Poor Man's Pultec EQP1-A Build Support Thread

[baadc0de]

Thanks for the explanation.. I'll just leave it out then I suppose, or maybe have one funky 3kHz setting for that Ampeg SVT yowl..

 

[gemini86]

that's quite the mish-mash of pots you have there...

Just curious though, where does one get 470k/47k/4.7K pots in the US?

Someone else had this problem in the original thread. It seem 5K, 50K and 500K are more common in the USA so I would suggest you use those values and tweak the capacitor values accordingly. If you check the original thread you should find that the person who originally asked the question tweaked the spreadsheet for those values of pot in order to calculate new cap values. I promised to update the spreadsheet so you could feed in different pot values and I have now done this. Just for you I have set the initial values to 5k etc.  All you have to do is set the Hi Cut pot value and the spreadsheet calculates the other pot values and all the capacitor values (except for the inductor ones). If you set the Hi Cut pot to 1000 for example you get very close to the classic Pultec circuit.

For those interested in using the single inductor position, the spreadsheet now also includes a section that calculates the required L and series C values for a variety of frequencies for a Q of 0.6.

http://www.ianbell.ukfsn.org/pics/capcalc.xls

Cheers

Ian

yeah that was me, Ian... I'm still curious why they seem to be uncommon in the US. I found that Newark has stock though, for anyone who wants to build to your recommended cap values.

 

[ruffrecords]

Thanks for the explanation.. I'll just leave it out then I suppose, or maybe have one funky 3kHz setting for that Ampeg SVT yowl..

No problem. You need to short it out rather than leave it out though.

Cheers

Ian

 

[ruffrecords]

yeah that was me, Ian... I'm still curious why they seem to be uncommon in the US. I found that Newark has stock though, for anyone who wants to build to your recommended cap values.

I have no idea why 4K7, 47K and 470K pots should be hard to find in the US. Maybe its because, although most passive parts are these days made in the Far East, pots still tend to be made locally. I know I use Omeg pots made here in the UK.

Cheers

Ian

 

[gemini86]

I tend to use alpha, cause their cheap and mouser stocks them. They're from Japan, i think?

 

[ruffrecords]

I tend to use alpha, cause their cheap and mouser stocks them. They're from Japan, i think?

I think they are Taiwanese. The switches I use in this project are made by Alpha and the data sheet says their company name is "Taiwan Alpha Electronic Co".

Cheers

Ian

 

[gemini86]

those are the ones...

Well, I've found that newark has 47/470/4.7 pots in stock, just their search function is not as filter-able? anyway, thanks again for this great project, Ian.

 

[AudioHammer]

Does anyone else get this issue when opening the Capacitor Calc spreadsheet?  I am using Excel 2003 and 2007 versions running on WinXP.

Excel found unreadable content in 'capcalc.xls'. Do you want to recover the contents of this workbook?

Clicked 'Yes" and got the following:

Excel recovered your formulas and cell values, but some data may have been lost.

When I open the spreadsheet with the recovery option, I check the formulas which seem to be wrong:

Hi Cut
Cell  I9 5000 Log  
Hi Boost
Cell I10 50000 Lin  
Low Cut
Cell I11 500000 Log  
Low Boost
Cell I12 50000 Lin


Low Boost/Cut
C cut(nF)
=1/(2*3.14*I10*B8*0.000000001) Formula references I10 cell which is Hi Boost 50k Linear Pot.  Should this be I11 cell instead?

Hi Boost/Cut
Cboost(nF)
=1/(2*3.14*47000*C24*0.000000001)          Should this formula reference I10 cell instead of being hard coded to 47000?

 

[ruffrecords]

Does anyone else get this issue when opening the Capacitor Calc spreadsheet?  I am using Excel 2003 and 2007 versions running on WinXP.

Low Boost/Cut
C cut(nF)
=1/(2*3.14*I10*B8*0.000000001) Formula references I10 cell which is Hi Boost 50k Linear Pot.  Should this be I11 cell instead?

Strictly speaking the time constant of the low cut cap and the 56K Lo bump resistor should be used here. I have created a new version of the spreadsheet which calculates the Bump R and uses it to calc the low cut caps - it is called capcalc3.xls http://www.ianbell.ukfsn.org/pics/capcalc3.xls

Hi Boost/Cut
Cboost(nF)
=1/(2*3.14*47000*C24*0.000000001)           Should this formula reference I10 cell instead of being hard coded to 47000?

Yes it should be I10. The new capcalc 3 reflects this change.

Sorry for the errors.

Note, the original is an Open Office spreadsheet (.ods) so I have uploaded this too (it is just called capcalc.ods) http://www.ianbell.ukfsn.org/pics/capcalc.ods

Cheers

Ian

 

[ruffrecords]

I have measured the frequency response of my prototype and it seems to perform pretty much as expected. Here are the Lo frequency measurments. You can clearly see the Pultec 'bump' with simultaneous max boost and cut.


http://www.ianbell.ukfsn.org/pics/LFresponse.pdf


Cheers

Ian

 

[pigsnoot]

Hey ian

I've been searching for the parts list everywhere! I forgot in wich thread it was....I guess it wouldn't be bad if it would be mentioned somewhere in this thread....?

I can get almost everything from the schematics posted here, but it is nice and easy if you would mention all the parts in a list...

Sorry if I missed it somewhere! 

 

[ruffrecords]

Hey ian

I've been searching for the parts list everywhere! I forgot in wich thread it was....I guess it wouldn't be bad if it would be mentioned somewhere in this thread....?

I can get almost everything from the schematics posted here, but it is nice and easy if you would mention all the parts in a list...

Sorry if I missed it somewhere!  

I have added all the documentation to the very first post of this thread so it is easy to find.

Cheers

Ian

 

[safe as milk]

hi
thanks for these great little boards
i got all 8 stuffed caps/switches yesterday whilst that rain was
coming down...

how do i go about loading the input trafo's so that the eq
gets it's prefered impedance?

cheers

 

[ruffrecords]

hi
thanks for these great little boards
i got all 8 stuffed caps/switches yesterday whilst that rain was
coming down...

how do i go about loading the input trafo's so that the eq
gets it's prefered impedance?

cheers

The EQ pretty much does it itself. All you really need is a 12K across the 10K:10K bridging input transformer to make its total load close to the desired 10K so whatever you drive it with 'sees' a 10K load. The EQ really likes a nice low source driving it but most of today's gear has an output resistance of a few hundred ohms at most which will be reflected straight through the transformer and will be what the EQ 'sees'. The original Pultec was much more picky about the impedance that drove it simply because the Q of the LC filters depends directly on it. We don't have and inductors so it is not really an issue.

Cheers

Ian

 

[Biasrocks]

we have this Peerles S 217 D all blueprinted,
who needs one?

CJ

I'd love to have one.

Mark

 

[safe as milk]

hi
thanks for the information ian.
what are your thoughts on placing the eq in the feedback loop
of whatever op amp i end up using? where a vol/trim pot would normally be.

cheers

 

[ruffrecords]

hi
thanks for the information ian.
what are your thoughts on placing the eq in the feedback loop
of whatever op amp i end up using? where a vol/trim pot would normally be.

cheers

In a nutshell, don't ;-) This is a passive EQ and is definitely not intended to be included as part of a feedback network. Not only that but if it did work at all, because it is in a feedback circuit, it would do the inverse of what you expect so the boost controls would cut and vice versa.

Cheers

Ian

 

[safe as milk]

cheers
i think i was getting mixed up with placing it between 2 op-amps?
i do have a tendency to type first ask questions later!

thanks again for your advice ian

 

[ruffrecords]

cheers
i think i was getting mixed up with placing it between 2 op-amps?

Between 2 op amps is fine. The first would act as a line in buffer and could provide a  balanced input and a nice low impedance drive to the passive EQ. The second could be used as a gain make up amplifier.

Cheers

Ian

 

[ilfungo]

is possible to use:
5k instead 4k7
50k instead 47k
500k instead 470k?
Thanks