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Question about AMP modifications
Question about AMP modifications
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Old 14th June 2019, 05:56 PM   #31
JMFahey is offline JMFahey  Argentina
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Cool build

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Quote:
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?
follow my math, basically same as that from others, and youŽll be happy that you practically reached maximum possible ... you canŽt go beyond rails or ground.
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Old 14th June 2019, 06:04 PM   #32
donpetru is offline donpetru  Romania
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Originally Posted by indianajo View Post
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.
I think resistors R418 and R420 are a little big !!! For this reason, insufficient current is injected into the NTE 184 and 185 transistor bases and the output does not saturate to a higher peak-to-peak value. Normally, on the amplifier output before saturation you should record a ~36Vpp. Of course, those 36Vpp would be recorded if the PSU has adequate power.
So, a first step would be to slightly reduce the values of R418 and R420.
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Old 14th June 2019, 08:22 PM   #33
wiseoldtech is offline wiseoldtech  United States
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Originally Posted by indianajo View Post
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.

Thanks.
But I'm against using SMPS supplies with anything analog audio.
I much prefer "old school" linear power, much quieter, even if being bulky.
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Old 15th June 2019, 04:56 AM   #34
PRR is offline PRR  United States
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Quote:
Originally Posted by indianajo View Post
Well the experts are somewhere else again....
I dunno about them. My excuse is the forum ate my post. Quickly again....

You get 42V peak-peak from a "52V" supply. Assuming the supply sags 15% under full load, assuming a couple V loss each side, this is as good as it gets.

A 4700u cap won't lose much over the audio band. If you think it might, re-check the AC voltage at the other side of the cap. Probably about the same.

The circuit has worked well for 50 years. Fisher built tons of these. There was a Yamaha bass amp we had used nearly the same scheme.

Not using an output cap is elegant fashion and economics. If you build stereo, bipolar supply is two big caps, unipolar supply is three big caps. In one-off economics the extra cap is not a profit-buster; against that cost, cap-coupled amps often get-by without protection circuits which turn out to be "essential" in direct coupled amps.
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Old 15th June 2019, 05:28 AM   #35
wiseoldtech is offline wiseoldtech  United States
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Quote:
Originally Posted by donpetru View Post
I think resistors R418 and R420 are a little big !!! For this reason, insufficient current is injected into the NTE 184 and 185 transistor bases and the output does not saturate to a higher peak-to-peak value. Normally, on the amplifier output before saturation you should record a ~36Vpp. Of course, those 36Vpp would be recorded if the PSU has adequate power.
So, a first step would be to slightly reduce the values of R418 and R420.

I believe you're talking about R523/R525, if you're going by my revised schematic, (shown in later post) and the current design.
Thanks.
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Old 15th June 2019, 05:36 AM   #36
wiseoldtech is offline wiseoldtech  United States
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Quote:
Originally Posted by JMFahey View Post
Cool build

As of:
follow my math, basically same as that from others, and youŽll be happy that you practically reached maximum possible ... you canŽt go beyond rails or ground.

Indeed, thanks, I'm aware of the rail limitations and clipping factors.
As I mentioned, I primarily designed this to be used for "normal" listening with high quality, non-fatiguing sonics, and with an indestructable output section and hefty power supply.
So far, it's what I have, and should last decades.
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