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

[Greg]

Better yet... what is your e-mail (PM me)?

I will e-mail it to you then you can post it.

 

[rodabod]

[quote author="khstudio"]OK,
I just looked at the DINGO Schematic & it just confused me more... sorry :sad:

http://jlmaudio.com/JLM dINgO complete schematic.pdf
[/quote]

Copy IC1a, along with C1, C2, C3, C4 and R3, R4, R5, R6, R7. Inputs at C1 and C2, output at R7. R3-R6 set to 10K. R7 could either be omitted or uped to a larger value to increase the output impedance here if that is necessary (would also limit the loading on the opamp, not that that's necessary for the DOAs).

Basically what I posted before in that example. Maybe you should read about opamps on the net - in particular the "comaparator".

 

[rodabod]

[quote author="drpat"]
We're talking about driving the 600ohm load of a Pultec EQ aren't we?[/quote]

It has been pointed out that it doesn't present 600 Ohms at certain settings (since the circuit changes). Apparently it can look as low as 75 Ohms, hence the need for good current drive to avoid distortion.

 

[Greg]

[quote author="drpat"]We're talking about driving the 600ohm load of a Pultec EQ aren't we?[/quote]
I believe the concern here is with HF Boost at maximum, the preceeding driver will "see" 75 ohms reflected from the passive circuitry at the seconday of the input transformer.

Also, Kevin... coupling capacitors would be wise addition to the circuit I sent you.

 

[drpat]

[quote author="rodabod"]It has been pointed out that it doesn't present 600 Ohms at certain settings (since the circuit changes). Apparently it can look as low as 75 Ohms, hence the need for good current drive to avoid distortion.[/quote]

You're joking right? Pultec... avoid distortion? Am I really missing something? :green:

 

[clintrubber]

[quote author="drpat"][quote author="clintrubber"]The datasheet for the SSM2142 for instance goes down to 600.. 300 load, but doesn't tell much about below that.
Perhaps the other types are more sturdy, but I'd say the '2142 won't do it right by the book.... which might mean nothing in practice of course, and that 75 is worst case, but I thought we were after a 'safe on all accounts'-solution here.[/quote]

We're talking about driving the 600ohm load of a Pultec EQ aren't we?[/quote]
Yes and no...
... there's info from Jakob that the load of the passive network can be as low as 75 Ohms @ certain settings.

Again, this might be at unlikely settings and a frequency at which there's little level anyway & worst worst case, but since the tube-gain makeup stage won't make this a battery operated device anyway why not make it able to handle this 75 as well while we're at it ?

I can't argue to the hands-on-experience of people happily using these driver-chips in this position of course - as I've said before that 75 might be unlikely, but tackling it won't give me much additional work w.r.t. using a chip, so that's why I'll be using the Tori-driver here myself.

 

[drpat]

[quote author="clintrubber"][quote author="drpat"]We're talking about driving the 600ohm load of a Pultec EQ aren't we?[/quote]
Yes and no...
... there's info from Jakob that the load of the passive network can be as low as 75 Ohms @ certain settings.[/quote]

I think the key words here are "passive network"...

 

[rodabod]

[quote author="drpat"]
You're joking right? Pultec... avoid distortion? Am I really missing something? :green:[/quote]

Well, there's good distortion and there's bad distortion. I'll say no more... Other than as we always say, if it sounds good, it is good.

 

[clintrubber]

[quote author="drpat"][quote author="rodabod"]It has been pointed out that it doesn't present 600 Ohms at certain settings (since the circuit changes). Apparently it can look as low as 75 Ohms, hence the need for good current drive to avoid distortion.[/quote]

You're joking right? Pultec... avoid distortion? Am I really missing something? :green:[/quote]
It'd be sad if the 'inherent' pretty distortion got company from unpretty nasties from the solid state driver I'd say :sam:

 

[clintrubber]

[quote author="drpat"]... there's info from Jakob that the load of the passive network can be as low as 75 Ohms @ certain settings.[/quote]

I think the key words here are "passive network"...[/quote]

:roll:

What's the difference between 75 Ohms worth of load-impedance realized by a passive or active load ? Both will make a stage driving it sweat as hard.

 

[clintrubber]

[quote author="rodabod"][quote author="drpat"]
You're joking right? Pultec... avoid distortion? Am I really missing something? :green:[/quote]

Well, there's good distortion and there's bad distortion. I'll say no more... Other than as we always say, if it sounds good, it is good.[/quote]
We've been thinking along the same lines at the same time, let's reveal the rest of the community here that we are twin-brothers :wink:


I'm out of this thread now as well, cheers,

Peter

 

[khstudio]

I just tried every possible setting to get the Impedance down to 75 ohms & NO settings brought it down this low.

The resistance at the input sat around 450 ohms & didn't drift much at ALL when trying several EXTREME settings. :?

Test it yourself & then PLEASE post the magic combo for the 75 ohm load... cause I don't see it.
My unit does work properly & sweeps well with the correct input setup, so...

(Lets PLEASE not make this a big debate & ruin the point of the thread)

I've got some great feedback from everyone & learned a lot.
Now I'm going to build & test.

When I'm done, I'll post back the results to help others that may be in the same situation as me... or just like to learn. :wink:

 

[clintrubber]

[quote author="khstudio"]I just tried every possible setting to get the Impedance down to 75 ohms & NO settings brought it down this low.

The resistance at the input sat around 450 ohms & didn't drift much at ALL when trying several EXTREME settings. :?
[/quote]
Hmm, said I was out of here, but ok...

How did you measure ? Not by a multimeter on its resistance-setting, correct ? (apologies for suspecting this).

Than 75 was just based on info from Jakob, but I see no reason not to trust the validity of this.

I'm with you, let's not get overboard here, but as it seems we've been busy to make this thread with correct info and that can only be a good thing for next builders.

Cheers,

Peter

 

[syn]

[quote author="khstudio"]
Still wondering about that 620 ohm load resistor from + to GND :?:
Joe never said if they used it or not... oh well... me try it & listen/sweep. :green:[/quote]

According to RMAA I have a bit better specs on my Pultec with 620r loading the output of the "Big Bloak" feeding the input of the Pultec than if feeding it with unloaded amp...in a fact loading of 300r gave even better results than 620r, again according to RMAA, 10k (as a loading resistor) was a few dB worse/same, compared to either above... I use 4:1Stancor, Big Bloak, Pultec, Big Bloak, 1:1Stancor ...

Very nice looking unit you have. :thumb:

 

[Greg]

[quote author="clintrubber"]How did you measure ? Not by a multimeter on its resistance-setting, correct ? (apologies for suspecting this).[/quote]
Correct, we are talking impendance here. There is an inductive element to consider.

Z = R + jwL

 

[khstudio]

[quote author="Greg"][quote author="clintrubber"]How did you measure ? Not by a multimeter on its resistance-setting, correct ? (apologies for suspecting this).[/quote]
Correct, we are talking impendance here. There is an inductive element to consider.

Z = R + jwL[/quote]

AH HA!... this is where I'M outta here :green:

I was just measuring across the 620 ohm input resistor... sorry.

Either way... I feel I have enough info to get started.

Thanks to everyone for helping.
This thread may have just put me back to "Newbie Status" :razz:

 

[drpat]

[quote author="rodabod"]Well, there's good distortion and there's bad distortion. I'll say no more... Other than as we always say, if it sounds good, it is good.[/quote]

Please don't take this the wrong way, because, I am genuinely curious, and I'm not the knower of all things (or an evil doer). Firstly, what bad distortion should I be looking for when I test a Pultec's performance after the 2142 chip?

As a side note, when you use a device like this on a regular basis, you know what it's limits and capabilities are. These devices are not surgical by any stretch of the imagination. According to what's being said here, I'm quite sure that I've been patching these things wrong every day for the past twenty years. I've used them behind all sorts of IC driven sources, and I can NEVER remember saying, "gee... this EQ really sucks".

 

[khstudio]

[quote author="RogerFoote"]I have never seen a 75 Ohm load presented by the passive EQ, but it can happen at certain settings.

All of the balancer chips will crap out with a 75 Ohm load, but it sounds like they are being used that way successfully. Maybe that point where you hit very low Z is a non musical setting anyway...

I do know however, that 5532 based balacer output stages don't drive it correctly at all.

If it is really possible to get a 75 Ohm input Z, maybe a 990 is best. No nasty surprises that way.[/quote]

Thanks Roger.
You guys are probably right about the Loads crapping out but like you said... it seems to be working out for Pat. & if it sounds good I sounds good.

I plan on getting some of those Line Drivers but really don't want the 6dB boost.

If you or anyone can post a schematic of how to properly PAD them or set them up to be UNITY GAIN... I'd really appreciate it.

 

[clintrubber]

[quote author="khstudio"]I plan on getting some of those Line Drivers but really don't want the 6dB boost.

If you or anyone can post a schematic of how to properly PAD them or set them up to be UNITY GAIN... I'd really appreciate it.[/quote]
Two resistors in front of the ehh, perhaps not to be used linedriver-chip. Topology: Google for resistive divider. Use equal values or perhaps a bit larger for the lower one to take the input impedance of the chip into account (too lazy now to look up what that figure is).

Alternative can be to pad the *ultec-output, but it'll run the tube-section & coils at higher levels, you might not want that.

 

[clintrubber]

[quote author="RogerFoote"]I have never seen a 75 Ohm load presented by the passive EQ, but it can happen at certain settings.[/quote]
We might need Jakob here to comment further, since he's the sole source of the 75 of which I'll stop typing about now.

Again, let me add (for the last time) that it'll be fine for most stuff, if people are driving their filters with whatever and it sounds fine then everything will be OK, don't let me spoil the fun with that 75-figure. But me being still in my pre-finalization-G-Pultec-days myself I will be addressing it.

Kevin, if you measured with a DMM the resistance of the passive network was measured for DC, so say at zero Hz.
The assumed impedance drop happens elsewhere, at some other (non-zero) frequency, something most multimeters can't measure.
It's indeed initially confusing that Ohms are used for both. If you want to know more then there's the Meta Meta, I better not OT this thread any further.


Regards,

Peter