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SKA GB150D now public domain...
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Old 11th August 2020, 04:33 PM   #961
amplidude is offline amplidude  Denmark
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Here are some specs.
Recommended Supply Voltage: ± 50V
Output power (± 50V regulated, < 1% THD): 150W at 4Ω or 8Ω
Voltage gain: 28
Input sensitivity: 1V RMS for 100W at 8Ω
Input impedance: 27K Ω || 330pf
Load impedance: 4 ohms or 8 ohms
Idle Dissipation: ~11W at ± 50V DC
THD 1kHz, 8 ohms load: 0.025% (100W), ~0.01XXX% Typical
Your voltage is a bit on the high side. Is there a switch to set different ac voltages on your amp?
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Old 11th August 2020, 04:49 PM   #962
Bigun is offline Bigun  Canada
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SKA GB150D now public domain...
Quote:
Originally Posted by amplidude View Post
Thanks for your reply bigun.
Yes I saw many opinions regarding setting bias, and I tried with 150 ma per device, couldn't really hear any difference, and heatsink got more hot than good, also when I start it up cold with this high bias setting it draws alot of current in 5 minutes before it stabilizes, I will aim for lower bias and longer lifespan.
I live in north maybe have a flip switch with summer/winter bias .
Thanks again.
Well, I didn’t think the thermal control in Greg’s design was bad, but I certainly improved on it when I redesigned for my clone and with a far better pcb layout than his. (probably why he deleted my posts from his forum and locked the thread without any explanation!).
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Old 11th August 2020, 05:30 PM   #963
Summitp is offline Summitp  United States
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Location: Capital City of Salt, UT USA
Quote:
Originally Posted by amplidude View Post
Here are some specs.
Recommended Supply Voltage: ± 50V
Output power (± 50V regulated, < 1% THD): 150W at 4Ω or 8Ω
Voltage gain: 28
Input sensitivity: 1V RMS for 100W at 8Ω
Input impedance: 27K Ω || 330pf
Load impedance: 4 ohms or 8 ohms
Idle Dissipation: ~11W at ± 50V DC
THD 1kHz, 8 ohms load: 0.025% (100W), ~0.01XXX% Typical
Your voltage is a bit on the high side. Is there a switch to set different ac voltages on your amp?
It does not have a switch to drop voltages for the ac. The transformer is just the Yamaha unit, 630va, and supplied t channels with 120wpc continuous. Looks like actual rail voltage from the yammie schemammie is +-58.3v. so yeah, kind of high.

So I am still quite a newb at this stuff, so maybe you can tell me if I am mistaken here. Higher rail voltages will lead to higher output by virtue of a larger signal. There will be additional heat generated by the higher voltage. If the heatsink/fan are able to control the heat, I should be ok, no? Would I want to make changes to the circuit or bom to account for the higher rail voltages?

Thanks for your help.
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Old 11th August 2020, 05:46 PM   #964
amplidude is offline amplidude  Denmark
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Well bigun I might just try your version some day, I had some stability issues, oscillation, I think the protection diodes caused it, once I removed them, it went silent, does your iteration have those diodes?
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Old 11th August 2020, 06:31 PM   #965
Bigun is offline Bigun  Canada
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SKA GB150D now public domain...
I included the protection diodes as they are a good idea and I had no issues with the final version, but I did include RC networks on the gate-drain as dictated by good engineering practice to forestall potential oscillations. I used a tight layout to keep the parts close to the MOSFET’s.


Sometime, I should try cranking the bias again, let ‘em run as hot as my heatsink allows and see how it sounds in case it becomes a contender for a high sensitivity speaker project where Class A would be nice to have, along with the dynamic headroom of switching up into Class B.
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Old 11th August 2020, 07:13 PM   #966
amplidude is offline amplidude  Denmark
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Summit.
I believe you can run it of those rails, an idea could be to insert a capacitance multiplier after rectifier, that would give you very clean DC, and it would lower rails 2 to 3 volts
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Old 2nd September 2020, 06:42 PM   #967
Summitp is offline Summitp  United States
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So, moving forward, I have the boards, and have started stuffing resistors. I need to source a few parts though, and have a couple of questions.

Input transistors, specifically, the c2240bl and a970bl transistors. These are getting difficult to find, and need to be matched. BD ent has them for $3-4 a pop. The thread discussing alternatives for c2240/a970 indicates that the c1845 and a992 transistors are good alternatives. I have a bunch of these. Can these work in this design? If not, do you guys have a good source for finding these in matched pairs? Popular unnamed auction site? (ugh)...

Output mosfets, ideas to source well matched Vgs pairs of p fets and n fets? The p mosfets need to match each other, and the n mosfets need to match each other, it's not about matching complementary pairs. Is this correct?

Thinking about the power supply. I plan on using the transformer from the Yamaha. And given the layout constraints imposed by the heat sink location, I am thinking about creating two capacitor banks, 40,000uf each. Is this adequate for this design, or too high? If I am using two separate banks, I would be setting them up to operate basically as mono supplies (driven by common transformer). So two rectifiers, two capacitor banks, etc. I am learning that AB amplifiers don't necessarily need the CRC layout, and a straight capacitor bank can be used, and I have set my HB up this way. I believe the same would apply here as well, yes?

Thanks for the advice!
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Old 2nd September 2020, 07:18 PM   #968
Bigun is offline Bigun  Canada
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SKA GB150D now public domain...
this maybe heresy, but the LTP’s in this amplifier are not at all balanced by design, matching transistors may not be critical. And mismatch produces a bit of 2nd harmonic, most of which will be compensated for by the symmetrical topology - in short, if finding transistors is a struggle, don’t sweat the matching.
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Old 2nd September 2020, 07:22 PM   #969
amplidude is offline amplidude  Denmark
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Could you post the schematic you're using.
Here's the schematic I used it uses easy and cheap to find jelly beans but with suffix c meaning high Hfe
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Old 2nd September 2020, 08:12 PM   #970
Summitp is offline Summitp  United States
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My boards are the Jims boards, (I looked for the real deal from Greg Ball, but his site is gone...) So I am going by the schematic and BOM provided by Jims, as posted earlier.
I believe I am after these mosfets IRFP240/9240. The schematic shows the BC parts, but it is called out to use the c2240/a970 transistors.

Thanks guys!
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File Type: pdf GB150D BC part list v1.1 (2).pdf (58.9 KB, 8 views)
File Type: pdf GB150D Sch v1.1 (2).pdf (65.7 KB, 18 views)
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