Mr.Sneis
Headphoneus Supremus
- Joined
- Feb 5, 2004
- Posts
- 3,355
- Likes
- 380
***I am re-posting this information at the behest of a friend, hence the separate parts and timeline, there are also some minor transposition edits from there to here. Keep an eye out for clickable links to old threads for bonus info. I hope you enjoy the read and please post if you may have feedback or suggestions!***
I am still learning the ropes for the DIY audio realm but this has been a project that I’ve wanted to tackle for several years now and only recently have decided I would actually take the plunge and just go for it. My apologies that I am not committing to provide detailed sound impressions for the amp as this project for me is more for nostalgia than anything else. I am positive that most amps you can buy today are better built and probably better sounding. My purpose here really is to document what I am doing and hopefully to learn in the process so your input is greatly appreciated!
Melos SHA History, a lot of this is skimmed from web searching, since I don’t see it mentioned much here’s my notes on the matter which may or may not even be factual. (Super cool reads http://www.head-fi.org/t/127867/an-interview-with-john-grado and http://www.head-fi.org/t/36656/melos-sha-x-modified-by-carlo-comments)
For those not familiar with the Melos SHA amps, the abridged lore goes something like this… back in the early 90’s before the headphone amp market was even a thing, Joe Grado (RIP) wanted a dedicated amplifier to power his HP-1000’s. Through some arrangement he contracted the then reputable Melos Audio to design said amp. Though I am not clear on the details, apparently he was not happy with the two prototypes they came up with (eventually dubbed the SHA-X) and eventually Joe created and sold his own Grado HPA-1 amp instead which is also quite a rare piece of history. The irony is that, based on what I’ve read, Joe actually kept an all Melos based gear chain for his own personal use.
I have no facts to back this up but Melos had a rocky history as an audio company, they sold many different iterations of the SHA amp (they also appear to have sold monoblocks, phono stage, R2R DAC, CDP) and like some amp builders something changed for the worse in the production quality standard along the way. The design of the SHA is pretty bad as far as I know, the PCB is a rats nest of traces, the higher end models add another rats nest of wires and jumped traces in addition, the parts selection was never top tier or even consistent. The Sha Gold added a Pho-Tentiometer which is a light dependent volume attenuator, on paper it sounds really cool but in reality it’s kind of a piece of junk. All of this resulted in poor reliability of the amp and probably by today standards mediocre sound. I can venture a guess that the reliability problems and skimping on parts were the beginning of the end for Melos as they closed their doors probably sometime around the early 2000's, there were even some failed promises to come back bigger and better but nothing ever panned out.
At Head-Fi there were some individuals who modded the hell out of these amps, carlo, nikongod,FallenAngel, nattonrice, these guys certainly had better chops than I do. Even ECP Doug was in on the action! Sadly half of these guys are in retirement from the HF scene near as I can tell.
Melos SHA Gold
(Schematic here: http://www.eserviceinfo.com/downloadsm/21578/Melos_Melos Sha Gold.html)
(Questionable info for tube plate voltage adjustment here: http://www.head-fi.org/t/63652/compiled-melos-tube-impressions)
Before diving in I already had a rough idea of what I wanted to do with the amp, namely to see what exactly the hoopla back in the early HF days was all about with regards to the modding potential and its synergy with the HP-2. This particular unit was snagged on A’gon in stock form; many of these were upgraded (read: repaired and shoddily so) by Melos over the years to “reference” level. I actually had an after the fact non-officially modded reference in my heyday in superb condition which I remember fondly. I am not even clear on what the reference upgrade meant in terms of parts or modifications but it is very clear there are a slew of changes evident in the pics you can scour from the web. The faceplate is in decent condition and just slightly bronzing, kind of like the picture shown in the old fluffed Stereophile review. Sadly a lot of these suffered pretty gross anodizing failure. This one must have not seen a ton of use as the amp itself came to me fully functional and complete in the old beat up box with an owner’s manual and infrared remote.
The sound in stock form is not bad even when running some super common JJ 6922 gold pin tubes. The Melos has a reputation for eating tubes, I read up on adjusting the plate voltages but I don’t even know if adjustment will make a major difference. Maybe I got lucky with this one but hum or hiss is not a major issue (yet?) though it is certainly not the quietest or blackest background amp I have heard. The sound I would characterize as relaxed and warm and only decent in the details department. Bass is not well defined but bumped up just slightly so which works out great for HP-2’s and HD800. Soundstaging and placement is better than average which also makes for some engaging listening sessions.
Getting to Work Pt 1
The amp didn’t last long in stock form and I spent way too much time researching old threads, poring over the inadequate schematics, and ordering parts from pcx, sonic craft, and mouser. Worth noting is that this amp has soldered wires holding everything together; just to free the amp PCB requires almost 20 different wires to be de-soldered. Working on it really sucks but at least I am pretty handy with a de-soldering pump now!
I focused on the amp board first, simple drop-in replacements that seem to add up even when on a budget. Electrolytics are replaced with modern fancy Nichicon’s as the Black Gates that were once the standard are NLA and very expensive. I purchased a bunch of Sonicap Gen 1’s for bypasses and input. Not so much of a drop in replacement, the High Voltage and Low Voltage rectifier bridges in stock form are maybe enough to get the job done but probably run at their limits. With the guidance of some super awesome friends, I replaced these with modern silicon carbide schottky diodes (about $3 each times 8) which are a popular audio tweak however they are still going to be constrained by a resistor capacitor snubber network. I am actually told these will be holding the schottky diodes’ performance back but I am a little afraid to simply remove them thanks to the convoluted pcb traces.
I swapped most of the caps over the course of a weekend and when I got the amp back up and running the sound was surprisingly much different. In my book the amp lost a lot of the warmth and instead moved noticeably closer towards neutrality, the bass loses a little on the impact and trades up for actual texture. The OK detail retrieval was a notch or two better which also further enhanced the soundstage and placement.
Some quirks about my amp I still need to figure out; on most pics I can find around the web there is a low value mylar cap tying the top tabs of two darlington transistors on the right side of the amp board, around the tube area I see jumper wires wrapping around the tubes that I can only guess are done to circumvent poorly laid out board traces (like the “left channel mod”), for reference models I also see a giant film cap acting as a shunt for the power supply. These small differences might add up to something so I want to wrap my head around them if I can. My particular board also has the HV TL783 voltage regulator pcb mounted with a heatsink whereas I am seeing most others remotely located to the chassis for better heat dissipation. I will relocate the TL783 later and add the cap on the darlingtons however the shunt cap and wire rerouting are to be determined.
Pt 2 Volume Board
My next step is to figure out a volume control solution which in stock form is pretty bad. The Pho-tentiometer is a weird beast, my board had 2 motorized Alpha pots for volume and balance; these pots fit super tight so upgrading them is likely not possible or even financially feasible. At 0 volume there is bleed through of audio which is documented in the user manual as normal operation. In turn Melos added a (stupid) mute component to the amp circuit triggered by the infrared remote (which is also required to kick it out of mute after power up). In operation you can see a light source on the underside of the volume board decrease as volume increases; what you get in reality is a frustratingly non-linear volume attenuation with limited range.
The popular solution is to remove the board and swap in a high quality attenuator or pot, however I have recently come across some light-dependent resistor (LDR) solutions that pointed me towards the Tortuga Audio LDR3x DIY controller. One user I have also been in contact with, 33na3rd has some great pics up of his installation. If I can get this implemented then the idea of a light dependent volume control will at least remain as will a functional remote for volume control.
It doesn't look like much but a million man hours later and right now this is where I stand, the Tortuga volume board should arrive any day so I am excited to move forward but I still have a ton of unanswered questions, such as where I could tap the VU meter into and what other functionality I may end up losing to still figure out!
Pt 2 Continued
Made some progress over the weekend with the Tortuga board in-hand. Install proved to be a little tricky but it went alright. The Tortuga board is very compact and the screw-down posts are super tiny so it doesn't like the thick gauge stock wires, it is also two PCB's stacked so it needs about 1.5" of clearance in the chassis. The board indicators for the input and output are not as clear as they were with the v1 version so I actually hooked up these wires incorrectly on the first try.
I made a small change to the right hand DPDT "Monitor" switch by wiring the active pre- amp mute wires to it but I am not sure I will keep it that way. From the factory it serves as a tape input switch which is useless considering the sheer amount of inputs available, who the hell really needs that ONE more input in addition to the other 6 on the selector dial? With the unused tape input I added jumpers for a second set of loop outs.
The good news is it seems to be working OK. The passive pre- performance of the Tortuga LDR3x is actually pretty impressive, I can't understand how but it indeed made a very pleasing sonic difference with my old Yamaha CR-1020 receiver. The LDR3x draws 12v power from the Melos' amp board right off the secondaries, I've also attached additional grounds going to the chassis for power and Left and Right channels. The output then gets split for a passive output via rear RCA's and also into the Melos' amp board. There is additionally an alps encoder dial which acts as an interface/volume control and an IR module which receives inputs from an Apple remote.
While it is technically up and running I still have a laundry list of things I need to get hammered out. The most annoying at the moment is the Tortuga power on sequence. While the board gets power from the Melos' power switch, the LDR3x board must in addition be turned on by de-pressing in the encoder dial. If you turn the encoder knob right away without powering it on (like any normal human being would) it goes into an auto-calibration mode and behaves very obnoxiously if you happen to have headphones or speakers on. I also failed to get the Apple IR remote functional; I suspect I could have hooked it up wrong possible destroying the sensor and have already ordered a few new IR receievers from Mouser to try. The shape of the provided one is useless for the Melos anyways.
Another small annoyance is that the LDR3x does not completely mute at 0 volume just like the Photentiometer board, I intend to turn the tape monitor switch into functional a headphone mute of sorts but will have to see if it is possible.
I am now somewhat regretting not having plunged into the left- channel mod, it may be due to the lower 20k input impedence of the LDR3x (Photentiometer was 50k?) but I think the noise floor is a little more noticeable now. This can be adjusted to anything from 1 to 99k but requires a separate display module that I don't want to deal with mounting or purchasing at the moment.
Tearing it all back down again is going to suck that's for sure!
Pt 2 Conclusion
Happy to say, with some minor adjustments from my last update, both the amp and LDR3x is finally in a usable state! Thanks to some tips from a few internet pen pals and a call to Tortuga, the status LED, remote, and headphone mute switch are now sorted out.
The sound of the amp at this point is completely different from stock. It may no longer sound as good with the HD800 (stock), but I do like what I am hearing with HP-2's, HD600's, and even TH-X00's. The soundstaging capability and detail retrieval of the amp, especially with the HP-2's, are definitely its strongest points. It has lost the original warmth and bass bump with the stock parts, but this is probably the tradeoff due to some bonus distortion to begin with. With any luck I will be trying out some cheapie reflektor 6h23-eb tubes to replace the gold pin jj 6922 to see if there are any more gains to be had.
Below are my notes to wrap up part 2:
The previously unmentioned status LED problem I was running into was due to the factory Melos LED which I overlooked. It is a "flashing" type so when it gets normal dcv it does not stay continuously lit but rather blinks all the damn time and is very flaky in operation. Needless to say with a half functional status LED and no remote it was very difficult to oeprate the volume board! I threw in a regular old 5mm red LED and it came to life as it was supposed to. Later I will swap in a closer color matched LED from clear red glass to opaque red, an easy fix.
The new IR receiver from Mouser also worked right away and fits perfectly into the stock holder with a dab of hot glue added. The Tortuga supplied part is Vishay TSOP31138 which will not fit the Melos bulb holder anyways so I ended up with TSOP32138 which matches the pinout. I must have hooked up the supplied part incorrectly or shorted it out somehow
A final tweak, the 2.2uf capacitor across the top tabs of the position 106 and 108 - 2N6045 darlington transistors also proved to be fruitless. Soldering the tabs seemed to be incredibly difficult even with a temperature controlled solder station at my disposal. I don't believe to-220 tabs are meant to be soldered onto anyways. Even with my attempt, the cap seemed like it exacerbated background noises so I ripped it out right away.
To summarize what I've done up to now:
Silicon Carbide Schottky Rectifier bridges to replace the HV and LV bridges
Replaced all electrolytic caps with Nichicon's
Sonicap Gen 1's for inputs and the bypasses all over the amp
Big honking 100uf 400v Solen PB to replace the "last" power supply electrolytic
Left-Channel mod/reroute
Remote locate TL783 regulator from pcb to chassis
Swapped JRC NE5332N for a Signetics NE5332N
Jumpered Tape In to an additional Tape (loop) Out
Rewired Tape In monitor switch to a manual Headphone mute
Replace Photentiometer with Tortuga LDR3x v 2.1 board and wired output to passive pre-out and Melos input
What lies ahead?
For me this has been pretty exhausting work but at least I am happy to have picked up a few new tricks along the way. The amp in its current iteration sounds great to my ears and hopefully it will remain in service for the forseeable future; the Melos SHA may not win any amp measurement (or design) contests or offer world beating performance but it does indeed carry a nice sound with it that pairs nicely with the right headphones. It is a shame the Melos brand ended up in obscurity with such a poor reputation, many of today's headphone enthusiasts may never have any real idea where the whole idea of the commercial dedicated headphone amp came from nor will they care.
There are still some small quirks and there remains a wishlist for this amp but I may not be able to achieve any of this without some outside help. Firstly, rigging up a timed relay to trigger the Tortuga board power, second determining if the Melos VU balance meter can be tapped in elsewhere without the Photentiometer board, and finally the above board wire trace re-route near the input caps and tubes. I almost forgot, I also have a 10R/5W resistor and .1uf X2 capacitor for a ground loop breaker that I may decide throw in at a later date, but I believe that will first require re-doing the entire ground setup for the amp by isolating the RCA's and re-examining the star ground points.
In use there are also a few small oddities when used as a pre-amp rather than just a headphone amp. Commercially, the Tortuga board is meant to act firstly as a passive pre- and the Melos in stock config is actually wired for a passive pre- out. While I have retained this wiring layout, I have determined that under certain circumstances it seems the headphone out and passive pre- out can split their load and likely decreases the sound quality as a result.
Also when functioning as an active pre- it seems that I have to choose either headphone priority or active pre- priority; with any headphones plugged in (and even with the manual mute switch up) the active pre- will not semi-muted like it used to be with the photentiometer, but rather it functions just at a lower volume. To get around this I will have to physically unplug the headphones, then the active pre- will operate at regular volume.
Another likely ground related issue; when I have my phono pre- plugged into the Melos directly it seems to introduce a ground loop. To get around this, having the phono pre- feeding to my stereo amp first and then using the loop from the stereo to the Melos however is perfectly fine! I suspect this grounding behavior was likely the same even before I ripped into the amp, but it's too late to know now.
Despite these (imo) small glitches, I find in use the Melos still performs very nicely as a pre-amp. I use it in either of these configurations 1) Passive pre- into a standard input of my Yamaha CR-1020 (utilizes the stereo tone and volume controls aka double amping) OR 2) feeding the active pre- to the "main in" of the receiver (bypassing tone and volume controls). Much to my chagrin, both methods will actually transcend the sound I am getting out of the CR-1020 alone!
If you have gotten this far, kudos for putting up with my long winded posts and thank you for having a look at my project! I hope that maybe it can help keep a few more of these old clunkers from the junk bin!
I am still learning the ropes for the DIY audio realm but this has been a project that I’ve wanted to tackle for several years now and only recently have decided I would actually take the plunge and just go for it. My apologies that I am not committing to provide detailed sound impressions for the amp as this project for me is more for nostalgia than anything else. I am positive that most amps you can buy today are better built and probably better sounding. My purpose here really is to document what I am doing and hopefully to learn in the process so your input is greatly appreciated!
Melos SHA History, a lot of this is skimmed from web searching, since I don’t see it mentioned much here’s my notes on the matter which may or may not even be factual. (Super cool reads http://www.head-fi.org/t/127867/an-interview-with-john-grado and http://www.head-fi.org/t/36656/melos-sha-x-modified-by-carlo-comments)
For those not familiar with the Melos SHA amps, the abridged lore goes something like this… back in the early 90’s before the headphone amp market was even a thing, Joe Grado (RIP) wanted a dedicated amplifier to power his HP-1000’s. Through some arrangement he contracted the then reputable Melos Audio to design said amp. Though I am not clear on the details, apparently he was not happy with the two prototypes they came up with (eventually dubbed the SHA-X) and eventually Joe created and sold his own Grado HPA-1 amp instead which is also quite a rare piece of history. The irony is that, based on what I’ve read, Joe actually kept an all Melos based gear chain for his own personal use.
I have no facts to back this up but Melos had a rocky history as an audio company, they sold many different iterations of the SHA amp (they also appear to have sold monoblocks, phono stage, R2R DAC, CDP) and like some amp builders something changed for the worse in the production quality standard along the way. The design of the SHA is pretty bad as far as I know, the PCB is a rats nest of traces, the higher end models add another rats nest of wires and jumped traces in addition, the parts selection was never top tier or even consistent. The Sha Gold added a Pho-Tentiometer which is a light dependent volume attenuator, on paper it sounds really cool but in reality it’s kind of a piece of junk. All of this resulted in poor reliability of the amp and probably by today standards mediocre sound. I can venture a guess that the reliability problems and skimping on parts were the beginning of the end for Melos as they closed their doors probably sometime around the early 2000's, there were even some failed promises to come back bigger and better but nothing ever panned out.
At Head-Fi there were some individuals who modded the hell out of these amps, carlo, nikongod,FallenAngel, nattonrice, these guys certainly had better chops than I do. Even ECP Doug was in on the action! Sadly half of these guys are in retirement from the HF scene near as I can tell.
Melos SHA Gold
(Schematic here: http://www.eserviceinfo.com/downloadsm/21578/Melos_Melos Sha Gold.html)
(Questionable info for tube plate voltage adjustment here: http://www.head-fi.org/t/63652/compiled-melos-tube-impressions)
Before diving in I already had a rough idea of what I wanted to do with the amp, namely to see what exactly the hoopla back in the early HF days was all about with regards to the modding potential and its synergy with the HP-2. This particular unit was snagged on A’gon in stock form; many of these were upgraded (read: repaired and shoddily so) by Melos over the years to “reference” level. I actually had an after the fact non-officially modded reference in my heyday in superb condition which I remember fondly. I am not even clear on what the reference upgrade meant in terms of parts or modifications but it is very clear there are a slew of changes evident in the pics you can scour from the web. The faceplate is in decent condition and just slightly bronzing, kind of like the picture shown in the old fluffed Stereophile review. Sadly a lot of these suffered pretty gross anodizing failure. This one must have not seen a ton of use as the amp itself came to me fully functional and complete in the old beat up box with an owner’s manual and infrared remote.
The sound in stock form is not bad even when running some super common JJ 6922 gold pin tubes. The Melos has a reputation for eating tubes, I read up on adjusting the plate voltages but I don’t even know if adjustment will make a major difference. Maybe I got lucky with this one but hum or hiss is not a major issue (yet?) though it is certainly not the quietest or blackest background amp I have heard. The sound I would characterize as relaxed and warm and only decent in the details department. Bass is not well defined but bumped up just slightly so which works out great for HP-2’s and HD800. Soundstaging and placement is better than average which also makes for some engaging listening sessions.
Getting to Work Pt 1
The amp didn’t last long in stock form and I spent way too much time researching old threads, poring over the inadequate schematics, and ordering parts from pcx, sonic craft, and mouser. Worth noting is that this amp has soldered wires holding everything together; just to free the amp PCB requires almost 20 different wires to be de-soldered. Working on it really sucks but at least I am pretty handy with a de-soldering pump now!
I focused on the amp board first, simple drop-in replacements that seem to add up even when on a budget. Electrolytics are replaced with modern fancy Nichicon’s as the Black Gates that were once the standard are NLA and very expensive. I purchased a bunch of Sonicap Gen 1’s for bypasses and input. Not so much of a drop in replacement, the High Voltage and Low Voltage rectifier bridges in stock form are maybe enough to get the job done but probably run at their limits. With the guidance of some super awesome friends, I replaced these with modern silicon carbide schottky diodes (about $3 each times 8) which are a popular audio tweak however they are still going to be constrained by a resistor capacitor snubber network. I am actually told these will be holding the schottky diodes’ performance back but I am a little afraid to simply remove them thanks to the convoluted pcb traces.
I swapped most of the caps over the course of a weekend and when I got the amp back up and running the sound was surprisingly much different. In my book the amp lost a lot of the warmth and instead moved noticeably closer towards neutrality, the bass loses a little on the impact and trades up for actual texture. The OK detail retrieval was a notch or two better which also further enhanced the soundstage and placement.
Some quirks about my amp I still need to figure out; on most pics I can find around the web there is a low value mylar cap tying the top tabs of two darlington transistors on the right side of the amp board, around the tube area I see jumper wires wrapping around the tubes that I can only guess are done to circumvent poorly laid out board traces (like the “left channel mod”), for reference models I also see a giant film cap acting as a shunt for the power supply. These small differences might add up to something so I want to wrap my head around them if I can. My particular board also has the HV TL783 voltage regulator pcb mounted with a heatsink whereas I am seeing most others remotely located to the chassis for better heat dissipation. I will relocate the TL783 later and add the cap on the darlingtons however the shunt cap and wire rerouting are to be determined.
Pt 2 Volume Board
My next step is to figure out a volume control solution which in stock form is pretty bad. The Pho-tentiometer is a weird beast, my board had 2 motorized Alpha pots for volume and balance; these pots fit super tight so upgrading them is likely not possible or even financially feasible. At 0 volume there is bleed through of audio which is documented in the user manual as normal operation. In turn Melos added a (stupid) mute component to the amp circuit triggered by the infrared remote (which is also required to kick it out of mute after power up). In operation you can see a light source on the underside of the volume board decrease as volume increases; what you get in reality is a frustratingly non-linear volume attenuation with limited range.
The popular solution is to remove the board and swap in a high quality attenuator or pot, however I have recently come across some light-dependent resistor (LDR) solutions that pointed me towards the Tortuga Audio LDR3x DIY controller. One user I have also been in contact with, 33na3rd has some great pics up of his installation. If I can get this implemented then the idea of a light dependent volume control will at least remain as will a functional remote for volume control.
It doesn't look like much but a million man hours later and right now this is where I stand, the Tortuga volume board should arrive any day so I am excited to move forward but I still have a ton of unanswered questions, such as where I could tap the VU meter into and what other functionality I may end up losing to still figure out!
Pt 2 Continued
Made some progress over the weekend with the Tortuga board in-hand. Install proved to be a little tricky but it went alright. The Tortuga board is very compact and the screw-down posts are super tiny so it doesn't like the thick gauge stock wires, it is also two PCB's stacked so it needs about 1.5" of clearance in the chassis. The board indicators for the input and output are not as clear as they were with the v1 version so I actually hooked up these wires incorrectly on the first try.
I made a small change to the right hand DPDT "Monitor" switch by wiring the active pre- amp mute wires to it but I am not sure I will keep it that way. From the factory it serves as a tape input switch which is useless considering the sheer amount of inputs available, who the hell really needs that ONE more input in addition to the other 6 on the selector dial? With the unused tape input I added jumpers for a second set of loop outs.
The good news is it seems to be working OK. The passive pre- performance of the Tortuga LDR3x is actually pretty impressive, I can't understand how but it indeed made a very pleasing sonic difference with my old Yamaha CR-1020 receiver. The LDR3x draws 12v power from the Melos' amp board right off the secondaries, I've also attached additional grounds going to the chassis for power and Left and Right channels. The output then gets split for a passive output via rear RCA's and also into the Melos' amp board. There is additionally an alps encoder dial which acts as an interface/volume control and an IR module which receives inputs from an Apple remote.
While it is technically up and running I still have a laundry list of things I need to get hammered out. The most annoying at the moment is the Tortuga power on sequence. While the board gets power from the Melos' power switch, the LDR3x board must in addition be turned on by de-pressing in the encoder dial. If you turn the encoder knob right away without powering it on (like any normal human being would) it goes into an auto-calibration mode and behaves very obnoxiously if you happen to have headphones or speakers on. I also failed to get the Apple IR remote functional; I suspect I could have hooked it up wrong possible destroying the sensor and have already ordered a few new IR receievers from Mouser to try. The shape of the provided one is useless for the Melos anyways.
Another small annoyance is that the LDR3x does not completely mute at 0 volume just like the Photentiometer board, I intend to turn the tape monitor switch into functional a headphone mute of sorts but will have to see if it is possible.
I am now somewhat regretting not having plunged into the left- channel mod, it may be due to the lower 20k input impedence of the LDR3x (Photentiometer was 50k?) but I think the noise floor is a little more noticeable now. This can be adjusted to anything from 1 to 99k but requires a separate display module that I don't want to deal with mounting or purchasing at the moment.
Tearing it all back down again is going to suck that's for sure!
Pt 2 Conclusion
Happy to say, with some minor adjustments from my last update, both the amp and LDR3x is finally in a usable state! Thanks to some tips from a few internet pen pals and a call to Tortuga, the status LED, remote, and headphone mute switch are now sorted out.
The sound of the amp at this point is completely different from stock. It may no longer sound as good with the HD800 (stock), but I do like what I am hearing with HP-2's, HD600's, and even TH-X00's. The soundstaging capability and detail retrieval of the amp, especially with the HP-2's, are definitely its strongest points. It has lost the original warmth and bass bump with the stock parts, but this is probably the tradeoff due to some bonus distortion to begin with. With any luck I will be trying out some cheapie reflektor 6h23-eb tubes to replace the gold pin jj 6922 to see if there are any more gains to be had.
Below are my notes to wrap up part 2:
The previously unmentioned status LED problem I was running into was due to the factory Melos LED which I overlooked. It is a "flashing" type so when it gets normal dcv it does not stay continuously lit but rather blinks all the damn time and is very flaky in operation. Needless to say with a half functional status LED and no remote it was very difficult to oeprate the volume board! I threw in a regular old 5mm red LED and it came to life as it was supposed to. Later I will swap in a closer color matched LED from clear red glass to opaque red, an easy fix.
The new IR receiver from Mouser also worked right away and fits perfectly into the stock holder with a dab of hot glue added. The Tortuga supplied part is Vishay TSOP31138 which will not fit the Melos bulb holder anyways so I ended up with TSOP32138 which matches the pinout. I must have hooked up the supplied part incorrectly or shorted it out somehow
A final tweak, the 2.2uf capacitor across the top tabs of the position 106 and 108 - 2N6045 darlington transistors also proved to be fruitless. Soldering the tabs seemed to be incredibly difficult even with a temperature controlled solder station at my disposal. I don't believe to-220 tabs are meant to be soldered onto anyways. Even with my attempt, the cap seemed like it exacerbated background noises so I ripped it out right away.
To summarize what I've done up to now:
Silicon Carbide Schottky Rectifier bridges to replace the HV and LV bridges
Replaced all electrolytic caps with Nichicon's
Sonicap Gen 1's for inputs and the bypasses all over the amp
Big honking 100uf 400v Solen PB to replace the "last" power supply electrolytic
Left-Channel mod/reroute
Remote locate TL783 regulator from pcb to chassis
Swapped JRC NE5332N for a Signetics NE5332N
Jumpered Tape In to an additional Tape (loop) Out
Rewired Tape In monitor switch to a manual Headphone mute
Replace Photentiometer with Tortuga LDR3x v 2.1 board and wired output to passive pre-out and Melos input
What lies ahead?
For me this has been pretty exhausting work but at least I am happy to have picked up a few new tricks along the way. The amp in its current iteration sounds great to my ears and hopefully it will remain in service for the forseeable future; the Melos SHA may not win any amp measurement (or design) contests or offer world beating performance but it does indeed carry a nice sound with it that pairs nicely with the right headphones. It is a shame the Melos brand ended up in obscurity with such a poor reputation, many of today's headphone enthusiasts may never have any real idea where the whole idea of the commercial dedicated headphone amp came from nor will they care.
There are still some small quirks and there remains a wishlist for this amp but I may not be able to achieve any of this without some outside help. Firstly, rigging up a timed relay to trigger the Tortuga board power, second determining if the Melos VU balance meter can be tapped in elsewhere without the Photentiometer board, and finally the above board wire trace re-route near the input caps and tubes. I almost forgot, I also have a 10R/5W resistor and .1uf X2 capacitor for a ground loop breaker that I may decide throw in at a later date, but I believe that will first require re-doing the entire ground setup for the amp by isolating the RCA's and re-examining the star ground points.
In use there are also a few small oddities when used as a pre-amp rather than just a headphone amp. Commercially, the Tortuga board is meant to act firstly as a passive pre- and the Melos in stock config is actually wired for a passive pre- out. While I have retained this wiring layout, I have determined that under certain circumstances it seems the headphone out and passive pre- out can split their load and likely decreases the sound quality as a result.
Also when functioning as an active pre- it seems that I have to choose either headphone priority or active pre- priority; with any headphones plugged in (and even with the manual mute switch up) the active pre- will not semi-muted like it used to be with the photentiometer, but rather it functions just at a lower volume. To get around this I will have to physically unplug the headphones, then the active pre- will operate at regular volume.
Another likely ground related issue; when I have my phono pre- plugged into the Melos directly it seems to introduce a ground loop. To get around this, having the phono pre- feeding to my stereo amp first and then using the loop from the stereo to the Melos however is perfectly fine! I suspect this grounding behavior was likely the same even before I ripped into the amp, but it's too late to know now.
Despite these (imo) small glitches, I find in use the Melos still performs very nicely as a pre-amp. I use it in either of these configurations 1) Passive pre- into a standard input of my Yamaha CR-1020 (utilizes the stereo tone and volume controls aka double amping) OR 2) feeding the active pre- to the "main in" of the receiver (bypassing tone and volume controls). Much to my chagrin, both methods will actually transcend the sound I am getting out of the CR-1020 alone!
If you have gotten this far, kudos for putting up with my long winded posts and thank you for having a look at my project! I hope that maybe it can help keep a few more of these old clunkers from the junk bin!
