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Old 1st February 2014, 04:06 PM   #1
gannaji is offline gannaji  India
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Default Bigun's TGM8 with Single Supply

Dear Friends,
My interest in single supply low power <50W amplifiers started in the middle of 70s when I made a single channel of JLLH 15-20 Watt Class AB amp. I continued to use it for around 3-4 years.
When I saw Biguns single ended TGM8 started, I wanted to make a stereo amp of TGM8 with single supply. I made a request in his thread for such a retro mod of his TGM8 and he agreed to consider it.
I posted in his thread in post no 312 ( TGM8 - an amplifier based on Rod Elliot P3a ) a possible schematic and we had exchanged some PMs and he gave some suggestions.
I made modifications to the original Schematic as per his suggestions and it is as posted below.

--gannaji
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File Type: asc tgm8v5-lowpower retro v2.asc (9.9 KB, 82 views)
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Old 1st February 2014, 04:16 PM   #2
gannaji is offline gannaji  India
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Here are answers to some of the questions raised by Bigun in our PMs.

  1. I will be interested in only through hole build. Using single power supply and output Elcos helps in DC protection of LS. That is my main reason.
  2. I am interested in building this amp. I have all the small signal semis and shall be procuring the 1943/5200 shortly.
  3. I will not be able to make PCB myself and an alternate arrangement has to be seen.
  4. I would prefer the power amp PCB seperate and PS PCB seperate. The Power spply can be similar to Dr Evils CM type with controlled rise time and fall time. This can reduce the risks of switch on thump as well as switch off unstability. This aspect requires to be studied in depth and a definitive solution found. In Bigun's thread on On / Off thumps, PMA suggested some good solutions.
  5. I would like the Amp Pcb to suit proper mounting of the Power BJTs on a heat sink of 196*100*33 mm. Ref http://www.diycomponents.in/ProductDetails.xhtml?current_SKU=1120 . Or 204*100*42 Ref: http://diyaudiocart.com/Heat-Sink-DAC-204-100
  6. These heat sinks are available easily in India. I hope either of them will be suitable for one channel.
  7. I was thinking a transformer sec voltage of 36 Volts AC (say 18-0-18) should be suitable giving audio output of 16 Volts AC and a power just above 30 watts into 8 ohms. Or a slightly higher voltage to give a max of 50 watts into 8 ohms.

    --gannaji.
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Last edited by gannaji; 1st February 2014 at 04:22 PM.
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Old 1st February 2014, 04:46 PM   #3
maouna is offline maouna  Greece
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what is the purpose of resistors R7 and R8?
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Old 1st February 2014, 09:49 PM   #4
Bigun is offline Bigun  Canada
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I'm not sure what R7 does either - I think it's redundant.

As for R8, it provides local 'nested feedback' from the VAS. It can be left off if you want to keep it as simple as possible.

I'm posting a picture of the schematic for easier viewing
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File Type: jpg tgm8v5-lowpower retro v2.jpg (186.6 KB, 478 views)
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Old 2nd February 2014, 06:31 AM   #5
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Old 2nd February 2014, 09:08 AM   #6
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Is R24 and R27 really necessary? What purpose do they serve? I've never seen Rod use these resistors on any of his CFP output stages.
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Old 2nd February 2014, 09:13 AM   #7
MiiB is offline MiiB  Denmark
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Base stoppers are always beneficial for stability, small PCB inductances paired with BJT-cob may cause oscillation.
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Old 2nd February 2014, 10:40 AM   #8
AndrewT is online now AndrewT  Scotland
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R24 & R27 can be any value from a 0r0 wire link to the 10r suggested.
Always leave pads for base stoppers in an unknown design.
You rarely know in advance whether the stopper effect will be required or not.

I would also allow base stoppers on drivers and if fitted pre-drivers.

The RC of C3 + R3 seems unusually low. Is this deliberate?

Allow pads to add D.Self compensating resistor between R13 and R17.
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Old 2nd February 2014, 02:20 PM   #9
gannaji is offline gannaji  India
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Quote:
Originally Posted by maouna View Post
what is the purpose of resistors R7 and R8?
Dear maouna,
R7 is in the same place as R10 in your circuit, Need help in finding stability margins/loop stability in LTspice for my amp
The value is wrong and requires to be adjusted in relation to R10.
--gannaji
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Last edited by gannaji; 2nd February 2014 at 02:22 PM.
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Old 2nd February 2014, 02:34 PM   #10
gannaji is offline gannaji  India
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Quote:
Originally Posted by AndrewT View Post
R24 & R27 can be any value from a 0r0 wire link to the 10r suggested.
Always leave pads for base stoppers in an unknown design.
You rarely know in advance whether the stopper effect will be required or not.

I would also allow base stoppers on drivers and if fitted pre-drivers.

The RC of C3 + R3 seems unusually low. Is this deliberate?

Allow pads to add D.Self compensating resistor between R13 and R17.
The various resistor and capacitor values are as per original Bigun's circuit, and are to be finalized. The circuit is posted as is to start off the discussion. Point about compensating R between R13 & R17 is noted. Thank you.
--gannaji
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Last edited by gannaji; 2nd February 2014 at 02:39 PM.
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