Feeler: Classic Solo

[ruffrecords]

Thanks for the insight Holger. It is good to know I am on the right track.

Cheers

Ian

 

[ruffrecords]

Holger, is the Sowter 2400:600 transformer you used the 1380?

Cheers

Ian

 

[Holger]

Sorry Ian, didn't saw this post. Yes, it's the 1380.

 

[ruffrecords]

I have now tried the prototype in the Hammond case with the custom mains toroid and using an Edcor output transformer instead of the Carnhill.. This time there is a slightly different hum problem. No hum from coupling the toroid to the Edcor as there was with the Carnhill. Instead there is a residual hum only at high gain settings. Again this is cured by changing the orientation of the mains toroid BUT the orientation required is different to that required to cure the toroid to Carnhill hum problem. SO I took the electronics out of the Hammond case and tested again. This time no hum unless you hang the mians toroid right on top of the Cinemag mic input transformer. So I conclude in this case that it is the steel of the Hammond enclosure that is coupling the toroid to the Cinemag. I seem to get this problem every time I use a steel enclsure. You would think I would have learnt that lesson by now. Anyway, I have now ordered a modushop.biz  2U Galaxy Maggiorato which is 230mm by 230mm in plan view. This is a little bit bigger than the Hammond, despite being a lot cheaper, and should ensure we can guarantee a decent separation of the mains toroid from the other transformers. The standard version has aluminium front, back and side but steel top and bottom. However, you can get aluminium top and bottom plates to replace the steel ones. I have ordered both so I can compare.

I also made a couple of small mods to the schematic. I have reduced the value of the 48V reservoir capacitor from 470uF to 300Uf. This makes little difference to the output ripple/noise but means we can use a cheaper 5mm spacing capacitor. I also realaised that quite a few people may want to use this to make a DI box in which case you do not need the phantom power.  There are plenty of small, cheap, low power  mains toroids that give 240VAC and 6.3VAC (even Carnhill do one) but none many that give 240VAC and 12VAC. So I have decide to modify the PCB so that by setting links the heaters can be powered from either a 6.3V or a 12V secondary.

Cheers

Ian

 

[ruffrecords]

I have just completed the build of the prototype in the modushop case:

https://drive.google.com/file/d/0B_n67A1hN3qtUWJHZnVHYUprT3c/view?usp=sharing

I fitted the mains toroid to the rear panel which makes it into a nice convenient sub-assembly. As you can see there is plenty of room in the box. I have routed the mic and line cables along the left hand edge to the 4 toggles PCB at the front; a couple of screened leads connect to the gain pot and another to the output transformer. Mains toroid plugs into the PCB using 0.2 inch Molex connectors. I still had a bit of hum trouble with the Carnhill output transformer but moving it to the front and bolting it the the right hand side wall fixed that. Performance is no pretty good. It is worth selecting both tubes for lowest noise.  With the gain turned right down the output noise is now -85dBu and at 60dB gain and above the EIN is better than -120dBu with the input shorted. Distortion is the same as with the original classic.

I have a few changes to do to the PCB to allow the use of 6.3V or 12V AC heaters to give more flexibility in the choice of transformer, especially if you don't want the phantom power section (e.g. for DI box or ribbon mic use). I also want to change the two up in the air resistors in the heater elevation section because I gave myself a shock off them more than once!! and I need to change the PCB mounting hole positions to line up with the slots in the bottom of the modushop case. Lastly I want to come up with an easier method of attaching the flying chassis lead to the AGND point on the PCB - basically a 1 pin connector.

I have added a ClassicSolo folder to the DIY tab of my web site with a few pics of the prototype. I need to put together some documentation and update the PCBs and then it will be ready to go.

Cheers

Ian

 

[fishdrop]

veeery interesting project  ;D
which classic pres is it similar to? to get an idea about it.

 

[bernbrue]

Hi Ian,
Although I'm quite eager to build the mic preamp I really appreciate your accurate testing and enormous efforts to improve the preamp before releasing the pcb. Excellent!

P.s.: I've noticed somewhere that you were looking for some p&g fader. If this is still the case you should have a look at this and contact me via pm.
http://groupdiy.com/index.php?topic=61171.0
Best
Bernd

 

[ruffrecords]

veeery interesting project  ;D
which classic pres is it similar to? to get an idea about it.

I call it the Classic because the design uses the same principle of fixed gain stages with intermedicate pads to set the overall gain as was used in many early early tube consoles.  A typical example would be the RCA 76 series ,RCA 3B consolette and the REDD 51. Theother reason is the design of the classic solo is very similar to one of my earliest mic pre designs which used a couple of 6CG7 tubes arranged as mu followers. In this design the 6CG7 has been replaced by 6922 tubes.

Cheers

Ian

 

[fishdrop]

veeery interesting project  ;D
which classic pres is it similar to? to get an idea about it.

I call it the Classic because the design uses the same principle of fixed gain stages with intermedicate pads to set the overall gain as was used in many early early tube consoles.  A typical example would be the RCA 76 series ,RCA 3B consolette and the REDD 51. Theother reason is the design of the classic solo is very similar to one of my earliest mic pre designs which used a couple of 6CG7 tubes arranged as mu followers. In this design the 6CG7 has been replaced by 6922 tubes.

Cheers

Ian

great
i'm really into 60s music.
mentioning those old pres and consoles turn me on this project even more  ;D

how about ribbon mics. are they gonna work god with your pre?

 

[ruffrecords]

great
i'm really into 60s music.
mentioning those old pres and consoles turn me on this project even more  ;D

how about ribbon mics. are they gonna work god with your pre?

Yes, ribbon mics will work fine with this pre. If you only want to use ribbon mics then you save a but of money by leaving out the phantom supply and buying a cheaper mains transformer.

Cheers

Ian

 

[Rocinante]

Okay so first I need to say thank you Ian. This is an amazing mic preamp.  I figured this somewhat belonged in this thread since it was a prototype 'classic'.  I had bugged Ian awhile ago and he had one if his prototype dual classic pcbs he relinquished to me. It's been working for a bit but I wanted to tidy it up before taking a few pics. I've also used it with a few clients as well as myself and didn't want to stop using it while taking it out of the rack.
Simply put this preamp is awesome.  It holds its own against my other high end preamps as well as from what i recall of other tube preamps I used in the past. I do need to get a better case because this beautiful beast deserves the best which I'll get to down the road. I honestly had been using it too much and didnt want to stop just yet.  I spent some time tweaking (obsessing) on the wiring and will probably tighten it up some more but its pretty quiet with no hum so it feels like I should leave well enough alone. 
If i were to build another I'd make it as an front end summing amp as it just is so thick yet transparent.  I've been really impressed with the detail and warmth the pre has.
The inputs are Jensens and the outputs are Carnhills. The tubes are matching electro-harmonix 6922. Everything from electric guitars to female vocals sound great through this. Really everything sounds good going through it.
So thanks again Ian for another awesome ruff records design to add to the rack. I learned a ton too.



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image posting

 

[ruffrecords]

Many thanks for the kind words. I should point out that Rocinate has  a unique dual version of the the original classic design for the 6CG7 tube.. I built one of these many years ago and sent it to a guy in the USA who was going to hawk it around and get orders for it.  After a few months it was mysteriously 'stolen; and I never heard from him again. Here is a pic of the original board assembled.

This is inside the one that was stolen:

and the front panel looks like this:

If anyone comes across this I would like it back.

@Rocinate: would you email me those pics so I can ask Pierre to add them to his gallery of DIY ruffrecords projects?

Cheers

Ian

 

[Rocinante]

My pleasure.

 

[ruffrecords]

I keep looking for low cost enclosures and just came across this one by Schroff:

https://enclosures4u.com/Electronics-Desktop-Cases/Standard-Cases/Metal-Enclosure-Case-2u-88-221-221

which is  just £16 plus tax and shipping. The only real draw back with it is that its front panel is only 1mm thick aluminium which I think is a bit flimsy but most people would probably get their own custom front panel made.

Cheers

Ian

 

[ding]

I'm in for 2 plus the switch boards. I've had a tranny sitting around to build a dual Mila but this looks fun, I like your designs and I can learn how to p2p another time. It's this one...
http://www.antekinc.com/as-1t250/
I'm thinking it should work.

I'm thinking of doing a 2u dual. I would like to add a 80hz HPF if possible, stepped input and an rotary output gain plus DI inputs and maybe meters.  Doesn't sound to involved, does it? 

 

[ruffrecords]

I'm in for 2 plus the switch boards. I've had a tranny sitting around to build a dual Mila but this looks fun, I like your designs and I can learn how to p2p another time. It's this one...
http://www.antekinc.com/as-1t250/
I'm thinking it should work.

I'm thinking of doing a 2u dual. I would like to add a 80hz HPF if possible, stepped input and an rotary output gain plus DI inputs and maybe meters.  Doesn't sound to involved, does it? 

That transformer will easily power a couple. In fact it is a bit of an overkill - it will power at least half a dozen Classic Solos. The only other thing is it does not have a 50VAC winding for the phantom power supply. If you do not need phantom power then it is not a problem.

Adding an HPF is no problem. I will be including details of it and the four toggles PCB in the documentation (it is identical to the one the EZTube MIxer version of the Classic pre.)

There is provision on the PCB for adding a DI input.

The main gain control is a straightforward pot so making this a stepped control should be no problem. At the input, the four toggles PCB provides a switched 20dB pad on the mic input. Currently there is no provision for a stepped pad at the input however I am looking at incorporating a continuously variable mic input pad to replace the fixed 20dB pad on the four toggles PCB. You could make this stepped if you want. This was discussed here recently:

http://groupdiy.com/index.php?topic=61094.msg773945#msg773945

Again, I will include details in the documentation.

If you use the usual transformer balanced output scheme then you should be able to hang a VU meter directly across the output. Be aware that a standard VU meter does have a non-linear impedance which will add some distortion to the output.

Cheers

Ian

 

[ding]

That transformer will easily power a couple. In fact it is a bit of an overkill - it will power at least half a dozen Classic Solos. The only other thing is it does not have a 50VAC winding for the phantom power supply. If you do not need phantom power then it is not a problem.

Adding an HPF is no problem. I will be including details of it and the four toggles PCB in the documentation (it is identical to the one the EZTube MIxer version of the Classic pre.)

There is provision on the PCB for adding a DI input.

The main gain control is a straightforward pot so making this a stepped control should be no problem. At the input, the four toggles PCB provides a switched 20dB pad on the mic input. Currently there is no provision for a stepped pad at the input however I am looking at incorporating a continuously variable mic input pad to replace the fixed 20dB pad on the four toggles PCB. You could make this stepped if you want. This was discussed here recently:

http://groupdiy.com/index.php?topic=61094.msg773945#msg773945

Again, I will include details in the documentation.

If you use the usual transformer balanced output scheme then you should be able to hang a VU meter directly across the output. Be aware that a standard VU meter does have a non-linear impedance which will add some distortion to the output.

Cheers

Ian

Yeah, I know its overkill but it's what I have at hand. I was thinking of getting a second small transformer for 48v. Maybe I'll build it into an external 1U PS to power 6 of these  One could only dream.  :'( I'm also considering a 1 Solo+1 REDD EQ 2U channel that might be nice.  I imagine I would also need the PMTubeMakeup if I was going that rout. I guess for the extra makeup gain I don't need extra transformers, is that right? It will all come down to Iron and how big of a hole they will put in my wallet. Looking forward to all your detailed documentation for this project.

Thanks,
Alain

 

[ruffrecords]

Yeah, I know its overkill but it's what I have at hand. I was thinking of getting a second small transformer for 48v. Maybe I'll build it into an external 1U PS to power 6 of these  One could only dream.  :'( I'm also considering a 1 Solo+1 REDD EQ 2U channel that might be nice.  I imagine I would also need the PMTubeMakeup if I was going that rout. I guess for the extra makeup gain I don't need extra transformers, is that right? It will all come down to Iron and how big of a hole they will put in my wallet. Looking forward to all your detailed documentation for this project.

Thanks,
Alain

A solo + REDD EQ is entirely possible. The good thing about the REDD is that because it does not give huge boosts its insertion loss is more modest than a Pultec for example. The Classic with its normal input transformer(s) and no output transformer has a good 76dB of gain available. Normally it is fine to add a 2K4:600 output transformer and lose 6dB of gain. If you did that AND added in a REDD EQ there should still be at least 55dB of gain available which for most applications is plenty.  To get the gain up to just over 60dB you could use a 600:600 output transformer as long as you don't load it with more than a couple or so 10K bridging inputs However, if you really need 70dB+ of gain then adding a PMTGMU is the way to go. For either of these options you do not need any more transformers.

By the way, I am working on new version of all three of my passive EQs with built in gain make up amplifiers. I have just completed and am about to test the Pultec version. REDD and Helios will follow in due course.

Cheers

Ian

 

[ruffrecords]

I have just finished the mods to the PCB described earlier and sent off for 5 prototype PCBs. I expect them to arrive in about a week. In the meantime I I have written an assembly and configuration guide. Apart from the technical and constructional details, this documents sets out the different projects you can build using this PCB from a simple ribbon mic pre to single and dual DI boxes. It is a work in progress so let me know  of anything that is missing or not clear. The document is in the Classic Solo folder of the DIY tab of my web site. Direct link is:

https://drive.google.com/file/d/0B_n67A1hN3qtRzFaOUNpVTczUkk/view?usp=sharing

Cheers

Ian

 

[ding]

Nice docs as always! Very thorough. I like all the options you built into this. HPF and continuous input pad are great. The SW PCB is really nice. Its going to be a fun build. A few questions...

1. To build as a pre with DI the DI input would go into J5 and J4 would be shorted? Or would it be plugged into J4 like the DI box? Or either? On the PCB it looks like J5 is going into the TX.
2. Can the "Drive" pot be connected to J4 even if I am building the pre just to get some dirty drive when required?
3. If I wanted to add the REDD EQ or any of your other EQs would it be before the 10k gain pot like the HPF setup?
4. I know this will mess with the impedance but is it possible to add an output attenuator after the output transformer in the style of the SR124 or in the style of the attached image?
This question is more about wanting to drive the hell of of this preamp if need be and get a little tube dirt for example on a DI bass. I know that this is a clean tube pre design and that you have taken measures to get a high gain, clean sound but options are always good. I guess if a "Drive" pot is possible than this question is moot anyways.
5. If I were to build this in a dual configuration is there a simple way to attach power, 48v and heater from 1 board to the other so as not to build 2 power supplies?
6. Again messing with impedance but if a meter is required it would go after the output TX?

I really like stepped gain pots so I would probably build a stepped 10k switch for the gain. Someething like http://diy-tubes.com/gain-switch-kit.html or P2P

Thanks,
Alain