Ausiospecific's Hotrod mod of the Klark Teknic EQP-KT

  I'm getting busy again with holiday stuff. But while we are waiting on things, Let me discuss how incorrect they designed their input circuit.

A passive EQ like this has to have a certain amount of current capacity from the voltage source so that the filter network maintains constant voltage drop ratio that changes when its series adjustment of its boost control is adjusted. Starving the transformer in front changes the potential current in the secondary due to transformer core loss. So what they should have done is pad the transformer afterwards to hit their target voltage range. Without the pad, the voltage scale would be too high for some of the parts they used in the filter I believe, but I would need their parts list/service data to confirm that, or bypass the attenuation and see what parts heat up in circuit with a thermal camera, which is the sign of pushing the limits of the wattage rating of the part.  We also have to see if they trimmed back resistances in the circuit, since they did starve a transformer and lowered the relative delivery capacity of current in the secondary.

Looking online others commented about increasing the high boost resistance leg, either by adding a resistor or changing the control to a higher value. It would work, to a point where there is not enough signal current for the voltage drop so somewhere in the middle of the control, the filter output would start drop signal voltage with too high of a replacement resistance value on the control. So they are not going to go very far boosting it the way its set up currently.

The two capacitors C15 and C18 shouldn't be there, but a test to see if the power supply is radiating RF noise should be checked also. At the end they should be removed for the final mod.

So, investigation of limiting factors should be examined at the full voltage scaling (at 1V input the filter circuit's signal would be at ~10V)  Because I don't care if the mod takes it to where we have more range in boost and cut than the original. If that becomes too course, we can install a range switch with a resistors to get back adjustment range contour or use a three turn potentiometer. I don't care if the clone is not the same. Because its not the same anyways, so let us see how far the design wants to go. The tubes at the end wouldn't care, and bias, gain, and negative feedback can be adjusted if needed.

I left off analyzing the input.The next step is to do signal analysis to determine if we are still within intended specs, even though impedance is different.

Picture 1: The signal at the input connector. Using a scope with a 2.5Vp-p signal @ 50 ohms My signal seems to be loaded down even through a 300 ohm H-pad attenuator. Resistance is low of the two primary coils at 35 ohms where the original was 210 ohms on the primary and secondary and the input impedance was 600 ohms instead of 300 ohms. Some may say "what is the difference". Well output circuits tend to have a higher noise floor when driven down in ohms too much and the bandwidth amplification product of the amp is also effected.

Now we see what is on the primary of the transformer after the H bridge.

Picture 2: The H-pad attenuated the signal from 2.21V p-p to 1.86V. So, it looks like they built an attenuator so their transformer wouldn't get overloaded. So what Klark really did was scale down and redesigned this product instead of copying the circuit like how everyone makes a "clone" model. Problem is that changing gain structure here has its negative impact on the circuit, as the current flowing through the primary is lower than the signal at the input connector that would cause a change in noise floor and loss of signal strength as relative core loss will occur with this circuit.

Now, let's look at the Secondary Signal

Picture 3: We see here that our secondary is 1.33V measured without load So the 1:1 transformer in the original design is not being simulated here. What I also find is that the Midas transformer is not 1:1 either, its actually a step up, but they starved the transformer and we are seeing the results. By looking at this transformer and what they were trying to accomplish, is the same voltage level as the input, but did not factor in the losses occurred in the primary. The logical course is to use a more suitable transformer.



Next slide, we see what the signal is at half of the load of their EQ circuit. (10K resistor across the secondary)

Picture 4. We observe half loading of this starved transformer as we see the voltage dropped again to 1.23V p-p. The reason why I say this is half load is the boost controls are in parallel in Ac signal analysis that have a 10K in impedance. When I measured the current it is only 42.8 uA through the 10K terminator that will be 21.4uA going through the circuit as the series boost controls will half this voltage.

In the original design the terminated at 560 ohms and a higher voltage drop was developed across the 25K boost circuit. Some people said they had limited success increasing the boost resistance of the filters. But that is because this circuit has poor current delivery, and as a result, the boost starts starving the circuit's overall current which effects filtering range.



Replacement transformer suggestions.

I am going to try an Edcor XS4400 transformer. Its a little better designed than the original, 1:1:1:1 quad-filar wound 110 ohms each coil, with a F3 specification of 10Hz-57Khz, +/-1db 20-20000 which is almost exact to a very popular UTC transformer, the A-20 without the encapsulation. At 1/4 the price of the UTC, this sleeper transformer is a modder's dream come true.

The original Triad transformer is a 600 to 600 bifilar wound which isn't currently made, but rumor has it they will be coming back to release that series sometime in 2024. Its a good transformer with a F3 specification (bandwidth measured to -3 db roll off points) is 10hz-30Khz, with a +/- 1db at 20Hz-20KHz.

Removed the transformer and read the values then double check them with the values written on them. Their transformer is like a mic transformer.

and still removing the rfi caps because they are the wrong type which is one of several places I imagine they deployed elsewhere in the circuit.

Besides that does it really matter anyways?

 I never heard of an RFI filter used before a passive base band filter, and the active circuit has that afterwards.

You act like this is your 'creation' .

Is it?

Because if it is you are clueless in selection of parts.

Surface mount in tube circuits makes it a garbage build.

The output circuit has a bunch of unnecessary loading and diodes? This thing actually has a turn on plop? I'll have to see this. But will have to draw this output circuit so I can understand this one better.

I'm glad I didn't pay much for them when they came out.

I decided to look ahead to the output circuit. Another modern circuit that will have to be undone.

Also on the front board I'm removing the cap and resistor across the secondary output(c42,r27). As that is adjusted for the transformer and not all need that.

So now if all of that is gone, the seconds and thirds of the tube harmonics shouldn't be subtracted anymore.

 Choosing the resistor and determine it will do better in a certain circuit takes time and experience. Its the construction of the resistor and how well it inter reacts with the rest of the components.

There is certain parallels to this, but its because of the effects it has on the circuit function of the amplifying device. 

So, I have a Berhinger console I'll trade you for your Neve console if you think parts don't matter.

But this one  and their LA2a copy suffer and most of it comes from them modernizing it with circuits without considering what its doing. SMD technology included.