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Old 4th September 2013, 01:35 AM   #1951
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Originally Posted by kgrlee View Post
Anyone have good SPICE models for small VAS devices with good Ccb?...
My current preference is the well known 2SC3503/2SA1381.
Not exactly small, but any reason not to use these?
There are Sanyo video drivers like 2SA17772SC4623 that are faster but more Cob. Mostly moot since apparently unotainable. I would be interested if anyone knows a source.
I have basic models of the 3503/1381 if you need them.

Best wishes
David
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Old 4th September 2013, 06:22 AM   #1952
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Thanks dvv. The new thread should be a great help to us all.

Cheers,
Bob
I hope so, Bob. It's downright amazing at what we collectively know. Also, it's good to sometimes face reality and take a step back in some humility.

Not long ago, I asked myself - if cascode is good, and if parallel is good, why not parallelled cascode? Oh, I was feeling so proud of myself just then. Then, a day or two later, I discover that Nelson Pass had used excactly that way back in the early 90ies in his Threshold 400A/4000 amps and probably elsewhere too.

Oh well, such is life.
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Old 4th September 2013, 06:38 AM   #1953
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Thanks dvv. The new thread should be a great help to us all.

Cheers,
Bob
Hi Bob , On that thread Steven asked if someone would keep a list of the transistors . I wondered if you might show your list every month ? I know you plan to publish the list in your next book so thought it would be OK to ask ?

Nigel
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Old 4th September 2013, 06:56 AM   #1954
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Hi Bob , On that thread Steven asked if someone would keep a list of the transistors . I wondered if you might show your list every month ? I know you plan to publish the list in your next book so thought it would be OK to ask ?

Nigel
"Questions are never indiscrete - answers sometims are". O. Wilde
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Old 4th September 2013, 10:57 AM   #1955
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Originally Posted by nigel pearson View Post
Hi Bob , On that thread Steven asked if someone would keep a list of the transistors . I wondered if you might show your list every month ? I know you plan to publish the list in your next book so thought it would be OK to ask ?

Nigel
Hi Nigel,

My list right now is just static, and I'm not sure when I'll get to that part of the second edition. As I learn more about new transistors available, I'll be happy to post that information, but I think it is pretty far down the road. Unfortunately, I'm no further along than anyone else on that thread, at this point - maybe behind many.

Cheers,
Bob
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Old 4th September 2013, 11:03 AM   #1956
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"Questions are never indiscrete - answers sometims are". O. Wilde
I prefer Mr G Marx . I don't care to belong to any club that will have me as a member"

That reflects badly on me perhaps . I do care to be in this one if asking .

Thank Bob .
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Old 18th September 2013, 05:20 PM   #1957
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Continuing with this theme: Audio Power Amplifier Design book- Douglas Self wants your opinions

Douglas, on page 265 you show the schematic of the "load invariant" design. I note, however, that you used a deeply compromised single-slope SOA protection circuit that should really never be used at all.

This is because it is exceedingly inefficient and wasteful of perfectly useable transistor SOA.

I have run a SICE simulation (Simetrix) of you arrangement with a properly calibrated simulation fixure (see attachment) and found that the maximum available current from your pair of output devices is slightly less than 6A, as predicted by simple calculation.

Additionally, the amplifier cannot be used with supply rails greater than +/-24V (~35W@8Ohms) as the SOA protection would be activated even with no load connected to the output. Moreover, the supply rails would have to be reduced even further if ambient temperatures in excess of 25 degrees Celcius are anticipated in the vicinity of the SOA protection transistors.
Attached Files
File Type: pdf SOA_simulation.pdf (7.0 KB, 31 views)

Last edited by michaelkiwanuka; 18th September 2013 at 05:32 PM.
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Old 18th September 2013, 05:36 PM   #1958
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Mike, don't be such a scientist!
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Old 18th September 2013, 05:44 PM   #1959
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Hi Jan. Good to hear from you; hope you're OK.
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Old 18th September 2013, 06:08 PM   #1960
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Good info . I leave the protection devices out anyway . If using TO3 on flying leads and design for disaster it's a 20 minute repair and rather good fun . I like capacitor coupling as it is the ultimate DC offset protection for real world people who smoke other than cigarettes . Humble Rotel amplifiers seldom gave trouble with 4AT fuses . The student customers had many new fuses on the house . 99% of the time that was all and we are not talking once . Knowing that a 6AT3 fuse will not blow after months on our workshop kettle ( 10 A ) I realized fuses allow vastly greater currents than we imagine . I think using the same transistors as Rotel and 4AT it should be OK . For 6 amps use two sets . I don't know if it was just me ? Quad 405 with blown fuses always meant transistors also . Rotel cheap and bulletproof . Ultra careful Quad less so . Lets be frank Quad were excellent as mini PA . Rotel were better albeit at lower voltage output average ( 21 Vrms , just fixed a RA 931 , just fuses ) .

My friend uses Simetrix . He was blowing transistors . The data has to be correct for the simulation to work . He says if a complex load double the current you think the maximum . This meant 50 A for his application . John is a scientist and a very good one ( too good , a bit intense ) . He got it wrong . The big mistake was to make the design able to run speakers . That isn't the company requirement . The device costs £50 000 the last time I asked and there is a waiting list to buy one . It wasn't for sale even . People heard they had one and wanted one . It is for calibration of magnetic field measuring equipment ( Magnetometer ) . Magnetometers are wonderful . How can you measure a static field ? Not easy . Asymmetry and saturation . The rest is a deadly secret .

A DC crowbar I do approve of . Dam the amplifier , save the speakers .
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