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Building MONGREL amp using recycled parts
I recently scapped a JVC (Victor) Surround amp and that gave me heaps of parts for DIY projects. First at all, the amp is about 20 years old and it cost me an arm & a leg when I bought it new. It gave crackening sound when I turned it on, I had it repaired but the problem resurfaced after a while.
I have gutted the amp and used the case & heatsink for a SymAsym and I have got 5 pairs of Sanken OTs & 2SB649 & 2SD669 plus heaps of mylar caps.
For the MONGREL project, I can use the following parts in the pictures:
2SA970 & 2SC2240 SSTs
Neatly wound output coil
White right-angled pins to connect the VB to the PB
Mounting bolt for OTs
10 Ohm (2W) resistors
May be 3900pF Mylar (Blue) capacitor for CCS C-B bypass, I think that is close enough to the 5600pF for bypass purpose.
I have some questions about the capacitor types:
Maroon - value 334 - mylar (polyester)
Blue - value 392 - I think it is also mylar (polyester)
Yellow with foil inside - value 392 - I don't know what this is??
Cheers, Stanley
I recently scapped a JVC (Victor) Surround amp and that gave me heaps of parts for DIY projects. First at all, the amp is about 20 years old and it cost me an arm & a leg when I bought it new. It gave crackening sound when I turned it on, I had it repaired but the problem resurfaced after a while.
I have gutted the amp and used the case & heatsink for a SymAsym and I have got 5 pairs of Sanken OTs & 2SB649 & 2SD669 plus heaps of mylar caps.
For the MONGREL project, I can use the following parts in the pictures:
2SA970 & 2SC2240 SSTs
Neatly wound output coil
White right-angled pins to connect the VB to the PB
Mounting bolt for OTs
10 Ohm (2W) resistors
May be 3900pF Mylar (Blue) capacitor for CCS C-B bypass, I think that is close enough to the 5600pF for bypass purpose.
I have some questions about the capacitor types:
Maroon - value 334 - mylar (polyester)
Blue - value 392 - I think it is also mylar (polyester)
Yellow with foil inside - value 392 - I don't know what this is??
Cheers, Stanley
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In what I've read, clear with visible foil usually means polystyrene, but don't know about the yellow color. The blue might be polypropylene but since it's not indicated I wouldn't bet on it. I'm sure the red ones are mylar.
- keantoken
- keantoken
Yellow caps
Thanks keantoken,
I have seen polystyrene caps - they are transparent cylindrical case with visible foil inside. These yellow ones are a bit different and they are also used in the LP filter section after DAC inside my blu-ray player, the value ranges from 1500pF to 47nF. I have attached another picture here.
Cheers, Stanley
Thanks keantoken,
I have seen polystyrene caps - they are transparent cylindrical case with visible foil inside. These yellow ones are a bit different and they are also used in the LP filter section after DAC inside my blu-ray player, the value ranges from 1500pF to 47nF. I have attached another picture here.
Cheers, Stanley
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The GX lives ... ha ha ha
I fired up the GX's , at first forgot to hook up the NFB wire. What I did.. is use a 35-0-35v supply to "test fire" the circuit. On first try ,one led would not light up , no smoke but something was wrong. 10R safety resistors were cool as was everything else. Forgot to give the poor amp NFB !!! Fixed that , have 32mv offset at OP without adjustment.
I used "fake" 33R emitter resistors to just power the drivers up and I get +651mv and -685mV on the driver emitters. This seems to be close to spec even at half rail voltage. Final pix is a voltage test of the whole GX at 35-0-35V.
OS
I fired up the GX's , at first forgot to hook up the NFB wire. What I did.. is use a 35-0-35v supply to "test fire" the circuit. On first try ,one led would not light up , no smoke but something was wrong. 10R safety resistors were cool as was everything else. Forgot to give the poor amp NFB !!! Fixed that , have 32mv offset at OP without adjustment.
I used "fake" 33R emitter resistors to just power the drivers up and I get +651mv and -685mV on the driver emitters. This seems to be close to spec even at half rail voltage. Final pix is a voltage test of the whole GX at 35-0-35V.
OS
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Hi OS,
Good works, I have been wondering where NFB goes. I guessed that I should add a jumper to the PB120 layout from NFB to somewhere near the output coil pickup point, roughly midway between the OTs.
I may add the voltage regulating section from the PB250 to the PB120 layout.
Do I lose a few volts by using regulator for the VB?
Cheers, Stanley
Good works, I have been wondering where NFB goes. I guessed that I should add a jumper to the PB120 layout from NFB to somewhere near the output coil pickup point, roughly midway between the OTs.
I may add the voltage regulating section from the PB250 to the PB120 layout.
Do I lose a few volts by using regulator for the VB?
Cheers, Stanley
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You lose 1.7v with 35.8-0-35.8VDC (that is exactly what the little trafo gives me) with 75v+ I expect about a 2v loss.
I am now at the full power 77-0-77VDC stage of my "burn-in" WITH 1 pair of outputs. Wish me luck. 🙄
OS
NFB, optimally, should come say a mm after the output takeoff point, so distorted currents don't squeeze past the NFB takeoff into the output takeoff. That's how I understand it.
- keantoken
- keantoken
quite right , man.. like below , right at the inductor(Below). On the horizontal layout it is more critical since the distance the current has to travel across traces is far greater.
OS
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Is that what kind of trimmer they have "down under", up here the trimmers are 3 X 2.5mm inline style ... This
or this..
OS
Is that what kind of trimmer they have "down under", up here the trimmers are 3 X 2.5mm inline style ...
OS
Hi OS,
You have reminded me - I have changed the trimpot footprint to the universal one; so that it can fit more than one type of trimpot, including the 3 x 2.5mm in-line style.
BTW, do we have minimum bias current for maximum resistance (200 ohm in this case) ??
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Tried that my self , with the GX/PB250 , an open trimpot (which will be 200R) gives + and - 500mv at the driver emitters which would be fully class B (no bias)
A shorted trimpot will bias the OP's at over 250mA , so we have a healthy range as well as a safety margin. I have 1 pair OP's hooked to the "monster" (below) running 1 small 2-way (sounds good). I must "break-in" the other amp with 1 Pair as well to ensure electrical and thermal stability as well before I populate all 16 OP devices. I will never again smell the "magic smoke". I am sure a small value change will be done to the VBe before I am done. I miss the massive headroom of my "supersym" already , my little Dx clone just doesn't cut it... 🙁
OS
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Parts list
Hi, OS
I am checking the BOM against the schematics for GX1.2VB & found some discrepancies. I believed that the BOM is more up-to-date.
I found most of the parts from Farnell & they do not charge shipping for local deliveries.
I have found some challenges with some capacitors:
C2 - 330pF, 100V ceramic - I have Wima FKP2 (1000V) in my parts bin so I will use this Polypropylene type.
C3 - 220uF/35V Non-polar. Farnell does not have these at reasonable price. I normally use low-voltage polar electroyltics in this position & I have got a couple of 470uF (16V) Panasonic FC type in my parts bin.
C8 & C9 - 0.68uF ceramic 100V - I can only find Kemet Z5U 0.68uF (50V) on Farnell & my power supply is only 42V DC.
C5 - 0.01uF ceramic 100V - I can only find Kemet COG 0.01uF (200V) on Farnell & I think that this is spot on.
C7 & C9 - 47pF silver mica - again I have Wima FKP2 in my parts bin so I will use them for test & I can buy Silver mica from a local shop later.
All up, my Farnell order is less than $20 and I have to buy some 2SC3503/2SA1381 transistors & trimpots from the local shops.
Thanks & regards, Stanley
Hi, OS
I am checking the BOM against the schematics for GX1.2VB & found some discrepancies. I believed that the BOM is more up-to-date.
I found most of the parts from Farnell & they do not charge shipping for local deliveries.
I have found some challenges with some capacitors:
C2 - 330pF, 100V ceramic - I have Wima FKP2 (1000V) in my parts bin so I will use this Polypropylene type.
C3 - 220uF/35V Non-polar. Farnell does not have these at reasonable price. I normally use low-voltage polar electroyltics in this position & I have got a couple of 470uF (16V) Panasonic FC type in my parts bin.
C8 & C9 - 0.68uF ceramic 100V - I can only find Kemet Z5U 0.68uF (50V) on Farnell & my power supply is only 42V DC.
C5 - 0.01uF ceramic 100V - I can only find Kemet COG 0.01uF (200V) on Farnell & I think that this is spot on.
C7 & C9 - 47pF silver mica - again I have Wima FKP2 in my parts bin so I will use them for test & I can buy Silver mica from a local shop later.
All up, my Farnell order is less than $20 and I have to buy some 2SC3503/2SA1381 transistors & trimpots from the local shops.
Thanks & regards, Stanley
SNG, you could try photoflash caps if you have some. I wonder if these aren't the "next step up" so to say from normal electrolytics. I'm using them in the feedback shunt cap currently and the difference in sound worth investigating I think.
- keantoken
- keantoken
Continuing my SLOW , thorough tests on the GX1.2VB/PB250 combo. With both 77V rails and 36V rails (SNG001 , I would not use the cap multipliers.. you lose 3 v AND 2V through the hawksford cascodes) , The Vbe is ROCK solid with a range of 0 Mv (last turn on trimmer) to 84mv (300ma bias) Pix 1 is after a 10C cold morning .. stayed at 22-23mV all day even after the 32C high (hot day). NO mods need to be done to this important circuit. It is very fast , use a lighter to check it..(pix 2) Ha Ha. 😀
It sounds great ... with only 1 pair OP's , about the same if not a little more detailed than the symasyms .. VERY nice high frequency reproduction.
With the modular design , once the PB250's are locked in , I will adjust any compensations easily. I don't expect very many issues with this .. as EVERY simulated aspect has panned out exactly as LT predicted , right down to the mV.
Edit , this is what I envisioned for the VAS heatsink(pix 3),made from a small piece of roof flashing , runs nice and cool.. 😎
OS
OS
It sounds great ... with only 1 pair OP's , about the same if not a little more detailed than the symasyms .. VERY nice high frequency reproduction.
With the modular design , once the PB250's are locked in , I will adjust any compensations easily. I don't expect very many issues with this .. as EVERY simulated aspect has panned out exactly as LT predicted , right down to the mV.
Edit , this is what I envisioned for the VAS heatsink(pix 3),made from a small piece of roof flashing , runs nice and cool.. 😎
OS
OS
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I've just tested a slew of caps in the feedback shunt network. Photoflash won, I can speak from experience now. My cap came from a clip-on flash unit, and is more powerful than what you usually find, 330V 200uF.
So I encourage others to try and want to hear the results, if so...
Originally posted here:
http://www.diyaudio.com/forums/head...phone-amp-jlh-output-stage-8.html#post2303403
- keantoken
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Don't have any of those caps , Keen.. what I do have is two killer amplifiers 😱😱. Below is the final tested 78 volt rail readings off the actual circuit.
For those using 35-50v , NO adjustment is needed , save for a slight tweak in the CCS and Vbias trimmers. NO turn on thump , NO noise at all (silence) , 1mv offset with just half turn of the trimmer and best of all ..no stability issues AT ALL. 😎
The 35v readings are just a matter of subtracting 43 volts from every 78V reading. LED Vf drops slightly at 35V rails (2.2@78 -1.9@35v).
I forgot the NFB hookup TWICE , amps did not blow but just went to the positive rail making one green LED not illuminate. A VERY forgiving circuit. BEWARE , do not touch input with finger , 60-70VAC at OP will fry your speakers ... 38DB gain is scary.. Total success , now I have to machine my heatsinks (layout,drill ,and tap) with all 4 Op pairs ...this will be one good project 🙂🙂.
A few ideas/examples on how to have a safe power board are pix 2 and 3. Black insulator is a polyethylene notebook cover cut to cover the board while allowing for power device thermal contact. Outputs/drivers should be mounted like in last picture 3-4mm off of board , try to space the driver holes 102mm (4") and the output holes 115mm (4 1/2") apart for mounting on a 5" /120-125mm heatsink.
OS
For those using 35-50v , NO adjustment is needed , save for a slight tweak in the CCS and Vbias trimmers. NO turn on thump , NO noise at all (silence) , 1mv offset with just half turn of the trimmer and best of all ..no stability issues AT ALL. 😎
The 35v readings are just a matter of subtracting 43 volts from every 78V reading. LED Vf drops slightly at 35V rails (2.2@78 -1.9@35v).
I forgot the NFB hookup TWICE , amps did not blow but just went to the positive rail making one green LED not illuminate. A VERY forgiving circuit. BEWARE , do not touch input with finger , 60-70VAC at OP will fry your speakers ... 38DB gain is scary.. Total success , now I have to machine my heatsinks (layout,drill ,and tap) with all 4 Op pairs ...this will be one good project 🙂🙂.
A few ideas/examples on how to have a safe power board are pix 2 and 3. Black insulator is a polyethylene notebook cover cut to cover the board while allowing for power device thermal contact. Outputs/drivers should be mounted like in last picture 3-4mm off of board , try to space the driver holes 102mm (4") and the output holes 115mm (4 1/2") apart for mounting on a 5" /120-125mm heatsink.
OS
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Cap multiplier
Hi OS,
Do you mean the voltage regulator that I added from the PB250?
I can easily bypass it by using the latest layout by installing only the R78 (22R) & C72. I have highlighted yellow in the attached picture.
- Stanley
Hi OS,
Do you mean the voltage regulator that I added from the PB250?
I can easily bypass it by using the latest layout by installing only the R78 (22R) & C72. I have highlighted yellow in the attached picture.
- Stanley
