I am confronted by this issue constantly and am always amazed by how people are so anal about correct impedance matching.. (The Secondary) that is..The term impedance was coined by Oliver Heaviside in July 1886. Arthur Kennelly was the first to represent impedance with complex numbers in his paper, Impedance (IEEE, 1893).
At the time it was mainly used by power companies to measure loss due to extremely long runs at a given frequency... As the term applies to audio transformers it is rather antiquated..Since the frequency's in audio amplifiers are in constant flux.....Impedance is frequency dependent with a 1k signal applied the impedance would match... But we all know when we are applying a guitar signal it is much broader..82-1050..
So in reality output impedance is constantly shifting.. I am not saying the property shouldn't be ignored..It no doubt still applies to a more efficient transfer of energy from the plates to the speakers... However I don't really care about maximum transfer of energy.... I care about tone..As long as the output transformer doesn't get hot on on you... Knock yourself out...(Keep in mind I am referring to slight mismatches here not a 16 ohm tap in to a 2 ohm load common sense!!)
If you want an open sweeter compressed top end use the 4 ohm tap...As you go up the sound will have a meatier more harmonically richer tone.. But the high's suffer a bit.. Sort of like a pickup!!!
Being a guitar amp manufacturer I have to stick closely to the laws placed there by the industry..But you DIY's have the luxury of breaking those rules in the name of tone... Just like in music know the implications of the rules you are breaking...
Paul at Mercury Magnetics concur es...
OK all you engineers you can shake your head now!! LOL!!!!
Tony VVT
Output Impedance Matching My Take
Moderators: pompeiisneaks, Colossal
Re: Output Impedance Matching My Take
Yes, I understand how the impedance is dependant on frequency.
I wonder how the, "you must match speaker impedance with secondary impedance" started?
So the impedance of the OT changes dynamically as well as the speakers impedance. I wonder how the two coincide with each other?
In other words, say on a graph, does the tranny impedance follow the speaker impedance or do they react differently to frequencies?
When you said use the 4 ohm tap, are you talking about using a 8 ohm speaker load with the 4 ohm tap?
And would it be safe to run the amp off the 4 ohm tap to a 16 ohm speaker load?
Because I always read that you can mismatch by 100% but you shouldn't go beyond that.
Interesting topic.
I wonder how the, "you must match speaker impedance with secondary impedance" started?
So the impedance of the OT changes dynamically as well as the speakers impedance. I wonder how the two coincide with each other?
In other words, say on a graph, does the tranny impedance follow the speaker impedance or do they react differently to frequencies?
When you said use the 4 ohm tap, are you talking about using a 8 ohm speaker load with the 4 ohm tap?
And would it be safe to run the amp off the 4 ohm tap to a 16 ohm speaker load?
Because I always read that you can mismatch by 100% but you shouldn't go beyond that.
Interesting topic.
Tom
Don't let that smoke out!
Don't let that smoke out!
Re: Output Impedance Matching My Take
What's it all about?
I guess the other side of this is related to the ends people will go to in trying
to tweak the thing into oblivion, for oblivion sake. No doubt I and most all on this site are here for some of that enjoyment. If the end of all is in owning the most tweaked amp on the planet, cool. If your gig is in what music is played through your ultra fine tuned rig, way cool. This is oft the most overlooked aspect of a lot of threads, only a few examples exist. Like C Parker said,' first, master your instrument, then forget all that shit and play'.
I guess the other side of this is related to the ends people will go to in trying
to tweak the thing into oblivion, for oblivion sake. No doubt I and most all on this site are here for some of that enjoyment. If the end of all is in owning the most tweaked amp on the planet, cool. If your gig is in what music is played through your ultra fine tuned rig, way cool. This is oft the most overlooked aspect of a lot of threads, only a few examples exist. Like C Parker said,' first, master your instrument, then forget all that shit and play'.
Re: Output Impedance Matching My Take
I was mainly referring to slight mis-matching hence the word anal!!
As for how the term impedance was introduced my guess would be back in the 50's when guitar amps were hitting the market a more efficient transfer of energy was closely watched.. since everyone wanted louder more powerful amps all the way up through the 70's.. The early designs were pushed to there limits..Now with the PA systems doing the work nobody goes into a club anymore and dimes there amps..If you are running your amp on 3 or 4 slight mis matching is not going to harm a good output transformer...I ran a Blackface bassman 4ohm on 2-8-16 ohm cabs 4 nights a week for 10 years.. The amp sounds as good as the day I bought it..I never had to run it past 5..
Like I said know the rules..The OPT needs to see a load.. it is rougher on them if your load is lower than the tap...As long as you don't really push the amp for long periods of time and the OPT doesn't get hot...Have a nice day!!
Tony VVT
As for how the term impedance was introduced my guess would be back in the 50's when guitar amps were hitting the market a more efficient transfer of energy was closely watched.. since everyone wanted louder more powerful amps all the way up through the 70's.. The early designs were pushed to there limits..Now with the PA systems doing the work nobody goes into a club anymore and dimes there amps..If you are running your amp on 3 or 4 slight mis matching is not going to harm a good output transformer...I ran a Blackface bassman 4ohm on 2-8-16 ohm cabs 4 nights a week for 10 years.. The amp sounds as good as the day I bought it..I never had to run it past 5..
Like I said know the rules..The OPT needs to see a load.. it is rougher on them if your load is lower than the tap...As long as you don't really push the amp for long periods of time and the OPT doesn't get hot...Have a nice day!!
Tony VVT
Re: Output Impedance Matching My Take
I know this was dicussed in another thread but here's some more info on the mismatch:
http://www.geofex.com/tubeampfaq/taffram.htm
Authored, assembled and edited by R.G. Keen
Quote " It's almost never low impedance that kills an OT, it's too high an impedance.
The power tubes simply refuse to put out all that much more current with a lower-impedance load, so death by overheating with a too-low load is all but impossible - not totally out of the question but extremely unlikely. The power tubes simply get into a loading range where their output power goes down from the mismatched load. At 2:1 lower-than-matched load is not unreasonable at all.
If you do too high a load, the power tubes still limit what they put out, but a second order effect becomes important.
There is magnetic leakage from primary to secondary and between both half-primaries to each other. When the current in the primary is driven to be discontinuous, you get inductive kickback from the leakage inductances in the form of a voltage spike.
This voltage spike can punch through insulation or flash over sockets, and the spike is sitting on top of B+, so it's got a head start for a flashover to ground. If the punchthrough was one time, it wouldn't be a problem, but the burning residues inside the transformer make punchthrough easier at the same point on the next cycle, and eventually erode the insulation to make a conductive path between layers. The sound goes south, and with an intermittent short you can get a permanent short, or the wire can burn though to give you an open there, and now you have a dead transformer.
So how much loading is too high? For a well designed (equals interleaved, tightly coupled, low leakage inductances, like a fine, high quality hifi) OT, you can easily withstand a 2:1 mismatch high.
For a poorly designed (high leakage, poor coupling, not well insulated or potted) transformer, 2:1 may well be marginal. Worse, if you have an intermittent contact in the path to the speaker, you will introduce transients that are sharper and hence cause higher voltages. In that light, the speaker impedance selector switch could kill OT's if two ways - if it's a break befor make, the transients cause punch through; if it's a make before break, the OT is intermittently shorted and the higher currents cause burns on the switch that eventually make it into a break before make. Turning the speaker impedance selector with an amp running is something I would not chance, not once. "
http://www.geofex.com/tubeampfaq/taffram.htm
Authored, assembled and edited by R.G. Keen
Quote " It's almost never low impedance that kills an OT, it's too high an impedance.
The power tubes simply refuse to put out all that much more current with a lower-impedance load, so death by overheating with a too-low load is all but impossible - not totally out of the question but extremely unlikely. The power tubes simply get into a loading range where their output power goes down from the mismatched load. At 2:1 lower-than-matched load is not unreasonable at all.
If you do too high a load, the power tubes still limit what they put out, but a second order effect becomes important.
There is magnetic leakage from primary to secondary and between both half-primaries to each other. When the current in the primary is driven to be discontinuous, you get inductive kickback from the leakage inductances in the form of a voltage spike.
This voltage spike can punch through insulation or flash over sockets, and the spike is sitting on top of B+, so it's got a head start for a flashover to ground. If the punchthrough was one time, it wouldn't be a problem, but the burning residues inside the transformer make punchthrough easier at the same point on the next cycle, and eventually erode the insulation to make a conductive path between layers. The sound goes south, and with an intermittent short you can get a permanent short, or the wire can burn though to give you an open there, and now you have a dead transformer.
So how much loading is too high? For a well designed (equals interleaved, tightly coupled, low leakage inductances, like a fine, high quality hifi) OT, you can easily withstand a 2:1 mismatch high.
For a poorly designed (high leakage, poor coupling, not well insulated or potted) transformer, 2:1 may well be marginal. Worse, if you have an intermittent contact in the path to the speaker, you will introduce transients that are sharper and hence cause higher voltages. In that light, the speaker impedance selector switch could kill OT's if two ways - if it's a break befor make, the transients cause punch through; if it's a make before break, the OT is intermittently shorted and the higher currents cause burns on the switch that eventually make it into a break before make. Turning the speaker impedance selector with an amp running is something I would not chance, not once. "
Former owner of Music Mechanix
www.RedPlateAmps.com
www.RedPlateAmps.com
Re: Output Impedance Matching My Take
Henry
There are plenty of articles out there written by engineers that will preach to the dangers of both..I doubt they were written about running a guitar amp with a 4 ohm tap into an 8 ohm load to enhance tone ..Like I said extreme mismatches in either direction under extreme conditions will no doubt cause serious damage to tubes and transformers
They are...
If the load impedance is too low this will increase the current in the transformer windings. This also reduces the primary impedance that the output tube sees which increases tube current flow causing overheating.
if the load impedance is too high, this reflects a higher than normal impedance in the primary for the output tube.
poor sound quality and lack of volume.
The way I see it or interpret it
The impedance load seen by the tube and output transformer is not constant. The frequency of the audio signal will vary over a wide range. The inductance in the windings will have a different impedance at different frequencies. At a certain frequency an 8 ohm voice coil may have an impedance of 10 ohms or at low frequencies it my have an impedance of 4 ohms. This varying load impedance is reflected back to the primary, so the tube, and output transformer must work into a varying impedance range. When you set the impedance you set the range...Within that range certain frequencies are amplified more efficiently effecting the frequency response..4 or 5 ohms in either direction will still keep you within the operating parameters of the speaker just shifting the range...Ive replaced more OPT from amps with people who use attenuators then guys who run mismatched impedances w guitar cables..
Tony VVT
There are plenty of articles out there written by engineers that will preach to the dangers of both..I doubt they were written about running a guitar amp with a 4 ohm tap into an 8 ohm load to enhance tone ..Like I said extreme mismatches in either direction under extreme conditions will no doubt cause serious damage to tubes and transformers
They are...
If the load impedance is too low this will increase the current in the transformer windings. This also reduces the primary impedance that the output tube sees which increases tube current flow causing overheating.
if the load impedance is too high, this reflects a higher than normal impedance in the primary for the output tube.
poor sound quality and lack of volume.
The way I see it or interpret it
The impedance load seen by the tube and output transformer is not constant. The frequency of the audio signal will vary over a wide range. The inductance in the windings will have a different impedance at different frequencies. At a certain frequency an 8 ohm voice coil may have an impedance of 10 ohms or at low frequencies it my have an impedance of 4 ohms. This varying load impedance is reflected back to the primary, so the tube, and output transformer must work into a varying impedance range. When you set the impedance you set the range...Within that range certain frequencies are amplified more efficiently effecting the frequency response..4 or 5 ohms in either direction will still keep you within the operating parameters of the speaker just shifting the range...Ive replaced more OPT from amps with people who use attenuators then guys who run mismatched impedances w guitar cables..
Tony VVT
-
CaseyJones
- Posts: 856
- Joined: Fri Mar 17, 2006 1:29 pm
Re: Output Impedance Matching My Take
Aw, shucks y'all...
You're preachin' to the choir. This would be a lot more fun on other sites where this topic comes up several times per day and no one has a clue...
I went over to the farm next door when I was a kid. The farmer's kid was a tech wannabe, I became a tech, the farmer's kid is still a farmer. He had some crappy old phonograph (tube of course) wired up with every speaker he could scrounge from the local dump. Judging by the dozens of hand-me-down speakers wired up with salvaged lamp cord he had been plannin' this for a while. Impedance... how low can ya go?!
Well... they have around a hundred speakers up in the ceiling down at the mall. How bad can it be?!
Oh, joy. Speak of the Devil! This post is number 666 for me.
Should I quit now and leave it at that?!
You're preachin' to the choir. This would be a lot more fun on other sites where this topic comes up several times per day and no one has a clue...
I went over to the farm next door when I was a kid. The farmer's kid was a tech wannabe, I became a tech, the farmer's kid is still a farmer. He had some crappy old phonograph (tube of course) wired up with every speaker he could scrounge from the local dump. Judging by the dozens of hand-me-down speakers wired up with salvaged lamp cord he had been plannin' this for a while. Impedance... how low can ya go?!
Well... they have around a hundred speakers up in the ceiling down at the mall. How bad can it be?!
Oh, joy. Speak of the Devil! This post is number 666 for me.
Should I quit now and leave it at that?!