My next project -- Heavy

[CurtZHP]

LS-10 and LS-50 are far too nice to build a fuzzbox. 

Of that I'm keenly aware. 

I think that's part of the problem.  Some of this circuit may be based on guitar preamp notes I had around (and had used on an earlier project for guitar and bass...).
I'm going to take a break and rethink this whole thing.

 

[abbey road d enfer]

OK, this has me scratching my head....

According to one set of calculations I did, I came up with a gain of 30 on the first stage (and pretty much the second stage too, since it's all but identical...).  Then I found another formula that factors in the cathode bypass cap, and when that's factored in, the gain goes way up to 55!  That's just on one stage!

You have 20+dB in the input xfmr and 30+dB in the first triode. That is enough to bring your mic level to a level that's compatible with most outboard gear. That is if you buffer the signal with a voltage follower. If you don't, you may lose about 20dB in the mismatch between the triode plate and the load.
The 1st stage without cathode decoupling has about 30dB gain; with cathode-decoupling, gain increases by about 4-5 dB.
The output stage, with a 12AU7 has less gain than the first, about 25dB. The output xfmr steps down 5:1 (-14dB), so the overall gain from insert RTN to secondary is about 10dB.

 

[emrr]

Look at RCA BA-2, would have to modify to parallel feed (See OP-6 output), could be done with 1 or 2 (humbucking) Hammond chokes.  Look at Gates SA-70.  Look at Collins 6Q-1.  Look at Altec A420. 

 

[CurtZHP]

You have 20+dB in the input xfmr and 30+dB in the first triode.

That's the other Amateur Hour mistake I was making, forgetting to consider the input transformer.  (Duh!)
(Previous build didn't have an input transformer...)

 

[CurtZHP]

OK, let's take it all apart and start over...

Forget the insert point, forget the EQ stage, forget the whole "channel strip" idea.  Like abbey suggested earlier, just hook it up to outboard stuff for that.

Anyway, I hacked this up this morning.  My sticking point is the second stage.  Right now, as you can see, it's basically a "copy-paste" of the first stage.  I know it's wrong, but I'm not sure how to make it right.  Cathode follower instead?  12AU7 for that stage instead of 12AX7 (lower gain factor...)?

Crunched the numbers for the first stage, and while a couple components might be overkill, I think I got that part right.

Lucky for all of us I don't do this for a living!  :-[

 

[abbey road d enfer]

My sticking point is the second stage. 

Just drive directly your SRPP output stage from the pot.

 

[CurtZHP]

Just drive directly your SRPP output stage from the pot.

Would it be possible, or advisable, to do an output stage with just one triode rather than two?

 

[emrr]

Look at 12DW7

 

[CurtZHP]

Look at 12DW7

Very interesting!  Didn't know such a thing existed.
So, I could use one side of it for the first stage, and the other side for the second stage.
Or....
If I scrounged another set of transformers, I could just use a 12AX7 for two input stages and a 12AU7 for two output stages, right?  Or two 12DW7's, one for each channel.


I suppose if I stayed with a SRPP output stage (needing two triodes...), I could do stereo with one 12AX7 handling the first stage of both channels, and then each channel gets its own 12AU7, both sides of which would form the SRPP for that channel.  So, three physical tube packages altogether.

My previous question was more about whether to have a SRPP output stage or something else that just used one side of one tube.

 

[emrr]

Single tube stage output would be the standard classic thing with an LS-50, which already has overkill response and inductance specs.  It needs less help than almost any transformer I can think of.  A true 15K source will still deliver spec, unlike many others. 

 

[CurtZHP]

Single tube stage output would be the standard classic thing with an LS-50, which already has overkill response and inductance specs.  It needs less help than almost any transformer I can think of.  A true 15K source will still deliver spec, unlike many others.

So, I can get away with a single stage 12AU7?  At what gain so I'm not blowing things up elsewhere?  Like Abbey suggested, my original plan was going to cause radiation burns!

Since the input transformer is providing considerable boost right out of the gate, a 20-30dB first stage will already give plenty of gain.  So, the second stage would only need to put out about 5-6dB, right?

 

[ruffrecords]

You don't really need two stages of 12AX7. SRPP output is a good idea because it has excellent drive capability. Here is the circuit of one half of my Eurochannel. It will provide up to  54 dB gain via a 2:1 output transformer (and 60dB into a 10K load with a 1:1)

Cheers

Ian

 

[abbey road d enfer]

  So, the second stage would only need to put out about 5-6dB, right?

Correct; that would take into account losing 14dB in the OT, so you need about 20 dB gain for the stage.

 

[CurtZHP]

Correct; that would take into account losing 14dB in the OT, so you need about 20 dB gain for the stage.

So, 20-30dB for the first stage and 20 for the second?

Here's what I have so far...

 

[emrr]

C1 can be 0.1mfd.  Consider using a choke instead of R6, which looks rather high in value as-is.  50k would be more typical I think....as I ride down the road in a car......choke will vastly improve max output compared to resistor.  There are good reports using a pair of the cheap Hammond chokes in series and physically oriented for humbucking to increase L and lower hum potential. 

I don't have a problem with 12AU7 for the whole thing, and I haven't met a 12AX7 I like the sound of in a mic preamp. Ian OTOH does like them, and has not liked 12AU7 for preamp front ends I think. 

A lot of this depends on how 'brown' sounding you want the extremes to get.  Ian's design should be significantly less brown, if that's what you want, though that will vary somewhat depending on the gain setting.  Something like you've drawn last can sound both clean and euphonic and also lean into  very forgiving overload.

 

[abbey road d enfer]

So, 20-30dB for the first stage and 20 for the second?

Here's what I have so far...

I would use smaller values of R4 and R6 for a higher idle current and thus better headroom. Try with 1k and 33k, for about 5mA idle.

 

[CurtZHP]

Thanks, guys!  I think we're starting to pin this down.

Got the basic circuit in place.  I changed R4 to 1K and R6 to 47K (sort of a compromise between emrr's and abbey's suggestions...).

It looks like I may have found another set of transformers, so a stereo preamp may be possible.

Now, as far as added features....
Phantom power is a no-brainer.  48V supply through a matched pair of 6.81K resistors, upstream of the input transformer, yes?
If I wanted to add a HPF at around 80-100Hz, where would be the best place to put that?  Before or after the gain pot?
And if I wanted to include an instrument input (switchable jack south of the input transformer...), would I then need a grid resistor?

 

[emrr]

LS-10 can take phantom as a 3K4 into the primary center tap, if you want. HPF after V1 before pot if you are going for more than 6dB/oct.  If 6 is all you want just make the coupling cap or V1 cathode cap variable.  Offhand I can't say I've thought lately about the tradeoff of one versus the other.  I like the 33K over the 47K, like I said, in a car......

 

[abbey road d enfer]

Phantom power is a no-brainer.  48V supply through a matched pair of 6.81K resistors, upstream of the input transformer, yes?

correct

If I wanted to add a HPF at around 80-100Hz, where would be the best place to put that?  Before or after the gain pot?

I would put it before, so the signal is hot enough to avoid noise issues, and the impedance there is stable enough.

And if I wanted to include an instrument input (switchable jack south of the input transformer...), would I then need a grid resistor?

Yes, even if only to make sure the grid is never left floating.

 

[abbey road d enfer]

LS-10 can take phantom as a 3K4 into the primary center tap, if you want.

That's right; solves the problem of finding 6.8k resistors matched better than 0.1%.

  after V1 before pot if you are going for more than 6dB/oct.

Going for more than 6dB/octave makes things more complicated, for a result that is not drastically better. Cascading RC cells doesn't increase slope arithmetically. RLC is expensive. Alternative is reconfiguring topology Sallen & Key style. Not easy here, would probably require one more tube.

  If 6 is all you want just make the coupling cap or V1 cathode cap variable.  Offhand I can't say I've thought lately about the tradeoff of one versus the other. 

Two problems with cathode cap:
- it takes big capacitors, 'cause you don't want electrolytics in that position, too much distortion, too high tolerance, too much variation with temperature.
-does not make a real HPF; it's a LF shelf of limited amplitude (max 10dB)

I like the 33K over the 47K

+1