Poor Man's Pultec EQP1-A

[ruffrecords]

I have had an idea for a poor man's version of the Pultec EQP1-A. Low boost/cut  and hi cut just as the EQP1-A with the hi boost modified to not need an inductor (based on the Helios Type 69 EQ). So just four pots (no bandwidth control) and a couple of cheap Lorlin 2 pole 6 way switches will give us 6 lo and 6 hi frequencies (more than the EQP1-). I have done some sims of the circuit and it still produces the classic Pultec bump in the response so I think I will knock one up and see what it sounds like.  Schematic attached - and I have changed the pot values so that it has a nominal 10K input impedance so it can be driven by a normal 10K:10K bridging transformer. Insertion loss is about 21dB so plenty of options for gain make up.

Cheers

Ian

 

[strangeandbouncy]

Hi Ian,



  are you reading my mind . . . . Fantastic stuff . . . .


    Brilliant. Thank you


    Kindest regards,



      ANdyP

 

[ruairioflaherty]

Excellent Ian!  I have something related on the bench right now but it's all peaking filters and laden with inductors, in fact a parcel of 6 nice Sowters just arrived yesterday.  I think the passive idea has great merit but I'm not a fan of needing huge make up gain for my specific application - in the mastering chain.  With that my version has just 6dB insertion loss, the compromise of course being less range.

Your idea will work wonderfully.  Once you get a handle on cap values please post an update, I'm visiting a huge surplus place soon and I might pick up what's needed to knock up a few channels.

Thanks,
Ruairi

 

[zebra50]

Nice!

I'm a bit short on inductors right now  , and this looks like a fun afternoon's project! Thanks for sharing.

 

[ruairioflaherty]

Nice!

I'm a bit short on inductors right now  , and this looks like a fun afternoon's project! Thanks for sharing.

;D

 

[ruffrecords]

Thanks some very encouraging replies. My only concern right now is that with maximum simultaneous top boost and cut the equaliser impedance drops continuously at very high frequencies which would probably be bad news for the driving source. So I think I might at a 4K7 in series with the hi boost wiper to make it into a shelving EQ just like the Helios which will limit to boost to about 15dB (low boost is limited to 14 to 15dB anyway). With a 750R in series with the hi cut wiper that also becomes a shelving type with about 15dB cut. Worst case EQ load then becomes about 5K4 which should be OK.

I'll do some more sims to check this out. What frequencies in addition to the normal EQP1-A ones do you think we should have?

Cheers

Ian

 

[strangeandbouncy]

Hi Ian,


  sorry to be a pedant, but there are actually 7 freq in the hf boost on an EQP1a . . . .


    I would have to muck about, but I think going higher in the top end, say 22kHz or even 26kHz, and maybe come up to something like 150 or 200hz . . . even up to 300 or 400hz would be nice for guitars in conjunction with the lo cut . . . anything would be a bonus.

    bloomin marvelous . . .


  I built  the HF section of a Helios (10kHz) in a box once upon a time . . . . Lordy knows where it is now . . .


    ANdyP

 

[ruffrecords]

Hi Ian,


   sorry to be a pedant, but there are actually 7 freq in the hf boost on an EQP1a . . . .

No problem. There's quite a few EQP1-? models. As you say the 1A has seven hi boost frequencies but only 3 hi cut ones. The 1R has five frequencies for both hi boost and cut. The 1R goes 3K, 5K 8K 10K 12K so we could add one at say 20K or one at 16K and 20K and lose the 3K for example.

    I would have to muck about, but I think going higher in the top end, say 22kHz or even 26kHz, and maybe come up to something like 150 or 200hz . . . even up to 300 or 400hz would be nice for guitars in conjunction with the lo cut . . . anything would be a bonus.

The 1A does 20, 30, 60 and 100 at the low end and the 1R omits the 20 so we could for example do 30, 60, 100, 200, 300 and 400. I just need to check we don't end up with silly capacitance values.

Cheers

Ian

 

[strangeandbouncy]

Hi,


  see what values are reasonable, Id say . . .


  Yay Ian . . ..


    ANdyP

 

[ruffrecords]

Hi,


   see what values are reasonable, Id say . . .

    ANdyP

I just checked out the Manley 'enhanced' Pultec. That has 20, 30, 60, 90, 120 on the lo, an amazing 11 frequencies for hi boost and 4K, 8K, 12K, 16K and 20K for cut. Since it is hard to get caps in odd values and its a pain to combine them in pairs instead, I think I'll continue in the spirit of this being a poor man's EQ by seeing what frequencies we get using standard values of capacitor.

Cheers

Ian

 

[lassoharp]

No problem. There's quite a few EQP1-? models. As you say the 1A has seven hi boost frequencies but only 3 hi cut ones. The 1R has five frequencies for both hi boost and cut. The 1R goes 3K, 5K 8K 10K 12K so we could add one at say 20K or one at 16K and 20K and lose the 3K for example.

Nice work Ian - look forward to trying this one out.  The 15db boost seems plenty on top and bottom.  I seldom find myself needing more than 11 or 12 o'clocks worth of low boost and 1 or 2 oclock on the high on the standard EQP1A.

I vote for at least adding the 16K.  I find that point most useful for adding presence without the 'chewiness' associated with the lower highs. 

Thanks for sharing.

 

[gemini86]

A few legacy EQs only give you +-12 for anything so I think we're on the right track. +1 on keeping to standard caps, us poor men can't be too picky.

+1 yay Ian, you're a good chap for doing the leg work here. If I could be more useful, I would...but I'm not, so no.

 

[Ptownkid]

I applaud this kind of thinking...I hate over complicated gear with way too many knobs...

 

[pucho812]

looking good. if you ask me, gotta have a 30K boost to add air. makes for perfect mp3

 

[chrispbass]

Good work Ian. I presume the valve section from the Gyraf pultec will still compliment this nicely?

Chris

PS +1 on adding a high 'air band'

 

[zayance]

Nice Stuff! 8)

 

[ruffrecords]

Good work Ian. I presume the valve section from the Gyraf pultec will still compliment this nicely?

Probably not in its current form. Since I have raised the input impedance of the EQ so we can use a 10K:10K input transformer, this means the output impedance has also been raised from about 1K to about 5K. This means the gain make up input impedance also needs to be about five times higher to avoid loading the EQ. The Gyraf uses a 10K:10K+10K transformer connected directly to the grids of the input tubes. This means the reflected primary impedance will be nice and high which is good but the LF response depends on the primary inductance of this transformer. If, for example, this is high enough for the existing design to be -1dB at 10Hz then it means in this design the -1dB point will likely be at 50Hz which is not so good.

Cheers

Ian

 

[strangeandbouncy]

Hi,


  does it need an interstage transformer . . .

  could you use 2520 like this . .

 

[strangeandbouncy]

or SRPP like this . . .

 

[strangeandbouncy]

  that srpp is 27.5dB gain, so just about perfect with 2 to 1 output tranny . . . .