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 Post subject: Holiday Projects
PostPosted: December 31st, 2019, 12:50 am 
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Joined: February 28th, 2013, 1:19 pm
Posts: 371
Nothing like having a week or two of relatively unscheduled time to fire up the ol' soldering iron and smell that holiday rosin aroma. I have a pair of DIY mono PP IT coupled 2A3 amplifiers (with Hashimoto PP iron!) that were sitting around idle for the longest time (several years) waiting for me to finally get back to them to debug and fix an intermittent noise problem in one of the amps. I wanted to have another pair of amplifiers that would drive my new Elsinore speakers, besides the Moskido hybrid amp and GM70 SET monoblocks. (always like double redundancy). Eyeing the easiest way of pushing up close to the 20W output capability to drive my speakers, I bought a quad of JJ 2A3-40 tubes, reworked the power supply to get 400V across the tubes, changed the driver from a 5842 to a D3A that can swing more volts into the grid, and installed a much more reliable way of nulling out residual hum. If you are using AC heaters on your DHTs (I know, old school), using a Helipot multiturn 50 ohm pot across the filaments one tube of a PP pair (the other tube having a pair of 1% 25ohm 3W WW resistors) allows one to adjust to incredibly low level of hum at the speaker terminals (less than 3 mV). Dead Quiet. Anyway, I have the 2A3s cooking at 85mA per tube, 72V bias, i.e., heavy Class A. I figure the JJ tubes like to run hard anyway (85% of rated dissipation). Anyway, these amps sound really, REALLY good. I did a partial rebuild keeping the 5842 driver at first, which was pretty sweet, but the D3A provides more texture and spaciousness. The amp is much more dynamic than I expected or remembered. In fact, I think the "jump" factor is comparable to my hybrid amp. I think I was using Shughang 2A3C's last these were working at a more conservative bias (i.e. closer to classic 2A3), so maybe the combo of the more powerful JJ version and more aggressive operating point makes all the difference. Definitely a keeper, and will be in the system for a while until I get the urge to switch things out again.

Also, dug out my MAC MR71 tuner which I had sidelined after getting a Sony HD Radio tuner (HD-1D?) a few years ago. I recently found the Sony stopped receiving any HD radio stations (It would lock on momentarily, and then lose the signal to static). Anyway, knocked the dust off the MR-71, reseated the tubes, replaced the burned out lamp for the stereo light, and replaced a weak 6U8A in the MPX section that resulted in poor stereo separation. I get a good signal on a classical radio station in Baltimore, WBJC, 91.5, that has decent 3D sound and great programming. I get great reception on WETA, but crappy 2D sound and poor programming (top 100 classical). Need to explore some decent Jazz stations (any suggestions?).

Also, relubed my Teres turntable bearing. Nice to have a steady speed again. And getting my retipped Benz Ruby III from Soundsmith back in a couple days! Woohoo! Eying one of the new Jelco knife bearing tonearms, but that will have to wait.

Good audio holiday season! And still have time for plenty of listening.

David


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 Post subject: Re: Holiday Projects
PostPosted: December 31st, 2019, 9:36 am 
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Joined: February 28th, 2013, 3:31 pm
Posts: 218
Damn Dave, you're getting it done! Congratulations getting another set of amplifiers in rotation.

Where you're located you should get a good signal for WPFW 89.3. You'll have to check their schedule for show times, but the Sunday lineup there from 9:00 am till 6:00 pm is unmatched. In particular, Tom Cole's show G-strings, and Larry Applebaum's show Sound of Surprise, are exceptional. I have to stream off the interwebs here in Annapolis (mp3 sadly), but you should be good there.


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 Post subject: Re: Holiday Projects
PostPosted: December 31st, 2019, 10:40 am 
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Joined: January 15th, 2015, 7:19 am
Posts: 967
Location: Baltimore MD
I’ve listened to Tom Cole for years and really enjoy his selections. I tried various FM tuners and find the streamed show over the web always better


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 Post subject: Re: Holiday Projects
PostPosted: December 31st, 2019, 12:10 pm 
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Joined: March 2nd, 2013, 2:43 pm
Posts: 165
Location: Potomac, MD
Dave- I with you on the WBJC vs WETA.

David


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 Post subject: Re: Holiday Projects
PostPosted: December 31st, 2019, 12:48 pm 
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Joined: February 28th, 2013, 3:31 pm
Posts: 218
Pelliott321 wrote:
I’ve listened to Tom Cole for years and really enjoy his selections. I tried various FM tuners and find the streamed show over the web always better

I'm only streaming if necessary. My Scott tuner brings in WBJC beautifully, but I'm too far from DC to get WPFW consistently.

WEAA 88.9 in B'more has great jazz programs as well but may be too far away for Dave.


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 Post subject: Re: Holiday Projects
PostPosted: December 31st, 2019, 12:57 pm 
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Joined: July 24th, 2015, 4:17 pm
Posts: 994
Location: Parkville, Maryland
Also, dug out my MAC MR71 tuner which I had sidelined after getting a Sony HD Radio tuner (HD-1D?) a few years ago. I recently found the Sony stopped receiving any HD radio stations (It would lock on momentarily, and then lose the signal to static). Anyway, knocked the dust off the MR-71, reseated the tubes, replaced the burned out lamp for the stereo light, and replaced a weak 6U8A in the MPX section that resulted in poor stereo separation. I get a good signal on a classical radio station in Baltimore, WBJC, 91.5, that has decent 3D sound and great programming. I get great reception on WETA, but crappy 2D sound and poor programming (top 100 classical). Need to explore some decent Jazz stations (any suggestions?).

Good audio holiday season! And still have time for plenty of listening.

David[/quote]

I have a MAC MR-67 that preceded the MR-71. Basically -- schematically they're the same with some upgrades. Because of age I had to rebuild the power supply, but otherwise my MR-67 was mint. I got lucky to say the least. Anyhow, if you really want to hear what your MR-71 is capable of you want to upgrade the MPX board, upgrade the audio section, add some nice film caps to the power supply.


Attachments:
MR-67 Power and Audio Out.JPG
MR-67 Power and Audio Out.JPG [ 455.38 KiB | Viewed 509 times ]
MR-67 Upgraded MPX Section.JPG
MR-67 Upgraded MPX Section.JPG [ 364.43 KiB | Viewed 509 times ]

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Walt
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 Post subject: Re: Holiday Projects
PostPosted: December 31st, 2019, 1:41 pm 
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Joined: February 28th, 2013, 1:19 pm
Posts: 371
Walt,

Nice work on the MPX boards! There is a site "Dark Lantern" that walks through some modifications that appear similar to yours. Bill Thallman years ago did a modification and alignment of my MR71, putting in a regulated supply for the MPX and output section, replacing the 6BL8 output section with a 6DJ8 cathode follower with some nice quality output caps since the output level from the MPX section is sufficient. I am thinking about touching up what he did (like adding a current sink to the cathode follower), but it is so damn tight to get to the cathode resistor (I would have to desolder almost everything on the socket just to get to the resistors). He did some mods to the MPX board, but I did not look closely at them. For now, I better leave it alone, since not clear what he may have done to make the MPX section double as part of the output.

Thanks to all for the radio station suggestions. It is a shame that streaming at MP3 level quality is better than FM in this day and age. My observation is that multi channel HD Radio killed decent analog FM, limiting the available signal bandwidth, as well as widespread use of compression or peak limiting. WETA is a good example of how bad it can be.

I was kicking around the idea of getting a DBX unit to play around with expanding the dynamics (and try to undo some of the signal abuse). Anyone play with one on FM?

David


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 Post subject: Re: Holiday Projects
PostPosted: December 31st, 2019, 7:44 pm 
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Joined: July 24th, 2015, 4:17 pm
Posts: 994
Location: Parkville, Maryland
David McGown wrote:
Walt,

Nice work on the MPX boards! There is a site "Dark Lantern" that walks through some modifications that appear similar to yours. Bill Thallman years ago did a modification and alignment of my MR71, putting in a regulated supply for the MPX and output section, replacing the 6BL8 output section with a 6DJ8 cathode follower with some nice quality output caps since the output level from the MPX section is sufficient. I am thinking about touching up what he did (like adding a current sink to the cathode follower), but it is so damn tight to get to the cathode resistor (I would have to desolder almost everything on the socket just to get to the resistors). He did some mods to the MPX board, but I did not look closely at them. For now, I better leave it alone, since not clear what he may have done to make the MPX section double as part of the output.

Thanks to all for the radio station suggestions. It is a shame that streaming at MP3 level quality is better than FM in this day and age. My observation is that multi channel HD Radio killed decent analog FM, limiting the available signal bandwidth, as well as widespread use of compression or peak limiting. WETA is a good example of how bad it can be.

I was kicking around the idea of getting a DBX unit to play around with expanding the dynamics (and try to undo some of the signal abuse). Anyone play with one on FM?

David


Hey David! Thanks for the complement! The MPX mods consisted of exactly matched resistors and capacitors. What was original was carbon-comp resistors and cermic caps. I replaced these with matched Roderstein metal-film resistors and stacked-film polycarbonate caps among other upgrades to the MPX section that you may have observed in my photos. There were many other changes but that was most influential on the sound quality.

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 Post subject: Re: Holiday Projects
PostPosted: January 1st, 2020, 9:50 am 
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Joined: February 28th, 2013, 1:19 pm
Posts: 371
Walt,

I need to take a closer look at the MPX board on my MR71, since not sure if Bill Thallman went to the level of detail you did. I noticed a few places where he replaced capacitors, but did not immediately notice any resistor changes. How extensive of a rework did you perform, i.e. just the MPX oscillator, or the entire board.

By exactly matched, did you mean to the (a) original in-place components, (b) the values on the schematic/parts list and between left/right channels (i.e., after the 38kHz oscillator). Carbon comps will drift, of course, so (a) is unlikely. For (b), the values seem to be for 10% (some 5%) tolerance components, so using 1% resistors and 5% (or better) capacitors would be a lot better than what is there. Did you need to readjust the MPX section after you worked it? I have the service manual (of course), but lack some of the test equipment (I.e., RF signal generators) necessary.

Was there a technical reason for using polycarbonate vs styrene or PP capacitors. Is that what you had on hand?

As you can tell, this intrigues me. Thanks!

David


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 Post subject: Re: Holiday Projects
PostPosted: January 1st, 2020, 3:18 pm 
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Joined: January 30th, 2018, 1:55 pm
Posts: 15
I also did a project over the break and built a new digital music streamer PC to replace my MacBook Air. I wanted to clean up my audio set up by going to a headless system. I also wanted to play around with Roon. Found a YouTube video on building a digital music streamer. One thing lead to another and now I have a fanless PC built on an ASUS thin mini board using a 4th generation Intel Celeron processor and running Windows 10. I added a MX Audio HiFi PCMCIE board for USB output. At the end of the day what I’ve really done is replaced my laptop using an external USB drive for my library with a headless PC doing the same. Still I like the set up and cleaner look.

All went smooth except that a cheap 9v power supply I used with the MX Audio board made some kind of a ground loop or floating ground situation. Wound up frying the USB input on my McIntosh C48 preamp. The silver lining was that I upgraded the DAC with a new Chord Qutest DAC that sounds worlds better than the built in McIntosh DAC.

Does the new PC sound better than the MacBook Air? If it does then it is subtle. I do think that the MX Audio board provides a cleaner signal to the DAC. Without a doubt though, Roon gives me a better experience navigating through my digital library and Tidal. Can’t tell a difference in sound quality between JRiver MC 26 and Roon.

It was a fun digital journey over the holiday.


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