Reckhorn F-1 and others; suggestions.

[Duo]

There are a couple of Reckhorn units here which Planet10 has asked me to check out. In my usual procrastination it's taken me a while (understatement) to get to them.

The F-1 is my first subject; another one, the model number of which I don't recall, coming next when I am able to locate it in the stash.

The F-1 is a line-level active crossover unit designed to allow separation of the input signal into frequency bands suitable for a subwoofer and for the rest of the hifi. It is provided with four controls; Sub Level, High Cut A, High Cut B, and Sat Level. The high cuts being to set the rolloff frequency for the subwoofer output and are variable between 40Hz and 150Hz. The Sat Level being the rolloff frequency for the rest of the system. High cuts A and B are the two sub outputs. As far as I know both A and B are mixdowns of the left and right inputs, but I may be wrong.

My concern with this crossover is the excessive sensitivity to power line noise, EMI, etc.

I opened ours up and found two main problems with the design. First was the hum and noise problem. The reason being that the chassis was left floating. The potentiometers are bolted to the chassis and thus their cases will be at whatever potential the chassis is at (which is undefined).

Given the audio signals pass through these potentiometers, they are a likely source of noise pickup if their cases are not grounded with respect to the signals they control and if the signal lines of are high impedance.

My solution to the grounding problem was to remove the washers insulating the RCA connectors from the chassis. This causes the entire chassis and everything bolted to it to be held to signal ground level. The hum and noise problem was entirely eliminated by this one change.

The next problem was only noticed after I pulled the lid: the power supply voltage for the opamps is too high and isn't even regulated. A simple bridge rectifier and pair of caps makes the split supply which on my line voltage produces just over +/-18V. The TL082 op-amps inside are rated for a maximum of +/-18 so this is cutting it too close. The lack of regulation is also not a nice thing to see when regulators are so cheap to implement.

I took the cheap/simple route and added 7812/7912 regulators and another set of electrolytic caps before them; leaving the PCB original and sending the outputs from the regulators directly to the original filter caps. A 100nF cap bypasses the input of each regulator and is soldered directly to them.

I have taken no measurements and done no further modifications but just wish to give this a start (mainly at Planet10's request).

[planet10]

Thanx Daniel,

I'm glad that we are finally getting to these as the price & funcionality make them very interesting pieces.

The other unit is a B1.

AFAIK the F1 has a summed mono output (a good thing if you read Toole)

dave

[Duo]

I think a summed output is good no matter who you read if you are using only one subwoofer.

[Duo]

I take it Toole's dissertation is that when using a pair of subwoofers a summed output for them is best?

[planet10]

Yes. Toole uses at least 1 additional subwoofer in such away that the various nulls & peaks in room response for each woofer at out-of-phase with those from the others, flattening out response in quite a dramatic manner. Easier said than done unless you have a rectangular room (with no openings), or have the not-yet-for-sale real slick, measure, computate, and XO/EQ/time delay tool he used at Harmon that can figure it out majikally & set it up (JBL Synthesis says Flloyd when asked what/when)

dave

[Duo]

That makes sense, though I bet Reckhorn wasn't thinking about that.

[planet10]

Probably not...

dave

[PB2]

Does anyone know if this schematic of the F-1 is accurate?
http://www.reckhorn.com/graphics/f1/...putVoltage.jpg

C3 is obviously wrong, and I've read that the OP amps are TL082 types - but it looks a bit strange.
The unit is inverting, I've not seen mention of this, and it is the first stage that inverts
which is difficult to interface to the input. Seems that the designer forgot that the negative
input is a virtual ground as wired. The input impedance is 1.5K - VERY low.

Pete B.

[PB2]

I have the F-1 on order and should have some comments once it arrives and I confirm the schematic. It looks like a good value for the money and worth a few simple mods.

Pete B.

[planet10]

Originally Posted by PB2

Does anyone know if this schematic of the F-1 is accurate?

That is the one i've seen, Duo hasn't had a chance to confirm it.

dave