squarewave

Pultec HLF3C Correct Source and Load Impedance
« on: October 02, 2017, 12:33:57 AM »
I'm doing a Pultec HLF3C:



but I'm not using transformers. This will be driven by and buffered by a pair simple op amp circuits. It now occurs to me that this will not work correctly without setting the source and output load impedance.

The question is, what sort of resistor network should I add before and after this to get the correct response? How do I mimic the impedance of the input and output transformers of the original unit using op amps? Just a 600R in series with the input and 600R to ground on the output? Do I want to throw some caps in here somewhere? Then I suppose my op amp buffer needs +6dB makeup gain?


ruffrecords

Re: Pultec HLF3C Correct Source and Load Impedance
« Reply #1 on: October 02, 2017, 03:10:25 AM »
These filters have a characteristic impedance, usually 600 ohms. This means they need to be fed from a 600 ohm source and loaded with 600 ohms. So your series resistor at the input and parallel one at the output is correct and you will need 6dB gain make up for unity overall.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

EmRR

Re: Pultec HLF3C Correct Source and Load Impedance
« Reply #2 on: October 03, 2017, 06:22:15 PM »
It wouldn't matter if you had transformers or not.  Just to say it out loud. 

Find my posts with curves about Langevin and Altec filter set response.  There's a great Altec white paper about filter loading impedances, and how they vary per circuit type.  That's online too. 

Shortest answer is drive Z + series buildout to reach intended input Z, shunt R across output to reach intended Z.  But for some circuit types none of that matters.  Build and measure reality. 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

squarewave

Re: Pultec HLF3C Correct Source and Load Impedance
« Reply #3 on: October 03, 2017, 07:31:03 PM »
I just noticed another potential issue. This is going to be the EQ section in a channel strip that will be Neve 1073 -> insert -> LA3A -> HLF3C. But when the insert and LA3A are bypassed, it up to the Neve BA283.P output to drive the HLF3C. I just tried loading the BA283.P output with a 600 ohm load in LTSpice and the bottom swing clips below -2.5V or so. Oops.

Of course with the filters out the load is 1.2k which is no problem. But with a filter in, the load can dip to a little over 700 ohms or so (but then goes back up and beyond 1.2k - so the 700 ohms is a narrow band). And using something other than 600 ohms, like 1k for example, changes the damping / Q (or whatever the correct term is). So it needs to be very close to 600 it seems. My plan was to use 680 as a compromise to alleviate the load slightly without affecting the Q much.

So now I'm wondering if I need to buffer the input of the HLF3C as well. The 1073 has an inductor EQ driven by BA284.M or P outputs which don't look they have great drive capability either (although it looks like controls have to be in the extreme position to have a low load).

What do you think? Maybe a little narrow band distortion is a feature and not a bug?

ruffrecords

Re: Pultec HLF3C Correct Source and Load Impedance
« Reply #4 on: October 04, 2017, 04:16:17 AM »
A Neve 1073 has the 2N3055 output stage (BA283AV) and a transformer balanced output that will drive +26dBu into a 600 ohm load:



Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

diggy fresh

Re: Pultec HLF3C Correct Source and Load Impedance
« Reply #5 on: October 05, 2017, 09:28:53 PM »
I built a hlf3c without transformer. The loading affects the "resonance" of the filter. At 600ohm the cutoff is normal and as you go up you see a little bump on the knee of the curve. I added a multipos switch to tweak it, works like a charm.  I suggest testing it out, adjust to taste.
English is my second language, apologies

Autodidact audio geek, constantly learning.

Very interested in old technology, i mostly learn from the info you guys share on here, so thanks!

squarewave

Re: Pultec HLF3C Correct Source and Load Impedance
« Reply #6 on: October 07, 2017, 06:34:45 PM »
I built a hlf3c without transformer. The loading affects the "resonance" of the filter. At 600ohm the cutoff is normal and as you go up you see a little bump on the knee of the curve. I added a multipos switch to tweak it, works like a charm.  I suggest testing it out, adjust to taste.
Yeah, that's a good idea. I should have added two toggles for "peak" options. Although that's not the best name maybe. Maybe "RESONANCE". The HLF3C is otherwise so devoid of controls it would be a nice option.

mjrippe

Re: Pultec HLF3C Correct Source and Load Impedance
« Reply #7 on: October 08, 2017, 12:46:02 PM »
I have done this on other filters with a 620 ohm resistor in series with a 10k pot across the output so you can adjust to taste.  Folks seem to like it.  :)

Re: Pultec HLF3C Correct Source and Load Impedance
« Reply #8 on: September 18, 2020, 11:17:41 AM »
hello good afternoon everyone i am trying to recreate this resonant filter but not getting too much of the schematic.
how many mh does the inductor need to apply at each frequency cutoff?
I'm especially interested in the high-pass filter part.
thank you

squarewave

Re: Pultec HLF3C Correct Source and Load Impedance
« Reply #9 on: September 18, 2020, 12:22:08 PM »
It says on the schem 800, 500, 390 ...

Unfortunately it seems audiomaintenance.com no longer has the VTA1430 and VTA1421 multi-tapped inductors for this. That's a shame. I love the HLF-3C. A lot of early records band-passed tracks.

Note that the HLF-3C is not a "resonant" filter. It's supposed to be critically damped with 600 ohms of source impedance and loaded with 600 ohms. If you don't do that it will have a peaky resonant response at the cutoff frequencies. But I played around with that with some other filter sets that I have and decided against it in the end. It just sounded nasal. IMO these filters must be critically damped. And then followed by a +6dB output buffer to compensate for insertion loss and get a low Z out.


Re: Pultec HLF3C Correct Source and Load Impedance
« Reply #10 on: September 18, 2020, 08:18:22 PM »
I thought it was a chebyshev filter like the altec 9069 or langevin 255, I think the latter are quite simple, it is reduced to a multitap inductor, a 3-pole switch and double capacitors, the inductor is to enhance the peak in the cutoff frequency, I'm not sure though.
with respect to the mh values of the scheme 800,500,390 ... ect
Are these values the same for the low pass filter and the high pass filter?