Poor Man's Pultec EQP1-A Build Support Thread

[ruffrecords]

Hi Ian
It's possible to use your tube make up stage in original pultec filter section?
I need to change something?
I' using sowter  3603 as input
Carnhill 2281 as output.
Thanks!

I am guessing you mean the poor man's tube gain make up stage - yes you can use this. The Sowter 3603 will be fine at the input. The Carnhill VTB2281 may not be so good at the output. The reason is that this particular tube amp design does not work well into a low impedance load. The VTB2281 is fine across most of the audio band but its impedance drops rapidly at low frequencies so you may lose low end response. If you want to use the amplifier then it is best to use the recommended Sowter output transformer since this has a much higher inductance than the VTB2281 and so it suffers a lot less from low end loss.

A better tube gain make up amp would be one half of my twin line amp design. This uses a VTB2291 output transformer wired 2K4:600 and will drive a 600 ohm load to well over +20dBu. It also has a simple adjustment for gain which simplifies set up. But if you only need one amp it is a bit of an overkill.

The alternative is to modify the original poor man's tube gain make up amp to use an 6922 tube instead of a 6CG7. This has extra gain and can also use a VTB2291 output transformer.

Cheers

Ian

 

[ilfungo]

Yup, that's the way it should be!

Cheers

Ian

Hi Ian


I did some experiments and
I do not know why,
if I put a 100k resistor from the G point (mid inductor) on the ground,
all problems with the middle band are solved.
the 1db bump at 16000 is gone.
The strange curve that came out with extreme middle frequencies has disappeared.
I don't understand what I did ... can it create problems?

 

[ilfungo]

Hi Ian just for info:
The bump at 16KZ come out only with mid on.
With eq in and mid out the responce is near flat...
Thanks!

Sorry...

 

[ruffrecords]

Hi Ian


I did some experiments and
I do not know why,
if I put a 100k resistor from the G point (mid inductor) on the ground,
all problems with the middle band are solved.
the 1db bump at 16000 is gone.
The strange curve that came out with extreme middle frequencies has disappeared.
I don't understand what I did ... can it create problems?

pin G on the  mid inductor is supposed to be connected to 0V. It is on the 3BPultec PCB. Which PCB are you using?

Cheers

ian

 

[ilfungo]

The 3 band green pcb.

 

[ruffrecords]

The 3 band green pcb.

You mean this one?

https://drive.google.com/open?id=0B_n67A1hN3qta3ZNVkNqMWpGclk

Cheers

Ian

 

[ilfungo]

Yes!
Sorry not the G point but the H point 
100k to  ground...

 

[ruffrecords]

Yes!
Sorry not the G point but the H point 
100k to  ground...

OK, that makes more sense.

The 3 band Pultec design was always a bit of a compromise. Originally I designed the poor man's EQP1A. Then I got loads of requests to add a mid band but unfortunately there is no straightforward way to do this. I tried to resist adding it but there were so many requests I felt I had to come up with something. The result is what is now the 3Band Pultec. The mid band of this is a compromise because when the boost/cut is engaged you can never turn the boost/cut down to zero. You can only turn it down to a small value. This is why the mid band has a centre off position. This means that with the mid engaged there will always be some small boost or cut but it should not happen around 16KHz. I think the problem is due to a spurious resonance in the inductor which can occur due to the capacitance between turns of the inductor. Other people have reported something similar.

Exactly why your 100K from pin H to 0V fixes it I do not know. The interesting thing is that pin H is the highest value inductance so the one most likely to create a spurious response in the  audio band. I have checked the schematic and I do not think the 100K will materially affect the overall performance so if it fixes the spurious 16KHz response I think you should leave it in.

Cheers

Ian

 

[ilfungo]

Thanks Ian
Th 100k remote the 16khz bump and create a more gentle curve between the mid and hi band...
Thanks!!!

 

[3nity]

ian.
Thanks for creating this project!
Im using the basic config. and its very smooth eq.
Thanks God for analog gear. What a difference it makes.

 

[ruffrecords]

ian.
Thanks for creating this project!
Im using the basic config. and its very smooth eq.
Thanks God for analog gear. What a difference it makes.

Glad you like it. Pierre Petit hosts a gallery of completed DIY projects based on my designs. So if you have any pics of your completed EQ you can email them to me and I will pass them on to him for inclusion in the gallery.

The gallery is here:

http://theworldin35mm.com/projectgallery/

Cheers

Ian

 

[spaceludwig]

Greetings all,

Is there a schematic available that includes the mid boost or cut circuit? I have searched all over and cannot source the one specific to this project as there doesn't seem to be a consolidated repository (not that there should be one) that includes the 3 band version of the PMP EQP1-A.

Yes, I know how to do a search but if someone can point me in the right direction, even just confirming which version of the MEQ is appropriate, I would be very appreciative.

Muchos gracias and cheers to all on this beautiful saturday!

 

[ruffrecords]

Go to the DIY tab of my web site:

https://www.customtubeconsoles.com/diy

Scroll down and click on the PMEQP1A folder. All the info you need should be in there.

Cheers

Ian

 

[spaceludwig]

Thank you very much for the quick reply, Ian! I may have some questions down the line (all pun intended...)

Cheers!

Go to the DIY tab of my web site:

https://www.customtubeconsoles.com/diy

Scroll down and click on the PMEQP1A folder. All the info you need should be in there.

Cheers

Ian

 

[spaceludwig]

Can I connect into the circuit from a buffer without the input transformer?

Can I substitute the inductors in the mid EQ with gyrators?

(or should I be posting this in the Drawing Board thread?)

 

[ruffrecords]

Can I connect into the circuit from a buffer without the input transformer?

Yes you can

Can I substitute the inductors in the mid EQ with gyrators?

I have no idea. I guess in theory it should be possible but I have no experience of gyrators. You should be aware that the Q of the mid EQ curves depend on the circuit resistances including that of the the inductor itself. I do not know if gyrators actually create inductors with a series resistive element. You are in completely new territory here.

If you get it working let me know.

Cheers

Ian

 

[spaceludwig]

Ian,

thanks again for the quick reply.

The inductor's intrinsic resistance is substituted in a gyrator circuit by a series resistance so I believe in theory this can be replicated. Is there a way to find out what the equivalent resistance is in the inductor you have chosen for your PCB?

 

[ruffrecords]

Ian,

thanks again for the quick reply.

The inductor's intrinsic resistance is substituted in a gyrator circuit by a series resistance so I believe in theory this can be replicated. Is there a way to find out what the equivalent resistance is in the inductor you have chosen for your PCB?

The value is not critical. The Q is mainly determined by the sum of this resistance and other circuit resistances. A good design ensures the inductor resistance is only a small percentage of the total so the other circuit resistances dominate the Q and the designer can properly control Q independently of the inductor. In practice, the actual resistance depends on the tap selected so it is not constant. Someone once measured the dcr of the inductor and I think they published the results here. Might be worth a search. If that fail, a value of 100 ohms would probably do.

Cheers

Ian

 

[spaceludwig]

ok, will be breadboarding in next 2 weeks and will let you know how it works out. Certainly in keeping with the Poor Man's ethos if the inductor can be swapped out for a quad + dual opamp with a few resistors and caps, though I expect the purists to be mortified at the very suggestion of substituting silicon for iron...

 

[ruffrecords]

ok, will be breadboarding in next 2 weeks and will let you know how it works out. Certainly in keeping with the Poor Man's ethos if the inductor can be swapped out for a quad + dual opamp with a few resistors and caps, though I expect the purists to be mortified at the very suggestion of substituting silicon for iron...

Absolutely; you should of course be using tubes 

Cheers

Ian