Another tube preamp/lineamp circuit

[sismofyt]

There's no such thing as original design today. Everything have been done before :wink:

I'm sure that no manufacturer uses the same design as me. My latest and greatest uses a WE417A & 12B4A combo (for a mic pre) but a mfg would be daft to use that for a commercial pre. So they don't.

Rip off or no rip off, there's nothing new under the sun. (Unless you're into DSP)

You can rip off what ever you want. Behringer got away pretty cleanly.

It's just paranoia :roll:

Maybe someone discovers a cool way to do a currentsource or whatever, but I'm sure it's been done before. I haven't seen anything here, or at the ol' place, that's _new_.

Relax. The majors are tied up in cheap opamps and fancy faceplates :wink:

 

[PRR]

> no such thing as original design today. Everything have been done before

In another field: about 10 years ago, car engine designers got stoked about "Stratified Charge". You can't ignite a lean mixture with a spark; the concept was to have a layer of rich mixture above a cylinder full of lean air.

I picked up a 1930s engine book, and found that Ricardo had built it back then.

The current "progress" in audio design is all along Moore's Law. Chips double in capacity every 18 months. This is great for repetitive systems like memory, and helpful for CPUs. It seems to be no big help in straight analog design: we don't want a lot of circuits, it takes more than 18 months to think of a good use for double the devices, and we typically need a lot of I/O and control (pins) rather than a lot of guts. DSP and PCs are the exception... kinda takes the hands-on out of audio.

> If they can understand the idea and then bring it to production at a price that is fair...

I think both the DIYers and the Companies need to face reality. Ideas are cheap. Good ideas are worth a little. Polished designs may even be worth money, but only IF you can produce it and MARKET it and sell in enough volume. Start-up cost of a product is very high, and design isn't a large part of that. Getting the word out to a thousand or a million potential customers, so that a hundred or ten-thousand of them BUY that darn box, is usually the hard part. You have to buy ads, you have to sneak it past magazine reviewers, you have to get on the good side of influential people, you have to bully and bribe your way into GuitarCenter and other mega-marketers to get shelf-space and flyer-space and sales-staff awareness.

For anybody to significantly "profit" from the ideas here, they need a LOT more investment to get the box sold and the cash collected.

 

[NewYorkDave]

Had a little lab time this evening, and I just can't stop tweaking! :guinness:

Here's another schematic.
9.8kB GIF

I incorporated PRR's suggestions to remove the input grid stopper and increase the size of C1. I made some other minor changes to double the headroom of the input stage. The performance is the same otherwise.

 

[bluebird]

Hey Dave,

Why the 12bh7 and not a 12au7? more output? Is this an improvement on the SRPP stage in the G9? I think I'm going to try this one.

 

[NewYorkDave]

The 12BH7 is beefier than the 12AU7, and the White Cathode follower is better at driving low-Z loads than SRPP.

 

[rafafredd]

Why are you using the pot between the 2 first stages instead of the global feedback/gain control? Does it sounds better this way, or is there another reason?

 

[Guest]

Did you omit the 1MEG grig resistor on V1b?

 

[NewYorkDave]

Raf: I did it so that you wouldn't ask me where to put a volume pot :wink:

Seriously, though, there are two typical approaches. One way is to select your tube to give just the gain you need with acceptable linearity without the use of overall feedback. The other approach is to use as much gain as possible and then enclose it in a feedback loop to force the gain to the desired value, reducing distortion and dependence on individual tube characteristics in the process. Both approaches are valid, but this preamp is an example of approach #1. Basically, my design goal was to build a decent preamp/lineamp using the cheap and cheerful 12AV7, and this seemed the most effective use of the tube. For maximum versatility with a number of different mics, I was aiming for 40dB of gain exclusive of the input transformer stepup, to give a maximum net gain of up to 60dB.

If I were following the second approach and needed 40dB or more of closed-loop gain, there are better tubes to use than the 12AV7, the 12AX7 being the obvious example. A good rule of thumb to follow is that your open-loop gain should be at least 20dB (and preferably as much as 40dB) above your desired closed-loop value of gain. Two 12AX7s in cascade can give as much as 66dB of open-loop gain.

This can be reconfigured for overall feedback; but since the open-loop gain of the circuit is fairly low--about 46dB--it would only be worth doing if your required closed-loop gain is 20dB or less (including the 6dB loss of the output transformer). With a 1:10 mic input transformer and 20dB of gain in the amplifier, it could be used as a 40dB mic preamp. Here's an example of the previous version of the circuit with overall feedback configured as a line amp, with a level pot on the input:
http://electronicdave.myhosting.net/miscimages/lineampD2.gif
For mic preamp use, you would eliminate the pot.

Even without overall feedback, the distortion is low, and is almost entirely second harmonic up to the clip point. As a gross generalization, it would tend to sound "warmer" or "more tubey" or whatever--at least at higher output levels, as distortion rises--as opposed to a cleaner, more open sound that you'd get with properly-applied overall feedback. (Cleaner and more open until the point of clipping, that is). :wink:

Tom: Yes, I did take out the 1M grid resistor on V1B. This helps to net a tiny bit more output from V1A. But it doesn't make a substantial difference--it can be put back in with no noticeable detriment.

 

[rafafredd]

[quote author="NewYorkDave"]Raf: I did it so that you wouldn't ask me where to put a volume pot :wink: [/quote]

Yeah, but you couldn´t get away without answering one more of my questions. :green: And I need to thank you one more time!

:thumb:

 

[pstamler]

[quote author="NewYorkDave"]Tom: Yes, I did take out the 1M grid resistor on V1B. This helps to net a tiny bit more output from V1A. But it doesn't make a substantial difference--it can be put back in with no noticeable detriment.[/quote]

And by putting it in, you get a lot less noise when the pot gets old and scratchy, as all pots eventually do. Slight rattle rather than speaker-busting BANG.

Peace,
Paul

 

[zebra50]

This looks really good! Don't have anything to add really, except that I am reading and learning lots from this discussion.

And I'm reluctantly coming round to thinking that this separate design area is a good idea.

:thumb:

 

[NewYorkDave]

The latest iteration with overall feedback:
http://electronicdave.myhosting.net/miscimages/nydave_lineamp1.gif

As per Paul's comment, a fixed grid resistor on V1A in addition to the pot would be a good idea. Replace the pot with a fixed grid resistor if you add an input transformer and use it as a mic preamp (or no grid resistor if your transformer is designed to work into an unterminated grid).

 

[bovox]

Why should I try to change a very nice circuit, thanks NY-Dave.
But... I have a few E80CC that I thought actually could be put in
stead of the 12 BH7 ??. They are not similiar, but have you
tried them in a WCF stage? They should have a little less R i than
the 12 AU7. Cheers Bo

 

[bovox]

Another thought about the splendid N-Y Dave pre-amp.
I´ve used the E88CC on both places with good results.
Havent loaded the output down to 600 ohms, but nearby 2k ohms.
The R8 i changed to about 900 ohms, I actually put 500ohm in series with 470ohms. R7 is also raised to 330 ohms.
The dc voltage around the circuit is pretty close to Dave´s when I
calculate to his B+ supply. Actually mine is 230V where his is 283V.

At cathode V1A the dc is 3.0V ,that gives you +7dBu max input. I´m
using 2 output´s one with and one without Tx. The output Tx is an Altec
15356 wired backwards. 2:1. The one without Tx cannot ,of course, be loaded under about 10kohms.
I´m using MicInput Tx 1:4 from Jörgen Schou, a Danish builder. His not in the busieness any more,but NTP and even Tube-Tech used his stuff.
I fortuned that you certainly need clean DC to get it quiet from hum and noise. The 100ohms bridge to DC ground made huge differences.
Well thank you Dave and PRR for this fine subject.
Cheers Bo

 

[cannikin]

Hey NYD

I'm starting to build your Tube DI.. better sound Good! :grin: HAHAHA

Probably have a few hundred questions for you though :?

 

[NewYorkDave]

What's my "Tube DI?"

By the way, regarding my line amp circuit, someone pointed out to me (correctly) that the heather-cathode voltage rating of the 12BH7 would be exceeded if a ground-referenced heater supply was used. So the heater supply should be biased to about +50VDC.

http://groupdiy.twin-x.com/albums/userpics/10031/heaterbias.jpg

 

[cannikin]

[quote author="NewYorkDave"]
This could be used as-is for a lineamp or an instrument DI, or add a good 1:10 mic input transformer for a 0-60dB mic preamp. [/quote]

:grin: :grin: :wink:

I figured this would be a good place to use my UTC A-11 thats collecting dust on the shelf.

whatcha think?

 

[bovox]

Thanks Dave, for pointing the heating bias out.
If ? I wanted a little less couloration, is there any point to put
in a series resistor on the cathode of V1A, not bypassed with C1.
I get a little less gain of course but the benefit could also be a little more max level on the input.
Cheers Bo

 

[NewYorkDave]

I figured this would be a good place to use my UTC A-11 that's collecting dust on the shelf.

Ah, OK. So you're looking to build the version without loop feedback, then.

http://electronicdave.myhosting.net/miscimages/lineampE.gif

Add a 1M resistor from Pin 7 of the 12AV7 to common, even though it's not shown on the schematic. (I'll add it later if I have the time).

Just for reasons of lower THD, I prefer the version with loop feedback, myself, but that gives 20dB less maximum gain. Try the loop-less version first and if it's quiet/clean enough, so be it.

Bo:

Yes, you can eliminate the cathode bypass cap on the first stage, if you like. But this will make that stage more susceptible to hum pickup unless you use a DC heater supply.

 

[cannikin]

thanks Dave.. much appreciated.. As Always - you rock.

Got quick non-related question for you: Would it be possible to take a 270-0-270 power transformer and strap some power resistors to the secondary to make it a 175-0-175?