Poor Man' s Tube Mic Pre Channel Strip (Eurochannel)
- Thread starter ruffrecords
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ruffrecords
Well-known member
I have now tried connecting a Carnhill VTB2291 wired as a 2:1 step down to the output of the tube mic pre. I tested it at 2KHz and a 600 ohm load with 10V rms across it (+20dBv or +22dBu). Distortion products were:
2H -63dB
3H -57dB
4H -70dB
5H -76dB
6H and beyond were commensurably low.
So the THD is still around 0.2% but the relative amplitudes of the 2H and 3H have changed a bit most likely because the load seen by the amp is now about 2K4 rather than the 2K7 I used for the tests without a transformer. Overall gain is obviously 6dB less
2H -63dB
3H -57dB
4H -70dB
5H -76dB
6H and beyond were commensurably low.
So the THD is still around 0.2% but the relative amplitudes of the 2H and 3H have changed a bit most likely because the load seen by the amp is now about 2K4 rather than the 2K7 I used for the tests without a transformer. Overall gain is obviously 6dB less
ruffrecords
Well-known member
naku said:
All the documentation for this project is here:
http://www.ianbell.ukfsn.org/EzTubeMixer/docs/EzTubeMixer/
By the way, I am currently working on a backplane PCB for this project.
Cheers
Ian
rafafredd
Well-known member
I don't see tube sockets on the PCBs... Are you using stand alone pins?
ruffrecords
Well-known member
rafafredd said:
Yes, I am. They are listed in the assembly notes. They are 1mm pins made by Harwin, Farnell part number 149319.
Cheers
Ian
ruffrecords
Well-known member
I have just finished the draft layout of the backplane PCB. It is not yet complete but I have managed to squeeze in a pair of Neutrik XLRs for mic an line inputs per channel so it could be used rather like a 500 series rack. It is rather large (16.8 inches by by 5.1 inches) and it needs to be thicker than usual for strength so it is not cheap. In prototype quantities is comes in at over 60 GBP each. I cannot afford to keep significant stocks of this PCB so I was thinking of open sourcing the Gerber files so home etchers can make their own. It is double sided but the majority of the tracks are on the solder side so you could probably get away with a single sided version and some wire links.
Cheers
Ian
Cheers
Ian
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chrispsound
Well-known member
Is it possible to use just the mic pre section with a 12AX7LPS , 6922, Jensen 1:10 input transformer and Edcor 10k/600 output trans. If so then you would not have to include the 10uf filtering electrolytic between stages? Would you ground the unused pins from the unused half of the 12AX7 or just strap them together. Have you tested it in this arrangement? I don't need a whole lot of gain and I love the design and parts count. Thanks, ChrisP
Let me know if you want me to post in the EZ Tube Mixer thread instead of here, it just seemed more appropriate.
Let me know if you want me to post in the EZ Tube Mixer thread instead of here, it just seemed more appropriate.
ruffrecords
Well-known member
chrispsound said:
This should be possible but you would need to make a few alterations to the PCB to make it work. At the moment, the two 6922 tubes have their heaters wired in series and the 12AX7 has its heaters wired for 12V. If you use just one 6922 you would need to rewire the heaters for 6.3V operation.
The HT decoupling is still required because the common cathode first stage has a poor power supply rejection. I have been considering a stand alone version of this amp with a single 6922 and the two halves of the 12AX7 wired in parallel for better noise performance. I have not tried this yet but I see no reason why it should not work. The resistor values in the 12AX7 circuit will need tweaking a little but if you just use one half then you can simply ground the plate , grid and cathode of the unused half as you suggested. This will work fine with Edcor 10k:600 transformer but you will lose some headroom - a 2k4:600 would be better and give you 6dB more headroom and gain.
I think this is exactly the right thread in which to post this question.
Cheers
Ian
chrispsound
Well-known member
Thanks Ian for the response. I was wondering if it would be OK for me to do a point to point version (for my own personal use) of the mic pre section of your Eurocard topology. I am an extremely poor man at the moment but would like to reimburse you someday(when employed) the cost of at least the PCB. I have just about everything to make a point to point version and can post my results here as well. I understand that a lot of time and money goes into designing building and refining audio gear and I appreciate everything you post. Thanks, ChrisP
ruffrecords
Well-known member
chrispsound said:
Just to be clear Chris, all my circuits are free for individuals to build for personal use any way they like. If you want to build a point to point version of my mic pre that is absolutely fine by me. All I would ask is you send me a couple of pictures of it and post them here too if you like.
Don't worry about the time and money I put in. I am retired and live comfortably on my pension without having to work. Tube audio electronics is my hobby and the PCB sales just about cover the costs so I get to indulge in my favourite pastime more or less for free. If as a result others get to enjoy it too than that's a happy bonus as far as I am concerned.
Enjoy!
Ian
rmaier
Well-known member
Just one of the many reasons I love this place! 
chrispsound
Well-known member
Built the mic pre section with the Jensen JT-115K-E input and Edcor 4:1 output transformer. 12AX7 (half) and 6922 tubes. This is a clean pre (probably should be, being in a mixer) with a sound that is pleasing to my ear. At the level that I like to record vocals at the noise floor is around -91 dBA and THD @ .08 (very unscientific testing using RMAA), and to my ear this seems close. I am experimenting with a pot in between stages so I can use the gain switch as a range switch like the NYD MILA pre. I put in a 500K log pot but it only controls volume until half way and then ceases to be effective. I will try a 250k pot next. In RMAA when I look at the frequency response (multitone) graph there is a slight half decibel rise that starts at 10kHz and ends at 20kHz. I notice that same bump at 10 kHz in another tube micpre ( NYD's one bottle) I have built using the Jensen JT-115K-E and Edcor 4:1 output transformer. They all sound nice to me but it would be nice to figure out where that bump is coming from. Maybe the output Iron? Thanks again for the mic pre design and another tool to bring happy electrons into my A/D converter. ChrisP
A very nice build! Nicely done turret boards always look like work of art to me, I should put some on the wall
Did you connect the input tube in parralell, or leave one side unconnected? I dont think a pot will work inside the feedback loop...Ian will shurly chime in on this.
Did you connect the input tube in parralell, or leave one side unconnected? I dont think a pot will work inside the feedback loop...Ian will shurly chime in on this.
ruffrecords
Well-known member
@crisp bass
A level control between stages will not work because there is negative feedback around them. The pit reduces the gain and the NFB compensates. If you want a level control, the simplest way to doing is at the output. Just add a 1K log pot across the transformer secondary and take the output from the slider and the bottom of the pot.
Cheers
Ian
A level control between stages will not work because there is negative feedback around them. The pit reduces the gain and the NFB compensates. If you want a level control, the simplest way to doing is at the output. Just add a 1K log pot across the transformer secondary and take the output from the slider and the bottom of the pot.
Cheers
Ian
chrispsound
Well-known member
Didn't NewYorkDave put a pot in between 2 stages with negative feedback around them in his MILA-1? I put a 100K pot between the stages and switch the gain switch between 48dB-57db and it seems I get some versatility in the sound and no output sound when the pot is turned all the way down and high gain when it is turned up(seems to be functioning properly). I will definitely try the output pot and compare sounds. I am still getting a bump around 10kHz with or without the pot between stages, any ideas? Thanks again Ian, ChrisP
And Ian, no big deal but I don't think "crisp bass" would like me "chrispsound" associated with him because he is way smarter than me.
Joechris thanks, that turret board setup is just my universal prototype 2 bottle board in my beater box, I love the turrets because you can keep on using them, you can remove them or add them as well, if you drill them out, save them and use them on a thinner board.
And Ian, no big deal but I don't think "crisp bass" would like me "chrispsound" associated with him because he is way smarter than me.
Joechris thanks, that turret board setup is just my universal prototype 2 bottle board in my beater box, I love the turrets because you can keep on using them, you can remove them or add them as well, if you drill them out, save them and use them on a thinner board.
chrispsound
Well-known member
"Didn't NewYorkDave put a pot in between 2 stages with negative feedback around them in his MILA-1? "
No he didn't. Took a closer look. :
So if I strap the 12AX7 in parallel the cathode and plate resistor must be halved, but since this is a different setup at the cathode which resistors should be halved? Will the bypass cap be OK left the same? Thanks
No he didn't. Took a closer look. :
So if I strap the 12AX7 in parallel the cathode and plate resistor must be halved, but since this is a different setup at the cathode which resistors should be halved? Will the bypass cap be OK left the same? Thanks
ruffrecords
Well-known member
chrispsound said:
Do you have a 150K load on the mic transformer secondary? For the Jensen, Rz needs to be 150K and Cz needs to be a shorting link.
Cheers
Ian
ruffrecords
Well-known member
chrispsound said:"Didn't NewYorkDave put a pot in between 2 stages with negative feedback around them in his MILA-1? "
No he didn't. Took a closer look. :
So if I strap the 12AX7 in parallel the cathode and plate resistor must be halved, but since this is a different setup at the cathode which resistors should be halved? Will the bypass cap be OK left the same? Thanks
I am working on a version right now where you wire the two halves of the 12AX7 in parallel as this theoretically lowers the noise by 3dB. It would make a nice low noise point to point design - but you already know that! I need to work out the component value changes. Hang on. I'll post them here as soon as I work them out.
Cheers
Ian
