Could someone please answer this Transistor question - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 9th January 2009, 01:21 PM   #1
OMNIFEX is offline OMNIFEX  Jamaica
diyAudio Member
 
OMNIFEX's Avatar
 
Join Date: Mar 2003
Location: Locked Up In The Amp Rack
Question Could someone please answer this Transistor question

If I can trouble the amplifier developers on what is considered an easy answer from your perspective.

When an amplifier offers 12 Transistors per channel, how would one determine the lowest impedance it can handle?

I've read using multiples in parallel enables a more stable amplifier when low impedance is the goal. However, quantity is not discussed.

The Transistors in question are MJ15024.



Cheers!
__________________
OMNIFEX
  Reply With Quote
Old 9th January 2009, 01:24 PM   #2
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
download one of the SOAR spreadsheets.
Bensen, Janneman etc.

It's not a stability issue, it's overheating of the output device junctions.
__________________
regards Andrew T.
  Reply With Quote
Old 10th January 2009, 12:38 AM   #3
OMNIFEX is offline OMNIFEX  Jamaica
diyAudio Member
 
OMNIFEX's Avatar
 
Join Date: Mar 2003
Location: Locked Up In The Amp Rack
Cheers AndrewT!
__________________
OMNIFEX
  Reply With Quote
Old 11th January 2009, 12:07 AM   #4
djk is offline djk
diyAudio Member
 
djk's Avatar
 
Join Date: Feb 2001
Location: USA
What's the rail voltage?

How good is the heatsinking?

Ultimately these all go into the SOA calculations.

12 MJ15024 per channel suggests a quasi-comp PA amplifier with 5 pair as outputs, one pair as drivers. With real good heatsinking and under 80V rails it could probably run a 2R load. This is essentially how the old BGW750, Peavey CS800, etc., were.

The Phase Linear 700B was also as described, but running on 100V it was a bit marginal at 4R with the MJ15024, but would be fine at 4R with the MJ21196 (if you could keep it cool).
__________________
Candidates for the Darwin Award should not read this author.
  Reply With Quote
Old 11th January 2009, 12:53 AM   #5
fotios is offline fotios  Greece
diyAudio Member
 
fotios's Avatar
 
Join Date: Feb 2007
Location: Δραμα - North Greece
You must don't forget as well the VI limitter of output included as defacto in such type amplifiers.
Under this option, you can connect enough speakers in output, down to 0,5 and when the V X I exceeds the SOA as said Andrew (the sum of all output devices predicted by the constructor of amplifier) then the output power is reduced accordingly.
In reality, you can drive enough speakers without problem but only from their low to mid power level.

Regs
Fotios
  Reply With Quote
Old 11th January 2009, 03:11 PM   #6
djk is offline djk
diyAudio Member
 
djk's Avatar
 
Join Date: Feb 2001
Location: USA
"You must don't forget as well the VI limitter of output included as defacto in such type amplifers"

By all means, remeber that it is there, because when it kicks it it will sound like h3ll, and if using full-range speakers it will probably blow the tweeters.

SOA limiting done with VI limiters is not designed to be engaged in the normal use of the amplifier. It is there to protect the amplifier in the case of a short.
__________________
Candidates for the Darwin Award should not read this author.
  Reply With Quote
Old 11th January 2009, 06:15 PM   #7
fotios is offline fotios  Greece
diyAudio Member
 
fotios's Avatar
 
Join Date: Feb 2007
Location: Δραμα - North Greece
Quote:
Originally posted by djk
SOA limiting done with VI limiters is not designed to be engaged in the normal use of the amplifier. It is there to protect the amplifier in the case of a short.
I think we discuss about this issue exactly. Your question in your first post, was:
"When an amplifier offers 12 transistors per channel, how would one determine the lowest impedance it can handle?"
The VI limiter has relation with this, i.e. to protect the output from heavy loading - very low impedance load - and not to protect tweeters or woofers.

Regs
Fotios
  Reply With Quote
Old 11th January 2009, 06:18 PM   #8
djk is offline djk
diyAudio Member
 
djk's Avatar
 
Join Date: Feb 2001
Location: USA
A member called me on the carpet for using h3ll.

Thank you, I was just too lazy to describe how a VI limiter sounds driving a reactive load-line.

It can be best described as a farting and popping sound, very nasty. The leading edge of the rail-to-rail voltage spikes are passed through the crossover to the HF units, many of which cannot handle this without damage (and they blow up).

Crest employes a circuit they call IGM, on low impedance loads it instantaniously modulates the gain of the amplifier (IGM) to prevent excurions outside of the SOA limits. This sounds very anemic, and changes the tonal balance in multi-amped systems.

Crown uses ODEP. On the vz5000 and vz3600 it cuts the rail voltage in half, effectively cutting the available power by 6dB.

In practice, amplifiers rated at 2R may geneally drive three 8R drivers, but seldom can handle four (without the protection becoming active, and the audible consequences of same).

VI limiting is like the air-bag in your car, do you really want to see if it works?
__________________
Candidates for the Darwin Award should not read this author.
  Reply With Quote
Old 11th January 2009, 09:07 PM   #9
fotios is offline fotios  Greece
diyAudio Member
 
fotios's Avatar
 
Join Date: Feb 2007
Location: Δραμα - North Greece
Hi djk

By the chance given from your report for "Output Devices Emulation Protection" of Crown and "Instantaneous Gain Modulation" of Crest, i have to reffer that tose are types of error computers comparing input and output. The named VI limiters are other thing. These included between the drivers and output devices. Their operation is by some way continuous, as they track the current flow thru emiter resistors, and when the current exceeds some limit, then start to thieve current from the bases of drivers.
Instead, the error computers are of different operation, and can produce significant amount of distortion when activated. I have a lot of experience from Peavey amplifiers. In these included a simillar computer, the known DDT (Distortion Detection Technic) which i can confirm you that when activated, the signal bellow 120Hz becomes very distorted, particularly in its positive going edge .

Regs
Fotios
  Reply With Quote
Old 12th January 2009, 09:46 AM   #10
djk is offline djk
diyAudio Member
 
djk's Avatar
 
Join Date: Feb 2001
Location: USA
"Their operation is by some way continuous, as they track the current flow thru emiter resistors, and when the current exceeds some limit, then start to thieve current from the bases of drivers. "

Those are the ones that sound horrible.

"Instead, the error computers are of different operation, and can produce significant amount of distortion when activated. "

Are you sure? The IGM in the Crest, the McIntosh power guard, the Crown PIP-Clip, these kind of circuits are virtually distortion free. The QSC and Carver optical limiters use exactly the same part as the McIntosh, and are also free of distortion.

The Peavey DDT is a very different circuit. I have never observed the behavior you described. I will say that it is not instantaneous, it takes quite a while after you drive it into clipping before it pulls the signal back, perhaps this is what you see/hear?
__________________
Candidates for the Darwin Award should not read this author.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Anyone want to answer a stupid question about ESL's? jzh797s Planars & Exotics 12 1st September 2006 07:52 PM
A question I couldn't answer... johnthedoctor Instruments and Amps 4 1st April 2006 09:53 AM
question that i cant seem to find the answer to: jaygeorge1979 Multi-Way 34 28th January 2006 04:21 AM
Dumb Question!! but i need the answer scooby1 Multi-Way 2 7th September 2005 11:20 AM


New To Site? Need Help?

All times are GMT. The time now is 09:55 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2