Mbira

Can you help me turn this tube headphone amp into a preamp?
« on: January 28, 2007, 10:07:27 PM »
I built a Cavalli-Jones headphone amp a few years back and honestly have used it once.  I really have no need for it as a headphone amp, but it sure would be cool to turn it into something I could use!  

Any of you gurus know what would be required to turn this into a usable preamp-either as a mic pre, or even a guitar/bass pre with line level out would be cool!

I'm thinking that really I just need a different output transformer.  Is this correct?

Oh, here it is:
http://mysite.verizon.net/vze22yzp/id25.html
Joel Laviolette

Rattletree   |  https://www.rattletree.com
The Rattletree School of Marimba | https://www.learnmarimba.com


walter

Can you help me turn this tube headphone amp into a preamp?
« Reply #1 on: January 29, 2007, 01:29:39 AM »
I didn't see an output transformer on that schematic, is there one on your unit? Have you tried using it as a pre-amp the way it is? Maybe add an XLR jack, phantom power, and a tone circuit.
Blown like a fuse

rafafredd

Can you help me turn this tube headphone amp into a preamp?
« Reply #2 on: January 29, 2007, 12:22:01 PM »
It will work nice as is with no output transformer, if you don´t need balanced output. In fact, the output will be pretty low Z, with a 6DJ8, and if it can drive most headphones, it will drive ANY line input, for shure.

It´s already an intrument preamp, or active DI, witha line output, as you might call it.

For a mic preamp, just add a good quality input transformer, and maybe pad, phase, phantom garbage, etc...

Mbira

Can you help me turn this tube headphone amp into a preamp?
« Reply #3 on: January 29, 2007, 12:56:25 PM »
I realized after I wrote that that there is no OT. :roll:
So what impedance do line inputs like?  The neg feedback is variable right now (and effects impedance, right?).  I wonder if I'm better off leaving it as is, or putting a fixed resistor there?

Thanks guys!  THis is exciting!
Joel Laviolette

Rattletree   |  https://www.rattletree.com
The Rattletree School of Marimba | https://www.learnmarimba.com

NewYorkDave

Can you help me turn this tube headphone amp into a preamp?
« Reply #4 on: January 29, 2007, 01:32:32 PM »
The voltage gain is pretty low. Also, the ~10K input impedance would be too low of a load for the secondary of a mic input xfmr unless the turns ratio was very low (1:3 or less).

You could use it as a mic pre with a 1:2 input transformer but the net gain would only be enough for hot mics.

Of course, if you're willing to rewire it into a different circuit, the options multiply substantially. There's the MILA and a hundred others you could implement with two 9-pin tubes and the same power supply.

Mbira

Can you help me turn this tube headphone amp into a preamp?
« Reply #5 on: January 29, 2007, 02:13:53 PM »
Quote from: "NewYorkDave"
Less talk.  More action.


New years resolution? :green:

I like it.

Poverty tells me:
I have no money for an input transformer right now.
I have no money for a different PT right now.  

Guess I'll bugger this into a DI thingy.
 :green:
Joel Laviolette

Rattletree   |  https://www.rattletree.com
The Rattletree School of Marimba | https://www.learnmarimba.com

NewYorkDave

Can you help me turn this tube headphone amp into a preamp?
« Reply #6 on: January 29, 2007, 02:25:54 PM »
You don't need to change the PT. The one shown in the schematic is fine.

As for the input xfmr... Check your PM box.

Mbira

Can you help me turn this tube headphone amp into a preamp?
« Reply #7 on: January 29, 2007, 03:24:17 PM »
Thanks dave.  I think I'm gonna check me out some MILA!
Joel Laviolette

Rattletree   |  https://www.rattletree.com
The Rattletree School of Marimba | https://www.learnmarimba.com

CJ

Can you help me turn this tube headphone amp into a preamp?
« Reply #8 on: January 29, 2007, 08:44:28 PM »
Wire it into a customers Fender Concert.
When he comes back with his friends, because they want the same sound that "Mibra-Master Tech from Planet Austin" gave him, soak em for the big cash.


Is Jovitas still open with the CHB playing every thursday? i guess I could get on th4 net,Doh!
Need to know your neighbor's toliet paper preferance?
Get on the net.  :green:
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

Mbira

Can you help me turn this tube headphone amp into a preamp?
« Reply #9 on: January 29, 2007, 08:53:33 PM »
:green:
Ah CJ, you always put a smile on my face!  Dude, check yer PM!  I did some groveling over there...
Joel Laviolette

Rattletree   |  https://www.rattletree.com
The Rattletree School of Marimba | https://www.learnmarimba.com


rafafredd

Can you help me turn this tube headphone amp into a preamp?
« Reply #10 on: January 30, 2007, 03:07:18 PM »
really, I would get rid of the feedback, put a volume control between stages, and very high ratio mic input transformer, like 1:18 or so, and maybe a 470k grid resistor at the input. Minimum work and you have now a nice mic preamp, with enought gain for most uses... OK, maybe not for tracking mosquitos with a ribbon mic...

NewYorkDave

Can you help me turn this tube headphone amp into a preamp?
« Reply #11 on: January 30, 2007, 03:17:16 PM »
If you went that route, you'd have to provide DC bias for the upper grid in the output stage as well. You could do this with fixed bias from a voltage divider... or you could do it like the White cathode follower in the LA2A output stage, using cathode bias.

Without looking up the curves for the 6DJ8: a single common-cathode medium-mu triode stage with a high-Z load can usually manage between 20 and 30dB of gain; then figure on 20 to 26dB of step-up from the input xfmr.The gain should be adequate for most medium-output mics as long as you don't need to use a stepdown output xfmr--in other words, if you don't have to drive a low-Z line.

rafafredd

Can you help me turn this tube headphone amp into a preamp?
« Reply #12 on: January 30, 2007, 03:31:34 PM »
Yes! Were was my mind?

I would use an extra cap and two more resistors for fixed grid bias...

Like cap, pot, cap, bias, grid...

I say it because I really like a simple gain stage followed by a white cathode follower, with no overall feedback, and this amp has all it takes to easily become something like this.

Alternatively, you could keep the original bias circuit, and terminate the input transformer with a pot for volume control. Some people will say bad things against doing something like this, but I did it many times, and it also works cool for most applications. Then the only thing you will need is a proper pot, that will depend on the input transformer you will use, and get rid of the feedback path and you are rocking...

Also, without the feedback and with no headphones to drive, you might also take that big 470uF cap out and use something like a good 20uF polypropylene, or maybe, if you are only driving 10k loads or higher, any common 2.2uf - 4.7uf polyester will do better than the best lytics out there.

Mbira

Can you help me turn this tube headphone amp into a preamp?
« Reply #13 on: January 30, 2007, 03:43:10 PM »
For this moment, I'm thinking another guitar and bass DI type thing will be really helpful.  Maybe more in the future...

OK, my beginner question of the minute.  How do I determine the output impedance of this circuit?  I looked in the tube datasheet online and I didn't see it.  

The delta says it can take a minimum 10K impedance.  

 :guinness:
Joel Laviolette

Rattletree   |  https://www.rattletree.com
The Rattletree School of Marimba | https://www.learnmarimba.com


 

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