Poor Man's Pultec EQP1-A

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Jean Clochet said:
Back again.

I don't see why you couldn't use a Direct box traffo on the out but I'll let the designer answer for himself  8)

Jean

It depends what impedance load the trafo presents to the output of the equaliser. The output impedance of the EQ is about 56K so the load it works into needs to be considerably bigger than this or the EQ response will be affected. I recommend a load of no less than 470K. If a Direct Box trafo can provide this load then it will work OK.

Cheers

Ian
 
ruffrecords said:
The output impedance of the EQ is about 56K

I don't know if this is the right topographie yes?
PMEQP1-AMKII.png


If I thevenize in my head I see a different output impedance:
With a signal source connected there is the source output Z in series with the 47K Hi-Boost Pot.  These two are in parallel with the 4K7 Hi-Cut pot and the 56K output resistor.  So 4K something total output impedance.
The Low boost and cut pots do not change the output Z, they just define the amount of Lo boost and cut but we could just put caps in by themselves for 6dB per octave simple filters or short them out. 
I do notice that you use the 56K to work your lo cut into so that bit is important but doesn't define the overall Z.
So, a typical DI traffo might present a load of 200K - if you adjust your 56K up a bit to accomodate for the parallel 200K then it should be OK.
Maybe I miss something here so please let me know  :)
Thanks Ian,

Jean


Edits:  I see your earlier schematic didn't rely on the 56K and used a topographie closer to pultec which would not need to account for the 200K in parallel.
I do also realise that what I propose for the parallel of 200K traffo and a slightly raised 56K is a bit loose for really spot on accurate low cut frequency but I still think you could make it work if you knew the traffo ratio, maybe loaded the output traffo a bit for more defined reflected Z? - And it would be cool if someone could use an existing mic pre for just the cost of a 30 Euro traffo  :D
 
Hi,
I attached a screen capture drawing of what I meant above.  The changed/added resistor are in red.  The added 15K series R on the output is there to satisfy the transformer which is looking for a source of 20K for good response.  I used a Jensen traffo as model.
Assuming ideal pots and traffo, total loss is 43dB.

Jean
 

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Here's one attached with a very simple buffer so you can keep your original 470K load.  You could pull power for it from Mic Phantom supply.  It would not need much current to drive the transformer so adjust the Fet's source load R for a few mA by ohm's law:  Supply voltage minus the 4V torn on voltage of the Fet divided by 3 or 4 or ? mA. 

Jean
 

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The load placed across the EQ output does affect the shape of the Lo boost/cut EQ curves and the maximum boost or cut that can be obtained. For example:

With full lo boost AND cut applied:

470K load gives a 5dB dip (the Pultec bump) and 14dB boost.
200K load gives a 6dB dip and a 12dB boost
100K load gives a 7dB dip and 10 dB boost
50K  load gives a 9dB dip and 7dB boost

Full Lo boost alone:

470K load gives  15.5dB boost
200K load gives  14.9dB boost
100K load gives  14.0dB boost
50K  load gives  12.5dB boost

Full Lo cut alone:

470K load gives 20dB cut
200K load gives 20.7dB cut
100Kload  gives 22dB cut
50K load gives 24.7dB cut

Changing the load from 470K to 50K makes only 0.5dB difference in the Hi and Lo Boost/Cut curves.

So I conclude that:

1. The load makes the greatest difference to the low frequency simultaneous boost/cut (bump)
2. The difference made by a 200K is quite small.
3. Most people could probably live with the curves produced with a 100K load.
4. 50K load messes up the lo bump and reduces the maximum boost by over 3dB

I think using a FET buffer powered by phantom supply is a very clever innovation.

Cheers

Ian
 
ruffrecords said:
The load placed across the EQ output does affect the shape of the Lo boost/cut EQ curves and the maximum boost or cut that can be obtained.

Thanks for doing that Ian :) 
Yes I thought that the low would be the one to be changed the most and with a typical 20K:150 ohm traffo presenting 200K to primary maybe not too much to care?

ruffrecords said:
So I conclude that:

1. The load makes the greatest difference to the low frequency simultaneous boost/cut (bump)
2. The difference made by a 200K is quite small.

Yes.  And if we push it a bit by adding my build out from your EQ of 15K for 215K total load,  even a tiny, tiny bit  less small of a difference ;)


ruffrecords said:
3. Most people could probably live with the curves produced with a 100K load.
4. 50K load messes up the lo bump and reduces the maximum boost by over 3dB

I think you are right that 100K is even fine for Rock 'N' Roll.  However, 200K is better myself but:

ruffrecords said:
I think using a FET buffer powered by phantom supply is a very clever innovation.

Thank you Ian.  I would use this option myself as it costs so little and gives flexibility.  Maybe add a 'buffer out' switch to provide for sometimes not having phantom? 
I think using a C.C.S. on the source would be better than a resistor for a few reasons but I didn't flesh it out.  I wanted to quickly throw out an idea for people. 
I'll be glad to finish it and post it for anyone to use if you want?  Or someone else can chime in if they have better thoughts?

Thanks again Ian for taking the time to do plots for differing loads and good for you for doing the design in the first place :)

Jean.

Oops, Edit:  Ian, when you simulated, did you adjust upwards the original 56K so that total load for lo eq is the same as your 56K//470K = 50K? 
Maybe it only makes a very small difference?
 
Wow!
This EQ was already awesome. Looks like this can turn every mic-pre into a line-level EQ/channel-amp...
Keeping it PM with edcor's on the I/O and it can be completely passive with a 48V option with a single FET? That is completely outta control!
I wonder what 16 or 32 channels would sound like on a mix?
Ian, I haven't checked your WM thread but are you doing additional runs of these?

Cheers,
jb
 
0dbfs said:
Ian, I haven't checked your WM thread but are you doing additional runs of these?

Cheers,
jb

I did one extra run in the BM. If there is enough interest I am happy to do another run on the same basis as last time.

Cheers

Ian
 
  "I am interested in learning what I might need to change around or add to make the output of this low impedance like a microphone. 150 or 200 ohms... 40dB of overall attenuation would be fine. It would be awfully convenient to feed line level to these and patch into a mic-pre-amp for makeup similar to the passive-summing-mixers that are all the rage." 

  "I don't see why you couldn't use a Direct box traffo on the out.." 

We are talking about the makeup gain, ok I'm following.  Using a stand alone preamp, after the PMEQP1-A for the makeup gain?

Would the chain look like:  mic--->Pre---->PMEQP1-A---->Pre(makeup?)--->interface etc etc...??

If this is correct, is there readily available (affordable, PM spirit  ;D ) transformers to achieve this?  And would we need to still have the output transformer, and then step down with a second transformer to mic level??

I'm on board, I like where this is going.
 
Would the chain look like:  mic--->Pre---->PMEQP1-A---->Pre(makeup?)--->interface etc etc...??

Yes. Or this for mix/returns:

DAW/DA --->> Comprerssor101 --->> PMEQP1A --->> CoolMicPre-XYZ-MakeUpGain --->> Mixer

I'm thinking you could probably hit it pretty hard with level depending on the transformer spec's so that after 40dB of attenuation you don't need too as much makeup....

Cheers,
jb
 
0dbfs said:
DAW/DA --->> Comprerssor101 --->> PMEQP1A --->> CoolMicPre-XYZ-MakeUpGain --->> Mixer

I'm thinking you could probably hit it pretty hard with level depending on the transformer spec's so that after 40dB of attenuation you don't need too as much makeup....

Most DAWs seem to be set so +4dBu at the D/A output corresponds to -16dBFS which means 0dBFS corresponds to +20dBu. That means you can hit the EQ pretty hard, and as it is passive it can take it. So the EQ output is around 0dBu and you can afford to lose  another 30 to 40dB before it gets to the following gain make up mic pre.

Yes, looks like a good idea.

Cheers

Ian
 
Here's a simple version of the FET buffer showing powering from 48V Phantom.  The ZVP3310A is available worlwide from Digikey/Mouser etc. and cost is less than a U.S. dollar.  There are exotic J-Fets from Toshiba etc. of course but they're hard to get and expensive  I think the Zetex 3310A is a fine choice for this job
 

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Here's a better (?) buffer using a constant current source load on the FET's source pin.  The current source will linearize the follower more and also seperates the buffer from any noise on the 48V line.  I think this is maybe worth trying but, up to you.
Like the FET, there are better PNP transistors to use instead of PN2907 but here also these are more than good enough for the job. 
AND, they are available from Digikey/Mouser for pennies/cents each so...
This follower has a bit more current through it than the simpler one, the ZVP3310A likes more current even though the transformer load doesn't need it. 
Maybe someone can try both and see?
Jean

Edit for changed values.
 

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Jean Clochet said:
Oops, Edit:  Ian, when you simulated, did you adjust upwards the original 56K so that total load for lo eq is the same as your 56K//470K = 50K? 
Maybe it only makes a very small difference?

All I did was keep the EQ the same and change the load. The 56K is key to producing the low frequency Pultec bump with simultaneous max Lo boost and cut. The extra load already makes the bump smaller so increasing the 56K would make it smaller still.

Cheers

ian
 
ruffrecords said:
Most DAWs seem to be set so +4dBu at the D/A output corresponds to -16dBFS which means 0dBFS corresponds to +20dBu. That means you can hit the EQ pretty hard, and as it is passive it can take it. So the EQ output is around 0dBu and you can afford to lose  another 30 to 40dB before it gets to the following gain make up mic pre.

Sounds good to me.
I had assumed though the level out of the Eq was lower than 0dBu so, if you wanted to use a Fet buffer before a transformer, it would be better to bias up the gate a bit to allow for bigger signals.  You would need to cap couple from the Eq into it then of course to block the bias V from the Eq pots etc.
When I have more time later I'll post an update schematic showing what I mean. 
 
Jean Clochet said:
Hi Ian,
I posted here instead: http://www.groupdiy.com/index.php?topic=45054.0  so I wouldnt mess up your thread any more  :)

Jean.

Good idea. It is a topic worth discussing in its own right.

Cheers

Ian
 
ruffrecords said:
It is a topic worth discussing in its own right.

Maybe so.  Just didn't want to0 mess with your thread too much!

I did have this idea:
Sometimes when I use a pultec, I have to use maximum boosts and cuts to push the freq. of operation outwards and leave the middle range unchanged.  But also, this sometimes gives too much boost, so what I do is blend in an uneq'ed amount of the same signal to fill it out and just leave the humps at the outer frequency ends. 
So I just figured that with the circuit I posted in the other thread you could add a 'blend in' for the uneq'ed signal and not upset anything.  I'll post this there when I have time

Jean.
 

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