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Question about AMP modifications
Question about AMP modifications
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Old 12th June 2019, 02:19 AM   #21
indianajo is offline indianajo  United States
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Warning DVM except for the $180 Fluke RMS version produce random numbers on AC volts scales on music. They are designed to measure power line frequencies. I use an analog VOM to measure AC voltages, which is why I quote Average voltages.
You can calibrate the scope with a zener diode driven by the AC signal through a power limiting resistors. Many AC sources like FM radio earphone jacks can put out 7 vac. Note a sine wave seen on a scope, Vav is 0.707 Vpp. RMS is close but more rigorous. Buy the zener from a ligitimate source like newark, mouser, digikey, RS. Put 1000 ohms over a 6 v zener on the line end, run a FM radio to the top of the resistor and the non-line end of the zener, turn radio all the way up, measure at the junction of the resistor & the zener. Scope should show a 6 v shelf from bottom of wave form (gnd is shaft of 1/8" phone plug) to the zener turn on point.
My point previous is that the circuit is designed to protect MJE21194 mounted on inadequate heat sinks, from excessive speaker current. With that heat sink & a stiff power supply, you can take off the limiters. Copy the experienced 7 transistor designers , Bigun TGM8 & John ellis basic 50. You don't need 21 transistors to make good sound, 6 is enough. If you're listening on speakers, those HD improvements in the 2nd & 3rd digit after the decimal point of the 21 transistors are for bragging, not listening IMHO. When you wire point to point as I do, the fewer the parts, the fewer wires you have to hook up.
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Last edited by indianajo; 12th June 2019 at 02:27 AM.
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Old 13th June 2019, 08:41 PM   #22
wiseoldtech is offline wiseoldtech  United States
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Ok, so I did some calibrating of the test equipment to insure accuracy, and here's some of my findings.
I'm a bit dissapointed at the output though, I thought I'd have more, it sure sounds that way hooked up to speakers.
So it appears that I'm only getting around 29 watts RMS/channel from this thing?
Attached Images
File Type: jpg audio-wattmeter-table.jpg (49.8 KB, 65 views)
File Type: jpg output info.JPG (84.8 KB, 64 views)
File Type: jpg T504 reading.JPG (30.4 KB, 66 views)
File Type: jpg 330_mod_FINAL.JPG (176.0 KB, 66 views)
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Old 14th June 2019, 01:47 PM   #23
indianajo is offline indianajo  United States
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Well the experts are somewhere else again. It's been 15 hours. Not much interest in speaker cap amps, people just love to burn up their speakers with split supply amps. They buy "speaker protector" boards that relies on a 15 A AC rated relay to disconnect the 1000 Adc fault current to their speaker. There is no zero volts to quench the arc through an AC rated relay trying to stop DC. Real relays used by Peavey are about $50 each. **** speaker protector boards have as many parts as this whole amp board.
What input voltage did you measure with the output just before clipping? As I asked in post 12, what voltages are at the collectors of tr504 (input transistor) and TR505 (VAS) when the clipping begins? Music, AC, not the DC voltages.
common emitter transistor gain is -Rc/Re so with 56:1 at tr504 and 50:1 at VAS tr505 the design might be doomed to clip. Besides the input transistor is running off 18v so there is no way you can get 25 vac out of it. The 2sc828 might be some *****y 25 vceo rated transistor that would overheat at 35 v supply, but that supply is what I am using for input transistor in my AX6 that puts out 70 W. If 80 v rated 2n5401/5551 or MPSA06/56 transistors are too expensive, try MPS8098/8099 which I got for $8 a hundred. I have my suspicions of the 2sc538 VAS, too. For VAS 120 v rated MJE15028/29 were on sale under $.50 each at newark two weeks ago. With MJL21194 outputs and a serious heat sink, you could also run the main supply up to 70 and go for 150 W peaks on the cannon shot of 1812 overture the way I am doing with my single output pair AX6. The 70 W I measured was over 5 loud seconds of a Rhianna song.
You're going to have to change some things to get better performance if your measurements are accurate. you'll have to pull R518 & R524 to increase the value and decrease the gain. I'm not sure that will do it, copying a TGM8 AX6 AX8 or John Ellis Basic 50 would be a surer path. I think there are some TGM8 boards out there on ali or ebay or something. There are patterns to make AX6 boards on the thread above but make a mask, etching and disposing of the waste chemicals are a PIT*. I found it easier to build AX6 on bare NEMA C board from mcmaster (they call it garolite) with 28 ga wirewrap wire & a soldering iron. The brown canvas reinforced stuff is 1000% easier to drill than glass reinforced board. Use a #46 drill, in a yankee hand crank drill if your electric drill chuck won't go that small.
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Last edited by indianajo; 14th June 2019 at 02:14 PM.
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Old 14th June 2019, 02:43 PM   #24
Ian Finch is offline Ian Finch  Australia
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Quote:
Originally Posted by wiseoldtech View Post
Ok, so I did some calibrating of the test equipment to insure accuracy, and here's some of my findings.
I'm a bit dissapointed at the output though, I thought I'd have more, it sure sounds that way hooked up to speakers.
So it appears that I'm only getting around 29 watts RMS/channel from this thing?
14.5VRMS is a lot more power than than 8.4V peak would deliver if that had been the case. Let's say its near enough to 30 watts from +/- 25V rails which is not too shabby @ a nominal 4 ohm load with 89 db/W 1m sensitivity speakers. Yes, that would shake the room, like it does mine.

To get a worthwhile volume increase on that already respectable level, you'd need a lot bigger amplifier with supply rails of at least +/-40V even with 4 ohm speakers. That's around 120W and I think you'd have to completely rebuild or simply replace the HK amplifier to get to the next level of house-shaker.

I've only seen the LSTs at shows and a couple of retailer demos many years ago. They were always out of my price and size range. Here's some facts and figures for anyone else reading: https://www.vintageshifi.com/reperto...T-Brochure.pdf
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Last edited by Ian Finch; 14th June 2019 at 02:46 PM.
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Old 14th June 2019, 02:51 PM   #25
wiseoldtech is offline wiseoldtech  United States
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Originally Posted by indianajo View Post
What input voltage did you measure with the output just before clipping? As I asked in post 12, what voltages are at the collectors of tr504 (input transistor) and TR505 (VAS) when the clipping begins?.....

Besides the input transistor is running off 18v so there is no way you can get 25 vac out of it.

If you look at the photo that I posted for TR504, you'll see my mention of 16.15 volts RMS off collector at clipping. (listed as Vk on the screen)


My thinking is that since it's a capacitor-coupled output, I'm losing some "power" because of that coupling capacitor (4,700uF).
Otherwise, the amp sounds fantastic, clear, and dynamic.
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Old 14th June 2019, 02:56 PM   #26
Mark Tillotson is offline Mark Tillotson
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If there's 1000A fault current to the speaker, the voice coil is vaporizing on a timescale of microseconds and a disconnect relay would be purely a spectator to the event! Speaker fault currents are limited by the resistance of the speaker coil to something much more managable, but is still DC at reasonably high voltages (most relays only have 24Vdc ratings for good reason, its hard to break DC and this difficult rises steeply with voltage).



A hard short will generate much higher currents (1kA is possible this way, but only for 10's or 100's of microseconds), but drain the filter caps rapidly and blow fuses. Shorts are likely to destroy output devices and would weld the relay contacts of even large relays if they reacted fast enough (which they can't - all the action is likely to be over before the relay moves).


Output device protection can prevent hard shorts leading to high currents in the first place - the combination of speaker protection and output device protection is indeed quite a lot of circuitry.


1kA is the sort of current that magnetizes nearby steel, and explodes thick copper wire. Oversizing filter caps is one way to risk this kind of damage!
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Old 14th June 2019, 03:21 PM   #27
wiseoldtech is offline wiseoldtech  United States
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Quote:
Originally Posted by Ian Finch View Post
14.5VRMS is a lot more power than than 8.4V peak would deliver if that had been the case. Let's say its near enough to 30 watts from +/- 25V rails which is not too shabby @ a nominal 4 ohm load with 89 db/W 1m sensitivity speakers. Yes, that would shake the room, like it does mine.

To get a worthwhile volume increase on that already respectable level, you'd need a lot bigger amplifier with supply rails of at least +/-40V even with 4 ohm speakers. That's around 120W and I think you'd have to completely rebuild or simply replace the HK amplifier to get to the next level of house-shaker.

I've only seen the LSTs at shows and a couple of retailer demos many years ago. They were always out of my price and size range. Here's some facts and figures for anyone else reading: https://www.vintageshifi.com/reperto...T-Brochure.pdf

Indeed, Ian, I originally built this amp to be used as a "Normal Listening Volume" type of amp, certainly not expecting to be a "powerhouse" with only a 52 volt supply.
I expected and so far have gotten a nice, clean-sounding, plenty of headroom amp with an indestructable output section and built-in speaker protection due to the capacitive-coupling design.
Not shown is an additional Zobel (R/L) network for stability, and speaker delay relay to the speakers to eliminate any "thump" at turn-on.


Going by my RMS wattage meter, during peaks of music, it hits about 50+ watts on occasion into 8 ohm speakers.
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Old 14th June 2019, 03:24 PM   #28
indianajo is offline indianajo  United States
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Originally Posted by wiseoldtech View Post
My thinking is that since it's a capacitor-coupled output, I'm losing some "power" because of that coupling capacitor (4,700uF).
I'm getting 70w for 4 seconds out of a 3300 uf speaker cap. Nearly 24 vac on a simpson 260xlpm analog meter parallel a 8 ohm SP2-XT speaker. That was on bass notes of Shut Up & Drive. At 40 hz 3300 uf is 1.3 ohms impedance? 1/(2*pi*f*C) Less impedance at higher frequencies.
From a 3300 uf supply cap shared 2 channels. ST120 chassis does have a stiff 6.5 A 80 v transformer regulated to 70 v.
Mark T, the PV 1.3K has 10000 uf caps at 85 v connected through 10 ga wire to the output transistor board. It melted the lands to the crowbar triac off the board when the OT's shorted. Thick wide lands.
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Old 14th June 2019, 03:28 PM   #29
wiseoldtech is offline wiseoldtech  United States
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The fruits of my labor.......
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Old 14th June 2019, 03:41 PM   #30
indianajo is offline indianajo  United States
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Looks great.
Less gain, more voltage headroom & rail current. You've spent the money on the output transistors & heat sink to deserve 70 w/ch. Hint, 400w switcher supplies like connexelectronic are more compact than a transformer, but require an inner steel box to keep the RF interferance down. May need a fan.
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Last edited by indianajo; 14th June 2019 at 03:47 PM.
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