mixing ohms ratings

[dreamchaser]

Hello, My new vhf radio has a 4 ohm output rating for the external speaker, my exsisting external speaker is 8 ohms, Do i need to worry about anything burnin up?

 

[QuadManiac]

Re: mixing ohms ratings

Absolutely not. The radio's rating means don't use a speaker with LESS than 4 ohms impedence. The output power will be halved into an 8 ohm speaker, but unless its too quiet for you to hear, who cares?

 

[dchris]

Re: mixing ohms ratings

Going lower in value could be a problem, reduced volume in what you will have now.

 

[QuadManiac]

Re: mixing ohms ratings

Going lower in value could be a problem, reduced volume in what you will have now.

Incorrect... Power is voltage squared divided by resistance and volume goes up by about the square root of power increase. Most audio amplifiers are voltage amps... at a given volume control setting, they attempt to hold their voltage gain fixed.

So, halving the resistance (speaker impedance from 8 to 4 Ohms) would double the power at the same voltage, increasing the apparent volume (if the speakers both have the same dB/W rating) by square root of 2, so it sounds about 1.4 times louder. Just don't go below the rated 4 ohms.

Thank you for putting up with 'The Engineering Of Audio Amplifiers - 101'

 

[Texasmark]

Re: mixing ohms ratings

Gotta jump in here from the old school.

Back in the tube days, that was probably true (high outpedance output transformers as they were voltage switchers, not current switchers).

In the interim transistor era, the output was usually directly coupled via a coupling capacitor, being the transistor was a current switcher, and 4 ohms usually gave the best impedance match at the lower transistor voltages, hence the best power transfer, hence volume/efficiency.

With todays "everything on one chip" I don't have a clue.

If you are concerned, you can go to Radio Shack and purchase an impedance matching transformer...but you will have to know other particulars, like power rating, maximum voltage and other things not on the tip of my tongue at the time.

I'm retired. Trying to forget all that "stuff" I used to have to know and implement.

HTH

Mark

 

[Barnacle_Bill]

Re: mixing ohms ratings

Gotta jump in here from the old school.

Back in the tube days, that was probably true (high outpedance output transformers as they were voltage switchers, not current switchers).

In the interim transistor era, the output was usually directly coupled via a coupling capacitor, being the transistor was a current switcher, and 4 ohms usually gave the best impedance match at the lower transistor voltages, hence the best power transfer, hence volume/efficiency.

With todays "everything on one chip" I don't have a clue.

If you are concerned, you can go to Radio Shack and purchase an impedance matching transformer...but you will have to know other particulars, like power rating, maximum voltage and other things not on the tip of my tongue at the time.

I'm retired. Trying to forget all that "stuff" I used to have to know and implement.

HTH

Mark

Hey Mark, I didn't have to try to forget my electronics training. It just come naturally with age. LOL

 

[QuadManiac]

Re: mixing ohms ratings

Gotta jump in here from the old school.

Back in the tube days, that was probably true (high outpedance output transformers as they were voltage switchers, not current switchers).

In the interim transistor era, the output was usually directly coupled via a coupling capacitor, being the transistor was a current switcher, and 4 ohms usually gave the best impedance match at the lower transistor voltages, hence the best power transfer, hence volume/efficiency.

With todays "everything on one chip" I don't have a clue.

If you are concerned, you can go to Radio Shack and purchase an impedance matching transformer...but you will have to know other particulars, like power rating, maximum voltage and other things not on the tip of my tongue at the time.

I'm retired. Trying to forget all that "stuff" I used to have to know and implement.

HTH

Mark

Virtually all present day transistor (or IC) audio amplifier circuits use voltage feedback loops, not current. Voltage gain is controlled by the volume control pot.

Now with the prevalence of class 'D' (PWM switching) amps, the pulse width and therefore the integrated amplitude, or voltage, across the characteristic impedence is contolled by the volume setting. The capacitor, in this case, is used for de-coupling to allow speaker reference to ground. Many newer systems now use a switching inverter to generate higher than supply voltages, and an H-bridge, directly coupled to the speaker to allow pulling both legs of the speaker both positive and negative relative to ground.

Bottom line, they're all still voltage, or equivalent, output (not current) devices. As long as the driven impedence is within the design paramaters, they hum happily along, ignorant of the percieved volume of the speaker.

BTW, this is NOT the case for motor drive/positioning amplifiers, of which I am much more familiar. They are typically current feedback, which allows for much higher bandwidth and therefore, more accurate positioning.

 

[JUSTINTIME]

Re: mixing ohms ratings

you are fine, might be a little quite

 

[byacey]

Re: mixing ohms ratings

I see this an older post, but using an 8 ohm load versus a 4 ohm will only drop your audio power level by 3 dB, not a huge difference as far as loudness goes.

The reason they specify 4 ohm minimum output impedance is due to the capabilities of the output transistors, if you go lower than 4 ohms the output transistors are called upon to dissipate more power than what is being utilized by the speaker load to produce audio. This causes reduced volume from the speaker and much heat produced by the excessive current through the output transistors and emitter resistors, usually followed by "the burning up" of the output stage.

I would say that unless you are operating the radio in an extremely noisy environment an 8 ohm load will probably suit your purposes just fine. I doubt you will find an 8 ohm to 4 ohm impedance matching transformer at Radio Shack, and in any case it's easier just to replace the speaker with a 4 ohm unit if required.

Just a side note: changing the speaker load from 8 ohm to 4 ohm will not always yield a 3 dB increase in audio level due to the efficiency differences of the particular speaker. Higher power speakers are typically less efficient than low power units due to stiffer compliance of the cone, more mass, and a heavier coil.

 

[Have Fun Today]

Re: mixing ohms ratings

Does the manual that came with the radio give any more details in the specifications page, normally in the back of the manual? My manual says output impedance of 4 ohms per channel, but the manufacturer didn't intend for this to limit me to 4 ohm speakers. In the section on speaker installation, it says "do not use speakers of less than 4 ohms". 8 or 16 ohm speakers are fine, I doubt any power loss would even be noticeable.

To answer your question, nothing will burn up!

 

[Texasmark]

Re: mixing ohms ratings

One final comment from me here. Back when I could hear, I never ran my radio at full volume. One of the main reasons was distortion that wasn't present at the lower levels, and the loud volume hurt my (and others') ears.

With that said, with you driving your stereo at a reduced volume you'd never know the difference.

I know 3 dB is the gain/reduction in power of 2:1. At 100 dBA, I doubt that your ear would know the difference in 100 and 103. Grin

Mark

 

[byacey]

Re: mixing ohms ratings

You're correct in that Mark, and I agree with listening at reasonable levels (I'm an audio engineer as well as an electronics technician and maintaining my hearing is important to me) but he's talking about a VHF com receiver / transceiver? which typically don't have a high power output, perhaps 14W if it's a bridge output running on 12VDC. My only concern would be that he has sufficient audio power (without clipping the outputs) to deliver loud enough audio over whatever ambient noise levels where the radio speaker is located.

The true test is to try the 8 Ohm speaker and see if it's sufficiently loud enough under all normal operating conditions. If not then maybe the extra 3dB might make the difference. We're probably talking about an SPL of around 80 to 95 dB.

Might be a useful reference for future questions but I find many people don't bother to search, but rather repost the same questions 100's of times.