Purpose of Speaker Transformer?

[Mike B.]

What is the purpose of the transformer (or coil) mounted on the back of early speakers? Was it to protect the tube amplifier in some way or to perhaps better match impedence? Some after market sellers insist that the speakers must have the transformer but give no reason why.

Thanks,
Mike

[Gary C.]

Transformer matches impedance *NM*

[Jack H.]

Noo.....who told you that?

The transformer isn't there to 'match impedance'... It's there to work around the drawback(s) of the archane DS-501 transistor that Delco Radio used to pump current to speaker.

The DS-501 is one of the oldest transistors on earth! It was made under the original license Bell Laboratories auctioned off when they 'perfected' the transistor in the mid-50's. It's based on germanium vs. silicon and that introduces some limitations to its performance profile.

Specifically, if you use it as a current pump and EVER drive it above its supply rails, kiss the sucka bye bye! The purpose of an audio output transistor is to convert voltage waveforms to current waveforms and drive the electro-magnet in the speaker. The problem is the speaker's electro magnet is an engergy storage device and it presents 'flyback' to the drive transistor.

Only a portion of the transformer that's mounted on the speaker is actually used and it's configured to construct an R/L filter between the speaker and the drive transistor to solve this problem. That's why you can either blow out or damage the DS-501 audio drive transistor if you connect one of these radios to an ordinary speaker that lacks the in-line filter!

On some other Delco radios using the same DS-501 drive transistor, there was enough space inside the radio chassis to mount the filter there. With these radios, you can freely substitute speakers and only suffer a modest loss of optimum power transfer if the substitute speaker's impedance profile isn't dead nuts on to that of the original Delco speaker...

The other complication of using a speaker that lacks the protective filter, is poor sound quality. Since the Delco designers KNEW the R/L filter was there, the upstream audio is intentionally skewed (bass/treble balance) to accommodate it. That's why you get a low volume and 'tinny' sound if you connect the radio to any speaker that lacks the protective filter circuit on it!

[Gary C.]

Re: Noo.....who told you that?

Jack, you're correct about the speaker transformer doing multi functions. Just remembered the differences between the primary and secondary windings of the transformer and forgot about the isolation functionality for the pair of 2N173 transistors. AT&T Bell Labs was required to license all patents by a consent agreement with the Federal Government. Thank you for correcting my post. Gary....

[Mike B.]

Re: Purpose of Speaker Transformer?

Thank you both for the informative explaination.

I wonder what would happen if such a speaker was used on a radio that did not require the transformer. I suspect the tone quality would be affected, but the electrical filter function would remain passive. Just a guess.

[Jack H.]

Re: Noo.....who told you that?

Methinks they used half of a transformer because it had, by serendipity, the inductance profile they needed for the filter and it was an off-the-shelf part built in substantial qty that required no tooling-$ to obtain vs. winding a special inductor for the filter.

The old wife's tale about the transformer being there to accomplish impedance matching really gets me chuckling! That's saying the electrical engineers at Delco Radio were absolute Bozo designers... They made an 'opps' in the design of the speaker's impedance profile (heck, it was their speaker; they could make it have any impedance they wanted!) and they solved the 'problem' by tacking on these extra cost, exterally mounted, components!

That's, of course, crazy nonsense. While being a radio designer at Delco in the 50-60's may not have been a 'lunar landing' design challenge to spiffy EE's graduating from college, GM and Delco didn't hire Bozo engineers... They were solid, well-trained, skilled professionals that I had the PLEASURE of interacting with when I worked at Texas Instruments.

[Jack H.]

Re: Purpose of Speaker Transformer?

That speaker would sound pretty 'hokey' connected to a conventional radio designed for nominal 'flat' audio response and traditional 4, 8, 16 ohm impedance....

The typical syndrome was:

The speaker (typically exposed to water infiltration from straight axle convertible configuration) aged and wore out. The current 'Bubba' owner of the car decided he'd replace the speaker and 'soup up' the audio system by installing a cheaper than Delco original but superior spec off-the-shelf 6x9.

Once connected, the new 'hot shot' speaker sounded TERRIBLE. There was almost no volume and the sound quality sucked big time. Well, by that time, it's highly likely that Bubba either burned out the DS-501 drive xsistor OR damaged it...

So, the original radio hit the garage floor with a spiffy after market fully solid-state AM/FM radio with more feature functions installed in its place. You can find these 'pull' radios on eBay just about every day of the week.

They're ususally sold with some kind of disclaimer like 'was working when removed'. But you're best off viewing these offerings as a 'core' purchase--good enough to send out for professional restoration.

[Alan Drake]

Re: Purpose of Speaker Transformer?

If you take a convential 8 ohn speaker add the transformer back will it work as expected or close and what damage may result to drive trans?

[Jack H.]

Yes,,,

but look again and you'll see it's not just the transformer that forms the protective filter...it's the inductance component of one side of the transformer + a resistor to achieve an RL filter. So, you need both components for the system to be right.

The local radio service tech at our United-Delco shop here in Denver (does speedometers, radios and general auto electric) has EXACTLY the setup you suggest on his work bench. He 'lifted' a transformer/resistor network from an old Caddy radio speaker (Corvette wasn't the only GM car that moved the protective circuit out of the radio chassis and onto the speaker to conserve space) and mounted it in-line to an off-the-shelf radio speaker of roughly the same native impedance rating as the original Delco speaker. That's what he uses to bench test the older radios...

[Robert P.]

Re: Purpose of Speaker Transformer?

Mike, I'm amazed at all the confusing opinions on the purpose of the transformer on Corvette speakers. It's main purpose is to absorb the DC current comming out of the output of the radio & bypass it to ground to keep most of that DC current from passing through the speaker. It's second purpose is to generate the negative voltage to pull the speaker cone inward. Without the transformer, you will have only the positive parts of the sound's wave form & will be missing the negative half of the sound's wave form. It will be very distorted & very low in volume. Also,without the transformer to bypass that current,most of it will flow into the speaker & will eventually damage the speaker,not the radio. However,if a substitute transformer is used that is smaller than the 1.5"x1.5"x1.5" Delco transformer,it will sound poor & damage the radio because it has less wire inside which will pass too much current. The origional Delco transformer is 1.5 ohms & idles at 1 amp DC. All the radio's I've repaired that came in with an aftermarket speaker & transformer measured only .5 to 1 ohm. They draw 2 to 3 times the current out of the radio & over heat the output transistor in the radio. For that reason I always use an origional Delco transformer on the new Corvette speakers I sell.

So in summary, you need a correct transformer or it will sound distorted & damage the speaker,not the radio. However,if a substitute transformer is used that is smaller than the 1.5"x1.5"x1.5" Delco transformer,it will sound poor & damage the radio,not the speaker.

Robert Pacini at:
Precision Stereo Repair 8441 Seneca Turnpike New Hartford,NY.13413
Ph.315-797-5219 email: oldstereo@hotmail.com




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