Reckhorn F-1 Active Crossover - Home Theater Forum and Systems -

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post #1 of 18 Old 05-25-10, 07:24 AM Thread Starter
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Reckhorn F-1 Active Crossover

This is post that I attached to the original brucek post about his modifications.
Nobody has answered or commented, so I will try my luck with a new post.

Hello, I am new to the forum and was wondering if someone could help me with some questions.
All answers are greatly appreciated.

I am attempting to modify my Reckhorn F-1 to get unity gain from the satellite output.
I modified the R3, R12 - change from 3.9K to 10.7K

The problem is that I am using a scythe kama amp with a TEC pre-amp and the volume is still a lot less with the Reckhorn in the chain that without the Reckhorn.

I am new to this modding game and would like to ask, if it is possible to get more than unity gain from the sat outputs? If so, what are the affects on clarity, humming etc.

Thanks alot and look forward your comments.
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post #2 of 18 Old 05-26-10, 02:21 AM
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Re: Reckhorn F-1 Active Crossover

Do you have a schematic for the unit? Bit hard to comment otherwise.
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post #3 of 18 Old 05-26-10, 05:06 AM Thread Starter
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Re: Reckhorn F-1 Active Crossover

Here is the schematic.

Any thought would be greatly appreciated.
Attached Thumbnails
Reckhorn F-1 Active Crossover-f1higheroutputvoltage.jpg  

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post #4 of 18 Old 05-26-10, 09:36 AM
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Re: Reckhorn F-1 Active Crossover

if it is possible to get more than unity gain from the sat outputs?
I'm waiting for a reply from a guy who does F1 modifications, we'll see what he says.

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post #5 of 18 Old 05-26-10, 09:38 AM Thread Starter
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Re: Reckhorn F-1 Active Crossover

Hello Mike P,

Thanks for the help and looking forward to hearing what information you get.

I am new, so I can't respond to your mail. (I believe)
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post #6 of 18 Old 05-28-10, 12:20 PM
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Re: Reckhorn F-1 Active Crossover

You should be able to respond now. After 5 posts, you can PM and post links and images.
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post #7 of 18 Old 06-25-10, 12:14 PM
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Re: Reckhorn F-1 Active Crossover

Looking at the schematic provided; the resistors R3 and R12 set the gain for the subwoofer channels. The two channels are joined together after R1 and R10 but can be separated by cutting a trace and adding a wire so that R1 continues onto VR2 and R10 to VR1 to allow stereo operation.

For the satellite output, gain is set by R18 and R27. All of the gain-setting resistors in this unit will have the effect of increasing gain if their value is increased. With the values shown in the schematic, the satellite gain should be about 1.4. A test with my un-modified F-1 by ear shows that its gain is less than 1 though, so this may be a revised schematic or I'm missing something.

Regardless of all else, you can definitely increase the gain beyond unity quite a ways by increasing the values of R18 and R27. You must consider the gain of the input networks (R35, R5, R38, and R20 for left and R37, R14, R36, and R29 for right). Their gain in the schematic is .322 (for example, R5 and R20 appear as if connected to ground (virtual ground) and thus are in parallel with R38 in the math. A resistor divider is then formed between the parallel resistance of R5, R20, and RR38 and the input resistance R35). This is for the left channel and the same applies to the right.

Once you know the gain of that network you can calculate the gain out of the first op-amp by considering R18 for the left channel and R27 for the right. Consider putting a volt into the input network: 1V * .322 = 322mV. Thus 322mV across R20 or R29 makes 322ľA which the op-amp must drop across R18 or R27 to bring its inverting input to zero. 322ľA across 6.2k as shown in the schematic makes 1.996V and thus the total gain to this point is 1.996. The reason I calculated 1.4 for the whole circuit on the satellite filter is that you see another resistor divider between the first and second filters because of R63 and R67 for the left channel, and R64 and R68 for the right. The values shown (2.2k and 4.7k) give a gain of .681 and thus .681 * 1.996 = 1.36 (rounds to 1.4).

The total gain of the system once inside your audio system will be slightly less due to the 1k output resistor. This will be reasonably insignificant with most amplifiers, but some will present a low enough input impedance to cause a noticeable effect. For example; if you have an amp with 10k input impedance you will get a reduction in gain by a factor of .909 ((1000+10000)/10000) bringing the complete gain from 1.36 down to 1.24.

This may all seem complex, so if you don't want to do the math you are welcome to experiment with different values for R18 and R27 in the circuit. A good place to start would be to double the resistance and see what the effect is. Then pick a value in between if you find that to be too much gain.

As for hum and noise: the crossover itself is unlikely to generate significant levels of this but it will amplify any junk from the previous piece of equipment by its gain so you have to think of that. I wouldn't worry about it most situations. The only big issue you might run into is clipping in the crossover itself but this is highly unlikely. You'd have to have an output swing from rail to rail, which for an unmodified F-1 this should be around 15 to 18 volts peak (far more than you normally ever see at line level for RCA interconnects).

I hope this is helpful.
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post #8 of 18 Old 06-25-10, 04:28 PM Thread Starter
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Re: Reckhorn F-1 Active Crossover

Thank you very much for the reply. I really appreciate it.
I am quite illiterate, when it comes to schematics.
Something I didn't write in my initial post.
I changed the R18 and R27 from 1.5K to 3.9K and the output was still too low.
Should I than go with a higher figure?
Any suggestions on what for a figure?

Thanks again.
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post #9 of 18 Old 06-25-10, 05:25 PM
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Re: Reckhorn F-1 Active Crossover

Henry: I'm glad I can help.

If 3.9k wasn't enough then there's no trouble going further. I'd try the 6.2k as seen in the schematic on this page. That should get you just over unity gain.

Best of luck.
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post #10 of 18 Old 07-05-10, 12:37 PM Thread Starter
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Re: Reckhorn F-1 Active Crossover

Hello W1ngs,

I finally found the time to change the resistors and I would like to report that all is well, but I have encountered a new problem. When hooking up the reckhorn to my system, i experienced a dreadful hum and distortion. I tried to play around with the connections, but to no avail. The amp I am using is the scythe kama bay amp 1100, preamp tcc tc754 connected to a power supply and an REL quake subwoofer. Is this caused by a grounding issue? I don't want to give up on the Reckhorn, but finding it hard not to think, it would be better to get rid of the thing. Any help would be greatly appreciated.
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