RECKHORN F-1 Active Crossover Modifications

[brucek]

Anyone with an older analog preamp knows the problem of trying to add a subwoofer onto their system. You need bass management. Without it, the full range mains usually combine poorly with the sub. With a DIY sub and no internal LPF, the problem gets worse.

The second system that I use in my back room office has an older Harmon Kardon analog preamp driving a set of old JBL L36 monitors and an Energy 12" sub. It suffers badly from not having any bass management. I use a BFD on the sub, but it sure isn't enough to overcome the low end extension of the mains combining with the sub to produce a poor overall response, including a big hump at 60Hz.

I decided I needed an active crossover between the preamp and mains amp. I wanted to be able to cross the mains amp and the subwoofer signal at about 80Hz using an analog device. They do sell passive crossovers, but they suffer from insertion loss and usually aren't variable. I suppose I could have purchased a Behringer CX2310, but I wanted to test out an analog unit.

The other requirement I had was that the device had to be inexpensive, and if it was necessary I could modify it to meet my needs.

I decided on the Reckhorn F-1 active crossover. The specs seemed fine and the price at about $45 was great.

I'm always interested in low end devices such as this to see the design decisions they make. There's a trade off between meeting needs and cost for the price point. The F-1 supplied both pleasant and unpleasant surprises.

The F-1 accepts line level, left and right, full range signals and outputs variable (50Hz-150Hz) left and right high passed signals, along with two mono mixed subwoofer variable signals, low passed at (40Hz-150Hz). The two mono mixed subwoofer outputs have independent low pass adjustment.

The two subwoofer outputs are gain controlled by a single gain dial on the front panel. The mains satellite left and right outputs are not adjustable.

The specs claim that both the LPF and HPF enjoy a slope of 24dB/octave and that the subwoofer distortion was 0.001% with a S/N ratio for sub and sat outputs at 95dB.


PICTURE OF FRONT AND REAR PANEL




My test equipment at home is basic at best, so I'm not here to pass the specs through a ringer. I will only reveal what I found in my testing and the modifications I carried out to meet my requirements.

I took the F-1 apart and tested it using REW software as the signal generator and used a standard oscilloscope and DVM.

Here's a pic of the unit apart during testing.




After testing the F-1, I had a list of the things I liked and a list of things I didn't like.

Here's what I didn't like:

1. Typical of these type of devices is the low frequency rolloff on the subwoofer outputs. They've incorporated a subsonic second order filter with a corner frequency that I calculate to 25Hz. This is simply too high for most users today. I feel 10Hz corner would be more appropriate.
The strange thing about the design was that the input stage was DC coupled and so they relied on the capacitors in the high pass and subsonic filters to provide the AC coupling in the second stages. Not a big deal given that the supply has a large swing (I'll talk about that below), so a bit of DC offset at the input wouldn't be a problem.

2. The satellite outputs were down by ~6dB (half) from the input level. This was obviously a design decision to support the variable subwoofer output gain pot. As an example, if I supply 2.1 voltsRMS @ 30Hz into both the left and right inputs of the F-1, the subwoofer outputs A and B are variable from ~ 0.0 - 2.6 vRMS. That's fine - a bit more than unity gain - no problems there.
But, if I supply 2.1 vRMS @2000Hz into both the left and right inputs of the F-1, the satellite left and right outputs produce ~ 1.0 vRMS. This isn't acceptable in my system. I want a device such as this to supply at least a unity gain. When I input 2.1 volts, I want 2.1 volts out.
I understand their decision. They wanted the sub pot to be able to swing the level above and below the satellite signal. Fine, but the box is full of op amps that can be set to any gain you like. Set the satellite signal to unity gain and then create a gain swing for the subwoofer that goes above and below. Easy.

3. They surprisingly used a plus and minus power supply instead of the standard cheap split supply and virtual ground technique that most of these devices use. That's the good part.
They didn't use any regulators, (which is understandable in an inexpensive device), but the show stopper was that they under spec'd their filter capacitors. This is a +/- 20 volt DC supply and the two filter capacitors used have a WVDC of 16 volts - whoops, not allowed. It wouldn't be long before these undersized capacitors were leaking excessively or arcing through. Premature failure is guaranteed. What were they thinking?

4. The slope of the low and high pass filters don't both conform to -24dB/octave. The subwoofer channels are second order -12dB/octave. The satellite outputs match -24dB/octave when the variable frequency pot is positioned less than about 3 o'clock. Above that there is some distortion in the waveform below the corner frequency and so results in about -12dB/octave (even though there are clearly a couple of second order filters cascaded together). You can see an example of the filter slopes and cutoff frequencies in the response graph below. Personally, I wasn't fussed about the slopes for my application. Others may have an issue with it though.

5. There are always phase issues with analog filters, so I can't say there's a big problem here. When the satellite outputs were within their pass band, they were 180 degrees out of phase with the input. They crossed the zero phase mark at the about the filters corner frequency. The subwoofer outputs behaved about the same, but the zero phase cross came a bit higher past the corners. So, this is fine in a stereo plus sub system, but if you had any surround speakers in a system, the decision to put the passband signals at 180 degrees might present a problem.

On the plus side:

1. The workmanship is quite good on the F-1. The printed circuit is high quality and the soldering and wiring is A-1. The dials and RCA connectors are all good quality.

2. Even though I have issues with the power supply filters, and I really don't like an unregulated supply (what do I expect for $45), I do have to give them kudos for using a full +/- power supply. They also used proper ceramic bypass on all IC's and in other spots that they were needed.

3. The circuit design is well done, and given the price point, I was pleased with the parts used and the method of accomplishing the end result.

---------------------------------

So, to correct the things I didn't like about the device demanded that I first draw the schematic diagram for the areas of interest. This is a real pain as anyone who has drawn a circuit diagram from a printed circuit board knows. There's no way around it though. Once accomplished, the modifications were relatively easy.

Here's a list of my modifications and the results:

1. I modified the gains of the appropriate stages to result in a unity gain through the satellite ports while providing enough gain in the sub stage to swing above and below the satellite signal. Operational amplifiers make it easy once identified and the new gains calculated. After the mod, if I input 2.1 vRMS, the sat output is ~2.5 vRMS, and the sub output at full clockwise on the gain pot is ~2.8 vRMS. This gain structure works well in my system. Certainly if I need more out of my subwoofer it has its own amplifier gain control.

2. As mentioned above, I plugged the values for the subsonic second order filter into the formula and came up with ~25Hz for the fc of the circuit. This is about exactly what I measured. So I modified the values in the formula to result in a 10Hz value of fc (given the parts I had on hand) and it measured that after the mod. I had a couple standard polyester film capacitors around at the correct size and used those in place of the metallized polyester film. The end result is that the signal at 10Hz just begins to drop, while before the mod, the signal was down -16dB at 10Hz. I've attached below the response graph comparing the low end rolloff before and after the mod.

3. The power supply proved to be the biggest investment of time. I replaced the incorrectly sized filter capacitors (that were originally 2200uf@16wvdc) with a pair of 3300uf@25wvdc. Then I added a couple of voltage regulators and associated tantalum output filters. The regulators were + and - 15volts, which was suffice for the op amps while taking advantage of the +/- 20 volt input level. I had to mount these on a small point to point PC board on standoffs and then run wires to and from the existing supply. This involved the cutting of runs on the existing PC board to be able to 'insert' my regulators. No big deal really.
The unregulated supply had a ripple of ~50mvolts before the modification and was unmeasurable after the mod. It makes quite a difference.

Here's a picture of the finished product. You can see the addition of the small PC board with its extra wires running to and from the main board. You can also see the rather large subsonic filter capacitors. Then there are some resistor changes.



Here is the comparison of the low end rolloff before and after the mod.



Here is a graph of the low pass subwoofer output and the high pass satellite output at three potentiometer positions of (1) full CCW (2) 12 o'clock (3) full CW. You can observe the distortion in the satellite slope when the pot is CW - not too bad really.



Anyway, I'm using the device now in my second system and it works quite nice. It does the job with a very low noise floor and good response for the sub. For me, it wasn't usable in its original form. It originally added a bit of noise, especially since it had insertion loss and I had to turn up my preamp to get back to unity gain. It also was limiting my bottom end a bit. So with a better power supply, response change and gain change, it's a pretty good active analog crossover.

A lot of people won't care about this kinda stuff, but it's a good record if anyone starts playing around with this unit.....

brucek

[Mike P.]

Since the F-1 is considered an active x-over, would the B-1 have been a better choice for bass management?
http://www.reckhorn.com/index.php?ln=en&prod=b1

[brucek]

No, I wanted to control both the mains and sub separately with their own variable crossovers.......

[Mike P.]

Ok. Gotcha!

[tkc]

Thanks for doing the work on the measurements! I just was looking at this device tonight, and was wondering if anyone had done any testing with it. This write-up was what I was looking for!

[drf]

brucek wrote:

A lot of people won't care about this kinda stuff, but it's a good record if anyone starts playing around with this unit.....

brucek

I care, I love this stuff. I must say I'm not very surprised they under spec'd the PS caps, most things these days are designed to stop working after the warranty runs out.

Unfortunately we don't have cheap stuff in Australlia, we only have stuff and expensive stuff. Soi we have to either build froom scratch or find secondhand.


EDIT: Curious to know what op-amps they used and what the noise is like with inputs open?
well done, good write up.

Dr F

[brucek]

Curious to know what op-amps they used and what the noise is like with inputs open?

They used standard Texas Instrument TI-TL082 CP JFET OP AMP in an 8 pin plastic DIP package. They're quite good if provided a proper supply. You can see the spec sheet I referenced for distortion etc....

brucek

[shr-t]

This is good stuff brucek

[drf]

brucek wrote:

They used standard Texas Instrument TI-TL082 CP JFET OP AMP in an 8 pin plastic DIP package. They're quite good if provided a proper supply. You can see the spec sheet I referenced for distortion etc....

brucek

Awesome, they are the ones I use for everything.

When I asked about noise, I was more interested in the results after your modifications, especially with the regulators, new 10Hz second order filter and uping the sat gain to unity.

Cheers
Dr F

[brucek]

I don't have the ability to accurately measure noise floor in the uvolt range, but the noise from the supply was corrected from easily measurable to below what I can measure. Most of the noise I experienced in its unmodified state came as a result of having to dial up the volume on my receiver since the F-1 lowered the level on the sat outputs so much. I certainly don't hear any noise from the unit in my system now. I use it on my old analog system.

The change to the fc of the subsonic filter simply allowed my sub to come through a bit more. I don't have the greatest sub on that system, but the filter was certainly too high for me. I had a couple capacitors that I figured would be good, so I plugged the values into the formula for 2nd order op amp high pass filters (1/2pi(R1*R2*C1*C2)^.5) and it came to about 10Hz, so that's what I used.

brucek

[benemens]

My mods.



I make subsonic filter about 50Hz LPF and move original LPF fmin under to 20Hz



Blue is before mod and greens after the mod, LPF fmin and LPF fmax.

Now i like

[brucek]

Yeah, looks good. I see from the parts you changed that you also modified the gain structure. Looks like you bypassed their PS bridge and just completed the whole supply on the extra board.

This is such a great device for the money if you're into electronics at all, because you can create a ton of different special purpose equipment from the basic block. You just can't buy all these parts seperately for the price they sell this unit at. It can be modified to do so many things..

brucek

[benemens]

brucek wrote:

Looks like you bypassed their PS bridge and just completed the whole supply on the extra board.

brucek

My extra board is between transformer and original ps bridge.

[brucek]

Smart. I should have done it that way. I wouldn't have needed the multiple run cuts I used.

does anyone has a schematic of this crossover?

dabeatisflo wrote:

does anyone has a schematic of this crossover?

+1 for that request....

Thanks a lot for the review and info on this device. I have been looking at it wondering if I could make use of it for a tube amp being fed by a CDP. I don't have your level of skills but with a little info, time and effort I might just try this.

[Mike Cason]

Hi Brucek,

I've just purchased and installed the B-1 in my system. I've got a couple of QSC amps and needed some protection so I left them at 30hz crossover point (30,50, or none). Now I've turned off the crossover point on the amps and am relying on the B1 for my bass control in conjunction with my internal Rotel 1065 settings. I'm getting terrific results now.

I'm a bit concerned about your modifications to the F-1 and am wondering if I should expect some problems with my stock B-1. Should I modify my power supply and change out some caps? I'm not that techie, but did the Arts Box Modification with the 1uf caps and built my own crossovers for my mains and center speaker. (I built all of my front speakers). I pulled the Arts Clean Box out because it was a horrible match for my system.

Thanks for any feedback

[brucek]

Now I've turned off the crossover point on the amps and am relying on the B1 for my bass control in conjunction with my internal Rotel 1065 settings.

Yeah, I would probably use REW to do a sweep of the B1 by itself and determine the dial position you want for the subsonic cutoff. You could document the dial position versus -3dB cutoff and then you would be better armed with where you want to set the B1 (rather than relying on their posted graphs).

I'm a bit concerned about your modifications to the F-1 and am wondering if I should expect some problems with my stock B-1. Should I modify my power supply and change out some caps?

I have never taken a B1 apart, so I can't say - I can only speak to my F1. You could take the device apart and read the WVDC of the filter capacitors used and then measure the plus minus supply and decide what you would like to do. You would need to find replacements of similar size, such that they would fit into the chassis and into the circuit board.

brucek

[Mike Cason]

Brucek,

I've forwarded on the link to your rebuild and the comments to Bob at CSS earlier this afternoon. I addressed the heat and voltage issues and told him about those caps you changed out. He'll be able to read that in the link. Maybe he will have an update from the mfgr on these issues.

I should hear back from him next week.

I didn't mention that it gets from very warm to slightly hot, even with no load on it...So does my H20 DTV receiver and techs say it's normal for their units.

I don't have a computer set up in the house to run any tests on the system, so it's kinda pot luck on the subsonic filters.

I appreciate your return post

Mike

[nmuntz]

I've had an F-1 (from CSS) for a while and it doesn't get very warm at all. The case is slightly warmer than room temperature. I took it apart to see if it's any different. It has the 16V capacitors and what appears to be the same transformer. It's marked 12V on the output side. My DMM measures 16 VAC coming directly off the transformer on both the + and - sides. This is with no load.

[Mike Cason]

I've now had the B-1 for a month or so and it's awesome.

It quit running warm to hot after a few days. Perhaps burn in time.

I'm recommending its use for anyone who integrates home audio to pro audio amps for excellent bass control, much better than simply using the gains on the pro amps or the freq cutoff of the receiver.

Happy camper here.....

[brucek]

It has the 16V capacitors and what appears to be the same transformer. It's marked 12V on the output side. My DMM measures 16 VAC coming directly off the transformer on both the + and - sides.

The DMM is measuring the RMS value from the transformer (not peak).

To choose capacitors for that RMS AC voltage, it requires converting to peak and adding about 10% for line variations. This value would then be the minimum capacitor voltage allowed in the design.

So: 16 volts x 1.414 x 1.1 = ~ 24.8 volts WVDC capacitors required. (25 volt capacitors are the next standard size)

brucek

[drf]

brucek wrote:

The DMM is measuring the RMS value from the transformer (not peak).

To choose capacitors for that RMS AC voltage, it requires converting to peak and adding about 10% for line variations. This value would then be the minimum capacitor voltage allowed in the design.

So: 16 volts x 1.414 x 1.1 = ~ 24.8 volts WVDC capacitors required. (25 volt capacitors are the next standard size)

brucek

Do you actually get variations in the line voltage of ten percent? :raped:

[brucek]

Do you actually get variations in the line voltage of ten percent?

Not likely, but this a design limit to determine the minimum size for a capacitor. Another rule of thumb in sizing capacitors is twice the RMS value of the secondary (which would 32 volts in this case). The next standard size would then be 35 volts. I think peak plus 10% is quite adequate in this case though. The spec is working and not surge (which they don't usually specify anymore), so 10% overspec should be fine.

The problem (as you know) with under spec'd capacitors is that they don't just fail, they explode. The dielectric (insulator) can only withstand so much voltage and then the DC will pass between the plates causing the capacitors electrolyte to heat up. This creates internal pressure that can cause the can to explode. Most capacitors have vents though (either plugs or stressed marks on the end of the can) to reduce the risk of an explosion. I've taken lots of dead equipment apart in my time that looked like a mouse was living in there, only to find it was a demolished capacitor.

There's next to nothing in that F-1 box that should get hot. If it's hot, check those filters to see if that's the source. If it is - shut it off.......

brucek