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Gemini Amp - and an idea for yet another headphone amp output stage

Posted 25th April 2014 at 11:13 PM by rjm
Updated 26th April 2014 at 08:19 AM by rjm

As a companion post to the GeminiPS I thought I'd throw the amplifier circuit out there too...

It's not something you'd have any reason to built today I think, but some of the ideas are worth revisiting.

The output stage is what is normally referred to as a complimentary Sziklai pair. The LTSpice circuit below uses the same output, but with the diamond buffer type bias, with it all scaled down to headphone-amplifier voltages and loads. It would be interesting to compare it against i.e. the conventional diamond buffer used in the Sapphire headphone amp. Maybe I'll get around to it. The simulation shows a bit more transient peaking than the straight diamond buffer, ideally there could be some way of adding compensation / reducing the bandwidth to more reasonable levels.
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Attached Files
File Type: asc geminiamp simple.asc (2.7 KB, 122 views)
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Old

Voltage regulators for (line level?) audio: The GeminiPS

Posted 22nd April 2014 at 02:40 AM by rjm
Updated 24th April 2014 at 12:40 AM by rjm

Part of a series.

The GeminiPS is another discrete series voltage regulator, with a Zener reference and bipolar pass transistor. It's an old circuit, published in Practical Electronics in 1970-71, and written by D.S. Gibbs and I.M. Shaw. I happen to have a reprint, but there's a nice overview here.

For reference it might be worth checking back to the two transistor regulator. The GeminiPS circuit is related in the sense that it is a more sophisticated take on the same basic principle. With just a handful of components we have a stabilized, 30 W output with soft turn on and short circuit protection. The circuit can be scaled up and down relatively easily, and the complimentary (negative output) version is an easy modification.

The pass transistor (TR2/3, Q2/3) is between the circuit common and the rectifier anodes. This may seem odd, but it was relatively common back in the day when high voltage transistors were both expensive and rare. The...
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File Type: asc voltageregulator-gemini.asc (4.3 KB, 120 views)
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Old

Voltage Regulators for Line Level Audio. Part 10 : Simple Shunts

Posted 11th April 2014 at 07:59 AM by rjm
Updated 16th April 2014 at 12:52 AM by rjm

Part of a series.

I've been meaning to take up shunt regulators for some time. I've never got around to building one myself to try, so I'll have to make do by playing in simulation.

Today's circuit is the shunt analog of the Z-reg series regulator: no feedback, Zener reference, single transistor regulation. The output impedance and ripple rejection-characteristics are similar too, with about 40 dB of RR and an output impedance of just a few ohms. It can be built equivalently from either an pnp or pnp transistor. (See attached LTSpice .asc files.)

The difference between shunt and series regulation can best be explained by considering the upstream power supply: In a series regulator an increase in current demand by the load causes the regulator to increase the current to compensate. In a shunt regulator an increase in current demand by the load causes the regulator to decrease the shunt current to balance, so there is no net change in current flowing...
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File Type: asc voltageregulator8-npn.asc (1.5 KB, 112 views)
File Type: asc voltageregulator8-pnp.asc (1.5 KB, 111 views)
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Old

The case of the crazy Sapphire amp.

Posted 3rd April 2014 at 02:24 AM by rjm
Updated 3rd April 2014 at 12:07 PM by rjm

Case report:

A set of Sapphire boards gave the proper V+, V- voltages out of the Z-reg, providing about 10.5 and -10.5 to op amp power pins. The output offsets were unusually high however, apparently at about 2 V in one board, and somewhat less in the other. Typically the offsets are in the order of +/-15 mV.

Changing out transistors and op amps did not help, and to all inspection the passive components were installed correctly and working properly. The offset voltages were extremely temperature sensitive. Measurements for the various circuit voltages were just screwy enough to be inconclusive.

I could ask for no more tests, so requested the boards be sent back to me. I found the circuit basically worked as expected, but the offsets were indeed high on both boards, though I measured 0.6 V max rather than 2 V.

***** stop here and make a guess *****

Blowing on the board through a soda straw, the offset shot up when I blew on...
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