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Tube Amp

Audioromy M828A amp based on 829B tube (GU-29/FU-29)
Audioromy M-828A push pull tube amp based on 829B tube

I have dipped my toe into the world of tube (or valve) audio.  The first thing that I learned was that in general, tube amps are expensive.  It seems that the least expensive amps run about $1,000 US, and from there it seems the sky is the limit.

There are a number of less expensive Chinese versions floating around, most of the tube audio experts call them garbage.  Myself; I am not so sure.  There are also a lot of somewhat dubious claims made by the same experts about speaker cable, AC power conditioning and so on.

I was going to build a single ended tube amp based on the KT88 design found here:

That is a whole series of videos, eighteen in all I think, on the design and construction of a single ended KT88 audio amp.  If you have the time, well worth it to watch.

Then I decided that I really do not have a lot of time for that and I just wanted to try a tube amp and see if there is really that big of a difference.  Thus, I purchased one of the Chinese designs based on the RCA 829B tube, which is kind of exotic looking:

FU-29 Chinese equivalent to RCA 829B dual pentode tube

FU-29 equivalent to RCA 829B dual pentode tube

That is the Chinese version FU-29, there is also a Russian радиолампа ГY-29.  The good news is that there are lots of these tubes available for not too much money.  New Old Stock (NOS) RCA 829Bs run about $25-30 each.  A Ulyanovsk GU-29 (NOS) runs about $10.00 (made in the USSR).  Somewhat more rare are the 3E29 tubes, which were designed for VHF pulsed radar.  These are dual pentode tubes which can be run ether parallel (single ended) or push pull.  They were originally designed for VHF transmitters, but have been put into use in HF transmitters and audio amplifiers.  The USSR versions are long life militarized versions and designed for aircraft radar; flying upside down at Mach 2 in -50 C temperatures 18,000 meters AMSL…  My Russian friend tells me I am joking.  I am joking.

Reflector factory, 6N3P-E dual triode tube

Reflector factory, 6N3P-E dual triode tube

The driver tubes and phase inverters are 6N3P-E (6N3, 6N3P, 6N3P-EV, 5670, 2C51 or 396A can also be used) which is a double triode tube, made by Reflector in Sartov, Russia.  These tubes are also militarized long life versions.

Audioromy M828A push pull tube amp

Audioromy M828A, power transformer and output transformers

The Audioromy M-828A amplifier seemed like a good compromise between price, power and workmanship.  I ordered the amp from Amazon and it took about a week to arrive.  The first thing I did was take it apart and look at it.  I was expecting poor workmanship and cheap components, etc.   Overall, it seems to be pretty well made. There are two printed circuit boards; one for the power supply, the other for the front end before the two power amp tubes.  The power supply uses solid state diodes, which some view as a compromise to a tube amp design.  There are also several power supplies on one board; 440 VDC B+ for power tubes, 220 VDC screen supply, a -25 VDC grid bias supply, 12 VDC for the audio switching relay, +6 VDC for the driver/phase inverter filaments.  I like the idea of DC filament voltage on the driver tubes.

Audioromy M828A underside

Audioromy M828A underside

This amp is configured for push pull and rated at 30 watts per channel.  I will test all of that plus measure THD, frequency response and so on.

There is no manual, which I find a little bit annoying.  Also, there is a lack of a schematic diagram nor any instructions on biasing and balancing the tubes when they are replaced.

Being thus annoyed, I did some deep diving on the intertubes and found that some people had posted on how to re-bias and re-balance the thing after tube replacement.  There where also several modifications suggested.

  1. Replace the input potentiometer with something a little more substantial.  It does seem to be a little bit cheap and I do not like the notches in the volume adjustment.  I will do this mod.
  2. Replace the coupling caps with oil filled units.  Not so sure about this one, but I might try it just to see if it makes a difference.
  3. Install a bias regulating circuit using an LM317 voltage regulator between the output tube cathode and ground.  This seems like a good idea.
  4. Roll (replace) the input and power tubes with better versions of US made or Russian made tubes.  The input tubes are 6N3P-E tubes from Reflector (Sartov, Russia) which are already pretty good tubes.  I might replace the FU-29’s with a set of GU-29’s at some point.

There appear to be several schematic diagrams with slight variations based on the changes in design over the years.  Several designs have different input and phase inverter tubes.  Some have different power supplies, still others show no anode resistors or a cathode resistor.  This is the diagram for the amp that I own, which was produced circa 2018 or so:

Audioromy M-828A schematic diagram

Audioromy M-828A schematic diagram

After all my investigations where finished, I put the amp back together and plugged it in.  I then ran my known CD’s though it and it sounded a bit rough.  I was a little bit disappointed until someone said that it takes about 10 hours or so for a tube to break in.  I connected it to my speaker test load (8 ohm, 50 watt resistors) and let it run for a day.

What a difference a day makes.  The second listen to the same CD proved to be much, much better.  There is definitely some coloration from the tubes.  A side to side comparison between my solid state Kenwood VR-309 amp and the Audioromy M-828A has the tube amp sounding much richer.  There is no real way to say it; it sounds full while detailed and clean all at the same time.  Playing though my homemade speakers, which are mid range deluxe, stringed instruments sound very detailed.  You can hear the pick hit the strings on an acoustic guitar.  You can hear the bow scrape across the strings on a cello.  It is unlike any amp that I have ever owned.

I am enjoying very much listening to Dave Mathews and Tim Reynolds Live at Luther College CD as I am typing this.

Now, I don’t know what the difference between this amp and the $10,000.00 version of the same tube amp made in Canada, other than the $9,500.00 difference in price.

A few comments about this amp and the 829B push pull amp design.  First of all, since the screen grids are connected internally, there is no way to run this tube in ultra linear mode.  Usually, ultra linear mode involves taking feedback from the output (anode) or the output transformer and feeding it into the screen of the power tube.  

Secondly, it is widely commented on that these amps are notoriously difficult to bias and balance.  One or both sides of the output tube will red plate due to over current.  I am hopeful the the LM-317 bias regulator circuit will take some of the difficulty out of this.  With an ordinary push pull amplifier, the balancing issue is taken care of with matched tubes.  Since both tubes in this push pull circuit are in the same envelope, getting a matched pair is not likely.  So, the tricky act of balancing the two outputs from the same tube will have to be carried out each time the tubes are replaced.  That being said, hopefully a set of those Soviet tubes will last for a long time.

One thing that I did do is make a bunch of voltage measurements and noted them on the schematic diagram.  If there are every any problems with this unit, having a set of base voltage measurements should go a long way toward troubleshooting and repairing it.

Finally, while the 829B is a rather exotic tube, it likely does not perform to the level of an EL86 or KT88 single ended design.  That being said, I have no problems with purchasing this amp and I am enjoying the toob audio sound very much.

Hearing test

This was on the Wheatstone news letter a few months ago. NPR has an interesting test to see if one can hear the difference between various quality .mp3 and .wav files.  There are six cuts with three versions each; a 128 kbps .mp3, a 256 kbps .mp3 and a .wav file in no particular order.

That NPR article can be found here: How Well Can You Hear Audio Quality?

I listened to all of them and found the 128 kbps .mp3 was pretty easy to pick out.  On the newer material, it was sometimes difficult to tell the difference between the .wav file and the 256 kbps .mp3.  Keep in mind that most radio stations stream at 64-128 kbps.  Online music services like Pandora (64 kbps for free listeners, 192 kbps for subscribers), Spotify (96-160 kbps for free listeners, 320 kbps for subscribers) and Apple (256 kbps for everybody) offer slightly better quality, especially for paid subscribers.

It is too bad one cannot simulate 15 IPS analog tape.  I would bet that a well mastered recording on analog tape would stand out above anything even remotely compressed.

Others have compared streaming audio to analog FM audio and found that fairly high bit rates are needed to make the quality equal: What bitrate is needed to sound like analog FM?

One more thing to keep in mind, HD Radio runs 96-144 kbps on the main channel and 20-60 kbps on the sub channels.

Cable Porn

On occasion, the company I currently work for does installation work. Thus, I am always keeping my eyes open for new equipment and tools to make that job easier. The cable comb seems like it is just such a thing:

ACOM tools cable comb

ACOM tools cable comb

Instructional video from youtube:

Then there is this:

Which is simply amazing. It is described as “1320 Category 6 cables, dressed and terminated.”

Incidentally, there is an entire sub-reddit: reddit.com/r/cableporn for all those cable geeks that like to look at neat cabling work.

What bitrate is needed to sound like analog FM?

As it turns out, 300 kbp/s or greater.  At least in critical listening environments according to the paper titled Perceived Audio Quality of Realistic FM and DAB+ Radio Broadcasting Systems (.pdf) published by the Journal of the Audio Engineering Society. This work was done by group in Sweden and made various observations with different program material and listening subjects. Each person was given a sample of analog FM audio to listen to, then they listened to various audio selections which were using bit reduction algorithms (AKA CODEC or Compression) and graded each one.  The methodology is very thorough and there is little left for subjective interpretation.

In less critical listening environments, bit rates of 160-192 kbp/s will work.

I made a chart and added HD Radio’s proprietary CODEC HDC, which is similar to, but not compatible with AAC:

System Codec Bit Rate (kbp/s)
HD Radio FM; HD1 channel* HDC (similar to AAC) 96 – 144
HD Radio FM; HD2 channel* HDC 24-48
HD Radio FM; HD3 channel* HDC 24-48
HD Radio AM* HDC 20-60
DRM30 (MF-HF) AAC/HE-AAC 34-72
DRM+ (VHF) AAC/HE-AAC 700
DAB+ AAC/HE-AAC 32 – 128
DAB MPEG II, Dolby digital 192 – 256
Blu-ray PCM** ≥6 Mbp/s
DVD PCM, DTS, Dolby digital >800
CD-A PCM 1,411
Web Streaming MPEG I,II,III, WMA, AAC, etc 32-320, 128 typical
iTunes AAC 128 – 256
Spotify Ogg Vorbis 96 – 320
Wimp AAC/HE-AAC 64 – 256

*Hydbrid mode
**PCM: uncompressed data

This is the composite Mean Basic Audio Quality and 95% confidence intervals for system across all excerpts:

digital-analog-audio-compar

Over the years, we have simply become accustomed to and now accept low quality audio from mp3 files being played over cheap computer speakers or through cheap ear buds.  Does this make it right?  In our drive to take something good and make it better, perhaps it should be, you know: Better.

Special thanks to Trevor from Surrey Electronics Limited.

Analog Sauce

A little blast from the past. This was found in a transmitter manual at one of the sites we take care of:

CCA Optomod 8000

CCA Optomod 8000

I thought I would scan it and make it available here.  As luck would have it, there is also a corresponding piece of equipment to go along with it.  I had never seen a “CCA Optomod” (.pdf) before I was working at one of the radio stations in Trenton, Florida.  This unit was rescued from under a pile of garbage out in the lawn shed.  It was full of mud was nests and mouse droppings.  Needless to say, it required a bit of TLC to return it to operation.  I replaced the electrolytics, cleaned it up and ran some audio through it.  It is probably as good as the day it left the factory.  Bob Orban made some really good stuff in his day.

CCA Optomod 8000

CCA Optomod 8000

The original Optomod 8000 was an evolutionary design that made FM radio processing what it is today.  The idea of combining broadband limiter, AGC and stereo generator in one box was a radical departure from the norm.  The audio limiter functioned as a 15 KHz low pass filter and broadband AGC.

Orban Optomod 8000 audio limiter block diagram

Orban Optomod 8000 audio limiter block diagram

The stereo generator used very modest amounts of composite clipping to reduce overshoot and transients. Many people disparage composite clippers. If done correctly, it is transparent to the listener and increases perceived loudness by stripping off modulation product that is non-productive.

Orban Optomod 8000 Stereo Generator block diagram

Orban Optomod 8000 Stereo Generator block diagram

Some thirty five or so years later, there are still many of these units in service in various stations around the world.

The Raytheon RL10 Limiting Amplifier

Update: Apparently this is quite interesting to a number of people.  I have rescanned the manual, properly compressed it and which you may find it here.

Found this manual at one of the older transmitter sites:

Raytheon RL10 limiting amplifier manual cover

Raytheon RL10 limiting amplifier manual cover

Entire manual is available for your reading pleasure here: Raytheon RL10 limiting amplifier

As this is an older design than either the Gates Sta level or the Collins 26U, it may not be as useful to tube audio enthusiasts.

Raytheon RL-10 Schematic diagram

Raytheon RL-10 Schematic diagram

The main issue with the Gates and Collins unit is the GE 6386 remote cutoff triode used, which were great tubes, but very difficult to come by these days.  This design calls for a 1612 or 6L7, which is a pentagrid amplifier.  Feedback is provided by the screen of the following stage, a 6SJ7GT.  Anyway, perhaps it will give somebody some idea on how to make a good tube compressor limiter.

New Studio Project, Part II

The finished product:

SAS Rubicon console, WAJZ Albany, NY

SAS Rubicon console, WAJZ Albany, NY

This is the finished product from an earlier post.  Currently, it it the studio for WAJZ in Albany, but that is not permanent.  The SAS studio goes together fairly quickly, as most of the trunking between the TOC and studio is done over the SAS data channel.

The studio monitors (Tanoy Reveal) are set on little posts under the computer screens. I like this set up as the DJ’s are less likely to rock the house if they decide to crank up the volume on their favorite tune.  I am also kind of digging the lack of a table top equipment pod.  That takes up a lot of counter top space and always seems to be in the way.  There are two CD players rack mounted below the counter (lower left), which are almost never used.

New Studio project

It is time, once again, to replace some very old Pacific Recorders BMXII consoles. The Pacific Recorders consoles were very expensive when new, but after 30 years of continuous use, have more than paid for themselves.  The replacement console of choice for this installation is a SAS Rubicon.  I have installed these units elsewhere and they are the modern equivalent of the PRE BMX.

The heart of the Rubicon system is the 32KD router.  Routed audio systems can save a lot of time and effort in a large studio facility installation.  Not having to run and terminate multiple analog and digital trunk cables between rack room and studio is a huge deal in a six or ten studio installation project.

The SAS 32KD router and Rubicon console system uses a serial TDM buss to communicate and transport audio around.  This is a simpler system than packet switched IP data.  Basically, the console surface is a very large, fancy computer control interface.  Here are some pictures of the start of the project:

New Studio room, furniture installed

New Studio room, furniture installed

This is the view from the entry door. The furniture was placed last week and the counter top cut in for the console. The furniture is made by Studio Technology.  The pile of yet to be installed equipment:

New studio equipment to be installed

New studio equipment to be installed

For monitors, we are using the Tanoy 602p near field monitor placed on the table top above the computer screens.  This studio will not have a turret.  Turrets used to be necessary to hold things like cart machines and CD players.  These days the CD players are used so infrequently that it was decided to put them in the side rack under the counter top.  Turrets also take up a lot of counter top space that can be put to better use.

New studio punch blocks

New studio punch blocks

Punch blocks and power connections.  The red outlets are isolated ground UPS type, the back outlets are feed by the emergency generator power panel.  All electric wiring is inside of metal conduit.  The punch blocks are the inputs to the SAS RIO link unit, one 16 pair analog audio cable and ten category 5e shielded cables.  The cat 5e is used for computer and TDM data buss to the router.

New Studio Rubicon console

New Studio Rubicon console

The SAS Rubicon console cut into the counter top and protected by plastic sheets.

Rack room

Rack room

Rack room with 32KD routers.  This facility has 9 studios total plus a news room with three work areas.

SAS 32KD router on line

SAS 32KD router on line

The SAS 32KD router.  All audio from the automation systems, satellite feeds and other sources is connected directly to these units.  This unit is on line for other studios that have already been converted to the SAS gear.

Goodbye, ISDN

The imminent demise of ISDN has been talked about for some time.  There now appears to be a date attached which makes it semi-sort of official.  As of May 18, 2013, Verizon will no longer accept orders for new ISDN lines.  They will also not make any changes to existing lines and will start charging more for the service.

Taking the place of ISDN will be a variety of Ethernet/IP based audio transmission methods.  As technology evolves, this makes sense.  The quality of ISDN and the bidirectional nature was a vast improvement over the old system  5/7/10/15 KHz point to point analog lines.  The one downside, ISDN equipment was expensive and the service was expensive to install and operate.

High speed internet is available in almost every business and venue.  Many times, there is no cost to access it and equipment is relatively inexpensive.  Depending on the equipment, CODEC, and speed, it can sound almost as good as ISDN.  For those opposed to using the public network due to reliability issues, there is always frame relay.

Time moves on, so buy your IP CODECS now.

Sound Hound music discovery application

Sound hound, making us look smarter than we should

Sound hound, making us look smarter than we should

For all of us that work at radio stations but are not programmers, Sound Hound great app for those “WTF is the name of that song?” moments.  As I get older, this seems to happen more and more.  These are either senior moments or I am just not keeping up with the new music today.  Probably a little of both.

To use the application, one can play, sing or hum the song in question and if Sound Hound can match the audio to a known song, it will return the song title, version if more than one and artist to your mobile device.  It will also provide links to lyrics, chart information, artist concert dates and Youtube videos, which is pretty cool.

It comes as a free version with banner ads.  For those of us that hate banner ads, a paid version is available as well.

I fooled around with this for a while playing songs from youtube club videos.  If the audio is not too distorted (some of those club videos are pretty bad), it will work.  It will also pick out live performances (and include venue and date if available), club mixes, etc.

Best of all, it makes me look like a genius to my kids.  Any help I can get in that department is most welcome.

Axiom


A pessimist sees the glass as half empty. An optimist sees the glass as half full. The engineer sees the glass as twice the size it needs to be.

Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.
~1st amendment to the United States Constitution

Any society that would give up a little liberty to gain a little security will deserve neither and lose both.
~Benjamin Franklin

The individual has always had to struggle to keep from being overwhelmed by the tribe. To be your own man is hard business. If you try it, you will be lonely often, and sometimes frightened. But no price is too high to pay for the privilege of owning yourself.
~Rudyard Kipling

Everyone has the right to freedom of opinion and expression; this right includes the freedom to hold opinions without interference and to seek, receive and impart information and ideas through any media and regardless of frontiers
~Universal Declaration Of Human Rights, Article 19

...radio was discovered, and not invented, and that these frequencies and principles were always in existence long before man was aware of them. Therefore, no one owns them. They are there as free as sunlight, which is a higher frequency form of the same energy.
~Alan Weiner

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