, try the ths4631d instead (same arrangement as the 637) It's about half the price and sounds nearly as good, or better with sennheiser headphones.

 

[agile_one]

Dan - You are the man! I love it when someone comes up with a tweak that changes everything, and this simple mod to the venerabvle RKV seems just that.

Thanks for sharing, and for the enthusiastic reporting style. If I had an RKV, I'd be breaking out the soldering iron as I type ...

 

[Zanth]

That is incredible! One of the last amps I really want to try... EMP anniversary, RKV MK II, SP SDS or Supra and a Cary 300SEI (in my system, have heard it before) and I think I'm just about done. Of course, getting my hands on a Holmes-Powell DCT-2 would not be so bad either

congrats on your discovery, and your elation. Glad you are enjoying it!

 

[Jon L]

637 is indeed the opamp to use if you are trying to extend both extremes and add extra speed and transparency. What opamp was in RKV before?

And what does the cap between pin 2 and 6 do? I thought you use a cap between 4 and 6 (or 6 and 7) to bias it more into class A..

 

[00940]

Quote:

Originally Posted by Jazper considering the cap is a bandwidth restrictor.. I wonder if you were to use a smaller cap (33pf?) whether you'd get even more improvement, you could wire 2x 68pf in series to get about 34ish pf... And yeah the 637 sounds awesome, pity it's so expensive. For those trying this mod who don't have a whole heap of money, try the ths4631d instead (same arrangement as the 637) It's about half the price and sounds nearly as good, or better with sennheiser headphones.

Be careful while playing with the bandwith. The RKV is a good but unstable design. The tubes it uses are easy to turn into oscillators. That's why there is an opamp to keep the tube stage inside a strong overall feedback loop which stabilizes it.

The value of 68pf was fixed by Helmut Becker, the designer of the RKV, to be safe.

Btw, the standard RKV uses by default the LF351.

I'm glad to see this mod finally gets some attention since the first explanations by Sugano-San didn't get the attention they deserved. I'm using simply 2X OP134 in my RKV and it already made a huge difference.

 

[Blorton]

Thanks everyone for the feedback.

As double-naught stated, the 68pf value was arrived at by the amp's designer. No way will I try anything different with the 637.

(And I'm not likely to try any other chips anytime soon.)

Cost: eBay is your friend. I got a pair of new 637's for $9 and also went for a pair of 627's from the same seller for $8, just in case. Total with shipping was $21USD. The caps were around a buck for the pair at a local store(Fry's).

I should've mentioned in the first post that the noise floor was also dropped dramatically. It really does feel like a completely different amp.

One other thing - the other components in this amp are also basic grade parts. There are further improvements to be made by upgrading the caps. However, I'm so happy with how it sounds now, I'm not sure when I'll ever do that.

Someday I'll get another RKV like this one, mod it the same way, and run balanced mode.

Oh yea - when you crank it way up - the tubes get *real bright*.

Lots and lots of heat coming out of this thing. I'm thinking of setting up a gentle filtered forced air ductwork for it.

Cheers!
Dan

 

[Sugano-san]

Thanks a lot for starting this thread, Dan. And welcome to the ever-growing owners' club of maxxxed-out RKVs.

I am happy to hear that your experience is consistent with mine, and that your modding efforts have paid off so abundantly.

Yes, it is true, last year I came up with the idea of rolling the RKV's opamps (and some other mods). I discussed my ideas in depth with Helmut Becker of audiovalve who then agreed to perform the mods if I provided the required parts.

The result is the "Silver Signature" RKV MKII, as I like to call it, which arrived sometime in January, and which I have been thoroughly enjoying since then.

The technical background, in a nutshell, is as follows: The RKV is not a "pure" tube amp design. If anything, it's a hybrid amp, because it uses an opamp (the stock model is an LF351, as pointed out by Ben) to control "any and all DC and AC aspects of the RKV" (as Helmut told me), which includes power supply and tube bias, but most importantly the amplification characteristics of the tubes themselves.

The tubes used in the RKV are not audio tubes at all; they were designed for old style b&w TVs to control vertical synchronization, or the like. Their frequency response is not normally suited for audio applications, but their ability to deliver lots of juice and their wide availability (at the time) and low price made them interesting for Helmut, and so he came up with the idea to use an opamp in the signal chain and also in a feedback loop to tweak the tubes' frequency response and other characteristics.

Some of you may have read on head-fi about tuberolling experiments with the RKV. The general consensus appears to be that the RKV does not respond to tuberolling at all. This finding has been confirmed by Helmut. It is a direct result of the design principles applied by Helmut. This means:

THE RKV’S SOUND IS DETERMINED ENTIRELY BY THE OPAMPS AND NOT AT ALL BY THE TUBES.

More specifically, the tubes I am using in my "Silver Signature" RKV MkII at this very moment and that I have been using continuously since January are not NIB or NOS or anything special. These tubes were not even new when they were installed. They have seen years of use and were pulled out of old TV sets; I got them for free. But they sound just like the NIB Siemens ones for which I paid so much money last year. And the Siemens ones do not sound any different from the Valvos or the Telefunkens that I have. That's how effective this circuit works. More detailed information may be found on audiovalve's website.

The tubes deliver the juice, i.e., the voltage and current required for the 3 wpc output of the RKV, but they are not supposed to add any character to the sound. Rather, it is the opamp, and only the opamp, which determines the RKV’s sonic characteristics. This has been confirmed by Helmut as well.

So I thought, why not play around a little with different opamps, and I installed some spare 627s and 637s I had at home at the time. It was a disaster! My RKV suddenly sounded like a broken amp, producing lots of pumping and clicking noises, but through that noise I heard music, and it was much nicer and clearer than with the original opamps.

I called Helmut and urged him to study the data sheets for the 627 and 637 opamps. He told me two interesting things: (1) The 627 or 637 may be used in the RKV, but it needs some sort of compensation cap. The optimal value of that compensation cap as personally determined by Helmut is 68 pF, and that is what he installed. (2) For purposes of the RKV it is irrelevant if you use the AP or the BP variant of any of the above opamps.

Helmut performed some other mods as well, which I will only mention briefly in this context: 4 new capacitors (higher capacitance, higher voltage & much much lower internal resistance) in lieu of the blue stock caps, ladder-type stepped attenuator (Goldpoint), new transformer (shielded with mu-metal to minimize background hum), and some more things that have no direct influence on the sound.

As to sonics, I can only agree with the descriptions provided by Dan (who uses 637s) and Ben (who uses 134s), to which I have nothing much to add. Those who are interested in my earlier posts on this subject (as mentioned by Ben), may click here (this is the thread that started it all), here and here.

I used to be interested in Singlepower amps, but that is no longer the case. Dan's statement that "this configuration easily bests a Singlepower SDS [...] and any ss amp I've tried" is in line with my expectations, and I am happy to read that Dan confirmed this from personal experience.

In fact, the "Silver Signature" RKV MkII fed into a Senn HD-650 through a Zu Mobius cable sounds so utterly seductive and appealing that I have a hard time deciding between this setup and my Stax Omega 2 with 007t tube energizer. Yes, it is that good. The two setups do not sound identical, but the degree of enjoyment and satisfaction delivered is easily comparable. And the modded RKV "goes to eleven", as Dan has pointed out correctly, which is something you cannot get from a Stax.

In closing, I would like to mention this: There are lots of used RKV's on the market, and I believe that they are very robust designs. Get one (or get a new one--they're still available from audiovalve), have it modded (or DIY), and be happy.

So much for now. Au revoir.

 

[Blorton]

Quote:

Originally Posted by Sugano-san In fact, the combination of RKV MkII S.E. into a Senn HD-650 through a Zu Mobius cable sounds so utterly sedcutive and appealing that I have a hard time deciding between this configuration and my Stax Omega 2 with 007t tube energizer. Yes, it is that good.

Thanks for the wonderful post, Sugano. I just had to respond on this issue - the Zu Mobius cable is exactly what I'm using!

I really wish I still had the RS-2 that was here last week. That can played wonderfully with the stock RKV. I shudder to think how it would've done now.

Oh, and please don't buy up all the used RKV's - I'll need another one at some point!

Cheers!
Dan

 

[Sugano-san]

Quote:

Originally Posted by Blorton Thanks for the wonderful post, Sugano. I just had to respond on this issue - the Zu Mobius cable is exactly what I'm using! I really wish I still had the RS-2 that was here last week. That can played wonderfully with the stock RKV. I shudder to think how it would've done now.

You're most welcome, Dan. I am happy that this mod is finally getting some attention on head-fi.

I may have some time over the weekend to try my RKV with a pair of Alessandro MS-2i (Fatboys), and I will post my impressions forthwith.

 

[Blorton]

Quote:

Originally Posted by Sugano-san I may have some time over the weekend to try my RKV with a pair of Alessandro MS-2i (Fatboys), and I will post my impressions forthwith.

That would be great if you could do that. The RS-2's convinced me I need to get a high end Grado at some point. The PS-1's will take some mustering to move to.

Of course, I've got SA5k's due to arrive any day and I've heard reports from others that they've forgotten all about buying any other cans after getting those.

Cheers!
Dan

 

[Iron_Dreamer]

Quote:

Originally Posted by Blorton Of course, I've got SA5k's due to arrive any day and I've heard reports from others that they've forgotten all about buying any other cans after getting those.

Indeed, buddy!

It'll be fun to see hwo they play with your new uber-amp

 

[dmb367]

Quote:

Originally Posted by Sugano-san In closing, I would like to mention this: There are lots of used RKV's on the market, and I believe that they are very robust designs. Get one (or get a new one--they're still available from audiovalve), have it modded (or DIY), and be happy. So much for now. Au revoir.

Is there a better place to look for a used one then here?

After all the nonsense, I'm dying to get one... thanks alot Dan

 

[Jon L]

So who's tried the modded RVW with K1000? K1000 (stock) has, of course, been maligned and we are told it 'needs' stock RKV's tubiness..

I personally find that my (modded) K1000 sounds better, more 'live', every time I move to an even more resolving/accurate amp.