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Spica TC-50 Upgrade

Spica TC-50 upgrade

We no longer have matched pairs of our GR-T2 tweeter (that were used for this upgrade) in stock. They were phased out for the newer GR-T3 tweeters. So we can no longer offer this upgrade.  

The Spica TC-50 is a discontinued design built from 1984 till 1991 in two series TC50/50i. This upgrade was made on the standard TC-50. It was a popular speaker designed by John Bau that now has a bit of a cult following.

This was a very well designed little speaker for its budget price point ($420 to $550). It utilized time-aligned drivers and had a very smooth response. Drivers and components were matched to close tolerances as well.

Having modified a lot of commercial speakers, many of which are often an engineering mess, it is refreshing to see a designer take a high level of care to design a very accurate speaker at this price point.

Still, there are some issues with it.

One of the problems is that the drivers used are currently unavailable due to being discontinued.

I have no solution for those looking for replacement woofers, but replacing the tweeters was no problem. Our GR-T2 tweeter worked great with a minimum amount of box modifications.

When I was brought this pair there was one working tweeter between the two of them. I was able to take measurements of the intact stock unit as a base line. NOTE: The speakers modified were TC-50s.

Pictured below is the frequency response.

This was about as good a response as I got from the original version. The most ideal height was at a mid point between the woofer and tweeter. Moving up any higher caused the tweeter and woofer to begin to get further out of phase and cause cancelations in the response. In fact they were still a little bit out of phase enough to cause a slight cancelation in the 400 to 1,400Hz range. Allowing the tweeter to play this low might have been putting a little added stress on it if it were played very loud.

The dive in the response in the top octave is a result of being so far off axis. With the baffle tilted back 34 degrees you are 34 degrees off the axis of the tweeter when level with the speaker.

All measurements were taken at 1 meter with 1 watt input.

Below are the vertical off axis responses.

The Red line was made on tweeter axis. Each additional response curve was made by moving the mic up 4". Subsequent measurements were Orange, Yellow, and then Green.

Below are the Horizontal off axis responses.

The Red is on axis with Orange, Yellow, Green and Blue being 10, 20, 30, and 40 degrees off axis.

Below is the spectral decay. There was a very small resonance in the tweeters response at 3kHz but all in all very clean.

Below is the impedance response.

The minimum impedance dip on the woofer side comes close to 4 ohms.

Pictured below is the stock crossover network.


The Iron core inductors and electrolytic caps used were at the very bottom of the performance scale but they were also at the bottom of the cost scale and this was a budget priced speaker. Significant improvements can be had from higher quality components.

Getting them off of the hand cut Aluminum circuit board and point to point wiring them with a good quality wire will result in another performance improvement.

The binding posts were also bottom of the barrel in quality using a cheap steel screw with a plastic cap on it. This is another easy upgrade.

The only real problem that I noticed with the particle board box was that there was a little bit of a resonance issue with the back panel. This is easy to fix as well.

My first issue was getting our GR-T2 tweeter to fit the old hole in the baffle.







The shielding cup that covers the magnet wouldn't fit. With a little light milling with a die grinder the hole was easily opened up. A router could have been used as well.

The old gasket material had to be removed too. This was scraped off with a putty knife.

Then I carefully set the depth of the cutter on my router to remove about 1mm of surface area to make the counter sunk area deep enough for the GR-T2 tweeter. This kept it flush with the front surface when mounted. Care must be taken here not to get into the sides of the hole and to remove material in the floor only.

Next was a slight modification to our tweeter. The faceplate of the GR-T2 tweeter is a standard 104mm in diameter. The previous tweeter was slightly smaller. The solution was to remove the plastic faceplate from our tweeter and run the edges across an electric sander in a circular motion removing about 1mm of material all the way around the tweeter. Sanding down the plastic faceplate was actually pretty easy and only took about 5 minutes per tweeter.

Once the tweeter fit the hole I then mounted the tweeter and the woofer after filling the box with stuffing. The wire from the drivers was run out the hole left by removing the binding posts. I could then design an outboard crossover by the measured responses.

See new response curve below:

I left the woofer network almost the same as the stock one. The main inductor value was increased slightly to better match the output level of the tweeter.

I went with a little different configuration on the tweeter and a little steeper slope. This kept the drivers in phase and greatly improved the polar response. No longer do the drivers cancel each other out if you increase vertical listening height.

Below are the vertical off axis responses taken after modification.

The greatly improved vertical response will give a much more even in room response and not limit image height.

Below is the off axis horizontal responses after the mod.

Again the Red line is the on axis response and Orange, Yellow, Green, and Blue are 10, 20, 30, and 40 degrees off axis. The response in the horizontal plane is greatly improved as well.

The spectral decay seen below is again very clean and slightly improved.

The new impedance reflects the the steeper slope used in the tweeter circuit, and its slightly higher 6 ohm load.

The big question is how the steeper slope affected the step response. Seen below, the Red line is the original and the Black line is the upgraded one. There is a very marginal difference here.

By request this new network received all Alpha Core foil inductors and Sonicaps everywhere but in the woofers notch filter. Lynk resistors were used exclusively. Wiring was Vampire Wire continuous cast Copper. All components was of course point to point wired. All Sonicaps were measured and matched in pairs.

Here is a pic of the network after installing it. Note that it is installed on the bottom of the speaker to allow the rear panel to be coated with New Res. It is also more stable in this position.

The old binding posts were removed from the cup and replaced with our GR Research binding posts. Our binding post were removed from their cups and placed into the old cup. Re-using the old cup meant no new holes had to be made in the box. The GR Research binding posts are solid Brass and Gold plated.

As previously mentioned No Rez was also added to the rear wall to dampen out the panel vibrations.

Finally, the original damping material was placed back into the box.

The original woofer did not have a gasket and did not fit well into the original hole and leaked air from around the edges. To remedy this a seal was made with a bead of plumbers putty. It worked out very well.

I have put together a complete upgrade kit pictured below.

Parts include a tested, measured, and matched pair of GR-T2 tweeters. Unlike the picture above this kit now uses the Erse perfect lay inductors, Erse poly caps, Sonicap by-pass caps, Lynks resistors, OFC wire, high quality solder, screws, heat shrink, two pairs of binding post, and 1/3rd of a sheet of No Rez. 

Sorry, this upgrade kt is no longer available. But if you have one of these vintage speakers and would like to upgrade them then please give us a call and we will assist you in any way that we can. 


Your Price: Call for pricing
Item Number: Spica TC-50 Upgrade
Manufacturer: GR Research
Manufacturer Part No: Spica TC-50 Upgrade


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