krabbencutter

Cheap's 73 EQ (previously Poor Man's 1073)
« on: October 01, 2018, 02:17:32 PM »
Are you ready for a faithful 1073 EQ, that contains only resistors, capacitors and standard opamps?  Well, I'm glad you're here  ;D

I'm kinda obsessed fascinated by the 1073 EQ. Being a student, who has no money to spare for costly inductors, I was inspired by Ian's Poor Man's Pultec and started looking into gyrators (aka 'simulated inductors') as an alternative. I stumbled upon a very interesting & helpful article from Rod Elliott: http://sound.whsites.net/articles/gyrator-filters.htm

Putting that knowledge to use, I went through the original 1073 schematics and started to put this 'magical' EQ together on my breadboard.  One week, some Insufficient amount of sleep and a lot of headaches later, I have succesfully managed to rebuild the 1073 with gyrators. Using the Waves Scheps 73 and V-Series EQs as a reference, as well as information from all around the internet, I can confirm that the end result is pretty close!

You can find the full schematic, as well as a BoM and several other (hopefully) helpful bits - including an LTSpice simulation - on my GitHub:
https://github.com/ravettel/cheaps73

If you have any questions or ideas, how to further improve the circuit, schematic, resources etc. feel free to chime in :)
« Last Edit: October 28, 2018, 12:10:17 PM by krabbencutter »


shot

Re: Poor (& Clueless) Man's 1073 EQ
« Reply #1 on: October 01, 2018, 05:06:23 PM »
This is great!
Thumbs up!

I'll be following your research!

:)

Luka

Re: Poor Man's 1073 EQ - Gyrator Edition
« Reply #2 on: October 02, 2018, 09:46:40 PM »
Can you sketch a schematic for what you’re doing?

shot

Re: Poor Man's 1073 EQ - Gyrator Edition
« Reply #3 on: October 03, 2018, 06:19:51 AM »
...only leaves 1 pole for the gyrator. However there are 2 resistors that need to change with every setting....

Could you use a DPDT relay?
It's cheap and can do the work probably.

krabbencutter

Re: Poor Man's 1073 EQ - Gyrator Edition
« Reply #4 on: October 03, 2018, 07:39:43 AM »
Can you sketch a schematic for what you’re doing?

I've added the schematic to my GitHub.

 
Could you use a DPDT relay?
It's cheap and can do the work probably.

I built four stages with a single quad-opamp and it works fine so far :) That dpdt sounds interestingh tho. How would you implement it? gotta admit I'm pretty new to this whole DIY thing and I also have not much of an electronics background, besides physics class at school 10+ yrs ago  ;D
« Last Edit: October 07, 2018, 03:55:55 PM by krabbencutter »

krabbencutter

Re: Poor Man's 1073 EQ
« Reply #5 on: October 17, 2018, 06:58:12 PM »
Sh!t just got real! I've prototyped the High- & Low-Shelf on a perfboard - frequency measurements attached  :)

To give you an impression of what you can expect, I'll quote the 'Doctors' from Universal Audio, who did a nice summary of the 1073 circuit:
Quote
[...] On the shelving filters, the low shelves have varying degrees of resonance as different capacitors are switched in for various shelving frequencies. The high shelf has a very gradual slope for both boost and cut, because its poles and zeros are interpolated. This gives it a very warm sound, and it is never strident, even at maximum boost. Both the high and low shelves feature a small "divot," a region of inverse gain slightly before the shelving frequency. This is similar to the feature of the Pultec EQP1-A when low boost and cut are employed at the same time. As far as the overall response of the 1073, the high and low ends are mostly flat, with a tiny bit of roll-off on the high end. [...]

See: http://www.uaudio.com/webzine/2006/may/text/content2.html

« Last Edit: October 18, 2018, 05:37:22 AM by krabbencutter »

krabbencutter

Re: Poor Man's 1073 EQ
« Reply #6 on: October 21, 2018, 07:56:37 AM »
Finished soldering the HPF today, measurements attached.

Spencerleehorton

Re: Poor Man's 1073 EQ
« Reply #7 on: October 21, 2018, 06:48:51 PM »
Look forward to seeing what you have done and hopefully adding this to my 8 x 1290 pre’s.
website: www.mohawkstudios.co.uk
email: [email protected]

If it hisses its probably the wrong impedance!!!
Or a snake!!!

jarvis

Re: Poor Man's 1073 EQ
« Reply #8 on: October 21, 2018, 07:21:53 PM »
This looks like a really cool project, thanks for sharing!

Cheers
Jannis

krabbencutter

Re: Poor Man's 1073 EQ
« Reply #9 on: October 22, 2018, 04:45:48 PM »
Thanks a lot, really happy to hear that  :)

I've finished the presence circuit today, which means the EQ part is now complete. I'm still missing the 3 small capacitors (470pf & 180pf around the EQ amplifiers, parallel 220pf capacitor in the 7,2kHz band), which should arrive in the coming days. When I used equivalent ceramic capacitors in my tests however, they had only a very small impact on the overall frequency response.

To give you a better understanding, of what you're looking at in the frequency graph, I'd like to once again quote the Doctors from Universal Audio:

Quote
The midband peaking filters are also built as feedback around an active stage, and again have a symmetric response. Although it would be possible to build an active peaking filter without magnetics, the 1073 employs an inductor and capacitor to generate the resonance for these filters. The responses for the peaking filters have a nice dependence on the bandwidth as the gain is adjusted, and at higher center frequencies, the Q goes up, for a more focused peak. [...] in the midband peaking filters, the responses are drawn towards unity gain at high frequencies by parasitic capacitances in the inductor.
Source: http://www.uaudio.com/webzine/2006/may/text/content2.html

I was pleasantly surprised to see, that even with simulated inductors, the result shows the same 'imperfections' as the original circuit! However the 1,6kHz band has a bit of an accentuated high end roll-off. There are 2 slightly different versions of this circuit on the web, one containing a parallel 220K resistor for the 1,6kHz band, which might be the solution I'm looking for.
« Last Edit: October 22, 2018, 05:23:01 PM by krabbencutter »


krabbencutter

Re: Poor Man's 1073 EQ
« Reply #10 on: October 22, 2018, 04:48:21 PM »
And here's a closer look at the proportional & beautifully symmetrical bell curves:

krabbencutter

Re: Poor Man's 1073 EQ
« Reply #11 on: October 22, 2018, 05:17:47 PM »
Last but not least, I've attached the 'flat' frequency & phase response.
UAD Doctors again (last time, I swear!):
Quote
As far as the overall response of the 1073, the high and low ends are mostly flat, with a tiny bit of roll-off on the high end. On both the high and low end, there is some phase shift associated with the finite bandwidth of the unit.
src: http://www.uaudio.com/webzine/2006/may/text/content2.html

To get all potentiometers as close to zero as possible, I've set them up as voltage dividers on my breadboard and used a multimeter to find the mid point.

What's next?
  • I'm still missing a proper output stage. I'm thinking about a TL071 with a gain of 1.5, to compensate for the 3dB insertion loss of the EQ, followed by a 100R output resistor? Or I might try one of Rod Elliotts balanced line driver circuits, like http://sound.whsites.net/articles/balanced-io.htm
  • as mentioned above, I will have a look what that 220K resistor will do to the 1,6kHz band
  • in theory the peaking filter can easily be extended, to include the 10kHz setting from the 1078. This would only require a 1200pF capacitor after the 200mH gyrator.  My 2x6 Lorlin pot however is already maxed out and the available gain range at 10kHz would probably be even smaller, than at 7,2kHz
  • the Hi-Q setting from the 1084 on the other hand only requires a 7k5 resistor to ground, if I read the schematics correctly. I will definitely look into this!
« Last Edit: October 22, 2018, 05:22:41 PM by krabbencutter »

peterc

Re: Poor Man's 1073 EQ
« Reply #12 on: October 23, 2018, 01:34:11 AM »
Very nice project! Good contribution to our community. Thanks!

Peter
If opportunity doesn't knock, build a door.

Jarno

Re: Poor Man's 1073 EQ
« Reply #13 on: October 23, 2018, 04:18:44 AM »
Do you want to drive 600 ohm? If not, it doesn't need to be more complex than the schematic with the single opamp I attached, or the one with the THAT IC, but then maybe with a bit of safeguarding against accidental phantom supply.


krabbencutter

Re: Poor Man's 1073 EQ
« Reply #14 on: October 23, 2018, 05:06:11 AM »
If I understand this correctly, the 'modern' standard are low impedance outputs (100R) and high impedance inputs (10k), so 600R would not be neccessary?
https://www.rane.com/note124.html

Thanks for the schematic. Can I just swap the THAT IC for any standard opamp? I'd like to keep the basic circuit as accessible as possible.

john12ax7

Re: Poor Man's 1073 EQ
« Reply #15 on: October 23, 2018, 05:33:34 AM »
THAT 1646 is a dedicated output driver and is not compatible with a regular opamp.  If you want simple and inexpensive the question becomes do you "need" a dedicated driver? If your circuit overall output is from an opamp, using an impedance balanced output is simple and effective.

Since you mentioned TL071, I would avoid using that for an output, as it doesn't have much drive capability.

Newmarket

Re: Poor Man's 1073 EQ
« Reply #16 on: October 23, 2018, 06:56:29 AM »
What is the format / program for the schematic* .sch files to be able to view them ?
( I can see the .sch extension but of course nearly all schematic/pcb software uses 'sch' :-)

krabbencutter

Re: Poor Man's 1073 EQ
« Reply #17 on: October 23, 2018, 07:00:20 AM »
What is the format / program for the schematic* .sch files to be able to view them ?
( I can see the .sch extension but of course nearly all schematic/pcb software uses 'sch' :-)
I created them in KiCad, which is open source. There's also a pdf version: https://github.com/ravettel/cheaps73/blob/master/cheaps73.pdf
« Last Edit: October 28, 2018, 12:11:06 PM by krabbencutter »

Newmarket

Re: Poor Man's 1073 EQ
« Reply #18 on: October 23, 2018, 07:15:52 AM »
Great. Thanks for the quick response  :)

krabbencutter

Re: Poor Man's 1073 EQ
« Reply #19 on: October 23, 2018, 01:50:53 PM »
Last but not least, I've attached the 'flat' frequency & phase response.
UAD Doctors again (last time, I swear!):src: http://www.uaudio.com/webzine/2006/may/text/content2.html

To get all potentiometers as close to zero as possible, I've set them up as voltage dividers on my breadboard and used a multimeter to find the mid point.

What's next?
  • I'm still missing a proper output stage. I'm thinking about a TL071 with a gain of 1.5, to compensate for the 3dB insertion loss of the EQ, followed by a 100R output resistor? Or I might try one of Rod Elliotts balanced line driver circuits, like http://sound.whsites.net/articles/balanced-io.htm
  • as mentioned above, I will have a look what that 220K resistor will do to the 1,6kHz band
  • in theory the peaking filter can easily be extended, to include the 10kHz setting from the 1078. This would only require a 1200pF capacitor after the 200mH gyrator.  My 2x6 Lorlin pot however is already maxed out and the available gain range at 10kHz would probably be even smaller, than at 7,2kHz
  • the Hi-Q setting from the 1084 on the other hand only requires a 7k5 resistor to ground, if I read the schematics correctly. I will definitely look into this!

I played around with the peaking filter some more. The results are kinda interesting, but actually not very exciting  ;D
  • that out-of-shape 1,6kHz boost turned out to be a simple glitch. When I repeated the measurements with the soldered circuit and a breaboard version, everything went back to normal.
  • as it turns out, that Hi-Q mode from the 1084 is kinda useless when compared to a 1073. As you can see in the attached measurements, the 1073 has an even higher Q, than a 1084. You can find a pretty good explanation why that is over here: https://groupdiy.com/index.php?topic=25053.msg462966#msg462966
  • the 220k feedback resistor does lift the high end a bit, but we're only talking <0,5dB in the 1073. I'm also not yet sure whether this affects only the 1,6kHz band or 0,7 - 1,6kHz  or all bands
  • on the 1084 however, that 220k resistor has a very noticable effect at low Q settings, as you can see in the image. (I repeated the measurements a couple times to confirm, that it's not an anomaly)


 

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