Just what the world needs, Another 2CH mic Pre

[Dave G.]

Hello Guys & Gals

I've been hanging around learning what I can, for awhile now. It's time I put my latest project out there, and get some feedback. I hope this is the right place.

Back in 93, I converted my Bogen, MX30A, into a guitar amp. Having removed 4, TM200 transformers, I thought some day I'll build a mic preamp. Last year I found a 6DJ8, and a 12AU7, in my parts supply at work. I haven't used a tube at work in 20 years, so with the manager's permission, I took them home and started to design a pre using those tubes.

My first design, was based on a Jenson schematic, with a cascode input stage. The Jenson input transformer, had a lower turns ratio than the TM200, so I thought the lower gain of the 6DJ8 would work well. The Jenson uses a 12AX7. Though the 6DJ8 was made for a cascode, I read that the cascode, worked well for RF, but not so well with audio. So I scrapped that design, and went with a simple cascade, with a gain control between gain stages. This is followed by a parallel cathode follower.

Wanting bells & whistles, I came up with way to switch input impedance. By changing the size of the grid leak resistor, It will change the reflected Z, plus it can be used with the Hi Z input when used as a DI.

I will be back with channel 2, it is completely different than CH1. After all variety is the spice of life.

Let me know if anything looks off, or if I'm just crazy(not that my wife needs confirmation).

 

[ruffrecords]

I have a couple of comments. It is a good idea to minimise noise sources in the first stage so I would decouple the cathode resistor and significantly reduce the value of the grid stopper.

The 12AU7 as a cathode follower will have limited drive and it is single ended. You could build this stage as an SRPP and get better drive capability and have some gain too.

Cheers

Ian

 

[emrr]

Lose the grid stoppers completely, unless you find a need.  I don't have a single tube preamp with a grid stopper in it, out of more than 60 types.

You didn't say what the output transformer was, or what it will feed.  The output stage may be more than adequate. 

Investigate the real world meaning of tube gain in circuit.  Surroundings dictate actual gain, and the textbook numbers reflect theoretical maximums seldom found in reality. 

I'm guessing you don't actually have any 300K audio pots, or you are making a stepped control?

I wouldn't use a maximum value grid leak resistor, I'd have a choice of open grid.  That's what most of the oldies use.  Add Z selection steps starting with a high value after the open position.  In fact, build it without the Z selection and play with a 1M rheostat across the secondary to see if you care about the differences.  Don't waste the effort until you've heard it. 

Have fun. 

 

[Dave G.]

Thanks for your replies

You didn't say what the output transformer was, or what it will feed.

Output transformers are Edcor 1000:600ohm. Most of the time it will feed a DBX 266XL, or direct to a Delta 44.

I took out the grid stoppers.

I wouldn't use a maximum value grid leak resistor, I'd have a choice of open grid.  That's what most of the oldies use.  Add Z selection steps starting with a high value after the open position.  In fact, build it without the Z selection and play with a 1M rheostat across the secondary to see if you care about the differences.  Don't waste the effort until you've heard it.

Thanks I'll do that

I'm guessing you don't actually have any 300K audio pots, or you are making a stepped control?

I'm using Mouser part #652-PDA241-SrT01304A2

The 12AU7 as a cathode follower will have limited drive and it is single ended. You could build this stage as an SRPP and get better drive capability and have some gain too.

If I get time tomorrow i'll work on that.
The grid stoppers are gone.
Would a 22uf be good on the cathode resistor?

 

[okgb]

Looks a bit like an altec , do a survey of schematics , not to cut & paste but see common
idea's what prog did you use to make the schematic ?

 

[emrr]

I'd use at least 100mfd on the cathode, sometimes up to 470 can make the bottom look better. Measure and see what satisfies. 

 

[ruffrecords]

I'd use at least 100mfd on the cathode, sometimes up to 470 can make the bottom look better. Measure and see what satisfies.

+1 on that. My go to value is 220uF.

Cheers

Ian

 

[Dave G.]

Looks a bit like an altec , do a survey of schematics , not to cut & paste but see common
idea's what prog did you use to make the schematic ?

I remember looking at an Altec schematic back when I was drawing this up. That's probably where I got my idea.

I'm using Expresspcb
http://www.expresspcb.com/expresspcbhtm/Free_schematic_software.htm

It is intended for pcb layout. I had to make my own tube symbols, but it comes with the tools to make your own.

+1 on that. My go to value is 220uF.

I have lots of 220uf 35V caps, I'll try that first

Thanks again

 

[ruffrecords]

C11 is perhaps a little on the small side. My calculator reckons its -3dB point with 300K is about 11Hz. I would also be tempted to bypass the 12AU7 cathode.

Otherwise looking good.

Cheers

ian

 

[Dave G.]

Thanks Ian. I spent my time calculating resistor values to match the compressor. After posting the schematic I remembered that I forgot to put in the bypass capacitor.

I also changed C11 to 100nf

Channel 2 is a loose copy of a Tube Tech pre. Rather than stick with the odd size, precision resistors. I changed them to the nearest standard values. After all I don't need exact 5db steps in the gain control, and I realize copying the circuit, with different transformers and layout. It will never sound the same as the original. I also used 2 switches for the gain to cut cost.