JRDG (Jeff Rowland Design Group) model 735 monoblock amps

[QuadDiffuser]

The replacement / upgrade to the JRDG 725 monoblock amps were announced a few months ago:
https://www.jeffrowlandgroup.com/wp-content/uploads/2020/05/735-Announcement.pdf
https://www.hifi-advice.com/blog/jeff-rowland-power-amplifiers/jrdg-company-special-model-735/

I just received a pair for home audition, and will be comparing them with my JRDG 725S2 monoblocks.

 

[shakti]

looking forward to your findings!

 

[Sa683s]

Bravo!

 

[danielk141]

QuadDiffusor,

I'm looking forward to this, also. I just recently had my 625 updated to Series 2, after 8 years of ownership. I could not be happier with this amp.
The only step I might take next is getting a used pair of 725S2 monoblocks. These will appear on the market, as the hardcore Jeff Rowland fans move up to the 735 monoblocks.
Everybody wins!

 

[ricjor1]

danielk141,
I've owned the JR 625 S2, for several years. I'm trying to decide if upgrading my JR Corus preamp to something else or going from 625 S2 to 725 S2 will yield better results.

 

[danielk141]

ricjor1,
Have you tried the Rowland PSU power supply, for your Corus? I have a friend here in the Phoenix area, with both pieces and 725S2s, driving Wilson Benesch speakers. He says the PSU was a big move up...

 

[ricjor1]

I haven't tried the PSU, but one of my audio buddy owns one. He has the 625 S2, Corus, and Aeris. Based on my conversation with him I think changing my amplifier or preamp may yield better results. Of course I won't know for certain unless I hear the PSU in my system.

 

[QuadDiffuser]

The 735s have barely been broken in, with less than 100 hours so far; I suspect that the Lundahl input transformers take an especially long time (1,000+ hours) to break in.

Let me start with the good news:
1) The form factor and input/output connectors are identical to the 725S2s - making the transition seamless and simple
2) The finish on the non-anodized cover is beautiful; the mirror-like but satiny surface which diffracts light in a seductive way that is elegant is very First-Class
3) The small on/off button and correspondingly small white LED is both easier to actuate and cute/elegant. The soft turn on/off circuitry eliminates the loud raspy burst of random noise - but I'm not sure what audible penalty there is the insertion the muting circuit - is it 100% transparent, sonically?
4) The ultra-annoying (and inexcusable) clicking and clacking noise emanating from the power supply of the 725S2s located near the XLR inputs, audible from even three feet away, is GONE - FINALLY !!!!!

Some things that may be worth fixing:
1) The flat/thin "isolation" pads stuck at the bottom of the chassis is awful, in terms of user-friendliness; 1/4 inches is not enough clearance to accommodate one's fingers under the chassis, making it impossible to grab and lift the chassis intuitively and securely. Grabbing the heat sink at the top is an alternative, but that doesn't prevent the chassis from constantly bumping against the substrate shelf material every time the perimeter of the amp is touched (even gently).
2) The XLR signal inputs were wired out-of-phase (pin 2 and pin 3 were reversed); hello, QC !

Sound-wise...
1) Caveat - still in its early days. The 735's bass is relatively deeper and punchier, but this frequency tilt seems to mask out some of the upper-mid to high frequency details, rendering the soundstage flatter and smaller. Cymbals and metallic strings lack "bite".
2) The expansive and holographic imaging of the 725S2s - I can't hear it in the 735s (yet). The size-appropriate imaging and attack/decay of the fragile, scintillating ambiance attack/decay envelope in music is the product of correct amplitude and phase. I'm not sure whether the purported benefits of the new materials (and winding?) in the input transformer is an improvement over the old version.

Again, early days... but my expectations are (justifiably) high for these $40,000 pair of monoblock amps, as it's supposed to be JRDG's flagship Class AB amp, incorporating decades of evolution and refinement in components, design, and execution. Fingers crossed that over the next few weeks, the performance gap will not only close, but exceed my 725S2's.

 

[wespruett]

danielk141,
I've owned the JR 625 S2, for several years. I'm trying to decide if upgrading my JR Corus preamp to something else or going from 625 S2 to 725 S2 will yield better results.

I decided to biamp w/ 2 x 625s2. Great decision!

 

[Ric Schultz]

Yes, please not only leave them on all the time but also play music through them continuously.....even at low volumes.....say, at night. You probably have maybe thousands? of hours on the older amps. Guido said it took forever to burn in his big Rowland Integrated. We always hope the latest is the best yet.....but you won't know for quite a while.

 

[QuadDiffuser]

Well, I'm not sure I want to "burn" tens of thousands of watt-hours just to begin to have a glimpse of the amp's potential sonic performance. I'm wonder if JRDG can provide the service of connecting a bunch of input transformers to line-level pink/white noise sources to "cook" them BEFORE installing them in their amps?

Assuming that the Lundahl input transformers are presently the bottlenecks to hearing the purported performance gains of the "improved" circuitry and overall design of the new 735s, what's the best way to break them in more painlessly? My rudimentary knowledge of transformers suggests that even playing line-level (1-2V) signals into the input side will result in a near 1:1 output on the other side, assuming that there is a circuit (with conductivity) downstream. In the old days, this can be done with an FM tuner playing "static". The problem is that I don't have an unbalanced RCA to balanced XLR adapter.

It would be useful for those who are intimately familiar with JRDG's Lundahl input transformers to comment on just what one should expect to hear during the first couple-of-hundred hours of break-in. The LL 7902 transformer (designed for microphone tube amplifiers) used in the 725 amp cost about US$230 each.

Lundahl LL7902 Line Input / General Purpose Transformer, 163,09 ̈́

Turns ratio: 1 + 1 + 1 + 1 : 1 + 1 + 1 + 1

www.don-audio.com

Here's the lineup of audiophile transformers from Lundahl's website:

Line input and line output audio transformers • Lundahl Transformers

Lundahl Transformers line input and line output audio transformers for galvanic isolation, balanced-to-unbalanced conversion and for impedance conversion.

www.lundahltransformers.com

The price of Lundahl's LL 1931 amorphous-core MC cartridge transformer (with Cardas wire), which I'm assuming is used in the JRDG phono preamplifier, is about US$375 each:

Lundahl 1931 Amorphous Core Moving Coil Input Transformer, 270,55

LL1931 is a high performance moving coil step-up transformer

www.don-audio.com

 

[danielk141]

QuadDiffusor,

I received my 625 with the S2 upgrade back from Jeff Rowland on June 22, 2020. The upgrade included new input transformers. I sent a thank you email to Jeff Rowland, he sent me a reply stating it would take at least 400 hours of playing time to break in my upgraded amp.
I have a Pass XP-30 preamp. I use a Yamaha tuner to break in anything I add to my system. I left my system on 24/7 since I got the amp back. At night I play various FM music channels at low level.
This amp took longer to break in than any other component I've ever owned. The sound opened up over the next couple of months. When the amp sounded completely broken in to me, I found that putting on a 4K or Blue Ray video with a hard transient scene (like the strafing attack in the movie Valkyrie) would result in a bright/forward sound. To me, this demonstrated something new in the amp was still breaking in.
Sure enough, after a couple more months of breaking in the amp on my favorite movies, the amp sounds better than anything I've ever owned, including my Pass XA-60.8 monoblocks. That's saying a lot!

 

[QuadDiffuser]

QuadDiffusor,

I received my 625 with the S2 upgrade back from Jeff Rowland on June 22, 2020. The upgrade included new input transformers. I sent a thank you email to Jeff Rowland, he sent me a reply stating it would take at least 400 hours of playing time to break in my upgraded amp.
I have a Pass XP-30 preamp. I use a Yamaha tuner to break in anything I add to my system. I left my system on 24/7 since I got the amp back. At night I play various FM music channels at low level.
This amp took longer to break in than any other component I've ever owned. The sound opened up over the next couple of months. When the amp sounded completely broken in to me, I found that putting on a 4K or Blue Ray video with a hard transient scene (like the strafing attack in the movie Valkyrie) would result in a bright/forward sound. To me, this demonstrated something new in the amp was still breaking in.
Sure enough, after a couple more months of breaking in the amp on my favorite movies, the amp sounds better than anything I've ever owned, including my Pass XA-60.8 monoblocks. That's saying a lot

Daniel,
Thanks for posting your experience/observations! The 735 I’m auditioning sounds constricted, lacking high frequency extension and soundstaging depth/height/width. In your case, you describe a hardness, an acoustic characteristic I’m not hearing.

Would you by chance know which Lundahl input transformers were used in the 625 and 625S2?

My 725 amps were upgraded to the 725S2 about three years ago - I don’t recall whether the Lundahl input transformers are identical before/after, or were changed (through the S2 upgrade).

 

[KeithR]

My 625s2 took less than 200 hrs to break in. Ymmv.

 

[QuadDiffuser]

Hi Folks,
Trust you've all had a great weekend. Many of you have sent me PMs, asking about my final verdict on the JRDG 735s; suffice it to say that my observations (described previously) did not change at all during my audition, and two highly respected audiophile friends also heard the same characteristics. The 735s are being returned tomorrow, and I'm totally delighted to be keeping my amazingly musical and transparent 725S2s.

 

[rau]

I’m looking at purchasing a pair of 725’s also , from what I was told they need 900-1000 hrs of break in . My current
amplifier is constellation centaur II 500 stereo but would like to change to monoblocks for placement issues , but don’t want to sacrifice sound quality