MEQ mid-range EQ pultec style (was meq500)

looks like a insufficient input transformer indeed.
even though this project is outdated and two years ago, just send me the defective input transformers and i will rewind them fo you (and ben)

-max

Thank you Max.  Please provide an address, and I'll send them your way.  My PM box should have space. 

adress for replacements send via email. it will take some time to source materials, pls note that this project was manufactured two years ago.

pm´s reached me asking about the status of this project: it´s closed, no parts available.

-max

Much appreciated Max.

Had a think about the upper cut 4k/5k/7k freq issue, this is the easy fix:

move wire 11 to main board pad 10
move wire 10 to main board pad 9
move wire 9 to main board pad 11 and change 0.01 (10nF) cap to 0.047=3981Hz or (3) parallel 0.015=4069Hz or (2) parallel 0.022=4115Hz

my freq measurements are all trending higher than specified, so I think the single 0.047 will be the ticket. 

Thanks max.

Some new curves.  45 Hz and 112 Hz on bottom, shown at full, half pot travel, and 1 dB boost.  Upper bands show additional 7K and 10K boost positions.  After hearing and seeing the bandwidth, I will next check out additional 700 and 1000 Hz boosts in the upper boost section.  I think I'll likely use those more than 7K/10K, and in combination with lower freq boosts.  

Extensively compared tantalum with standard electrolytic in those bottom two freq, they are different but seems mostly hairsplitting.   I can find things I prefer slightly with each type.  The tants are physically better suited, and slightly better tolerances in my tests, so in they go.  


I notice the gain difference between my pair disappears (at band centers) at least when the lows are at full boost.  Points to tolerances within the resistances in the EQ (pad) network.    Probably the pots.  Toyed with a gain switch rather than bypass, abandoning both ideas.  

clintrubber said:

these hybrid-opamp PCBs......
some potential issues show up. I was wondering how many have actually
built this hybrid.

As I see it it might be good to 'define' the unused half of the 5532. As it is now
all pins are not connected, might be better to short 'out' to '-' & connect
'+' to the (so far unused) GND-pin (remember to connect it at the mother-PCB).

People indeed using LEDs for biasing ? What measured current-draw do you obtain ?

Click to expand...

I measure 48 mA per rail with the hybrid amp.  Seems high, explains why it's running hot.  I've had them on for at least an hour at a go so far without failure.  Will probably explore alternative implementation if I use these here or elsewhere.  Is there a source for this circuit that can be referenced Max?  

Adding a second boost set of 700 and 1K is much cooler than adding 7K and 10K.  

Both 1K shapes are the same, but added high band is same boost level (+9) as other high bands. Low band 1K goes to +11.   I got lucky and had some 0.039 mfd caps in my stock, which are about perfect.  

Added high band 700 is a much broader shape than stock low band 700.  It resembles the width of the Langevin 251A or Electrodyne boost bands, rather than the relatively narrow boosts of the Pultec MEQ.    Math says 700 Hz cap should be 0.082 for 625mH, bench shows 0.1 actual to hit that center with these inductors.  

The 7K/10K boosts I'm abandoning are like dog whistles, they are so narrow.

buschfsu said:

one question, what does the 47r across the dc blocking cap on the output do for the amp?  inc output impedance?

Click to expand...

API-ish circuits do this as well and I must admit I never understood this too well.....

I'm used to seeing C with R in series for control of source Z to following device, or current limiting in case of output short.   I suppose parallel gives more limited DC Z relative to AC Z, but for what purpose?

I am a little confused, I have three of these EQ's, do I need new input transformers?

are your's built ?
mine isn't yet , but maybe
Emmr will mention if it is obvious
when you listen to it ,
more motivation to finish mine
but no one else has mentioned any problems
so maybe an isolated case

one is done, it does seem to be EQ'ing, though I don't have an original to compare it to

> what does the 47r across the dc blocking cap on the output do for the amp?

Say you hang a perfect transformer on the output.

It has impedance above 20Hz, but at DC it has ZERO ohms.

If the opamp has ANY DC offset, "infinite" DC current flows.

So stick a resistor in there.

Taking 50mV offset across 50 ohms, only 1mA of DC current flows.

But.... you set levels with a 10K load, then cut-in a 600 ohm load. The 50 ohm output resistance causes a 1dB level drop. Such little inconsistencies drive some engineers wild.

Also the distortion of a real transformer rises with source impedance. You want very-low drive impedance.

So AC-bypass the 50 ohms with a huge cap, to keep AC impedance far below 50 ohms.

PR You Rule.  I wondered about this for a long time on my 550 EQ's

I am an idiot, how does this apply to me?

I tried moving the 3K boost back to the 477mH tap (use 0.0056-0.006 mfd), and like it better, as it's wider.   The freq is off, as my test cap was reading 0.0074 mfd.  Going to mod that permanently.  Each option shown and then shown together, for your consideration.  Stock 625, alt 477, both.  

IMO, bandwidth switches are useless.  They hardly change the shape at all.  For boosts, they simply restrict the boost amount.  At the shallowest settings you get a bit more overall width (significant maybe with max +3 or less!), but hardly enough to be worth the extra switching.  
Stock, 1K, 3K1, 12K8 series resistance values shown, corresponding to +9, +7.7, +6, +3 max boost ranges.  As you can also guess, changing the bandwidth will bump your total boost, so it's not even simple to compare apples to apples.  


Shorting 270R in the attenuation band simply extends the depth of the existing shape.  If I want a real notch, I'll let the computer do it deeper and tighter.  

Some graphics for my added 700 and 1K frequencies in the high boost band, as compared to the low boost bands.  Actual capacitance still to be tweaked tighter.  The added high band freq's are the +9 max plots.  

A confusing compilation of all these boost options, with 300 thrown in for bandwidth comparison.  

If you haven't figured it out, to wire for extra low boosts in each band requires wiring 1-3/2-4/3-5/4-6/5-7 so you have 1 and 2 on the switch free for the extra freq's.

Revised drawing of the beast with Max's op amp included.  Switching shown as actually connected in this build.  My additional frequency choices are shown, and the actual measured frequency is listed for both units built.  4K cut position is still not stuffed or documented.    Interesting to note the frequency spread between two similar units.  I imagine there is no such thing as a well matched stereo Pultec, without a lot of trimming up using additional small caps.  I doubt there are well matched multi-tap inductors. 

I drew the additional strapping recommended for the unused 5532 side.  There is a Gyraf headphone amp schematic that somewhat resembles Max's op amp plan. 

----------2/25/12 edit---------

Jensenmann just noticed a transcription error regarding the opamp in the drawing posted here before 2/25/12.  I have revised and replaced the drawing.

Just a quick note on Doug´s handdrawn schematic: the second half of the 5532 hybrid amp is drawn with the inputs reversed. Negative input (Pin 6) should be tied to the output (Pin7) and positive input (Pin5) should be tied to ground.

Click to expand...

Is the coupling cap from Filter to 5532 shown in the right direction?

That is indeed how it is placed. 

Been looking at adding some frequencies in the high boost section.  Originally I'd just wanted a single boost point at 16K added.  Still considering inclusion of several of the other points.

I generated this chart for reference purposes. It uses the 'Mid cut' inductor and covers common points from 1K-16K.  NOTE - 8K is missing - could use .0068 on 61mH(7.81K) or .012//.012 on 61mH(7.93K) also .022//.022 on 34mH (8.23K)