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1970s Design Indulgence

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Graham Slee View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: Yesterday at 3:56pm
@ Bruce, you said,

"double-checked
64.5V good?
actual will be +/- 0.5V"

(was he right?)

Actual measured voltages,
Ch1: 64.2V
Ch2: 64.8V

Output DC, solid! 37.8V no signal to full out (75.6V average HT)

Max out: 48WPC, 8 ohms

THD(1) At 43WPC: 0.023%
THD(1) At 1WPC: 0.033%

Note: my mains voltage is 248 - 253V

Both PCB/heatsink assemblies had to be removed to modify the boards for the voltage regulator. We'd concluded that regular metal film resistors were best to set the voltage, and then to use a separate ballast resistor for the minimum regulator current and the reverse bootstrap current (15mA + 17mA), and the nearest value, a 1k8 TO-220 35W power film provides a little extra ballast at 35 - 36mA.

The ballast resistor was first tried without a heatsink, but the resistance was unstable. It improved with a clip-on heatsink, but I decided to tie them both to the chassis.

Best FFT harmonics trace were obtained at roughly 13mV per emitter resistor.

Amp is playing as I type.

No SNAFU! Wink
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Fatmangolf View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: Yesterday at 6:36pm
That is an impressive engineering calculation Bruce and Graham.
Jon

Open mind and ears whilst owning GSP Genera, Accession M, Accession MC, Elevator EXP, Solo ULDE, Proprius amps, Cusat50 cables, Lautus digital cable, Spatia cables and links, and a Majestic DAC.
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 7 hours 46 minutes ago at 3:38pm
In all unregulated single supply transistor power amplifier circuits, the output's DC level is bound to change gradually with load. Now, where have I heard that before? I haven't!

Why didn't JLH say it? Did Self say it and I didn't understand? All they ever said was the output capacitor caused distortion, as well as saying class-B causes distortion.

JLH did say that it appears that there's more bass when a regulated power supply is used, but without explanation.

All I did was put a DVM between the OP stage DC and ground, and swept the signal amplitude up and down, and noticed how the DC changed with the droop on the HT (due to load).

And all I've done (with Bruce's measure thrice to the power of lots - to cut once) is to stop that happening by giving the voltage stage a regulator.

But I do understand they didn't have TL783's in the 60's or 70's, after which amplifier design had moved on anyway. And here it only just fits the required voltages. And you do have to fully grasp what a bootstrap does (negative current on positive swings).

It hasn't broke yet after being on for 24 hours, so I hope it has dealt with the unmeasurable distortion my ears were complaining of.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote BAK Quote  Post ReplyReply Direct Link To This Post Posted: 6 hours 55 minutes ago at 4:29pm
I have seen valve (tube) amplifiers use an OA2 or OA3 gas discharge valve as a shunt voltage regulator for the voltage stage valve circuit. The OA2 valves were 7-pin miniature valves, which 1st came out in the 1950's. (For the 1st time encountered, these OA2 circuits were hard to understand and repair.Wink)
 It comprised of an OA2 with constant current characteristics driving a triode to convert the constant current to a constant voltage, often with very close tolerance resistors like 1%. The regulated voltage from this would supply HT power to the 1st valve stages before the power-driving output valve stage. The power-driving output valve stage was supplied by the full, unregulated, HT power.

 This is basically what Graham has done in this SS power amplifier. Only, the SS circuits do this job much more accurately.


Edited by BAK - 6 hours 24 minutes ago at 5:00pm
Bruce
AT-14SA, Pickering XV-15/625, Hana EL, Technics SL-1600MK2, Reflex M or C, Lautus, Technics SH-8066, Eminence Beta 8A in custom cabs; Using Majestic DAC & Proprius
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