SonoruS modified reVox PR99 /B77

[Hibernian]

As some of you are aware, On A Higher Note imports Luxman electronics & Vivivd Audio loudspeakers, so we are continually travelling around the country playing music & demonstrating the gear we represent. My own personal passion is music & I am continually on the hunt for interesting new music.  Since I arrived in the USA in 1991, I have bought on average 500 pieces of music every year.

I met Paul Stubbelbine a few years ago at CES & we hit it off; Paul is a wonderful person. He mentioned his Tape Project & I immediately offered to help in any way that I could. The concept of having access to master tapes in my own home was v. appealing. It would also allow me to use the tapes as a goal post to determine how turntables & digital players should sound.

I started off by buying a Technics RS1520 from J-Corder and shortly afterwards shipped it to the UK to Tim de Pavarachini for his full modifications. While I was waiting & waiting for this machine to return I came across Arian Jansen who mentioned that he had modified a reVox PR99 and would I like to have a listen. The reVox is much nicer functionally than the Technics, the PR99 mk II and III both have a real time digital display that shows hours, minutes, seconds that can be read even across the room. The Z function allows the machine to be set in repeat mode for a portion of music, a whole song or a whole tape. For demonstration purposes its v handy to be able to locate tacks, just by rewinding to a predetermined point.

As for sonics, once Arian demonstrated the diferences between his modified Studer A810 and his modifed PR99 mk I; we pressed ahead and bought a MK II and a mk III and had SonoruS modify the mk II for Playback & Record and the mk III for Playback only. Our experience of the SonoruS mods is that the modified player has better defined & more atriculate bass, it is also more extended. The midrange & high frequencies benefit from much greater transparency.




American Revox  B77 /PR 99 are not ready for The Tape Project Tapes; as a normal American market machine's EQ is set up for NAB. The Tape Project are using the European IEC or CCIR EQ.
 
Here is a brief over-view of what was done to my reVox PR99 mk II and mk III by SonoruS:
 
1.    Service the basic machine; lubricate the pots, replace light bulbs, switches - bring back to original spec.
2.    Modify the playback board; change NAB to IEC EQ, the audio / record(optional) PCBs are completely revised - 150+ new parts are added.
3.    Modify the Power Supply so that the Audio electronics are buffered from any noise on the AC.
4.    Remove the muting circuit, while the tape is fast forwarding / rewinding. Arian feels that the muting circuit veils playback somewhat so he has removed it entirely.
5.    Rewire the monitor circuit so that the playback is now direct from the heads, leave all the existing inputs /outputs alone, add a new pair of RCA outputs. OFC Copper in Teflon dielectric cabling used.
 
The major change between the MK I and the later MK II and III is that the earlier machine had a much simpler circuit for the playback PCB, so the sound travels through less components. So in the case of the Sonorus PR99 mkII or III, it uses a heavily modified mk I Playback PCB.
 
The power supply is buffered so that the ground signal no longer passes through the audio circuit. On a stock machine, noise in the AC line could corrupt the audio signal.
 
A modified Sonorus PR99 is more articulate sounding than a stock Studer A810. However, as the latter is  a better machine, later on I intend to ask Sonorus to modify a Studer A810 for me. Arian at SonoruS can best be reached at [email protected]

Scott Johnson supplied the RC receiver, its mounted on top, its the black box between the reels, with the red LED. Scott supplies a basic Sony RC that controls, STOP, PLAY, FF & REVERSE. Its enough - a pity that it does not control the Z functions. Scott can be reached at
[email protected]  However Arian needed to modify power supply line on the RC connector to get the Receiver to work properly.
 
Hope this is of help.

Philip O'Hanlon
http://www.onahighernote.com/music/

[ironbut]

Man, that's a beauty! Can't wait to hear it.
Thanks for the post Phillip.

[microstrip]

Beautiful job with the Revox PR99. I am located in Europe, and considering the scarce availability of spares for the Otari here, bought a PR99 mk3 to play the Tape Project tapes. Have you considered upgrading the tape path, e.g. putting tension arm roller bearings or an improved fixed tape guide?

[astrotoy]

I saw Philip's PR99 at Music Lovers in Berkeley about a year ago. It is a beautiful machine and I thought the soiund was great. We heard the Bruch TP tape as well as some others. As I remember, Arian's mod changed the EQ to IEC only, so it is great for TP tapes, but not for NAB 2 track tapes. I don't know which PR99 I heard - but it was about a week after the Bruch/Hindemith was sent out. Philip hadn't received his yet, so I brought my copy to play.  Larry

[Tubes n tapes]

With the Sonorus modification the PR99 or B77 has different EQ's for the two speeds. At 15ips, the machine records and plays back with IEC EQ. At 7.5ips, the machine records and plays back with NAB EQ.

On top of that there is the option to have the machine play back 15 ips tapes with NAB EQ, by plugging a dongle into a connector on the back. There is no 15ips NAB recording option.

Concerning mechanical upgrades. I haven't done much in that area. The only very minor mechanical modification in the Sonorus PR99 or B77 is the bearing in the left rolling tape guide. The original SKF bearing is replaced by a heavier and smoother Minebea bearing. 

[Cyrano]

Hi,

Could you share more details regarding the power supply being buffered so that the ground signal no longer passes through the audio circuit (all PCBs have their own return loops in star rather than the common bus?).  I am trying to understand the concept, any links to applied technique to read about will be appreciated!

Best
ANdrew

[Tubes n tapes]

Andrew,

If you'd analyze most amplifier circuits in general you'll see that the power supply is partly or often even completely in the audio path.  On a few of the cards in the PR99/B77 the +21V supply voltage is hardly buffered which means that the audio signal goes all the way through the power supply card before it returns to ground. To avoid that, I'm putting large power supply decoupling capacitors on the individual boards and add small resistors in series to avoid the power supply board being in the audio path.

 

[Cyrano]

Arian,

I was surprised that they did so many variants of local decoupling.  I posted here the comment that when moding PS this is a hard part to deal with as any change affects different boards in a different way.  I must say this is very crucial redesign work you undertook. 

I cant calculate snubers like that but I did try different values of the local decoupling caps with very poor results, when just moving from 100u to 220u capacitance change, the sound was getting local cap sound signature overpowering the mains, and the main PS is the place where I could put some really better and therefore larger in size caps.  So I decided to go for a merge of certain caps sound qualities i just could not distribute locally with maxed out PS configuration.  I put during the development so many variants of caps in PS with a goal to standardize on only one type of cap on PCB local decoupling to get it right and it worked. 

I am left with Record amp being totally dependent on the mains.  Can you drop me a msg with options to obtain RC network configuration and values from you.  I drifted from Revox original parts too far so types of parts I would have to select to fit existing sound frame.

Regards
ANdrew