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Driving 8 pairs of MJL21194/93
Driving 8 pairs of MJL21194/93
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Old 24th December 2018, 04:09 AM   #1
rhythmsandy is offline rhythmsandy
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Default Driving 8 pairs of MJL21194/93

Hi bob I read your book and I have few questions regarding driving 8 pairs of MJL21194/93. At +/-80V The application is that I have currently made an amplifier single drivers with 2sc5171 driving 6 pairs of 2sc5949 and eventually figured out that the current gain is not sufficient to drive the OPS properly. Now instead of using 2sc5171 Im thinking of using 5 pairs of MJE15032/33 or drive the 6 pairs of 2sc5949, 2sa2121 but to my surprise even with very little input less than 1V the amplifier is showing clipping on the clip indicator LED at +/-80V supply and amp gain being 29db. Im assuming the current from VAS is not sufficient to drive the OPS with ease. Now I would like to drive the subwoofer with super authority at its deepest frequencies. Im now considering using Triple EF and now would that solve the problem?

I always use to feel the sloppy bass coming out and have just excursions but no slam in bass. I believe using 8 pairs of MJL21194/93 with one pair of 2sc5949 2sa2121 as drivers and then use 2sc5171 1930 as predrivers would solve the problem?
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Old 24th December 2018, 04:49 PM   #2
Mark Johnson is offline Mark Johnson  United States
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Driving 8 pairs of MJL21194/93
An amp with 80 V supply rails, and six pairs of output transistors, justifies the purchase of an oscilloscope for debugging and testing. A scope will let you quickly discover which stage clips first: input stage, voltage amplifier stage, driver stage, or output stage? Then you can post photos of your scope traces when asking folks for their opinions and advice.
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Old 24th December 2018, 06:23 PM   #3
anatech is offline anatech  Canada
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Driving 8 pairs of MJL21194/93
Hi Sandy,
I have to agree with Mark on this. By now you should have an oscilloscope and good meter along with dummy loads. You would also need a THD analyser like a sound card with front end (THD analyser or Bob`s front end).

-Chris
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Old 24th December 2018, 06:27 PM   #4
Bob Cordell is offline Bob Cordell  United States
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Quote:
Originally Posted by rhythmsandy View Post
Hi bob I read your book and I have few questions regarding driving 8 pairs of MJL21194/93. At +/-80V The application is that I have currently made an amplifier single drivers with 2sc5171 driving 6 pairs of 2sc5949 and eventually figured out that the current gain is not sufficient to drive the OPS properly. Now instead of using 2sc5171 Im thinking of using 5 pairs of MJE15032/33 or drive the 6 pairs of 2sc5949, 2sa2121 but to my surprise even with very little input less than 1V the amplifier is showing clipping on the clip indicator LED at +/-80V supply and amp gain being 29db. Im assuming the current from VAS is not sufficient to drive the OPS with ease. Now I would like to drive the subwoofer with super authority at its deepest frequencies. Im now considering using Triple EF and now would that solve the problem?

I always use to feel the sloppy bass coming out and have just excursions but no slam in bass. I believe using 8 pairs of MJL21194/93 with one pair of 2sc5949 2sa2121 as drivers and then use 2sc5171 1930 as predrivers would solve the problem?
Do you have experience building and testing smaller amplifiers? 6 or 8 pairs and an 80V supply is a big amplifier and can be a challenge. For example, have you built something like a 2-pair amplifier with 50-60V rails?

As you know, I am a fan of output triples, and believe that there is just not enough current gain in a 2 EF output stage to make a really good amplifier. This is even more important if you are driving 8 triples to get lots of watts and output current. Let's say you are driving a 4 ohm load and your amplifier can deliver 64 V peak into that load, after supply sag and headroom are considered at that high power. That is 16A peak into the load. That comes to 512 W average power. With a 2 EF output stage with minim current gains of 50 and 20 in the drivers and output transistors, respectively, you have output stage current gain of only 1000 and very nonlinear. You would need at least 16 mA peak from the VAS to drive it.

For that big an amplifier, use something like TO126 VAS, TO220 pre-drivers, TO264 drivers and TO 264 outputs. At that power level, it is appropriate to use output-size transistors for the drivers. Always beware of the SOA needs of the drivers, especially considering fault conditions. Use a power transistor like the 2sc3503 for the driver, as it is faster than the MJL21194/93. Maybe for the power transistors as well.

Simulate the heck out of the amplifier before you build it.

In my view, amplifying on what Mark said, a scope is a virtual necessity if you are building a power amplifier. If you don't want to spend a lot of money on a scope, get a modest cost PC-based scope OR a vintage TEK scope from Ebay that works. Something like a TEK465 or 475 would do. You need to have eyes!

Merry Christmas,
Bob
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Old 24th December 2018, 07:45 PM   #5
rhythmsandy is offline rhythmsandy
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Quote:
Originally Posted by Mark Johnson View Post
An amp with 80 V supply rails, and six pairs of output transistors, justifies the purchase of an oscilloscope for debugging and testing. A scope will let you quickly discover which stage clips first: input stage, voltage amplifier stage, driver stage, or output stage? Then you can post photos of your scope traces when asking folks for their opinions and advice.
but usually is that above triple configuration good enough?
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Old 25th December 2018, 12:31 AM   #6
Krisfr is online now Krisfr  United States
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I would also suggest a Variac to bring the amp up slowly in steps...Good Luck and Listening.
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Old 25th December 2018, 03:53 AM   #7
rhythmsandy is offline rhythmsandy
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I do have the CRO, Function Gen, Variac but not THD analyzer. Ill check it but in general is that above config of driving multiple output transistors good enough?

Quote:
Originally Posted by Bob Cordell View Post
Do you have experience building and testing smaller amplifiers? <snip>
Hi Bob,
Thank you very much for the reply. Yes I have built the amps with 2 3 and 5 pairs at 50, 70 and 85V but in above cases I have used everywhere as single 5171 and 1930 as drivers which was actually a big mistake and the day when I found that the sub wasnt getting slam I was thinking that it could be a impedance matching of the amp at the input hence using a low output imp preamp and eventually that was sounding relatively better but still it didnt had that slam and when I acoustically measured the sub the rolloff was relatively faster as the frequency was getting lower. I couldnt understand. Checked with labgruppen and it was awesome i realised that something was wrong double checking the heatsink temperatures and found that the heatsink was just running warm even at clipping levels. The sound was better with a lateral mosfet amp. I couldnt understand at first and then realized that is the ouput stage not getting enough drive current to take full control and after seeing the datasheets of 5171 to my surprise the output current is hardly about 0.2Amp on the continuous output scale ( I do consider DC is not always happens as the signal is music but I see some bass notes are prolonged in duration even though we take 50% of duty cycle or less of a sinewave ). I read your book and there you have mentioned about considering the design to be at 2ohm load at worst as well as back emf problem makes it even more harder to drive. That made the conclusion that the current drive wasnt sufficient. Im working on the newer version on the PCB with 5 output pairs and using same as drivers and using 2sc5171 and 1930 as pre drivers. I will test on my existing 5 pair board and let know the result.

Bob you have mentioned in the above post that 2sc3503 for the driver? or VAS driver. Im thinking the following all together
2sc3503/1381 VAS output
2sc5171/1930 Pre drivers
2sc5949/2121 drivers
MJL21194/93 output.

I have a doubt what if I use 15 pairs of IRFP240? can one 2sc5949 drive 15 pairs of IRFP240/9240?

Hi Bob I have a question regarding the driver idle current that how much should it actually be? Consider If i use 5 pairs of MJL21194/93 at about 80V with load imp 3.5ohm how much should be the driver idle current consider a TO 264 package driver like 2sc5949. Its said that the drivers shouldnt turn off. Can you please give us the input about those values which connects between the two emitters of the drivers.
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Old 27th December 2018, 01:53 PM   #8
rhythmsandy is offline rhythmsandy
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Hi Bob, I have a question about a line in the book: In the " Safe Operating Area of the Driver " topic there is a last line.
" The caveat here is that the predriver bias current must be sufficient to drive the larger collector base capacitance of the driver transistor"
Asfar as I understood that its important to see that if we have a larger driver then the predriver should have the capacity to drive the driver transistor and its capacitance. But how do you choose the value of the resistor which connects the emiters of drivers?

Consider two cases: One is I have tried to drive the MJE15032/33 with 2sc3503/1381 VAS transistors at about 80V and I found that its clipping because of under current, I have added 2sc5171 as pre driver 2sc5949( TO-264 OPS transistor ) as driver and it sounds good now and no clipping observed at lower input voltage. Now If i see if we use 2sc3503 as predriver role to drive the MJE15032/33 and then to drive the 4 or 5 pairs of MJL21194/93 at 70V will the 2sc3503 be able to drive the MJE15032?
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Old 29th December 2018, 01:00 AM   #9
poldaaudio is offline poldaaudio
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Quote:
Originally Posted by Bob Cordell View Post
As you know, I am a fan of output triples, and believe that there is just not enough current gain in a 2 EF output stage to make a really good amplifier. This is even more important if you are driving 8 triples to get lots of watts and output current. Let's say you are driving a 4 ohm load and your amplifier can deliver 64 V peak into that load, after supply sag and headroom are considered at that high power. That is 16A peak into the load. That comes to 512 W average power. With a 2 EF output stage with minim current gains of 50 and 20 in the drivers and output transistors, respectively, you have output stage current gain of only 1000 and very nonlinear. You would need at least 16 mA peak from the VAS to drive it.
Yes because most of the amps in selfs book are 2 EF so none of them designs that he ever did are really good obviously and never have good slam sound and even with 6-8 parallel pairs all of those 16A will be going through just one pair of course and this is super big beta droop territory at 16A bro! What about (you don't mention) a 2 EF output stage with minim current gains of 5 and 2 in the drivers and output transistors? You have a current gain of 10 (if my maths right) and super nonlinear now and need 1.6A peak from the vas to drive it! WOW!!!!!!!!!!!!!! vas will die transistors and 2 EF sux totally unusable in any respectable design.
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Old 30th December 2018, 03:04 PM   #10
Bob Cordell is offline Bob Cordell  United States
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Quote:
Originally Posted by rhythmsandy View Post
Hi Bob, I have a question about a line in the book: In the " Safe Operating Area of the Driver " topic there is a last line.
" The caveat here is that the predriver bias current must be sufficient to drive the larger collector base capacitance of the driver transistor"
Asfar as I understood that its important to see that if we have a larger driver then the predriver should have the capacity to drive the driver transistor and its capacitance. But how do you choose the value of the resistor which connects the emiters of drivers?

Consider two cases: One is I have tried to drive the MJE15032/33 with 2sc3503/1381 VAS transistors at about 80V and I found that its clipping because of under current, I have added 2sc5171 as pre driver 2sc5949( TO-264 OPS transistor ) as driver and it sounds good now and no clipping observed at lower input voltage. Now If i see if we use 2sc3503 as predriver role to drive the MJE15032/33 and then to drive the 4 or 5 pairs of MJL21194/93 at 70V will the 2sc3503 be able to drive the MJE15032?
My most common configuration for the 3EF (Locanthi) output triple is as follows:

pre-drivers: 2SC3503/2SA1381 biased at about 12 mA with Re = 240.
Drivers: MJ1E5032/MJE15023 biased at about 20-30 mA with Re = 33-47.
Outputs: MJL3281/MJL1302 biased at about 120 mA/pair with Re = 0.22.

I always center-tap the Re for the pre-drivers and drivers for use as test points. For some testing, I will close the feedback loop from the pre-driver Re center tap. This provides a low-distortion drive to the output stage and then exposes output stage distortion alone when an analyzer is connected to the amplifier output.

The above is typical for a 2-pair output stage. The Oliver condition of Vq = 26 mV across each RE results in bias current of about 118 mA. Due to ohmic components in the emitter resistance of the output transistors (including base resistance divided by beta), optimum bias current for lowest distortion is usually lower, more like Vq = 20 mV or less.

For a very large amplifier, the drivers would be replaced with MJL3281/MJL1302 biased at 60+ mA. In this case, predriver current would also be increased to perhaps 20 mA and MJE15032/15033 might be used as predrivers. Even if the output transistors in the large amp are MJE21194/93, I would use the MJL3281/1302 as drivers because they are faster.

Cheers,
Bob
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