Feeler: Classic Solo

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
How about building a miniature replica RCA console using one of these:

http://www.newark.com/hammond/scem351619wh/enclosures/dp/94F4787?ost=94F4787&mckv=sXgvRXFbB_dc%7Cpcrid%7C76495671515%7Cplid%7C%7Ckword%7C+hammond++scem351619wh%7Cmatch%7Cb&selectedCategoryId=&CMP=KNC-GUSA-GEN-SKU-MDC&gclid=Cj0KEQiA4qSzBRCq1-iLhZ6Vsc0BEiQA1qt-zlZE3CgmkEhFP99AFxrOOanj-1kdokvqkcFz3hFoW8IaAhgC8P8HAQ

Ignore the picture, this is the 14 inch wide version. All aluminium construction so easy to work. I think you should be able to squeeze three EZTube Mixer boards and a PSU in there. You could have six simple tube mic pres in a box or four mic pres and a couple of DIs.

Cheers

Ian
 
The V2 PCBs arrived in the post yesterday. I will build one up  and probably fit it into the Hammond box. I have got a small Carnhill mains toroid that provides 30mA at 240VAC and 6.3VAC at 1 amp - just enough for the Classic Solo. I think it is the Carnhill VTT2202. Handy because it has dual primaries so it works world wide. I plan to make this one a DI box/ ribbon mic pre so I do not need the phantom supply. However, I do need to make sure the small mods I made to the phantom power supply work properly so I will still build it. I have a tiny toroid I can use to power it.

Cheers

Ian
 
I have just finished putting together the V2 prototype configured as a DI box. I tried it with my Yamaha guitar. Try as I might I could not get the input stage to distort, much less clip, it has so much headroom. No problem winding up the output stage but the actual output level is then enormous. Now I know why the REDD DI has a pot on the secondary of the output transformer! Time to dig out a 1K log pot.

Cheers

Ian
 
Hi Ian,
Did you cascade both tube stages or did you use only one? I'm going to build a dual DI with one single board and would use a 1 Meg Ohm in front of the tube.
Best
Bernd
 
bernbrue said:
Hi Ian,
Did you cascade both tube stages or did you use only one? I'm going to build a dual DI with one single board and would use a 1 Meg Ohm in front of the tube.
Best
Bernd

I cascaded the two stages. I did not measure the output level from the first stage but I could not hear it distort when I played my guitar. However, the output level must have bee quite high because the level control between stages had to be turned down really low to stop the second stage clipping. I will make some more precise measurements and post them here.

Cheers

Ian
 
Last night I did some more detailed tests using my Lindos test set. The set up was that  the first stage is loaded only with the 10K pot. The second stage drives an Edcor 2.4K/600 transformer loaded with 600 ohms.

I fed a -10dBu 1KHz sine wave into the DI input and adjusted the 10K pot for 0dBu at the output. The measured distortion was 0.08%. I then increased the input level to 0dBu and again adjusted the pot for 0dBu at the output. This time the distortion was  about 3 times higher at 0.23%. This is the normal distortion proportional to level you expect with a triode. However, as the first stage has about 29dB gain, the level across the 10K resistor is about +29dBu (about 22V rms), the first stage must be getting close to clipping. I then increased the input to +6dBu and the distortion rose to 0.57% so still roughly proportional to level. So I increased the input to +12dBu and the distortion shot up to 7.1%. With +12dBu input, the level at the 10K pot should be +41dBu which is 87V rms!!! I measured across the pot with my DVM and it was 80V rms which is +40dBu so it is within a dB of what was expected.

The bottom line is that the mu follower loaded with 10K can output a huge level before it really starts to distort.

The next test was with low input levels but with the gain pot turned up so we get higher levels form the output. So I started again with a -10dBu input and the gain pot set for 0dBu at the output and a distortion of 0.08%. I increased the gain pot so the output level rose to +10dBu and the distortion rose to 0.29%. At +15dBu out it rose to 0.43% . At +17dBu it was 0.63,and at +20 it was 6%. At+21dBu this shoots up to 24% so clearly this is where clipping occurs.

All this means is that, as expected, the distortion you get and the level at which you get it depends mostly on the load. With a 10K load, each stage can output a phenomenal +40dBu before clipping. However, with a 2K4 load (the 600 ohms reflected back via the Edcor transformer) it starts to clips at +20dBu output. For a single stage with a transformer, as in a dual DI unit, you need an input of about -10dBu or 300mV to reach clipping. Most modern guitar pick ups will achieve this level. SO, I would recommend loading the output with 600 ohms and adding an output attenuator to set the level and use your 1Meg as the DI input to set the drive level.

if you find you need more distortion at lower levels than there are other options. The mu follower is designed to give as little distortion as possible. If you change it to an SRPP stage and lower the quiescent current then you should get quite a bit more distortion at lower levels. If that is of interest I will add the details of how to do it to the build document.

Cheers

Ian
 
This sounds like an amazing project! Just wanted to confirm that the Classic Mic Pre V3 from the PCB Emporium is the PCB that's being discussed here in this thread. If so, I'm definitely ordering some.  ;)

Best,

Martin
 
madriaanse said:
This sounds like an amazing project! Just wanted to confirm that the Classic Mic Pre V3 from the PCB Emporium is the PCB that's being discussed here in this thread. If so, I'm definitely ordering some.  ;)

Best,

Martin

The one being discussed is a new PCB. It is the same audio circuit as the Classic Mic Pre V3 but with a built in power supply so you can easily build a stand alone mic pre or DI box. Check out the documentation in the classic solo folder of the DIY tab of my web site:

http://www.customtubeconsoles.com/diy

Cheers

Ian
 
madriaanse said:
Very nice! Where can I buy the PCBs?

M.

As soon as I have finished testing the latest version PCB I will get a batch made and offer them for sale through my web site. I'll post here when they are available.

Cheers

Ian
 
I'm putting a mouser cart together for this project. C3 has the wrong Farnell number, it should be the same as C7. Can you point me to an insulated 4 position phono jack socket? I'm having a hard time figuring out how to find this? It's the last thing on my list.  :D Also the 2.4k/600 Edcor has a CT. Leave it Un-connected right?

Alain
 
ding said:
I'm putting a mouser cart together for this project. C3 has the wrong Farnell number, it should be the same as C7. Can you point me to an insulated 4 position phono jack socket? I'm having a hard time figuring out how to find this? It's the last thing on my list.  :D Also the 2.4k/600 Edcor has a CT. Leave it Un-connected right?

Alain

Thanks for finding the error. I have a number of other changes to add so I will include that in the next issue. Switched jack socket here:

http://uk.farnell.com/cliff-electronic-components/s4bnb/socket-1-4-jack-switched-2pole/dp/148267

Yes, leave the CT unconnected on the Edcor.

And thanks for creating a Mouser cart. When it is done I am happy to include it on the web site if you wish.

Cheers

Ian
 
No problem I will. About the jack, is this just a 2 position switched jack? I thought it needed to have 4 positions or at least 3. I guess I didn't get that from the diagram on the manual. It looked like it needed at least 3.  Or maybe I just don't understand the data sheet from the manufacturer.
 
ding said:
No problem I will. About the jack, is this just a 2 position switched jack? I thought it needed to have 4 positions or at least 3. I guess I didn't get that from the diagram on the manual. It looked like it needed at least 3.  Or maybe I just don't understand the data sheet from the manufacturer.

It's a regular  unbalanced mono jack for guitar use. You only need one switched contact on the tip contact. If it is a pure DI box then you just use this contact to short the input when the plug is pulled out. If it is a mic pre with a DI input, you use the contact to disconnect the mic transformer secondary when you plug into the DI socket and to switch back to the mic transformer  when you unplug. If that is not clear I will add a more detailed description to the documentation as it is crucial that builders understand how this works.

Cheers

ian
 
Yes, I see. Thats what I get for doing these things late at night.  :eek:

Here is the mouser cart
http://www.mouser.com/ProjectManager/ProjectDetail.aspx?AccessID=327003dea4

I'm not sure if I'm sharing it right. I used all 1w resistors on the main board except for the ones that needed to be over that. I also picked several 6k8's and 3k3's so I can match them and several 150r's for R9 to get the closest to 150 on the switch board. There is 1 backorder item on there I noticed this morning, the dual 1k pot for the pad.
 
I have not checked the cart thoroughly but I did notice a 68 ohm resistor and 22nF capacitor that are not on the schematic.

The linked worked fine.

Cheers

Ian
 
Those are 220nf for the filter switch and 68r for the input pad pot. This is for the full upgrade with high pass, DI and input pad pot. I wouldn't doubt there are some mistakes in there but those 2 are ok. I can make you a version just for the main board if you like.
 
ding said:
Those are 220nf for the filter switch and 68r for the input pad pot. This is for the full upgrade with high pass, DI and input pad pot. I wouldn't doubt there are some mistakes in there but those 2 are ok. I can make you a version just for the main board if you like.

No problem. They just stuck out is all. I had completely forgotten about the variable input pad. Since that is an option it might be better if it were separate. There will be lots of variations so it will be hard to make a one size fits all BOM. I would appreciate it if you would do one just for the main board.


Cheers

Ian
 
I have now finished all the tests on the V2 PCB prototype which I have built as a DI box and ribbon mic pre. I am pleased to say it all works and in the Hammond box I abandoned previously. I do not know how I did it but when I had the initial hum problems, I got it into my that the Hammond enclosure had a steel top and bottom. It does not, it is aluminium throughout. The hum problems were due to pick up in the output transformer. The Edcor XSM2.4K/600 appears not to have this problem so it would be my recommended output transformer. The only other difficulty I encountered was when using cheap plastic XLR connectors for the mic input. These have no connection to the shell of the plugged in XLR and so its shielding does not work. I found this out when trying to measure the EIN and started getting hum pick up. Changing to a good quality metal bodied XLR fixed that problem.

So, the V2 PCB is OK so I shall place an order for 20 or so. They should be here in a couple of weeks. In the meantime I will update the documentation. When the boards arrive I will list them in the emporium. Price will be £14.

Cheers

Ian
 
Back
Top