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Superphon Revelation MK II PreAmp
Superphon Revelation MK II PreAmp
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Old 26th October 2007, 04:05 PM   #11
eeeehaw is offline eeeehaw  United States
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Thanks, Beeger. Just tried to email you via diyAudio, but couldn't do it as my account "is still under moderation" (!?!). Drop me an email to stephen@kangas.com when you have a spare moment.
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Old 31st October 2007, 02:20 PM   #12
eeeehaw is offline eeeehaw  United States
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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?
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Old 2nd December 2007, 01:49 AM   #13
SierraTech is offline SierraTech  United States
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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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File Type: jpg revalation-ii_555.jpg (83.2 KB, 1402 views)
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Old 2nd December 2007, 11:44 AM   #14
beeger is offline beeger  United States
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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.
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Old 2nd December 2007, 06:08 PM   #15
eeeehaw is offline eeeehaw  United States
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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?
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Old 3rd December 2007, 12:18 AM   #16
beeger is offline beeger  United States
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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.
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Old 3rd December 2007, 07:31 AM   #17
eeeehaw is offline eeeehaw  United States
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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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File Type: jpg superphon transistors.jpg (96.8 KB, 1559 views)
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Old 3rd December 2007, 08:15 AM   #18
beeger is offline beeger  United States
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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.
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Old 4th December 2007, 07:36 AM   #19
eeeehaw is offline eeeehaw  United States
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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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File Type: jpg superphon bias adjust 1.jpg (98.2 KB, 1403 views)
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Old 4th December 2007, 08:01 AM   #20
eeeehaw is offline eeeehaw  United States
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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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