Pultech EQP-1a: Building only the low end section.

[expat]

Hello all...

I'd like to build just the low-end section of the EQP-1A with different frequencies - (Boost/Attenuate; 60hz, 80hz, 100hz, 120hz).

My question is this:  Is this done with just LC networks or would inductors be needed? 

After looking at some of the schemos out there it appears the inductors aren't needed for the low-end, but I wanted to check with the pros.

 

[ruffrecords]

Both the bass boost and cut in the EQP1A are done with simple RC networks,

Cheers

Ian

 

[expat]

Hi Ian!!!

Been following your various posts for years -thanks for chiming in, I was hoping you would!!

So the rest of the eq circuit isn't making an impact on the low-end side?

What would be the other most crucial components in your opinion (or anyone else's?) for the low-end section:

Input AND/OR output transformers?
Tube make-up gain?

Much thanks.

 

[expat]

I do understand that tubes will add their harmonics, etc, and both in & out transformers would imbue their character.  I'm just wondering what of these things (besides the passive EQ circuit) would be considered most important for the low-end of it.  Like, if it was on a list in order of importance/most essential.

 

[ruffrecords]

Transformers tend to be more coloured at the low end so I would expect them the be a significant influence. Tubes tend to distort more as the output level gets higher but it is no worse at low frequencies than any others.

Cheers

Ian

 

[expat]

Thanks, Ian!!!!

Is the input transformer more or less important than the OT for this or do you really need to go for both in your opinion?

If this unit is to be used mostly for kick and bass guitar, could you make a suggestion as to the transformer core?
Steel?  Nickel?  Mix?

GREATLY appreciate your expert input!!!

 

[ruffrecords]

Thanks for the endorsement but I have to say I am not a transformer expert. It is a very complex subject and I am picking it up in dribs and drabs, mainly from great guys like CJ.

Go and look up BH curve and see if you can find a picture of a typical one. You will see it has a fairly straight bit in the middle and curved bits at the top and bottom. The curved bits are what cause distortion and as they occur at the top and bottom of the curve,  this means transformers tend to distort at very low levels and very high levels. Different core materials have different shaped BH curves and produce different amounts of distortion at different levels. From here on in its complicated but, as a rule, iron/steel distorts more than mumetal and radiometal

In a line level device like an EQ, the input and output levels are pretty much identical so both the input and output transformers will contribute to the distortion. In good quality transformers at normal operating levels the distortion is very low; generally below the level of audibility. However, transformers do tend to distort more at low frequencies so kick and bass guitar are likely to produce more distortion,

To summarise; input and output transformers have similar levels so in this instance both are contributors. Iron/steel is likely to produce the greatest amount of distortion.

Cheers

Ian

 

[ChrioN]

Just use big enough cores with enough inductances in  the windings