Motorola 3-Channel Crossover Frequency

[Moto_TJ]

Does anyone know where Motorola crosses everything to the bass channel and the Treble left and right channels?

I know Magnavox did their bi-amps around 1200hz, primarily to protect the horns that were rated down to 1000hz.

I have the amp and tuner to a mid-level '62 Motorola, and woofers and horns for Magnavox and was thinking about building a Motorola powered, Magnavox speakered system.

[Motorola minion]

I may look for cutoff on my 63 Motorola as a fun exercise. I'm sure it's lower for the reason you stated.

The left and right channels use a midrange 8" and high frequency 3-1/2". Center channel is a 15".

Put the two Magnavox woofers in parallel for center channel. Add a good 8" round or 6x9 speaker to extend range of horns, use the Mag crossover caps for horns. I think this is a good combination.

[Firedome]

A great 8" round are these JBL 8" D216s, some of the best ever, very efficient but unfortunately quite pricey in recent years apparently driven up by guitar nuts as similar 8" JBLs may have been used in Fender Tweeds or some such.

Dad built a ported corner speaker using one of these as the only driver for use with his low-power mono Harman-Kardon Recital II tube receiver around 1957 and it sounded quite amazing for a simple 8" full range speaker.

https://www.ebay.com/itm/185213058861?h ... SwplJht3fM

They'd be super nice in my Dumont console to improve the mid-range but too rich for my blood.

[electra225]

When I worked for Mr. Wells in his Zenith TV dealership, he parted out a Zenith TV and gave me the 8" speaker to tinker with. I still use that speaker in my shop for a test speaker. I can't kill that thing, and I've had some potent amplifiers powering it. If you could find a couple 8" Zenith TV speakers, you might find they may not sound like the fancy ones, but might be quite adequate, none the less.....

[hermitcrab]

I noticed on my Motorola the higher the volume , the more the bass rolls off , and unfortunately the single output 6BQ5 outputs require volume to fill the room ...

[Motorola minion]

1963 circuit to roll-off HF going into bass amp

1964 circuit for comparison

[TC Chris]

They look the same? Have you calculated the frequency at which it starts to roll off?

Chris Campbell

[Motorola minion]

They look the same? Have you calculated the frequency at which it starts to roll off?

Chris Campbell

Yes! You see values are the same, a second order filter, having two RC time constants. I will calculate that once I find my notes from senior year of college when we studied transfer functions. The math in filters/networks came AFTER the experimental labs unlike other courses, which helped me understand it better but that was 34 years ago

[Motorola minion]

My preliminary calculation yields 476 Hertz for the first time constant and 1200 Hertz for the second one.

Bode Plots are the graphical representation of a second-order filter's transfer function.

Notice the HS-1200 (A&B) amplifier has different negative feedback loop components for 12" and 15" bass drivers.
The HS-1200A is used in my SKR151 which has a 15" Jensen driver. The HS-1048 has a 10 K resistor, for use with a 12", which my SK112 has.

Good bass response roll-off up to 476 Hz with more aggressive roll-off of the frequencies up to 1200 Hz.

To measure the actual response would require a dual-trace oscilloscope and audio generator, which I might do.

[Motorola minion]

And in a related thread, here is the schematic for the 1962 models using the HS-968, another 6V6 3-channel amp.

HS968.jpg (116.43 KiB) Viewed 15045 times

I will eventually add the HS-775 from 1960 which is a different animal - 4 x 6BQ5, the original 3-channel stereophonic amplifier using single-ended L+R channels and a push-pull bass channel.

[stbasil]

Old thread, but I recently obtained the owner's manual for my SK-111, circa 1963, and it says 200 hz is the crossover frequency. It's now in the downloads section.

[Motorola minion]

Funny, I must have read my owners manual long before this thread and forgot that fact.

What I find interesting is a generous tone control range for bass but is measured at 50 Hz. I wonder how much boost/cut at 200 hZ

The other item of interest is that my SK112 and several others of this model family use a Sonotone 9t cart.

The cartridge shown in the operators manual is an E-V 5595, aka Astatic 133,155,485 etc.

I did have an SK107, the "junior" 3 channel amp, which had a less deluxe trim on the VM changer,
which DID have the Astatic type