Superphon Revelation MK II PreAmp

Recently the Superphon Revelation MK II preamp has come my way. Was designed and built by Stan Warren, at one time the other half of PS Audio. Cant' find out much about it though. Performance wise a very good preamp, phono section exceptional.

However the Volume and Balance pots are quite pedestrian and should be upgraded. I have in the parts bin a TKD 25Kohm volume pot. Anybody know if this would be of correct value? Any other help greatly appreciated.
 

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Memory Serves

I had a Rev 2G back in the late 80's. It was a very nice preamp, but mine suffered from a low level buzz. Suspect grounding, but muting the unit would make it dead silent.
I think mine had 50k pots. It was dual mono, no balance. I would rewire the unit and go dual mono controls.
The preamp was very nice, low level hash was not that bad. I think today I could sort it out. Sent it back and they said it was silent, so it was most likely one of my sources caused it.
To check yours, only measure the volume control. The three pins are in-out-ground. Measure the two outside pins to find impedance of pot. I would ditch the balance.
If it is a 50K control, get two 100k stereo log taper controls. Parallel the two sections. This should track a little better and with two wipers should be a little quieter.
The value of the pot is not critical. Just get it in the same ballpark as the original.


George

George
 
Revelation II

I have already sent Ye Olde Audio Fan a note, but for the sake of clarity to other viewers of this subject I will attempt to explain a few things. The controls in the Rev II were custom made by Alps. We auditioned several of their pots and found the 16MM metal enclosed units to have about the same grain as the larger and more expensive square bodied types, with a more open and spacious sound stage. The problem was tracking. The best case channel to channel match was about 2 dB and some were worse. We paid a small surcharge and bought a thousand units with less than one dB error. At least, the ones we checked met that spec! Of course, Alps wanted around 24-26 weeks to build them and that was kind of a pain. Still, what are you gonna do? We got a better sounding control with good tracking and it cost about half of the bigger and darker sounding ones from the same company. The balance control is more or less a standard unit from Alps with resistance elements that are metal from one side to the twelve o'clock position. Thus, if you have the control at top center there is no resistance in series with the signal and if you turn the balance to one side, only the attenuated side gets a little resistance inserted. We did do an A-B test and no one in the shop at the time could tell which units had dual mono volume and which were volume/balance. Not much has changed since then. Beware of expensive controls with great specs, most did poorly in a blind test. These last few years, I have been very impressed by the Mouser/Alpha 24 MM units sold by Mouser Electronics. They are not great trackers either, but at less than three dollars for a stereo unit you can buy a few of them and use the best one. Of course, YMMV;)
 
Try making a stereo a mono

Those cabon Alpha pots ae prime candidates for converting a stereo to mono. Even if you cannot tell a stereo and balance control from dual mono by listening, the dual mono takes tracking out of the equation.
The Superphon gear was noted for having wimpy power supplies, cheap RCA jacks, and now controls. But they did sound and operate very well for quite reasonable money.


George
 
Replacement relay for Superphone Revelation II

I have a Superphon Revelation II preamp (s/n 2520F) that's served me quite well since 1984ish. But now the Play/Mute relay has gone bad, contacts have degraded and adds a lot of noise to the signal, particularly in one channel. I have confirmed that the relay itself is the problem.

However, I can't find a source for the relay. Its blue plastic housing is marked with p/n "OUB-SS-212D" and "ES Taiwan". Anyone have ideas on how to locate, or substitute equivalent, part?
 
Au contraire. It would have been great if Superphone had used a "conventional" relay. This is an unusual relay. Altho its pin spacing is standard, and uses a common PCB-mount configuration of 8 pins of standard 16DIP dimension, the pin-out is non-standard. The 12VDC coil is on pins 6 & 12, instead of 1 & 16, and the commons of the relay switches are on 1 & 16.

Certainly, there are several choices of low-signal PCB relays available with similar coil current and sensitivity, but they will require re-drilling the PCB board in an open space, gluing it on, and then hand-wiring it via soldering directly to the pins and the original PCB pads in the correct locations...something I'd like to avoid if there's a drop-in replacement. Instead of grass, this one looks to be a rare orchid.
 
Didn't hear anything further from Beeger, and more searching yielded no joy, so I took the plunge and installed an alternative relay. Fortunately, there's space on the PCB next to the old relay so I drilled the necessary holes for the new relay pins, and then glued the new relay onto the board. After removing the old relay, I soldered/connected the new relay pins to the old relay's PCB pads/holes, using 30ga wire-wrap wire. The new relay is an Omron G5V-2-H1-DC12, which is a low-signal high sensitivity DPDT relay (12VDC/400ohm/30ma coil), gold plated contacts, 120.5mmL x 10.1mmW x 11.5mmH, Mouser.com p/n 653-G5V-2-H1-DC12 & price $2.01. I also bought an Axiom V23105A5403A201 relay (Mouser.com p/n 655-23105A5403A201 & price $1.83) to compare; it should also work well & is similar size & construction; I chose to use the Omron based on past experience with their high-quality products.

While I was in there, I replaced the Volume & Balance pots with the larger Alpha carbon-composition dual ganged audio (log) taper 50K assemblies, Mouser.com p/n 313-2420F-50K, $2.73 ea. I had to drill a 1/8" key hole in the proper locations in the front panel for these new pots, but otherwise they drop in nicely. I also replaced all of the electrolytic capacitors with higher quality Nichicon low impedance hi-reliability, higher temp 105degC. I also added 0.1uf polypropelene film bypass capacitors onto each of the electrolytics (Vishay/Roederstein, Mouser.com p/n 75-MKT1817410064 price $0.26 ea). This is my standard practice capacitor replacements when "souping up" audio gear that doesn't have the best components already. All of the other capacitors (and resistors) in the Superphon actually look pretty good, so left them alone. I'm impressed by the circuit design of this preamp.

Upon doing some testing & measurements, I was reminded about a problem I had with this preamp since it was new: the balance is off. I have to leave the balance control at almost 1:30 to correct it (even with the new pot). There are some trimmer pots on the PCB, 2 per channel. Perhaps someone out there knows best practices on adjustment of these???

Also, anyone had any success mod'ing the phono preamp to reduce the noise floor?
 
One of the problems with the Revalation II (NON-MK) was the Power-up delay circuit (based on a 555 timer) which was designed on the edge. As the components age (i.e. electrolytic capacitor off of bridge rectifier), problems crop up. I capture the waveform of the trigger to the 555 and the ripple seems excessive. If a negative pulse drops below 4V (1/3 of VCC) the RELAY will turn off muting the outputs.

I replaced the 12.1K resistor (in series with a 100K Resistor) with a 15K to raise the resting voltage from 4.84 to an average of 5.8V. The preamp now turns energizes the relay, but still subject to AC line glitches. There is a 220uF 50V cap filtering the Bridge Raw DC and I suspect that this cap is leaking, exacerbating the ripple thus causing false brown outs to the timer.

The circuit has gotten progressively worse over the last 2 years, and I decided to finally address it since I cannot do without listening to music.

I attached a waveform which reveals the following:

Channel 1 - 555 (Pins 6 & 7) Threshold/Discharge Pins, CH2 Relay Output (from Front Panel Switch and 555 Pin-3), CH3 – Trigger (falling below 4 V after initial timeout causing a re-trigger).

I will need to purchase a new Electrolytic CAP and install to see if this cures the problem. By the way, I changed the 555 first just in case the trigger drifted on the original chip.

I had modified this unit when I first purchased it back in 1986, because the series dropping resistor to the +12V regulator was too low of a value cause the regulator to "OVER-VOLTAGE". I Spoke with Superphon's Technical Engineer (Name was Mark) and told them what I had to do, and they had found the same problem in production (I purchased one of their first units). I change the 5W 560 ohms to a 600 ohm resistor which resolved that issue way back when.
 

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Hello eeehaw; Sorry, didn't hear from you and failed to check back here for a reply. Still, you have apparently already fixed your relay problem so all is well.

To Sierra tech; The relay circuit is supposed to drop out quickly in the event of a power interruption. The idea is to trip the grounding relay before the supply rails can sag and put a big nasty voltage on your amp. It then resets the turn on timer and makes sure the caps are fully discharged before enabling play. The hair trigger is deliberate. The only time unwanted dropouts usually occur is when the line voltage is lower than normal. Probably all you really needed to do was replace the 220 mFd cap. It has been seventeen (?) years and the caps do age. I would use a newer, low impedance cap for this application. It will heat less with the high ripple and last longer than the original. Without seeing the unit, I don't know why the gain would be different on your two channels. The transistors in the phono are low noise, and the FET differential input pair is fairly low as well. If you know of a lower noise FET pair, you could try a substitution. You might want to try to make sure the transconductance of the replacements is at least as high as the originals, or the equalization will be affected. (Not in a good way) Also, extensive comparisons revealed that the metal can devices sounded better than the plastic versions. Keep that in mind when shopping for other types. You might make the noise lower and hurt the sound. You know, the operation was a success but the patient died.:xeye:
 
beeger:
Thanks for the reply. I'll look around for better FETs. The noise floor is actually pretty low, just doesn't come near my primary preamp (Musical Fidelity A306cr) & always interested in souping up; otherwise, the sound is excellent.

Where can I find a procedure for adjusting those trimmer pots?
 
eeeehaw,
I wish I knew for sure which variant you had in front of you. Does your model have an IRFF110 near each line stage output? It will be in a TO-39 metal can. If you have said mosfets, then the lowest distortion is usually achieved at 29 Volts on the output resistor. You should find a resistor in the line stage with one lead bent into an upside down U shape. That is for a voltmeter probe or clip. The other voltmeter probe goes to ground. Slowly turn the trimpot until you see about 29 Volts and you are done. Detuning the trimpot to lower voltages will increase the second harmonic distortion. I wouldn't go lower than 24 Volts as you risk premature clipping. Some folks actually liked the sound with a little second harmonic and it is easily reversed if you don't. The phono stage voltage should be 19-19.5 Volts. Don't mess with that one, it is a different type of circuit and needs to be biased at half supply. I hope this will help you get your preamp sounding like a new one.
 
beeger: I have UFNF110, and there are 2 such resistor test points for each channel (each with one bent lead). I am attaching a photo with one channel (only) labeled for xstr type & with resistor test points indicated. Which resistor should I be looking at the voltage? Does the UFNF suggest different voltage setting as compared to IRFF?
 

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eeehaw;
The 110 numbered parts you have are fine, just another brand than International Rectifier. The phono stages are nearest the back panel and the line stages are closer to the front panel. The phono stage voltages should be 19-19.5 Volts and the line stages should read 29 Volts, plus or minus .2 Volts. That is the lowest distortion setting.

Be careful working around the semiconductors. The metal cans are electrically 'hot' and shorts can be fatal to the circuit. The 2N6661s are running about $40 each these days when you can find them. You wouldn't want to kill one by accidental contact with a probe.
 
Thanks, again beeger. I finally located the test points for the Line stage (had to look closer & put on my glasses). Both channels were set at about 29.9V previously, so lowered them to 29.0V. The Phono stages were 19.4V and 18.4V; they're both now at 19.4. I still have a gain difference between the two channels, so looks like more looking around for that cause with o'scope etc. It would help to have a schematic.

For the benefit of others following this, I updated the photo to better show the test points and their respective trim pots; that's attached.
 

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For the benefit of others facing replacement of the Mute Relay, attached is a photo of my new relay, shown from the component side. The old relay was removed, and its terminal solder pads reused for the 30ga wire-wrap wires going to the new relay. Of course, you must observe the differences in pin-out functions with the new relay, so if you don't already have a diagram stamped on the old relay you should use your VOM to figure it out & sketch it before replacement. I carefully measured & marked the terminal pin locations for the new relay on the circuit board, then drilled them with a 1/16in bit. Then I simply glued the new relay onto the board, tinned the terminal posts and wired it up. Took about an hour, including removing top & bottom covers (removing the bottom cover requires removal of the 2 screws securing the PCB, and 1 screw securing the fuse holder).
 

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