passive crossover output impedance change (M&K)

Sonance'84

Sonance'84

moe.ron
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I have an M&K HP-80 passive crossover (high pass 80 Hz filter). The early model came set from the factory as version A, B or C, which would match the input impedance of your power amp. Later models came with a 3-position switch, so you could switch between A, B or C, depending on the amp you're using.

Mine is an early version C, which can be seen in the quote from the manual and pic below as being optimized for 47K nominal. This worked great with my previous amp, but my new amp has an input impedance of 11 Kohms, which means I need version A now. So, I'd like to modify mine to be version A.

> "SETTING THE "AMP INPUT IMPEDANCE" SWITCH
On the HP-80 back panel is a three-position switch, marked A, B, and C. Set this switch to match the input impedance of your power amp. Consult your amplifier's manual to find the input impedance of the power amplifier section only. Most amps and receivers use the C position. Consult your dealer or the amplifier's manufacturer if you can't find this spec. If you are unable to identify the impedance, set the switch to the C position.

Optimized for / Use for input impedances
A position - 10 Kohms / 10K ohms to 19.9 Kohms
B position - 22 Kohms / 20K ohms to 29.9 Kohms
C position - 47 Kohms / 30K ohms and over" <

This crossover has 2 resistors per channel, which I'm guessing correspond to the input Z of the power amp. I'm not sure if they're in parallel or series though. I'll attach a pic of the PCB, which shows that the resistors go from ground to the output RCA's.

R1 = 34.7K ohms
R2 = 20K ohms

*Could I change these resistors and make this a version A? Or, is the inductor an influence on the impedance as well? I hope with the explanation and pics, this can be figured out. I really appreciate any help, thanks in advance!
Resized_20180130_143900.jpeg P1070640.JPG
vmjmWCh.jpg
Resized_20180130_143843.jpeg
 
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Can anyone help me figure out what parts would actually differentiate between the A, B and C versions? What parts influence the output impedance?
 
It would help if we had a schematic to look at.

But the input impedance of an amp is AC impedance, and probably varies across the frequency spectrum just like the impedance of a speaker does. 47k is 'nominal' on the label there for that reason.

I'm no expert but if those resistors go to ground they appear to me to be a parallel load on the M&K's output, along with the amp input load.

As to your question, "Could I change these resistors and make this a version A?", can't you just set the switch on A? If you're concerned that the resistors don't seem to match the stated impedance ranges, I think it's because the amp input impedance has to be figured in.
 
toxcrusadr said:
It would help if we had a schematic to look at.

But the input impedance of an amp is AC impedance, and probably varies across the frequency spectrum just like the impedance of a speaker does. 47k is 'nominal' on the label there for that reason.

I'm no expert but if those resistors go to ground they appear to me to be a parallel load on the M&K's output, along with the amp input load.

As to your question, "Could I change these resistors and make this a version A?", can't you just set the switch on A? If you're concerned that the resistors don't seem to match the stated impedance ranges, I think it's because the amp input impedance has to be figured in.
Click to expand...

As mentioned above, I have an early version which doesn't have the 3-way switch. That's why I'm here asking. :biggrin:

I'm pretty sure those resistors are parallel to each other as well, both going to ground.
 
Oops, sorry, I missed that yours doesn't have the switch.

I think you could certainly do this as long as you hook it up right, and we won't be much help with that, at least not without a schematic. If you need to draw out the current arrangement and what you're proposing we could look at it and confirm it.
 
I'll see if I can try to draw it up...or maybe have a local friend do it. Is there a "how to" write up on drawing out a schematic for the first time?
 
None that I can think of offhand. You don't need the whole circuit of course, only this little part. So all you need is some zigzags for resistors and a small bag of plusses and minuses.:bigok:
 
Here's a blown up and cropped pic of 1 channel of the crossover, to simplify things. This is the same thing I'm looking at to draw the schematic, so hopefully someone else can help me along.

Red dot = input signal rca (+)
Yellow dot = output signal rca (+)
Blue dot = ground (-)
Purple boxes = caps, resistors & inductor
R1 = 34.7K ohms
R2 = 20K ohms

vmjmWCh.jpg
 
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The rca's are the inputs and outputs...it's not a speaker level crossover, it goes in between a preamp and power amp.

Please refer to the last pic in the first post, it's shows the rear panel.

In my post just above, the inputs and outputs are labeled. The + signal comes in the top rca and the + output goes out of the bottom rca. They share the same ground. I don't think L1 has multiple windings.
 
Not quite, I'm not sure of the right layout but:

C1,2,3 are in PARALLEL with each other, one side connected to Red (+in) and the other side connected to Yel (+out).

L1 seems to have multiple windings? 5 terminals at left, and two separate sets of two at right that go different places. It should show on the schematic as maybe 2 parallel inductors with one end in common and the other ends going to R1/R2 and Yellow respectively. Maybe. I wish I was better at this stuff.

The left side of L1 does not seem to be connected to anything on the part of the board we can see so I don't know where that goes.
 
Just crossposted with you and I deleted my earlier post about the inputs and outputs because I had missed the list above the pics that explained all that. :D
 
The original goal was to convert this from a 47KOhm to 10 KOhm output impedance. At this point I do not know how to do that. The two resistors (or at least one of them) would have to change but it's not obvious what the value should be, at least to this rookie. The AC impedance of the circuit is not equal to the sum of DC resistance of those resistors.
 
I really appreciate the help!

I think the left side of L1, in the pic of the bottom of the board, is just connected to ground. One of the solder pads touches the mounting screw hole, which is ground.
 
Ah, OK.

I took the liberty of posting in Solid State with a link here, asking for more eyes. It may not be possible to figure out the correct resistors here but if anyone knows, someone over there will.
 
Awesome, thanks. I posted over on diyaudio forum, but only asking for help turning this into a schematic. I figured once we had that, we could focus more on the mod and if it can be done.
 
You can't just change the resistors on the output, it will affect the crossover frequency. You would have to change the caps as well. Alternatively you could add a resistor in series with the 2 output resistors and the caps, such that the impedance seen by the caps stays the same when the output resistors are 10K total (20K each.). They are in parallel.
 
The combination of R1 and R2 = 12.6K. Now knowing nothing else about this other than it is designed to have the correct crossover frequency when loaded with 47K, 12.6K in parallel with 47K = 10K. So the simple solution is to remove R1 and R2 for use with your amp with a 10K load. Should work fine.
 
dr*audio said:
The combination of R1 and R2 = 12.6K. Now knowing nothing else about this other than it is designed to have the correct crossover frequency when loaded with 47K, 12.6K in parallel with 47K = 10K. So the simple solution is to remove R1 and R2 for use with your amp with a 10K load. Should work fine.
Click to expand...

I think I see where you're going, but could you expand on that a bit further?

Also, the HP-80 is an LC filter, if that makes any difference.
 
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