Want to build tube mic pre without buying kit. Suggestions.

[gibsonripper]

I would like to build a tube mic-pre using transformer in/out. I know which Jensen XFMRs to use...

HOWEVER: I want to build a CLASS-A, ALL TUBE, TRANSFORMER COUPLED, NO CAPACITORS IN THE SIGNAL PATH, point-to-point mic-preamp....
and without buying someone's kit, since I don't mind building from scratch.

I built a solid-state pre-amp using someone else's popular (and excellent) design just from the schematic. I used point-to-point design (!) instead of a circuit board and it sounds and works excellent.

I spent about 50% of what I would have paid had I bought the kit for that particular design. While this is not for most people, it is my preference.

I'm just looking for any suggestions or feedback on this exact objective outlined above.

Thanks very much.

[JohnDavisNYC]

i like your message board name. i have a really wonderful old gibson ripper. they are great... under-rated basses.

that is all... i know nothing about tube amp design...

john

[GravityRobert]

I take it you've been to prodigy-pro... check out the 'tube' section in this mic pre meta
http://www.prodigy-pro.com/forum/viewtopic.php?t=16909

[A-Barr]

You check out the schematics section on the Jensen website? They may not be cap-less, but they look really good and are designed around the transformers you'll likely be using, I'd think.

[ckeene]

how do you even do a tube amp without having DC blocking caps? I'm not doubting it can be done, just proclaiming my ignorance!

[???????]

I was wondering the same thing. Maybe there's another way of blocking DC that I'm not smart enough to know about.

[A-Barr]

Here is one way.

The grid of the second tube is at the plate voltage of the first gain stage, which means making the first gain stage's plate voltage pretty low - a pretty big compromise, I would think.

You could DC block with transformers but it sure will cost a lot.

[???????]

and wouldn't transformers introduce just as many non-linearities as blocking caps?

[A-Barr]

and wouldn't transformers introduce just as many non-linearities as blocking caps?

Yup!

I can understand the desire to use fancy topologies, but when you look at all hte fantastic gear that uses "mundane" (for lack of a better word) topologies, it falls to the side I think. Blocking caps didn't hurt the V76 or Ampex, or any other classic piece of gear.

[gibsonripper]

You check out the schematics section on the Jensen website? They may not be cap-less, but they look really good and are designed around the transformers you'll likely be using, I'd think.

Yep, I've checked it out. Actually, their design is really good, because it is simple. The less parts, the better. And they know they specs of their own product so well that the impedance is sure to be matched, thereby having the least amount of possible distortion and tonal variations. I think I'll just start with those schematics and expand from there, if necessary.

[gibsonripper]

how do you even do a tube amp without having DC blocking caps? I'm not doubting it can be done, just proclaiming my ignorance!

Direct coupling and transformers.

Many old (really old) radios had no coupling capacitors. The used a combo of direct coupling and transformers.

I'm going to attempt it this way.

[gibsonripper]

I was wondering the same thing. Maybe there's another way of blocking DC that I'm not smart enough to know about.

transformers block DC, however, they aren't an ideal choice, as too much DC current in either the primary or secondary, cause undesirable anomalies.

[gibsonripper]

and wouldn't transformers introduce just as many non-linearities as blocking caps?

Actually, a good transformer (and design) will have less non-linearities than a blocking cap. A capacitor (high end or not) still exhibits some phase shift (albeit slight) and small to moderate amounts of ESR.

How much less? Probably not enough for our ears to detect on average, so obviously we draw the line at some point as transformers cost a lot compared to caps.

I wonder if any one has built an all tube mic-pre with an XFMR at the plate of the final tube stage? hmmmm....

[gibsonripper]

and wouldn't transformers introduce just as many non-linearities as blocking caps?

Yup!

I can understand the desire to use fancy topologies, but when you look at all hte fantastic gear that uses "mundane" (for lack of a better word) topologies, it falls to the side I think. Blocking caps didn't hurt the V76 or Ampex, or any other classic piece of gear.

Good point. I may just use a cap for the last stage to the o/p xfmr. I think I'll save the experiment for the next project.

[???????]

Don't transformers introduce phase shift as well, when implemented in the audio path?

I have right here in front of me a data sheet from a Jensen transformer I just used in a microphone build, and on the graph that shows "deviation from linear phase," it's up about 1.2 degrees at 20 Hz, with a gradual slope, linear from about 1k on up. And this is a nice transformer.