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Question about AMP modifications
Question about AMP modifications
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Old 11th June 2019, 05:57 PM   #11
wiseoldtech is offline wiseoldtech  United States
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Quote:
Originally Posted by indianajo View Post
You need to use the scope to check whether the VAS Q404 is clipping, or the input transistor no name. If so you need to reduce the gain or increase the supply voltage at that point. A 80 v or 120 v Vceo rated transistor may be required at that point. Sanyo may not have wanted to spend the extra $.02 for a higher rated transistor, or in those early days more voltage rating might have bought a lot more noise. I use MPSA06/56 for input and MJE15028/29 for VAS & drivers. Furthermore MJE21194 output transistors are I suspect TO220 parts and limiting voltage swing helped prevent violating lower power capability on those cheaper parts. MJL21194 are the TO3P versions of that line. 4 A 50 v 1 s soa rating.
More to the point a successful 7 transistor single supply design is Bigun's TGM8, with the same npn input transistor. You may need to make wiring changes to copy that design that is different in detail.

Thanks indianajo for that info.
Please ignore the previous schematic diagrams, and go by THIS one below.
My mistake, surely.


This one includes measured voltage readings, the proper values/parts.
I'm curious but hesitant about jumping R524 with a 220u cap - would it give some boost, or make things worse.
The main PS is 20,000Uf 80V caps - at no signal 52V, at clipping 49V.
The input transistor, TR504 is fed by a seperate 18V regulated supply.
Output idle current is about 25mA.



Any more thoughts, suggestions - feel free, thanks.
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Last edited by wiseoldtech; 11th June 2019 at 06:02 PM.
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Old 11th June 2019, 07:35 PM   #12
indianajo is offline indianajo  United States
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So what maximum AC voltages (average or rms) are you getting out of the input transistor and the vas tr505 at max? Since you have a scope there you can see any clipping. You're not going to get more than that out of the drivers & output transistors. They are not set up for voltage gain. The john ellis basic 50 has input transistor collector resistor of 1.5 k from the center line, and an emitter resistor of 470. I think this would give more voltage gain. Bigun TGM8 has on the input transistor a collector resistor of 1 k and an emitter resistor of 4.7k to the center line, which also might give more gain. I run the input transistor of my AX6 from the center line, which with a 70 v rail is about 35 volts. Plenty of gain there.
Basic 50 has driver current limiters of 68 ohms and tgm8 has 220 ohms which gives harder current drive of the output transistors than this sanyo. Sanyo used 330 ohms, and I suspect they left out the driver heat sinks since they cost $1 each with installation labor. Some of my driver VAS heat sinks were bits of window screen sawed off & drilled, & I run my amp 14 hours a day. I did use real heat compound.
Like I said, I think sanyo was limiting the voltage swing to the output transistors to limit the current out into the speaker, since they were using cheap TO220 output transistors. If you go for more gain & power out, at least upgrade to output transistors MJL4281 or 2sc5200(inferior in power and same price in US). You can put output transistors off board at least 4" away on a heat sink, at least I did & got away with it without oscillation. Separate the wires from the heat sensors (double diode) as much as possible from the OT drive wires. Close parallel inputs & output lines to a gain point like the VAS cause oscillation.
note TGM8 has a fully complementary output transistor pair. I don't think 2 npn output transistors would mess it up much. BTW .33 ohm emitter resistors on OT's are safer from thermal runaway than .22 ohm. I use .47 ohm actually, plus a fan since my ST120 amp has such a history of burning output transistors.
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Last edited by indianajo; 11th June 2019 at 07:46 PM.
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Old 11th June 2019, 09:13 PM   #13
wiseoldtech is offline wiseoldtech  United States
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Quote:
Originally Posted by indianajo View Post
So what maximum AC voltages (average or rms) are you getting out of the input transistor and the vas tr505 at max? Since you have a scope there you can see any clipping. You're not going to get more than that out of the drivers & output transistors. They are not set up for voltage gain. The john ellis basic 50 has input transistor collector resistor of 1.5 k from the center line, and an emitter resistor of 470. I think this would give more voltage gain. Bigun TGM8 has on the input transistor a collector resistor of 1 k and an emitter resistor of 4.7k to the center line, which also might give more gain. I run the input transistor of my AX6 from the center line, which with a 70 v rail is about 35 volts. Plenty of gain there.
Basic 50 has driver current limiters of 68 ohms and tgm8 has 220 ohms which gives harder current drive of the output transistors than this sanyo. Sanyo used 330 ohms, and I suspect they left out the driver heat sinks since they cost $1 each with installation labor. Some of my driver VAS heat sinks were bits of window screen sawed off & drilled, & I run my amp 14 hours a day. I did use real heat compound.
Like I said, I think sanyo was limiting the voltage swing to the output transistors to limit the current out into the speaker, since they were using cheap TO220 output transistors. If you go for more gain & power out, at least upgrade to output transistors MJL4281 or 2sc5200(inferior in power and same price in US). You can put output transistors off board at least 4" away on a heat sink, at least I did & got away with it without oscillation. Separate the wires from the heat sensors (double diode) as much as possible from the OT drive wires. Close parallel inputs & output lines to a gain point like the VAS cause oscillation.
note TGM8 has a fully complementary output transistor pair. I don't think 2 npn output transistors would mess it up much. BTW .33 ohm emitter resistors on OT's are safer from thermal runaway than .22 ohm. I use .47 ohm actually, plus a fan since my ST120 amp has such a history of burning output transistors.

Thanks for the additional info, much appreciated.
This original amp board was taken from a Harman Kardon 330A (or B) receiver, just so you know, not a Sanyo.

I will do another 'scoping and get back to you on my findings from T504 and T505 at the onset of clipping. (while driving an 8 ohm dummy load)

Additionally, I want to track down the source where clipping originates, be it at the input, or down the chain towards the outputs.
I just want to maximize performance without sacrificing quality.

As for "upgrading" the outputs, aren't the (newly installed) Motorola MJL21194's (TO3P-250V-16A) hefty enough?
And the new NTE 184/85's are a Matched Pair capable of 4A collector current.
There is no heat sink on them as they only get slightly warm.
The bias diodes are mounted between each output pair, with some grease against the heatsink.
The bias is not a problem, that I know. (25mA idle - 40-50mA warmed up)



Musically, at this time, the amp sounds superb driving hungry AR LST's (4 ohms) to room shaking levels.
And if you're familier with the LST's, you'll know they're beasts to drive.
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Old 12th June 2019, 12:18 AM   #14
indianajo is offline indianajo  United States
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MJL21194 is state of the art.
NTE184/185 should be fairly capable.
If you're hot rodding it (increasing output power) you want more heat sink, and probably a fan. Receivers were made to a price point and huge power was not a typical selling point. What was the power rating? 8.2 vav into 8 ohms is 8.4 watts. That is not much.
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Old 12th June 2019, 12:27 AM   #15
Mark Tillotson is offline Mark Tillotson
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NJW3281/NJW1302 are considerably better(*) than the MJL22193/4, so you can't really call those state of the art any more.



(*) _much_ more bandwidth, improved gain linearity at high current
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Old 12th June 2019, 12:34 AM   #16
wiseoldtech is offline wiseoldtech  United States
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Quote:
Originally Posted by indianajo View Post
MJL21194 is state of the art.
NTE184/185 should be fairly capable.
If you're hot rodding it (increasing output power) you want more heat sink, and probably a fan. Receivers were made to a price point and huge power was not a typical selling point. What was the power rating? 8.2 vav into 8 ohms is 8.4 watts. That is not much.

This amp board from the HK330 is now custom installed into a Hammond chassis, along with the power supply I designed, a delay relay board, and an RIAA preamp I designed.
The chassis is 14 inches across, 8 of those inches are for a heatsink for the outputs, which only get mildly warm.


Here's the layout so you can see my work.
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File Type: jpg IMG_8263.jpg (376.5 KB, 60 views)
File Type: jpg IMG_8259.jpg (396.0 KB, 50 views)
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Old 12th June 2019, 12:41 AM   #17
wiseoldtech is offline wiseoldtech  United States
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Quote:
Originally Posted by Mark Tillotson View Post
NJW3281/NJW1302 are considerably better(*) than the MJL22193/4, so you can't really call those state of the art any more.
(*) _much_ more bandwidth, improved gain linearity at high current

Well, that may be, but I've got a whole bag of the MJL22193/4's in matched sets and decided they'd be hefty enough to withstand hard use if need be.
at 250v and 16 amps rating, they should be fine for this situation.


I'd be happy with a solid 50W/per chaannel, but am interested in possible tweaks as long as I don't have to go crazy.

Last edited by wiseoldtech; 12th June 2019 at 12:45 AM.
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Old 12th June 2019, 12:52 AM   #18
Ian Finch is offline Ian Finch  Australia
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It would be good to know the facts of the reading first before proposing changes.

As you say that you have room shaking volume levels, it would suggest that you have more power than 8 volts peak-peak would produce. I think there may be an error in the scaling of what you see and perhaps read on the the oscilloscope. I don't mean with the switchable x10 probe setting but perhaps there is some confusion with the method of measurement. To check, you need a voltage standard and most scopes have a 1V squarewave reference available on the front panel. If you determine that this does indeed scale to 1V peak-peak, then there is a problem with the amplifier or perhaps with the maximum level that the signal source produces but if not, then you have a reading error.
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Old 12th June 2019, 01:01 AM   #19
JMFahey is online now JMFahey  Argentina
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1) 53V raw / 49V loaded means you have an excellent PSU

2) given supply drop plus Vce saturation you can count with 22V peaks, and that being optimistic, so some 15.5V RMS.
Given what you say about your scope abilities Id trust more what a plain meter shows , at least up to 1 kHz, so injecting a 440 Hz or 1kHz tone and just reaching clipping you should see at least 15V RMS.

Which means nice 28W RMS into 8 ohm.

3) no clue on your NTE which are always "iffy" but the MJL are WELL rated (and then some) for that duty.
Dont worry about "state of the art parts", the circuit itself is fine but dated.

You can use these test tones.
They last 30 seconds so set player to "repeat" :

https://www.mediacollege.com/audio/t...6bit_30sec.mp3

https://www.mediacollege.com/audio/t...6bit_30sec.mp3
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Last edited by JMFahey; 12th June 2019 at 01:06 AM.
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Old 12th June 2019, 01:14 AM   #20
wiseoldtech is offline wiseoldtech  United States
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Quote:
Originally Posted by JMFahey View Post
1) 53V raw / 49V loaded means you have an excellent PSU

2) given supply drop plus Vce saturation you can count with 22V peaks, and that being optimistic, so some 15.5V RMS.
Given what you say about your scope abilities Id trust more what a plain meter shows , at least up to 1 kHz, so injecting a 440 Hz or 1kHz tone and just reaching clipping you should see at least 15V RMS.

Which means nice 28W RMS into 8 ohm.

3) no clue on your NTE which are always "iffy" but the MJL are WELL rated (and then some) for that duty.
Dont worry about "state of the art parts", the circuit itself is fine but dated.

You can use these test tones.
They last 30 seconds so set player to "repeat" :

https://www.mediacollege.com/audio/t...6bit_30sec.mp3

https://www.mediacollege.com/audio/t...6bit_30sec.mp3



Thanks for that info, appreciated.
I already have a CD with 1Kz sine waves at 0, -10, --20 dB, and an benchtop audio signal generator, so I'm good to go as far as testing.
But I will get back on some 'scope readings as mentioned and let you all know the results.
The NTE184/5's are original stock authentic, I don't trust a lot of "newer" parts due to counterfeiting these days.

Last edited by wiseoldtech; 12th June 2019 at 01:17 AM.
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