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Old 29th September 2010, 03:05 PM   #1
wicked1 is offline wicked1  United States
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Default 7w 300b vs 350w Car audio observation (car loses)

I built a 300b amp several years ago.. Also a little 5watt F2 (pass labs) solid state amp.
Both can be played as loud as Im comfortable listening, and sound GREAT!

I just got a new car w/ upgraded sound system, 350 watts per channel. Figured it would be way way way more than I ever needed.
But, driving around listening to it, at levels comparable to what I listen to my tube amp in my living room, I get TONS of distortion out of the car system. And this system gets GREAT reviews.
It sounds good in general.. But its a world away from being great.

I guess this is a sort of pointless post, but I was just shocked at the discrepancy
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Old 29th September 2010, 04:21 PM   #2
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The 350 watts per channel is probably bogus.
I've seen amplifiers which are rated at 1200 watts. When I looked at the specs, I noticed that the total power consumption is only 60 watts.
So much for "truth in advertising."
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Old 29th September 2010, 04:28 PM   #3
TheGimp is offline TheGimp  United States
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Peak measurement using a half cycle of 1KHZ with the heat sinks covered with dry ice?
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Old 29th September 2010, 04:28 PM   #4
taj is offline taj
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Yep, car audio is (generalizing of course) marketed to the <30 crowd who care about nothing more than BIG numbers, and not the pragmatic application thereof. The 70-something db sensitive speakers attached to your car amp would be virtually inaudible on your home system.

You want it done right, you gotta do it yourself.

..todd
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Old 29th September 2010, 05:04 PM   #5
kevinkr is offline kevinkr  United States
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OEM car stereo systems with digital amps generally do produce very high power levels, but the speakers used in such systems tend to high on power handling and rather low on efficiency.. (A single Bose switching amp module was rated 50Wrms into 1 ohm, and I worked a lot with these when I was there 11+ yrs ago and they really did produce that amount of power delivered into a puny 4.5" driver - it went loud, but not that loud. Typical applications would have at least 4 of these, plus a couple of analog tweeter amps and a much bigger digital amp for the system woofer.)

The Harman Kardon 450W premium sound system in my Subaru sounds quite good, far better in fact than the Bose system in my previous Acura.. Like you however I find my home system is able to achieve deeper bass extension and higher spls simultaneously with lower overall audible distortion than the system in my car which has about 25X the rated output power.. In fairness the cone area of all of the drivers in the car still comes nowhere close to the radiating area of just one of my home system woofers..
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Old 1st October 2010, 12:15 PM   #6
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7W from 300Bs, shouldn't that be 107W? these were used in a few PA amplifiers even a pair of 6V6s gives about 18W or am I missing something?
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Old 1st October 2010, 05:29 PM   #7
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by metalsculptor View Post
7W from 300Bs, shouldn't that be 107W? these were used in a few PA amplifiers even a pair of 6V6s gives about 18W or am I missing something?
Yeah, you are.. A pair of 300B in pushpull when pushed hard will give you around 40Wrms max, in my designs 20 - 30W is the norm.

A single ended 300B given the typical triode SE efficiency of 25% or less will net you about 10W with a dissipation of around 40W and with mediocre linearity at that. Anything above 8Wrms without lots of global feedback and/or a design meant to push the 300B to its limits is not going to make more clean power. (There is a Japanese design that gets nearly 17W out of a single 300B, but I've never seen one in person, and tube life would be extremely short given its operating conditions - and it is doubtful that any current standard 300B could survive operation in that design for long. I believe it runs deep into class A2 which is a no no IMHO for a 300B.)

My preferred amp is a 0fdbk 300B SE amplifier with about 8W out per channel.

I can't think of any PA that used PP 300B, there were WE theater amps that did, but power output was ~ 20W..
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Old 1st October 2010, 11:32 PM   #8
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Quote:
Yeah, you are.. A pair of 300B in pushpull when pushed hard will give you around 40Wrms max, in my designs 20 - 30W is the norm.
Cheers a single valve in class A that explains it, it was a long time ago that I saw these devices AFAIR the PA was an AWA popular in Australian schools. I thought they were around 25W plate dissipation each, why only 30 to 40W output for a pair? AFAIR AB1 push pull amps usually get efficiencies over 55% but that was with tetrodes.
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Old 3rd October 2010, 01:28 AM   #9
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by metalsculptor View Post
Cheers a single valve in class A that explains it, it was a long time ago that I saw these devices AFAIR the PA was an AWA popular in Australian schools. I thought they were around 25W plate dissipation each, why only 30 to 40W output for a pair? AFAIR AB1 push pull amps usually get efficiencies over 55% but that was with tetrodes.
300B has a 40W dissipation rating, and yes efficiencies of 55% are achievable with PP tetrodes, but not low mu power triodes.. Something like 35% - 40% is achievable under ideal conditions without over-running the output tubes. (And they should not be run at rated dissipation, 80% seems fine for good service life.) Note that I design amps that do not use global feedback and so need good linearity in the output stage - I guess if one used a 2.5K primary one could get maybe 45 - 50Wrms at 5% thd or so, but IMHO this I think would be pushing the outer limits... (I used 3.4K - 5K depending on tube, class of operation, target power and other [market] related things. )

I am surprised that a very fragile and hideously expensive tube would be used in an amplifier targeted at a school system.. The main use for these tubes was in Western Electric theater amps which is what they were in fact designed for, some telco power supply and amplifier applications and oddly enough voltage regulators at NASA in the 1950s and 1960s. (Why I have no idea) It was even too expensive for most hobbyists outside of Japan where it enjoyed an early and ongoing popularity amongst those well heeled enough to afford it.

STC in the UK (IIRC) made the 4300B which was equivalent and was used in similar applications..
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Old 3rd October 2010, 03:14 PM   #10
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I may have got it wrong but the school amplifiers were probably late 50's to mid 60's vintage. I was into valve circuit in my teens which was the 70's about the time a lot of valve equipment was being replaced by transistors. I do not understand the use of triodes in the output stage a PA amplifier either but I have seen stranger things, I have a 70's vintage Japanese ultrasonic welder with 20 Audio output valves in parallel as a single ended class C amplifier when a single force cooled ceramic tetrode such a 4cx250A would have been ample and 1/20 the size.
The lack of feedback in your designs would explain a few things, the output stage would have to run close to class A to minimise crossover distortion in the PP designs but at least there would be no DC to worry about in the output transformer unlike in single ended designs.
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