Kits useable with a M500 300B Monoblock amp
I recently got a pair of older (Early 2000's) Consonance M500 300B monoblock amps and after reading a lot of posts, many suggested that I could upgrade the amp significantly thru parts or upgrade kits from DIY.
I am interested in the Power supply and ABS modules, but am not sure what if any of your modules would be appropriate for the M500s.
The output transformers appear to be of suitably high quality that changing them doesn't appear to be neccesary. However, the power supply and bias system appear to be areas where I could significantly improve the overall sound and reliability of the amp if any of the modules are useable.
If anyone could point me in the proper direction of what might be useable, and possibly a preference order of what might be the order of upgrade, I could take it from there and would be extremely grateful.
Thank you in advance for anyones time and effort to help me out.
I have a copy of the Original Billy Kit in my archive which is basically absolutely identical with the M500. If Brian is agreeable, I can post the schematic, which would make discussing modifications easier.
The ABS would be an option, but then you would need to replace the mains transformer, as the PSU voltage is otherwise too high for a 300B in fixed bias.
I would not especially consider the power supply and output stage biasing the greatest weakness of the M500. The 300B Filament supply is a major achilles heel of the this design (it is a single capacitor plus a generic rectifier bridge). This puts around 900mV Peak-Peak of noise across the heater. It is cheap and works with less hum than AC heating, but the sound generally is not so great.
I would suggest you have a look if there is space to fit the filament supply Module. The space may not allow it. If you fit the Filament supply module you need to add around six turns of isolated wire on the torroid transformer to get enough voltage for the module to work.
DIYHFS Ultra Low Noise Filament Supply (Rev B2) 2-12V or 12-24v DC jumper selectable (Pair)
Installation instructions here:
Otherwise at least change the bridge rectifier for individual schottky diodes and add a pair of 0.68 Ohm Resistor in the plus and minus lines (needs trimming to get 5V at the 300B heater) and a second high quality 10,000uF capacitor directly across the heater (at the 300B Socket), to reduce noise. With this setup you will get much lower noise (over ten times improvement) and significantly better sound, though not as good as the Filament Supply Module.
The second "easy win" in terms of sound quality is to apply the full "Western Electrics" connection to the 300B Output stage.
The following is taken from Troel's Gravensen's site, who surprisingly did not find it a significant upgrade, however others differ, hard to say why it did not give significant improvements in his case.
The parts are the 47R resistor and 22uF/500V Capacitor drawn in green. The 22uF Capacitor should be the best you can afford.
The actual value of this capacitor scales with the value of the cathode capacitor on the 300B (C4 in the schematic), I have seen variations in this capacitor value in opera gear over time. In the Billy I empirically established 12uF/47uF as the correct ratio, so if C4 is 68uF the capacitor is 17uF (15uF with a 2.2uF Capacitor in parallel), for 100uF C4 it is 25.5uF (22uF in parallel with 3.3uF). I personally would likely use an Obbligate oil cap for the larger value and a gold Obbligato capacitor to make up the rest.
Obbligato Film Oil Caps
Obbligato Gold Premium Caps
Connecting in this capacitor significantly reduced the influence of the powersupply on the sound quality and it also reduced the impact of C4 (Cathode capacitor).
As the powersupply has too much voltage anyway, modifying it as I did for my 91 Project would probably stress rectifiers less and would allow you to use some exotic NOS rectifiers without risking them to blow up (which can then take out the mains transformer).
The above is taken from the DMS Audio site, which also has some details on Billy/M500 Upgrades:
Billie Ladyday 300B Kit
The SRPP Frontend in the M500 is okay. The biggest issue is the 100uF Cathode Capacitor. If this is not a Black Gate 100uF/16V try finding one or use at least 2 of the small size 22uF/35V Film Capacitors from the diyhifisupply store instead (I like these better than BG BTW).
Obbligato Film Cap 22uF 35v
You can also convert the frontend to a hybrid Mu-Follower as discussed here:
DIY HiFi Forum: hybrid mu follower by bcherry
The CCS Kit you need is this:
diyhifisupply CCS Kit
Finally I would suggest to change the coupling capacitor and to place a gridchoke at the 300B instead of the grid resistor. The Gridchoke subjectively improves dynamics and power delivery, the amplifiers play cleaner and louder.
Grid Choke pair
The coupling capacitor are a matter of taste. The Teflon & Tinfoil Capacitors deliver neutrality and detail in spades, but some find them a little matter of fact, I like them though.
Teflon Tin Foil 0.22uF 1000V
An alternative are the Audio Note Copper capacitors which have a more warm and laid back sound, some might say slightly lacking in detail and not neutral, but very musical.
Audio Note Copper In Oil 0.22uF 630V
Audio Note Silvers are the dogs bollocks, musical, detailed and pretty neutral but they cost accordingly and then some. I find the Tinfoil/Teflons a valid alternative at lower cost.
Audio Note Silver Foil 0.22uF 630V
If you do all the above and use some nice quality resistors where called for, you will get your Amp's most of the way to a premium version of the current Lady Day 300B Amp's, which are rather nice Amp's by anyones count.
Dear Herr Thunderstone,
you seem to know a bit about Brian and what he sells... some of his Chinese tubes are incredibly cheap, are they worth the money?
I do like the Shugang EL34B, it sounds very good and I like the TJ 300B's.
Consider yourself "geknutscht" and donīt tell about the ī34...as long as fellow Germans believe "itīs German steam radio, so it must be ****", it will remain wonderfully undervalued. EL84 too.
Thanks for all the info. I have a couple of questions.
It would be very easy to increase the overall depth of the chassis as I have access to some commercial equipment to add a "spacer" to the chassis that would be effectively invisible, and could make the chassis pretty much any depth I want.
If I had the room, would it then be best to put in the Univ. PS and ABS?
I would think that would allow use of the filament heater circuit without issues? I could still change out the coupling caps and cathode cap.
What do you think of this? It does raise the question of if I could fit the UPS and ABS, what mods does it "replace" in your original list? or change?
Lastly, I'm hazy on the CCS? What does it do and where does it fit into the audio scheme of the M500? From looking over the instructions, it seems to deal primarily with the driver tube (6sl7). Is it basically a circuit to furnish cleaner regulated power to the 6sl7?
I very much appreciate the time and incredible information you have provided. I really like the M500's, and already, just changing the tubes to Sophia Princess Mesh and GZ34's have made a significant difference.
As the DMS site and your own comments have mentioned, this has got to be the quietest 300B amp I have ever owned so if I can mod it to bring out its full potential, it got to really be a killer of an amp.
Thanks again for your help!!
If some of the info (like schematics, etc) are too difficult or inappropriate for the forum, please feel free to email me directly. This is fun and fascinating information and I truly appreciate your time and effort to help me.
Thorsten, another question came up as I was following your suggestions.
You mention converting to a hybrid Mu-follower using the CCS board.
Does that replace the 100uf coupling cap you mentioned replacing earlier or would I replace that and do the conversion? If it replaces it, better to convert or replace?
And I assume that you would want to replace the 100uf with 4-5 22uf to reach the same value and is it better to go higher (110) or lower (88)?
Sorry for the questions, just trying to follow the upgrade path.
Best regards, Bruce
Alternatively in order to use this you would need a new mains transformer, one with the right voltages and that fits the chassis.
So to use the FCUPS would need a lot of mechanical modifications and may still not fit mechanically.
To use the ABS would need a new mains transformer and of course the FCUPS to work first.
At the same time these two changes alone will not transform the Amplifier. In fact, I would consider them "icing on the cake" compared to the other modifications covered. Nice to have sure and it will be better, but first you need the cake to put the icing on.
And with the heater circuit remaining stock the (negative) impact of that on the sound will remain.
If using the hybrid Mu-Follower circuit you still need cathode bypass capacitors (actually, not strictly NEED, but they still help).
If you use 2pcs 22uF capacitors instead the -3dB point for the first stage becomes 5Hz, which is still very low, but the quality of the capacitors will much higher. So in this case less is more.
In my books the biggest gains will be the WE connection and improved heater supply for the 300B, the driver cathode capacitor and the coupling Cap, all of which will significantly improve tonality, transparency and musicality.
Mu-Follower and Gridchoke help, but mainly in terms of drive and large scale dynamics, which to me is actually rather important.
The suggested modifications to the Powersupply (not FCUPS and ABS) have more to do with helping the rectifiers live longer and bringing down the excess HT voltage to improve life expectancy for the 300B (with Mu-Follower and Gridchoke you get more apparent power back than you loose).
After all that the ABS and FCUPS would help further on the side of dynamics and transparency, but much less than any of the above.
In the FCUPS Manual there is a Circuit for what I call "Simple 300B Amplifier".
This in essence has all the mods suggested above included and then the FCUPS and the ABS implemented. But the mains transformer has very different ratings to that from the M500.
Thorsten, you are an angel. Thanks so much. Its a lot clearer now. I didn't know essentially who was on first, whats on second etc in terms of the mods.
So I will follow you advice and build my "cake" and then see what I have.
I suspect that the sound quality will be so much better that implementing the FCUPS and ABS will not be worth the extreme mods required to use them.
Thanks and I'll let you know how it sounds!!
Help with 300B cathode coupling cap
Thorsten, I'm lost on the 100uf Cathode coupling cap. You mentioned its c4 in the schematic. But I can't find anything like that in the amp.
There is a 10000uf Rubycon across the + and - of the rectifier, .47 Mcap supreme from the driver tube to where the negative lead attaches to the 300B, and a 200uf cap that goes across the large resistor that is in the gold heatsink. I've attached some pics to help.
If you can help point out where the cap I'm missing is, I'd appreciate it. Everything else I think I have figured out.
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