Pultec question (extended into pultec alternatives)

A stupid question perhaps, but can the passive bits of a pultec(sans tube stage) be driven by say 436. Since the latter has crazy gain I mean, and can even be used as a mic pre. Also has 10K(or is it 15K?) input.

I'm sure there is some way to do it. But you will probably need make sure you terminate the inputs/outputs with the right value resistor so the frequencies don't change on you.
AC

eskimo said:

A stupid question perhaps, but can the passive bits of a pultec(sans tube stage) be driven by say 436. Since the latter has crazy gain I mean, and can even be used as a mic pre. Also has 10K(or is it 15K?) input.

Click to expand...

I believe that with Pultec-type circuits, the gain is "made up" AFTER the EQ, so you don't "drive" the circuit, the amp makes up the gain after the EQ.  Someone with more knowledge of Pultec should chime in.
Best,
Bruno2000

Ah, sloppy choice of word, I meant use the 436 for make up gain.

yes the pultec has a make up gain amp.

I think he means using the altec 436 compressor as a make up gain stage.

He should just be able to slap a 600ohm terminating resistor on the output of the pultec circuit and then wire it straight into the 436 without too much trouble. no?

Try it. I'm curious

abechap024 said:

He should just be able to slap a 600ohm terminating resistor on the output of the pultec circuit and then wire it straight into the 436 without too much trouble.

Click to expand...

The response will be completely out of wack like that. A lot of bandwidth lost, gain too, quite unnecessarily. You don't just slap a resistor at random on a passive filter network output.

Pultec passive section likes to work in to 5-15K loads, maybe even higher. I'm not familiar with 436 compressor at all, but if you make the input a bridge from, say, 10k to whatever that 436 first tube stage likes to see, you'll be fine.

input transformer -> passive filter network -> 10k primary/into whatever the 436 likes to see secondary (interstage transformer) -> first 436 tube stage.

Make sure your wiring and shielding are absolutely top notch.

By the way make sure that "crazy gain" is enough for this task. a pultec type filter network needs at least 20-25dB of make up gain to get it at unity.

The interstage would be a utc A-18, but yeah, the 436 might not have enough juice, wouldn't be fun if you weren't able to push it into compression...

Anyway, was just an idea, might try it out.

Nevermind the previous idea, I think I'll build a buffer amp anyway. I'm interested though in alternatives to the pultec, a bit simpler if you will. Something like a baxandall, cut/boost shelf highs and lows(perhaps switchable bell-shelf) and a cut/boost mid with some frequency alternatives. The NYD went secret I suppose, so no info there.
Plenty of simple bass + treble design. I'm thinking passive, but wouldn't mind looking at some active ones either.
Oh, and I'm talking tube design.

What are there?

Kingston said:

The response will be completely out of wack like that. A lot of bandwidth lost, gain too, quite unnecessarily. You don't just slap a resistor at random on a passive filter network output.

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Ahhh I stand corrected

AC

eskimo said:

Nevermind the previous idea, I think I'll build a buffer amp anyway. I'm interested though in alternatives to the pultec, a bit simpler if you will. Something like a baxandall, cut/boost shelf highs and lows(perhaps switchable bell-shelf) and a cut/boost mid with some frequency alternatives. The NYD went secret I suppose, so no info there.
Plenty of simple bass + treble design. I'm thinking passive, but wouldn't mind looking at some active ones either.
Oh, and I'm talking tube design.

What are there?

Click to expand...

Yeah, me too I'd like to know how to build a passive valve EQ design. I'd like something like the pultec but with 4 bands. More for mastering. The NYD would be perfect... but that was before my time.

Is it as simple as putting 4 LRC networks in series for the 4 bands and then adding makeup gain? I could work that out, is there anything else to watch out for?

J

eskimo said:

a bit simpler if you will. Something like a baxandall, cut/boost shelf highs and lows(perhaps switchable bell-shelf) and a cut/boost mid with some frequency alternatives.

Click to expand...

That's not simpler than the standard pultec EQP1 network. It's actually nearly double the complexity with the mid band cut and boost. More than double the cost with all the added inductors. Much more make up gain needed. Much more attention needed for layout.

There's an overload of material in the passive EQ META thread. Time to start reading.

If you want simple, don't do it passive.

A Pultec EQP1 has a nominal attenuation of 22dB and needs to see a load equal or higher than 40kohms.
You can't make a 4 band passive EQ by connecting 4 LCR sections and a make up amp.
Two reasons:
a) each section needs to see a high load impedance and be driven by a low impedance
b) if you put 4 bands in series, you get more than 80dB attenuation, so the signal will be lost in noise
That's why you have to have a make-up gain amp/buffer for each section.

Kingston said:

eskimo said:

a bit simpler if you will. Something like a baxandall, cut/boost shelf highs and lows(perhaps switchable bell-shelf) and a cut/boost mid with some frequency alternatives.

Click to expand...

That's not simpler than the standard pultec EQP1 network. It's actually nearly double the complexity with the mid band cut and boost. More than double the cost with all the added inductors. Much more make up gain needed. Much more attention needed for layout.

There's an overload of material in the passive EQ META thread. Time to start reading.

If you want simple, don't do it passive.

Click to expand...

I guess I mean simpler control wise rather than circuit wise.
I found this active circuit, which might be fun to try out, no selectable mids though.


http://www.angelfire.com/electronic/funwithtubes/Amp-Tone-A.html

Looks pretty cool actually. I have been looking for a new tube EQ project for some while. There's some great ideas in that website. Thanks for the link. I'm not too fond of the 12AX7 output stage for example, or 12AX7's in general (might as well do solid state instead of this boring tube), but the filter networks or similar variants all seem like good candidates for prototyping. This network along some 6SN7 and 6SL7's and transformers would turn to a badass EQ.

eskimo said:

I found this active circuit, which might be fun to try out, no selectable mids though.

Click to expand...

This circuit is called a Baxendall, after the name of its inventor. The basic circuit existed before, but he was the first to include it in a feedback loop, which solved a number of problems.
This one has an additional MF circuit. But it is very different in terms of performance than the Pultec. The slopes are very soft (about 4dB/octave for the shelving sections and about 4 octaves wide for the MF) so you cannot think of using it for surgical EQ, just broadband sweetening. It has its purpose...

Yeah, I'm not looking for surgery at this moment. Mainly it's for recording with a 4038, needs some extra air and low attentuation at times.

Btw on a circuit like this one, where's the output impedance at? (ball park)

Roughly I am guessing 50K. Why? Because it is driving into 500K load and I am assuming rule of thumb of x 10. Otherwise a bit of a calculation required. However, it is quite a complex network there. I was about to go to bed as I have been working till very late in the last week, but let's see if I can work it out before I fall a sleep.

Edit. I have re-modelled it but not tonight. I am going to bed.

Without doing any heavy lifting, we can say 100K or greater based on the 0.1 coupling cap, for full frequency audio.    Since we see 500K as an output shunt, it really wants to see that or higher; really the next tube grid.