PULTEC - Input Options -Buffers- Imp - TX - JLM Hybrid opamp

I'm pulling my hair out with this one.
I didn't realize the Pultecs were so sensitive to input loading.
When I looked at all the schematics & saw that Jacob & the Manley's had 10k resistors to GND before the circuit... I "assumed" they were using 1:1 (10k:10k) input trannys.(until I did more research )
So that's what I did... I got Cinemag 15k:15k and loaded the secondary with 10k.
After I started sweeping the unit I noticed I couldn't get the high boost to boost enough & spent days looking for wiring errors... then, the LAST thing I tried was dropping the 10k on the secondary of my input tranz to 620 ohms & BANG... the whole EQ changed & sounded great + fix the high boost... what a difference.

So now I'm a little pissed at myself for not researching this further & have two Nice Cinemag inputs that probably aren't optimal being loaded by the 600 ohm.

What I'd like to do :idea:

Build a few universal 10k:600 buffering circuits that I can patch thru when needed for my Pultecs & Black Face 1176's.
But even after researching this I'm still confused on HOW to do this.

This is a PM I sent to Matt from JLM:

Funny you mention this as Ive just finished 2 pultec type eqs for mastering similar to what you describe.

They use JLM pultec boards that we no longer make and a dual 99V MB PCB with two JLM Hybrid opamps to provide electronically balanced in and quasi balanced out. We did some rewiring and used the PAD relay to bypass the eq. I chose the Hybrid opamp to drive the eq circuit because it stays very linear with unchanged THD into any load down to 50 ohms, so it can easily cope with the low and changing impedances of the pultec circuit no matter what the settings. I was going to use a 99V for the output amp but tried some Hybrids to start and loved the sound so Ive left them in.

M

Click to expand...

Could you PLEASE tell me how you set up the JLM Hybrid opamps as the Input Buffer to your Pultecs or even better, send me a schematic that shows how to set the JLM Hybrid opamps as a buffer for this.
I just finished a dual Pultec & I need to be able to use it with my 10k inserts on my board (& other 10k gear) & with my mic pre's (which don't have a problem driving it as is.

Click to expand...

[quote author="khstudio"]But even after researching this I'm still confused on HOW to do this.[/quote]
The worst case as held in mind around here is that the preceeding stage needs to be able to drive a load of 75 Ohms well. The weird ~Namley circuit as discussed here a while ago ('certain class-A driver') might be unnecessary complex and/or not even work healthy, but as far as I remember it the Tori-amp from PRR came out as a nice, cheap, easy & very suited circuit:

one ordinary IC-opamp, one TIP-BJT and a power resistor & some small stuff and you're done. It might not be what you're asking for but I myself would look no further. And yes, it's a lot of class A if that was a requirement :wink:
You could reconfigure the opamp for balanced in or preceed it with a now uncritical TX.

Regards,

Peter

I would just like to make the Pultec's inputs as universal as possible.

But what I really want to do is set it up "Stock" with 600:600 input trannys & loading... then have some sort of a "Buffering box" to go between my consoles insert sends (TL074's - Un-Balanced 10k) to the Pultec - 600 ohm balanced input. I'm not even sure if it HAS to be balanced coming out of the "Buffering box" because the input TX on the Pultec can handle either... like joining pin 3 to 1 for unbalanced signals.

How should I wire this to the input if I decide to remove the input TX's all together? seems very simple & a "Modern" way to do what I need.
http://jlmaudio.com/JLM Hybrid Opamp circuit.pdf

I have some 2604's here... can't I just make a basic buffer circuit out of them to go between my 10k gear to 600 ohm gear?

& How?

[quote author="khstudio"]I have some 2604's here... can't I just make a basic buffer circuit out of them to go between my 10k gear to 600 ohm gear?[/quote]

I don't think the OPA2604 would be very happy driving the 75 Ohms that the Pultec looks like at certain settings. The JLM hybrid would probably be fine with this though (as well as other discrete opamps which can drive plenty current).

I think you could simply debalance via your transformers then configure the opamp as a unity gain buffer with 10K input impedance and 600 Ohms out if I'm understanding your problem correctly.

What is the "Tori-amp from PRR" :?:

I did a search & found nothing.
Post a link if you can.

[quote author="rodabod"]

I don't think the OPA2604 would be very happy driving the 75 Ohms that the Pultec looks like at certain settings. The JLM hybrid would probably be fine with this though (as well as other discrete opamps which can drive plenty current).

I think you could simply debalance via your transformers then configure the opamp as a unity gain buffer with 10K input impedance and 600 Ohms out if I'm understanding your problem correctly.[/quote]

Yes... I have 2 options here:

1- Make a buffer to go between my board & Pultec (which is what I prefer)
2- or Leave my 10k trannys on the input of the Pultec & build a buffer circuit after it.

The problem is I don't know HOW to do it... i need some help or a schematic that shows me what's up.

I also have ALL the components to make the JLM Hybrid Amp but need help with that too.

The JLM schematic only shows the AMP, not the surrounding components or connections I need to make.

Thank for helping.

Here's how steffen did his solid state pultec.

Is Steffen just presenting the output impedance of the opamps then (~0 Ohms) instead of providing a 600 Ohm source as described?

KHStudio I think you could possibly just do a standard de-balancing stage like you see in figure 9 here:

http://www.dself.dsl.pipex.com/ampins/balanced/balanced.htm

You'd maybe want something like 3u3 caps on the hot and cold input off the top of my head, and set the JLM opamp to use the servo so as to cancel any DC offset at the opamp's output. I'd maybe consider strapping a 600R resistor to the output to present the "correct" source impedance, but I'm not familiar with how the Pultec interacts.

in general , i wire xlrs for balenced and 1/4"s for unbalenced ,
so you can have both options available , in & out
you could of course add a switch in the back for loading or
make a patch cord with the resister in it .
If it's mostly at your place you'll discover the combinations
soon enough , but resisters on the outputs of pultecs
[ and some neve modules ] has been one of those secrets for a long time

[quote author="khstudio"]What is the "Tori-amp from PRR" :?: [/quote]
A design PRR did, originally for phones I think.

I did a search & found nothing.
Post a link if you can.

Click to expand...

Search outside this site as well, here's more:


from http://headwize.com/ubb/showpage.php?fnum=3&tid=2649&fpage=1

And using a power-BJT i.s.o. a FET:
http://headwize.com/ubb/showpage.php?fnum=3&tid=4272

Greg, Kevin is talking about the input here... not the output.

Kevin, I am currently building one of these. I am doing what you describe as option 1. I am building the Pultec with 600:600 input transformer. I have other 600 ohm gear. I am going to build an external box for interfacing with modern gear.

[quote author="Greg"]Greg, Kevin is talking about the input here... not the output.[/quote]
Hi,

In case this was meant for the Tori above, yep, despite being a headphone driver it was here meant to drive the input of the Pultec. Overkill possibly, but it won't make the box bigger and better safe than sorry - FWIW, the mentioned worst case 75 Ohms won't suit various other driving schemes. Again, FWIW, can be a rare setting.

somebody suggested using a 10k/600 xfmr in front of the pultec and increasing the gain makeup by 12dB. i cant think of a more straightforward way to deal with the low-z input problem of the pultec.

a lot of devices dont have the sack to drive the pultec filter section directly- its a difficult load. i wonder if the 'characteristic sound' could be partly from driving devices distorting under the load!

there was definitely a thread about input buffers for pultec.

aha- found it

http://www.groupdiy.com/index.php?topic=134221

Oh, sorry for the interuption , carry on !

Sure and i was generalizing about inputs and outputs ,
as the oppositte of overkill .

Kev had said he didn't know if loading the the cinemag with
600 ohms was the best thing to do for it , but no one said
spoke firmly about if it was detrimental or not .

come to think of it I have a gpultec that i was never happy with ,
[ Lundalh in , no output xfmr ]
sure you could hear hi boost but it was almost as if the was a blanket
above that , I put it aside thinking it was the design or parts ,
No comparison to even a tube tech which is quite satisfying

[quote author="Greg"]Greg, Kevin is talking about the input here... not the output.

Kevin, I am currently building one of these. I am doing what you describe as option 1. I am building the Pultec with 600:600 input transformer. I have other 600 ohm gear. I am going to build an external box for interfacing with modern gear.[/quote]

Cool, I'm not alone. :green:

How are you going to to it?
I just got off the phone with Patrick (DrPat) & he mentioned the balanced line drivers & receivers like:
SSM 2141/2142 (THAT) or DRV 139
Said he uses them to do the same thing & that they drive the Pultecs well.

I found out first hand how much the Pultec is affected by the input loading.
I do not understand why or how Jacob used a 10k secondary loading resistor on his input... BUT, I did finally find a few post from Jacob saying how the input loading DOES directly affect the Pultecs circuit... mostly affecting (& what I first noticed) the maximum Hi Boost being "Limited"
I could only get about 11dB out of mine with the 10k input load.
But then it jumped to about 17-18dB after changing the input resistor to 620 ohms. :shock:

3 damn days of tracking it down & go figure... it was the FIRST thing I though about but the LAST thing I tried. :sad:

Kev had said he didn't know if loading the the cinemag with
600 ohms was the best thing to do for it , but no one said
spoke firmly about if it was detrimental or not .

Click to expand...

The loading of Cinemags with the 600 ohm resistor is MORE a problem with the fact that they are rated 15k:15k (High impedance)
Yes they work but are probably not optimal with the 600 load.

It's not the transformers "Specs" that are affecting the Pultec as much as the actual LOAD resistor that the Pultec circuit WANTs to see to work correctly...
I thought I could use 10k:10k TX with the 10k to GND to make it more universal in my studio... but I learned the hard way & should have done more research FIRST!
The Gyraf & M@nley schematics confused me... showing that 10k to GND.

To sum things up...

I'm going to build my Pultecs STOCK... with 600:600 Input TX's & the correct 620 ohm secondary loading.

Then build some Unity gain Impedance matching boxes that I can patch thru that can except high (10k) impedance outputs
& drive LOW impedance inputs of older (sensitive & load demanding) gear like the Pultec, 1176's & LA2A's.

The BEST (& easies/cheapest) way to do this is still undetermined.
But I am starting to gather positive info.

Help out if you can. :thumb:

The SSM 2141/2142 (THAT) or DRV 139 line drivers & receives are looking like the way to go!!!

[quote author="drpat"]Kevin,

The AD SSM2142 is the balanced line driver that I spoke to you about. Stick that between your unbalanced insert output and the 600Ω transformer on your Pultec, and you should be golden. I e-mailed you the datasheet for this chip...[/quote]

Thanks Patrick,
I have been looking over all the line driver data sheets... very cool, cheap & simple.
I didn't get your email yet but I did find it.

Is it really as simple as just adding power to the 2142 & hooking up the I/O?
Or are there other needed component to add... Caps, Resistors, etc...?
I know you told me but I've been chewin on all my options & forgot if you mentioned any else... sorry.

Using the "External" buffer is what I'm mainly shooting for, so I can keep the input stock = 600:600

*** I've got a few recommendation to use a 10k:600 TX but I must say I feel it's got too many drawbacks & I also believe it won't "HIT" the input enough... not to mention the gain loss.***

Joel (JC) also gave me a nice simple buffer circuit to try & since I have all the parts in stock, I will try it first... & If it works & it's OK with him, I'll post it to help others.

...I must say, I'm also interested in trying Matt Moogus's idea too...

Using the JLM Hybrid opamp in place of the input TX. It sound like a great idea... I just don't know how he hooked it up. I'm waiting for his or Joe's response to my email.