Question for Ed...

oldphaser

Chief Journeyman
Joined
Sep 8, 2012
Messages
567
#21
More on the FTC Amplifier Rule (1974) 1 Hr Pre-conditioning & Mfr Model Numbers

A.P. Van Meter had told me (a number of years ago) about his phone calls he had with the FTC in an effort to meet the FTC 1/3rd power preconditioning spec.
He was regularly asked what Phase Linear was doing in an effort to meet it. One day he re-read the spec and the next time they called he stated that he re-read the spec and it said "cumulative". They never called him again. It may have been his greatest contribution to audio?

After I went home, I read the spec myself and couldn't find the word cumulative, accumulative or accumulated anywhere.
A.P. also told me that the FTC didn't like the model number to imply the power output of the amplifier. I didn't see that anywhere in the FTC spec either.

I had been searching for anything related to this for years and finally today I found some more information on the subject that confirms what A.P. told me.

DB Magazine January 1976 pg 40:
http://www.americanradiohistory.com/Archive-DB-Magazine/70s/DB-1976-01.pdf see page 40 (42 of 44 in pdf)

"An additional interpretation of the audio amplifier rule for the high fidelity industry has been incorporated in a letter addressed to Leonard Feldman, technical editor of the Institute of High Fidelity, and signed by Carthon E. Aldhizer of the division of special statutes, F.T.C. “… use of the automatic recycling methods, ie., permitting a piece of equipment to recycle (thermally cut off or on) automatically until ‘on time’ of one hour is accumulated is not inconsistent with the language of Regulation (3c) testing, and would therefore constitute compliance with the rule. Where thermal build-up presents a problem at the point of 3(e) testing, tests may commence at, for example, 250 milliwatts to permit cooling” This section (3c) requires that amplifiers deliver one third of their rated power output for one hour before measurements are made to determine final publishable power ratings."


Billboard magazine October 26, 1974 page 50 (NOV.4 FTC DEADLINE) Audio Firms Must Back Up 'RMS'

https://books.google.com/books?id=s...EIQDAF#v=onepage&q=Aldhizer amplifier&f=false


“…Another practice by some manufacturers to name the wattage output after their own brand name (ie: 400 Smithphonic watts) in an effort to circumvent the rule has also been outlawed…..”


Years later the spec's (FTC, EIA etc) were relaxed to 1/8th power pre-conditioning for 1/2 hour.

There was also a review of the Phase Linear 700 series II amplifier in Stereo Review April 1979
on page 64:

"Laboratory Measurements. When we tested the original Phase Linear Model 700 about seven years ago, the FTC ruling on amplifier power ratings had not been issued. The ruling resulted in the "beefing up" of many high powered amplifiers, Phase Linear's included, to withstand the severe heating conditions imposed by the "preconditioning" operation at one-third power for one hour."

"The Model 700 Series Two specifically requires the cooling fan accessory for this type of testing. Even with it, the thermal-protection circuit cycled on and off regularly, with a duty cycle of five minutes on and 1-1/2 minutes off when we drove both channels to one-third power into 8 ohms. To accumulate the required one hour of operation, we had to run the amplifier for 1 hour and 20 minutes."

"At the end of that time, the amplifier was very hot. Nevertheless, it seemed to suffer no ill effects from this treatment. The outputs clipped at 473 watts per channel (IHF clipping headroom = 1.18dB) and the IHF dynamic-headroom rating of 1.48dB corresponded to a short-term output of 506 watts per channel into 8 ohms at the clipping point. We could not measure the amplifier with 4 ohm loads, since the 10-amp line fuse blew before we reached the clipping point."

Ed
 
Last edited:

oldphaser

Chief Journeyman
Joined
Sep 8, 2012
Messages
567
#23
That last paragraph was most informative Ed!!
Lee,

I thought so too! The power output numbers are interesting.

When I drive both channels into clipping into a 4 ohm load, I replace the 10A line supply fuse with a 15A fuse and make sure the B+ and B- fuses are AGX 8 fuses. I perform testing quickly at 4 ohms so as to not cause the Zobel network resistors to fail.

NOTE: The Audio Analysts 700's I went through for Darrel Fisher years ago had 15A line fuses and 10A B+ and B- fuses. The amps were fan cooled with (4) large industrial fans mounted to the back of the amp racks. As a result of the fan cooling, the pc boards also didn't see any signs of being heat effected.

P.S. Also be sure that your power cord is rated for 15A as well. There are some amplifiers that have been modified/restored in the last year or two out there now that have 10A line cords on them.

Ed
 
Last edited:

Gepetto

Administrator
Staff member
Joined
May 15, 2011
Messages
6,955
Location
Sterling, MA
Tagline
Old 'Arn Enthusiast
#24
Lee,

I thought so too! The power output numbers are interesting.

When I drive both channels into clipping into a 4 ohm load, I replace the 10A line supply fuse with a 15A fuse and make sure the B+ and B- fuses are AGX 8 fuses. I perform testing quickly at 4 ohms so as to not cause the Zobel network resistors to fail.

NOTE: The Audio Analysts 700's I went through for Darrel Fisher years ago had 15A line fuses and 10A B+ and B- fuses. The amps were fan cooled with (4) large industrial fans mounted to the back of the amp racks. As a result of the fan cooling, the pc boards also didn't see any signs of being heat effected.

P.S. Also be sure that your power cord is rated for 15A as well. There are some amplifiers that have been modified/restored in the last year or two out there now that have 10A line cords on them.

Ed
Hi Ed
Why are you concerned about the Zoebel resistors failing? That comment puzzles me.
 

oldphaser

Chief Journeyman
Joined
Sep 8, 2012
Messages
567
#25
Hi Ed
Why are you concerned about the Zoebel resistors failing? That comment puzzles me.
When running the Phase Linear amp into a 4 ohm load at rated output (or to establish max power output) at high frequencies around 20KHz can cause the Zobel network resistor(s) to explode. I have seen it happen first hand. Dean explained to me years ago that this was also more likely to happen with slower output transistors (i.e. PL909, FPL909, XPL909's) because of the common mode conduction.

The book Audio Power Amplifier Design by Douglas Self page 254 states the following:

"HF instability is probably the most difficult problem that may confront the amplifier designer, and
there are several reasons for this:


"1. The most daunting feature of HF oscillation is that under some circumstances it can cause
the destruction of the amplifier in relatively short order. It is often most inadvisable to let the
amplifier sit there oscillating while you ponder its shortcomings."


"BJT amplifiers will suffer overheating because of conduction overlap in the output devices; it takes time to clear the charge carriers out of the device junctions.
Some designs deal with this better
than others, but it is still true that subjecting a BJT design to prolonged sine-wave testing above 20 kHz should be done with great caution.
Internal oscillations may of course have much higher
frequencies than this, and in some cases the output devices may be heated to destruction in a few
seconds. The resistor in the Zobel network will probably also catch fire."

FET amplifiers are less vulnerable to this overlap effect, due to their different conduction
mechanism, but show a much greater tendency to parasitic oscillation at high frequencies, which
can be equally destructive. Under high-amplitude oscillation plastic-package FETs may fail
explosively; this is usually a prompt failure within a second or so and leaves very little time to
hit the off switch."


The book
High Performance Audio Amplifiers by Ben Duncan page 382 states the following:

"The OPN (OutPut Network) is a perennial weak spot in semiconductor amplifiers.
The resistors here can smoke, burst into flames or even explode if driven with sufficient
HF/VHF level. It’s easily done by accident when playing a test CD, or sweeping
a test generator. Or RF may be present on the incoming signal."


"In amplifier servicing, discovering slightly singed zobel resistors is not unusual.
They are best looked at, whenever amplifiers are overhauled; and also after anyone
discovers that a faulty console (or preamp or other equipment) has been driving RF
up the system. Thoughtful makers will place the Zobel resistors where they can be
readily inspected and replaced. Ironically, once the Zobel network is at all damaged,
the stabilizing resistance value usually increases, thus the amplifier is all the
more likely to go unstable, generate its own RF output, and finish off the burnout."


Apparently Clair Brothers had a problem with the factory Zobel networks in Phase Linear 700 series amplifiers. So they specified Dale 5 ohm 50 watt resistors instead of (2) 10 ohm 2 watt resistors in parallel. Later 700 series 2's had (1) 5 ohm 5 watt resistor. So for a Clair Brothers amplifier to be considered a Clair Brothers amplifier it must have the Dale 5 ohm 50 watt resistors installed.

Ed

 

Gepetto

Administrator
Staff member
Joined
May 15, 2011
Messages
6,955
Location
Sterling, MA
Tagline
Old 'Arn Enthusiast
#26
When running the Phase Linear amp into a 4 ohm load at rated output (or to establish max power output) at high frequencies around 20KHz can cause the Zobel network resistor(s) to explode. I have seen it happen first hand. Dean explained to me years ago that this was also more likely to happen with slower output transistors (i.e. PL909, FPL909, XPL909's) because of the common mode conduction.

The book Audio Power Amplifier Design by Douglas Self page 254 states the following:

"HF instability is probably the most difficult problem that may confront the amplifier designer, and
there are several reasons for this:


"1. The most daunting feature of HF oscillation is that under some circumstances it can cause
the destruction of the amplifier in relatively short order. It is often most inadvisable to let the
amplifier sit there oscillating while you ponder its shortcomings."


"BJT amplifiers will suffer overheating because of conduction overlap in the output devices; it takes time to clear the charge carriers out of the device junctions.
Some designs deal with this better
than others, but it is still true that subjecting a BJT design to prolonged sine-wave testing above 20 kHz should be done with great caution.
Internal oscillations may of course have much higher
frequencies than this, and in some cases the output devices may be heated to destruction in a few
seconds. The resistor in the Zobel network will probably also catch fire."

FET amplifiers are less vulnerable to this overlap effect, due to their different conduction
mechanism, but show a much greater tendency to parasitic oscillation at high frequencies, which
can be equally destructive. Under high-amplitude oscillation plastic-package FETs may fail
explosively; this is usually a prompt failure within a second or so and leaves very little time to
hit the off switch."


The book
High Performance Audio Amplifiers by Ben Duncan page 382 states the following:

"The OPN (OutPut Network) is a perennial weak spot in semiconductor amplifiers.
The resistors here can smoke, burst into flames or even explode if driven with sufficient
HF/VHF level. It’s easily done by accident when playing a test CD, or sweeping
a test generator. Or RF may be present on the incoming signal."


"In amplifier servicing, discovering slightly singed zobel resistors is not unusual.
They are best looked at, whenever amplifiers are overhauled; and also after anyone
discovers that a faulty console (or preamp or other equipment) has been driving RF
up the system. Thoughtful makers will place the Zobel resistors where they can be
readily inspected and replaced. Ironically, once the Zobel network is at all damaged,
the stabilizing resistance value usually increases, thus the amplifier is all the
more likely to go unstable, generate its own RF output, and finish off the burnout."


Apparently Clair Brothers had a problem with the factory Zobel networks in Phase Linear 700 series amplifiers. So they specified Dale 5 ohm 50 watt resistors instead of (2) 10 ohm 2 watt resistors in parallel. Later 700 series 2's had (1) 5 ohm 5 watt resistor. So for a Clair Brothers amplifier to be considered a Clair Brothers amplifier it must have the Dale 5 ohm 50 watt resistors installed.

Ed

It should not be doing this Ed. This is RF energy at above 300KHz that indicates instability is occurring.
 

oldphaser

Chief Journeyman
Joined
Sep 8, 2012
Messages
567
#27
It should not be doing this Ed. This is RF energy at above 300KHz that indicates instability is occurring.
Whether it should be happening or not it does. Even with amplifiers that met Phase Linear's requirements as outlined in their service manuals. Prolonged testing above 8 kHz into a 4 ohm load particularly with older slower output transistors should be avoided. In the case of 400 series 1 amps you will also blow the B+ and B- fuses.

The Clair Brothers 700 series II amps didn't use slower output devices like the series 1 amps did and yet Clair Brothers must still have seen problems. Perhaps it was due to some external device(s) with HF oscillations driving into Clair Brothers (Phase Linear) amp(s). P.A. amps see conditions much different from our home use application.


Ed
 
Last edited:

Gepetto

Administrator
Staff member
Joined
May 15, 2011
Messages
6,955
Location
Sterling, MA
Tagline
Old 'Arn Enthusiast
#28
Whether it should be happening or not it does. Even with amplifiers that met Phase Linear's requirements as outlined in their service manuals. Prolonged testing above 8 kHz into a 4 ohm load particularly with older slower output transistors should be avoided. In the case of 400 series 1 amps you will also blow the B+ and B- fuses.

The Clair Brothers 700 series II amps didn't use slower output devices like the series 1 amps did and yet Clair Brothers must still have seen problems. Perhaps it was due to some external device(s) with HF oscillations driving into Clair Brothers (Phase Linear) amp(s). P.A. amps see conditions much different from our home use application.


Ed
Did the Clair amps operate in quasi-comp mode?
 

Northwinds

Veteran and General Yakker
Joined
Mar 18, 2013
Messages
7,271
Location
Coventry, CT
Tagline
Fondler errrr... fan of all Nav's avatars
#29
Did the Clair amps operate in quasi-comp mode?
Perry would probably be a good one to ask also since he bought pretty much all of them left in the World and gutted many of them to make WOPLs. Of course Ed probably ended up with all the stuff Perry pulled out so he could probably tell also. I do know the Clair's ran the Flying S4 rigs but never found out what impedance the speakers were but I believe they used one amp for each flying speaker system cab
 

oldphaser

Chief Journeyman
Joined
Sep 8, 2012
Messages
567
#30
Did the Clair amps operate in quasi-comp mode?
Joe,

If my memory serves me correct:

Clair Brothers went through a progression of Phase Linear amps. First starting off with the 700B. I don't think there were any 700B's with Clair Brothers silk-screened on the front panel. Clair Brothers then went to the 700 series II which from approximately 1978 to late 1980 were quasi-comp amps. Amps produced after January 1981 would have been fully comp. Sometime during the production of the 700 series II's Phase Linear began making different front panels for Clair Brothers eventually ending up with the black front panels that were on some of Perry's amps. Somewhere among all my pictures are photos of the front panels which would have appeared on Clair Brothers amps. The Dale 5 ohm 50 watt resistors only appeared on later(?) Clair Brothers amps and not the later PRO700's(?)

Phase Linear also made some 700 series II's for Audio Analysts with Audio Analysts silk-screened on the front panel. I went through approx 25 or 30 Audio Analysts amps for Darrel Fisher. Those amps that were not already fully-comp I converted into fully-comp for him. I also converted any series 1 amps to fully comp for him as well. Albeit, the fully comps were more reliable (the use of better output transistors also had something to do with it), but the series 1's were quieter. It appears that everything in life is a compromise, in other words; to gain one thing you have to give up something else).

So much for my memory test. Can I now enter the old folks home before its too late?

Ed
 
Last edited:

oldphaser

Chief Journeyman
Joined
Sep 8, 2012
Messages
567
#31
Perry would probably be a good one to ask also since he bought pretty much all of them left in the World and gutted many of them to make WOPLs. Of course Ed probably ended up with all the stuff Perry pulled out so he could probably tell also. I do know the Clair's ran the Flying S4 rigs but never found out what impedance the speakers were but I believe they used one amp for each flying speaker system cab
Ron,

Right you are. We will have to wait for Perry to chime in. I think he had both quasi and fully-comp amps as I also received a fair number of PL36 pc boards from him. If my memory serves me correct, some were set up for quasi-comp and others were fully-comp. I received as well as (17) pairs of Dale 5 ohm 50 watt resistors which would have been removed from (17) amplifiers. Needless to say, those amplifiers no longer have 5 ohm 50 watt resistors installed in them.

Probably by now you and everyone else knows I hedge my bets when I say: "If my memory serves me correct" just in case I am proven wrong. LOL!!!!! Just to be clear I meant that as a joke as no one is perfect and we can all be proven wrong from time to time.


Ed
 
Last edited:

Gepetto

Administrator
Staff member
Joined
May 15, 2011
Messages
6,955
Location
Sterling, MA
Tagline
Old 'Arn Enthusiast
#32
Ron,

Right you are. We will have to wait for Perry to chime in. I think he had both quasi and fully-comp amps as I also received a fair number of PL36 pc boards from him. If my memory serves me correct, some were set up for quasi-comp and others were fully-comp. I received as well as (17) pairs of Dale 5 ohm 50 watt resistors which would have been removed from (17) amplifiers. Needless to say, those amplifiers no longer have 5 ohm 50 watt resistors installed in them.

Probably by now you and everyone else knows I hedge my bets when I say: "If my memory serves me correct" just in case I am proven wrong. LOL!!!!! Just to be clear I meant that as a joke as no one is perfect and we can all be proven wrong from time to time.


Ed
Were the 5 ohm 50W resistors WW?
 

oldphaser

Chief Journeyman
Joined
Sep 8, 2012
Messages
567
#35
FTC "Amplifier Rule" & Dynaco's 2 Page Ad in "The Audio Amateur" 4/1974

On October 16, 2018 americanradiohistorymuseum added (98) issues of "The Audio Amateur" from 1970 to 1999 https://www.americanradiohistory.com/Audio_Amateur.htm


I went through every issue. There are many interesting articles.

I found an ad that Dynaco took out to be interesting as it relates to the FTC amplifier rule and 1/3rd power, etc.........
https://www.americanradiohistory.com/Archive-Amateur-Audio/Audio-Amateur-1974-4.pdf see pages 28, 29
 

Attachments

Last edited:
Top